Professor Philip St.J. Russell, FRS

Valleytronics in bulk MoS2 with a topologic optical field

Igor Tyulnev, Álvaro Jiménez-Galán, Julita Poborska, Lenard Vamos, Philip Russell, Francesco Tani, Olga Smirnova, Misha Ivanov, Rui E. F. Silva, et al.

The valley degree of freedom of electrons in materials promises routes towards energy-efficient information storage with enticing prospects for quantum information processing. Current challenges in utilizing valley polarization are symmetry conditions that require monolayer structures or specific material engineering non-resonant optical control to avoid energy dissipation and the ability to switch valley polarization at optical speed. We demonstrate all-optical and non-resonant control over valley polarization using bulk MoS2, a centrosymmetric material without Berry curvature at the valleys. Our universal method utilizes spin angular momentum-shaped trefoil optical control pulses to switch the material’s electronic topology and induce valley polarization by transiently breaking time and space inversion symmetry through a simple phase rotation. We confirm valley polarization through the transient generation of the second harmonic of a non-collinear optical probe pulse, depending on the trefoil phase rotation. The investigation shows that direct optical control over the valley degree of freedom is not limited to monolayer structures. Indeed, such control is possible for systems with an arbitrary number of layers and for bulk materials. Non-resonant valley control is universal and, at optical speeds, unlocks the possibility of engineering efficient multimaterial valleytronic devices operating on quantum coherent timescales.

Frequency conversion of vortex states by chiral forward Brillouin scattering in twisted photonic crystal fibre

Xinglin Zeng, Philip St.J. Russell, Birgit Stiller

Optical vortex states-higher optical modes with helical phase progression and carrying orbital angular momentum-have been explored to increase the flexibility and capacity of optical fibres employed for example in mode-division-multiplexing, optical trapping and multimode imaging. A common requirement in such systems is high fidelity transfer of signals between different frequency bands and modes, which for vortex modes is not so straightforward. Here we report intervortex conversion between backward-propagating circularly polarised vortex modes at one wavelength, using chiral flexural phonons excited by chiral forward stimulated Brillouin scattering at a different wavelength. The experiment is carried out using chiral photonic crystal fibre, which robustly preserves circular polarisation states. The chiral acoustic wave, which has the geometry of a spinning single-spiral corkscrew, provides the orbital angular momentum necessary to conserve angular momentum between the coupled optical vortex modes. The results open up new opportunities for interband optical frequency conversion and the manipulation of vortex states in both classical and quantum regimes.

Protecting Quantum Modes in Optical Fibers

Muhammad Abdullah Butt, Paul Roth, Gordon Wong, Michael Frosz, Luis Sanchez-Soto, E. A. Anashkina, A. V. Andrianov, Peter Banzer, Philip Russell, et al.

Polarization-preserving fibers maintain the two polarization states of an orthogonal basis. Quantum communication, however, requires sending at least two nonorthogonal states and these cannot both be preserved. We present an alternative scheme that allows for using polarization encoding in a fiber not only in the discrete, but also in the continuous-variable regime. For the example of a helically twisted photonic crystal fiber, we experimentally demonstrate that using appropriate nonorthogonal modes, the polarization-preserving fiber does not fully scramble these modes over the full Poincaré sphere, but that the output polarization will stay on a great circle; that is, within a one-dimensional protected subspace, which can be parametrized by a single variable. This allows for more efficient measurements of quantum excitations in nonorthogonal modes.

Modulational instability and spectral broadening of vortex modes in chiral photonic crystal fibers

Paul Roth, Philip Russell, Michael Frosz, Yang Chen, Gordon Wong

We report on intra- and inter-modal four-wave-mixing (FWM) in N-fold rotationally symmetric (C_N) single- and multi-core chiral photonic crystal fiber (PCF), created by spinning the preform during fiber drawing. The non-circular modal field is forced to rotate as it propagates along the fiber, resulting in circular birefringence and robust maintenance of circular polarization state. Multi-core chiral C_N PCF supports vortex-carrying helical Bloch modes (HBMs) in which the degeneracy between clockwise and counter-clockwise vortices is lifted. This makes possible new kinds of intermodal polarization modulational instability (PMI). We develop PMI theory for vortex HBMs, and illustrate the results by a series of experiments in which two or more PMI sidebands with different vorticities and polarization states are selectively generated by adjusting the polarization state and topological charge of the pump light. In every case both the topological charge and the spin of the pump light are conserved. We also report generation of a broadband supercontinuum in a single circularly polarized vortex mode.

Compact Yb fiber few-cycle pulse source based on precision pulse compression and shaping with an adaptive fiber Bragg grating

Jacob Lampen, Francesco Tani, Peng Li, Kevin F. Lee, Jie Jiang, Philip Russell, Martin E. Fermann

We generate bandwidth limited 10 µJ pulses of 92 fs pulse width using an adaptive fiber Bragg grating stretcher (FBG) in conjunction with a Lyot filter. The temperature controlled FBG is used to optimize the group delay, whereas the Lyot filter counteracts gain narrowing in the amplifier chain. Soliton compression in a hollow core fiber (HCF) allows for access to the few-cycle pulse regime. Adaptive control further enables the generation of nontrivial pulse shapes.

Selective phase filtering of charged beams with laser-driven antiresonant hollow-core fibers

Luca Genovese, Max Kellermeier, Frank Mayet, Klaus Floettmann, Gordon Wong, Michael Frosz, Ralph Assmann, Philip Russell, Francois Lemery

Emerging accelerator concepts increasingly rely on the combination of high-frequency electromagnetic radiation with electron beams, enabling longitudinal phase space manipulation which supports a variety of advanced applications. The handshake between electron beams and radiation is conventionally provided by magnetic undulators which unfortunately require a balance between the electron beam energy, undulator parameters, and laser wavelength. Here we propose a scheme using laser-driven large-core antiresonant optical fibers to manipulate electron beams. We explore two general cases using TM01 and HE11 modes. In the former, we show that large energy modulations O(100 keV). can be achieved while maintaining the overall electron beam quality. Further, we show that by using larger field strengths O(100 MV/m) the resulting transverse forces can be exploited with beam-matching conditions to filter arbitrary phases from the modulated electron bunch, leading to the production of ≈100 attosecond FWHM microbunches. Finally, we also investigate the application of the transverse dipole HE11 mode and find it suitable for supporting time-resolved electron beam measurements with sub-attosecond resolution. We expect the findings to be widely appealing to high-charge pump-probe experiments, metrology, and accelerator science.

Optical Vortex Brillouin Laser

Xinglin Zeng, Philip Russell, Yang Chen, Zheqi Wang, Gordon Wong, Paul Roth, Michael Frosz, Birgit Stiller

Optical vortices, which have been extensively studied over the last decades, offer an additional degree of freedom useful in many applications, such as optical tweezers and quantum control. Stimulated Brillouin scattering (SBS), providing a narrow linewidth and a strong nonlinear response, has been used to realize quasi-continuous wave lasers. Here, stable oscillation of optical vortices and acoustic modes in a Brillouin laser based on chiral photonic crystal fiber (PCF) is reported, which robustly supports helical Bloch modes (HBMs) that carry circularly polarized optical vortex and display circular birefringence. A narrow-linewidth Brillouin fiber laser that stably emits 1st- and 2nd-order vortex-carrying HBMs is implemented. Angular momentum conservation selection rules dictate that pump and backward Brillouin signals have opposite topological charge and spin. Additionally, it is shown that when the chiral PCF is placed within a laser ring cavity, the linewidth-narrowing associated with lasing permits the peak of the Brillouin gain that corresponds to acoustic mode to be measured with resolution of 10 kHz and accuracy of 520 kHz. The results pave the way to a new generation of vortex-carrying SBS systems with applications in optical tweezers, quantum information processing, and vortex-carrying nonreciprocal systems.

Temporal Self-Compression and Self-Frequency Shift of Submicrojoule Pulses at a Repetition Rate of 8 MHz

Francesco Tani, Jacob Lampen, Martin Butryn, Michael Frosz, Jie Jiang, Martin E. Fermann, Philip Russell

We combine soliton dynamics in gas-filled hollow-core photonic crystal fibers with a state-of-the-art fiber laser to realize a turnkey system producing few-femtosecond pulses at 8-MHz repetition rate at pump energies as low as 220 nJ. Furthermore, by exploiting the soliton self-frequency shift in a second hydrogen-filled hollow-core fiber, we efficiently generate pulses as short as 22 fs, continuously tunable from 1100 to 1474 nm.

Nonreciprocal vortex isolator via topology-selective stimulated Brillouin scattering

Xinglin Zeng, Philip Russell, Christian Wolff , Michael Frosz, Gordon Wong, Birgit Stiller

Optical nonreciprocity, which breaks the symmetry between forward and backward propagating optical waves, has become vital in photonic systems and enables many key applications. So far, all the existing nonreciprocal systems are implemented for linearly or randomly polarized fundamental modes. Optical vortex modes, with wavefronts that spiral around the central axis of propagation, have been extensively studied over the past decades and offer an additional degree of freedom useful in many applications. Here, we report a light-driven nonreciprocal isolation system for optical vortex modes based on topology-selective stimulated Brillouin scattering (SBS) in chiral photonic crystal fiber. The device can be reconfigured as an amplifier or an isolator by adjusting the frequency of the control signal. The experimental results show vortex isolation of 22 decibels (dB), which is at the state of the art in fundamental mode isolators using SBS. This device may find applications in optical communications, fiber lasers, quantum information processing, and optical tweezers.

Strong circular dichroism for the HE11 mode in twisted single-ring hollow-core photonic crystal fiber: erratum

Paul Roth, Yang Chen, Mehmet Can Günendi, Ramin Beravat, Nitin Edavalath, Michael Frosz, Goran Ahmed, Gordon Wong, Philip Russell

Recent work has revealed that the dispersion relation, given inOptica 5, 1315 (2018), for helicalBloch modes in a ring of capillaries surrounding a central hollowcore, is incorrect.Herewe correct this error and provide a revised version of Fig. 2. The overall conclusions of the original paper are unaffected.

Erratum to “Bragg Reflection and Conversion Between Helical Bloch Modes in Chiral Three-Core Photonic Crystal Fiber”

Sébastien Loranger, Yang Chen, Paul Roth, Michael Frosz, Gordon Wong, Philip Russell

The dispersion relation for the helical Bloch modes in this paper contains an error, which affects Equation (3) in the original manuscript, as well as Fig. 2. Otherwise the conclusions of the paper are unaffected.

Tunable and state-preserving frequency conversion of single photons in hydrogen

Rinat Tyumenev, Jonas Hammer, Nicolas Joly, Philip St.J. Russell, David Novoa

In modern quantum technologies, preservation of the photon statistics of quantum optical states upon frequency conversion holds the key to the viable implementation of quantum networks, which often require interfacing of several subsystems operating in widely different spectral regions. Most current approaches offer only very small frequency shifts and limited tunability, while suffering from high insertion loss and Raman noise originating in the materials used. We introduce a route to quantum-correlation–preserving frequency conversion using hydrogen-filled antiresonant-reflecting photonic crystal fibers. Transient optical phonons generated by stimulated Raman scattering enable selective frequency up-conversion by 125 terahertz of the idler photon of an entangled pair, with efficiencies up to 70%. This threshold-less molecular modulation process preserves quantum correlations, making it ideal for applications in quantum information.

Backward jet propulsion of particles by femtosecond pulses in hollow-core photonic crystal fiber

Maria N. Romodina, Shangran Xie, Francesco Tani, Philip St.J. Russell

A dielectric microparticle, optically trapped within an air-filled hollow-core photonic crystal fiber PCF), is accelerated backwards close to the speed of sound when a single guided femtosecond pulse is incident upon it. Acting as a spherical lens, the particle focuses a fraction of the pulse energy onto its inner rear surface, causing the material to ablate. The resulting plasma and vapor jet act like a rocket motor, driving the particle backward at peak accelerations conservatively estimated at more than a million times gravity. Using counter-propagating pulses to suppress particle motion, the effect may permit the inner core walls to be coated locally with different materials, allowing optical devices to be created at otherwise inaccessible points inside long lengths of hollow-core PCF.

Stimulated Brillouin scattering in chiral photonic crystal fiber

Xinglin Zeng, Wenbin He, Michael Frosz, Andreas Geilen, Paul Roth, Gordon Wong, Philip Russell, Birgit Stiller

Stimulated Brillouin scattering (SBS) has many applications; for example, in sensing, microwave photonics, and signal processing. Here, we report the first experimental study of SBS in chiral photonic crystal fiber (PCF), which displays optical activity and robustly maintains circular polarization states against external perturbations. As a result, circularly polarized pump light is cleanly backscattered into a Stokes signal with the orthogonal circular polarization state, as is required by angular momentum conservation. By comparison, untwisted PCF generates a Stokes signal with an unpredictable polarization state, owing to its high sensitivity to external perturbations. We use chiral PCF to realize a circularly polarized continuous-wave Brillouin laser. The results pave the way for a new generation of stable circularly polarized SBS systems with applications in quantum manipulation, optical tweezers, optical gyroscopes, and fiber sensors.

Synchronization of gigahertz core resonances in multiple photonic crystal fiber cores by timing-modulated harmonic mode locking

Dung-Han Yeh, Wenbin He, Meng Pang, Xin Jiang, Philip St.J. Russell

Synchronization of mechanical oscillators by optical forces is a topic that has been much explored in recent years, for example, in the context of SiN microdisk resonators. Here we report stable long-term synchronization of the core vibrations of three different photonic crystal fibers, driven intra-cavity by a 2 GHz train of timing-modulated pulses in a high harmonic opto-acoustically mode-locked fiber laser. The core resonances are equally spaced in frequency and are coupled purely by the optical field. Under the correct conditions, they become stably synchronized, being simultaneously driven by the timing-modulated pulse train. Floquet–Bloch theory, in which the pulses are treated as particles trapped in potential wells and coupled by optomechanical back-action, describes the complex temporal dynamics observed in the experiments. This unique system provides a novel means of modifying the temporal structure of pulse trains running at few-gigahertz repetition rates.

Twist and strain tuning of third harmonic generation in glass nanostrand with two sub-wavelength hollow channels

Yang Chen, Jonas Hammer, Nicolas Joly, Philip Russell

A major challenge in third harmonic generation and its converse, parametric down-conversion, is how to arrange phase matching between signals at omega and 3 omega while maintaining a high nonlinear overlap. In this Letter, we present a design consisting of a nanostrand of glass with two hollow channels. The fundamental and third harmonic modal fields, enhanced in the region between the channels, have high nonlinear overlap, while the phase-matching wavelength can be coarse-tuned by gas pressure and fine-tuned by axial strain and mechanical twist, which, remarkably, have opposite effects. The ability to adjust the phase-matching condition may facilitate efficient generation of entangled photon triplets. (C) 2021 Optical Society of America.

Deep-UV-enhanced supercontinuum generated in a tapered gas-filled photonic crystal fiber

Mallika Irene Suresh, Jonas Hammer, Nicolas Y. Joly, Philip Russell, Francesco Tani

We present the use of a linearly down-tapered gas-filled hollow-core photonic crystal fiber in a single stage, pumped with pulses froma compact infrared (IR) laser source, to generate a supercontinuum (SC) carrying significant spectral power in the deep ultraviolet (UV) [200-300 nm]. The generated SC extends from the near IR down to similar to 213 nm with 0.58 mW/nm and down to similar to 220 nm with 0.83 mW/nm in the deepUV. (C) 2021 Optical Society of America

Optical signatures of the coupled spin-mechanics of a levitated magnetic microparticle

Vanessa Wachter, Victor A. S. V. Bittencourt, Shangran Xie, Sanchar Sharma, Nicolas Joly, Philip Russell, Florian Marquardt, Silvia Viola-Kusminskiy

We propose a platform that combines the fields of cavity optomagnonics and levitated optome-<br>chanics in order to control and probe the coupled spin-mechanics of magnetic dielectric particles. We theoretically study the dynamics of a levitated Faraday-active dielectric microsphere serving as an optomagnonic cavity, placed in an external magnetic field and driven by an external laser. We find that the optically driven magnetization dynamics induces angular oscillations of the particle with low associated damping. Further, we show that the magnetization and angular motion dynamics<br>can be probed via the power spectrum of the outgoing light. Namely, the characteristic frequencies attributed to the angular oscillations and the spin dynamics are imprinted in the light spectrum by two main resonance peaks. Additionally, we demonstrate that a ferromagnetic resonance setup with an oscillatory perpendicular magnetic field can enhance the resonance peak corresponding to<br>the spin oscillations and induce fast rotations of the particle around its anisotropy axis.

Reconfigurable millimeter-range optical binding of dielectric microparticles in hollow-core photonic crystal fiber

Abhinav Sharma, Shangran Xie, Philip St.J. Russell

Optical binding of microparticles offers a versatile playground for investigating the optomechanics of levitated multi-particle systems. We report millimeter-range optical binding of polystyrene microparticles in hollow-core photonic crystal fiber. The first particle scatters the incident LP₀₁ mode into several LP_0n modes, creating a beat pattern that exerts a position-dependent force on the second particle. Particle binding results from the interplay of the forces created by counterpropagating beams. A femtosecond trapping laser is used so that group velocity walk-off eliminates disturbance caused by higher order modes accidentally excited at the fiber input. The inter-particle distance can be optically switched over 2 orders of magnitude (from 42 µm to 3 mm), and the bound particle pairs can be translated along the fiber by unbalancing the powers in the counterpropagating trapping beams. The frequency response of a bound particle pair is investigated at low gas pressure by driving with an intensity-modulated control beam. The system offers new degrees of freedom for manipulating the dynamics and configurations of optically levitated microparticle arrays.

Tumbling and anomalous alignment of optically levitated anisotropic microparticles in chiral hollow-core photonic crystal fiber

Shangran Xie, Abhinav Sharma, Maria N. Romodina, Nicolas Y. Joly, Philip Russell

The complex tumbling motion of spinning nonspherical objects is a topic of enduring interest, both in popular culture and in advanced scientific research. Here, we report all-optical control of the spin, precession, and nutation of vaterite microparticles levitated by counterpropagating circularly polarized laser beams guided in chiral hollow-core fiber. The circularly polarized light causes the anisotropic particles to spin about the fiber axis, while, regulated by minimization of free energy, dipole forces tend to align the extraordinary optical axis of positive uniaxial particles into the plane of rotating electric field. The end result is that, accompanied by oscillatory nutation, the optical axis reaches a stable tilt angle with respect to the plane of the electric field. The results reveal new possibilities for manipulating optical alignment through rotational degrees of freedom, with applications in the control of micromotors and microgyroscopes, laser alignment of polyatomic molecules, and study of rotational cell mechanics.

Scaling rules for high quality soliton self-compression in hollow-core fibers

Daniel Schade, Felix Köttig, Johannes Köhler, Michael H. Frosz, Philip St.J. Russell, Francesco Tani

Soliton dynamics can be used to temporally compress laser pulses to few fs durations in many different spectral regions. Here we study analytically, numerically and experimentally the scaling of soliton dynamics in noble gas-filled hollow-core fibers. We identify an optimal parameter region, taking account of higher-order dispersion, photoionization, self-focusing, and modulational instability. Although for single-shots the effects of photoionization can be reduced by using lighter noble gases, they become increasingly important as the repetition rate rises. For the same optical nonlinearity, the higher pressure and longer diffusion times of the lighter gases can considerably enhance the long-term effects of ionization, as a result of pulse-by-pulse buildup of refractive index changes. To illustrate the counter-intuitive nature of these predictions, we compressed 250 fs pulses at 1030 nm in an 80-cm-long hollow-core photonic crystal fiber (core radius 15 µm) to ∼5 fs duration in argon and neon, and found that, although neon performed better at a repetition rate of 1 MHz, stable compression in argon was still possible up to 10 MHz.

Synthesis and dissociation of soliton molecules in parallel optical-soliton reactors

W He, M Pang, D.-H. Yeh, J Huang, Philip St. J. Russell

Mode-locked lasers have been widely used to explore interactions between optical solitons, including bound-soliton<br>states that may be regarded as “photonic molecules”. Conventional mode-locked lasers normally, however, host at<br>most only a few solitons, which means that stochastic behaviours involving large numbers of solitons cannot easily be<br>studied under controlled experimental conditions. Here we report the use of an optoacoustically mode-locked fibre<br>laser to create hundreds of temporal traps or “reactors” in parallel, within each of which multiple solitons can be<br>isolated and controlled both globally and individually using all-optical methods. We achieve on-demand synthesis and<br>dissociation of soliton molecules within these reactors, in this way unfolding a novel panorama of diverse dynamics in<br>which the statistics of multi-soliton interactions can be studied. The results are of crucial importance in understanding<br>dynamical soliton interactions and may motivate potential applications for all-optical control of ultrafast light fields in<br>optical resonators.

Doppler optical frequency domain reflectometry for remote fiber sensing

Max Koeppel, Abhinav Sharma, Jasper Podschus, Sanju Sundaramahalingam, Nicolas Y. Joly, Shangran Xie, Philip Russell, Bernhard Schmauss

Coherent optical frequency domain reflectometry has been widely used to locate static reflectors with high spatial resolution. Here, we present a new type of Doppler optical frequency domain reflectometry that offers simultaneous measurement of the position and speed of moving objects. The system is exploited to track optically levitated "flying" particles inside a hollow-core photonic crystal fiber. As an example, we demonstrate distributed temperature sensing with sub-mm-scale spatial resolution and a standard deviation of similar to 10 degrees C up to 200 degrees C. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Efficient self-compression of ultrashort near-UV pulses in air-filled hollow-core photonic crystal fibers

Jie Luan, Philip St. J. Russell, David Novoa

We report generation of ultrashort near-UV pulses by soliton self-compression in kagomé-style hollow-core photonic crystal fibers filled with ambient air. Pump pulses with the energy of 2.6 µJ and duration of 54 fs at 400 nm were compressed temporally by a factor of 5, to a duration of ∼11 fs. The experimental results are supported by numerical simulations, showing that both Raman and Kerr effects play a role in the compression dynamics. The convenience of using ambient air and the absence of glass windows that would distort the compressed pulses makes the setup highly attractive as the basis of an efficient table-top UV pulse compressor.

Broadband mid-infrared supercontinuum generation in dispersion-engineered As2S3-silica nanospike waveguides pumped by 2.8 μm femtosecond laser

Pan Wang, Jiapeng Huang, Shangran Xie, Johann Troles, Philip St. J. Russell

Broadband mid-infrared (IR) supercontinuum laser sources are essential for spectroscopy in the molecular fingerprint region. Here, we report generation of octave-spanning and coherent mid-IR supercontinua in As2S3-silica nanospike hybrid waveguides pumped by a custom-built 2.8 μm femtosecond fiber laser. The waveguides are formed by pressure-assisted melt-filling of molten As2S3 into silica capillaries, allowing the dispersion and nonlinearity to be precisely tailored. Continuous coherent spectra spanning from 1.1 μm to 4.8 μm (30 dB level) are observed when the waveguide is designed so that 2.8 μm lies in the anomalous dispersion regime. Moreover, linearly tapered millimeter-scale As2S3-silica waveguides are fabricated and investigated for the first time, to the best of our knowledge, showing much broader supercontinua than uniform waveguides, with improved spectral coherence. The waveguides are demonstrated to be long-term stable and water-resistant due to the shielding of the As2S3 by the fused silica sheath. They offer an alternative route to generating broadband mid-IR supercontinua, with applications in frequency metrology and molecular spectroscopy, especially in humid and aqueous environments.

Frenet–Serret analysis of helical Bloch modes in N-fold rotationally symmetric rings of coupled spiraling optical waveguides

Yang Chen, Philip St.J. Russell

The behavior of electromagnetic waves in chirally twisted structures is a topic of enduring interest, dating back at least to the 1940s invention of the microwave travelling-wave-tube amplifier and culminating in contemporary studies of chiral metamaterials, metasurfaces, and photonic crystal fibers (PCFs). Optical fibers with chiral microstructures, drawn from a spinning preform, have many useful properties, exhibiting, for example, circular birefringence and circular dichroism. It has recently been shown that chiral fibers with N-fold rotationally symmetric (symmetry group CN) transverse microstructures support families of helical Bloch modes (HBMs), each of which consists of a superposition of azimuthal Bloch harmonics (or optical vortices). An example is a fiber with N coupled cores arranged in a ring around its central axis (N-core single-ring fiber). Although this type of fiber can be readily modeled using scalar coupled-mode theory, a full description of its optical properties requires a vectorial analysis that takes account of the polarization state of the light, which is particularly important in studies of circular and vortical birefringence. In this paper, we develop, using an orthogonal 2D helicoidal coordinate system embedded in a cylindrical surface at constant radius, a rigorous vector coupled-mode description of the fields using local Frenet–Serret frames that rotate and twist with each of the N cores. The analysis places on a firm theoretical footing a previous HBM theory in which a heuristic approach was taken, based on physical intuition of the properties of Bloch waves. After a detailed review of the polarization evolution in a single spiraling core, analysis of the N-core single-ring system is carefully developed step by step. Accuracy limits of the analysis are assessed by comparison with the results of finite element modeling, focusing in particular on the dispersion, polarization states, and transverse field profiles of the HBMs. We believe this study provides clarity into what can sometimes be a rather difficult field and will facilitate further exploration of real-world applications of these fascinating waveguiding systems.

Post-recombination effects in confined gases photoionized at megahertz repetition rates

Johannes Köhler, Felix Köttig, Daniel Schade, Philip Russell, Francesco Tani

Recombination-driven acoustic pulses and heating in a photoionized gas transiently alter its refractive index. Slow thermal dissipation can cause substantial heat accumulation and impair the performance and stability of gas-based laser systems operating at strong-field intensities and megahertz repetition rates. Here we study this effect by probing the pulse-by-pulse buildup of refractive index changes in gases spatially confined inside a capillary. A high-power repetition-rate-tunable femtosecond laser photoionizes the gas at its free-space focus, while a transverse-propagating probe laser interferometrically monitors the resulting time-dependent changes in refractive index. The system allows convenient exploration of the nonlinear regimes used to temporally compress pulses with durations in the ∼30 to ∼300 fs range. We observe thermal gas-density depressions, milliseconds in duration, that saturate to a level that depends on the peak intensity and repetition rate of the pulses, in good agreement with numerical modelling. The dynamics are independently confirmed by measuring the mean speed-of-sound across the capillary core, allowing us to infer that the temperature in the gas can exceed 1000 K. Finally, we explore several strategies for mitigating these effects and improving the stability of gas-based high-power laser systems at high repetition rates.

Optofluidic photonic crystal fiber microreactors for in-situ studies of carbon nanodot-driven photoreduction

Philipp Koehler, Takashi Lawson, Julian Neises, Janina Willkomm, Benjamin C. M. Martindale , Georgina A. M. Hutton, Daniel Antón-García, Ava Lage, Alexander S. Gentleman, et al.

Performing quantitative in situ spectroscopic analysis on minuscule sample volumes is a common difficulty in photochemistry. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liquid channel and acts as an optofluidic microreactor with a reaction volume of less than 35 nL. The system was used to demonstrate in situ optical detection of photoreduction processes that are key components of many photocatalytic reaction schemes. The photoreduction of viologens (XV2+) to the radical XV•+ in a homogeneous mixture with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradiation cycles, are interpreted with a quantitative kinetic model to determine photoreduction and photobleaching rate constants. The powerful combination of time-resolved, low-volume absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quantitative, and rapid screening platform for novel photocatalysts and flow chemistry in general.

Cross-phase modulational instability of circularly polarized helical Bloch modes carrying optical vortices in a chiral three-core photonic crystal fiber

Paul Roth, Michael Frosz, Linda Weise, Philip Russell, Gordon Wong

We report the first, to the best of our knowledge, observation of cross-phase modulational instability (XPMI) of circularly polarized helical Bloch modes carrying optical vortices in a twisted photonic crystal fiber with a three-fold symmetric core, formed by spinning the fiber preform during the draw. When the fiber is pumped by a superposition of left-circular polarization (LCP) and right-circular polarization (RCP) modes, a pair of orthogonal circularly polarized sidebands of opposite topological charge is generated. When, on the other hand, a pure LCP (or RCP) mode is launched, the XPMI gain is zero, and no sidebands are seen. This observation has not been seen before in any system and is unique to chiral structures with N-fold rotational symmetry. The polarization state and topological charge of the generated sidebands are measured. By decomposing the helical Bloch modes into their azimuthal harmonics, we are able to deduce the selection rules for the appearance of modulational instability sidebands. We showed that the four waves in the nonlinear mixing process must exhibit the same set of azimuthal harmonic orders.

Seven-octave high-brightness and carrier-envelope-phase-stable light source

Ugaitz Elu, Luke Maidment, Lenard Vamos, Francesco Tani, David Novoa, Michael H. Frosz, Valeriy Badikov, Dmitrii Badikov, Valentin Petrov, et al.

High-brightness sources of coherent and few-cycle-duration light waveforms with spectral coverage from the ultraviolet to the terahertz would offer unprecedented versatility and opportunities for a wide range of applications from bio-chemical sensing1 to time-resolved and nonlinear spectroscopy, and to attosecond light-wave electronics. Combinations of various sources with frequency conversion and supercontinuum generation can provide relatively large spectral coverage, but many applications require a much broader spectral range and low-jitter synchronization for time-domain measurements. Here, we present a carrier-envelope-phase (CEP)-stable light source, seeded by a mid-infrared frequency comb, with simultaneous spectral coverage across seven optical octaves, from the ultraviolet (340 nm) into the terahertz (40,000 nm). Combining soliton self-compression and dispersive wave generation in an anti-resonant-reflection photonic-crystal fibre with intra-pulse difference frequency generation in BaGa2GeSe6, the spectral brightness is two to five orders of magnitude above that of synchrotron sources. This will enable high-dynamic-range spectroscopies and provide numerous opportunities in attosecond physics and material sciences.

Sub-40  fs pulses at 1.8  µm and MHz repetition rates by chirp-assisted Raman scattering in hydrogen-filled hollow-core fiber

Sébastien Loranger, Philip Russell, David Novoa

The possibility to perform time-resolved spectroscopic studies in the molecular fingerprinting region or extending the cutoff wavelength of high-harmonic generation has recently boosted the development of efficient mid-infrared (mid-IR) ultrafast lasers. In particular, fiber lasers based on active media such as thulium or holmium are a very active area of research since they are robust, compact, and can operate at high repetition rates. These systems, however, are still complex, are unable to deliver pulses shorter than 100 fs, and are not yet as mature as their near-infrared counterparts. Here, we report the generation of sub-40 fs pulses at 1.8 µm, with quantum efficiencies of 50% and without the need for post-compression, in hydrogen-filled, hollow-core photonic crystal fiber pumped by a commercial high-repetition-rate 300 fs fiber laser at 1030 nm. This is achieved by pressure-tuning the dispersion and avoiding Raman gain suppression by adjusting the chirp of the pump pulses and the proportion of higher-order modes launched into the fiber. The system is optimized using a physical model that incorporates the main linear and nonlinear contributions to the optical response. The approach is average power-scalable, permits adjustment of the pulse shape, and can potentially allow access to much longer wavelengths.

Covariance spectroscopy of molecular gases using fs pulse bursts created by modulational instability in gas-filled hollow-core fiber

Mallika Irene Suresh, Philip Russell, Francesco Tani

We present a technique that uses noisy broadband pulse bursts generated by modulational instability to probe nonlinear processes, including infrared-inactive Raman transitions, in molecular gases. These processes imprint correlations between different regions of the noisy spectrum, which can be detected by acquiring single shot spectra and calculating the Pearson correlation coefficient between the different frequency components. Numerical simulations verify the experimental measurements and are used to further understand the system and discuss methods to improve the signal strength and the spectral resolution of the technique.

Modulational-instability-free pulse compression in anti-resonant hollow-core photonic crystal fiber

Felix Köttig, Francesco Tani, Philip Russell

Gas-filled hollow-core photonic crystal fiber (PCF) is used for efficient nonlinear temporal compression of femtosecond laser pulses, two main schemes being direct soliton-effect self-compression and spectral broadening followed by phase compensation. To obtain stable compressed pulses, it is crucial to avoid decoherence through modulational instability (MI) during spectral broadening. Here, we show that changes in dispersion due to spectral anti-crossings between the fundamental-core mode and core wall resonances in anti-resonant-guiding hollow-core PCF can strongly alter the MI gain spectrum, enabling MI-free pulse compression for optimized fiber designs. The results are important, since MI cannot always be suppressed by pumping in the normal dispersion regime.

Narrowband Vacuum Ultraviolet Light via Cooperative Raman Scattering in Dual-Pumped Gas-Filled Photonic Crystal Fiber

Rinat Tyumenev, Philip Russell, David Novoa

Many fields such as biospectroscopy and photochemistry often require sources of vacuum ultraviolet (VUV) pulses featuring a narrow line width and tunable over a wide frequency range. However, the majority of available VUV light sources do not simultaneously fulfill those two requirements and few if any are truly compact, cost-effective, and easy to use by nonspecialists. Here we introduce a novel approach that goes a long way to meeting this challenge. It is based on hydrogen-filled hollow-core photonic crystal fiber pumped simultaneously by two spectrally distant pulses. Stimulated Raman scattering enables the generation of coherence waves of collective molecular motion in the gas, which together with careful dispersion engineering and control over the modal content of the pump light, facilitates cooperation between the two separate Raman combs, resulting in a spectrum that reaches deep into the VUV. Using this system, we demonstrate the generation of a dual Raman comb of narrowband lines extending down to 141 nm using only 100 mW of input power delivered by a commercial solid-state laser. The approach may enable access to tunable VUV light to any laboratory and therefore boost progress in many research areas across multiple disciplines.

Sub-two-cycle octave-spanning mid-infrared fiber laser

Jiapeng Huang, Meng Pang, Xin Jiang, Felix Köttig, Daniel Schade, Wenbin He, Martin Butryn, Philip Russell

Compact and powerful ultrafast light sources at high pulse repetition rates, based on mode-locked near infrared fiber lasers, are now widely available and are being used in applications such as frequency metrology, molecular spectroscopy, and laser micro-machining. The realization of such lasers in the mid-infrared has, however, remained a challenge for many years. Here we report a record-breaking three-stage fiber laser system that uses an Er-doped fluoride fiber as gain medium, delivering W-level few-cycle pulses at 2.8 µm at a repetition rate of 42.1 MHz. A fiber-based seed oscillator, cavity dispersion-managed by a pulse-stretcher, generates near-100-fs mid-infrared pulses with >110nm spectral bandwidth. These pulses are amplified to an average power of ∼1 W in a chirp-engineered fiber amplifier, and then compressed to ∼16 fs in a short length of highly nonlinear ZBLAN fiber, resulting in a more-than-octave-wide spectrum reaching from 1.8 µm to 3.8 µm with a total power of 430 mW.

Optomechanical cooling and self-stabilization of a waveguide coupled to a whispering-gallery-mode resonator

Riccardo Pennetta, Shangran Xie, Richard Zeltner, Jonas Hammer, Philip Russell

Laser cooling of mechanical degrees of freedom is one of the most significant achievements in the field of optomechanics. Here, we report, for the first time to the best of our knowledge, efficient passive optomechanical cooling of the motion of a freestanding waveguide coupled to a whispering-gallery-mode (WGM) resonator. The waveguide is an 8 mm long glass-fiber nanospike, which has a fundamental flexural resonance at Ω/2π=2.5  kHz and a Q-factor of 1.2×10^5. Upon launching ∼250  μW laser power at an optical frequency close to the WGM resonant frequency, we observed cooling of the nanospike resonance from room temperature down to 1.8 K. Simultaneous cooling of the first higher-order mechanical mode is also observed. The strong suppression of the overall Brownian motion of the nanospike, observed as an 11.6 dB reduction in its mean square displacement, indicates strong optomechanical stabilization of linear coupling between the nanospike and the cavity mode. The cooling is caused predominantly by a combination of photothermal effects and optical forces between nanospike and WGM resonator. The results are of direct relevance in the many applications of WGM resonators, including atom physics, optomechanics, and sensing.

Three-photon head-mounted microscope for imaging deep cortical layers in freely moving rats

Alexandr Klioutchnikov, Damian J Wallace, Michael H. Frosz, Richard Zeltner, Jürgen Sawinski, Verena Pawlak, Kay-Michael Voit, Philip St. J. Russell, Jason N. D. Kerr

We designed a head-mounted three-photon microscope for imaging deep cortical layer neuronal activity in a freely moving rat. Delivery of high-energy excitation pulses at 1,320 nm required both a hollow-core fiber whose transmission properties did not change with fiber movement and dispersion compensation. These developments enabled imaging at >1.1 mm below the cortical surface and stable imaging of layer 5 euronal activity for >1 h in freely moving rats performing a range of behaviors.

Three-photon head-mounted microscope for imaging deep cortical layers in freely moving rats

Alexandr Klioutchnikov, Damian James Wallace, Michael H. Frosz, Richard Zeltner, Jürgen Sawinski, Verena Pawlak, Kay-Michael Voit, Philip St. J. Russell, Jason Kerr

We designed a head-mounted three-photon microscope for imaging deep cortical layer neuronal activity in a freely moving rat. Delivery of high-energy excitation pulses at 1,320 nm required both a hollow-core fiber whose transmission properties did not change with fiber movement and dispersion compensation. These developments enabled imaging at >1.1 mm below the cortical surface and stable imaging of layer 5 euronal activity for >1 h in freely moving rats performing a range of behaviors.

Thermally tunable whispering-gallery mode cavities for magneto-optics

Serge Vincent, Xin Jiang, Philip Russell, Frank Vollmer

We report the experimental realization of magneto-optical coupling between whispering-gallery modes in a germanate (56GeO2-31PbO9Na2O-4Ga2O3) microspherical cavity due to the Faraday effect. An encapsulated gold conductor heats the resonator and tunes the quasitransverse electric (TE) and quasi-transverse magnetic (TM) polarized modes with an efficiency of 65 fm/V at a peak-to-peak bias voltage of 4 V. The signal parameters for a number of heating regimes are quantified to confirm sensitivity to the generated magnetic field. The quasi-TE and quasi-TM resonance frequencies stably converge near the device’s heating rate limit (equivalently, bias voltage limit) in order to minimize inherent geometrical birefringence. This functionality optimizes Faraday rotation and thus enables the observation of subsequent magneto-optics.

Bragg Reflection and Conversion Between Helical Bloch Modes in Chiral Three-Core Photonic Crystal Fiber

Sébastien Loranger, Yang Chen, Paul Roth, Michael Frosz, Gordon Wong, Philip Russell

Optical fiber modes carrying orbital angular momentum (OAM) have many applications, for example in mode-division-multiplexing for optical communications. The natural guided modes of N-fold rotationally symmetric optical fibers, such as most photonic crystal fibers, are helical Bloch modes (HBMs). HBMs consist of a superposition of azimuthal harmonics (order m) of order l_A(m)=l_A(0)+mN. When such fibers are twisted, these modes become circularly and azimuthally birefringent, that is to say HBMs with equal and opposite values of l_A(0) and spin s are non-degenerate. In this article we report the use of Bragg mirrors to reflect and convert HBMs in a twisted three-core photonic crystal fiber, and show that by writing a tilted fiber Bragg grating (FBG), reflection between HBMs of different orders becomes possible, with high wavelength-selectivity. We measure the near-field phase and amplitude distribution of the reflected HBMs interferometrically, and demonstrate good agreement with theory. This new type of FBG has potential applications in fiber lasers, sensing, quantum optics, and in any situation where creation, conversion, and reflection of OAM-carrying modes is required.

Robust excitation and Raman conversion of guided vortices in a chiral gas-filled photonic crystal fiber

Sona Davtyan, Yang Chen, Michael Frosz, Philip Russell, David Novoa

The unique ring-shaped intensity patterns and helical phase fronts of optical vortices make them useful in many applications. Here we report for the first time, to the best of our knowledge, efficient Raman frequency conversion between vortex modes in a twisted hydrogen-filled single-ring hollow core photonic crystal fiber (SR-PCF). High-fidelity transmission of optical vortices in an untwisted SR-PCF becomes<br>more and more difficult as the orbital angular momentum (OAM) order increases, due to scattering at structural imperfections in the fiber microstructure. In a helically twisted<br>SR-PCF, however, the degeneracy between left- and righthanded versions of the same mode is lifted, with the result<br>that they are topologically protected from such scattering. With launch efficiencies of ∼75%, a high damage threshold and broadband guidance, these fibers are ideal for performing nonlinear experiments that require the polarization<br>state and azimuthal order of the interacting modes to be preserved over long distances. Vortex coherence waves of internal molecular motion carrying angular momentum are excited in the gas, permitting the polarization and OAM of the Raman bands to be tailored, even in spectral regions where conventional solid-core waveguides are opaque or susceptible to optical damage.

Efficient single-cycle pulse compression of an ytterbium fiber laser at 10 MHz repetition rate

Felix Köttig, Daniel Schade, Johannes Köhler, Philip Russell, Francesco Tani

Over the past years, ultrafast lasers with average powers in the 100 W range have become a mature technology, with a multitude of applications in science and technology. Nonlinear temporal compression of these lasers to few- or even single-cycle duration is often essential, yet still hard to achieve, in particular at high repetition rates. Here we report a two-stage system for compressing pulses from a 1030 nm ytterbium fiber laser to single-cycle durations with 5 µJ output pulse energy at 9.6 MHz repetition rate. In the first stage, the laser pulses are compressed from 340 to 25 fs by spectral broadening in a krypton-filled single-ring photonic crystal fiber (SR-PCF), subsequent phase compensation being achieved with chirped mirrors. In the second stage, the pulses are further compressed to single-cycle duration by soliton-effect self-compression in a neon-filled SR-PCF. We estimate a pulse duration of ∼3.4 fs at the fiber output by numerically back-propagating the measured pulses. Finally, we directly measured a pulse duration of 3.8 fs (1.25 optical cycles) after compensating (using chirped mirrors) the dispersion introduced by the optical elements after the fiber, more than 50% of the total pulse energy being in the main peak. The system can produce compressed pulses with peak powers >0.6 GW and a total transmission exceeding 66%.

Formation of optical supramolecular structures in a fibre laser by tailoring long-range soliton interactions

Wenbin He, Meng Pang, Dung-Han Yeh, Jiapeng Huang, Curtis Menyuk, Philip Russell

Self-assembly of fundamental elements through weak, long-range interactions plays a central role in both supramolecular DNA assembly and bottom-up synthesis of nanostructures. Optical solitons, analogous in many ways to particles, arise from the balance between nonlinearity and dispersion and have been studied in numerous optical systems. Although both short- and long-range interactions between optical solitons have attracted extensive interest for decades, stable soliton supramolecules, with multiple aspects of complexity and flexibility, have thus far escaped experimental observation due to the absence of techniques for enhancing and controlling the long-range inter-soliton forces. Here we report that long-range soliton interactions originating from optoacoustic effects and dispersive-wave radiations can be precisely tailored in a fibre laser cavity, enabling self-assembly of large numbers of optical solitons into highly-ordered supramolecular structures. We demonstrate several features of such optical structures, highlighting their potential applications in optical information storage and ultrafast laser-field manipulation.

Pump-Probe Study of Plasma Dynamics in Gas-Filled Photonic Crystal Fiber Using Counterpropagating Solitons

Mallika Irene Suresh, Felix Köttig, Johannes Köhler, Francesco Tani, Philip Russell

We present a pump-probe technique for monitoring ultrafast polarizability changes. In particular, we use it to measure the plasma density created at the temporal focus of a self-compressing higher-order pump soliton in a gas-filled hollow-core photonic crystal fiber. This is done by monitoring the wavelength of the dispersive wave emission from a counterpropagating probe soliton. By varying the relative delay between pump and probe, the plasma density distribution along the fiber can be mapped out. Compared with recently introduced interferometric side probing for monitoring the plasma density, our technique is relatively immune to instabilities caused by air turbulence and mechanical vibration. The results of two experiments on argon- and krypton-filled fiber are presented and compared to numerical simulations. The technique provides an important tool for probing photoionization in many different gases and gas mixtures, as well as ultrafast changes in dispersion in many other contexts.

Sustained Self-Starting Orbital Motion of a Glass-Fiber “Nanoengine” Driven by Photophoretic Forces

Shangran Xie, Riccardo Pennetta, Zheqi Wang, Philip Russell

Controllable optically driven rotation of microscopic objects is desirable in many applications, but is difficult to achieve. Here we report a sustained self-starting orbital motion of a clamped elongated nanostructure, a glass-fiber nanospike, when a CW laser<br>beam is focused axially onto its tip. Analysis shows that photophoretic antitrapping forces,<br>acting on the nanospike with a delayed response, introduce optomechanical gain into the mechanical motion, overcoming the intrinsic mechanical dissipation and resulting in growth from noise of oscillations at the resonant frequency of the nanospike. These photophoretic forces further enable phase-locking of the orthogonal fast and slow vibrations of the nanospike (induced by slight mechanical anisotropy), giving rise to a self-sustained orbital motion. The locked phase of orbital motion can be changed by tuning the gas pressure and adjusting the geometrical asymmetry of the system. This light-driven<br>nanoengine opens up a new degree of freedom for controlling the rotational motion of elongated nano-objects.

On-the-fly particle metrology in hollow-core photonic crystal fibre

Abhinav Sharma, Shangran Xie, Richard Zeltner, Philip Russell

Efficient monitoring of airborne particulate matter (PM), especially particles with aerodynamic diameter less than 2.5 µm (PM2.5), is crucial for improving public health. Reliable information on the concentration, size distribution and chemical characteristics of PMs is key to evaluating air pollution and identifying its sources. Standard methods for PM2.5 characterization require sample collection from the atmosphere and post-analysis using sophisticated equipment in a laboratory environment, and are normally very time-consuming. Although optical methods based on analysis of scattering of free-space laser beams or evanescent fields are in principle suitable for real-time particle counting and sizing, lack of knowledge of the refractive index in these methods not only leads to inevitable sizing ambiguity but also prevents identification of the particle material. In the case of evanescent wave detection, the system lifetime is strongly limited by adhesion of particles to the surfaces. Here we report a novel technique for airborne particle metrology based on hollow-core photonic crystal fibre. It offers in situ particle counting, sizing and refractive index measurement with effectively unlimited device lifetime, and relies on optical forces that automatically capture airborne particles in front of the hollow core and propel them into the fibre. The resulting transmission drop, together with the time-of-flight of the particles passing through the fibre, provide unambiguous mapping of particle size and refractive index with high accuracy. The technique offers unique advantages over currently available real-time particle metrology systems, and can be directly applied to monitoring air pollution in the open atmosphere as well as precise particle characterization in a local environment such as a closed room or a reaction vessel.

Highly efficient deep UV generation by four-wave mixing in gas-filled hollow-core photonic crystal fiber

Federico Belli, Amir Abdolvand, John Travers, Philip Russell

We report on a highly efficient experimental scheme for the generation of deep-ultraviolet (UV) ultrashort light pulses using four-wave mixing in gas-filled kagomé-style photonic crystal fiber. By pumping with ultrashort, few microjoule pulses centered at 400 nm, we generate an idler pulse at 266 nm and amplify a seeded signal at 800 nm. We achieve remarkably high pump-to-idler energy conversion efficiencies of up to 38%. Although the pump and seed pulse durations are ∼100  fs, the generated UV spectral bandwidths support sub-15 fs pulses. These can be further extended to support few-cycle pulses. Four-wave mixing in gas-filled hollow-core fibers can be scaled to high average powers and different spectral regions such as the vacuum UV (100–200 nm).

Full-field characterization of helical Bloch modes guided in twisted coreless photonic crystal fiber

Paul Roth, Gordon Wong, Michael Frosz, Goran Ahmed, Philip Russell

It was recently reported that a photonic crystal fiber (PCF) with no structural core guides light if a permanent chiral twist is introduced by spinning the fiber preform during the draw. The intriguing guidance mechanism behind this novel effect has many remarkable features; for example, it intrinsically supports circularly polarized helical Bloch modes (HBMs) that carry multiple optical vortices, making twisted PCFs of interest in fields such as optical micromanipulation, imaging, quantum optics, and optical communications. Here we report for the first time, to the best of our knowledge, that a twisted coreless PCF supports not just one but a family of guided HBMs, each member of which has a unique transverse field distribution and harmonic spectrum. By making detailed interferometric measurements of the near-field phase and amplitude distributions of HBMs, and expanding them as a series of Bessel beams, we are able to extract the amplitude of each azimuthal and radial HBM harmonic. Good agreement is found with the numerical solutions of Maxwell’s equations. The results shed light on the properties of this curious new optical phenomenon.

Carrier-envelope-phase-stable soliton-based pulse compression to 4.4  fs and ultraviolet generation at the 800  kHz repetition rate

Alexey Ermolov, Christian Heide, Philip Dienstbier, Felix Köttig, Francesco Tani, Peter Hommelhoff, Philip Russell

In this Letter, we report the generation of a femtosecond supercontinuum extending from the ultraviolet to the near-infrared spectrum and detection of its carrier-envelope-phase (CEP) variation by f-to-2f interferometry. The spectrum is generated in a gas-filled hollow-core photonic crystal fiber, where soliton dynamics allows the CEP-stable self-compression of the optical parametric chirped-pulse amplifier pump pulses at 800 nm to a duration of 1.7 optical cycles, followed by dispersive wave emission. The source provides up to 1 μJ of pulse energy at the 800 kHz repetition rate, resulting in 0.8 W of average power, and it can be extremely useful, for example in strong-field physics, pump–probe measurements, and ultraviolet frequency comb metrology.

Non-invasive real-time characterization of hollow-core photonic crystal fibers using whispering gallery mode spectroscopy

Michael Frosz, Riccardo Pennetta, Michael Enders, Goran Ahmed, Philip Russell

Single-ring hollow-core photonic crystal fibers, consisting of a ring of one or two thin-walled glass capillaries surrounding a central hollow core, hold great promise for use in optical communications and beam delivery, and are already being successfully exploited for extreme pulse compression and efficient wavelength conversion in gases. However, achieving low loss over long (km) lengths requires highly accurate maintenance of the microstructure—a major fabrication challenge. In certain applications, for example adiabatic mode transformers, it is advantageous to taper the fibers, but no technique exists for measuring the delicate and complex microstructure without first cleaving the taper at several positions along its length. In this Letter, we present a simple non-destructive optical method for measuring the diameter of individual capillaries. Based on recording the spectrum scattered from whispering gallery modes excited in the capillary walls, the technique is highly robust, allowing real-time measurement of fiber structure during the draw with sub-micron accuracy.

Optically Addressable Array of Optomechanically Compliant Glass Nanospikes on the Endface of a Soft-Glass Photonic Crystal Fiber

Zheqi Wang, Shangran Xie, Xin Jiang, Fehim Babic, Jiapeng Huang, Riccardo Pennetta, Johannes Köhler, Philip Russell

Arrays of elongated nanoscale structures with suitable optical and mechanical properties can act as probes of numerous physical processes at the nanoscale, with applications in, for example, high-resolution optical imaging and atomic force microscopy. They can also be used to investigate optomechanical phenomena such as synchronization among large assemblies of mechanical oscillators. Here we report a novel and versatile technique for fabricating two-dimensional light-guiding arrays of mechanically compliant glass nanospikes with lengths up to several hundred micrometers. The procedure starts with a multicore fiber made by stacking and drawing capillaries and rods of two different germanate glasses with markedly different acid etching rates. After a suitable etching step, a free-standing nanospike array is created at the fiber endface. The parameters are chosen so that there is evanescent coupling between adjacent nanospikes, which gives rise to strong optomechanical forces that can be exploited to drive and control the mechanical motion of the nanospikes and thus the optical properties.

Route from single-pulse to multi-pulse states in a mid-infrared soliton fiber laser

Jiapeng Huang, Meng Pang, Xin Jiang, Wenbin He, Philip Russell

State-of-the-art ultrafast mid-IR fiber lasers deliver optical solitons with durations of several hundred femtoseconds. The Er- or Ho-doped fluoride gain fibers generally used in these lasers have strong anomalous dispersion at ∼3 µm, which generally forces them to operate in the soliton regime. Here we report that a pulse-energy clamping effect, caused by the buildup of intracavity nonlinearities, limits the shortest obtainable pulse durations in these mid-infrared soliton fiber lasers. Excessive intra-cavity energy results in soliton instability, collapse and fragmentation into a variety of stable multi-pulse states, including phase-locked soliton molecules and harmonically mode-locked states. We report that the spectral evolution of the mid-IR laser pulses can be recorded between roundtrips through stretching their second-harmonic signal in a 25-km-length of single-mode fiber. Using a modified dispersive Fourier transform set-up, we were able to perform for the first time spectro-temporal measurements of mid-IR laser pulses both in the pulsed state and during pulse collapse and fragmentation. The results provide insight into the complex nonlinear dynamics of mid-IR soliton fiber lasers and open up new opportunities for obtaining a variety of stable multi-pulse mode-locked states at mid-IR wavelengths.

Generation of 1.5 cycle pulses at 780 nm at oscillator repetition rates with stable carrier-envelope phase

Philip Dienstbier, Francesco Tani, Takuya Higuchi, John Travers, Philip Russell, Peter Hommelhoff

We demonstrate a spectral broadening and compression setup for carrier-envelope phase (CEP) stable sub-10-fs Ti:sapphire oscillator pulses resulting in 3.9 fs pulses spectrally centered at 780 nm. Pulses from the oscillator with 2 nJ energy are launched into a 1 mm long all-normal dispersive solid-core photonic crystal fiber and spectrally broadened to more than one octave. Subsequent pulse compression is achieved with a phase-only 4f pulse shaper. Second harmonic frequency resolved optical gating with a ptychographic reconstruction algorithm is used to obtain the spectral phase, which is fed back as a phase mask to the shaper display for pulse compression. The compressed pulses are CEP stable with a long term standard deviation of 0.23 rad for the CEP noise and 0.32 rad for the integrated rms phase jitter. The high total throughput of 15% results in a remaining pulse energy of about 300 pJ at 80 MHz repetition rate. With these parameters and the ability to tailor the spectral phase, the system is well suited for waveform sensitive photoemission experiments with needle tips or nanostructures and can be easily adapted to other sub-10 fs ultra-broadband Ti:sapphire oscillators.

Generation of broadband circularly polarized supercontinuum light in twisted photonic crystal fibers

Rafal Sopalla, Gordon Wong, Nicolas Joly, Michael Frosz, Xin Jiang, Goran Ahmed, Philip Russell

We compare the properties of the broadband supercontinuum (SC) generated in twisted and untwisted solid-core photonic crystal fibers when pumped by circularly polarized<br>40 picosecond laser pulses at 1064 nm. In the helically twisted fiber, fabricated by spinning the preform during the draw, the SC is robustly circularly polarized across its entire<br>spectrum whereas, in the straight fiber, axial fluctuations in linear birefringence and polarization-dependent nonlinear effects cause the polarization state to vary randomly with the wavelength. Theoretical modelling confirms the experimental results. Helically twisted photonic crystal fibers permit the generation of pure circularly polarized SC light with excellent polarization stability against fluctuations in input power and environmental perturbations.

Optical traps and anti-traps for glass nanoplates in hollow waveguides

Mehmet Can Günendi, Shangran Xie, David Novoa, Philip Russell

We study theoretically the optical forces acting on glass nanoplates introduced into<br>hollow waveguides, and show that, depending on the sign of the laser detuning relative to the nanoplate resonance, optomechanical back-action between nanoplate and hollow waveguide can create both traps and anti-traps at intensity nodes and anti-nodes in the supermode field profile, behaving similarly to those experienced by cold atoms when the laser frequency is red or blue detuned of an atomic resonance. This arises from dramatic distortions to the mode profile in the hollow waveguide when the nanoplate is off-resonant, producing gradient forces that vary strongly with nanoplate position. In a planar system, we show that when the nanoplate is constrained by an imaginary mechanical spring, its position exhibits strong bistability as the base position is varied. We then treat a two-dimensional system consisting of an anti-resonant nanoplate in the hollow core of a photonic crystal fiber, and predict the stable dark trapping of nanoplate at core center against both translational and rotational motion. The results show that spatial and angular position of nano-scale objects in hollow waveguides can be optically controlled by launching beams with appropriately synthesized transverse field profiles. <br>

Thresholdless deep and vacuum ultraviolet Raman frequency conversion in hydrogen-filled photonic crystal fiber

Manoj K. Mridha, David Novoa, Pooria Hosseini, Philip St. J. Russell

Coherent ultraviolet light has many uses, for example, in the study of molecular species relevant in biology and chemistry. Very few, if any, laser materials offer ultraviolet transparency along with damage-free operation at high-photon energies and laser power. Here we report efficient generation of narrowband deep and vacuum ultraviolet light using hydrogen-filled hollow-core photonic crystal fiber. Pumping above the stimulated Raman threshold at 532 nm, coherent molecular vibrations are excited in the gas, permitting thresholdless wavelength conversion in the ultraviolet with efficiencies close to 60%. The system is uniquely pressure tunable, allows spatial structuring of the out-coupled radiation, and shows excellent performance in the vacuum ultraviolet. As the underlying scattering process is effectively linear, our approach can also in principle operate at the single-photon level, when all other alternatives are extremely inefficient.

Fabrication and non-destructive characterization of tapered single-ring hollow-core photonic crystal fiber

Riccardo Pennetta, Michael T. Enders, Michael H. Frosz, Francesco Tani, Philip St. J. Russell

We report on the properties of tapered single-ring hollow-core photonic-crystal fibers, with a particular emphasis on applications in nonlinear optics. The simplicity of these structures allows the use of non-invasive side-illumination to assess the quality of the tapering process, by<br>observing the scattered far-field spectrum originating from excitation of whispering-gallery modes in the cladding capillaries. We investigate the conditions that ensure adiabatic propagation in the up- and down-tapers, and the scaling of loss-bands (created by anti-crossings between the core mode and modes in the capillary walls) with taper ratio. We also present an analytical model for the pressure profile along a tapered hollow fiber under differential pumping

Pump-probe multi-species CARS in a hollow-core PCF with a 20 ppm detection limit under ambient conditions

Rinat Tyumenev, Luisa Späth, Barbara M. Trabold, Goran Ahmed, Michael H. Frosz, Philip St. J. Russell

We report coherent anti-Stokes Raman spectroscopy (CARS) in a gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF) using a pump-probe configuration. The long collinear path length offered by an SR-PCF strongly enhances the efficiency of the Raman interactions. Pressure tuning the zero-dispersion wavelength (ZDW) of the SR-PCF allows the Raman coherence prepared by seeded pumping at 515 nm to be used in the visible for phase-matched generation of an anti-Stokes signal from a probe in the ultraviolet. The unique dispersion profile in the vicinity of the ZDW enables simultaneous phase matching of all known Raman transitions. We demonstrate that simultaneous multi-species CARS with a detection limit of 20 ppm is possible with only 20 kW of peak pump power delivered by a single laser source.

Spatio-temporal measurement of ionization-induced modal index changes in gas-filled PCF by prism-assisted side-coupling

Barbara M. Trabold, Mallika I. Suresh, Johannes R. Köhler, Michael H. Frosz, Francesco Tani, Philip St. J. Russell

We report the use of prism-assisted side-coupling to investigate the spatio-temporal dynamics of photoionization in an Ar-filled hollow-core photonic crystal fiber. By launching four different LP core modes we are able to probe temporal and spatial changes in the modal refractive index on timescales from a few hundred picoseconds to several hundred microseconds after the ionization event. We experimentally analyze the underlying gas density waves and find good agreement with quantitative and qualitative hydrodynamic predictions. Moreover, we observe periodic modulations in the MHz-range lasting for a few microseconds, indicating nanometer-scale vibrations of the fiber structure, driven by gas density waves.

Polarization-Tailored Raman Frequency Conversion in Chiral Gas-Filled Hollow-Core Photonic Crystal Fibers

Sona Davtyan, David Novoa, Yang Chen, Michael H. Frosz, Philip St. J. Russell

Broadband-tunable sources of circularly polarized light are crucial in fields such as laser science, biomedicine, and spectroscopy. Conventional sources rely on nonlinear wavelength conversion and polarization control using standard optical components and are limited by the availability of suitably transparent crystals and glasses. Although a gas-filled hollow-core photonic crystal fiber provides pressuretunable dispersion, long well-controlled optical path lengths, and high Raman conversion efficiency, it is unable to preserve a circular polarization state, typically exhibiting weak linear birefringence. Here we report a revolutionary approach based on a helically twisted hollow-core photonic crystal fiber, which displays circular birefringence, thus robustly maintaining a circular polarization state against external perturbations. This makes it possible to generate pure circularly polarized Stokes and anti-Stokes signals by rotational Raman scattering in hydrogen. The polarization state of the frequency-shifted Raman bands can be continuously varied by tuning the gas pressure in the vicinity of the gain-suppression point. The results pave the way to a new generation of compact and efficient fiber-based sources of broadband light with a fully controllable polarization state.

Pulse-repetition-rate tuning of a harmonically mode-locked fiber laser using a tapered photonic crystal fiber

Dung-Han Yeh, Wenbin He, Meng Pang, Xin Jiang, Gordon K. L. Wong, Philip St J. Russell

Strong enhancement of optoacoustic interactions in the micrometer-sized core of a photonic crystal fiber (PCF) enables stable, harmonic mode locking of a soliton fiber laser<br>at GHz frequencies. Here we report that by tapering the PCF during the draw, the optoacoustic gain bandwidth can be broadened to ∼47 MHz, more than 3 times wider than in the untapered fiber. This made possible broad pulse-repetition-rate tuning over 66 MHz (from 2.042 to 2.108 GHz) of an optoacoustically mode-locked soliton fiber laser. Within this tuning range, the harmonically mode-locked pulse trains at the laser output were observed to be quite robust, with better than 40 dB supermode suppression ratio, sub-ps pulse timing jitter, and <0.2% relative intensity noise. This gigahertz-rate, near-infrared soliton fiber laser has remarkable pulse-rate tunability and low noise level, and has important potential applications in frequency metrology, high-speed optical sampling, and fiber telecommunications.

Direct characterization of tuneable few-femtosecond dispersive-wave pulses in the deep UV

Christian Brahms, Dane R. Austin, Francesco Tani, Allan S. Johnson, Douglas Garratt, John. C. Travers, John W. G. Tisch, Philip Russell, Jon P. Marangos

Dispersive wave emission (DWE) in gas-filled hollow-core dielectric waveguides is a promising source of tuneable coherentand broadband radiation, but so far the generation of fewfemtosecond pulses using this technique has not been demonstrated. Using in-vacuum frequency-resolved optical gating, we directly characterize tuneable 3 fs pulses in the deep ultraviolet generated via DWE. Through numerical simulations, we identify that the use of a pressure gradient in the waveguide is critical for the generation of short pulses.

Long-Lived Refractive-Index Changes Induced by Femtosecond Ionization in Gas-Filled Single-Ring Photonic-Crystal Fibers

Johannes R. Koehler, Felix Köttig, Barbara M. Trabold, Francesco Tani, Philip St. J. Russell

We investigate refractive-index changes caused by femtosecond photoionization in a gas-filled hollow-core photonic-crystal fiber. Using spatially-resolved interferometric side-probing, we find that these changes live for tens of microseconds after the photoionization event — eight orders of magnitude longer than the pulse duration. Oscillations in the megahertz frequency range are simultaneously observed, caused by mechanical vibrations of the thin-walled capillaries surrounding the hollow core. These two nonlocal effects can affect the propagation of a second pulse that arrives within their lifetime, which works out to repetition rates of tens of kilohertz. Filling the fiber with an atomically lighter gas significantly reduces ionization, lessening the strength of the refractive-index changes. The results will be important for understanding the dynamics of gas-based fiber systems operating at high intensities and high repetition rates, when temporally nonlocal interactions between successive laser pulses become relevant.

Excitation of higher-order modes in optofluidic photonic crystal fiber

Andrei Ruskuc, Philipp Koehler, Marius A. Weber, Ana Andres-Arroyo, Michael H. Frosz, Philip St. J. Russell, Tijmen G. Euser

Higher-order modes up to LP33 are controllably excited in water-filled kagomé- and bandgap-style hollow-core photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10–20% into the liquid-filled core. Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagomé HC-PCF and match numerical simulations. Mode indices are obtained by launching plane-waves at specific angles onto the fiber input-face and comparing the resulting intensity pattern to that of a particular mode. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation.

Strong circular dichroism for the HE11 mode in twisted single-ring hollow-core photonic crystal fiber

Paul Roth, Yang Chen, Mehmet Can Günendi, Ramin Beravat, Nitin N. Edavalath, Michael H. Frosz, Goran Ahmed, Gordon K. L. Wong, Philip St. J. Russell

We report a series of experimental, analytical, and numerical studies demonstrating strong circular dichroism for the HE11-like core mode in helically twisted hollow-core single-ring photonic crystal fiber (SR-PCF), formed by spinning the preform during fiber drawing. In the SR-PCFs studied, the hollow core is surrounded by a single ring of nontouching capillaries. Coupling between these capillaries results in the formation of helical Bloch modes carrying orbital angular momentum. When twisted, strong circular birefringence appears in the ring, so that coupling to the core mode is possible for only one circular polarization state. The result is a SR-PCF that acts as a circular polarizer, offering 1.4 dB/m for the low-loss polarization state and 9.7 dB/m for the high-loss state over a 25 nm band centered at 1593 nm wavelength. In addition, we report for the first time that the vector fields of the helical Bloch modes are perfectly periodic when evaluated in cylindrical coordinates. Such fibers have many potential applications, such as generating circularly polarized light in gas-filled SR-PCF and realizing polarizing elements in the deep and vacuum ultraviolet.

Broadband and tunable time-resolved THz system using argon-filled hollow-core photonic crystal fiber

Wei Cui, Aidan W. Schiff-Kearn, Emily Zhang, Nicolas Couture, Francesco Tani, David Novoa, Philip St. J. Russell, Jean-Michel Ménard

We demonstrate broadband, frequency-tunable, phase-locked terahertz (THz) generation and detection based on difference frequency mixing of temporally and spectrally structured near-infrared (NIR) pulses. The pulses are prepared in a gas-filled hollow-core<br>photonic crystal fiber (HC-PCF), whose linear and nonlinear optical properties can be adjusted by tuning the gas pressure. This permits optimization of both the spectral broadening of the pulses due to self-phase modulation (SPM) and the generated THz spectrum. The properties of the prepared pulses, measured at several different argon gas pressures, agree well with the results of numerical modeling. Using these pulses, we perform difference frequency generation in a standard time-resolved THz scheme. As the argon pressure is gradually increased from 0 to 10 bar, the NIR pulses spectrally broaden from 3.5 to 8.7 THz, while the measured THz bandwidth increases correspondingly from 2.3 to 4.5 THz. At 10 bar, the THz spectrum extends to 6 THz, limited only by the spectral bandwidth of our time-resolved detection scheme. Interestingly, SPM in the HC-PCF produces asymmetric spectral broadening that may be used to enhance the generation of selected THz frequencies. This scheme, based on a HC-PCF pulse shaper, holds great promise for broadband time-domain spectroscopy in the THz, enabling the use of compact and stable ultrafast laser sources with relatively narrow linewidths (<4 THz).

Long-range optical trapping and binding of microparticles in hollow-core photonic crystal fibre

Dmitry S. Bykov, Shangran Xie, Richard Zeltner, Andrey Machnev, Gordon K. L. Wong, Tijmen G. Euser, Philip St. J. Russell

Optically levitated micro- and nanoparticles offer an ideal playground for investigating photon–phonon interactions over macroscopic distances. Here we report the observation of long-range optical binding of multiple levitated microparticles, mediated by intermodal scattering and interference inside the evacuated core of a hollow-core photonic crystal fibre (HC-PCF). Three polystyrene particles with a diameter of 1 μm are stably bound together with an inter-particle distance of ~40 μm, or 50 times longer than the wavelength of the trapping laser. The levitated bound-particle array can be translated to-and-fro over centimetre distances along the fibre. When evacuated to a gas pressure of 6 mbar, the collective mechanical modes of the bound-particle array are able to be observed. The measured inter-particle distance at equilibrium and mechanical eigenfrequencies are supported by a novel analytical formalism modelling the dynamics of the binding process. The HC-PCF system offers a unique platform for investigating the rich optomechanical dynamics of arrays of levitated particles in a well-isolated and protected environment.

Dispersion tuning in sub-micron tapers for third-harmonic and photon triplet generation

Jonas Hammer, Andrea Cavanna, Riccardo Pennetta, Maria Chekhova, Philip St. J. Russell, Nicolas Joly

Precise control of the dispersion landscape is of crucial importance if optical fibers are to be successfully used for the generation of three-photon states of light—the inverse of third-harmonic generation (THG). Here we report gas-tuning of intermodal phase-matched THG in sub-micron-diameter tapered optical fiber. By adjusting the pressure of the surrounding argon gas up to 50 bars, intermodally phase-matched third-harmonic light can be generated for pump wavelengths within a 15 nm range around 1.38 μm. We also measure the infrared fluorescence generated in the fiber when pumped in the visible and estimate that the accidental coincidence rate in this signal is lower than the predicted detection rate of photon triplets

Dominance of backward stimulated Raman scattering in gas-filled hollow-core photonic crystal fibers

Manoj Kumar Mridha, David Novoa, Philip Russell

Backward stimulated Raman scattering in gases provides a promising route to the compression and amplification of a Stokes seed pulse by counter-propagating against a pump pulse, as has been demonstrated already in various platforms, mainly in free space. However, the dynamics governing this process when seeded by noise has not yet been investigated in a fully controllable collinear environment. Here we report, to the best of our knowledge, the first unambiguous observation of efficient noise-seeded backward stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber. At high gas pressures, when the backward Raman gain is comparable to, but lower than, the forward gain, we report quantum conversion efficiencies exceeding 40% to the backward Stokes at 683 nm from a narrowband 532 nm pump. Efficiency increases to 65% when the backward process is seeded by a small amount of back-reflected forward-generated Stokes light. At high pump powers, the backward Stokes signal, emitted in a clean fundamental mode and spectrally pure, is unexpectedly always stronger than its forward-propagating counterpart. We attribute this striking observation to the unique temporal dynamics of the interacting fields, which cause the Raman coherence (which takes the form of a moving fine-period Bragg grating) to grow in strength toward the input end of the fiber. A good understanding of this process, together with the rapid development of novel anti-resonant-guiding hollow-core fibers, may lead to improved designs of efficient gas-based Raman lasers and amplifiers operating at wavelengths from the ultraviolet to the mid-infrared.

UV Soliton Dynamics and Raman-Enhanced Supercontinuum Generation in Photonic Crystal Fiber

Pooria Hosseini, Alexey Ermolov, Francesco Tani, David Novoa, Philip Russell

Ultrafast broadband ultraviolet radiation is of importance in spectroscopy and photochemistry, since high photon energies enable single-photon excitations and ultrashort pulses allow time-resolved studies. Here we report the use of gas-filled hollow-core photonic crystal fibers (HC-PCFs) for efficient ultrafast nonlinear optics in the ultraviolet. Soliton selfcompression of 400 nm pulses of (unprecedentedly low) ∼500 nJ energies down to sub-6 fs durations is achieved, as well as resonant emission of tunable dispersive waves from these solitons. In addition, we discuss the generation of a flat supercontinuum extending from the deep ultraviolet to the visible in a hydrogen-filled HC-PCF. Comparisons with argon-filled fibers show that the enhanced Raman gain at high frequencies makes the hydrogen system more efficient. As HC-PCF technology develops, we expect these fiber-based ultraviolet sources to lead to new applications.

Stable Immobilization of Size‐Controlled Bimetallic Nanoparticles in Photonic Crystal Fiber Microreactor

Sebastian Ponce, Macarena Munoz, Ana M. Cubillas, Tijmen G. Euser, Gui-Rong Zhang, Philip St. J. Russell, Peter Wasserscheid, Bastian J. M. Etzold

The possibility of immobilizing ex situ‐synthesized colloidal bimetallic nanoparticles (NPs) of well‐defined characteristics inside hollow core photonic crystal fiber (HC‐PCF) microreactors is demonstrated. With the developed method, PtNi clusters remain strongly attached to the fiber core and can be used as active catalysts for the hydrogenation of an azobenzene dye. The study revealed that optical transmission exhibits a size‐dependent behavior, i.e., smaller NPs bring in less optical signal loss. Sufficient light transmission was achieved for all particle sizes. Furthermore, with these catalytic PCF microreactors, kinetic data can be obtained with a much lower amount of precious metals compared to a conventional batch reactor, opening a new pathway for in situ catalyst screening.

Highly Sensitive Luminescence Detection of Photosensitized Singlet Oxygen within Photonic Crystal Fibers

Gareth O. S. Williams, Tijmen G. Euser, Philip St. J. Russell, Alexander J. MacRobert, Anita C. Jones

Highly sensitive, quantitative detection of singlet oxygen (1O2) is required for the evaluation of newly developed photosensitizers and the elucidation of the mechanisms of many processes in which singlet oxygen is known or believed to be involved. The direct detection of 1O2 through its intrinsic phosphorescence at 1270 nm is challenging, because of the extremely low intensity of this emission, coupled with the low quantum efficiency of currently available photodetectors at this wavelength. We introduce hollow‐core photonic crystal fibers (HC‐PCF) as a novel optofluidic modality for photosensitization and detection of 1O2. We report the use of this approach to achieve highly sensitive detection of the luminescence decay of 1O2 produced by using two common photosensitizers, Rose Bengal and Hypericin, within the 60‐μm diameter core of a 15 cm length of HC‐PCF. We demonstrate the feasibility of directly detecting sub‐picomole quantities of 1O2 by using this methodology, and identify some aspects of the HC‐PCF technology that can be improved to yield even higher detection sensitivity.

Flying particle microlaser and temperature sensor in hollow-core photonic crystal fiber

Richard Zeltner, Riccardo Pennetta, Shangran Xie, Philip Russell

Whispering-gallery mode (WGM) resonators combine small optical mode volumes with narrow resonance linewidths, making them exciting platforms for a variety of applications. Here we report a flying WGM microlaser, realized by optically trapping a dye-doped microparticle within a liquid-filled hollow-core photonic crystal fiber (HC-PCF) using a CW laser and then pumping it with a pulsed excitation laser whose wavelength matches the absorption band of the dye. The laser emits into core-guided modes that can be detected at the endfaces of the HC-PCF. Using radiation forces, the microlaser can be freely propelled along the HC-PCF over multi-centimeter distances—orders of magnitude farther than in previous experiments where tweezers and fiber traps were used. The system can be used to measure temperature with high spatial resolution, by exploiting the temperature-dependent frequency shift of the lasing modes, and may also permit precise delivery of light to remote locations.

Effect of anti-crossings with cladding resonances on ultrafast nonlinear dynamics in gas-filled photonic crystal fibers

Francesco Tani, Felix Köttig, David Novoa, Ralf Keding, Philip Russell

Spectral anti-crossings between the fundamental guided mode and core-wall resonances alter the dispersion in hollow-core anti-resonant-reflection photonic crystal fibers. Here we study the effect of this dispersion change on the nonlinear propagation and dynamics of ultrashort pulses. We find that it causes emission of narrow spectral peaks through a combination of four-wave mixing and dispersive wave emission. We further investigate the influence of the anti-crossings on nonlinear pulse propagation and show that their impact can be minimized by adjusting the core-wall thickness in such a way that the anti-crossings lie spectrally distant from the pump wavelength.

Control of ultrafast pulses in a hydrogen-filled hollow-core photonic-crystal fiber by Raman coherence

Federico Belli, Amir Abdolvand, John Travers, Philip Russell

We present the results of an experimental and numerical investigation into temporally nonlocal coherent interactions between ultrashort pulses, mediated by Raman coherence, in a gas-filled kagome-style hollow-core photonic-crystal fiber. A pump pulse first sets up the Raman coherence, creating a refractive index spatiotemporal<br>grating in the gas that travels at the group velocity of the pump pulse. Varying the arrival time of a second, probe, pulse allows a high degree of control over its evolution as it propagates along the fiber through the grating. Of particular interest are soliton-driven effects such as self-compression and dispersive wave (DW) emission. In the experiments reported, a DW is emitted at ∼300 nm and exhibits a wiggling effect, with its central frequency oscillating periodically with pump-probe delay. The results demonstrate that a strong Raman coherence, created in a broadband guiding gas-filled kagome photonic-crystal fiber, can be used to control the nonlinear dynamics of ultrashort probe pulses, even in difficult-to-access spectral regions such as the deep and vacuum ultraviolet.

Three-dimensional holographic optical manipulation through a high-numerical-aperture soft-glass multimode fibre

Ivo T. Leite, Sergey Turtaev, Xin Jiang, Martin Siler, Alfred Cuschieri, Philip St. J. Russell, Tomas Cizmar

Holographic optical tweezers (HOT) hold great promise for many applications in biophotonics, allowing the creation and measurement of minuscule forces on biomolecules, molecular motors and cells. Geometries used in HOT currently rely on bulk optics, and their exploitation in vivo is compromised by the optically turbid nature of tissues. We present an alternative HOT approach in which multiple three-dimensional (3D) traps are introduced through a high-numerical-aperture multimode optical fibre, thus enabling an equally versatile means of manipulation through channels having cross-section comparable to the size of a single cell. Our work demonstrates real-time manipulation of 3D arrangements of micro-objects, as well as manipulation inside otherwise inaccessible cavities. We show that the traps can be formed over fibre lengths exceeding 100 mm and positioned with nanometric resolution. The results provide the basis for holographic manipulation and other high-numerical-aperture techniques, including advanced microscopy, through single-core-fibre endoscopes deep inside living tissues and other complex environments.

Generation of microjoule pulses in the deep ultraviolet at megahertz repetition rates

Felix Koettig, Francesco Tani, Christian Martens-Biersach, John C. Travers, Philip St J. Russell

Although ultraviolet (UV) light is important in many areas of science and technology, there are very few if any lasers capable of delivering wavelength-tunable ultrashort UV pulses at high repetition rates. Here we report the generation of deep UV laser pulses at megahertz repetition rates and microjoule energies by means of dispersive wave (DW) emission from self-compressed solitons in gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF). Pulses from an ytterbium fiber laser (similar to 300 fs) are first compressed to <25 fs in a SR-PCF-based nonlinear compression stage and subsequently used to pump a second SR-PCF stage for broadband DW generation in the deep UV. The UV wavelength is tunable by selecting the gas species and the pressure. Through rigorous optimization of the system, in particular employing a large-core fiber filled with light noble gases, we achieve 1 mu J pulse energies in the deep UV, which is more than 10 times higher, at average powers more than four orders of magnitude greater (reaching 1 W) than previously demonstrated, with only 20 mu J pulses from the pump laser. (C) 2017 Optical Society of America

Generation of broadband mid-IR and UV light in gas-filled single-ring hollow-core PCF

Marco Cassataro, David Novoa, Mehmet C. Guenendi, Nitin N. Edavalath, Michael H. Frosz, John C. Travers, Philip St. J. Russell

We report generation of an ultrafast supercontinuum extending into the mid-infrared in gas-filled single-ring hollow-core photonic crystal fiber (SR-PCF) pumped by 1.7 mu m light from an optical parametric amplifier. The simple fiber structure offers shallow dispersion and flat transmission in the near and mid-infrared, enabling the generation of broadband spectra extending from 270 nm to 3.1 mu m, with a total energy of a few mu J. In addition, we demonstrate the emission of ultraviolet dispersive waves whose frequency can be tuned simply by adjusting the pump wavelength. SR-PCF thus constitutes an effective means of compressing and delivering tunable ultrafast pulses in the near and mid-infrared spectral regions. (C) 2017 Optical Society of America

Fresnel-Reflection-Free Self-Aligning Nanospike Interface between a Step-Index Fiber and a Hollow-Core Photonic-Crystal-Fiber Gas Cell

Riccardo Pennetta, Shangran Xie, Frances Lenahan, Manoj Mridha, David Novoa, Philip St. J. Russell

We report a fully integrated interface delivering efficient, reflection-free, single-mode, and self-aligned coupling between a step-index fiber and a gas-filled hollow-core photonic crystal fiber. The device offers a universal solution for interfacing solid and hollow cores and can be sealed to allow operation either evacuated or at high pressure. Stimulated Raman scattering and molecular modulation of light are demonstrated in a H-2-filled hollow-core photonic crystal fiber using the device.

Analytical formulation for the bend loss in single-ring hollow-core photonic crystal fibers

Michael H. Frosz, Paul Roth, Mehmet C. Guenendi, Philip St. J. Russell

Understanding bend loss in single-ring hollow-core photonic crystal fibers (PCFs) is becoming of increasing importance as the fibers enter practical applications. While purely numerical approaches are useful, there is a need for a simpler analytical formalism that provides physical insight and can be directly used in the design of PCFs with low bend loss. We show theoretically and experimentally that a wavelength-dependent critical bend radius exists below which the bend loss reaches a maximum, and that this can be calculated from the structural parameters of a fiber using a simple analytical formula. This allows straightforward design of single-ring PCFs that are bend-insensitive for specified ranges of bend radius and wavelength. It also can be used to derive an expression for the bend radius that yields optimal higher-order mode suppression for a given fiber structure. (C) 2017 Chinese Laser Press

Generation of spectral clusters in a mixture of noble and Raman-active gases (vol 41, pg 5543, 2016)

Pooria Hosseini, Amir Abdolvand, Philip St. J. Russell

Universality of Coherent Raman Gain Suppression in Gas-Filled Broadband-Guiding Photonic Crystal Fibers

Pooria Hosseini, M. K. Mridha, D. Novoa, A. Abdolvand, P. St. J. Russell

As shown in the early 1960s, the gain in stimulated Raman scattering (SRS) is drastically suppressed when the rate of creation of phonons (via a pump-to-Stokes conversion) is exactly balanced by the rate of phonon annihilation (via a pump-to-anti-Stokes conversion). This occurs when the phonon coherence waves-synchronized vibrations of a large population of molecules-have identical propagation constants for both processes; i. e., they are phase-velocity matched. As recently demonstrated, hydrogen-filled photonic crystal fiber pumped in the vicinity of its zero-dispersion wavelength provides an ideal system for observing this effect. Here we report that Raman gain suppression is actually a universal feature of SRS in gas-filled hollow-core fibers and that it can strongly impair SRS even when the phase mismatch is high, particularly at high pump powers when it is normally assumed that nonlinear processes become more (not less) efficient. This counterintuitive result means that intermodal stimulated Raman scattering (for example, between LP01 and LP11 core modes) begins to dominate at high power levels. The results reported have important implications for fiber-based Raman shifters, amplifiers, or frequency combs, especially for operation in the ultraviolet, where the Raman gain is much higher.

Coherent control of flexural vibrations in dual-nanoweb fibers using phase-modulated two-frequency light

J. R. Koehler, R. E. Noskov, A. A. Sukhorukov, D. Novoa, P. St. J. Russell

Coherent control of the resonant response in spatially extended optomechanical structures is complicated by the fact that the optical drive is affected by the backaction from the generated phonons. Here we report an approach to coherent control based on stimulated Raman-like scattering, in which the optical pressure can remain unaffected by the induced vibrations even in the regime of strong optomechanical interactions. We demonstrate experimentally coherent control of flexural vibrations simultaneously along the whole length of a dual-nanoweb fiber, by imprinting steps in the relative phase between the components of a two-frequency pump signal, the beat frequency being chosen to match a flexural resonance. Furthermore, sequential switching of the relative phase at time intervals shorter than the lifetime of the vibrations reduces their amplitude to a constant value that is fully adjustable by tuning the phase modulation depth and switching rate. The results may trigger new developments in silicon photonics, since such coherent control uniquely decouples the amplitude of optomechanical oscillations from power-dependent thermal effects and nonlinear optical loss.

Effect of stray fields on Rydberg states in hollow-core PCF probed by higher-order modes

G. Epple, N. Y. Joly, T. G. Euser, P. St. J. Russell, R. Loew

The spectroscopy of atomic gases confined in hollow-core photonic crystal fiber (HC-PCF) provides optimal atom-light coupling beyond the diffraction limit, which is desirable for various applications such as sensing, referencing, and nonlinear optics. Recently, coherent spectroscopy was carried out on highly excited Rydberg states at room temperature in a gas-filled HC-PCF. The large polarizability of the Rydberg states made it possible to detect weak electric fields inside the fiber. In this Letter, we show that by combining highly excited Rydberg states with higher-order optical modes, we can gain insight into the distribution and underlying effects of these electric fields. Comparisons between experimental findings and simulations indicate that the fields are caused by the dipole moments of atoms adsorbed on the hollow-core wall. Knowing the origin of the electric fields is an important step towards suppressing them in future HC-PCF experiments. Furthermore, a better understanding of the influence of adatoms will be advantageous for optimizing electric-fieldsensitive experiments carried out in the vicinity of nearby surfaces. (C) 2017 Optical Society of America

Mid-infrared dispersive wave generation in gas-filled photonic crystal fibre by transient ionization-driven changes in dispersion

F. Koettig, D. Novoa, F. Tani, M. C. Guenendi, M. Cassataro, J. C. Travers, P. St. J. Russell

Gas-filled hollow-core photonic crystal fibre is being used to generate ever wider super-continuum spectra, in particular via dispersive wave emission in the deep and vacuum ultraviolet, with a multitude of applications. Dispersive waves are the result of nonlinear transfer of energy from a self-compressed soliton, a process that relies crucially on phase-matching. It was recently predicted that, in the strong-field regime, the additional transient anomalous dispersion introduced by gas ionization would allow phase-matched dispersive wave generation in the mid-infrared-something that is forbidden in the absence of free electrons. Here we report the experimental observation of such mid-infrared dispersive waves, embedded in a 4.7-octave-wide supercontinuum that uniquely reaches simultaneously to the vacuum ultraviolet, with up to 1.7W of total average power.

Rapid screening of photoactivatable metallodrugs: photonic crystal fibre microflow reactor coupled to ESI mass spectrometry

Ruth J. McQuitty, Sarah Unterkofler, Tijmen G. Euser, Philip St J. Russell, Peter J. Sadler

We explore the efficacy of a hyphenated photonic crystal fibre microflow reactor-high-resolution mass spectrometer system as a method for screening the activity of potential new photoactivatable drugs. The use of light to activate drugs is an area of current development as it offers the possibility of reduced side effects due to improved spatial and temporal targeting and novel mechanisms of anticancer activity. The di-nuclear ruthenium complex [{(eta(6)-indan) RuCl}(2)(mu-2,3-dpp)](PF6)(2), previously studied by Magennis et al. (Inorg. Chem., 2007, 46, 5059) is used as a model drug to compare the system to standard irradiation techniques. The photodecomposition pathways using blue light radiation are the same for PCF and conventional cuvette methods. Reactions in the presence of small biomolecules 50-guanosine monophosphate (5'-GMP), 5'-adenosine monophosphate (5'-AMP), L-cysteine (L-Cys) and glutathione (gamma-L-glutamyl-L-cysteinyl-glycine, GSH) were studied. The complex was found to bind to nucleobases in the dark and this binding increased upon irradiation with 488 nm light, forming the adducts [(eta(6)-indan) Ru2(mu-2,3-dpp) + 5'-GMP](2+) and [(eta(6)-indan) Ru + (5'-AMP)]+. These findings are consistent with studies using conventional methods. The dinuclear complex also binds strongly to GSH after irradiation, a possible explanation for its lack of potency in cell line testing. The use of the PCF-MS system dramatically reduced the sample volume required and reduced the irradiation time by four orders of magnitude from 14 hours to 12 seconds. However, the reduced sample volume also results in a reduced MS signal intensity. The dead time of the combined system is 15 min, limited by the intrinsic dead volume of the HR-MS.

Broadband high-resolution multi-species CARS in gas-filled hollow-core photonic crystal fiber

Barbara M. Trabold, Robert J. R. Hupfer, Amir Abdolvand, Philip St. J. Russell

We report the use of coherent anti-Stokes Raman spectros-copy (CARS) in gas-filled hollow-core photonic crystal fiber (HC-PCF) for trace gas detection. The long optical path-lengths yield a 60 dB increase in the signal level compared with free-space arrangements. This enables a relatively weak supercontinuum (SC) to be used as Stokes seed, along with a ns pump pulse, paving the way for broadband (> 4000 cm(-1)) single-shot CARS with an unprecedented resolution of similar to 100 MHz. A kagome-style HC-PCF provides broadband guidance, and, by operating close to the pressure-tunable zero dispersion wavelength, we can ensure simultaneous phase-matching of all gas species. We demonstrate simultaneous measurement of the concentrations of multiple trace gases in a gas sample introduced into the core of the HC-PCF. (C) 2017 Optical Society of America

Enhanced Control of Transient Raman Scattering Using Buffered Hydrogen in Hollow-Core Photonic Crystal Fibers

P. Hosseini, D. Novoa, A. Abdolvand, P. St. J. Russell

Many reports on stimulated Raman scattering in mixtures of Raman-active and noble gases indicate that the addition of a dispersive buffer gas increases the phase mismatch to higher-order Stokes and anti-Stokes sidebands, resulting in a preferential conversion to the first few Stokes lines, accompanied by a significant reduction in the Raman gain due to collisions with gas molecules. Here we report that, provided the dispersion can be precisely controlled, the effective Raman gain in a gas-filled hollow-core photonic crystal fiber can actually be significantly enhanced when a buffer gas is added. This counterintuitive behavior occurs when the nonlinear coupling between the interacting fields is strong and can result in a performance similar to that of a pure Raman-active gas, but at a much lower total gas pressure, allowing competing effects such as Raman backscattering to be suppressed. We report high modal purity in all the emitted sidebands, along with anti-Stokes conversion efficiencies as high as 5% in the visible and 2% in the ultraviolet. This new class of gas-based waveguide device, which allows the nonlinear optical response to be beneficially pressure-tuned by the addition of buffer gases, may find important applications in laser science and spectroscopy.

Continuously wavelength-tunable high harmonic generation via soliton dynamics

Francesco Tani, Michael H. Frosz, John C. Travers, Philip St. J. Russell

We report the generation of high harmonics in a gas jet pumped by pulses self-compressed in a He-filled hollow-core photonic crystal fiber through the soliton effect. The gas jet is placed directly at the fiber output. As the energy increases, the ionization-induced soliton blueshift is transferred to the high harmonics, leading to emission bands that are continuously tunable from 17 to 45 eV. (C) 2017 Optical Society of America

Broadband, Lensless, and Optomechanically Stabilized Coupling into Microfluidic Hollow-Core Photonic Crystal Fiber Using Glass Nanospike

Richard Zeltner, Shangran Xie, Riccardo Pennetta, Philip St J. Russell

We report a novel technique for launching broadband laser light into liquid-filled hollow-core photonic crystal fiber (HC-PCF). It uniquely offers self alignment and self-stabilization via optomechanical trapping of a,fused silica nanospike, fabricated by thermally tapering and chemically etching a single mode fiber into a tip diameter of 350 nm. We show that a trapping laser, deliirering similar to 300 mW at 1064 nm, can be used to optically align and stably maintain the iianospike at the core center. Once this is done, a weak broadband supercontinuum signal (similar to 575-1064 nm) can be efficiently and close to achromatically launched in the HC-PCF. The system is robust against liquid-flow in either direction inside the HC-PCF, and the Fresnel back-reflections are reduced to negligible levels compared to free-space launching or butt-coupling. The results are of potential relevance for any application where the efficient delivery of broadband light into liquid-core waveguides is desired.

PHz-Wide Spectral Interference Through Coherent Plasma-Induced Fission of Higher-Order Solitons

F. Koettig, F. Tani, J. C. Travers, P. St. J. Russell

We identify a novel regime of soliton-plasma interactions in which high-intensity ultrashort pulses of intermediate soliton order undergo coherent plasma-induced fission. Experimental results obtained in gas-filled hollow-core photonic crystal fiber are supported by rigorous numerical simulations. In the anomalous dispersion regime, the cumulative blueshift of higher-order input solitons with ionizing intensities results in pulse splitting before the ultimate self-compression point, leading to the generation of robust pulse pairs with PHz bandwidths. The novel dynamics closes the gap between plasma-induced adiabatic soliton compression and modulational instability.

Higher-order mode suppression in twisted single-ring hollow-core photonic crystal fibers

N. N. Edavalath, M. C. Guenendi, R. Beravat, G. K. L. Wong, M. H. Frosz, J. -M. Menard, P. St. J. Russell

A hollow-core single-ring photonic crystal fiber (SR-PCF) consists of a ring of capillaries arranged around a central hollow core. Spinning the preform during drawing introduces a continuous helical twist, offering a novel means of controlling the modal properties of hollow-core SR-PCF. For example, twisting geometrically increases the effective axial propagation constant of the LP01-like modes of the capillaries, providing a means of optimizing the suppression of HOMs, which occurs when the LP11-like core mode phase-matches to the LP01-like modes of the surrounding capillaries. (In a straight fiber, optimum suppression occurs for a capillary-to-core diameter ratio d/D = 0.682.) Twisting also introduces circular birefringence (to be studied in a future Letter) and has a remarkable effect on the transverse intensity profiles of the higher-order core modes, forcing the two-lobed LP11-like mode in the untwisted fiber to become three-fold symmetric in the twisted case. These phenomena are explored by means of extensive numerical modeling, an analytical model, and a series of experiments. Prism-assisted side-coupling is used to measure the losses, refractive indices, and near-field patterns of individual fiber modes in both the straight and twisted cases. (C) 2017 Optical Society of America

Helically twisted photonic crystal fibres

P. St. J. Russell, Ramin Beravat, G. K. L. Wong

Recent theoretical and experimental work on helically twisted photonic crystal fibres (PCFs) is reviewed. Helical Bloch theory is introduced, including a new formalism based on the tight-binding approximation. It is used to explore and explain a variety of unusual effects that appear in a range of different twisted PCFs, including fibres with a single core and fibres with N cores arranged in a ring around the fibre axis. We discuss a new kind of birefringence that causes the propagation constants of left-and rightspinning optical vortices to be non-degenerate for the same order of orbital angular momentum (OAM). Topological effects, arising from the twisted periodic 'space', cause light to spiral around the fibre axis, with fascinating consequences, including the appearance of dips in the transmission spectrum and low loss guidance in coreless PCF. Discussing twisted fibres with a single off-axis core, we report that optical activity in a PCF is opposite in sign to that seen in a step-index fibre. Fabrication techniques are briefly described and emerging applications reviewed. The analytical results of helical Bloch theory are verified by an extensive series of 'numerical experiments' based on finite-element solutions of Maxwell's equations in a helicoidal frame. This article is part of the themed issue 'Optical orbital angular momentum'.

Photochemistry in a soft-glass single-ring hollow-core photonic crystal fibre

Ana M. Cubillas, Xin Jiang, Tijmen G. Euser, Nicola Taccardi, Bastian J. M. Etzold, Peter Wasserscheid, Philip St. J. Russell

A hollow-core photonic crystal fibre (HC-PCF), guided by photonic bandgap effects or anti-resonant reflection, offers strong light confinement and long photochemical interaction lengths in a microscale channel filled with a solvent of refractive index lower than that of glass (usually fused silica). These unique advantages have motivated its recent use as a highly efficient and versatile microreactor for liquid-phase photochemistry and catalysis. In this work, we use a single-ring HC-PCF made from a high-index soft glass, thus enabling photochemical experiments in higher index solvents. The optimized light-matter interaction in the fibre is used to strongly enhance the reaction rate in a proof-of-principle photolysis reaction in toluene.

Extremely broadband single-shot cross-correlation frequency-resolved optical gating using a transient grating as gate and dispersive element

H. Valtna-Lukner, F. Belli, A. Ermolov, F. Koettig, K. F. Mak, F. Tani, J. C. Travers, P. St. J. Russell

Across-correlation frequency-resolved optical gating (FROG) concept, potentially suitable for characterizing few or sub-cycle pulses in a single shot, is described in which a counter-propagating transient grating is used as both the gate and the dispersive element in a FROG spectrometer. An all-reflective setup, which can operate over the whole transmission range of the nonlinear medium, within the sensitivity range of the matrix sensor, is also proposed, and proof-of-principle experiments for the ultraviolet and visible-to-near-infrared spectral ranges are reported. Published by AIP Publishing.

Characterization and shaping of the time-frequency Schmidt mode spectrum of bright twin beams generated in gas-filled hollow-core photonic crystal fibers

M. A. Finger, N. Y. Joly, P. St. J. Russell, M. V. Chekhova

We vary the time-frequency mode structure of ultrafast pulse-pumped modulational instability (MI) twin beams in an argon-filled hollow-core kagome-style photonic crystal fiber by adjusting the pressure, pump pulse chirp, fiber length, and parametric gain. Compared to solid-core systems, the pressure-dependent dispersion landscape brings increased flexibility to the tailoring of frequency correlations, and we demonstrate that the pump pulse chirp can be used to tune the joint spectrum of femtosecond-pumped.(3) sources. We also characterize the resulting mode content, not only by measuring the multimode second-order correlation function g((2)), but also by directly reconstructing the shapes and weights of time-frequency Schmidt (TFS) modes. We show that the number of modes directly influences the shot-to-shot pulse-energy and spectral-shape fluctuations in MI. Using this approach we control and monitor the number of TFS modes within the range from 1.3 to 4 using only a single fiber.

High average power and single-cycle pulses from a mid-IR optical parametric chirped pulse amplifier

Ugaitz Elu, Matthias Baudisch, Hugo Pires, Francesco Tani, Michael H. Frosz, Felix Koettig, Alexey Ermolov, Philip St J. Russell, Jens Biegert

In attosecond and strong-field physics, the acquisition of data in an acceptable time demands the combination of high peak power with high average power. We report a 21 W mid-IR optical parametric chirped pulse amplifier (OPCPA) that generates 131 mu J and 97 fs (sub-9-cycle) pulses at a 160 kHz repetition rate and at a center wavelength of 3.25 mu m. Pulse-to-pulse stability of the carrier envelope phase (CEP)-stable output is excellent with a 0.33% rms over 288 million pulses (30 min) and compression close to a single optical cycle was achieved through soliton self-compression inside a gas-filled mid-IR antiresonant-guiding photonic crystal fiber. Without any additional compression device, stable generation of 14.5 fs (1.35-optical-cycle) pulses was achieved at an average power of 9.6 W. The resulting peak power of 3.9 GW in combination with the near-single-cycle duration and intrinsic CEP stability makes our OPCPA a key-enabling technology for the next generation of extreme photonics, strong-field attosecond research, and coherent x-ray science. (C) 2017 Optical Society of America

Single-shot reconstruction of spectral amplitude and phase in a fiber ring cavity at a 80 MHz repetition rate

Jonas Hammer, Pooria Hosseini, Curtis R. Menyuk, Philip St. J. Russell, Nicolas Y. Joly

Femtosecond pulses circulating in a synchronously driven fiber ring cavity have complex amplitude and phase profiles that can change completely from one round-trip to the next. We use a recently developed technique, combining dispersive Fourier transformation) with spectral interferometry, to reconstruct the spectral amplitude and phase at each round-trip and, thereby, follow in detail the pulse reorganization that occurs. We focus on two different regimes: a period-two regime in which the pulse alternates between two distinct states and a highly complex regime. We characterize the spectral amplitude and phase of the pulses in both regimes at a repetition rate of 75.6 MHz and find good agreement with modeling of the system based on numerical solutions of the generalized nonlinear Schrodinger equation with feedback. (C) 2016 Optical Society of America

Hybrid photonic-crystal fiber for single-mode phase matched generation of third harmonic and photon triplets

Andrea Cavanna, Felix Just, Xin Jiang, Gerd Leuchs, Maria V. Chekhova, Philip St. J. Russell, Nicolas Y. Joly

All-fiber systems for third harmonic generation are of great interest because they can be used for the inverse process, namely, the generation of entangled photon triplets. Usually, chromatic dispersion prevents phase matching between the incident and generated radiation when they are both guided in an LP01-like mode. Here, we present a hybrid photonic crystal fiber that has been designed for phase matched third harmonic generation from 1596 to 532 nm in single-lobed modes. The third harmonic radiation is guided by an all-solid bandgap microstructure, while the pump frequency is confined by conventional total internal reflection. The fiber is also suitable for the generation of photon triplet states.

Broadband electric-field-induced LP01 and LP02 second harmonic generation in Xe-filled hollow-core PCF

Jean-Michel Menard, Felix Köttig, Philip St. J. Russell

Second harmonic (SH) generation with 300 fs pump pulses is reported in a xenon-filled hollow-core photonic crystal fiber (PCF) across which an external bias voltage is applied. Phase-matched intermodal conversion from a pump light in the LP01 mode to SH light in the LP02 mode is achieved at a particular gas pressure. Using periodic electrodes, quasi-phase-matched SH generation into the low-loss LP01 mode is achieved at a different pressure. The low linear dispersion of the gas enables phase-matching over a broad spectral window, resulting in a measured bandwidth of similar to 10 nm at high pump energies. A conversion efficiency of similar to 18%/ mJ is obtained. Gas-filled anti-resonant-reflecting hollow-core PCF uniquely offers pressure-tunable phase-matching, ultra-broadband guidance, and a very high optical damage threshold, which hold great promise for efficient three-wave mixing, especially in difficult-to-access regions of the electromagnetic spectrum. (C) 2016 Optical Society of America

Twist-induced guidance in coreless photonic crystal fiber: A helical channel for light

Ramin Beravat, Gordon K. L. Wong, Michael H. Frosz, Xiao Ming Xi, Philip St. J. Russell

Near-ionization-threshold emission in atomic gases driven by intense sub-cycle pulses

Wei-Chun Chu, John C. Travers, Philip St J. Russell

We study theoretically the dipole radiation of a hydrogen atom driven by an intense sub-cycle pulse. The time-dependent Schrodinger equation for the system is solved by ab initio calculation to obtain the dipole response. Remarkably, a narrowband emission lasting longer than the driving pulse appears at a frequency just above the ionization threshold. An additional calculation using the strong field approximation also recovers this emission, which suggests that it corresponds to the oscillation of nearly bound electrons that behave similarly to Rydberg electrons. The predicted phenomenon is unique to ultrashort driving pulses but not specific to any particular atomic structure.

Generation of spectral clusters in a mixture of noble and Raman-active gases

Pooria Hosseini, Amir Abdolvand, Philip St J. Russell

High-resolution wavefront shaping with a photonic crystal fiber for multimode fiber imaging

Lyubov V. Amitonova, Adrien Descloux, Joerg Petschulat, Michael H. Frosz, Goran Ahmed, Fehim Babic, Xin Jiang, Allard P. Mosk, Philip St. J. Russell, et al.

We demonstrate that a high-numerical-aperture photonic crystal fiber allows lensless focusing at an unparalleled resolution by complex wavefront shaping. This paves the way toward high-resolution imaging exceeding the capabilities of imaging with multi-core single-mode optical fibers. We analyze the beam waist and power in the focal spot on the fiber output using different types of fibers and different wavefront shaping approaches. We show that the complex wavefront shaping technique, together with a properly designed multimode photonic crystal fiber, enables us to create a tightly focused spot on the desired position on the fiber output facet with a subwavelength beam waist. (C) 2016 Optical Society of America

RF-dressed Rydberg atoms in hollow-core fibres

C. Veit, G. Epple, H. Kuebler, T. G. Euser, P. St J. Russell, R. Loew

The giant electro-optical response of Rydberg atoms manifests itself in the emergence of sidebands in the Rydberg excitation spectrum if the atom is exposed to a radio-frequency (RF) electric field. Here we report on the study of RF-dressed Rydberg atoms inside hollow-core photonic crystal fibres, a system that enables the use of low modulation voltages and offers the prospect of miniaturised vapour-based electro-optical devices. Narrow spectroscopic features caused by the RF field are observed for modulation frequencies up to 500 MHz.

Resolving the mystery of milliwatt-threshold opto-mechanical self-oscillation in dual-nanoweb fiber

J. R. Koehler, R. E. Noskov, A. A. Sukhorukov, A. Butsch, D. Novoa, P. St. J. Russell

Reducing losses in solid-core photonic crystal fibers using chlorine dehydration

Michael H. Frosz, Goran Ahmed, Nadezda Lapshina, Ralf Keding, Fehim Babic, Nicolas Y. Joly, Philip St. J. Russell

Tapered Glass-Fiber Microspike: High-Q Flexural Wave Resonator and Optically Driven Knudsen Pump

Riccardo Pennetta, Shangran Xie, Philip St. J. Russell

Solid-core and hollow-core photonic crystal fiber for generation of bright ultraviolet light (Conference Presentation)

Nicolas Y. Joly, Xin Jiang, John C. Travers, Alexey Ermolov, Philip St. J. Russell

Characterization of few-fs deep-UV dispersive waves by ultra-broadband transient-grating XFROG

Alexey Ermolov, Heli Valtna-Lukner, John Travers, Philip St J. Russell

Long-range optical binding in a hollow-core photonic crystal fiber using higher order modes

Dmitry S. Bykov, Richard Zeltner, Tijmen G. Euser, Shangran Xie, Philip St. J. Russell

Generation of a vacuum ultraviolet to visible Raman frequency comb in H-2-filled kagome photonic crystal fiber

M. K. Mridha, D. Novoa, S. T. Bauerschmidt, A. Abdolvand, P. St J. Russell

We report on the generation of a purely vibrational Raman comb, extending from the vacuum ultraviolet (184 nm) to the visible (478 nm), in hydrogen-filled kagome-style photonic crystal fiber pumped at 266 nm. Stimulated Raman scattering and molecular modulation processes are enhanced by higher Raman gain in the ultraviolet. Owing to the pressure-tunable normal dispersion landscape of the "fiber + gas" system in the ultraviolet, higher-order anti-Stokes bands are generated preferentially in higher-order fiber modes. The results pave the way toward tunable fiber-based sources of deep and vacuum ultraviolet light for applications in, e.g., spectroscopy and biomedicine. (C) 2016 Optical Society of America

Self-alignment of glass fiber nanospike by optomechanical back-action in hollow-core photonic crystal fiber

S. Xie, Riccardo Pennetta, P. St J. Russell

A topic of great current interest is the harnessing and enhancement of optical tweezer forces for trapping small objects of different sizes and shapes at relatively small powers. Here we demonstrate the stable trapping, inside the core of a hollow-core photonic crystal fiber (HC-PCF), of a mechanically compliant fused silica nanospike, formed by tapering a single-mode fiber (SMF). The nanospike is subwavelength in diameter over its similar to W50 mu m insertion length in the HC-PCF. Laser light, launched into the SMF core, adiabatically evolves into a mode that extends strongly into the space surrounding the nanospike. It then senses the presence of the hollow core, and the resulting optomechanical action and back-action results in a strong trapping force at the core center. The system permits lens-less, reflection-free, self-stabilized, and self-aligned coupling from SMF to HC-PCF with a demonstrated efficiency of 87.8%. The unique configuration also provides an elegant means of investigating optomechanical effects in optical tweezers, especially at very low pressures. (C) 2016 Optical Society of America

Gigahertz-repetition-rate Tm-doped fiber laser passively mode-locked by optoacoustic effects in nanobore photonic crystal fiber

M. Pang, W. He, P. St. J. Russell

Supercontinuum generation in ZBLAN glass photonic crystal fiber with six nanobore cores

Xin Jiang, Nicolas Y. Joly, Martin A. Finger, Fehim Babic, Meng Pang, Rafal Sopalla, Michael H. Frosz, Samuel Poulain, Marcel Poulain, et al.

Optically Driven Self-Oscillations of a Silica Nanospike at Low Gas Pressures

Shangran Xie, Riccardo Pennetta, Roman E. Noskov, Philip St. J. Russell

Sub-100-fs 1.87 GHz mode-locked fiber laser using stretched-soliton effects

W. He, M. Pang, C. R. Menyuk, P. St J. Russell

Broadband robustly single-mode hollow-core PCF by resonant filtering of higher-order modes

Patrick Uebel, Mehmet C. Guenendi, Michael H. Frosz, Goran Ahmed, Nitin N. Edavalath, Jean-Michel Menard, Philip St. J. Russell

We report a hollow-core photonic crystal fiber that is engineered so as to strongly suppress higher-order modes, i.e., to provide robust LP01 single-mode guidance in all the wavelength ranges where the fiber guides with low loss. Encircling the core is a single ring of nontouching glass elements whose modes are tailored to ensure resonant phase-matched coupling to higher-order core modes. We show that the resulting modal filtering effect depends on only one dimensionless shape parameter, akin to the well-known d/Lambda parameter for endlessly single-mode solid-core PCF. Fabricated fibers show higher-order mode losses some similar to 100 higher than for the LP01 mode, with LP01 losses <0.2 dB/m in the near-infrared and a spectral flatness similar to 1 dB over a >110 THz bandwidth. (C) 2016 Optical Society of America

Fluorescence-based remote irradiation sensor in liquid-filled hollow-core photonic crystal fiber

R. Zeltner, D. S. Bykov, S. Xie, T. G. Euser, P. St. J. Russell

We report an irradiation sensor based on a fluorescent "flying particle" that is optically trapped and propelled inside the core of a water-filled hollow-core photonic crystal fiber. When the moving particle passes through an irradiated region, its emitted fluorescence is captured by guided modes of the fiber core and so can be monitored using a filtered photodiode placed at the fiber end. The particle speed and position can be precisely monitored using in-fiber Doppler velocimetry, allowing the irradiation profile to be measured to a spatial resolution of similar to 10 mu m. The spectral response can be readily adjusted by appropriate choice of particle material. Using dye-doped polystyrene particles, we demonstrate detection of green (532 nm) and ultraviolet (340 nm) light. Published by AIP Publishing.

All-optical bit storage in a fibre laser by optomechanically bound states of solitons

M. Pang, W. He, X. Jiang, P. St. J. Russell

Soliton fibre lasers mode-locked at a high harmonic of their round-trip frequency have many potential applications, from telecommunications to data storage(1). Control of multiple pulses in passively mode-locked fibre lasers has, however, proved very difficult to achieve. This has recently changed with the advent of fibre lasers mode-locked by intense optomechanical interactions in a short length of photonic crystal fibre(2,3). Optomechanical coupling between cavity modes gives rise to highly stable, optomechanically bound, laser soliton states. The repetition rate of these states corresponds to the mechanical resonant frequency in the photonic crystal fibre core(4), which can be a few gigahertz. Here we show that this system can be successfully used for programmable generation and storage of gigahertz-rate soliton sequences over many hours.

Coherent octave-spanning mid-infrared supercontinuum generated in As2S3-silica double-nanospike waveguide pumped by femtosecond Cr:ZnS laser

Shangran Xie, Nikolai Tolstik, John C. Travers, Evgeni Sorokin, Celine Caillaud, Johann Troles, Philip St J. Russell, Irina T. Sorokina

A more than 1.5 octave-spanning mid-infrared supercontinuum (1.2 to 3.6 mu m) is generated by pumping a As2S3-silica "double-nanospike" waveguide via a femtosecond Cr:ZnS laser at 2.35 mu m. The combination of the optimized group velocity dispersion and extremely high nonlinearity provided by the As2S3-silica hybrid waveguide enables a similar to 100 pJ level pump pulse energy threshold for octave-spanning spectral broadening at a repetition rate of 90 MHz. Numerical simulations show that the generated supercontinuum is highly coherent over the entire spanning wavelength range. The results are important for realization of a high repetition rate octave-spanning frequency comb in the mid-infrared spectral region. (C)2016 Optical Society of America

Current sensing using circularly birefringent twisted solid-core photonic crystal fiber

R. Beravat, G. K. L. Wong, X. M. Xi, M. H. Frosz, P. St. J. Russell

Continuously twisted solid-core photonic crystal fiber (PCF) exhibits pure circular birefringence (optical activity), making it ideal for current sensors based on the Faraday effect. By numerical analysis, we identify the PCF geometry for which the circular birefringence (which scales linearly with twist rate) is a maximum. For silica-air PCF, this occurs at a shape parameter (diameter-to-spacing ratio of the hollow channels) of 0.37 and a scale parameter (spacing-to-wavelength) of 1.51. This result is confirmed experimentally by testing a range of different structures. To demonstrate the effectiveness of twisted PCF as a current sensor, a length of fiber is placed on the axis of a 7.6 cm long solenoid, and the Faraday rotation is measured at different values of dc current. The system is then used to chart the wavelength dependence of the Verdet constant. (C) 2016 Optical Society of America

An ion trap built with photonic crystal fibre technology

F. Lindenfelser, B. Keitch, D. Kienzler, D. Bykov, P. Uebel, M. A. Schmidt, P. St. J. Russell, J. P. Home

We demonstrate a surface-electrode ion trap fabricated using techniques transferred from the manufacture of photonic-crystal fibres. This provides a relatively straightforward route for realizing traps with an electrode structure on the 100 micron scale with high optical access. We demonstrate the basic functionality of the trap by cooling a single ion to the quantum ground state, allowing us to measure a heating rate from the ground state of 787 +/- 24 quanta/s. Variation of the fabrication procedure used here may provide access to traps in this geometry with trap scales between 100 mu m and 10 mu m. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Photoionization-Induced Emission of Tunable Few-Cycle Midinfrared Dispersive Waves in Gas-Filled Hollow-Core Photonic Crystal Fibers

D. Novoa, M. Cassataro, J. C. Travers, P. St. J. Russell

We propose a scheme for the emission of few-cycle dispersive waves in the midinfrared using hollow-core photonic crystal fibers filled with noble gas. The underlying mechanism is the formation of a plasma cloud by a self-compressed, subcycle pump pulse. The resulting free-electron population modifies the fiber dispersion, allowing phase-matched access to dispersive waves at otherwise inaccessible frequencies, well into the midinfrared. Remarkably, the pulses generated turn out to have durations of the order of two optical cycles. In addition, this ultrafast emission, which occurs even in the absence of a zero dispersion point between pump and midinfrared wavelengths, is tunable over a wide frequency range simply by adjusting the gas pressure. These theoretical results pave the way to a new generation of compact, fiber-based sources of few-cycle midinfrared radiation.

Angle-resolved photoemission spectroscopy with 9-eV photon-energy pulses generated in a gas-filled hollow-core photonic crystal fiber

H. Bromberger, A. Ermolov, F. Belli, H. Liu, F. Calegari, M. Chavez-Cervantes, M. T. Li, C. T. Lin, A. Abdolvand, et al.

A recently developed source of ultraviolet radiation, based on optical soliton propagation in a gasfilled hollow-core photonic crystal fiber, is applied here to angle-resolved photoemission spectroscopy (ARPES). Near-infrared femtosecond pulses of only few mu J energy generate vacuum ultraviolet radiation between 5.5 and 9 eV inside the gas-filled fiber. These pulses are used to measure the band structure of the topological insulator Bi2Se3 with a signal to noise ratio comparable to that obtained with high order harmonics from a gas jet. The two-order-of-magnitude gain in efficiency promises time-resolved ARPES measurements at repetition rates of hundreds of kHz or even MHz, with photon energies that cover the first Brillouin zone of most materials. (C) 2015 AIP Publishing LLC.

Compressing mu J-level pulses from 250 fs to sub-10 fs at 38-MHz repetition rate using two gas-filled hollow-core photonic crystal fiber stages

K. F. Mak, M. Seidel, O. Pronin, M. H. Frosz, A. Abdolvand, V. Pervak, A. Apolonski, F. Krausz, J. C. Travers, et al.

Compression of 250-fs, 1-mu J pulses from a KLM Yb:YAG thin-disk oscillator down to 9.1 fs is demonstrated. A kagome-PCF with a 36-mu m core-diameter is used with a pressure gradient from 0 to 40 bar of krypton. Compression to 22 fs is achieved by 1200 fs(2) group-delay-dispersion provided by chirped mirrors. By coupling the output into a second kagome-PCF with a pressure gradient from 0 to 25 bar of argon, octave spanning spectral broadening via the soliton-effect is observed at 18-W average output power. Self-compression to 9.1 fs is measured, with compressibility to 5 fs predicted. Also observed is strong emission in the visible via dispersive wave generation, amounting to 4% of the total output power. (C) 2015 Optical Society of America

Phase-matched electric-field-induced second-harmonic generation in Xe-filled hollow-core photonic crystal fiber

Jean-Michel Menard, Philip St. J. Russell

Second-order nonlinearity is induced inside a Xe-filled hollow-core photonic crystal fiber (PCF) by applying an external dc field. The system uniquely allows the linear optical properties to be adjusted by changing the gas pressure, allowing for precise phase matching between the LP01 mode at 1064 nm and the LP02 mode at 532 nm. The dependence of the second-harmonic conversion efficiency on the gas pressure, launched pulse energy, and applied field agrees well with theory. The ultra-broadband guidance offered by anti-resonant reflecting hollow-core PCFs, for example, a kagome PCF, offers many possibilities for generating light in traditionally difficult-to-access regions of the electromagnetic spectrum, such as the ultraviolet or the terahertz windows. The system can also be used for non-invasive measurements of the transmission loss in a hollow-core PCF over a broad spectrum, including the deep and vacuum UV regions. (C) 2015 Optical Society of America

Raman amplification of pure side-seeded higherorder modes in hydrogen-filled hollow-core PCF

Jean-Michel Menard, Barbara M. Trabold, Amir Abdolvand, Philip St J. Russell

We use Raman amplification in hydrogen-filled hollow-core kagome photonic crystal fiber to generate high energy pulses in pure single higher-order modes. The desired higher-order mode at the Stokes frequency is precisely seeded by injecting a pulse of light from the side, using a prism to select the required modal propagation constant. An intense pump pulse in the fundamental mode transfers its energy to the Stokes seed pulse with measured gains exceeding 60 dB and output pulse energies as high as 8 mu J. A pressure gradient is used to suppress stimulated Raman scattering into the fundamental mode at the Stokes frequency. The growth of the Stokes pulse energy is experimentally and theoretically investigated for six different higher-order modes. The technique has significant advantages over the use of spatial light modulators to synthesize higher-order mode patterns, since it is very difficult to perfectly match the actual eigenmode of the fiber core, especially for higher-order modes with complex multi-lobed transverse field profiles. (C) 2015 Optical Society of America

Supercontinuum generation in the vacuum ultraviolet through dispersive-wave and soliton-plasma interaction in a noble-gas-filled hollow-core photonic crystal fiber

A. Ermolov, K. F. Mak, M. H. Frosz, J. C. Travers, P. St. J. Russell

We report on the generation of a three-octave-wide supercontinuum extending from the vacuum ultraviolet (VUV) to the near infrared, spanning at least 113-1000 nm (i.e., 11-1.2eV), in He-filled hollow-core kagome-style photonic crystal fiber. Numerical simulations confirm that the main mechanism is an interaction between dispersive-wave emission and plasma-induced blue-shifted soliton recompression around the fiber zero dispersion frequency. The VUV part of the supercontinuum, the modeling of which proves to be coherent and possesses a simple phase structure, has sufficient bandwidth to support single-cycle pulses of 500 asec duration. We also demonstrate, in the same system, the generation of narrower-band VUV pulses through dispersive-wave emission, tunable from 120 to 200 nm with efficiencies exceeding 1% and VUV pulse energies in excess of 50 nJ.

Stable subpicosecond soliton fiber laser passively mode-locked by gigahertz acoustic resonance in photonic crystal fiber core

M. Pang, X. Jiang, W. He, G. K. L. Wong, G. Onishchukov, N. Y. Joly, G. Ahmed, C. R. Menyuk, P. St J. Russell

Ultrafast lasers with high repetition rates are of considerable interest in applications such as optical fiber telecommunications, frequency metrology, high-speed optical sampling, and arbitrary waveform generation. For fiber lasers mode-locked at the cavity round-trip frequency, the pulse repetition rate is limited to tens or hundreds of megahertz by the meter-order cavity lengths. Here we report a soliton fiber laser passively mode-locked at a high harmonic (similar to 2 GHz) of its fundamental frequency by means of optoacoustic interactions in the small solid glass core of a short length ( 60 cm) of photonic crystal fiber. Due to tight confinement of both light and vibrations, the optomechanical interaction is strongly enhanced. The long-lived acoustic vibration provides strong modulation of the refractive index in the photonic crystal fiber core, fixing the soliton spacing in the laser cavity and allowing stable mode-locking, with low pulse timing jitter, at gigahertz repetition rates. (C) 2015 Optical Society of America

Dramatic Raman Gain Suppression in the Vicinity of the Zero Dispersion Point in a Gas-Filled Hollow-Core Photonic Crystal Fiber

S. T. Bauerschmidt, D. Novoa, P. St J. Russell

In 1964 Bloembergen and Shen predicted that Raman gain could be suppressed if the rates of phonon creation and annihilation (by inelastic scattering) exactly balance. This is only possible if the momentum required for each process is identical, i.e., phonon coherence waves created by pump-to-Stokes scattering are identical to those annihilated in pump-to-anti-Stokes scattering. In bulk gas cells, this can only be achieved over limited interaction lengths at an oblique angle to the pump axis. Here we report a simple system that provides dramatic Raman gain suppression over long collinear path lengths in hydrogen. It consists of a gas-filled hollow-core photonic crystal fiber whose zero dispersion point is pressure adjusted to lie close to the pump laser wavelength. At a certain precise pressure, stimulated generation of Stokes light in the fundamental mode is completely suppressed, allowing other much weaker phenomena such as spontaneous Raman scattering to be explored at high pump powers.

Generation of three-octave-spanning transient Raman comb in hydrogen-filled hollow-core PCF

F. Tani, F. Belli, A. Abdolvand, J. C. Travers, P. St. J. Russell

A noise-seeded transient comb of Raman sidebands spanning three octaves from 180 to 2400 nm, is generated by pumping a hydrogen-filled hollow-core photonic crystal fiber with 26-mu J, 300-fs pulses at 800 nm. The pump pulses are spectrally broadened by both Kerr and Raman-related self-phase modulation (SPM), and the broadening is then transferred to the Raman lines. In spite of the high intensity, and in contrast to bulk gas-cell based experiments, neither SPM broadening nor ionization are detrimental to comb formation. (C) 2015 Optical Society of America

Vacuum-ultraviolet to infrared supercontinuum in hydrogen-filled photonic crystal fiber

Federico Belli, Amir Abdolvand, Wonkeun Chang, John C. Travers, Philip St. J. Russell

Although supercontinuum sources are readily available for the visible and near infrared (IR), and recently also for the mid-IR, many areas of biology, chemistry, and physics would benefit greatly from the availability of compact, stable, and spectrally bright deep-ultraviolet and vacuum-ultraviolet (VUV) supercontinuum sources. Such sources have, however, not yet been developed. Here we report the generation of a bright supercontinuum, spanning more than three octaves from 124 nm to beyond 1200 nm, in hydrogen-filled kagome-style hollow-core photonic crystal fiber (kagome-PCF). Few-microjoule, 30 fs pump pulses at wavelength of 805 nm are launched into the fiber, where they undergo self-compression via the Raman-enhanced Kerr effect. Modeling indicates that before reaching a minimum subcycle pulse duration of similar to 1 fs, much less than one period of molecular vibration (8 fs), nonlinear reshaping of the pulse envelope, accentuated by self-steepening and shock formation, creates an ultrashort feature that causes impulsive excitation of long-lived coherent molecular vibrations. These phase modulate a strong VUV dispersive wave (at 182 nm or 6.8 eV) on the trailing edge of the pulse, further broadening the spectrum into the VUV. The results also show for the first time that kagome-PCF guides well in the VUV. (C) 2015 Optical Society of America

Broadband-tunable LP01 mode frequency shifting by Raman coherence waves in a H-2-filled hollow-core photonic crystal fiber

S. T. Bauerschmidt, D. Novoa, A. Abdolvand, P. St. J. Russell

When a laser pump beam of sufficient intensity is incident on a Raman-active medium such as hydrogen gas, a strong Stokes signal, redshifted by the Raman transition frequency Omega(R), is generated. This is accompanied by the creation of a "coherence wave" of synchronized molecular oscillations with wave vector Delta beta determined by the optical dispersion. Within its lifetime, this coherence wave can be used to shift by Omega(R) the frequency of a third "mixing" signal, provided phase matching is satisfied, i.e., Delta beta is matched. Conventionally, this can be arranged using noncollinear beams or higher-order waveguide modes. Here we report the collinear phase-matched frequency shifting of an arbitrary mixing signal using only the fundamental LP01 modes of a hydrogen-filled hollow-core photonic crystal fiber. This is made possible by the S-shaped dispersion curve that occurs around the pressure-tunable zero dispersion point. Phase-matched frequency shifting by 125 THz is possible from the UV to the near IR. Long interaction lengths and tight modal confinement reduce the peak intensities required, allowing conversion efficiencies in excess of 70%. The system is of great interest in coherent anti-Stokes Raman spectroscopy and for wavelength conversion of broadband laser sources. (C) 2015 Optical Society of America

Wideband-tunable soliton fiber laser mode-locked at 1.88 GHz by optoacoustic interactions in solid-core PCF

Wenbin He, Meng Pang, Philip St J. Russell

We report a wavelength-tunable soliton fiber laser stably mode-locked at 1.88 GHz (the 389th harmonic of the cavity round-trip frequency) by a light-driven acoustic resonance in the core of a photonic crystal fiber. Stable high-harmonic mode-locking could be maintained when the lasing wavelength was continuously tuned from 1532 to 1566 nm by means of an optical filter placed inside the laser cavity. We report on the experimental performance of the laser, including its power scalability, super-mode noise suppression ratio, long-term repetition rate stability, short-term pulse amplitude noise and timing jitter, optical comb structure and pulse-to-pulse phase fluctuations. (C) 2015 Optical Society of America

Raman-Free, Noble-Gas-Filled Photonic-Crystal Fiber Source for Ultrafast, Very Bright Twin-Beam Squeezed Vacuum

Martin A. Finger, Timur Sh. Iskhakov, Nicolas Y. Joly, Maria V. Chekhova, Philip St. J. Russell

We report a novel source of twin beams based on modulational instability in high-pressure argon-filled hollow-core kagome-style photonic-crystal fiber. The source is Raman-free and manifests strong photonnumber correlations for femtosecond pulses of squeezed vacuum with a record brightness of similar to 2500 photons per mode. The ultra-broadband (similar to 50 THz) twin beams are frequency tunable and contain one spatial and less than 5 frequency modes. The presented source outperforms all previously reported squeezed-vacuum twin-beam sources in terms of brightness and low mode content.

Deep-ultraviolet to mid-infrared supercontinuum generated in solid-core ZBLAN photonic crystal fibre

Xin Jiang, Nicolas Y. Joly, Martin A. Finger, Fehim Babic, Gordon K. L. Wong, John C. Travers, Philip St J. Russell

Silica-based photonic crystal fibre has proven highly successful for supercontinuum generation, with smooth and flat spectral power densities. However, fused silica glass suffers from strong material absorption in the mid-infrared (>2,500 nm), as well as ultraviolet-related optical damage (solarization), which limits performance and lifetime in the ultraviolet (<380 nm). Supercontinuum generation in silica photonic crystal fibre is therefore only possible between these limits. A number of alternative glasses have been used to extend the mid-infrared performance, including chalcogenides, fluorides and heavy-metal oxides, but none has extended the ultraviolet performance. Here, we describe the successful fabrication (using the stack-and-draw technique) of a ZBLAN photonic crystal fibre with a high air-filling fraction, a small solid core, nanoscale features and near-perfect structure. We also report its use in the generation of ultrabroadband, long-term stable, supercontinua spanning more than three octaves in the spectral range 200-2,500 nm.

Flying particle sensors in hollow-core photonic crystal fibre

D. S. Bykov, O. A. Schmidt, T. G. Euser, P. St. J. Russell

Optical fibre sensors make use of diverse physical effects to measure parameters such as strain, temperature and electric field. Here we introduce a new class of reconfigurable fibre sensor, based on a 'flying-particle' optically trapped inside a hollow-core photonic crystal fibre and illustrate its use in electric field and temperature sensing with high spatial resolution. The electric field distribution near the surface of a multi-element electrode is measured with a resolution of similar to 100 mu m by monitoring changes in the transmitted light signal due to the transverse displacement of a charged silica microparticle trapped within the hollow core. Doppler-based velocity measurements are used to map the gas viscosity, and thus the temperature, along a hollow-core photonic crystal fibre. The flying-particle approach represents a new paradigm in fibre sensors, potentially allowing multiple physical quantities to be mapped with high positional accuracy over kilometre-scale distances.

Enhanced optical activity and circular dichroism in twisted photonic crystal fiber

G. K. L. Wong, X. M. Xi, M. H. Frosz, P. St. J. Russell

We demonstrate experimentally and theoretically that the core-guided mode in helically twisted photonic crystal fiber exhibits resonantly enhanced optical activity and circular dichroism in the vicinity of anti-crossings with leaky orbital angular momentum (OAM) modes in the cladding. This arises because the anti-crossings for left and right circularly polarized core modes occur at slightly different wavelengths. (C) 2015 Optical Society of America

As2S3-silica double-nanospike waveguide for mid-infrared supercontinuum generation

Shangran Xie, Francesco Tani, John C. Travers, Patrick Uebel, Celine Caillaud, Johann Troles, Markus A. Schmidt, Philip St J. Russell

A double-nanospike As2S3-silica hybrid waveguide structure is reported. The structure comprises nanotapers at input and output ends of a step-index waveguide with a subwavelength core (1 mu m in diameter), with the aim of increasing the in-coupling and out-coupling efficiency. The design of the input nanospike is numerically optimized to match both the diameter and divergence of the input beam, resulting in efficient excitation of the fundamental mode of the waveguide. The output nanospike is introduced to reduce the output beam divergence and the strong endface Fresnel reflection. The insertion loss of the waveguide is measured to be similar to 2 dB at 1550 nm in the case of free-space in-coupling, which is similar to 7 dB lower than the previously reported single-nanospike waveguide. By pumping a 3-mm-long waveguide at 1550 nm using a 60-fs fiber laser, an octave-spanning supercontinuum (from 0.8 to beyond 2.5 mu m) is generated at 38 pJ input energy. (C) 2014 Optical Society of America

Supercontinuum up-conversion via molecular modulation in gas-filled hollow-core PCF

S. T. Bauerschmidt, D. Novoa, B. M. Trabold, A. Abdolvand, P. St J. Russell

We report on the efficient, tunable, and selective frequency up-conversion of a supercontinuum spectrum via molecular modulation in a hydrogen-filled hollow-core photonic crystal fiber. The vibrational Q(1) Raman transition of hydrogen is excited in the fiber by a pump pre-pulse, enabling the excitation of a synchronous, collective oscillation of the molecules. This coherence wave is then used to up-shift the frequency of an arbitrarily weak, delayed probe pulse. Perfect phase-matching for this process is achieved by using higher order fiber modes and adjusting the pressure of the filling gas. Conversion efficiencies of similar to 50% are obtained within a tuning range of 25 THz. (C)2014 Optical Society of America

Accuracy of the capillary approximation for gas-filled kagome-style photonic crystal fibers

M. A. Finger, N. Y. Joly, T. Weiss, P. St. J. Russell

Precise knowledge of the group velocity dispersion in gas-filled hollow-core photonic crystal fiber is essential for accurate modeling of ultrafast nonlinear dynamics. Here we study the validity of the capillary approximation commonly used to calculate the modal refractive index in kagome-style photonic crystal fibers. For area-preserving core radius alpha(AP) and core wall thickness t, measurements and finite element simulations show that the approximation has an error greater than 15% for wavelengths longer than 0.56 root(alpha(AP)t), independently of the gas-filling pressure. By introducing an empirical wavelength-dependent core radius, the range of validity of the capillary approximation is extended out to a wavelength of at least 0.98 root(alpha(AP)t). (C) 2014 Optical Society of America

Real-time Doppler-assisted tomography of microstructured fibers by side-scattering

Alessio Stefani, Michael H. Frosz, Tijmen G. Euser, Gordon K. L. Wong, Philip St. J. Russell

We introduce the concept of Doppler-assisted tomography (DAT) and show that it can be applied successfully to non-invasive imaging of the internal microstructure of a photonic crystal fiber. The fiber is spun at similar to 10 Hz around its axis and laterally illuminated with a laser beam. Monitoring the time-dependent Doppler shift of the light scattered by the hollow channels permits the azimuthal angle and radial position of individual channels to be measured. An inverse Radon transform is used to construct an image of the microstructure from the frequency-modulated scattered signal. We also show that DAT can image sub-wavelength features and monitor the structure along a tapered fiber, which is not possible using other techniques without cutting up the taper into several short pieces or filling it with index-matching oil. The non-destructive nature of DAT means that it could potentially be applied to image the fiber microstructure as it emerges from the drawing tower, or indeed to carry out tomography on any transparent microstructured cylindrical object. (C) 2014 Optical Society of America

Orbital-angular-momentum-preserving helical Bloch modes in twisted photonic crystal fiber

X. M. Xi, G. K. L. Wong, M. H. Frosz, F. Babic, G. Ahmed, X. Jiang, T. G. Euser, P. St. J. Russell

In optical fiber telecommunications, there is much current work on the use of orbital angular momentum (OAM) modes for increasing channel capacity. Here we study the properties of a helically twisted photonic crystal fiber (PCF) that preserves the chirality of OAM modes of the same order, i.e., it inhibits scattering between an order +1 mode to an order -1 mode. This is achieved by thermally inducing a helical twist in a PCF with a novel three-bladed Y-shaped core. The effect is seen for twist periods of a few millimeters or less. We develop a novel scalar theory to analyze the properties of the twisted fiber, based on a helicoidal extension to Bloch wave theory. It yields results that are in excellent agreement with full finite element simulations. Since twisted PCFs with complex core structures can be produced in long lengths from a fiber drawing tower, they are of potential interest for increasing channel capacity in optical telecommunications, but the result is also of interest to the photonic crystal community, where a new kind of guided helical Bloch mode is sure to excite interest, and among the spin-orbit coupling community. (C) 2014 Optical Society of America

Broadband single-photon-level memory in a hollow-core photonic crystal fibre

M. R. Sprague, P. S. Michelberger, T. F. M. Champion, D. G. England, J. Nunn, X. -M. Jin, W. S. Kolthammer, A. Abdolvand, P. St J. Russell, et al.

Storing information encoded in light is critical for realizing optical buffers for all-optical signal processing(1,2) and quantum memories for quantum information processing(3,4). These proposals require efficient interaction between atoms and a well-defined optical mode. Photonic crystal fibres can enhance light-matter interactions and have engendered a broad range of nonlinear effects(5); however, the storage of light has proven elusive. Here, we report the first demonstration of an optical memory in a hollow-core photonic crystal fibre. We store gigahertz-bandwidth light in the hyperfine coherence of caesium atoms at room temperature using a far-detuned Raman interaction. We demonstrate a signal-to-noise ratio of 2.6:1 at the single-photon level and a memory efficiency of 27 +/- 1%. Our results demonstrate the potential of a room-temperature fibre-integrated optical memory for implementing local nodes of quantum information networks.

Multimode ultrafast nonlinear optics in optical waveguides: numerical modeling and experiments in kagome photonic-crystal fiber

Francesco Tani, John C. Travers, Philip St. J. Russell

We introduce a general full-field propagation equation for optical waveguides, including both fundamental and higher order modes, and apply it to the investigation of spatial nonlinear effects of ultrafast and extremely broad-band nonlinear processes in hollow-core optical fibers. The model is used to describe pulse propagation in gas-filled hollow-core waveguides including the full dispersion, Kerr, and ionization effects. We study third-harmonic generation into higher order modes, soliton emission of resonant dispersive waves into higher order modes, intermodal four-wave mixing, and Kerr-driven transverse self-focusing and plasma-defocusing, all in a gas-filled kagome photonic crystal fiber system. In the latter case a form of waveguide-based filamentation is numerically predicted. (C) 2014 Optical Society of America

Midinfrared frequency combs from coherent supercontinuum in chalcogenide and optical parametric oscillation

Kevin F. Lee, N. Granzow, M. A. Schmidt, W. Chang, L. Wang, Q. Coulombier, J. Troles, Nick Leindecker, Konstantin L. Vodopyanov, et al.

We observe the coherence of the supercontinuum generated in a nanospike chalcogenide-silica hybrid waveguide pumped at 2 mu m. The supercontinuum is shown to be coherent with the pump by interfering it with a doubly resonant optical parametric oscillator (OPO) that is itself coherent with the shared pump laser. This enables coherent locking of the OPO to the optically referenced pump frequency comb, resulting in a composite frequency comb with wavelengths from 1 to 6 mu m. (C) 2014 Optical Society of America

CW-pumped single-pass frequency comb generation by resonant optomechanical nonlinearity in dual-nanoweb fiber

A. Butsch, J. R. Koehler, R. E. Noskov, P. St. J. Russell

Recent experiments in the field of strong optomechanical interactions have focused on either structures that are simultaneously optically and mechanically resonant, or photonic crystal fibers pumped by a laser intensity modulated at a mechanical resonant frequency of the glass core. Here, we report continuous-wave (CW) pumped self-oscillations of a fiber nanostructure that is only mechanically resonant. Since the mechanism has close similarities to stimulated Raman scattering by molecules, it has been named stimulated Raman-like scattering. The structure consists of two submicrometer thick glass membranes (nanowebs), spaced by a few hundred nanometers and supported inside a 12-cm-long capillary fiber. It is driven into oscillation by a CW pump laser at powers as low as a few milliwatts. As the pump power is increased above threshold, a comb of Stokes and anti-Stokes lines is generated, spaced by the oscillator frequency of similar to 6 MHz. An unprecedentedly high Raman-like gain of similar to 4 x 10(6) m(-1) W-1 is inferred after analysis of the experimental data. Resonant frequencies as high as a few hundred megahertz are possible through the use of thicker and less-wide webs, suggesting that the structure can find application in passive mode-locking of fiber lasers, optical frequency metrology, and spectroscopy. (C) 2014 Optical Society of America

Selective excitation of higher order modes in hollow-core PCF via prism-coupling

Barbara M. Trabold, David Novoa, Amir Abdolvand, Philip St. J. Russell

Prism-coupling through the microstructured cladding is used to selectively excite individual higher order modes in hollow-core photonic crystal fibers (PCFs). Mode selection is achieved by varying the angle between the incoming beam and the fiber axis, in order to match the axial wavevector component to that of the desired mode. The technique allows accurate measurement of the effective indices and transmission losses of modes of arbitrary order, even those with highly complex transverse field distributions that would be extremely difficult to excite by conventional endfire coupling. (C) 2014 Optical Society of America

Hollow-core photonic crystal fibres for gas-based nonlinear optics

P. St J. Russell, P. Hoelzer, W. Chang, A. Abdolvand, J. C. Travers

Unlike the capillaries conventionally used for gas-based spectral broadening of ultrashort (<100 fs) multi-millijoule pulses, which produce only normal dispersion at usable pressure levels, hollow-core photonic crystal fibres provide pressure-adjustable normal or anomalous dispersion. They also permit low-loss guidance in a hollow channel that is about ten times narrower and has a 100-fold-higher effective nonlinearity than capillary-based systems. This has led to several dramatic results, including soliton compression to few-cycle pulses, widely tunable deep-ultraviolet light sources, novel soliton-plasma interactions and multi-octave Raman frequency combs. A new generation of versatile and efficient gas-based light sources, which are tunable from the vacuum ultraviolet to the near infrared, and of versatile and efficient pulse compression devices is emerging.

Taking Two-Photon Excitation to Exceptional Path-Lengths in Photonic Crystal Fiber

Gareth O. S. Williams, Tijmen G. Euser, Jochen Arlt, Philip St. J. Russell, Anita C. Jones

The well-known, defining feature of two-photon excitation (TPE) is the tight, three-dimensional confinement of excitation at the intense focus of a laser beam. The extremely small excitation volume, on the order of 1 mu m(3) (1 femtoliter), is the basis of far-reaching applications of TPE in fluorescence imaging, photodynamic therapy, nanofabrication, and three-dimensional optical memory. Paradoxically, the difficulty of detecting photochemical events in such a small volume is a barrier to the development of the two-photon-activated molecular systems that are essential to the realization of such applications. We show, using two-photon-excited fluorescence to directly visualize the excitation path, that confinement of both laser beam and sample solution within the 20 mu m hollow core of a photonic crystal fiber permits TPE to be sustained over an extraordinary path-length of more than 10 cm, presenting a new experimental paradigm for ultrasensitive studies of two-photon-induced processes in solution.

Rydberg atoms in hollow-core photonic crystal fibres

G. Epple, K. S. Kleinbach, T. G. Euser, N. Y. Joly, T. Pfau, P. St J. Russell, R. Loew

The exceptionally large polarizability of highly excited Rydberg atoms-six orders of magnitude higher than ground-state atoms-makes them of great interest in fields such as quantum optics, quantum computing, quantum simulation and metrology. However, if they are to be used routinely in applications, a major requirement is their integration into technically feasible, miniaturized devices. Here we show that a Rydberg medium based on room temperature caesium vapour can be confined in broadband-guiding kagome-style hollow-core photonic crystal fibres. Three-photon spectroscopy performed on a caesium-filled fibre detects Rydberg states up to a principal quantum number of n = 40. Besides small energy-level shifts we observe narrow lines confirming the coherence of the Rydberg excitation. Using different Rydberg states and core diameters we study the influence of confinement within the fibre core after different exposure times. Understanding these effects is essential for the successful future development of novel applications based on integrated room temperature Rydberg systems.

In Situ Heterogeneous Catalysis Monitoring in a Hollow-Core Photonic Crystal Fiber Microflow Reactor

Ana M. Cubillas, Matthias Schmidt, Tijmen G. Euser, Nicola Taccardi, Sarah Unterkofler, Philip St. J. Russell, Peter Wasserscheid, Bastian J. M. Etzold

Atomic mercury vapor inside a hollow-core photonic crystal fiber

Ulrich Vogl, Christian Peuntinger, Nicolas Y. Joly, Philip St. J. Russell, Christoph Marquardt, Gerd Leuchs

We demonstrate high atomic mercury vapor pressure in a kagome-style hollow-core photonic crystal fiber at room temperature. After a few days of exposure to mercury vapor the fiber is homogeneously filled and the optical depth achieved remains constant. With incoherent optical pumping from the ground state we achieve an optical depth of 114 at the 6(3)P(2) - 6(3)D(3) transition, corresponding to an atomic mercury number density of 6 x 10(10) cm(-3). The use of mercury vapor in quasi one-dimensional confinement may be advantageous compared to chemically more active alkali vapor, while offering strong optical nonlinearities in the ultraviolet region of the optical spectrum. (C) 2014 Optical Society of America

Damage-free single-mode transmission of deep-UV light in hollow-core PCF

F. Gebert, M. H. Frosz, T. Weiss, Y. Wan, A. Ermolov, N. Y. Joly, P. O. Schmidt, P. St. J. Russell

Transmission of UV light with high beam quality and pointing stability is desirable for many experiments in atomic, molecular and optical physics. In particular, laser cooling and coherent manipulation of trapped ions with transitions in the UV require stable, single-mode light delivery. Transmitting even similar to 2 mW CW light at 280 nm through silica solid-core fibers has previously been found to cause transmission degradation after just a few hours due to optical damage. We show that photonic crystal fiber of the kagome type can be used for effectively single-mode transmission with acceptable loss and bending sensitivity. No transmission degradation was observed even after >100 hours of operation with 15 mW CW input power. In addition it is shown that implementation of the fiber in a trapped ion experiment increases the coherence time of the internal state transfer due to an increase in beam pointing stability. (C) 2014 Optical Society of America

Multistability and spontaneous breaking in pulse-shape symmetry in fiber ring cavities

M. J. Schmidberger, D. Novoa, F. Biancalana, P. St J. Russell, N. Y. Joly

We describe the spatio-temporal evolution of ultrashort pulses propagating in a fiber ring cavity using an extension of the Lugiato-Lefever model. The model predicts the appearance of multistability and spontaneous symmetry breaking in temporal pulse shape. We also use a hydrodynamical approach to explain the stability of the observed regimes of asymmetry. (C) 2014 Optical Society of America

Photonic crystal fibres for chemical sensing and photochemistry

Ana M. Cubillas, Sarah Unterkofler, Tijmen G. Euser, Bastian J. M. Etzold, Anita C. Jones, Peter J. Sadler, Peter Wasserscheid, Philip St. J. Russell

In this review, we introduce photonic crystal fibre as a novel optofluidic microdevice that can be employed as both a versatile chemical sensor and a highly efficient microreactor. We demonstrate that it provides an excellent platform in which light and chemical samples can strongly interact for quantitative spectroscopic analysis or photoactivation purposes. The use of photonic crystal fibre in photochemistry and sensing is discussed and recent results on gas and liquid sensing as well as on photochemical and catalytic reactions are reviewed. These developments demonstrate that the tight light confinement, enhanced light-matter interaction and reduced sample volume offered by photonic crystal fibre make it useful in a wide range of chemical applications.

Mid-infrared supercontinuum generation in As2S3-silica "nano-spike" step-index waveguide

N. Granzow, M. A. Schmidt, W. Chang, L. Wang, Q. Coulombier, J. Troles, P. Toupin, I. Hartl, K. F. Lee, et al.

Efficient generation of a broad-band mid-infrared supercontinuum spectrum is reported in an arsenic trisulphide waveguide embedded in silica. A chalcogenide "nano-spike", designed to transform the incident light adiabatically into the fundamental mode of a 2-mm-long uniform section 1 mu m in diameter, is used to achieve high launch efficiencies. The nano-spike is fully encapsulated in a fused silica cladding, protecting it from the environment. Nano-spikes provide a convenient means of launching light into sub-wavelength scale waveguides. Ultrashort (65 fs, repetition rate 100 MHz) pulses at wavelength 2 mu m, delivered from a Tm-doped fiber laser, are launched with an efficiency similar to 12% into the sub-wavelength chalcogenide waveguide. Soliton fission and dispersive wave generation along the uniform section result in spectral broadening out to almost 4 mu m for launched energies of only 18 pJ. The spectrum generated will have immediate uses in metrology and infrared spectroscopy. (C) 2013 Optical Society of America

Efficient optical pumping and high optical depth in a hollow-core photonic-crystal fibre for a broadband quantum memory

Michael R. Sprague, Duncan G. England, Amir Abdolvand, Joshua Nunn, Xian-Min Jin, W. Steven Kolthammer, Marco Barbieri, Bruno Rigal, Patrick S. Michelberger, et al.

The generation of large multiphoton quantum states-for applications in computing, metrology and simulation-requires a network of high-efficiency quantum memories capable of storing broadband pulses. Integrating these memories into a fibre offers a number of advantages towards realizing this goal: strong light-matter coupling at low powers, simplified alignment and compatibility with existing photonic architectures. Here, we introduce a large-core kagome-structured hollow-core fibre as a suitable platform for an integrated fibre-based quantum memory with a warm atomic vapour. We demonstrate, for the first time, efficient optical pumping in such a system, where 90 +/- 1% of atoms are prepared in the ground state. We measure high optical depths (3 x 10(4)) and narrow homogeneous linewidths (6 +/- 2 MHz) that do not exhibit significant transit-time broadening, showing that we can prepare a Lambda-level system in a pure state. Our results establish that kagome fibres are suitable for implementing a broadband, room-temperature quantum memory, as well as a range of nonlinear optical effects.

Measuring mechanical strain and twist using helical photonic crystal fiber

Xiaoming Xi, Gordon K. L. Wong, Thomas Weiss, Philip St J. Russell

Solid-core photonic crystal fiber (PCF) with a permanent helical twist exhibits dips in its transmission spectrum at certain wavelengths. These are associated with the formation of orbital angular momentum states in the cladding. Here we investigate the tuning of these states with mechanical torque and axial tension. The dip wavelengths are found to scale linearly with both axial strain and mechanical twist rate. Analysis shows that the tension-induced shift in resonance wavelength is determined both by the photoelastic effect and by the change in twist rate, while the torsion-induced wavelength shift depends only on the change in twist rate. Twisted PCF can act as an effective optically monitored torque-tension transducer, twist sensor, or strain gauge. (C) 2013 Optical Society of America

Spectrofluorimetry with attomole sensitivity in photonic crystal fibres

Gareth O. S. Williams, Tijmen G. Euser, Philip St J. Russell, Anita C. Jones

We report the use of photonic crystal fibres (PCF) as spectrofluorimetric systems in which sample solutions are excited within the microstructure of the fibre. The use of intra-fibre excitation has several advantages that combine to enable highly sensitive measurements of fluorescence spectra and lifetimes: long path-lengths are achieved by the efficient guidance of the fundamental mode; sample volumes contained within the micron-scale structure are very small, only a few nanolitres per cm of path; collection and guidance of the emitted fluorescence is efficient and the fluorescence lifetime is unperturbed. Fluorophores in bulk solution can be studied in hollow core PCF, whereas the use of PCF with a suspended, solid core enables selective excitation of molecules in close proximity to the silica surface, through interaction with the evanescent field. We demonstrate the measurement of fluorescence spectra and fluorescence lifetimes in each of these excitation regimes and report the detection of attomole quantities of fluorescein.

Five-ring hollow-core photonic crystal fiber with 1.8 dB/km loss

M. H. Frosz, J. Nold, T. Weiss, A. Stefani, F. Babic, S. Rammler, P. St. J. Russell

A 19-cell hollow-core photonic crystal fiber reaching 1.8 +/- 0.5 dB/km loss at 1530 nm is reported. Despite expanded corner holes in the first ring adjacent to the core, and only five cladding rings, the minimum loss is close to the previously published record of 1.7 dB/km at a comparable wavelength, achieved in a fiber with seven cladding rings. Since each additional cladding ring requires a significant increase in fabrication time and complexity, it is highly desirable to use as few as possible while still achieving low loss. Modeling results confirm that further reducing cladding deformations would yield only a small decrease in loss. This demonstrates that loss comparable to the previously demonstrated lowest-loss bandgap fibers can be achieved with fiber structures that are significantly simpler and faster to fabricate. (C) 2013 Optical Society of America

Chemical and (Photo)-Catalytical Transformations in Photonic Crystal Fibers

Matthias Schmidt, Ana M. Cubillas, Nicola Taccardi, Tijmen G. Euser, Till Cremer, Florian Maier, Hans-Peter Steinrueck, Philip St. J. Russell, Peter Wasserscheid, et al.

The concept of employing photonic crystal fibers for chemical and (photo)-catalytical transformations is presented. These optofluidic microdevices represent a versatile platform where light and fluids can interact for spectroscopic or photoactivation purposes. The use of photonic crystal fibers in chemistry and sensing is reviewed and recent applications as catalytic microreactors are presented. Results on homogeneous catalysis and the immobilization of homogeneous and heterogeneous catalysts in the fiber channels are discussed. The examples demonstrate that combining catalysis and the excellent light guidance of photonic crystal fibers provides unique features for example, for photocatalytic activation and quantitative photospectroscopic reaction analysis.

Combined soliton pulse compression and plasma-related frequency upconversion in gas-filled photonic crystal fiber

W. Chang, P. Hoelzer, J. C. Travers, P. St. J. Russell

We numerically investigate self-frequency blueshifting of a fundamental soliton in a gas-filled hollow-core photonic crystal fiber. Because of the changing underlying soliton parameters, the blueshift gives rise to adiabatic soliton compression. Based on these features, we propose a device that enables frequency shifting over an octave and pulse compression from 30 fs down to 2.3 fs. (C) 2013 Optical Society of America

Low loss hollow optical-waveguide connection from atmospheric pressure to ultra-high vacuum

A. Ermolov, K. F. Mak, F. Tani, P. Hoelzer, J. C. Travers, P. St J. Russell

A technique for optically accessing ultra-high vacuum environments, via a photonic-crystal fiber with a long small hollow core, is described. The small core and the long bore enable a pressure ratio of over 10(8) to be maintained between two environments, while permitting efficient and unimpeded delivery of light, including ultrashort optical pulses. This delivery can be either passive or can encompass nonlinear optical processes such as optical pulse compression, deep UV generation, supercontinuum generation, or other useful phenomena. (C) 2013 AIP Publishing LLC.

A gold-nanotip optical fiber for plasmon-enhanced near-field detection

P. Uebel, S. T. Bauerschmidt, M. A. Schmidt, P. St. J. Russell

A wet-chemical etching and mechanical cleaving technique is used to fabricate gold nanotips attached to tapered optical fibers. Localized surface plasmon resonances (tunable from 500 to 850 nm by varying the tip dimensions) are excited at the tip, and the signal is transmitted via the fiber to an optical analyzer, making the device a plasmon-enhanced near-field probe. A simple cavity model is used to explain the resonances observed in numerical simulations. (C) 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Topological Zeeman effect and circular birefringence in twisted photonic crystal fibers

T. Weiss, G. K. L. Wong, F. Biancalana, S. M. Barnett, X. M. Xi, P. St. J. Russell

The propagation of light guided in optical fibers is affected in different ways by bending or twisting. Here we treat the polarization properties of twisted six-fold symmetric photonic crystal fibers. Using a coordinate frame that follows the twisting structure, we show that the governing equation for the fiber modes resembles the Pauli equation for electrons in weak magnetic fields. This implies index splitting between left and right circularly polarized modes, which are degenerate in the untwisted fiber. We develop a theoretical model, based on perturbation theory and symmetry properties, to predict the observable circular birefringence (i.e., optical activity) associated with this splitting. Our overall conclusion is that optical activity requires the rotational symmetry to be broken so as to allow coupling between different total angular momentum states. (C) 2013 Optical Society of America

Ultrafast nonlinear dynamics of surface plasmon polaritons in gold nanowires due to the intrinsic nonlinearity of metals

A. Marini, M. Conforti, G. Della Valle, H. W. Lee, Tr X. Tran, W. Chang, M. A. Schmidt, S. Longhi, P. St J. Russell, et al.

Starting from first principles, we theoretically model the nonlinear temporal dynamics of gold-based plasmonic devices resulting from the heating of their metallic components. At optical frequencies, the gold susceptibility is determined by the interband transitions around the X, L points in the first Brillouin zone, and thermo-modulational effects ensue from Fermi smearing of the electronic energy distribution in the conduction band. As a consequence of light-induced heating of the conduction electrons, the optical susceptibility becomes nonlinear. In this paper we describe, for the first time to our knowledge, the effects of the thermo-modulational nonlinearity of gold on the propagation of surface plasmon polaritons guided on gold nanowires. We introduce a novel nonlinear Schrodinger-like equation to describe pulse propagation in such nanowires, and we predict the appearance of an intense spectral red-shift caused by the delayed thermal response.

Efficient anti-Stokes generation via intermodal stimulated Raman scattering in gas-filled hollow-core PCF

B. M. Trabold, A. Abdolvand, T. G. Euser, P. St J. Russell

A strong anti-Stokes Raman signal, from the vibrational Q(1) transition of hydrogen, is generated in gas-filled hollow-core photonic crystal fiber. To be efficient, this process requires phase-matching, which is not automatically provided since the group velocity dispersion is typically non-zero and-inside a fiber-cannot be compensated for using a crossedbeam geometry. Phase-matching can however be arranged by exploiting the different dispersion profiles of higher-order modes. We demonstrate the generation of first and second anti-Stokes signals in higher-order modes by pumping with an appropriate mixture of fundamental and a higher-order modes, synthesized using a spatial light modulator. Conversion efficiencies as high as 5.3% are achieved from the pump to the first anti-Stokes band. (C) 2013 Optical Society of America

Nonlinear optics in Xe-filled hollow-core PCF in high pressure and supercritical regimes

M. Azhar, N. Y. Joly, J. C. Travers, P. St J. Russell

Supercritical Xe at 293 K offers a Kerr nonlinearity that can exceed that of fused silica while being free of Raman scattering. It also has a much higher optical damage threshold and a transparency window that extends from the UV to the infrared. We report the observation of nonlinear phenomena, such as self-phase modulation, in hollow-core photonic crystal fiber filled with supercritical Xe. In the subcritical regime, intermodal four-wave mixing resulted in the generation of UV light in the HE12 mode. The normal dispersion of the fiber at high pressures means that spectral broadening can be clearly obtained without influence from soliton effects or material damage.

Mode-based microparticle conveyor belt in air-filled hollow-core photonic crystal fiber

Oliver A. Schmidt, Tijmen G. Euser, Philip St. J. Russell

We show how microparticles can be moved over long distances and precisely positioned in a low-loss air-filled hollow-core photonic crystal fiber using a coherent superposition of two co-propagating spatial modes, balanced by a backward-propagating fundamental mode. This creates a series of trapping positions spaced by half the beat-length between the forward-propagating modes (typically a fraction of a millimeter). The system allows a trapped microparticle to be moved along the fiber by continuously tuning the relative phase between the two forward-propagating modes. This mode-based optical conveyor belt combines long-range transport of microparticles with a positional accuracy of 1 mu m. The technique also has potential uses in waveguide-based optofluidic systems. (C)2013 Optical Society of America

Optical Activity in Twisted Solid-Core Photonic Crystal Fibers

X. M. Xi, T. Weiss, G. K. L. Wong, F. Biancalana, S. M. Barnett, M. J. Padgett, P. St. J. Russell

In this Letter we show that, in spectral regions where there are no orbital cladding resonances to cause transmission loss, the core mode of a continuously twisted photonic crystal fiber (PCF) exhibits optical activity, and that the magnitude of the associated circular birefringence increases linearly with twist rate and is highly reproducible. In contrast to previous work on twist-induced circular birefringence, PCF has zero linear birefringence and an on-axis core, making the appearance of circular birefringence rather unexpected. A theoretical model based on symmetry properties and perturbation theory is developed and used to show that both spin and orbital angular momentum play a role in this effect. It turns out that the degenerate left-and right-circularly polarized modes of the untwisted PCF are not 100% circularly polarized but carry a small amount of orbital angular momentum caused by the interaction between the core mode and the hollow channels. DOI:10.1103/PhysRevLett.110.143903

PHz-wide Supercontinua of Nondispersing Subcycle Pulses Generated by Extreme Modulational Instability

F. Tani, J. C. Travers, P. St. J. Russell

Modulational instability (MI) of 500 fs, 5 mu J pulses, propagating in gas-filled hollow-core kagome photonic crystal fiber, is studied numerically and experimentally. By tuning the pressure and launched energy, we control the duration of the pulses emerging as a consequence of MI and hence are able to study two regimes: the classical MI case leading to few-cycle solitons of the nonlinear Schrodinger equation; and an extreme case leading to the formation of nondispersing subcycle pulses (0.5 to 2 fs) with peak intensities of order 10(14) Wcm(-2). Insight into the two regimes is obtained using a novel statistical analysis of the soliton parameters. Numerical simulations and experimental measurements show that, when a train of these pulses is generated, strong ionization of the gas occurs. This extreme MI is used to experimentally generate a high energy (> 1 mu J) and spectrally broad supercontinuum extending from the deep ultraviolet (320 nm) to the infrared (1300 nm).

Tunable vacuum-UV to visible ultrafast pulse source based on gas-filled Kagome-PCF

Ka Fai Mak, John C. Travers, Philipp Hoelzer, Nicolas Y. Joly, Philip St. J. Russell

An efficient and tunable 176-550 nm source based on the emission of resonant dispersive radiation from ultrafast solitons at 800 nm is demonstrated in a gas-filled hollow-core photonic crystal fiber (PCF). By careful optimization and appropriate choice of gas, informed by detailed numerical simulations, we show that bright, high quality, localized bands of UV light (relative widths of a few percent) can be generated at all wavelengths across this range. Pulse energies of more than 75 nJ in the deep-UV, with relative bandwidths of similar to 3%, are generated from pump pulses of a few mu J. Excellent agreement is obtained between numerical and experimental results. The effects of positive and negative axial pressure gradients are also experimentally studied, and the coherence of the deep-UV dispersive wave radiation numerically investigated. (C) 2013 Optical Society of America

Raman-free nonlinear optical effects in high pressure gas-filled hollow core PCF

M. Azhar, G. K. L. Wong, W. Chang, N. Y. Joly, P. St J. Russell

The effective Kerr nonlinearity of hollow-core kagome-style photonic crystal fiber (PCF) filled with argon gas increases to similar to 15% of that of bulk silica glass when the pressure is increased from 1 to 150 bar, while the zero dispersion wavelength shifts from 300 to 900 nm. The group velocity dispersion of the system is uniquely pressure-tunable over a wide range while avoiding Raman scattering-absent in noble gases-and having an extremely high optical damage threshold. As a result, detailed and well-controlled studies of nonlinear effects can be performed, in both normal and anomalous dispersion regimes, using only a fixed-frequency pump laser. For example, the absence of Raman scattering permits clean observation, at high powers, of the interaction between a modulational instability side-band and a soliton-created dispersive wave. Excellent agreement is obtained between numerical simulations and experimental results. The system has great potential for the realization of reconfigurable supercontinuum sources, wavelength convertors and short-pulse laser systems. (C)2013 Optical Society of America

Effects of squeezed-film damping on the optomechanical nonlinearity in dual-nanoweb fiber

J. R. Koehler, A. Butsch, T. G. Euser, R. E. Noskov, P. St. J. Russell

The freely-suspended glass membranes in a dual-nanoweb fiber, driven at resonance by intensity-modulated light, exhibit a giant optomechanical nonlinearity. We experimentally investigate the effect of squeezed-film damping by exploring the pressure dependence of resonant frequency and mechanical quality factor. As a consequence of the unusually narrow slot between the nanowebs (22 mu m by 550nm), the gas-spring effect causes a pressure-dependent frequency shift that is similar to 15 times greater than typically measured in micro-electro-mechanical devices. When evacuated, the dual-nanoweb fiber yields a quality factor of similar to 3 600 and a resonant optomechanical nonlinear coefficient that is similar to 60 000 times larger than the Kerr effect. (C) 2013 AIP Publishing LLC.

Long-distance laser propulsion and deformation-monitoring of cells in optofluidic photonic crystal fiber

Sarah Unterkofler, Martin K. Garbos, Tijmen G. Euser, Philip St. J. Russell

We introduce a unique method for laser-propelling individual cells over distances of 10s of cm through stationary liquid in a microfluidic channel. This is achieved by using liquid-filled hollow-core photonic crystal fiber (HC-PCF). HC-PCF provides low-loss light guidance in a well-defined single mode, resulting in highly uniform optical trapping and propulsive forces in the core which at the same time acts as a microfluidic channel. Cells are trapped laterally at the center of the core, typically several microns away from the glass interface, which eliminates adherence effects and external perturbations. During propagation, the velocity of the cells is conveniently monitored using a non-imaging Doppler velocimetry technique. Dynamic changes in velocity at constant optical powers up to 350 mW indicate stress-induced changes in the shape of the cells, which is confirmed by bright-field microscopy. Our results suggest that HC-PCF will be useful as a new tool for the study of single-cell biomechanics. ((c) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Amplification of higher-order modes by stimulated Raman scattering in H-2-filled hollow-core photonic crystal fiber

B. M. Trabold, A. Abdolvand, T. G. Euser, A. M. Walser, P. St. J. Russell

We report a method for amplifying higher-order guided modes, synthesized with a spatial light modulator, in a hydrogen-filled hollow-core photonic crystal fiber. The gain mechanism is intermodal stimulated Raman scattering, a pump laser source in the fundamental mode providing amplification for weak higher-order seed modes at the Stokes frequency. The gain for higher-order modes up to LP31 is calculated and verified experimentally. (C) 2013 Optical Society of America

Nonlinear amplification of side-modes in frequency combs

R. A. Probst, T. Steinmetz, T. Wilken, H. Hundertmark, S. P. Stark, G. K. L. Wong, P. St. J. Russell, T. W. Haensch, R. Holzwarth, et al.

We investigate how suppressed modes in frequency combs are modified upon frequency doubling and self-phase modulation. We find, both experimentally and by using a simplified model, that these side-modes are amplified relative to the principal comb modes. Whereas frequency doubling increases their relative strength by 6 dB, the growth due to self-phase modulation can be much stronger and generally increases with nonlinear propagation length. Upper limits for this effect are derived in this work. This behavior has implications for high-precision calibration of spectrographs with frequency combs used for example in astronomy. For this application, Fabry-Perot filter cavities are used to increase the mode spacing to exceed the resolution of the spectrograph. Frequency conversion and/or spectral broadening after non-perfect filtering reamplify the suppressed modes, which can lead to calibration errors. (C) 2013 Optical Society of America

Passive mode-locking of fiber ring laser at the 337th harmonic using gigahertz acoustic core resonances

M. S. Kang, N. Y. Joly, P. St. J. Russell

We report the experimental demonstration of a passively mode-locked Er-doped fiber ring laser operating at the 337th harmonic (1.80 GHz) of the cavity. The laser makes use of highly efficient Raman-like optoacoustic interactions between the guided light and gigahertz acoustic resonances trapped in the micron-sized solid glass core of a photonic crystal fiber. At sufficient pump power levels the laser output locks to a repetition rate corresponding to the acoustic frequency. A stable optical pulse train with a side-mode suppression ratio higher than 45 dB was obtained at low pump powers (similar to 60 mW). (C) 2013 Optical Society of America

Two techniques for temporal pulse compression in gas-filled hollow-core kagome photonic crystal fiber

K. F. Mak, J. C. Travers, N. Y. Joly, A. Abdolvand, P. St. J. Russell

We demonstrate temporal pulse compression in gas-filled kagome hollow-core photonic crystal fiber (PCF) using two different approaches: fiber-mirror compression based on self-phase modulation under normal dispersion, and soliton effect self-compression under anomalous dispersion with a decreasing pressure gradient. In the first, efficient compression to near-transform-limited pulses from 103 to 10.6 fs was achieved at output energies of 10.3 mu J. In the second, compression from 24 to 6.8 fs was achieved at output energies of 6.6 mu J, also with near-transform-limited pulse shapes. The results illustrate the potential of kagome-PCF for postprocessing the output of fiber lasers. We also show that, using a negative pressure gradient, ultrashort pulses can be delivered directly into vacuum. (C) 2013 Optical Society of America

Extreme supercontinuum generation to the deep UV

S. P. Stark, J. C. Travers, P. St. J. Russell

We report the formation of an ultrabroad supercontinuum down to 280 nm in the deep UV by pumping sharply tapered (5-30 mm taper lengths) solid-core photonic crystal fibers with 130 fs, 2 nJ pulses at 800 nm. The taper moves the point of soliton fission to a position where the core is narrower, a process that requires normal dispersion at the input face of the fiber. We find that the generation of deep-UV radiation is limited by strong two-photon absorption in the silica. (C) 2012 Optical Society of America

Excitation of a nanowire "molecule" in gold-filled photonic crystal fiber

H. W. Lee, M. A. Schmidt, P. St. J. Russell

A pair of gold nanowires, incorporated into a photonic crystal fiber, acts as a plasmonic "molecule." Hybridized modes are excited at specific wavelengths by launching light into the glass core. The formation of bonding and antibonding solutions results in a modal splitting of more than 100 nm, even though the spatial separation between the wires is larger than 3 mu m. The study provides insight into multiwire plasmonic devices with applications as polarizers or filters in near-field optics, nonlinear plasmonics, optical sensing, and telecommunications. (C) 2012 Optical Society of America

Ultra-Low Concentration Monitoring of Catalytic Reactions in Photonic Crystal Fiber

Ana M. Cubillas, Matthias Schmidt, Michael Scharrer, Tijmen G. Euser, Bastian J. M. Etzold, Nicola Taccardi, Peter Wasserscheid, Philip St. J. Russell

Excitation of Orbital Angular Momentum Resonances in Helically Twisted Photonic Crystal Fiber

G. K. L. Wong, M. S. Kang, H. W. Lee, F. Biancalana, C. Conti, T. Weiss, P. St J. Russell

Spiral twisting offers additional opportunities for controlling the loss, dispersion, and polarization state of light in optical fibers with noncircular guiding cores. Here, we report an effect that appears in continuously twisted photonic crystal fiber. Guided by the helical lattice of hollow channels, cladding light is forced to follow a spiral path. This diverts a fraction of the axial momentum flow into the azimuthal direction, leading to the formation of discrete orbital angular momentum states at wavelengths that scale linearly with the twist rate. Core-guided light phase-matches topologically to these leaky states, causing a series of dips in the transmitted spectrum. Twisted photonic crystal fiber has potential applications in, for example, band-rejection filters and dispersion control.

Polarisation-resolved near-field mapping of a coupled gold nanowire array

Patrick Uebel, Markus A. Schmidt, Howard W. Lee, Philip St. J. Russell

We report direct observation of the 2D transverse near-field intensity and polarisation distribution of surface plasmon polaritons guided on metal nanowires. Quadrupolar modes are excited on an array of coupled nanowires arranged around the central glass core in a photonic crystal fibre, with lobes whose orientation depends on the polarisation state of the launched core light. The radial electric field is resolved using a polarization sensitive near-field probe in light-collection mode. (C) 2012 Optical Society of America

Influence of timing jitter on nonlinear dynamics of a photonic crystal fiber ring cavity

M. Schmidberger, W. Chang, P. St. J. Russell, N. Y. Joly

We demonstrate that timing jitter has a strong influence on supercontinua generated in a photonic crystal fiber ring cavity synchronously pumped by 140 fs pulses. The global dynamics with respect to cavity detuning is analyzed both numerically and experimentally by tracking the cavity pulse energy. The results show that low-frequency timing jitter, induced by both the pump oscillator and the external cavity, masks the fine underlying bifurcation structure of the system. Numerical simulations in the absence of timing jitter reveal that the system dynamics fall into four qualitatively different regimes. The existence of these regimes is experimentally observed in first-return diagrams. (c) 2012 Optical Society of America

Reconfigurable Optothermal Microparticle Trap in Air-Filled Hollow-Core Photonic Crystal Fiber

O. A. Schmidt, M. K. Garbos, T. G. Euser, P. St. J. Russell

We report a novel optothermal trapping mechanism that occurs in air-filled hollow-core photonic crystal fiber. In the confined environment of the core, the motion of a laser-guided particle is strongly influenced by the thermal-gradient- driven flow of air along the core surface. Known as "thermal creep flow,'' this can be induced either statically by local heating, or dynamically by the absorption (at a black mark placed on the fiber surface) of light scattered by the moving particle. The optothermal force on the particle, which can be accurately measured in hollow-core fiber by balancing it against the radiation forces, turns out to exceed the conventional thermophoretic force by 2 orders of magnitude. The system makes it possible to measure pN-scale forces accurately and to explore thermally driven flow in micron-scale structures.

Intermodal stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fiber

M. Ziemienczuk, A. M. Walser, A. Abdolvand, P. St. J. Russell

Stimulated Raman scattering is investigated in a slightly multimode gas-filled hollow-core photonic crystal fiber. Although, second-order Stokes light appears in the fundamental mode below a certain threshold energy, it is observed to switch to a two-lobed higher order mode above this threshold. Conversion to the higher order mode is made possible by the creation of a two-lobed moving coherence wave in the gas that provides both phase-matching and a strong intermodal pump-Stokes overlap. A theoretical model is developed, based on this physical interpretation that agrees quantitatively with the experimental results. The results suggest new opportunities for all-fiber gas-based nonlinear processes requiring phase-matching, such as coherent anti-Stokes Raman scattering, as well as providing a means (for example) of efficiently converting light from a higher order pump mode to a fundamental Stokes mode. (C) 2012 Optical Society of America

Optomechanical Nonlinearity in Dual-Nanoweb Structure Suspended Inside Capillary Fiber

A. Butsch, M. S. Kang, T. G. Euser, J. R. Koehler, S. Rammler, R. Keding, P. St J. Russell

A novel kind of nanostructured optical fiber, displaying an extremely high and optically broadband optomechanical nonlinearity, is presented. It comprises two closely spaced ultrathin glass membranes (webs) suspended in air and attached to the inner walls of a glass fiber capillary. Light guided in this dual-web structure can exert attractive or repulsive pressure on the webs, causing them to be pushed together or pulled apart. The elastic deflection of the webs is, in turn, coupled to the electromagnetic field distribution and results in a change in the effective refractive index within the fiber. Employing a pump-probe technique in an interferometric setup, optomechanically induced refractive index changes more than 10(4) times larger than the Kerr effect are detected. Theoretical estimates of the optomechanical nonlinearity agree well with the experimental results. The dual-web fiber combines the sensitivity of a microoptomechanical device with the versatility of an optical fiber and could trigger new developments in the fields of nonlinear optics, optical metrology, and sensing.

Optomechanical Self-Channeling of Light in a Suspended Planar Dual-Nanoweb Waveguide

A. Butsch, C. Conti, F. Biancalana, P. St J. Russell

It is shown that optomechanical forces can cause nonlinear self-channeling of light in a planar dual-slab waveguide. A system of two parallel silica nanowebs, spaced similar to 100 nm and supported inside a fiber capillary, is studied theoretically and an iterative scheme developed to analyze its nonlinear optomechanical properties. Steady-state field distributions and mechanical deformation profiles are obtained, demonstrating that self-channeling is possible in realistic structures at launched powers as low as a few mW. The differential optical nonlinearity of the self-channeled mode can be as much as 10 x 10(6) times higher than the corresponding electronic Kerr nonlinearity. It is also intrinsically broadband, does not utilize resonant effects, can be viewed as a consequence of the extreme nonlocality of the mechanical response, and in fact is a notable example of a so-called accessible soliton.

Plasma-Induced Asymmetric Self-Phase Modulation and Modulational Instability in Gas-Filled Hollow-Core Photonic Crystal Fibers

Mohammed F. Saleh, Wonkeun Chang, John C. Travers, Philip St J. Russell, Fabio Biancalana

We study theoretically the propagation of relatively long pulses with ionizing intensities in a hollow-core photonic crystal fiber filled with a Raman-inactive noble gas. Because of photoionization, an extremely asymmetric self-phase modulation and a new kind of "universal" plasma-induced modulational instability appear in both normal and anomalous dispersion regions. We also show that it is possible to spontaneously generate a plasma-induced continuum of blueshifting solitons, opening up new possibilities for pushing supercontinuum generation towards shorter and shorter wavelengths.

Stabilised Biosensing Using Needle-Based Recess Electrodes

Salzitsa Anastasova, Anna-Maria Spehar-Deleze, Dale Bickham, Patrick Uebel, Markus Schmidt, Philip Russell, Pankaj Vadgama

A recess disk electrode for amperometric monitoring of oxygen and glucose is reported. The basic design of sensor is a needle structure for easy implantation, embodying a gold filled silica capillary with a 80 mu m inner diameter working electrode. Recess and inlaid disc electrodes, placed inside stainless steel tubes which served as a counter/reference electrode, were compared. The working electrode surface was modified with different treatments and barrier membranes to achieve selectivity for the analytes of interest. The basic needle format is ideal for in vivo use, and the format is easily extendable to other analyte targets. The sensors show linear working range for oxygen up to 160 mmHg partial pressure of oxygen, and for glucose from 1 to 10 mM. Bio-fouling, as assessed by exposure to bovine serum albumin, was significantly reduced. Response times for the recess construct was increased but remained within the acceptable range for physiological monitoring. The operational stability of the sensors is demonstrated as well as the interference-free detection of peroxide in the presence of physiologically relevant levels of ascorbic acid, uric acid, acetaminophen, and catechol. Preliminary in vivo tests showed excellent response towards glucose.

Dynamics of optomechanical spatial solitons in dual-nanoweb structures

C. Conti, A. Butsch, F. Biancalana, P. St J. Russell

We theoretically investigate the stability and dynamics of self-channeled beams that form via nonlocal optomechanical interactions in dual-nanoweb microstructured fibers. These beams represent a class of spatial soliton.

Kagome hollow-core photonic crystal fiber probe for Raman spectroscopy

Petru Ghenuche, Silke Rammler, Nicolas Y. Joly, Michael Scharrer, Michael Frosz, Jerome Wenger, Philip St J. Russell, Herve Rigneault

We demonstrate the use of a large-pitch Kagome-lattice hollow-core photonic crystal fiber probe for Raman spectroscopy. The large transmission bandwidth of the fiber enables both the excitation and Raman beams to be transmitted through the same fiber. As the excitation beam is mainly transmitted through air inside the hollow core, the silica luminescence background is reduced by over 2 orders of magnitude as compared to standard silica fiber probes, removing the need for fiber background subtraction. (C) 2012 Optical Society of America

Generation of a phase-locked Raman frequency comb in gas-filled hollow-core photonic crystal fiber

A. Abdolvand, A. M. Walser, M. Ziemienczuk, T. Nguyen, P. St J. Russell

In a relatively simple setup consisting of a microchip laser as pump source and two hydrogen-filled hollow-core photonic crystal fibers, a broad, phase-locked, purely rotational frequency comb is generated. This is achieved by producing a clean first Stokes seed pulse in a narrowband guiding photonic bandgap fiber via stimulated Raman scattering and then driving the same Raman transition resonantly with a pump and Stokes fields in a second broad-band guiding kagome-style fiber. Using a spectral interferometric technique based on sum frequency generation, we show that the comb components are phase locked. (C) 2012 Optical Society of America

Metrology of laser-guided particles in air-filled hollow-core photonic crystal fiber

O. A. Schmidt, M. K. Garbos, T. G. Euser, P. St. J. Russell

Micrometer-sized particles are trapped in front of an air-filled hollow-core photonic crystal fiber using a novel dual-beam trap. A backward guided mode produces a divergent beam that diffracts out of the core, and simultaneously a focused laser beam launches a forward-propagating mode into the core. By changing the backward/forward power balance, a trapped particle can be selectively launched into the hollow core. Once inside, particles can be optically propelled along several meters of fiber with mobilities as high as 19 cm. s(-1) W-1 (precisely measured using in-fiber Doppler velocimetry). The results are in excellent agreement with theory. The system allows determination of fiber loss as well as the mass density and refractive index of single particles. c 2011 Optical Society of America

Microfluidic integration of photonic crystal fibers for online photochemical reaction analysis

S. Unterkofler, R. J. McQuitty, T. G. Euser, N. J. Farrer, P. J. Sadler, P. St. J. Russell

Liquid-filled hollow-core photonic crystal fibers (HC-PCFs) are perfect optofluidic channels, uniquely providing low-loss optical guidance in a liquid medium. As a result, the overlap of the dissolved specimen and the intense light field in the micronsized core is increased manyfold compared to conventional bioanalytical techniques, facilitating highly-efficient photoactivation processes. Here we introduce a novel integrated analytical technology for photochemistry by microfluidic coupling of a HC-PCF nanoflow reactor to supplementary detection devices. Applying a continuous flow through the fiber, we deliver photochemical reaction products to a mass spectrometer in an online and hence rapid fashion, which is highly advantageous over conventional cuvette-based approaches. (C) 2012 Optical Society of America

Photonic crystal fibre as an optofluidic reactor for the measurement of photochemical kinetics with sub-picomole sensitivity

Gareth O. S. Williams, Jocelyn S. Y. Chen, Tijmen G. Euser, Philip St J. Russell, Anita C. Jones

Photonic crystal fibre constitutes an optofluidic system in which light can be efficiently coupled into a solution-phase sample, contained within the hollow core of the fibre, over long path-lengths. This provides an ideal arrangement for the highly sensitive monitoring of photochemical reactions by absorption spectroscopy. We report here the use of UV/vis spectroscopy to measure the kinetics of the photochemical and thermal cis-trans isomerisation of sub-picomole samples of two azo dyes within the 19-mu m diameter core of a photonic crystal fibre, over a path length of 30 cm. Photoisomerisation quantum yields are the first reported for "push-pull'' azobenzenes in solution at room temperature; such measurements are challenging because of the fast thermal isomerisation process. Rate constants obtained for thermal isomerisation are in excellent agreement with those established previously in conventional cuvette-based measurements. The high sensitivity afforded by this intra-fibre method enables measurements in solvents in which the dyes are too insoluble to permit conventional cuvette-based measurements. The results presented demonstrate the potential of photonic crystal fibres as optofluidic elements in lab-on-a-chip devices for photochemical applications.

Optofluidic refractive-index sensor in step-index fiber with parallel hollow micro-channel

H. W. Lee, M. A. Schmidt, P. Uebel, H. Tyagi, N. Y. Joly, M. Scharrer, P. St. J. Russell

We present a simple refractive index sensor based on a step-index fiber with a hollow micro-channel running parallel to its core. This channel becomes waveguiding when filled with a liquid of index greater than silica, causing sharp dips to appear in the transmission spectrum at wavelengths where the glass-core mode phase-matches to a mode of the liquid-core. The sensitivity of the dip-wavelengths to changes in liquid refractive index is quantified and the results used to study the dynamic flow characteristics of fluids in narrow channels. Potential applications of this fiber microstructure include measuring the optical properties of liquids, refractive index sensing, biophotonics and studies of fluid dynamics on the nanoscale. (C) 2011 Optical Society of America

An azimuthally polarizing photonic crystal fibre with a central gold nanowire

Patrick Uebel, Markus A. Schmidt, Michael Scharrer, Philip St J. Russell

An air-silica photonic crystal fibre with a gold nanowire at core centre is shown to support a low-loss azimuthally polarized mode. Since all the other modes have very high attenuation, the fibre effectively supports only this mode, acting as a single-polarization fibre with an extinction ratio >20 dB cm(-1) over a broad range of wavelengths (550-1650 nm in the device reported). It can be used as an effective azimuthal mode filter.

Bandgap guidance in hybrid chalcogenide-silica photonic crystal fibers

Nicolai Granzow, Patrick Uebel, Markus A. Schmidt, Andrey S. Tverjanovich, Lothar Wondraczek, Philip St J. Russell

We report a hybrid chalcogenide-silica photonic crystal fiber made by pressure-assisted melt-filling of molten glass. Photonic bandgap guidance is obtained at a silica core placed centrally in a hexagonal array of continuous centimeters-long chalcogenide strands with diameters of 1.45 mu m. In the passbands of the cladding, when the transmission through the silica core is very weak, the chalcogenide strands light up with distinct modal patterns corresponding to Mie resonances. In the spectral regions between these passbands, strong bandgap guidance is observed, where the silica core transmission loss is 60 dB/cm lower. The pressure-assisted fabrication approach opens up new ways of integrating sophisticated glass-based devices into optical fiber circuitry with potential applications in supercontinuum generation, magneto-optics, wavelength selective devices, and rare-earth-doped amplifiers with high gain per unit length. (C) 2011 Optical Society of America

Bright Spatially Coherent Wavelength-Tunable Deep-UV Laser Source Using an Ar-Filled Photonic Crystal Fiber

N.Y. Joly, J. Nold, W. Chang, P. Hoelzer, A. Nazarkin, G. K. L. Wong, F. Biancalana, P. St. J. Russell

We report on the spectral broadening of similar to 1 mu J 30 fs pulses propagating in an Ar-filled hollow-core photonic crystal fiber. In contrast with supercontinuum generation in a solid-core photonic crystal fiber, the absence of Raman and unique pressure-controlled dispersion results in efficient emission of dispersive waves in the deep-UV region. The UV light emerges in the single-lobed fundamental mode and is tunable from 200 to 320 nm by varying the pulse energy and gas pressure. The setup is extremely simple, involving <1 m of a gas-filled photonic crystal fiber, and the UV signal is stable and bright, with experimental IR to deep-UV conversion efficiencies as high as 8 %. The source is of immediate interest in applications demanding high spatial coherence, such as laser lithography or confocal microscopy.

Influence of ionization on ultrafast gas-based nonlinear fiber optics

W. Chang, A. Nazarkin, J. C. Travers, J. Nold, P. Hoelzer, N. Y. Joly, P. St. J. Russell

We numerically investigate the effect of ionization on ultrashort high-energy pulses propagating in gas-filled kagome-lattice hollow-core photonic crystal fibers by solving an established uni-directional field equation. We consider the dynamics of two distinct regimes: ionization induced blue-shift and resonant dispersive wave emission in the deep-UV. We illustrate how the system evolves between these regimes and the changing influence of ionization. Finally, we consider the effect of higher ionization stages. (C) 2011 Optical Society of America

Structural analysis of photonic crystal fibers by side scattering of laser light

L. Y. Zang, T. G. Euser, M. S. Kang, M. Scharrer, P. St. J. Russell

A side-scattering technique for investigating the inner microstructure of photonic crystal fibers (PCFs) is reported. Multiple scattering is reduced by filling the hollow PCF channels with index-matching fluid. The scattered signal is measured for fixed angles of incidence and detection while the fiber is rotated. A pattern of peaks, unique to each PCF, whether solid or hollow core, correlates closely with the symmetry planes of the PCF structure. As an example of the technique, the twist profile of a structural rocking filter is directly measured. (C) 2011 Optical Society of America

Entangling Different Degrees of Freedom by Quadrature Squeezing Cylindrically Polarized Modes

C. Gabriel, A. Aiello, W. Zhong, T. G. Euser, N. Y. Joly, P. Banzer, M. Foertsch, D. Elser, U. L. Andersen, et al.

Quantum systems such as, for example, photons, atoms, or Bose-Einstein condensates, prepared in complex states where entanglement between distinct degrees of freedom is present, may display several intriguing features. In this Letter we introduce the concept of such complex quantum states for intense beams of light by exploiting the properties of cylindrically polarized modes. We show that already in a classical picture the spatial and polarization field variables of these modes cannot be factorized. Theoretically it is proven that by quadrature squeezing cylindrically polarized modes one generates entanglement between these two different degrees of freedom. Experimentally we demonstrate amplitude squeezing of an azimuthally polarized mode by exploiting the nonlinear Kerr effect in a specially tailored photonic crystal fiber. These results display that such novel continuous-variable entangled systems can, in principle, be realized.

Birefringence and dispersion of cylindrically polarized modes in nanobore photonic crystal fiber

T. G. Euser, M. A. Schmidt, N. Y. Joly, C. Gabriel, C. Marquardt, L. Y. Zang, M. Foertsch, P. Banzer, A. Brenn, et al.

We demonstrate experimentally and theoretically that a nanoscale hollow channel placed centrally in the solid-glass core of a photonic crystal fiber strongly enhances the cylindrical birefringence (the modal index difference between radially and azimuthally polarized modes). Furthermore, it causes a large split in group velocity and group velocity dispersion. We show analytically that all three parameters can be varied over a wide range by tuning the diameters of the nanobore and the core. (C) 2010 Optical Society of America

Ultrafast nonlinear optics in gas-filled hollow-core photonic crystal fibers [Invited]

John C. Travers, Wonkeun Chang, Johannes Nold, Nicolas Y. Joly, Philip St. J. Russell

We review the use of hollow-core photonic crystal fibers (PCFs) in the field of ultrafast gas-based nonlinear optics, including recent experiments, numerical modeling, and a discussion of future prospects. Concentrating on broadband guiding kagome-style hollow-core PCF, we describe its potential for moving conventional nonlinear fiber optics both into extreme regimes-such as few-cycle pulse compression and efficient deep ultraviolet wavelength generation-and into regimes hitherto inaccessible, such as single-mode guidance in a photoionized plasma and high-harmonic generation in fiber. (C) 2011 Optical Society of America

Optofluidic immobility of particles trapped in liquid-filled hollow-core photonic crystal fiber

M. K. Garbos, T. G. Euser, P. St. J. Russell

We study the conditions under which a particle, laser-guided in a vertically-oriented hollow-core photonic crystal fiber filled with liquid, can be kept stationary against a microfluidic counter-flow. An immobility parameter-the fluid flow rate required to immobilize a particle against the radiation force produced by unit guided optical power-is introduced to quantify the conditions under which this occurs, including radiation, viscous and gravity forces. Measurements show that this parameter depends strongly on the ratio of particle radius a to core radius R, peaking at an intermediate value of a/R. The results follow fairly well the theoretical estimates of the optical (calculated approximately using a ray optics approach) and numerically simulated drag forces. We suggest that the system has potential applications in, e.g., measurement of the diameter, refractive index and density of particles, synthesis and biomedical research. (C) 2011 Optical Society of America

Multi-mJ carrier envelope phase stabilized few-cycle pulses generated by a tabletop laser system

A. Anderson, F. Luecking, T. Prikoszovits, M. Hofer, Z. Cheng, C. C. Neacsu, M. Scharrer, S. Rammler, P. St J. Russell, et al.

A compact system for the generation of few-cycle multi-mJ Carrier Envelope Phase (CEP) stabilized pulses is presented. At the output 1.9 mJ, 5.7 fs pulses were achieved after hollow fiber compression (HFC) of 5 mJ, 25 fs circularly-polarized pulses from a Ti:sapphire multipass chirped pulse amplifier (CPA). Polarization control of the generated pulses was done using all reflective phase retarders which can be nearly arbitrarily scaled for increasing energies. The CEP noise from the amplifier system is shown to be 190 mrad rms over a period of more than 7 hours. The full system, i.e., oscillator, amplifier, CEP stabilization, and HFC is compact enough to fit on a standard optical table.

Interfacial reactions between tellurite melts and silica during the production of microstructured optical devices

N. Da, A. A. Enany, N. Granzow, M. A. Schmidt, P. St. J. Russell, L. Wondraczek

Interfacial reactions between silica glass and tellurite melts were studied under confined conditions in the temperature regime of 400-700 degrees C, applying two different sampling techniques: isothermal heat-treatment of a several micrometer thick tellurite film, confined in a silica/tellurite/silica sandwich, and capillary filling of tellurite melts into silica microcapillaries. The sandwich technique provides detailed ex situ insights on the interface chemistry, microstructure and diffusion after given treatment times and temperatures. Data on dynamic viscosity, surface tension, wetting behaviour and eventual scaling effects was obtained from the capillary filling technique. For temperatures > 500 degrees C, silica is completely wet by the considered tellurite melts. At T > 600 degrees C and for a treatment time of 20 min or longer, cationic diffusion of Na(+) and Te(4+) into the silica substrate occurs to a depth of several micrometers. At the same time, the tellurite melt attacks the silica surface, leading to the formation of a stationary silica-tellurite reaction layer and silica dissolution. Dissolved silica was observed to re-precipitate from the tellurite melt by liquid-liquid phase separation. In the early reaction stages, as a result of alkali diffusion into the silica substrate, beta-quartz crystallizes at the interface (what can be avoided by using alkali-free filling glasses). Obtained data set the boundary conditions for the generation of tellurite-silica all-solid fiber waveguides by melt infiltration of silica photonic crystal fibers or microcapillaries. (C) 2011 Elsevier B.V. All rights reserved.

Theory of Photoionization-Induced Blueshift of Ultrashort Solitons in Gas-Filled Hollow-Core Photonic Crystal Fibers

Mohammed F. Saleh, Wonkeun Chang, Philipp Hoelzer, Alexander Nazarkin, John C. Travers, Nicolas Y. Joly, Philip St. J. Russell, Fabio Biancalana

We show theoretically that the photoionization process in a hollow-core photonic crystal fiber filled with a Raman-inactive noble gas leads to a constant acceleration of solitons in the time domain with a continuous shift to higher frequencies, limited only by ionization loss. This phenomenon is opposite to the well-known Raman self-frequency redshift of solitons in solid-core glass fibers. We also predict the existence of unconventional long-range nonlocal soliton interactions leading to spectral and temporal soliton clustering. Furthermore, if the core is filled with a Raman-active molecular gas, spectral transformations between redshifted, blueshifted, and stabilized solitons can take place in the same fiber.

Soliton Blueshift in Tapered Photonic Crystal Fibers

S. P. Stark, A. Podlipensky, P. St. J. Russell

We show that solitons undergo a strong blueshift in fibers with a dispersion landscape that varies along the direction of propagation. The experiments are based on a small-core photonic crystal fiber, tapered to have a core diameter that varies continuously along its length, resulting in a zero-dispersion wavelength that moves from 731 nm to 640 nm over the transition. The central wavelength of a soliton translates over 400 nm towards a shorter wavelength. This is accompanied by strong emission of radiation into the UV and IR spectral regions. The experimental results are confirmed by numerical simulation.

Pressure-assisted melt-filling and optical characterization of Au nano-wires in microstructured fibers

H. W. Lee, M. A. Schmidt, R. F. Russell, N. Y. Joly, H. K. Tyagi, P. Uebel, P. St. J. Russell

We report a novel splicing-based pressure-assisted melt-filling technique for creating metallic nanowires in hollow channels in microstructured silica fibers. Wires with diameters as small as 120 nm (typical aspect ration 50:1) could be realized at a filling pressure of 300 bar. As an example we investigate a conventional single-mode step-index fiber with a parallel gold nanowire (wire diameter 510 nm) running next to the core. Optical transmission spectra show dips at wavelengths where guided surface plasmon modes on the nanowire phase match to the glass core mode. By monitoring the side-scattered light at narrow breaks in the nanowire, the loss could be estimated. Values as low as 0.7 dB/mm were measured at resonance, corresponding to those of an ultra-long-range eigenmode of the glass-core/nanowire system. By thermal treatment the hollow channel could be collapsed controllably, permitting creation of a conical gold nanowire, the optical properties of which could be monitored by side-scattering. The reproducibility of the technique and the high optical quality of the wires suggest applications in fields such as nonlinear plasmonics, near-field scanning optical microscope tips, cylindrical polarizers, optical sensing and telecommunications. (C) 2011 Optical Society of America

Supercontinuum generation in chalcogenide-silica step-index fibers

N. Granzow, S. P. Stark, M. A. Schmidt, A. S. Tverjanovich, L. Wondraczek, P. St. J. Russell

We explore the use of a highly nonlinear chalcogenide-silica waveguide for supercontinuum generation in the near infrared. The structure was fabricated by a pressure-assisted melt-filling of a silica capillary fiber (1.6 mu m bore diameter) with Ga4Ge21Sb10S65 glass. It was designed to have zero group velocity dispersion (for HE11 core mode) at 1550 nm. Pumping a 1 cm length with 60 fs pulses from an erbium-doped fiber laser results in the generation of octave-spanning supercontinuum light for pulse energies of only 60 pJ. Good agreement is obtained between the experimental results and theoretical predictions based on numerical solutions of the generalized nonlinear Schrodinger equation. The pressure-assisted melt-filling approach makes it possible to realize highly nonlinear devices with unusual combinations of materials. For example, we show numerically that a 1 cm long As2S3:silica step-index fiber with a core diameter of 1 mu m, pumped by 60 fs pulses at 1550 nm, would generate a broadband supercontinuum out to 4 mu m. (C) 2011 Optical Society of America

Femtosecond Nonlinear Fiber Optics in the Ionization Regime

P. Hoelzer, W. Chang, J. C. Travers, A. Nazarkin, J. Nold, N. Y. Joly, M. F. Saleh, F. Biancalana, P. St. J. Russell

By using a gas-filled kagome-style photonic crystal fiber, nonlinear fiber optics is studied in the regime of optically induced ionization. The fiber offers low anomalous dispersion over a broad bandwidth and low loss. Sequences of blueshifted pulses are emitted when 65 fs, few-microjoule pulses, corresponding to high-order solitons, are launched into the fiber and undergo self-compression. The experimental results are confirmed by numerical simulations which suggest that free-electron densities of similar to 10(17) cm(-3) are achieved at peak intensities of 10(14) W/cm(2) over length scales of several centimeters.

Complex Faraday Rotation in Microstructured Magneto-optical Fiber Waveguides

Markus A. Schmidt, Lothar Wondraczek, Ho W. Lee, Nicolai Granzow, Ning Da, Philip St. J. Russell

Magneto-optical glasses are of considerable current interest, primarily for applications in fiber circuitry, optical isolation, all-optical diodes, optical switching and modulation. While the benchmark materials are still crystalline, glasses offer a variety of unique advantages, such as very high rare-earth and heavy-metal solubility and, in principle, the possibility of being produced in fiber form. In comparison to conventional fiber-drawing processes, pressure-assisted melt-filling of microcapillaries or photonic crystal fibers with magneto-optical glasses offers an alternative route to creating complex waveguide architectures from unusual combinations of glasses. For instance, strongly diamagnetic tellurite or chalcogenide glasses with high refractive index can be combined with silica in an all-solid, microstructured waveguide. This promises the implementation of as-yet-unsuitable but strongly active glass candidates as fiber waveguides, for example in photonic crystal fibers.

Nonlinear wavelength conversion in photonic crystal fibers with three zero-dispersion points

S. P. Stark, F. Biancalana, A. Podlipensky, P. St. J. Russell

In this theoretical study, we show that a simple endlessly single-mode photonic crystal fiber can be designed to yield, not just two, but three zero-dispersion wavelengths. The presence of a third dispersion zero creates a rich phase-matching topology, enabling enhanced control over the spectral locations of the four-wave-mixing and resonant-radiation bands emitted by solitons and short pulses. The greatly enhanced flexibility in the positioning of these bands has applications in wavelength conversion, supercontinuum generation, and pair-photon sources for quantum optics.

14 GHz visible supercontinuum generation: calibration sources for astronomical spectrographs

S. P. Stark, T. Steinmetz, R. A. Probst, H. Hundertmark, T. Wilken, T. W. Haensch, Th. Udem, P. St. J. Russell, R. Holzwarth

We report the use of a specially designed tapered photonic crystal fiber to produce a broadband optical spectrum covering the visible spectral range. The pump source is a frequency doubled Yb fiber laser operating at a repetition rate of 14 GHz and emitting sub-5 pJ pulses. We experimentally determine the optimum core diameter and achieve a 235 nm broad spectrum. Numerical simulations are used to identify the underlying mechanisms and explain spectral features. The high repetition rate makes this system a promising candidate for precision calibration of astronomical spectrographs. (C) 2011 Optical Society of America

Reconfigurable light-driven opto-acoustic isolators in photonic crystal fibre

M. S. Kang, A. Butsch, P. St. J. Russell

Dynamic optical isolation with all-optical switching capability is in great demand in advanced optical communications and all-optical signal processing systems. Most conventional optical isolators rely on Faraday rotation and are realized using micro/nanofabrication techniques, but it is not always straightforward to incorporate magneto-optical crystals into these compact systems. Here, we report the experimental demonstration of a reconfigurable all-optical isolator based on optical excitation of a gigahertz guided acoustic mode in a micrometre-sized photonic crystal fibre core. This device has remarkable advantages over its passive counterparts, including a large dynamic range of isolation, fast switching capability and reversibility, which provide new functionality that is useful in various types of all-optical systems. Devices based on similar physical principles could also be realized in CMOS-compatible silicon on-chip platforms.

Fiber Transport of Spatially Entangled Photons

W. Loeffler, T. G. Euser, E. R. Eliel, M. Scharrer, P. St J. Russell, J. P. Woerdman

Entanglement in the spatial degrees of freedom of photons is an interesting resource for quantum information. For practical distribution of such entangled photons, it is desirable to use an optical fiber, which in this case has to support multiple transverse modes. Here we report the use of a hollow-core photonic crystal fiber to transport spatially entangled qubits.

Doppler velocimetry on microparticles trapped and propelled by laser light in liquid-filled photonic crystal fiber

M. K. Garbos, T. G. Euser, O. A. Schmidt, S. Unterkofler, P. St. J. Russell

Laser Doppler velocimetry is used to measure very accurately the velocity and position of a microparticle propelled and guided by laser light in liquid-filled photonic crystal fiber. Periodic variations in particle velocity are observed that correlate closely with modal beating between the two lowest order guided fiber modes. (C) 2011 Optical Society of America

Single-mode hollow-core photonic crystal fiber made from soft glass

X. Jiang, T. G. Euser, A. Abdolvand, F. Babic, F. Tani, N. Y. Joly, J. C. Travers, P. St J. Russell

We demonstrate the first soft-glass hollow core photonic crystal fiber. The fiber is made from a high-index lead-silicate glass (Schott SF6, refractive index 1.82 at 500 nm). Fabricated by the stack-and-draw technique, the fiber incorporates a 7-cell hollow core embedded in a highly uniform 6-layer cladding structure that resembles a kagome-like lattice. Effective single mode guidance of light is observed from 750 to 1050 nm in a large mode area (core diameter similar to 30 mu m) with a low loss of 0.74 dB/m. The underlying guidance mechanism of the fiber is investigated using finite element modeling. The fiber is promising for applications requiring single mode guidance in a large mode area, such as particle guidance, fluid and gas filled devices. (C) 2011 Optical Society of America

High index-contrast all-solid photonic crystal fibers by pressure-assisted melt infiltration of silica matrices

Ning Da, Lothar Wondraczek, Markus A. Schmidt, Nicolai Granzow, Philip St. J. Russell

All-solid photonic crystal fibers (PCFs) are created by pressure-assisted filling of low-melting-point chalcogenide and tellurite glasses into silica matrix fibers with channel diameters as small as 200 nm. Overcoming to a large extent the problem of viscosity and, thus, process incompatibility of silica and non-silicate optical glasses, the technique provides a unique way of producing waveguiding devices with high core-cladding index-contrast, high optical non-linearity and a transmission range that extends into the mid infrared. In this paper, as a prerequisite for waveguide production, the rheologic properties and controlled flow of highly-viscous liquids under geometrically confined conditions are considered, and deviations from Newtonian behavior are discussed. Because the filling process requires only very small quantities of filling material that do not come into contact with the environment, and because ultra-high cooling rates can be achieved, the technique enables the use of difficult-to-handle or reactive optical glasses. (C) 2010 Elsevier B.V. All rights reserved.

Understanding Raman-shifting multipeak states in photonic crystal fibers: two convergent approaches

Alexander Hause, Truong X. Tran, Fabio Biancalana, Alexander Podlipensky, Philip St J. Russell, Fedor Mitschke

In this Letter we give theoretical explanations for the recent observations of the excitation of Raman-shifting pulse pairs in solid-core photonic crystal fibers. The formation of these pairs is surprisingly common in the deep anomalous dispersion regime of a large variety of highly nonlinear optical fibers, away from zero group-velocity dispersion points. We have developed two different theoretical models, which agree very well in their conclusions. A qualitative and a quantitative explanation of pair formation is provided, and the existence of multipeak states is predicted. (C) 2010 Optical Society of America

Bio-sensing using recessed gold-filled capillary amperometric electrodes

A. Kacanovska, Z. Rong, M. Schmidt, P. St. J. Russell, P. Vadgama

A novel recessed electrode is reported for amperometric detection of hydrogen peroxide and via glucose oxidase for the detection of glucose. The electrode utilised electrodeposited platinum over a gold wire surface, which proved to be an effective peroxide-detecting surface. Compared with a traditional exposed electrode surface, the recessed tip facilitated an extended linear range for glucose from 4 to over 14 mM. Bio-fouling, as assessed by exposure to bovine serum albumin, was also significantly reduced. Though response time at the recess was increased, it was within an acceptable range for physiological monitoring. Moreover, the recess enabled precise measurement of the hydrogen peroxide diffusion coefficient; this was based on a bipartite expression for the transient amperometric current at the recessed structure following a step change in ambient hydrogen peroxide concentration. An important aspect of the diffusion measurement was the curve fitting routine used to map on to the theoretical response curve.

Emergence of Geometrical Optical Nonlinearities in Photonic Crystal Fiber Nanowires

Fabio Biancalana, Truong X. Tran, Sebastian Stark, Markus A. Schmidt, Philip St J. Russell

We demonstrate analytically and numerically that a subwavelength-core dielectric photonic nanowire embedded in a properly designed photonic crystal fiber cladding shows evidence of a previously unknown kind of nonlinearity (the magnitude of which is strongly dependent on the waveguide parameters) which acts on solitons so as to considerably reduce their Raman self-frequency shift. An explanation of the phenomenon in terms of indirect pulse negative chirping and broadening is given by using the moment method. Our conclusions are supported by detailed numerical simulations.

Multiple hydrodynamical shocks induced by the Raman effect in photonic crystal fibers

C. Conti, S. Stark, P. St. J. Russell, F. Biancalana

We theoretically predict the occurrence of multiple hydrodynamical-like shock phenomena in the propagation of ultrashort intense pulses in a suitably engineered photonic crystal fiber. The shocks are due to the Raman effect, which acts as a nonlocal term favoring their generation in the focusing regime. It is shown that the problem is mapped to shock formation in the presence of a slope and a gravity-like potential. The signature of multiple shocks in cross-correlation frequency-resolved optical gating (XFROG) signals is unveiled.

Measurement of group-velocity dispersion of Bloch modes in photonic-crystal-fiber rocking filters

G. K. L. Wong, L. Zang, M. S. Kang, P. St. J. Russell

We use low-coherence interferometry to measure the group-velocity dispersion (GVD) of the fast and slow Bloch modes of structural rocking filters, produced by twisting a highly birefringent photonic crystal fiber to and fro while scanning a focused CO(2) laser beam along it. The GVD curves in the vicinity of the resonant wavelength differ dramatically from those of the unperturbed fiber, suggesting that rocking filters could be used in the optimization of, e.g., four-wave mixing and supercontinuum generation. Excellent agreement is obtained between theory and experiment. (C) 2010 Optical Society of America

Direct Observation of Self-Similarity in Evolution of Transient Stimulated Raman Scattering in Gas-Filled Photonic Crystal Fibers

A. Nazarkin, A. Abdolvand, A. V. Chugreev, P. St. J. Russell

A unique characteristic of transient stimulated Raman scattering, in which the spatiotemporal evolution of the fields and the molecular excitation follow a universal self-similarity law, is observed in gas-filled photonic crystal fibers. As the input laser power is increased, the coupled system "optical fields + molecular excitation" goes through the same phases of time evolution but at a higher rate. Using the self-similarity law we are able to completely reconstruct the evolution of the pump and Stokes fields from one measurement.

Approaching the full octave: Noncollinear optical parametric chirped pulse amplification with two-color pumping

D. Herrmann, C. Homann, R. Tautz, M. Scharrer, P. St J. Russell, F. Krausz, L. Veisz, E. Riedle

We present a new method to broaden the amplification range in optical parametric amplification toward the bandwidth needed for single cycle femtosecond pulses. Two-color pumping of independent stages is used to sequentially amplify the long and short wavelength parts of the ultrabroadband seed pulses. The concept is tested in two related experiments. With multi-mJ pumping pulses with a nearly octave spanning spectrum and an uncompressed energy of 3 mJ are generated at low repetition rate. The spectral phase varies slowly and continuously in the overlap region as shown with 100 kHz repetition rate. This should allow the compression to the Fourier limit of below 5 fs in the high energy system. (C) 2010 Optical Society of America

Photochemistry in Photonic Crystal Fiber Nanoreactors

Jocelyn S. Y. Chen, Tijmen G. Euser, Nicola J. Farrer, Peter J. Sadler, Michael Scharrer, Philip St. J. Russell

We report the use of a liquid-filled hollow-core photonic crystal fiber (PCF) as a highly controlled photochemical reactor. Hollow-core PCFs have several major advantages over conventional sample cells: the sample volume per optical path length is very small (2.8 nL cm(-1) in the fiber used), long optical path lengths are possible as a result of very low intrinsic waveguide loss, and furthermore the light travels in a diffractionless single mode with a constant transverse intensity profile. As a proof of principle, the (very low) quantum yield of the photochemical conversion of vitamin Bp, cyanocobalamin (CNCbl) to hydroxocobalamin ([H(2)OCbl](+)) in aqueous solution was measured for several pH values from 2.5 to 7.5. The dynamics of the actively induced reaction were monitored in real-time by broadband absorption spectroscopy. The PCF nanoreactor required ten thousand times less sample volume compared to conventional techniques. Furthermore. the enhanced sensitivity and optical pump intensity implied that even systems with very small quantum yields can be measured very quickly in our experiments one thousand times faster than in a conventional cuvette.

Spatiotemporal evolution of femtosecond laser pulses guided in air-clad fused-silica nanoweb

C. Kreuzer, A. Podlipensky, P. St. J. Russell

We investigate nonlinear propagation and self-focusing of femtosecond Ti:sapphire laser pulses in an 800-nm-thick silica nanoweb fiber. Different dispersion regimes are accessible by launching TE- or TM-polarized light. Excitation in the anomalous dispersion regime (TM) results in pulse splitting and spectral broadening, which lead to supercontinuum generation, whereas, for normal dispersion (TE, excited close to a zero dispersion wavelength), self-phase modulation causes spectral broadening, which leads at higher power to beam collapse and the creation of a damage track. (C) 2010 Optical Society of America

Ultraviolet-enhanced supercontinuum generation in tapered photonic crystal fiber

S. P. Stark, A. Podlipensky, N. Y. Joly, P. St. J. Russell

We investigate numerically and experimentally the propagation of visible sub-50 fs pulses in a tapered small core photonic crystal fiber. The fiber has anomalous dispersion between two closely spaced zero dispersion wavelengths at 509 and 640 nm, and the excitation wavelength was varied within this range. We find that the spectral evolution in the low power regime is dominated by higher-order soliton fission, soliton self-frequency shift, and dispersive wave generation. At higher powers, extremely wide spectral broadening of the input pulse occurs within the first few millimeters of fiber. The wavelength conversion into the blue and red spectral ranges is studied as a function of the input power and excitation wavelength. Conversions into the spectral range 300-470 nm at efficiencies as high as 40% are observed when pumping at 523 nm. (C) 2010 Optical Society of America

Highly Noninstantaneous Solitons in Liquid-Core Photonic Crystal Fibers

Claudio Conti, Markus A. Schmidt, Philip St J. Russell, Fabio Biancalana

The nonlinear propagation of pulses in liquid-filled photonic crystal fibers is considered. Because of the slow reorientational nonlinearity of some molecular liquids, the nonlinear modes propagating inside such structures can be approximated, for pulse durations much shorter than the molecular relaxation time, by temporally highly nonlocal solitons, analytical solutions of a linear Schrodinger equation. The physical relevance of these novel solitons is discussed.

Bridging visible and telecom wavelengths with a single-mode broadband photon pair source

C. Soeller, B. Brecht, P. J. Mosley, L. Y. Zang, A. Podlipensky, N. Y. Joly, P. St. J. Russell, C. Silberhorn

We present a spectrally decorrelated photon pair source bridging the visible and telecom wavelength regions. Tailored design and fabrication of a solid-core photonic crystal fiber (PCF) lead to the emission of signal and idler photons into only a single spectral and spatial mode. Thus no narrowband filtering is necessary and the heralded generation of pure photon number states in ultrafast wave packets at telecom wavelengths becomes possible.

Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic-bandgap fiber (vol 34, pg 3674, 2009)

T. G. Euser, M. K. Garbos, J. S. Y. Chen, P. St J. Russell

Dispersion of photonic Bloch modes in periodically twisted birefringent media

Leyun Zang, Myeong Soo Kang, Miroslav Kolesik, Michael Scharrer, Philip Russell

We investigate the polarization evolution and dispersive properties of the eigenmodes of birefringent media with arbitrarily twisted axes of birefringence. Analytical and numerical methods based on a transfer matrix approach are developed and used to study specifically helically twisted structures and the Bloch modes of periodically twisted media, as represented in particular by structural "rocking" filters inscribed in highly birefringent photonic crystal fibers. The presence of periodically twisted birefringence axes causes the group velocity dispersion curves to separate strongly from each other in the vicinity of the anti-crossing wavelength, where the inter-polarization beat-length equals an integer multiple of the rocking period. The maximum separation between these curves and the bandwidth of the splitting depend on the amplitude of the rocking angle. We also show that suitably designed adiabatic transitions, formed by chirping the rocking period, allow a broadband conversion between a linearly polarized fiber eigenmode and a single Bloch mode of a uniform rocking filter. The widely controllable dispersive properties provided by rocking filters may be useful for manipulating the phase-matching conditions in nonlinear optical processes such as four-wave mixing, supercontinuum generation, and the generation of resonant radiation from solitons. (C) 2010 Optical Society of America

Plasmon resonances on gold nanowires directly drawn in a step-index fiber

H. K. Tyagi, H. W. Lee, P. Uebel, M. A. Schmidt, N. Joly, M. Scharrer, P. St. J. Russell

We report the successful production of high-quality gold wires, with diameters down to 260 nm, by direct fiber drawing from a gold-filled fused-silica cane. The stack-and-draw technique makes it straightforward to incorporate a conventional step-index core, adjacent to the gold wire, in the cane. In the drawn fiber, strong coupling of light from the glass core to SPP resonances on the gold wire is observed at specific well-defined wavelengths. Such embedded wires have many potential applications, for example, as nanoscale electrodes, in nonlinear optical plasmonics, and as near-field scanning optical microscope tips. (C) 2010 Optical Society of America

All-Optical Control of Gigahertz Acoustic Resonances by Forward Stimulated Interpolarization Scattering in a Photonic Crystal Fiber

M. S. Kang, A. Brenn, P. St. J. Russell

We report the observation of a novel nonlinear optoacoustic phenomenon, that we name forward stimulated interpolarization scattering. When two frequency-offset laser signals are colaunched into orthogonally polarized guided modes of a birefringent small-core (1.8 mu m diameter) photonic crystal fiber, a pattern of axially moving polarization fringes is produced, with a velocity and spacing that depends on the frequency offset. At values of frequency offset in the few-GHz range, the pattern of moving fringes can perfectly match the phase velocity and axial wavelength (3.9 mm) of the torsional-radial acoustic mode tightly guided in the core. An intense optoacoustic interaction ensues, leading to efficient nonlinear exchange of power from the higher frequency (pump) mode to the orthogonally polarized lower frequency (Stokes) mode. A full-vectorial theory is developed to explain the observations.

Pressure-controlled phase matching to third harmonic in Ar-filled hollow-core photonic crystal fiber

J. Nold, P. Hoelzer, N. Y. Joly, G. K. L. Wong, A. Nazarkin, A. Podlipensky, M. Scharrer, P. St J. Russell

We report tunable third-harmonic generation (THG) in an Ar-filled hollow-core photonic crystal fiber, pumped by broadband < 2 mu J, 30 fs pulses from an amplified Ti:sapphire laser system. The overall dispersion is precisely controlled by balancing the negative dielectric susceptibility of the waveguide against the positive susceptibility of the gas. We demonstrate THG to a higher-order guided mode and show that the phase-matched UV wavelength is tunable by adjusting the gas pressure. (C) 2010 Optical Society of America

Theory of Raman multipeak states in solid-core photonic crystal fibers

Truong X. Tran, Alexander Podlipensky, Philip St. J. Russell, Fabio Biancalana

We provide a full theoretical understanding of the recent observations of excitation of Raman two-peak states in solid-core photonic crystal fibers. Based on a "gravity-like" potential approach we derive simple equations for the "magic" peak power ratio and the temporal separation between pulses forming these two-peak states. We develop a model to calculate the magic input power of the input pulse around which the phenomenon can be observed. We also predict the existence of exotic multipeak states that strongly violate the perturbative pulse splitting law, and we study their stability and excitation conditions. (C) 2010 Optical Society of America

Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic bandgap fiber

T. G. Euser, M. K. Garbos, J. S. Y. Chen, P. St. J. Russell

A great challenge in microfluidics is the precise control of laser radiation forces acting on single particles or cells, while allowing monitoring of their optical and chemical properties. We show that, in the liquid-filled hollow core of a single-mode photonic crystal fiber, a micrometer-sized particle can be held stably against a fluidic counterflow using radiation pressure and can be moved to and fro (over tens of centimeters) by ramping the laser power up and down. Accurate studies of the microfluidic drag forces become possible, because the particle is trapped in the center of the single guided optical mode, resulting in highly reproducible radiation forces. The counterflowing liquid can be loaded with sequences of chemicals in precisely controlled concentrations and doses, making possible studies of single particles, vesicles, or cells. (C) 2009 Optical Society of America

All-solid bandgap guiding in tellurite-filled silica photonic crystal fibers

Markus A. Schmidt, Nicolai Granzow, Ning Da, Mingying Peng, Lothar Wondraczek, Philip St. J. Russell

We report all-solid bandgap-guiding fibers formed by pumping molten tellurite glass into silica-air photonic crystal fiber at high pressure. The spectral positions of the guidance bands agree well with multipole simulations and bandgap calculations. The micrometer-diameter tellurite strands are found to contain microheterogeneities (most probably originating from devitrification), which increase the fiber attenuation, although no evidence of crystallization is seen in the bulk tellurite glass. The technique offers a potential route to employing difficult-to-handle glasses, or glasses unsuitable for fiber drawing, in fiber-based amplifiers, modulators, filters, and nonlinear devices. (C) 2009 Optical Society of America

Manipulation of coherent Stokes light by transient stimulated Raman scattering in gas filled hollow-core PCF

A. V. Chugreev, A. Nazarkin, A. Abdolvand, J. Nold, A. Podlipensky, P. St. J. Russell

Transient stimulated Raman scattering is investigated in methane-filled hollow-core photonic crystal fiber. Using frequency-chirped ps-pulses at 1.06 mu m as pump and tunable CW-radiation as Stokes seed, the vibrational excitation of the CH4 molecules can be controlled on the sub T-2 time-scale. In this way the generated Stokes pulse can be phase-locked to the pump pulse and its spectrum manipulated. (c) 2009 Optical Society of America

Solitary Pulse Generation by Backward Raman Scattering in H-2-Filled Photonic Crystal Fibers

A. Abdolvand, A. Nazarkin, A. V. Chugreev, C. F. Kaminski, P. St. J. Russell

Using a hydrogen-filled hollow-core photonic crystal fiber as a nonlinear optical gas cell, we study amplification of ns-laser pulses by backward rotational Raman scattering. We find that the amplification process has two characteristic stages. Initially, the pulse energy grows and its duration shortens due to gain saturation at the trailing edge of the pulse. This phase is followed by formation of a symmetric pulse with a duration significantly shorter than the phase relaxation time of the Raman transition. Stabilization of the Stokes pulse profile to a solitonlike hyperbolic secant shape occurs as a result of nonlinear amplification at its front edge and nonlinear absorption at its trailing edge (caused by energy conversion back to the pump field), leading to a reshaped pulse envelope that travels at superluminal velocity.

Optimizing anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers

A. Nazarkin, A. Abdolvand, P. St. J. Russell

Anti-Stokes Raman scattering in gas-filled hollow-core photonic crystal fibers is discussed. It is shown that the efficient anti-Stokes generation observed under conditions of significant wave mismatch is caused by phase locking of the interacting fields. This leads to the establishment of a phase difference that is independent of the optical path. An optimization technique, based on the adjustment of the wave mismatch along a gas-filled hollow fiber using pressure control, is proposed. Anti-Stokes conversion efficiencies close to the theoretical maximum of 50% are predicted.

Influence of air-filling fraction on forward Raman-like scattering by transversely trapped acoustic resonances in photonic crystal fibers

A. Brenn, G. S. Wiederhecker, M. S. Kang, H. Hundertmark, N. Joly, P. St. J. Russell

Raman-like forward scattering by acoustic phonons transversely trapped in birefringent silica-air photonic crystal fibers is studied. As the air-filling fraction increases, core-confined acoustic resonances become increasingly apparent at higher frequencies (> 1.1 GHz), while the number of cladding-confined acoustic modes involved in scattering falls. Two main types of scattering are observed: intramodal (scattering to new frequencies within the same optical mode) and intermodal (frequency-shifted scattering to a different optical mode). It is shown that the twofold symmetric microstructure in a birefringent fiber causes strongly polarization-dependent intramodal scattering. Good agreement is obtained between the experimental measurements and numerical solutions of both the acoustic and electromagnetic wave equations by using a full-vectorial finite-element approach. Phononic bandgaps are found to play a significant role at higher air-filling fractions, leading to the appearance of additional bands in the scattering spectrum. (C) 2009 Optical Society of America

Octave-spanning supercontinuum generated in SF6-glass PCF by a 1060 nm mode-locked fibre laser delivering 20 pJ per pulse

H. Hundertmark, S. Rammler, T. Wilken, R. Holzwarth, T. W. Haensch, P. St. J. Russell

We report the generation of an octave-spanning supercontinuum in SF6-glass photonic crystal fiber using a diode-pumped passively modelocked fs Yb-fiber laser oscillating at 1060 nm. The pulses (energy up to 500 pJ and duration 60 fs) were launched into a 4 cm length of PCF (core diameter 1.7 mu m and zero-dispersion wavelength similar to 1060 nm). Less than 20 pJ of launched pulse energy was sufficient to generate a supercontinuum from 600 nm to 1450 nm, which represents the lowest energy so far reported for generation of an octave-spanning supercontinuum from a 1 mu m pump. Since the laser pulse energy scales inversely with the repetition rate, highly compact and efficient sources based on SF6-glass PCF are likely to be especially useful for efficient spectral broadening at high repetition rates (several GHz), such as those needed for the precise calibration of astronomical spectrographs, where a frequency comb spacing > 10 GHz is required for the best performance. (C) 2009 Optical Society of America

Tightly trapped acoustic phonons in photonic crystal fibres as highly nonlinear artificial Raman oscillators

M. S. Kang, A. Nazarkin, A. Brenn, P. St. J. Russell

Interactions between light and hypersonic waves can be enhanced by tight field confinement, as shown in periodically structured materials(1), microcavities(2), micromechanical resonators(3) and photonic crystal fibres(4-6) (PCFs). There are many examples of weak sound-light interactions, for example, guided acoustic-wave Brillouin scattering in conventional optical fibres(7). This forward-scattering effect results from the interaction of core-guided light with acoustic resonances of the entire fibre cross-section, and is viewed as a noise source in quantum-optics experiments(8). Here, we report the observation of strongly nonlinear forward scattering of laser light by gigahertz acoustic vibrations, tightly trapped together in the small core of a silica-air PCF. Bouncing to and fro across the core at close to 90 degrees to the fibre axis, the acoustic waves form optical-phonon-like modes with a flat dispersion curve and a distinct cutoff frequency Omega(a). This ensures automatic phase-matching to the guided optical mode so that, on pumping with a dual-frequency laser source tuned to Omega(a), multiple optical side bands are generated, spaced by Omega(a). The number of strong side bands in this Raman-like process increases with pump power. The results point to a new class of designable nonlinear optical device with applications in, for example, pulse synthesis, frequency comb generation for telecommunications and fibre laser mode-locking.

Dynamic control of higher-order modes in hollow-core photonic crystal fibers

T. G. Euser, G. Whyte, M. Scharrer, J. S. Y. Chen, A. Abdolvand, J. Nold, C. F. Kaminski, P. St. J. Russell

We present a versatile method for selective mode coupling into higher-order modes of photonic crystal fibers, using holograms electronically generated by a spatial light modulator. The method enables non-mechanical and completely repeatable changes in the coupling conditions. We have excited higher order modes up to LP(31) in hollow-core photonic crystal fibers. The reproducibility of the coupling allows direct comparison of the losses of different guided modes in both hollow-core bandgap and kagome-lattice photonic crystal fibers. Our results are also relevant to applications in which the intensity distribution of the light inside the fiber is important, such as particle-or atom-guidance. (C) 2008 Optical Society of America

Polarization-dependent coupling to plasmon modes on submicron gold wire in photonic crystal fiber

H. W. Lee, M. A. Schmidt, H. K. Tyagi, L. Prill Sempere, P. St. J. Russell

We present experimental results on coupling to surface plasmon modes on gold nanowires selectively introduced into polarization-maintaining photonic crystal fibers. Highly polarization- and wavelength-dependent transmission is observed. In one sample 24.5 mm long, the transmission on and off resonance differs by as much as 45 dB. Near-field optical images of the light emerging from such a gold-filled fiber show light guided on the wire at surface plasmon resonances. Finite element simulations are in good agreement with the experimental results. These gold-filled fibers can be potentially used as in-fiber wavelength-dependent filters and polarizers and as near-field tips for sub-wavelength-scale imaging. (C) 2008 American Institute of Physics.

Anomalous pulse breakup in small-core photonic crystal fibers

A. Podlipensky, P. Szarniak, N. Y. Joly, P. St. J. Russell

Detailed numerical and experimental studies of propagation of 110 fs laser pulses at 800 am in small-core photonic crystal fibers (gamma = 100 W (1) km(-1)) reveal that pulse breakup occurs in two distinct regimes defined by the input power. At low peak power (soliton order N <= g7) higher-order soliton fission occurs: individual solitons being ejected from the input pulse one after the other and are at-ranged in wavelength and in time by peak power. At higher levels of peak power (N>8), pulse breakup results in ejection of bound soliton pairs and the formation of single solitons that collide during propagation. (C) 2008 Optical Society of America

Heat dissipative solitons in optical fibers

N. Akhmediev, P. St. J. Russell, M. Taki, J. M. Soto-Crespo

We propose a one-dimensional model governing the propagation of heat waves in an optical fiber (the "fiber fuse"). The model has solutions in the form of high temperature localized waves moving towards the input end of the fiber, fueled by the laser power. These waves can be ignited by local heating at any point along the fiber. The effect of such a wave is irreversible damage to the fiber core. The phenomenon was observed earlier by Hand and Russell, when locally heating a fiber through which CW light of modest intensity was propagating. This induced self-destruction of the optical fiber core. (c) 2007 Elsevier B.V. All rights reserved.

Velocity of heat dissipative solitons in optical fibers

A. Ankiewicz, Wenjing Chen, P. St. J. Russell, M. Taki, N. Akhmediev

In the fiber fuse, a pulse of high temperature travels toward the input end of the fiber, where high-power laser light is launched into the fiber. At any point along the fiber, the soliton can be ignited. The fiber core is damaged in the process so that light cannot propagate beyond the hot spot. This phenomenon is an example of a dissipative soliton that can exist only in the presence of an external energy supply and internal loss, We analyze this phenomenon, derive an expression for the velocity of the soliton, and determine its width as functions of the physical parameters of the laser and the fiber material. (C) 2008 Optical Society of America

Long-range spiralling surface plasmon modes on metallic nanowires

M. A. Schmidt, P. St. J. Russell

We discuss the characteristics of surface plasmon modes guided on metallic nanowires of circular cross-section embedded in silica glass. Under certain conditions such wires allow low-loss guided modes, full account being taken of ohmic losses in the metal. We find that these modes can be bound to the wire even when the real part of their axial refractive index is less than that of the surrounding dielectric. We assess in detail the accuracy of a simple model in which SPs are viewed as spiralling around the nanowire in a helical path, forming modes at certain angles of pitch. The results are relevant for understanding the behavior of light in twodimensional arrays of metallic nanowires in fiber form. (c) 2008 Optical Society of America.

Optical properties of photonic crystal fiber with integral micron-sized Ge wire

H. K. Tyagi, M. A. Schmidt, L. Prill Sempere, P. St. J. Russell

Using a selective hole closure technique, individual hollow channels in silica-air photonic crystal fibers are filled with pure Ge by pumping in molten material at high pressure. The smallest channels filled so far are 600 nm in diameter, which is 10x smaller than in previous work. Electrical conductivity and micro-Raman measurements indicate that the resulting cm-long wires have a high degree of crystallinity. Optical transmission spectra are measured in a sample with a single wire placed adjacent to the core of an endlessly single-mode photonic crystal fiber. This renders the fiber birefringent, as well as causing strongly polarization-dependent transmission losses, with extinction ratios as high as 30 dB in the visible. In the IR, anti-crossings between the glass-core mode and resonances on the high index Ge wire create a series of clear dips in the spectrum transmitted through the fiber. The measurements agree closely with the results of finite-element simulations in which the wavelength dependence of the dielectric constants is taken fully into account. A toy model based on a multilayer structure is used to help interpret the results. Finally, the temperature dependence of the anti-crossing wavelengths is measured, the preliminary results suggesting that the structure might form the basis of a compact optical thermometer. Since Ge provides electrical conductance together with low-loss guidance in the mid-IR, Ge-filled PCF seems likely to lead to new kinds of in-fiber detector and sensor, as well as having potential uses in ultra-low-threshold nonlinear optical devices. (C) 2008 Optical Society of America

Optical excitation and characterization of gigahertz acoustic resonances in optical fiber tapers

Myeong Soo Kang, Andre Brenn, Gustavo S. Wiederhecker, Philip St. J. Russell

Transverse acoustic resonances at gigahertz frequencies are excited by electrostriction in the few-micrometer-thick waists of low-loss optical fiber tapers of up to 40 cm long. A pump-probe technique is used in which the resonances are excited by a train of optical pulses and probed in a Sagnac interferometer. Strong radially symmetric acoustic resonances are observed and the dependence of their frequencies on taper thickness is investigated. Such easily reconfigurable acousto-optic interactions may have applications in the high-frequency mode locking of fiber lasers. (C) 2008 American Institute of Physics.

Quasi-phase-matched high harmonic generation in hollow core photonic crystal fibers

H. Ren, A. Nazarkin, J. Nold, P. St. J. Russell

The potential of hollow core photonic crystal fiber as a nonlinear gas cell for efficient high harmonic generation is discussed. The feasibility of phase-matching this process by modulating the phase of ionization electrons using a counter-propagating laser field is shown. In this way, harmonics with energies of several hundreds of eV can be produced using fs-laser pump pulses of mu J energy. (C) 2008 Optical Society of America

Waveguiding and plasmon resonances in two-dimensional photonic lattices of gold and silver nanowires

M. A. Schmidt, L. N. Prill Sempere, H. K. Tyagi, C. G. Poulton, P. St. J. Russell

We report the fabrication of triangular lattices of parallel gold and silver nanowires of high optical quality, with diameters down to 500 nm and length-to-diameter ratios as high as 100 000. The nanowires are supported by a silica glass matrix and are disposed around a central solid glass core, i.e., a missing nanowire. These centimeter-long structures make it possible to trap light within an array of nanowires and characterize the plasmon resonances that form at specific optical frequencies. Such nanowire arrays have many potential applications, e.g., imaging on the subwavelength scale.

Coherent control of ultrahigh-frequency acoustic resonances in photonic crystal fibers

G. S. Wiederhecker, A. Brenn, H. L. Fragnito, P. St. J. Russell

Ultrahigh frequency acoustic resonances (2 GHz) trapped within the glass core (1 mu m diameter) of a photonic crystal fiber are selectively excited through electrostriction using laser pulses of duration 100 ps and energy 500 pJ. Using precisely timed sequences of such driving pulses, we achieve coherent control of the acoustic resonances by constructive or destructive interference, demonstrating both enhancement and suppression of the vibrations. A sequence of 27 resonantly-timed pulses provides a 100-fold increase in the amplitude of the vibrational mode. The results are explained and interpreted using a semianalytical theory, and supported by precise numerical simulations of the complex light-matter interaction.

Quantitative broadband chemical sensing in air-suspended solid-core fibers

T. G. Euser, J. S. Y. Chen, M. Scharrer, P. St. J. Russell, N. J. Farrer, P. J. Sadler

We demonstrate a quantitative broadband fiber sensor based on evanescent-field sensing in the cladding holes of an air-suspended solid-core photonic crystal fiber. We discuss the fabrication process, together with the structural and optical characterization of a range of different fibers. Measured mode profiles are in good agreement with finite element method calculations made without free parameters. The fraction of the light in the hollow cladding can be tuned via the core diameter of the fiber. Dispersion measurements are in excellent agreement with theory and demonstrate tuning of the zero dispersion wavelength via the core diameter. Optimum design parameters for absorption sensors are discussed using a general parameter diagram. From our analysis, we estimate that a sensitivity increase of three orders of magnitude is feasible compared to standard cuvette measurements. Our study applies to both liquid and gas fiber sensors. We demonstrate the applicability of our results to liquid chemical sensing by measuring the broad absorption peak of an aqueous NiCl(2) solution. We find excellent agreement with the reference spectrum measured in a standard cuvette, even though the sample volume has decreased by three orders of magnitude. Our results demonstrate that air-suspended solid-core photonic crystal fibers can be used in quantitative broadband chemical-sensing measurements. (C) 2008 American Institute of Physics.

Numerical study of guided modes in arrays of metallic nanowires

C. G. Poulton, M. A. Schmidt, G. J. Pearce, G. Kakarantzas, P. St. J. Russell

We numerically investigate the band structure and guided modes within arrays of metallic nanowires. We show that bandgaps appear for a range of array geometries and that these can be used to guide light in these structures. Values of attenuation as low as 1.7 dB/cm are predicted for arrays of silver wires at communications wavelengths. This is more than 100 times smaller than the attenuation of the surface plasmon polariton modes on a single silver nanowire. (c) 2007 Optical Society of America.

Bound soliton pairs in photonic crystal fiber

A. Podlipensky, P. Szarniak, N. Y. Joly, C. G. Poulton, P. St. J. Russell

We demonstrate experimentally the formation and stable propagation of bound soliton pairs in a highly nonlinear photonic crystal fiber. The bound pairs occur at a particular power as the consequence of high-order soliton fission. They propagate over long distances with constant inter-soliton frequency and time separation. During propagation, the soliton self-frequency shift causes the central frequency of the pairs to move towards longer wavelength. The formation and characteristics of the bound soliton pairs are confirmed numerically. We believe this to be the first experimental observation of such bound soliton pairs. (c) 2007 Optical Society of America.

Models for guidance in kagome-structured hollow-core photonic crystal fibres

G. J. Pearce, G. S. Wiederhecker, C. G. Poulton, S. Burger, P. St. J. Russell

We demonstrate by numerical simulation that the general features of the loss spectrum of photonic crystal fibres (PCF) with a kagome structure can be explained by simple models consisting of thin concentric hexagons or rings of glass in air. These easily analysed models provide increased understanding of the mechanism of guidance in kagome PCF, and suggest ways in which the high-loss resonances in the loss spectrum may be shifted. (C) 2007 Optical Society of America.

Photonic-crystal fibers

Philip St. J. Russell

The history, fabrication, theory, numerical modeling, optical properties, guidance mechanisms, and applications of photonic-crystal fibers are reviewed.

Raman-like light scattering from acoustic phonons in photonic crystal fiber

P Dainese, PSJ Russell, GS Wiederhecker, N Joly, HL Fragnito, V Laude, A Khelif

Raman and Brillouin scattering are normally quite distinct processes that take place when light is resonantly scattered by, respectively, optical and acoustic phonons. We show how few-GHz acoustic phonons acquire many of the same characteristics as optical phonons when they are tightly trapped, transversely and close to modal cut-off, inside the wavelength-scale core of an air-glass photonic crystal fiber (PCF). The result is an optical scattering effect that closely resembles Raman scattering, though at much lower frequencies. We use photoacoustic techniques to probe the effect experimentally and finite element modelling to explain the results. We also show by numerical modelling that the cladding structure supports two phononic band gaps that contribute to the confinement of sound in the core. (c) 2006 Optical Society of America

Photonic sensing based on variation of propagation properties of photonic crystal fibres

John H. Rothwell, Donal A. Flavin, William N. MacPherson, Julian D. C. Jones, Jonathan C. Knight, Philip St. J. Russell

We report on a low-coherence interferometric scheme for the measurement of the strain and temperature dependences of group delay and dispersion in short, index-guiding, 'endlessly-single-mode' photonic crystal fibre elements in the 840 nm and 1550 nm regions. Based on the measurements, we propose two schemes for simultaneous strain and temperature measurement using a single unmodified PCF element, without a requirement for any compensating components, and we project the measurement accuracies of these schemes.

Stimulated Brillouin scattering from multi-GHz-guided acoustic phonons in nanostructured photonic crystal fibres

P. Dainese, P. St. J. Russell, N. Joly, J. C. Knight, G. S. Wiederhecker, H. L. Fragnito, V. Laude, A. Khelif

Wavelength-scale periodic microstructuring dramatically alters the optical properties of materials. An example is glass photonic crystal fibre(1) ( PCF), which guides light by means of a lattice of hollow micro/nanochannels running axially along its length. In this letter, we explore stimulated Brillouin scattering in PCFs with subwavelength-scale solid silica glass cores. The large refractive-index difference between air and glass allows much tighter confinement of light than is possible in all-solid single-mode glass optical fibres made using conventional techniques. When the silica-air PCF has a core diameter of around 70% of the vacuum wavelength of the launched laser light, we find that the spontaneous Brillouin signal develops a highly unusual multi-peaked spectrum with Stokes frequency shifts in the 10-GHz range. We attribute these peaks to several families of guided acoustic modes each with different proportions of longitudinal and shear strain, strongly localized to the core(2,3). At the same time, the threshold power for stimulated Brillouin scattering(4) increases fivefold. The results show that Brillouin scattering is strongly affected by nanoscale microstructuring, opening new opportunities for controlling light-sound interactions in optical fibres.

Spectrally smooth supercontinuum from 350 nm to 3 mu m in sub-centimeter lengths of soft-glass photonic crystal fibers.

FG Omenetto, NA Wolchover, MR Wehner, M Ross, A Efimov, AJ Taylor, VVRK Kumar, AK George, JC Knight, et al.

The conversion of light fields in photonic crystal fibers ( PCFs) capitalizes on the dramatic enhancement of several optical nonlinearities. We present here spectrally smooth, highly broadband supercontinuum radiation in a short piece of high-nonlinearity soft-glass PCF. This supercontinuum spans several optical octaves, with a spectral range extending from 350 nm to beyond 3000 nm. The selection of an appropriate propagation-length determines the spectral quality of the supercontinuum generated. Experimentally, we clearly identify two regimes of nonlinear pulse transformation: when the fiber length is much shorter than the dispersion length, soliton propagation is not important and a symmetric supercontinuum spectrum arises from almost pure self-phase modulation. For longer fiber lengths the supercontinuum is formed by the breakup of multiple Raman-shifting solitons. In both regions very broad supercontinuum radiation is produced. (c) 2006 Optical Society of America.

Competition between spectral splitting and Raman frequency shift in negative-dispersion slope photonic crystal fiber

Nicolas Y. Joly, Fiorenzo G. Omenetto, Anatoly Efimov, Antoinette J. Taylor, Jonathan C. Knight, Philip St.J. Russell

We report on the nonlinear behavior of high air-filling fraction solid-core photonic crystal fibers pumped with ultrashort pulses in the vicinity of a negative-slope zero-crossing of the group velocity dispersion. We observed dramatically different behavior when the pump wavelength lies in the normal or the anomalous dispersion range. When pumping at the zero-dispersion wavelength the combined effects of spectral splitting, self-phase modulation and soliton self-frequency shift result a “comma”-shape of the power-dependant spectra. This spectral feature is explained using a simple model.