Localized Phase Structures Growing Out of Quantum Fluctuations in a Quench of Tunnel-coupled Atomic Condensates

Clemens Neuenhahn, Anatoli Polkovnikov, Florian Marquardt

We investigate the relative phase between two weakly interacting 1D condensates of bosonic atoms after suddenly switching on the tunnel coupling. The following phase dynamics is governed by the quantum sine-Gordon equation. In the semiclassical limit of weak interactions, we observe the parametric amplification of quantum fluctuations leading to the formation of breathers with a finite lifetime. The typical lifetime and density of these "quasibreathers" are derived employing exact solutions of the classical sine-Gordon equation. Both depend on the initial relative phase between the condensates, which is considered as a tunable parameter.

Direct printing of nanostructures by electrostatic autofocussing of ink nanodroplets

Patrick Galliker, Julian Schneider, Hadi Eghlidi, Sascha Kress, Vahid Sandoghdar, Dimos Poulikakos

Nanotechnology, with its broad impact on societally relevant applications, relies heavily on the availability of accessible nanofabrication methods. Even though a host of such techniques exists, the flexible, inexpensive, on-demand and scalable fabrication of functional nanostructures remains largely elusive. Here we present a method involving nanoscale electrohydrodynamic ink-jet printing that may significantly contribute in this direction. A combination of nanoscopic placement precision, soft-landing fluid dynamics, rapid solvent vapourization, and subsequent self-assembly of the ink colloidal content leads to the formation of scaffolds with base diameters equal to that of a single ejected nanodroplet. The virtually material-independent growth of nanostructures into the third dimension is then governed by an autofocussing phenomenon caused by local electrostatic field enhancement, resulting in large aspect ratio. We demonstrate the capabilities of our electrohydrodynamic printing technique with several examples, including the fabrication of plasmonic nanoantennas with features sizes down to 50 nm.

Thermalization of interacting fermions and delocalization in Fock space

Clemens Neuenhahn, Florian Marquardt

We investigate the onset of "eigenstate thermalization" and the crossover to ergodicity in a system of one-dimensional fermions with increasing interaction. We analyze the fluctuations in the expectation values of most relevant few-body operators with respect to eigenstates. It turns out that these are intimately related to the inverse participation ratio of eigenstates displayed in the operator eigenbasis. Based on this observation, we find good evidence that eigenstate thermalization should set in even for vanishingly small perturbations in the thermodynamic limit.

White-light bias external quantum efficiency measurements of standard and inverted P3HT: PCBM photovoltaic cells

Thomas J. K. Brenner, Yana Vaynzof, Zhe Li, Dinesh Kabra, Richard H. Friend, Christopher R. McNeill

We have investigated the behaviour of inverted poly(3-hexylthiophene) : [6,6]-phenyl-C-61-butyric acid methyl ester (P3HT : PCBM) solar cells with different active layer thickness upon changing light intensity. Using white-light bias external quantum efficiency (EQE) measurements and photocurrent transient measurements we explain the different thickness dependence of device performance of inverted (ITO/ZnO/P3HT : PCBM/WO3/Ag) and standard (ITO/PEDOT : PSS/P3HT : PCBM/Ca/Al) cells. Whereas for inverted devices where high EQEs of up to 68% are measured under low light intensities (similar to 3.5 mW cm(-2)), a dramatic reduction in EQE is observed with increasing white-light bias (up to similar to 141.5 mW cm(-2)) accompanied by a severe distortion of the EQE spectrum. For the inverted device this spectral distortion is characterized by a dip in the EQE spectrum for wavelengths corresponding to maximum light absorption and becomes more prominent with increasing active layer thickness. For regular P3HT : PCBM devices, in contrast, a less dramatic reduction in EQE with increasing light intensity and only a mild change in EQE spectral shape are observed. The change in EQE spectral shape is also different for standard devices with a relative reduction in EQE for spectral regions where light is absorbed less strongly. This asymmetry in device behaviour is attributed to unbalanced charge transport with the lower mobility carrier having to travel further on average in the inverted device structure. Thus at high light intensities charge recombination is more pronounced at the front half of the device (close to the transparent electrode) for inverted cells where most of the light is absorbed, and more pronounced at the back half of the device for standard cells. Our results therefore indicate that bulk charge transport mobilities rather than vertical composition gradients are the dominant factor in determining the performance of standard and inverted P3HT : PCBM cells.

Plasmonic dimer antennas for surface enhanced Raman scattering

Katja Hoeflich, Michael Becker, Gerd Leuchs, Silke Christiansen

Electron beam induced deposition (EBID) has recently been developed into a method to directly write optically active three-dimensional nanostructures. For this purpose a metal-organic precursor gas (here dimethyl-gold(III)-acetylacetonate) is introduced into the vacuum chamber of a scanning electron microscope where it is cracked by the focused electron beam. Upon cracking the aforementioned precursor gas, 3D deposits are realized, consisting of gold nanocrystals embedded in a carbonaceous matrix. The carbon content in the deposits hinders direct plasmonic applications. However, it is possible to activate the deposited nanostructures for plasmonics by coating the EBID structures with a continuous silver layer of a few nanometers thickness. Within this silver layer collective motions of the free electron gas can be excited. In this way, EBID structures with their intriguing precision at the nanoscale have been arranged in arrays of free-standing dimer antenna structures with nanometer sized gaps between the antennas that face each other with an angle of 90 degrees. These dimer antenna ensembles can constitute a reproducibly manufacturable substrate for exploiting the surface enhanced Raman effect (SERS). The achieved SERS enhancement factors are of the order of 10(4) for the incident laser light polarized along the dimer axes. To prove the signal enhancement in a Raman experiment we used the dye methyl violet as a robust test molecule. In future applications the thickness of such a silver layer on the dimer antennas can easily be varied for tuning the plasmonic resonances of the SERS substrate to match the resonance structure of the analytes to be detected.

Universal quantum computation with continuous-variable Abelian anyons

Darran F. Milne, Natalia V. Korolkova, Peter van Loock

We describe how continuous-variable Abelian anyons, created on the surface of a continuous-variable analog of Kitaev's lattice model can be utilized for quantum computation. In particular, we derive protocols for the implementation of quantum gates using topological operations. We find that the topological operations alone are insufficient for universal quantum computation, which leads us to study additional nontopological operations such as offline squeezing and single-mode measurements. It is shown that these in conjunction with a non-Gaussian element allow for universal quantum computation using continuous-variable Abelian anyons.

Filtering of the absolute value of photon-number difference for two-mode macroscopic quantum superpositions

M. Stobinska, F. Toeppel, P. Sekatski, A. Buraczewski, M. Zukowski, M. V. Chekhova, G. Leuchs, N. Gisin

We discuss a device capable of filtering out two-mode states of light with mode populations differing by more than a certain threshold, while not revealing which mode is more populated. It would allow engineering of macroscopic quantum states of light in a way which is preserving specific superpositions. As a result, it would enhance optical phase estimation with these states as well as distinguishability of "macroscopic" qubits. We propose an optical scheme, which is a relatively simple, albeit nonideal, operational implementation of such a filter. It uses tapping of the original polarization two-mode field, with a polarization-neutral beam splitter of low reflectivity. Next, the reflected beams are suitably interfered on a polarizing beam splitter. It is oriented such that it selects unbiased polarization modes with respect to the original ones. The more an incoming two-mode Fock state is unequally populated, the more the polarizing beam-splitter output modes are equally populated. This effect is especially pronounced for highly populated states. Additionally, for such states we expect strong population correlations between the original fields and the tapped one. Thus, after a photon-number measurement of the polarizing beam-splitter outputs, a feed-forward loop can be used to let through a shutter the field, which was transmitted by the tapping beam splitter. This happens only if the counts at the outputs are roughly equal. In such a case, the transmitted field differs strongly in occupation number of the two modes, while information on which mode is more populated is nonexistent (a necessary condition for preserving superpositions).

Optical mesh lattices with PT symmetry

Mohammad-Ali Miri, Alois Regensburger, Ulf Peschel, Demetrios N. Christodoulides

We investigate a class of optical mesh periodic structures that are discretized in both the transverse and longitudinal directions. These networks are composed of waveguide arrays that are discretely coupled, while phase elements are also inserted to discretely control their effective potentials and can be realized both in the temporal and the spatial domain. Their band structure and impulse response are studied in both the passive and parity-time (PT)-symmetric regime. The possibility of band merging and the emergence of exceptional points, along with the associated optical dynamics, are considered in detail both above and below the PT-symmetry breaking point. Finally, unidirectional invisibility in PT-synthetic mesh lattices is also examined, along with possible superluminal light transport dynamics.

Ultrasensitive detection of a protein by optical trapping in a photonic-plasmonic microcavity

Miguel A. Santiago-Cordoba, Murat Cetinkaya, Svetlana V. Boriskina, Frank Vollmer, Melik C. Demirel

Microcavity and whispering gallery mode (WGM) biosensors derive their sensitivity from monitoring frequency shifts induced by protein binding at sites of highly confined field intensities, where field strengths can be further amplified by excitation of plasmon resonances in nanoparticle layers. Here, we propose a mechanism based on optical trapping of a protein at the site of plasmonic field enhancements for achieving ultra sensitive detection in only microliter-scale sample volumes, and in real-time. We demonstrate femto-Molar sensitivity corresponding to a few 1000 s of macromolecules. Simulations based on Mie theory agree well with the optical trapping concept at plasmonic hotspots locations. ((c) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical Resonator Biosensors: Molecular Diagnostic and Nanoparticle Detection on an Integrated Platform

Martin Baaske, Frank Vollmer

Optical resonator biosensors are emerging as one of the most sensitive microsystem biodetection technology that does not require amplification or labeling of the analyte. This minireview provides a scholarly introduction to this research area and reviews current advances in molecular diagnostics and nanoparticle detection.

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 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

Label-free detection with high-Q microcavities: a review of biosensing mechanisms for integrated devices

Frank Vollmer, Lan Yang

Optical microcavities that confine light in high-Q resonance promise all of the capabilities required for a successful nextgeneration microsystem biodetection technology. Label-free detection down to single molecules as well as operation in aqueous environments can be integrated cost-effectively on microchips, together with other photonic components, as well as electronic ones. We provide a comprehensive review of the sensing mechanisms utilized in this emerging field, their physics, engineering and material science aspects, and their application to nanoparticle analysis and biomolecular detection. We survey the most recent developments such as the use of mode splitting for self-referenced measurements, plasmonic nanoantennas for signal enhancements, the use of optical force for nanoparticle manipulation as well as the design of active devices for ultra-sensitive detection. Furthermore, we provide an outlook on the exciting capabilities of functionalized high-Q microcavities in the life sciences.

Ultra-narrow linewidth CW sub-THz generation using GS based OFCG and n-i-pn-i-p superlattice photomixers

A. R. Criado, C. de Dios, G. H. Doehler, S. Preu, S. Malzer, S. Bauerschmidt, H. Lu, A. C. Gossard, P. Acedo

A report is presented on the photonic synthesis of ultra-narrow line-width continuous-wave (CW) sub-THz signals using a gain-switching (GS) based optical frequency comb generator (OFCG), selective optical filtering and a n-i-pn-i-p superlattice photomixer. This setup provides continuous tunability with a tuning resolution in the range of 0.1 Hz at 120 GHz and full width at half maximum of the generated signals below the limits of the measurement setup (< 10 Hz). The advantages of this system make it a very good candidate for applications requiring extremely low phase noise and continuous tunability, such as high resolution spectroscopy in the sub-THz and THz range.

Compensation of Nonlinear Phase Noise Using the Effective Negative Nonlinearity of a Nonlinear Amplifying Loop Mirror

Klaus Sponsel, Christian Stephan, Georgy Onishchukov, Bernhard Schmauss, Gerd Leuchs

The nonlinear amplifying loop mirror (NALM) has been explored for use as a nonlinear phase-shift compensator (NPSC). Operation conditions for a tunable effective negative nonlinearity are considered and the NALM parameter optimization is discussed for direct bit-error-ratio (BER) improvement by post-compensation after a nonlinear transmission line. In this configuration, the fundamental limits for NPSC are estimated for differential quadrature phase-shift keying (DQPSK) using a simplified model. Numerical simulations of a 20 Gb/s RZ-DQPSK transmission system confirmed the applicability of this model and showed a significant BER improvement in a realistic transmission line. Alternatively, the fiber launch power per span could be increased by 2 dB for the same BER.

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

Observation of spontaneous Brillouin cooling

Gaurav Bahl, Matthew Tomes, Florian Marquardt, Tal Carmon

Although bolometric- and ponderomotive-induced deflection of device boundaries are widely used for laser cooling, the electrostrictive Brillouin scattering of light from sound was considered an acousto-optical amplification-only process(1-7). It was suggested that cooling could be possible in multi-resonance Brillouin systems(5-8) when phonons experience lower damping than light(8). However, this regime was not accessible in electrostrictive Brillouin systems(1-3,5,6) as backscattering enforces high acoustical frequencies associated with high mechanical damping(1). Recently, forward Brillouin scattering(3) in microcavities(7) has allowed access to low-frequency acoustical modes where mechanical dissipation is lower than optical dissipation, in accordance with the requirements for cooling(8). Here we experimentally demonstrate cooling via such a forward Brillouin process in a microresonator. We show two regimes of operation for the electrostrictive Brillouin process: acoustical amplification as is traditional and an electrostrictive Brillouin cooling regime. Cooling is mediated by resonant light in one pumped optical mode, and spontaneously scattered resonant light in one anti-Stokes optical mode, that beat and electrostrictively attenuate the Brownian motion of the mechanical mode.

Stroboscopic observation of quantum many-body dynamics

Stefan Kessler, Andreas Holzner, Ian P. McCulloch, Jan von Delft, Florian Marquardt

Recent experiments have demonstrated single-site resolved observation of cold atoms in optical lattices. Thus, in the future it may be possible to take repeated snapshots of an interacting quantum many-body system during the course of its evolution. Here we address the impact of the resulting quantum (anti-)Zeno physics on the many-body dynamics. We use the time-dependent density-matrix renormalization group to obtain the time evolution of the full wave function, which is then periodically projected in order to simulate realizations of stroboscopic measurements. For the example of a one-dimensional lattice of spinless fermions with nearest-neighbor interactions, we find regimes for which many-particle configurations are stabilized or destabilized, depending on the interaction strength and the time between observations.

Electrical circuit model of arrays of resonant elements

V. Lomanets, O. Zhuromskyy, G. Onishchukov, U. Peschel

We present an effective electrical circuit model that can be used for a quasianalytic analysis of electromagnetic oscillations in arrays of coupled elements, resonant in the microwave domain. The model accounts for electric and magnetic interactions between charges and currents excited in individual resonators. Respective coupling coefficients can be calculated from the field and current distributions in a subsystem of just one or two elements, provided by a finite-difference electromagnetic solver. The model was used to investigate current distributions and dispersion relations of wave propagation on chains of coupled split-ring resonators. The change of the dispersion characteristics from forward to backward propagating wave type observed experimentally is readily reproduced by the model.

Spray-Coating Route for Highly Aligned and Large-Scale Arrays of Nanowires

Ossama Assad, Alexander M. Leshansky, Bin Wang, Thomas Stelzner, Silke Christiansen, Hossam Haick

Technological implementation of nanowires (NWs) requires these components to be organized with controlled orientation and density on various substrates. Here, we report on a simple and efficient route for the deposition of highly ordered and highly aligned NW arrays on a wide range of receiver substrates, including silicon, glass, metals, and flexible plastics with controlled density. The deposition approach is based on spray-coating of a NW suspension under controlled conditions of the nozzle flow rate, droplet size of the sprayed NWs suspension, spray angle, and the temperature of the receiver substrate. The dynamics of droplet generation is understood by a combined action of shear forces and capillary forces. Provided that the size of the generated droplet is comparable to the length of the single NW, the shear-driven elongation of the droplets results presumably in the alignment of the confined NW in the spraying direction. Flattening the droplets upon their impact with the substrate yields fast immobilization of the spray-aligned NWs on the surface due to van der Waals attraction. The availability of the spray-coating technique in the current microelectronics technology would ensure immediate implementation in production lines, with minimal changes in the fabrication design and/or auxiliary tools used for this purpose.

Time-reversal symmetry in optics

G. Leuchs, M. Sondermann

The utilization of time-reversal symmetry in designing and implementing (quantum) optical experiments has become more and more frequent over the last few years. We review the basic idea underlying time-reversal methods, illustrate it with several examples and discuss a number of implications.

Adaptive Detection of Arbitrarily Shaped Ultrashort Quantum Light States

C. Polycarpou, K. N. Cassemiro, G. Venturi, A. Zavatta, M. Bellini

A quantum state of light is the excitation of a particular spatiotemporal mode of the electromagnetic field. A precise control of the mode structure is therefore essential for processing, detecting, and using photonic states in novel quantum technologies. Here we demonstrate an adaptive scheme, combining techniques from the fields of ultrafast coherent control and quantum optics, for probing the arbitrary complex spectrotemporal profile of an ultrashort quantum light pulse. The ability to access the modal structure of a quantum light state could boost the capacity of current quantum information protocols.

Entanglement witnesses and measures for bright squeezed vacuum

Magdalena Stobinska, Falk Toeppel, Pavel Sekatski, Maria V. Chekhova

Quantum entanglement is a fascinating phenomenon, especially if it is observed at the macroscopic scale. Importantly, macroscopic quantum correlations can be revealed only by accurate measurement outcomes and strategies. Here, we formulate feasible entanglement witnesses for bright squeezed vacuum in the form of the macroscopically populated polarization triplet Bell states. Their testing involves efficient photodetection and the measurement of the Stokes operators' variances. We also calculate the measures of entanglement for these states such as the Schmidt number and the logarithmic negativity. Our results show that the bright squeezed vacuum degree of polarization entanglement scales as the mean photon number squared. We analyze the applicability of an operational analog of the Schmidt number.

Probing guided modes in a monolayer colloidal crystal on a flat metal film

Sergei G. Romanov, Nicolas Vogel, Karina Bley, Katharina Landfester, Clemens K. Weiss, Sergej Orlov, Alexander V. Korovin, Gennady P. Chuiko, Alois Regensburger, et al.

Two-dimensional slab hybrid metal-dielectric photonic crystals, which are prepared by assembling polymer colloidal spheres into closely packed monolayers of hexagonal symmetry on a gold-coated glass substrate, show an improved confinement of light compared with a colloidal monolayer on a glass substrate. We demonstrated that the optical response of such hybrid crystals consists of diffractively coupled waveguiding modes, Fabry-Perot resonances, and Mie resonances. Correspondingly, two major mechanisms, namely, band transport and hopping of localized excitations, participate in the in-plane light transport in such hybrid crystals.

Entanglement of Gaussian states and the applicability to quantum key distribution over fading channels

Vladyslav C. Usenko, Bettina Heim, Christian Peuntinger, Christoffer Wittmann, Christoph Marquardt, Gerd Leuchs, Radim Filip

Entanglement properties of Gaussian states of light as well as the security of continuous variable quantum key distribution with Gaussian states in free-space fading channels are studied. These qualities are shown to be sensitive to the statistical properties of the transmittance distribution in the cases when entanglement is strong or when channel excess noise is present. Fading, i.e. transmission fluctuations, caused by beam wandering due to atmospheric turbulence, is a frequent challenge in free-space communication. We introduce a method of fading discrimination and subsequent post-selection of the corresponding sub-states and show that it can improve the entanglement resource and restore the security of the key distribution over a realistic fading link. Furthermore, the optimal post-selection strategy in combination with an optimized entangled resource is shown to drastically increase the protocol's robustness to excess noise, which is confirmed for experimentally measured fading channel characteristics. The stability of the result against finite data ensemble size and imperfect channel estimation is also addressed.

Collecting more than half the fluorescence photons from a single ion

Robert Maiwald, Andrea Golla, Martin Fischer, Marianne Bader, Simon Heugel, Benoit Chalopin, Markus Sondermann, Gerd Leuchs

We demonstrate the trapping of a single ion in the focus of a deep parabolic mirror that covers 81% of the solid angle surrounding the ion. Accounting for the reflectivity of the mirror we infer a photon collection efficiency of 54.8% for our setup. The underlying experimentally detected maximum fluorescence rate is 1.91 x 10(6) s(-1) from a single Yb-174(+) ion, mainly limited by the quantum efficiency of our photon detector. Besides the high collection efficiency, the integration of an ion trap into a parabolic mirror is a key ingredient for efficient coupling of light to a single ion in free space.

Tools for detecting entanglement between different degrees of freedom in quadrature squeezed cylindrically polarized modes

C. Gabriel, A. Aiello, S. Berg-Johansen, Ch Marquardt, G. Leuchs

Quadrature squeezed cylindrically polarized modes contain entanglement not only in the polarization and spatial electric field variables but also between these two degrees of freedom [C. Gabriel et al., Phys. Rev. Lett. 106, 060502 (2011)]. In this paper we present tools to generate and detect this entanglement. Experimentally we demonstrate the generation of quadrature squeezing in cylindrically polarized modes by mode transforming a squeezed Gaussian mode. Specifically, -1.2dB +/- 0.1 dB of amplitude squeezing are achieved in the radially and azimuthally polarized mode. Furthermore, theoretically it is shown how the entanglement contained within these modes can be measured and how strong the quantum correlations are, depending on the measurement scheme.

Metallodielectric Hybrid Antennas for Ultrastrong Enhancement of Spontaneous Emission

Xue-Wen Chen, Mario Agio, Vahid Sandoghdar

We devise new optical antennas that reduce the excited-state radiative lifetimes of emitters to the order of 100 fs while maintaining quantum efficiencies of about 80% at a broadband operation. Here, we combine metallic nanoparticles with planar dielectric structures and exploit design strategies from plasmonic nanoantennas and concepts from cavity quantum electrodynamics to maximize the local density of states and minimize the nonradiative losses incurred by the metallic constituents. The proposed metallodielectric hybrid antennas promise an important impact on various fundamental and applied research fields, including photophysics, ultrafast plasmonics, bright single-photon sources, and Raman spectroscopy.

Spontaneous emission enhancement of a single molecule by a double-sphere nanoantenna across an interface

K-G. Lee, H. Eghlidi, X-W. Chen, A. Renn, S. Goetzinger, V. Sandoghdar

We report on two orders of magnitude reduction in the fluorescence lifetime when a single molecule placed in a thin film is surrounded by two gold nanospheres across the film interface. By attaching one of the gold particles to the end of a glass fiber tip, we could control the modification of the molecular fluorescence at will. We find a good agreement between our experimental data and the outcome of numerical calculations. (C) 2012 Optical Society of America

Radial mode dependence of optical beam shifts

N. Hermosa, Andrea Aiello, J. P. Woerdman

It is known that orbital angular momentum (OAM) couples the Goos-Hanchen and Imbert-Fedorov shifts. Here, we present the first study of these shifts when the OAM-endowed LG(l,p) beams have higher-order radial mode index (p > 0). We show theoretically and experimentally that the angular shifts are enhanced by p while the positional shifts are not. (C) 2012 Optical Society of America

Early stages of oxide growth in H-terminated silicon nanowires: determination of kinetic behavior and activation energy

Muhammad Y. Bashouti, Kasra Sardashti, Juergen Ristein, Silke H. Christiansen

Silicon nanowires (Si NWs) terminated with hydrogen atoms exhibit higher activation energy under ambient conditions than equivalent planar Si(100). The kinetics of sub-oxide formation in hydrogen-terminated Si NWs derived from the complementary XPS surface analysis attribute this difference to the Si-Si backbond and Si-H bond propagation which controls the process at lower temperatures (T < 200 degrees C). At high temperatures (T >= 200 degrees C), the activation energy was similar due to self-retarded oxidation. This finding offers the understanding of early-stage oxide growth that affects the conductance of the near-gap channels leading towards more efficient Si NW electronic devices.

Molecular Gating of Silicon Nanowire Field-Effect Transistors with Nonpolar Analytes

Yair Paska, Thomas Stelzner, Ossama Assad, Ulrike Tisch, Silke Christiansen, Hossam Haick

Silicon nanowire field-effect transistors (Si NW FETs) have been used as powerful sensors for chemical and biological species. The detection of polar species has been attributed to variations in the electric field at the conduction channel due to molecular gating with polar molecules. However, the detection of nonpolar analytes with Si NW FETs has not been well understood to date. In this paper, we experimentally study the detection of nonpolar species and model the detection process based on changes in the carrier mobility, voltage threshold, off-current, off-voltage, and subthreshold swing of the Si NW FET. We attribute the detection of the nonpolar species to molecular gating, due to two Indirect effects: (i) a change in the dielectric medium close to the Si NW surface and (ii) a change in the charged surface states at the functionality of the SI NW surface. The contribution of these two effects to the overall measured sensing signal is determined and discussed. The results provide a launching pad for real-world sensing applications, such as environmental monitoring, homeland security, food quality control, and medicine.

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

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.

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.

Characteristics of displaced single photons attained via higher order factorial moments

Kaisa Laiho, Malte Avenhaus, Christine Silberhorn

We investigate up to the fourth order normalized factorial moments of free-propagating and pulsed single photons displaced in phase space in a phase-averaged manner. Due to their loss independence, these moments offer expedient methods for quantum optical state characterization. We examine quantum features of the prepared displaced states, retrieve information on their photon-number content and study the reliability of the state reconstruction method used.

Evaluation algorithms for multistep measurement of spatially varying linear polarization and phase

Andreas Berger, Vanusch Nercissian, Klaus Mantel, Irina Harder

Optical components manipulating both polarization and phase of wave fields find more and more applications in today's optical systems. In particular, the polarization orientation may vary across the aperture. New measurement techniques and evaluation algorithms are needed to simultaneously characterize the properties of such elements. In this Letter, a general measurement algorithm for locally linear polarization distributions is presented, extending the methods of phase shifting interferometry to the simultaneous determination of polarization and phase. A class of evaluation algorithms is derived, and some example algorithms are described and tested for their resilience against systematic and stochastic stepping errors. (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

The 8th International Comparison of Absolute Gravimeters 2009: the first Key Comparison (CCM.G-K1) in the field of absolute gravimetry

Z. Jiang, V. Palinkas, F. E. Arias, J. Liard, S. Merlet, H. Wilmes, L. Vitushkin, L. Robertsson, L. Tisserand, et al.

The 8th International Comparison of Absolute Gravimeters (ICAG2009) took place at the headquarters of the International Bureau of Weights and Measures (BIPM) from September to October 2009. It was the first ICAG organized as a key comparison in the framework of the CIPM Mutual Recognition Arrangement of the International Committee for Weights and Measures (CIPM MRA) (CIPM 1999). ICAG2009 was composed of a Key Comparison (KC) as defined by the CIPM MRA, organized by the Consultative Committee for Mass and Related Quantities (CCM) and designated as CCM.G-K1. Participating gravimeters and their operators came from national metrology institutes (NMIs) or their designated institutes (DIs) as defined by the CIPM MRA. A Pilot Study (PS) was run in parallel in order to include gravimeters and their operators from other institutes which, while not signatories of the CIPM MRA, nevertheless play important roles in international gravimetry measurements. The aim of the CIPM MRA is to have international acceptance of the measurement capabilities of the participating institutes in various fields of metrology. The results of CCM.G-K1 thus constitute an accurate and consistent gravity reference traceable to the SI (International System of Units), which can be used as the global basis for geodetic, geophysical and metrological observations of gravity. The measurements performed afterwards by the KC participants can be referred to the international metrological reference, i.e. they are SI-traceable. The ICAG2009 was complemented by a number of associated measurements: the Relative Gravity Campaign (RGC2009), high-precision levelling and an accurate gravity survey in support of the BIPM watt balance project. The major measurements took place at the BIPM between July and October 2009. Altogether 24 institutes with 22 absolute gravimeters(32) and nine relative gravimeters participated in the ICAG/RGC campaign. This paper is focused on the absolute gravity campaign. We review the history of the ICAGs and present the organization, data processing and the final results of the ICAG2009. After almost thirty years of hosting eight successive ICAGs, the CIPM decided to transfer the responsibility for piloting the future ICAGs to NMIs, although maintaining a supervisory role through its Consultative Committee for Mass and Related Quantities.

Quadrature phase shift keying coherent state discrimination via a hybrid receiver

C. R. Mueller, M. A. Usuga, C. Wittmann, M. Takeoka, Ch Marquardt, U. L. Andersen, G. Leuchs

We propose and experimentally demonstrate a near-optimal discrimination scheme for the quadrature phase shift keying (QPSK) protocol. We show in theory that the performance of our hybrid scheme is superior to the standard scheme-heterodyne detection-for all signal amplitudes and underpin the predictions with our experimental results. Furthermore, our scheme provides hitherto the best performance in the domain of highly attenuated signals. The discrimination is composed of a quadrature measurement, a conditional displacement and a threshold detector.

Nanowire Arrays in Multicrystalline Silicon Thin Films on Glass: A Promising Material for Research and Applications in Nanotechnology

Sebastian W. Schmitt, Florian Schechtel, Daniel Amkreutz, Muhammad Bashouti, Sanjay K. Srivastava, Bjoern Hoffmann, Christel Dieker, Erdmann Spiecker, Bernd Rech, et al.

Silicon nanowires (SiNW) were formed on large grained, electron-beam crystallized silicon (Si) thin films of only 6 pm thickness on glass using nanosphere lithography (NSL) in combination with reactive ion etching (RIE). Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) studies revealed outstanding structural properties of this nanomaterial. It could be shown that SiNWs with entirely predetermined shapes including lengths, diameters and spacings and straight side walls form independently of their crystalline orientation and arrange in ordered arrays on glass. Furthermore, for the first time grain boundaries could be observed in individual, straightly etched SiNWs. After heat treatment an electronic grade surface quality of the SiNWs could be shown by X-ray photoelectron spectroscopy (XPS). Integrating sphere measurements show that SiNW patterning of the multicrystalline Si (mc-Si) starting thin film on glass substantially increases absorption and reduces reflection, as being desired for an application in thin film photovoltaics (PV). The multicrystalline SiNWs directly mark a starting point for research not only in PV but also in other areas like nanoelectronics, surface functionalization, and nanomechanics.

Informational completeness of continuous-variable measurements

D. Sych, J. Rehacek, Z. Hradil, G. Leuchs, L. L. Sanchez-Soto

We justify that homodyne tomography turns out to be informationally complete when the number of independent quadrature measurements is equal to the dimension of the density matrix in the Fock representation. Using this as our thread, we examine the completeness of other schemes when continuous-variable observations are truncated to discrete finite-dimensional subspaces.

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.

On the use of a Continuous Wave Nd:YAG Laser in the Generation of Random Numbers

M. Sabuncu

The Nd:YAG laser is a solid state laser that has many applications in the industry, science and medicine. We in this paper use a continuous wave (CW) Nd:YAG laser emitting light at 1064 nm to generate random numbers. We first give a recipe of an experimental setup that is capable of measuring and recording the vacuum fluctuations with the help of our cw Nd:YAG laser. The basic scheme simply involves the Nd:YAG laser, detectors and some radio frequency electronic components. The laser source used in the demonstration provides us with a quantum noise limited CW at a radio frequency sideband of 14 MHz. The final data obtained after appropriate electronic signal processing are digitized voltage fluctuations that correspond to the vacuum fluctuations in the optical domain. The data randomly lies above or below zero due to its quantum mechanical probabilistic nature. We assign a '1' to the data that is positive and a '0' to the data that is negative, hence we are able to generate a random bit sequence by using a cw Nd:YAG laser. This result adds yet another interesting application to the many number of broad applications of the Nd:YAG laser system.

Generation of a wave packet tailored to efficient free space excitation of a single atom

A. Golla, B. Chalopin, M. Bader, I. Harder, K. Mantel, R. Maiwald, N. Lindlein, M. Sondermann, G. Leuchs

We demonstrate the generation of an optical dipole wave suitable for the process of efficiently coupling single quanta of light and matter in free space. We employ a parabolic mirror for the conversion of a transverse beam mode to a focused dipole wave and show the required spatial and temporal shaping of the mode incident onto the mirror. The results include a proof of principle correction of the parabolic mirror's aberrations. For the application of exciting an atom with a single photon pulse, we demonstrate the creation of a suitable temporal pulse envelope. We infer coupling strengths of 89% and success probabilities of up to 87% for the application of exciting a single atom for the current experimental parameters.

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

Spectral properties of high-gain parametric down-conversion

K. Yu Spasibko, T. Sh Iskhakov, M. V. Chekhova

High-gain parametric down-conversion (PDC) is a source of bright squeezed vacuum, which is a macroscopic nonclassical state of light and a promising candidate for quantum information applications. Here we study its properties, such as the intensity spectral width and the spectral width of pairwise correlations. In agreement with the theory, we observe an increase in the spectral width by 27% compared with the low-gain PDC. Frequency cross- and auto-correlations are registered by measuring the reduction of noise in the difference of PDC intensities at various pairs of wavelengths. The noise reduction plots also demonstrate super-bunching typical for collinear frequency-degenerate PDC. (C) 2012 Optical Society of America

Far field spectrum in surface plasmon-assisted Young's double-slit interferometer

Bhaskar Kanseri, Hem Chandra Kandpal, Ramesh Chandra Budhani

We derive an expression for the resultant spectral density (spectrum) at a point in the far zone for the surface plasmons modulated Young's double-slit interference setup. The resultant spectral interference law has the same form as the standard spectral interference law for the scalar fields. This resemblance in turn provides a means for determination of the modified spectral degree of coherence at the two slits. The mathematical results also show that in an interesting situation when the field is incident at one slit only, the interference can still be observed at the observation plane. These findings are verified theoretically using a wide-band source, i.e. a black-body, having a spectrum following Planck's radiation law. (C) 2012 Elsevier B.V. All rights reserved.

Spatial Coherence and Optical Beam Shifts

W. Loffler, Andrea Aiello, J. P. Woerdman

A beam of light, reflected at a planar interface, does not follow perfectly the ray optics prediction. Diffractive corrections lead to beam shifts; the reflected beam is displaced (spatial Goos-Hanchen type shifts) and/or travels in a different direction (angular Imbert-Fedorov type shifts), as compared to geometric optics. How does the degree of spatial coherence of light influence these shifts? We investigate this issue first experimentally and find that the degree of spatial coherence influences the angular beam shifts, while the spatial beam shifts are unaffected.

Superbunched bright squeezed vacuum state

T. Sh. Iskhakov, A. M. Perez, K. Yu. Spasibko, M. V. Chekhova, G. Leuchs

In this Letter, we experimentally study the statistical properties of a bright squeezed vacuum state containing up to 10(13) photons per mode (10 mu J per pulse), produced via high-gain parametric down conversion (PDC). The effects of bunching and superbunching of photons were observed for a single-mode PDC radiation by second-order intensity correlation function measurements with analog detectors. (C) 2012 Optical Society of America

Limits on the deterministic creation of pure single-photon states using parametric down-conversion

Andreas Christ, Christine Silberhorn

Parametric down-conversion (PDC) is one of the most widely used methods to create pure single-photon states for quantum information applications. However, little attention has been paid to higher-order photon components in the PDC process, yet these ultimately limit the prospects of generating single photons of high quality. In this paper we investigate the impact of higher-order photon components and multiple frequency modes on the heralding rates and single-photon fidelities. This enables us to determine the limits of PDC sources for single-photon generation. Our results show that a perfectly single-mode PDC source in conjunction with a photon-number-resolving detector is ultimately capable of creating single-photon Fock states with unit fidelity and a maximal state creation probability of 25%. Hence, an array of 17 switched sources is required to build a deterministic (>99% emission probability) pure single-photon source.

Parity-time synthetic photonic lattices

Alois Regensburger, Christoph Bersch, Mohammad-Ali Miri, Georgy Onishchukov, Demetrios N. Christodoulides, Ulf Peschel

The development of new artificial structures and materials is today one of the major research challenges in optics. In most studies so far, the design of such structures has been based on the judicious manipulation of their refractive index properties. Recently, the prospect of simultaneously using gain and loss was suggested as a new way of achieving optical behaviour that is at present unattainable with standard arrangements. What facilitated these quests is the recently developed notion of 'parity-time symmetry' in optical systems, which allows a controlled interplay between gain and loss. Here we report the experimental observation of light transport in large-scale temporal lattices that are parity-time symmetric. In addition, we demonstrate that periodic structures respecting this symmetry can act as unidirectional invisible media when operated near their exceptional points. Our experimental results represent a step in the application of concepts from parity-time symmetry to a new generation of multifunctional optical devices and networks.

Nonreciprocal behavior and switching in optical couplers with longitudinally varying coupling coefficient

Truong X. Tran, Fabio Biancalana

We report the nonreciprocal behavior of an optical coupler consisting of two straight waveguides forming a small angle. An optical diode action is theoretically demonstrated when light is launched along opposite directions. The switching power is lower than the case of parallel waveguides with a constant coupling coefficient. (C) 2012 Optical Society of America

Influence of the contacting scheme in simulations of radial silicon nanorod solar cells

Felix Voigt, Thomas Stelzner, Silke H. Christiansen

Silicon nanorod solar cells were simulated using the Silvaco Technical Computer Aided Design (TCAD) software suite. For reasons of speed optimization the simulations were performed in cylinder coordinates taking advantage of the model's symmetry. Symmetric doping was assumed with a dopant density of 1018 cm(-3) in the p-type core and in the n-type shell, and the location of the pn-junction was chosen such that the space charge region was located adjacent to the shell surface. Two contact configurations were explored. In configuration A the cathode contact was wrapped around the semiconductor nanorod, while in configuration B the cathode was assumed just on top of the nanorod. In both cases the anode was located at the bottom of the rod. Cell efficiency was optimized with regard to rod radius and rod length. Optimization was performed in a three-step procedure consisting in radius optimization, length optimization and again radius optimization. A maximum in efficiency with respect to rod length L was visible in configuration A. leading to an optimum value of L=48 mu m. This maximum is explained by the combination of an increase of short-circuit current density J(sc) and a decrease of open-circuit voltage U-oc with L. In configuration B.J(sc) also increases with L. but U-oc stays rather constant and the maximum in efficiency only appears at very large values of L approximate to 12 mm. We restricted the rod length to L <= 100 mu m for further optimization, in order to stay in an experimentally feasible range. During the optimization of rod radius R in configuration A the open circuit voltage increased continuously, while short circuit current density stayed rather constant. This leads to an increase in efficiency with R, which only stops at very large radii, where R starts to be comparable with L. In configuration B efficiency is almost independent of R, provided that the radius is large enough to comprise a well-formed space charge region, here only a shallow maximum can be estimated. With the demand of rod lengths being smaller than 100 mu m, optimum parameters L = 48 mu m, R=32 mu m and L = 96 mu m, R=2 mu m were extracted for configuration A and B, respectively. (C) 2011 Elsevier B.V. All rights reserved.

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.

Optical Gap solitons and Truncated Nonlinear Bloch Waves in Temporal Lattices

Christoph Bersch, Georgy Onishchukov, Ulf Peschel

We experimentally demonstrate the formation and stable propagation of various types of discrete temporal solitons in an optical fiber system. Pulses interacting with a time-periodic potential and defocusing nonlinearity are shown to form gap solitons and nonlinear truncated Bloch waves. Multi-pulse solitons with defects, as well as novel structures composed of a strong soliton riding on a weaker truncated nonlinear Bloch wave are shown to propagate over up to eleven coupling lengths. The nonlinear dynamics of all pulse structures is monitored over the full propagation distance which provides detailed insight into the soliton dynamics.

Far-Field Imaging for Direct Visualization of Light Interferences in GaAs Nanowires

Rachel Grange, Gerald Broenstrup, Michael Kiometzis, Anton Sergeyev, Jessica Richter, Christian Leiterer, Wolfgang Fritzsche, Christoph Gutsche, Andrey Lysov, et al.

The optical and electrical characterization of nanostructures is crucial for all applications in nanophotonics. Particularly important is the knowledge of the optical near-field distribution for the design of future photonic devices. A common method to determine optical near-fields is scanning near-field optical microscopy (SNOM) which is slow and might distort the near-field. Here, we present a technique that permits sensing indirectly the infrared near-field in GaAs nanowires via its second-harmonic generated (SHG) signal utilizing a nonscanning far-field microscope. Using an incident light of 820 nm and the very short mean free path (16 nm) of the SHG signal in GaAs, we demonstrate a fast surface sensitive imaging technique without using a SNOM. We observe periodic intensity patterns in untapered and tapered GaAs nanowires that are attributed to the fundamental mode of a guided wave modulating the Mie-scattered incident light. The periodicity of the interferences permits to accurately determine the nanowires' radii by just using optical microscopy, i.e., without requiring electron microscopy.

Fission of solitons in continuous-wave supercontinuum

E. J. R. Kelleher, M. Erkintalo, J. C. Travers

We use numerical simulations to revisit the generation of fiber supercontinua pumped by partially coherent continuous-wave (CW) sources. Specifically, we show that intensity fluctuations characteristic of temporal partial coherence can be described as a stochastic train of high-order solitons, whose individual dynamics drive continuum formation. For sources with sufficiently low coherence, these solitons actually undergo fission rather than modulation instability, changing the nature of the CW supercontinuum evolution. (C) 2012 Optical Society of America

Visualizing the quantum interaction picture in phase space

Bahar Mehmani, Andrea Aiello

We present a graphical example of the interaction picture-time evolution. Our aim is to help students understand in a didactic manner the simplicity that this picture provides. Visualizing the interaction picture unveils its advantages, which are hidden behind the involved mathematics. Specifically, we show that the time evolution of a driven harmonic oscillator in the interaction picture corresponds to a local transformation of a phase space-reference frame into the one that is co-rotating with the Wigner function.

Generation and Characterization of Multimode Quantum Frequency Combs

Olivier Pinel, Pu Jian, Renne Medeiros de Araujo, Jinxia Feng, Benoit Chalopin, Claude Fabre, Nicolas Treps

Multimode nonclassical states of light are an essential resource in quantum computation with continuous variables, for example, in cluster state computation. We report in this Letter the first experimental evidence of a multimode nonclassical frequency comb in a femtosecond synchronously pumped optical parametric oscillator. In addition to a global reduction of its quantum intensity fluctuations, the system features quantum correlations between different parts of its frequency spectrum. This allows us to show that the frequency comb is composed of several uncorrelated eigenmodes having specific spectral shapes, two of them at least being squeezed, and to characterize their spectral shapes.

Development of Broadband Cavity Ring-Down Spectroscopy for Biomedical Diagnostics of Liquid Analytes

S. -S. Kiwanuka, T. K. Laurila, J. H. Frank, A. Esposito, K. Blomberg von der Geest, L. Pancheri, D. Stoppa, C. F. Kaminski

We present a spectrometer for sensitive absorption measurements in liquids across broad spectral bandwidths. The spectrometer combines the unique spectral properties of incoherent supercontinuum light sources with the advantages of cavity ring-down spectroscopy, which is a self-calibrating technique. A custom-built avalanche photodiode array is used for detection, permitting the simultaneous measurement of ring-down times for up to 64 different spectral components at nanosecond temporal resolution. The minimum detectable absorption coefficient was measured to be 3.2 x 10(-6) cm(-1) Hz at 527 nm. We show that the spectrometer is capable of recording spectral differences in trace levels of blood before and after hemolysis.

Polarization-Entangled Light Pulses of 10(5) Photons

Timur Sh. Iskhakov, Ivan N. Agafonov, Maria V. Chekhova, Gerd Leuchs

We experimentally demonstrate polarization entanglement for squeezed vacuum pulses containing more than 105 photons. We also study photon-number entanglement by calculating the Schmidt number and measuring its operational counterpart. Theoretically, our pulses are the more entangled the brighter they are. This promises important applications in quantum technologies, especially photonic quantum gates and quantum memories.

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.

Role of spatial coherence in Goos-Hanchen and Imbert-Fedorov shifts: reply to comment

Andrea Aiello, J. P. Woerdman

Wang and Liu [Opt. Lett. 37, 1056 (2012)] comment on our previous Letter [Opt. Lett. 36, 3151 (2011)] regarding the validity of the theory we presented. We reply to their comment here. (C) 2012 Optical Society of America

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.

A 2D Quantum Walk Simulation of Two-Particle Dynamics

Andreas Schreiber, Aurel Gabris, Peter P. Rohde, Kaisa Laiho, Martin Stefanak, Vaclav Potocek, Craig Hamilton, Igor Jex, Christine Silberhorn

Multidimensional quantum walks can exhibit highly nontrivial topological structure, providing a powerful tool for simulating quantum information and transport systems. We present a flexible implementation of a two-dimensional (2D) optical quantum walk on a lattice, demonstrating a scalable quantum walk on a nontrivial graph structure. We realized a coherent quantum walk over 12 steps and 169 positions by using an optical fiber network. With our broad spectrum of quantum coins, we were able to simulate the creation of entanglement in bipartite systems with conditioned interactions. Introducing dynamic control allowed for the investigation of effects such as strong nonlinearities or two-particle scattering. Our results illustrate the potential of quantum walks as a route for simulating and understanding complex quantum systems.

Enhanced Quantum Nonlinearities in a Two-Mode Optomechanical System

Max Ludwig, Amir H. Safavi-Naeini, Oskar Painter, Florian Marquardt

In cavity optomechanics, nanomechanical motion couples to a localized optical mode. The regime of single-photon strong coupling is reached when the optical shift induced by a single phonon becomes comparable to the cavity linewidth. We consider a setup in this regime comprising two optical modes and one mechanical mode. For mechanical frequencies nearly resonant to the optical level splitting, we find the photon-phonon and the photon-photon interactions to be significantly enhanced. In addition to dispersive phonon detection in a novel regime, this offers the prospect of optomechanical photon measurement. We study these quantum nondemolition detection processes using both analytical and numerical approaches.

Single-Photon Spectroscopy of a Single Molecule

Y. L. A. Rezus, S. G. Walt, R. Lettow, A. Renn, G. Zumofen, S. Goetzinger, V. Sandoghdar

Efficient interaction of light and matter at the ultimate limit of single photons and single emitters is of great interest from a fundamental point of view and for emerging applications in quantum engineering. However, the difficulty of generating single-photon streams with specific wavelengths, bandwidths, and power as well as the weak interaction probability of a single photon with an optical emitter pose a formidable challenge toward this goal. Here, we demonstrate a general approach based on the creation of single photons from a single emitter and their use for performing spectroscopy on a second emitter situated at a distance. While this first proof of principle realization uses organic molecules as emitters, the scheme is readily extendable to quantum dots and color centers. Our work ushers in a new line of experiments that provide access to the coherent and nonlinear couplings of few emitters and few propagating photons.

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

Nonlinear fiber-optic strain sensor based on four-wave mixing in microstructured optical fiber

Bobo Gu, Wu Yuan, Michael H. Frosz, A. Ping Zhang, Sailing He, Ole Bang

We demonstrate a nonlinear fiber-optic strain sensor, which uses the shifts of four-wave mixing Stokes and anti-Stokes peaks caused by the strain-induced changes in the structure and refractive index of a microstructured optical fiber. The sensor thus uses the inherent nonlinearity of the fiber and does not require any advanced post-processing of the fiber. Strain sensitivity of -0.23 pm/mu epsilon is achieved experimentally and numerical simulations reveal that for the present fiber the sensitivity can be increased to -4.46 pm/mu epsilon by optimizing the pump wavelength and power. (C) 2012 Optical Society of America

Resonant metamaterials for contrast enhancement in optical lithography

Sabine Dobmann, Dzmitry Shyroki, Peter Banzer, Andreas Erdmann, Ulf Peschel

The transmission through ultra-thin metal films is noticeable and thus limits their potential for the formation of lithographic masks. By sub-wavelength patterning of a metal film with a post structure, a resonant metamaterial is formed, which can effectively suppress the transmission. Measurements as well as calculations identify the width of the metal islands as a critical geometrical feature. Hence, the extraordinarily low transmission effect can be explained by the resonant response of single scatterers known as Localized Surface Plasmon Resonances (LSPR). A potential application of this suppressed transmission effect to thin metal masks in optical lithography is experimentally investigated. (C) 2012 Optical Society of America

Optomechanical circuits for nanomechanical continuous variable quantum state processing

Michael Schmidt, Max Ludwig, Florian Marquardt

We propose and analyze a nanomechanical architecture where light is used to perform linear quantum operations on a set of many vibrational modes. Suitable amplitude modulation of a single laser beam is shown to generate squeezing, entanglement and state transfer between modes that are selected according to their mechanical oscillation frequency. Current optomechanical devices based on photonic crystals, as well as other systems with sufficient control over multiple mechanical modes, may provide a platform for realizing this scheme.

Optomechanical cooling of levitated spheres with doubly resonant fields

G. A. T. Pender, P. F. Barker, Florian Marquardt, J. Millen, T. S. Monteiro

Optomechanical cooling of levitated dielectric particles represents a promising new approach in the quest to cool small mechanical resonators toward their quantum ground state. We investigate two-mode cooling of levitated nanospheres in a self-trapping regime. We identify a structure of overlapping, multiple cooling resonances and strong cooling even when one mode is blue-detuned. We show that the best regimes occur when both optical fields cooperatively cool and trap the nanosphere, where cooling rates are over an order of magnitude faster compared to corresponding single-resonance cooling rates.

Tunable modulational instability sidebands via parametric resonance in periodically tapered optical fibers

Andrea Armaroli, Fabio Biancalana

We analyze the modulation instability induced by periodic variations of group velocity dispersion and nonlinearity in optical fibers, which may be interpreted as an analogue of the well-known parametric resonance in mechanics. We derive accurate analytical estimates of resonant detuning, maximum gain and instability margins, significantly improving on previous literature on the subject. We also design a periodically tapered photonic crystal fiber, in order to achieve narrow instability sidebands at a detuning of 35 THz, above the Raman maximum gain peak of fused silica. The wide tunability of the resonant peaks by variations of the tapering period and depth will allow to implement sources of correlated photon pairs which are far-detuned from the input pump wavelength, with important applications in quantum optics. (C) 2012 Optical Society of America

Numerical Solution and Experimental Validation of the Drawing Process of Six-Hole Optical Fibers Including the Effects of Inner Pressure and Surface Tension

Giovanni Luzi, Philipp Epple, Michael Scharrer, Ken Fujimoto, Cornelia Rauh, Antonio Delgado

Microstructured optical fibers (MOFs) achieve their desired performance via a pattern of holes that run trough the whole length of the fiber. The variation of the hole pattern allows the production of a variety of optical effects. However, the cross-sectional hole structure can be different from that designed in the preform, due to the combined effects of surface tension and internal pressure. The present paper focuses on the comparison between experiments and numerical calculation of a six hole-optical fiber taking into account the effects of surface tension and internal hole-pressure, since those are of essential importance during drawing. It is shown that the numerical computations deliver reliable results for practical applications and can be used as a predictive tool for fiber development, as long as the inner pressure or the temperature do not exceed too high values.

Field-Effect Transistors Based on Silicon Nanowire Arrays: Effect of the Good and the Bad Silicon Nanowires

Bin Wang, Thomas Stelzner, Rawi Dirawi, Ossama Assad, Nisreen Shehada, Silke Christiansen, Hossam Haick

Aligned arrays of silicon nanowires (aa-Si NWs) allow the exploitation of Si NWs in a scalable way. Previous studies explored the influence of the Si NWs' number, doping density, and diameter on the related electrical performance. Nevertheless, the origin of the observed effects still not fully understood. Here, we aim to provide an understanding on the effect of channel number on the fundamental parameters of aa-Si NW field effect transistors (FETs). Toward this end, we have fabricated and characterized 87 FET devices with varied number of Si NWs, which were grown by chemical vapor deposition with gold catalyst. The results show that FETs with Si NWs above a threshold number (n > 80) exhibit better device uniformity, but generally lower device performance, than FETs with lower number of Si NWs (3 <= n < 80). Complementary analysis indicates that the obtained discrepancies could be explained by a weighted contribution of two main groups of Si NWs: (i) a group of gold free Si NWs that exhibit high and uniform electrical characteristics; and (ii) a group of gold doped Si NWs that exhibit inferior electrical characteristics. These findings are validated by a binomial model that consider the aa-Si NW FETs via a weighted combination of FETs of individual Si NWs. Overall, the obtained results suggest that the criterions used currently for evaluating the device performance (e.g., uniform diameter, length, and shape of Si NWs) do not necessarily guarantee uniform or satisfying electrical characteristics, raising the need for new growth processes and/or advanced sorting techniques of electrically homogeneous Si NWs.

Measurement of two-mode squeezing with photon number resolving multipixel detectors

Dmitry A. Kalashnikov, Si-Hui Tan, Timur Sh. Iskhakov, Maria V. Chekhova, Leonid A. Krivitsky

The measurement of the two-mode squeezed vacuum generated in an optical parametric amplifier (OPA) was performed with photon number resolving multipixel photon counters (MPPCs). Implementation of the MPPCs allows for the observation of noise reduction in a broad dynamic range of the OPA gain, which is inaccessible with standard single photon avalanche photodetectors. (c) 2012 Optical Society of America

Goos-Hanchen and Imbert-Fedorov shifts: a novel perspective

Andrea Aiello

When a beam of light is reflected by a smooth surface its behavior deviates from geometrical optics predictions. Such deviations are quantified by the so-called spatial and angular Goos-Hanchen (GH) and Imbert-Fedorov (IF) shifts of the reflected beam. These shifts depend upon the shape of the incident beam, its polarization and on the material composition of the reflecting surface. In this paper we suggest a novel approach that allows one to unambiguously isolate the beam-shape dependent aspects of GH and IF shifts. We show that this separation is possible as a result of some universal features of shifted distribution functions which are presented and discussed.

Quantum polarization tomography of bright squeezed light

C. R. Mueller, B. Stoklasa, C. Peuntinger, C. Gabriel, J. Rehacek, Z. Hradil, A. B. Klimov, G. Leuchs, Ch Marquardt, et al.

We reconstruct the polarization sector of a bright polarization squeezed beam starting from a complete set of Stokes measurements. Given the symmetry that underlies the polarization structure of quantum fields, we use the unique SU(2) Wigner distribution to represent states. In the limit of localized bright states, the Wigner function can be approximated by an inverse three-dimensional Radon transform. We compare this direct reconstruction with the results of a maximum likelihood estimation, thus finding excellent agreement.

Homodyne detection for atmosphere channels

A. A. Semenov, F. Toeppel, D. Yu Vasylyev, H. V. Gomonay, W. Vogel

We give a systematic theoretical description of homodyne detection in the case where both the signal and the local oscillator pass through the turbulent atmosphere. Imperfect knowledge of the local-oscillator amplitude is effectively included in a noisy density operator, leading to postprocessing noise. Alternatively, we propose a technique with monitored transmission coefficient of the atmosphere, which is free of postprocessing noise.

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.

Three-dimensional quantum polarization tomography of macroscopic Bell states

Bhaskar Kanseri, Timur Iskhakov, Ivan Agafonov, Maria Chekhova, Gerd Leuchs

The polarization properties of macroscopic Bell states are characterized using three-dimensional quantum polarization tomography. This method utilizes three-dimensional (3D) inverse Radon transform to reconstruct the polarization quasiprobability distribution function of a state from the probability distributions measured for various Stokes observables. The reconstructed 3D distributions obtained for the macroscopic Bell states are compared with those obtained for a coherent state with the same mean photon number. The results demonstrate squeezing in one or more Stokes observables.

Grain structure of thin-film silicon by zone melting recrystallization on SiC base layer

T. Kunz, M. T. Hessmann, R. Auer, A. Bochmann, S. Christiansen, C. J. Brabec

The grain structure of thin-film silicon layers obtained by chemical vapor deposition and zone melting recrystallization (ZMR) on SiC barrier layers, as developed for thin-film solar cells, have been investigated by electron backscader diffraction (EBSD). The occurrence of subgrain boundaries was checked by defect etching. Twin boundaries form 1 to 100 mu m wide stripes, which are nearly parallel to the scan direction of ZMR. We find that stripe structure and the dominant grain orientations differ significantly from previously published ZMR layers grown on SiO2 surface. In a comprehensive model it is shown how the twinning structure and the dominant grain orientation can be related to the growth kinetics. The electronic activity of the defects was measured by electron beam induced current (EBiC). Contrary to other defects, the twin boundaries show no enhanced recombination. Therefore the found growth regime has potential advantages with respect of electronic properties of the layers. (C) 2012 Elsevier B.V. All rights reserved.

Probabilistic cloning of coherent states without a phase reference

Christian R. Mueller, Christoffer Wittmann, Petr Marek, Radim Filip, Christoph Marquardt, Gerd Leuchs, Ulrik L. Andersen

We present a probabilistic cloning scheme operating independently of any phase reference. The scheme is based solely on a phase-randomized displacement and photon counting, omitting the need for nonclassical resources and nonlinear materials. In an experimental implementation, we employ the scheme to clone coherent states from a phase covariant alphabet and demonstrate that the cloner is capable of outperforming the hitherto best-performing deterministic scheme. An analysis of the covariances between the output states shows that uncorrelated clones can be approached asymptotically. This simultaneously demonstrates how the effect of loss on coherent states can be compensated via noiseless preamplification.

Rare-earth ion doped TeO2 and GeO2 glasses as laser materials

Animesh Jha, Billy Richards, Gin Jose, Toney Teddy-Fernandez, Purushottam Joshi, Xin Jiang, Joris Lousteau

Germanium oxide (GeO2) and tellurium oxide (TeO2) based glasses are classed as the heavy metal oxide glasses, with phonon energies ranging between 740 cm(-1) and 880 cm(-1). These two types of glasses exhibit unique combinations of optical and spectroscopic properties, together with their attractive environmental resistance and mechanical properties. Engineering such a combination of structural, optical and spectroscopic properties is only feasible as a result of structural variability in these two types of glasses, since more than one structural units (TeO4 bi-pyramid, TeO3 trigonal pyramid, and TeO3+delta polyhedra) in tellurite and (GeO4 tetrahedron, GeO3 octahedron) in GeO2 based glasses may exist, depending on composition. The presence of multiple structural moities creates a range of dipole environments which is ideal for engineering broad spectral bandwidth rare-earth ion doped photonic device materials, suitable for laser and amplifier devices. Tellurite glasses were discovered in 1952, but remained virtually unknown to materials and device engineers until 1994 when unusual spectroscopic, nonlinear and dispersion properties of alkali and alkaline earth modified tellurite glasses and fibres were reported. Detailed spectroscopic analysis of Pr3+, Nd3+, Er3+ and Tm3+ doped tellurite glasses revealed its potential for laser and amplifier devices for optical communication wavelengths. This review summarises the thermal and viscosity properties of tellurite and germanate glasses for fibre fabrication and compares the linear loss for near and mid-IR device engineering. The aspects of glass preform fabrication for fibre engineering is discussed by emphasising the raw materials processing with casting of preforms and fibre fabrication. The spectroscopic properties of tellurite and germanate glasses have been analysed with special emphasis on oscillator strength and radiative rate characteristics for visible, near IR and mid-IR emission. The review also compares the latest results in the engineering of lasers and amplifiers, based on fibres for optical communication and mid-IR. The achievements in the areas of near-IR waveguide and mid-IR bulk glass, fibre, and waveguide lasers are discussed. The latest landmark results in mode-locked 2 mu m bulk glass lasers sets the precedence for engineering nonlinear and other laser devices for accessing the inaccessible parts of the mid-IR spectrum and discovering new applications for the future. (c) 2012 Elsevier Ltd. All rights reserved.

Resonant modulational instability and self-induced transmission effects in semiconductors: Maxwell-Bloch formalism

Oleksii A. Smyrnov, Fabio Biancalana

The nonlinear optical properties of semiconductors near an excitonic resonance are investigated theoretically by using the macroscopic J model [Ostreich and Knorr, Phys. Rev. B 48, 17811 (1993); 50, 5717 (1994)] based on the microscopic semiconductor Bloch equations. These nonlinear properties cause modulational instability of long light pulses with large gain and give rise to a self-induced transmission of short light pulses in a semiconductor. By an example of the latter well-studied effect, the validity of the used macroscopic model is demonstrated, and good agreement is found with both existing theoretical and experimental results.

Hybrid quantum repeater with encoding

Nadja K. Bernardes, Peter van Loock

We present an encoded hybrid quantum repeater scheme using qubit-repetition and Calderbank-Shor-Steane codes. For the case of repetition codes, we propose an explicit implementation of the quantum error-correction protocol. Moreover, we analyze the entangled-pair distribution rate for the hybrid quantum repeater with encoding and we clearly identify trade-offs between the efficiency of the codes, the memory-decoherence time, and the local gate errors. Finally, we show that in the presence of reasonable imperfections our system can achieve rates of roughly 24 Hz per memory for 20 km repeater spacing, a final distance of 1280 km, and final fidelity of about 0.95.

Novel Discovery of Silicon

Vladimir Sivakov, Silke Christiansen

The silicon based technologies are certainly favored because of material abundance and non-toxicity at a high level of materials control and understanding together with a huge industrial infrastructure to account for low production/processing costs and high production yields. This article comprises a comprehensive review of new observation in silicon material grown by top-down or bottom-up technologies which can be useful for the future optoelectronic applications, especially in energy sector.

Narrow-bandwidth high-order harmonics driven by long-duration hot spots

Maxim Kozlov, Ofer Kfir, Avner Fleischer, Alex Kaplan, Tal Carmon, Harald G. L. Schwefel, Guy Bartal, Oren Cohen

We predict and investigate the emission of high-order harmonics by atoms that cross intense laser hot spots that last for a nanosecond or longer. An atom that moves through a nanometer-scale hot spot at characteristic thermal velocity can emit high-order harmonics in a similar fashion to an atom that is irradiated by a short-duration (picosecond-scale) laser pulse. We analyze the collective emission from a thermal gas and from a jet of atoms. In both cases, the line shape of a high-order harmonic exhibits a narrow spike with spectral width that is determined by the bandwidth of the driving laser. Finally, we discuss a scheme for producing long-duration laser hot spots with intensity in the range of the intensity threshold for high-harmonic generation. In the proposed scheme, the hot spot is produced by a long laser pulse that is consecutively coupled to a high-quality micro-resonator and a metallic nano-antenna. This system may be used for generating ultra-narrow bandwidth extreme-ultraviolet radiation through frequency up-conversion of a low-cost compact pump laser.

Analytical expansion of highly focused vector beams into vector spherical harmonics and its application to Mie scattering

S. Orlov, U. Peschel, T. Bauer, P. Banzer

The analytical expansion of linearly, azimuthally, and radially polarized rigorous beam-type solutions of Maxwell's equations into vector spherical harmonics (VSHs) is presented. We report on the dominance of higher order multipoles in highly focused radially and azimuthally polarized beams compared to linearly polarized beams under similar conditions. Furthermore, we theoretically investigate a scenario in which highly focused azimuthally and radially polarized beams interact with a linear polarizer placed in the focal plane and expand the resulting fields into VSHs. The generalized Mie theory is used afterwards to investigate the scattering of the studied beams off a spherical gold nanoparticle.

Quantum stabilizer codes embedding qubits into qudits

Carlo Cafaro, Federico Maiolini, Stefano Mancini

We study, by means of the stabilizer formalism, a quantum error correcting code which is alternative to the standard block codes since it embeds a qubit into a qudit. The code exploits the noncommutative geometry of discrete phase space to protect the qubit against both amplitude and phase errors. The performance of this code is evaluated on Weyl channels by means of the entanglement fidelity as a function of the error probability. A comparison with standard block codes, like five- and seven-qubit stabilizer codes, shows its superiority.

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

Observation of Orbital Angular Momentum Sidebands due to Optical Reflection

W. Loffler, Andrea Aiello, J. P. Woerdman

We investigate how the orbital angular momentum of a paraxial light beam is affected upon reflection at a planar interface. Theoretically, the unavoidable angular spread of the beam leads to orbital angular momentum sidebands, which are found to be already significant for a modest beam spread (0.05). In analogy to the polarization Fresnel coefficients, we develop an analytical theory based upon spatial Fresnel coefficients; this allows a straightforward prediction of the strength of the sidebands. We confirm this by experiment and numerical simulation.

Quantum Signatures of the Optomechanical Instability

Jiang Qian, A. A. Clerk, K. Hammerer, Florian Marquardt

In the past few years, coupling strengths between light and mechanical motion in optomechanical setups have improved by orders of magnitude. Here we show that, in the standard setup under continuous laser illumination, the steady state of the mechanical oscillator can develop a nonclassical, strongly negative Wigner density if the optomechanical coupling is comparable to or larger than the optical decay rate and the mechanical frequency. Because of its robustness, such a Wigner density can be mapped using optical homodyne tomography. This feature is observed near the onset of the instability towards self-induced oscillations. We show that there are also distinct signatures in the photon-photon correlation function g((2))(t) in that regime, including oscillations decaying on a time scale not only much longer than the optical cavity decay time but even longer than the mechanical decay time.

Intrinsically disordered proteins as molecular shields

Sohini Chakrabortee, Rashmi Tripathi, Matthew Watson, Gabriele S. Kaminski Schierle, Davy P. Kurniawan, Clemens F. Kaminski, Michael J. Wise, Alan Tunnacliffe

The broad family of LEA proteins are intrinsically disordered proteins (IDPs) with several potential roles in desiccation tolerance, or anhydrobiosis, one of which is to limit desiccation-induced aggregation of cellular proteins. We show here that this activity, termed molecular shield function, is distinct from that of a classical molecular chaperone, such as HSP70 - while HSP70 reduces aggregation of citrate synthase (CS) on heating, two LEA proteins, a nematode group 3 protein, AavLEA1, and a plant group 1 protein, Em, do not; conversely, the LEA proteins reduce CS aggregation on desiccation, while HSP70 lacks this ability. There are also differences in interaction with client proteins - HSP70 can be co-immunoprecipitated with a polyglutamine-containing client, consistent with tight complex formation, whereas the LEA proteins can not, although a loose interaction is observed by Forster resonance energy transfer. In a further exploration of molecular shield function, we demonstrate that synthetic polysaccharides, like LEA proteins, are able to reduce desiccation-induced aggregation of a water-soluble proteome, consistent with a steric interference model of anti-aggregation activity. If molecular shields operate by reducing intermolecular cohesion rates, they should not protect against intramolecular protein damage. This was tested using the monomeric red fluorescent protein, mCherry, which does not undergo aggregation on drying, but the absorbance and emission spectra of its intrinsic fluorophore are dramatically reduced, indicative of intramolecular conformational changes. As expected, these changes are not prevented by AavLEA1, except for a slight protection at high molar ratios, and an AavLEA1-mCherry fusion protein is damaged to the same extent as mCherry alone. A recent hypothesis proposed that proteomes from desiccation-tolerant species contain a higher degree of disorder than intolerant examples, and that this might provide greater intrinsic stability, but a bioinformatics survey does not support this, since there are no significant differences in the degree of disorder between desiccation tolerant and intolerant species. It seems clear therefore that molecular shield function is largely an intermolecular activity implemented by specialist IDPs, distinct from molecular chaperones, but with a role in proteostasis.

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

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.

Quantum polarization characterization and tomography

J. Soederholm, G. Bjoerk, A. B. Klimov, L. L. Sanchez-Soto, G. Leuchs

We present a complete polarization characterization of any quantum state of two orthogonal polarization modes and give a systematic measurement procedure to collect the necessary data. Full characterization requires measurements of the photon number in both modes and linear optics. In the case where only the photon-number difference can be determined, a limited but useful characterization is obtained. The characteristic Stokes moment profiles are given for several common quantum states.

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

Central-moment description of polarization for quantum states of light

G. Bjork, J. Soederholm, Y-S Kim, Y-S Ra, H-T Lim, C. Kothe, Y-H Kim, L. L. Sanchez-Soto, A. B. Klimov

We present a moment expansion for the systematic characterization of the polarization properties of quantum states of light. Specifically, we link the method to the measurements of the Stokes operator in different directions on the Poincare sphere and provide a scheme for polarization tomography without resorting to full-state tomography. We apply these ideas to the experimental first-and second-order polarization characterization of some two-photon quantum states. In addition, we show that there are classes of states whose polarization characteristics are dominated not by their first-order moments (i.e., the Stokes vector) but by higher-order polarization moments.

Dipole pulse theory: Maximizing the field amplitude from 4 pi focused laser pulses

Ivan Gonoskov, Andrea Aiello, Simon Heugel, Gerd Leuchs

We present a class of exact nonstationary solutions of Maxwell equations in vacuum from dipole pulse theory: electric and magnetic dipole pulses. These solutions can provide for a very efficient focusing of electromagnetic field and can be generated by 4 pi focusing systems, such as parabolic mirrors, by using radially polarized laser pulses with a suitable amplitude profile. The particular cases of a monochromatic dipole wave and a short dipole pulse with either quasi-Gaussian or Gaussian envelopes in the far-field region are analyzed and compared in detail. As a result, we propose how to increase the maximum field amplitude in the focus by properly shaping the temporal profile of the input laser pulses with given main wavelength and peak power.

Exponentially enhanced quantum communication rate by multiplexing continuous-variable teleportation

Andreas Christ, Cosmo Lupo, Christine Silberhorn

A major challenge of today's quantum communication systems lies in the transmission of quantum information with high rates over long distances in the presence of unavoidable losses. Thereby the achievable quantum communication rate is fundamentally limited by the amount of energy that can be transmitted per use of the channel. It is hence vital to develop quantum communication protocols that encode quantum information as energy efficiently as possible. To this aim we investigate continuous-variable quantum teleportation as a method of distributing quantum information. We explore the possibility to encode information on multiple optical modes and derive upper and lower bounds on the achievable quantum channel capacities. This analysis enables us to benchmark single-mode versus multi-mode entanglement resources. Our research reveals that multiplexing does not only feature an enhanced energy efficiency, leading to an exponential increase in the achievable quantum communication rates in comparison to single-mode coding, but also yields an improved loss resilience. However, as reliable quantum information transfer is only achieved for entanglement values above a certain threshold a careful optimization of the number of coding modes is needed to obtain the optimal quantum channel capacity.

All photons are equal but some photons are more equal than others

Falk Toeppel, Andrea Aiello, Gerd Leuchs

Two photons are said to be identical if they are prepared in the same quantum state. Given the latter, there is a unique way to achieve this. Conversely, there are many different ways of preparing two non-identical photons: they may differ in frequency, polarization, amplitude, etc. Therefore, photon distinguishability depends upon the specific degree of freedom being varied. By means of a careful analysis of the coincidence probability distribution in a Hong-Ou-Mandel experiment, we can show that photon distinguishability can be actually quantified by the rate of distinguishability of photons, an experimentally measurable parameter that crucially depends on both the photon quantum state and the degree of freedom under control.

Polarization tomography of bright states of light

I. N. Agafonov, M. V. Chekhova, T. Sh. Iskhakov, B. Kanseri, G. Leuchs

Polarization quantum tomography is performed on 4-mode squeezed vacuum states. Three-dimensional polarization quasiprobability functions are obtained and compared to that of an equal intensity coherent state. These distributions clearly demonstrate the difference in the polarization properties of the considered states. The reconstruction quality of the coherent state distribution is also analyzed by comparing the theoretically and experimentally obtained shapes for this state.

The role of self-trapped excitons and defects in the formation of nanogratings in fused silica

Soeren Richter, Fei Jia, Matthias Heinrich, Sven Doering, Ulf Peschel, Andreas Tuennermann, Stefan Nolte

We investigate the role of self-trapped excitons (STEs) and defects in the formation of femtosecond laser pulse induced nanogratings (NGs) in fused silica. Our experiments reveal strongly enhanced NG formation for pulse separations up to the STE lifetime. In addition, the absorption spectra show that the weaker cumulative action of laser pulses for longer temporal separations is predominantly mediated by dangling-bond-type lattice defects that emerge from decaying STEs. (C) 2012 Optical Society of America

Studying free-space transmission statistics and improving free-space quantum key distribution in the turbulent atmosphere

C. Erven, B. Heim, E. Meyer-Scott, J. P. Bourgoin, R. Laflamme, G. Weihs, T. Jennewein

The statistical fluctuations in free-space links in the turbulent atmosphere are important for the distribution of quantum signals. To that end, we first study statistics generated by the turbulent atmosphere in an entanglement-based free-space quantum key distribution (QKD) system. Using the insights gained from this analysis, we study the effect of link fluctuations on the security and key generation rate of decoy state QKD concluding that it has minimal effect in the typical operating regimes. We then investigate the novel idea of using these turbulent fluctuations to our advantage in QKD experiments. We implement a signal-to-noise ratio filter (SNRF) in our QKD system which rejects measurements during periods of low transmission efficiency, where the measured quantum bit error rate is temporarily elevated. Using this, we increase the total secret key generated by the system from 78 009 bits to 97 678 bits, representing an increase of 25.2% in the final secure key rate, generated from the same raw signals. Lastly, we present simulations of a QKD exchange with an orbiting low earth orbit satellite and show that an SNRF will be extremely useful in such a situation, allowing many more passes to extract a secret key than would otherwise be possible.

Classifying, quantifying, and witnessing qudit-qumode hybrid entanglement

Karsten Kreis, Peter van Loock

Recently, several hybrid approaches to quantum information emerged which utilize both continuous-and discrete-variable methods and resources at the same time. In this work, we investigate the bipartite hybrid entanglement between a finite-dimensional, discrete-variable quantum system and an infinite-dimensional, continuous-variable quantum system. A classification scheme is presented leading to a distinction between pure hybrid entangled states, mixed hybrid entangled states (those effectively supported by an overall finite-dimensional Hilbert space), and so-called truly hybrid entangled states (those which cannot be described in an overall finite-dimensional Hilbert space). Examples for states of each regime are given and entanglement witnessing as well as quantification are discussed. In particular, using the channel map of a thermal photon noise channel, we find that true hybrid entanglement naturally occurs in physically important settings. Finally, extensions from bipartite to multipartite hybrid entanglement are considered.