New self-dual additive F4-codes constructed from circulant graphs

Markus Grassl, Masaaki Harada

In order to construct quantum [[n,0,d]] codes for (n,d)=(56,15), (57,15), (58,16), (63,16), (67,17), (70,18), (71,18), (79,19), (83,20), (87,20), (89,21), (95,20), we construct self-dual additive F4-codes of length n and minimum weight d from circulant graphs. The quantum codes with these parameters are constructed for the first time.<br>

Intensity-intensity correlations determined by dimension of quantum state in phase space: P-distribution

Gerd Leuchs, Roy J. Glauber, Wolfgang P Schleich

Strong, spectrally-tunable chirality in diffractive metasurfaces

Israel De Leon, Matthew J. Horton, Sebastian A. Schulz, Jeremy Upham, Peter Banzer, Robert W. Boyd

Metamaterials and metasurfaces provide a paradigm-changing approach for manipulating light. Their potential has been evinced by recent demonstrations of chiral responses much greater than those of natural materials. Here, we demonstrate theoretically and experimentally that the extrinsic chiral response of a metasurface can be dramatically enhanced by near-field diffraction effects. At the core of this phenomenon are lattice plasmon modes that respond selectively to the illumination’s polarization handedness. The metasurface exhibits sharp features in its circular dichroism spectra, which are tunable over a broad bandwidth by changing the illumination angle over a few degrees. Using this property, we demonstrate an ultra-thin circular-polarization sensitive spectral filter with a linewidth of ~10 nm, which can be dynamically tuned over a spectral range of 200 nm. Chiral diffractive metasurfaces, such as the one proposed here, open exciting possibilities for ultra-thin photonic devices with tunable, spin-controlled functionality.

Dimension of quantum phase space measured by photon correlations

Gerd Leuchs, Roy J. Glauber, Wolfgang P Schleich

Thermal characterisation of (bio)polymers with a temperature-stabilised whispering gallery mode microsensor

Eugene Kim, Matthew R. Foreman, Martin D. Baaske, Frank Vollmer

In this work, we theoretically and experimentally investigate the thermal response of whispering gallery mode microresonators operating in an aqueous glycerol medium. Thermal stabilisation of the resonance wavelength is realised by appropriate choice of the resonator radius and glycerol concentration, with a 60 fold reduction in thermal sensitivity demonstrated. Finally, we employ our stabilised system to determine the thermal dependence of the molecular polarisability of adsorbed bovine serum albumin molecules and the refractive index of dextran and poly(diallyldimethylammonium chloride) coatings. (C) 2015 AIP Publishing LLC.

Light microscopy: an ongoing contemporary revolution

Siegfried Weisenburger, Vahid Sandoghdar

The optical microscope is one of the oldest scientific instruments that is still used in forefront research. Ernst Abbe's nineteenth century formulation of the resolution limit in microscopy let generations of scientists believe that optical studies of individual molecules and resolving subwavelength structures were not feasible. The Nobel Prize in 2014 for super-resolution fluorescence microscopy marks a clear recognition that the old beliefs have to be revisited. In this article, we present a critical overview of various recent developments in optical microscopy. In addition to the popular super-resolution fluorescence methods, we discuss the prospects of various other techniques and imaging contrasts and consider some of the fundamental and practical challenges that lie ahead.

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

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

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

Curved singular beams for three-dimensional particle manipulation

Juanying Zhao, Ioannis D. Chremmos, Daohong Song, Demetrios N. Christodoulides, Nikolaos K. Efremidis, Zhigang Chen

For decades, singular beams carrying angular momentum have been a topic of considerable interest. Their intriguing applications are ubiquitous in a variety of fields, ranging from optical manipulation to photon entanglement, and from microscopy and coronagraphy to free-space communications, detection of rotating black holes, and even relativistic electrons and strong-field physics. In most applications, however, singular beams travel naturally along a straight line, expanding during linear propagation or breaking up in nonlinear media. Here, we design and demonstrate diffraction-resisting singular beams that travel along arbitrary trajectories in space. These curved beams not only maintain an invariant dark "hole" in the center but also preserve their angular momentum, exhibiting combined features of optical vortex, Bessel, and Airy beams. Furthermore, we observe three-dimensional spiraling of microparticles driven by such fine-shaped dynamical beams. Our findings may open up new avenues for shaped light in various applications.

3D Printing of Reduced Graphene Oxide Nanowires

Jung Hyun Kim, Won Suk Chang, Daeho Kim, Jong Ryul Yang, Joong Tark Han, Geon-Woong Lee, Ji Tae Kim, Seung Kwon Seol

Nondiffracting chirped Bessel waves in optical antiguides

Ioannis Chremmos, Melpomeni Giamalaki

Chirped Bessel waves are introduced as stable (nondiffracting) solutions of the paraxial wave equation in optical antiguides with a power-law radial variation in their index of refraction. Through numerical simulations, we investigate the propagation of apodized (finite-energy) versions of such waves, with or without vorticity, in antiguides with practical parameters. The new waves exhibit a remarkable resistance against the defocusing effect of the unstable index potentials, outperforming standard Gaussians with the same full width at half-maximum. The chirped profile persists even under conditions of eccentric launching or antiguide bending and is also capable of self-healing like standard diffraction-free beams in free space. (C) 2015 Optical Society of America

Superoscillatory field features with evanescent waves

Ioannis Chremmos, George Fikioris

We show how to obtain optical fields possessing superoscillatory features by superposing the evanescent tails of waves undergoing total internal reflection at a plane dielectric interface. In doing so, we essentially extend the definition of superoscillations to functions expressed as a continuum of slowly decaying exponentials, while not necessarily being bandlimited in the standard (Fourier) sense. We obtain such functions by complexifying the argument of standard bandlimited superoscillatory functions with a strictly positive spectrum. Combined with our recent method for superoscillations with arbitrary polynomial shape, the present approach offers flexibility for locally shaping the evanescent field near dielectric interfaces for applications such as particle or atom trapping. (C) 2015 Elsevier B.V. All rights reserved.

Microconstriction Arrays for High-Throughput Quantitative Measurements of Cell Mechanical Properties

Janina R. Lange, Julian Steinwachs, Thorsten Kolb, Lena A. Lautscham, Irina Harder, Graeme Whyte, Ben Fabry

We describe a method for quantifying the mechanical properties of cells in suspension with a microfluidic device consisting of a parallel array of micron-sized constrictions. Using a high-speed charge-coupled device camera, we measure the flow speed, cell deformation, and entry time into the constrictions of several hundred cells per minute during their passage through the device. From the flow speed and the occupation state of the microconstriction array with cells, the driving pressure across each constriction is continuously computed. Cell entry times into microconstrictions decrease with increased driving pressure and decreased cell size according to a power law. From this power-law relationship, the cell elasticity and fluidity can be estimated. When cells are treated with drugs that depolymerize or stabilize the cytoskeleton or the nucleus, elasticity and fluidity data from all treatments collapse onto a master curve. Power-law rheology and collapse onto a master curve are predicted by the theory of soft glassy materials and have been previously shown to describe the mechanical behavior of cells adhering to a substrate. Our finding that this theory also applies to cells in suspension provides the foundation for a quantitative high-throughput measurement of cell mechanical properties with microfluidic devices.

Magnon dark modes and gradient memory

Xufeng Zhang, Chang-Ling Zou, Na Zhu, Florian Marquardt, Liang Jiang, Hong X. Tang

Extensive efforts have been expended in developing hybrid quantum systems to overcome the short coherence time of superconducting circuits by introducing the naturally long-lived spin degree of freedom. Among all the possible materials, single-crystal yttrium iron garnet has shown up recently as a promising candidate for hybrid systems, and various highly coherent interactions, including strong and even ultrastrong coupling, have been demonstrated. One distinct advantage in these systems is that spins form well-defined magnon modes, which allows flexible and precise tuning. Here we demonstrate that by dissipation engineering, a non-Markovian interaction dynamics between the magnon and the microwave cavity photon can be achieved. Such a process enables us to build a magnon gradient memory to store information in the magnon dark modes, which decouple from the microwave cavity and thus preserve a long lifetime. Our findings provide a promising approach for developing long-lifetime, multimode quantum memories.

Topological Phases of Sound and Light

V. Peano, C. Brendel, M. Schmidt, F. Marquardt

Topological states of matter are particularly robust, since they exploit global features of a material's band structure. Topological states have already been observed for electrons, atoms, and photons. It is an outstanding challenge to create a Chern insulator of sound waves in the solid state. In this work, we propose an implementation based on cavity optomechanics in a photonic crystal. The topological properties of the sound waves can be wholly tuned in situ by adjusting the amplitude and frequency of a driving laser that controls the optomechanical interaction between light and sound. The resulting chiral, topologically protected phonon transport can be probed completely optically. Moreover, we identify a regime of strong mixing between photon and phonon excitations, which gives rise to a large set of different topological phases and offers an example of a Chern insulator produced from the interaction between two physically distinct particle species, photons and phonons.

Loss-compensated nonlinear modes and symmetry breaking in amplifying metal-dielectric-metal plasmonic couplers

Andrea Marini, Samudra Roy, Ajit Kumar, Fabio Biancalana

We theoretically investigate the propagation of surface plasmon polaritons in an amplifying plasmonic coupler (metal-amplifying dielectric-metal). We study the loss-compensated nonlinear stationary modes of the system by deriving coupled-mode equations for the optical amplitudes, predicting the existence of a mode with broken symmetry for gain values higher than a characteristic threshold. We analyze the stability of symmetric, antisymmetric, and nonsymmetric modes by solving the linearized system for small perturbations and by numerically integrating coupled-mode equations in propagation. We find that, while the antisymmetric mode stays always stable or marginally stable, the stability of symmetric and nonsymmetric modes is more involved.

Quantum-like nonseparable structures in optical beams

Andrea Aiello, Falk Toeppel, Christoph Marquardt, Elisabeth Giacobino, Gerd Leuchs

When two or more degrees of freedom become coupled in a physical system, a number of observables of the latter cannot be represented by mathematical expressions separable with respect to the different degrees of freedom. In recent years it appeared clear that these expressions may display the same mathematical structures exhibited by multiparty entangled states in quantum mechanics. In this work, we investigate the occurrence of such structures in optical beams, a phenomenon that is often referred to as 'classical entanglement'. We present a unified theory for different kinds of light beams exhibiting classical entanglement and we indicate several possible extensions of the concept. Our results clarify and shed new light upon the physics underlying this intriguing aspect of classical optics.

Contribution of third-harmonic and negative-frequency polarization fields to self-phase modulation in nonlinear media

Cristian Redondo Loures, Andrea Armaroli, Fabio Biancalana

We study the influence of third-harmonic generation (THG) and negative-frequency polarization terms in the self-phase modulation (SPM) of short and intense pulses in Kerr media. We find that THG induces additional symmetric lobes in the SPM process. The amplitude of these new sidebands are greatly enhanced by the contributions of the negative-frequency Kerr (NFK) term and the shock operator. We compare our theoretical predictions based on the analytical nonlinear phase with simulations carried out by using the full unidirectional pulse propagation equation (UPPE). (C) 2015 Optical Society of America

Engineering Nanoporous Iron(III) Oxide into an Effective Water Oxidation Electrode

Sandra Haschke, Yanlin Wu, Muhammad Bashouti, Silke Christiansen, Julien Bachmann

The geometric effects of nanostructuring a pure Fe2O3 surface on its electrochemical water oxidation performance at neutral pH were systematically explored. Atomic layer deposition was used to coat the inner walls of cylindrical "anodic" nanopores ordered in parallel arrays with a homogeneous Fe2O3 layer. Annealing and electrochemical treatments generated a roughened surface, as demonstrated by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy, the larger geometric area of which increases current densities. Combining these treatments with the "anodic" pore geometry delivered an effective increase in turnover by almost three orders of magnitude with respect to a smooth, planar Fe2O3 surface. However, the current density depended on the pore length in a non-monotonic manner. An optimal length was found that maximized turnover by equating the rate of transport in the electrolyte with that of charge transfer across the interface.

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

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

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

Projective filtering of the fundamental eigenmode from spatially multimode radiation

A. M. Perez, P. R. Sharapova, S. S. Straupe, F. M. Miatto, O. V. Tikhonova, G. Leuchs, M. V. Chekhova

Lossless filtering of a single coherent (Schmidt) mode from spatially multimode radiation is a problem crucial for optics in general and for quantum optics in particular. It becomes especially important in the case of nonclassical light that is fragile to optical losses. An example is bright squeezed vacuum generated via high-gain parametric down conversion or four-wave mixing. Its highly multiphoton and multimode structure offers a huge increase in the information capacity provided that each mode can be addressed separately. However, the nonclassical signature of bright squeezed vacuum, photon-number correlations, are highly susceptible to losses. Here we demonstrate lossless filtering of a single spatial Schmidt mode by projecting the spatial spectrum of bright squeezed vacuum on the eigenmode of a single-mode fiber. Moreover, we show that the first Schmidt mode can be captured by simply maximizing the fiber-coupled intensity. Importantly, the projection operation does not affect the targeted mode and leaves it usable for further applications.

Selective switching of individual multipole resonances in single dielectric nanoparticles

Pawel Wozniak, Peter Banzer, Gerd Leuchs

Following Mie theory, nanoparticles made of a high-refractive-index dielectric, such as silicon, exhibit a resonator-like behavior and very rich resonance spectra. Which electric or magnetic particle mode is excited depends on the wavelength, the refractive-index contrast relative to the environment, and the geometry of the nanoparticle itself. In addition, the spatial structure of the impinging light field plays a major role in the excitation of the nanoparticle resonances. Here, it is shown that, by tailoring the excitation field, individual multipole resonances can be selectively addressed while suppressing the excitation of other particle modes. This enables a detailed study of selected individual resonances without interference by the other modes.

Enhanced photovoltaics inspired by the fovea centralis

Gil Shalev, Sebastian W. Schmitt, Heidemarie Embrechts, Gerald Broenstrup, Silke Christiansen

The fovea centralis is a closely-packed vertical array of inverted-cone photoreceptor cells located in the retina that is responsible for high acuity binocular vision. The cones are operational in well-lit environments and are responsible for trapping the impinging illumination. We present the vertical light-funnel silicon array as a light-trapping technique for photovoltaic applications that is bio-inspired by the properties of the fovea centralis. We use opto-electronic simulations to evaluate the performance of light-funnel solar cell arrays. Light-funnel arrays present similar to 65% absorption enhancement compared to a silicon film of identical thickness and exhibit power conversion efficiencies that are 60% higher than those of optimized nanowire arrays of the same thickness although nanowire arrays consist of more than 2.3 times the amount of silicon. We demonstrate the superior absorption of the light-funnel arrays as compared with recent advancements in the field. Fabrication of silicon light-funnel arrays using low-cost processing techniques is demonstrated.

Ultrafast Dynamics of Lasing Semiconductor Nanowires

Robert Roeder, Themistoklis P. H. Sidiropoulos, Christian Tessarek, Silke Christiansen, Rupert F. Oulton, Carsten Ronning

Semiconductor nanowire lasers operate at ultrafast timescales; here we report their temporal dynamics, including laser onset time and pulse width, using a double-pump approach. Wide bandgap gallium nitride (GaN), zinc oxide (ZnO), and cadmium sulfide (CdS) nanowires reveal laser onset times of a few picoseconds, driven by carrier thermalization within the optically excited semiconductor. Strong carrier-phonon coupling in ZnO leads to the fastest laser onset time of similar to 1 ps in comparison to CdS and GaN exhibiting values of similar to 2.5 and similar to 3.5 ps, respectively. These values are constant between nanowires of different sizes implying independence from any optical influences. However, we demonstrate that the lasing onset times vary with excitation wavelength relative to the semiconductor band gap. Meanwhile, the laser pulse widths are dependent on the optical system. While the fastest ultrashort pulses are attained using the thinnest possible nanowires, a sudden change in pulse width from similar to 5 to similar to 15 ps occurs at a critical nanowire diameter. We attribute this to the transition from single to multimode waveguiding, as it is accompanied by a change in laser polarization.

Thermal characterisation of (bio)polymers with a temperature-stabilised whispering gallery mode microsensor

Eugene Kim, Matthew R. Foreman, Martin D. Baaske, Frank Vollmer

In this work, we theoretically and experimentally investigate the thermal response of whispering gallery mode microresonators operating in an aqueous glycerol medium. Thermal stabilisation of the resonance wavelength is realised by appropriate choice of the resonator radius and glycerol concentration, with a 60 fold reduction in thermal sensitivity demonstrated. Finally, we employ our stabilised system to determine the thermal dependence of the molecular polarisability of adsorbed bovine serum albumin molecules and the refractive index of dextran and poly(diallyldimethylammonium chloride) coatings. (C) 2015 AIP Publishing LLC.

Near-infrared single-photon spectroscopy of a whispering gallery mode resonator using energy-resolving transition edge sensors

Michael Foertsch, Thomas Gerrits, Martin J. Stevens, Dmitry Strekalov, Gerhard Schunk, Josef U. Fuerst, Ulrich Vogl, Florian Sedlmeir, Harald G. L. Schwefel, et al.

We demonstrate a method to perform spectroscopy of near-infrared single photons without the need of dispersive elements. This method is based on a photon energy resolving transition edge sensor and is applied for the characterization of widely wavelength tunable narrow-band single photons emitted from a crystalline whispering gallery mode resonator. We measure the emission wavelength of the generated signal and idler photons with an uncertainty of up to 2 nm.

Giant narrowband twin-beam generation along the pump-energy propagation direction

Angela M. Perez, Kirill Yu Spasibko, Polina R. Sharapova, Olga V. Tikhonova, Gerd Leuchs, Maria V. Chekhova

Walk-off effects, originating from the difference between the group and phase velocities, limit the efficiency of nonlinear optical interactions. While transverse walk-off can be eliminated by proper medium engineering, longitudinal walk-off is harder to avoid. In particular, ultrafast twin-beam generation via pulsed parametric down-conversion and four-wave mixing is only possible in short crystals or fibres. Here we show that in high-gain parametric down-conversion, one can overcome the destructive role of both effects and even turn them into useful tools for shaping the emission. In our experiment, one of the twin beams is emitted along the pump Poynting vector or its group velocity matches that of the pump. The result is markedly enhanced generation of both twin beams, with the simultaneous narrowing of angular and frequency spectrum. The effect will enable efficient generation of ultrafast twin photons and beams in cavities, waveguides and whispering-gallery mode resonators.

Practical implementation of mutually unbiased bases using quantum circuits

U. Seyfarth, L. L. Sanchez-Soto, G. Leuchs

The number of measurements necessary to perform the quantum state reconstruction of a system of qubits grows exponentially with the number of constituents, creating a major obstacle for the design of scalable tomographic schemes. We work out a simple and efficient method based on cyclic generation of mutually unbiased bases. The basic generator requires only Hadamard and controlled-phase gates, which are available in most practical realizations of these systems. We show how complete sets of mutually unbiased bases with different entanglement structures can be realized for three and four qubits. We also analyze the quantum circuits implementing the various entanglement classes.

Role of Silicon Nanowire Diameter for Alkyl (Chain Lengths C-1-C-18) Passivation Efficiency through Si-C Bonds

Muhammad Y. Bashouti, Carmelina A. Garzuzi, Maria de la Mata, Jordi Arbiol, Juergen Ristein, Hossam Haick, Silke Christiansen

The effect of silicon nanowire (Si NW) diameter on the functionalization efficiency as given by covalent Si-C bond formation is studied for two distinct examples of 25 +/- 5 and 50 +/- 5 nm diameters (Si NW25 and Si NW50, respectively). A two-step chlorination/alkylation process is used to connect alkyl chains of various lengths (C-1-C-18) to thinner and thicker Si NWs. The shorter the alkyl chain lengths, the larger the surface coverage of the two studied Si NWs. Increasing the alkyl chain length (C-2-C-9) changes the coverage on the NWs: while for Si NW25 90 +/- 10% of all surface sites are covered with Si-C bonds, only 50 +/- 10% of all surface sites are covered with Si-C bonds for the Si NW50 wires. Increasing the chain length further to C-14-C-18 decreases the alkyl coverage to 36 +/- 6% in thin Si NW25 and to 20 +/- 5% in thick Si NW50. These findings can be interpreted as being a result of increased steric hindrance of Si-C bond formation for longer chain lengths and higher surface energy for the thinner Si NWs. As a direct consequence of these findings, Si NW surfaces have different stabilities against oxidation: they are more stable at higher Si-C bond coverage, and the surface stability was found to be dependent on the Si-C binding energy itself. The Si-C binding energy differs according to (C1-9)-Si NW > (C14-18)-Si NW, i.e., the shorter the alkyl chain, the greater the Si-C binding energy. However, the oxidation resistance of the (C2-18)-Si NW25 is lower than for equivalent Si NW50 surfaces as explained and experimentally substantiated based on electronic (XPS and KP) and structure (TEM and HAADF) measurements.

Goos-Hanchen and Imbert-Fedorov shifts for astigmatic Gaussian beams

Marco Ornigotti, Andrea Aiello

In this work we investigate the role of the beam astigmatism in the Goos-Hanchen and Imbert-Fedorov shift. As a case study, we consider a Gaussian beam focused by an astigmatic lens and we calculate explicitly the corrections to the standard formulas for beam shifts due to the astigmatism induced by the lens. Our results show that the different focusing in the longitudinal and transverse direction introduced by an astigmatic lens may enhance the angular part of the shift.

Classically entangled optical beams for high-speed kinematic sensing

Stefan Berg-Johansen, Falk Toeppel, Birgit Stiller, Peter Banzer, Marco Ornigotti, Elisabeth Giacobino, Gerd Leuchs, Andrea Aiello, Christoph Marquardt

Tracking the kinematics of fast-moving objects is an important diagnostic tool for science and engineering. Here, we demonstrate an approach to positional and directional sensing based on the concept of classical entanglement in vector beams of light [Found. Phys. 28, 361 -374 (1998)]. The measurement principle relies on the intrinsic correlations existing in such beams between transverse spatial modes and polarization. The latter can be determined from intensity measurements with only a few fast photodiodes, greatly outperforming the bandwidth of current CCD/CMOS devices. In this way, our setup enables two-dimensional real-time sensing with temporal resolution in the GHz range. We expect the concept to open up new directions in metrology and sensing. (C) 2015 Optical Society of America

Maximizing the ultimate absorption efficiency of vertically-aligned semiconductor nanowire arrays with wires of a low absorption cross-section

Gil Shalev, Sebastian W. Schmitt, Gerald Broenstrup, Silke Christiansen

Single semiconducting nanowires with sub-wavelength diameters exhibit superior light absorption, and hence triggered a vivid discussion regarding the application of these nanostructures into future generations of high efficiency solar cells. We examine the transition from a single highly absorbing silicon wire into an array composed of such individuals in order to validate the application of these into solar harvesting devices. We use finite-difference time-domain simulations to show that the coupling of the Fabry-Perot oscillations with the waveguide resonances inside the wires has a significant effect on the array absorption. For example, the ultimate absorption efficiency of a square-tiled wire array under normal incidence (array period of 0.5 mu m, wire diameter of 0.4 mu m and wire height of 2) is 81% higher than a 2 mu m thin-film when the Fabry-Perot oscillations are considered and 37% higher when these oscillations are not considered. This coupling screens out the contribution of the waveguide modes to the array absorption and therefore, unlike previously published work, we eliminate the contribution of the Fabry-Perot oscillations. In this manner we demonstrate the absorption enhancement due to waveguide modes, and general correlations between the nanowire geometry and the overall array absorption are presented. First, we show that once an isolated wire with high absorption cross-section is nested inside an array its absorption decreases due to wire proximity effects. Secondly, the array absorption is maximized with relatively wide wires of low absorption cross-sections. We show that a 75 nm wire inside an square-tiled array with 2 pm period has an average absorption efficiency factor of 6.5 and the average relative absorption of the array is 0.5%, while the same wire nested inside an array of a 0.25 mu m period exhibits 2.3 average absorption efficiency factor and the array exhibits average relative absorption of 9.85%. Finally, there is an optimized wire diameter that once exceeded the array absorption converges to that of a continuous film. For example, the maximum absorption of 0.5 mu m array is obtained with wire diameter of 0.4 mu m where a decrease in relative absorption is recorded for arrays with wires exceeding 0.4 mu m. (C) 2015 Elsevier Ltd. All rights reserved.

Measuring the Transverse Spin Density of Light

Martin Neugebauer, Thomas Bauer, Andrea Aiello, Peter Banzer

We generate tightly focused optical vector beams whose electric fields spin around an axis transverse to the beams' propagation direction. We experimentally investigate these fields by exploiting the directional near-field interference of a dipolelike plasmonic field probe placed adjacent to a dielectric interface. This directionality depends on the transverse electric spin density of the excitation field. Near-to far-field conversion mediated by the dielectric interface enables us to detect the directionality of the emitted light in the far field and, therefore, to measure the transverse electric spin density with nanoscopic resolution. Finally, we determine the longitudinal electric component of Belinfante's elusive spin momentum density, a solenoidal field quantity often referred to as "virtual."

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

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

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

An ion trap built with photonic crystal fibre technology

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

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

Integration of plasmonic Ag nanoparticles as a back reflector in ultra-thin Cu(In,Ga)Se-2 solar cells

Guanchao Yin, Alexander Steigert, Patrick Andrae, Manuela Goebelt, Michael Latzel, Phillip Manley, Iver Lauermann, Silke Christiansen, Martina Schmid

Integration of plasmonic Ag nanoparticles as a back reflector in ultra-thin Cu(In,Ga)Se-2 (CIGSe) solar cells is investigated. X-ray photoelectron spectroscopy results show that Ag nanoparticles underneath a Sn:In2O3 back contact could not be thermally passivated even at a low substrate temperature of 440 degrees C during CIGSe deposition. It is shown that a 50 nm thick Al2O3 film prepared by atomic layer deposition is able to block the diffusion of Ag, clearing the thermal obstacle in utilizing Ag nanoparticles as a back reflector in ultra-thin CIGSe solar cells. Via 3-D finite element optical simulation, it is proved that the Ag nanoparticles show the potential to contribute the effective absorption in CIGSe solar cells. (C) 2015 Elsevier B.V. All rights reserved.

Carbon irradiated semi insulating GaAs for photoconductive terahertz pulse detection

Abhishek Singh, Sanjoy Pal, Harshad Surdi, S. S. Prabhu, S. Mathimalar, Vandana Nanal, R. G. Pillay, G. H. Doehler

We report here a photoconductive material for THz detection with sub-picosecond carrier lifetime made by C-12 (Carbon) irradiation on commercially available semi-insulating (SI) GaAs. We are able to reduce the carrier lifetime of SI-GaAs down to sub-picosecond by irradiating it with various irradiation dosages of Carbon (C-12) ions. With an increase of the irradiation dose from similar to 10(12) /cm(2) to similar to 10(15) /cm(2) the carrier lifetime of SI-GaAs monotonously decreases to 0.55 picosecond, whereas that of usual non-irradiated SI-GaAs is similar to 70 picosecond. This decreased carrier lifetime has resulted in a strong improvement in THz pulse detection compared with normal SI-GaAs. Improvement in signal to noise ratio as well as in detection bandwidth is observed. Carbon irradiated SI-GaAs appears to be an economical alternative to low temperature grown GaAs for fabrication of THz devices. (C) 2015 Optical Society of America

Risk Analysis of Trojan-Horse Attacks on Practical Quantum Key Distribution Systems

Nitin Jain, Birgit Stiller, Imran Khan, Vadim Makarov, Christoph Marquardt, Gerd Leuchs

An eavesdropper Eve may probe a quantum key distribution (QKD) system by sending a bright pulse from the quantum channel into the system and analyzing the back-reflected pulses. Such Trojan-horse attacks can breach the security of the QKD system, if appropriate safeguards are not installed or if they can be fooled by the Eve. We present a risk analysis of such attacks based on extensive spectral measurements, such as transmittance, reflectivity, and detection sensitivity of some critical components used in a typical QKD systems. Our results indicate the existence of wavelength regimes, where the attacker gains considerable advantage as compared to launching an attack at 1550 nm. We also propose countermeasures to reduce the risk of such attacks.

Whispering gallery mode sensors (vol 7, pg 168, 2015)

Matthew R. Foreman, Jon D. Swaim, Frank Vollmer

Position-Squared Coupling in a Tunable Photonic Crystal Optomechanical Cavity

Taofiq K. Paraiso, Mahmoud Kalaee, Leyun Zang, Hannes Pfeifer, Florian Marquardt, Oskar Painter

We present the design, fabrication, and characterization of a planar silicon photonic crystal cavity in which large position-squared optomechanical coupling is realized. The device consists of a double-slotted photonic crystal structure in which motion of a central beam mode couples to two high-Q optical modes localized around each slot. Electrostatic tuning of the structure is used to controllably hybridize the optical modes into supermodes that couple in a quadratic fashion to the motion of the beam. From independent measurements of the anticrossing of the optical modes and of the dynamic optical spring effect, a position-squared vacuum coupling rate as large as (g) over tilde'/2 pi = 245 Hz is inferred between the optical supermodes and the fundamental in-plane mechanical resonance of the structure at omega(m)/2 pi = 8.7 MHz, which in displacement units corresponds to a coupling coefficient of g'/2 pi = 1 THz/nm(2). For larger supermode splittings, selective excitation of the individual optical supermodes is used to demonstrate optical trapping of the mechanical resonator with measured (g) over tilde'/2 pi = 46 Hz.

Engineering of a Ge-Te-Se glass fibre evanescent wave spectroscopic (FEWS) mid-IR chemical sensor for the analysis of food and pharmaceutical products

Xin Jiang, Animesh Jha

Using an unclad multimode Ge-Te-Se based chalcogenide glass fibre, simple design robust fibre evanescent wave spectroscopic (FEWS) sensor is demonstrated. Methodologies adopted for material development and fibre drawing are discussed in the following steps: purification of raw materials for high spectral purity, fabrication of glass and fibre preform leading to fibre drawing. The fabricated fibre has a minimum loss of 1.4 dB/m at 4.2 mu m, and less than 3 dB/m between 1.5 and 6.3 mu m. The feasibility of using such a fibre for evanescent wave spectroscopic sensing has been verified by using the finite-element (FE) computation technique. Supported optical modes as well as corresponding penetration depths of evanescent fields from different modes are discussed. Based on the FE computation, a FEWS sensor consisting of a 40 cm Ge-Te-Se fibre, coupled with a Fourier transform infrared (FTIR) spectrometer and a liquid nitrogen cooled mercury-cadmium-tellurium (MCT) detector, is demonstrated. The active length along this fibre employed for sensing is 3 cm. Based on FEWS design, the fabricated fibre sensor was used for the analysis of chemicals, namely the acetone, ethanol, methanol, tocopherol (vitamin E), ascorbic acid (vitamin C), fresh orange and lemon juice. (C) 2014 Elsevier B.V. All rights reserved.

Intracavity Squeezing Can Enhance Quantum-Limited Optomechanical Position Detection through Deamplification

V. Peano, H. G. L. Schwefel, Ch. Marquardt, F. Marquardt

It has been predicted and experimentally demonstrated that by injecting squeezed light into an optomechanical device, it is possible to enhance the precision of a position measurement. Here, we present a fundamentally different approach where the squeezing is created directly inside the cavity by a nonlinear medium. Counterintuitively, the enhancement of the signal-to-noise ratio works by deamplifying precisely the quadrature that is sensitive to the mechanical motion without losing quantum information. This enhancement works for systems with a weak optomechanical coupling and/or strong mechanical damping. This can allow for larger mechanical bandwidth of quantum-limited detectors based on optomechanical devices. Our approach can be straightforwardly extended to quantum nondemolition qubit detection.

Stars of the quantum Universe: extremal constellations on the Poincare sphere

Gunnar Bjork, Markus Grassl, Pablo de la Hoz, Gerd Leuchs, Luis L. Sanchez-Soto

The characterization of the polarization properties of a quantum state requires the knowledge of the joint probability distribution of the Stokes variables. This amounts to assessing all the moments of these variables, which are aptly encoded in a multipole expansion of the density matrix. The cumulative distribution of these multipoles encapsulates in a handy manner the polarization content of the state. We work out the extremal states for that distribution, finding that SU(2) coherent states are maximal to any order, so they are the most polarized allowed by quantum theory. The converse case of pure states minimizing that distribution, which can be seen as the most quantum ones, is investigated for a diverse range of number of photons. Exploiting the Majorana representation, the problem appears to be closely related to distributing a number of points uniformly over the surface of the Poincare sphere.

Adjustable diffractive spiral phase plates

Walter Harm, Stefan Bernet, Monika Ritsch-Marte, Irina Harder, Norbert Lindlein

We report on the fabrication and the experimental demonstration of Moire diffractive spiral phase plates with adjustable helical charge. The proposed optical unit consists of two axially stacked diffractive elements of conjugate structure. The joint transmission function of the compound system corresponds to that of a spiral phase plate where the angle of mutual rotation about the central axis enables continuous adjustment of the helical charge. The diffractive elements are fabricated by gray-scale photolithography with a pixel size of 200 nm and 128 phase step levels in fused silica. We experimentally demonstrate the conversion of a TEM00 beam into approximated Laguerre-Gauss (LG) beams of variable helical charge, with a correspondingly variable radius of their ring-shaped intensity distribution. (C) 2015 Optical Society of America

Strong Raman-induced noninstantaneous soliton interactions in gas-filled photonic crystal fibers

Mohammed F. Saleh, Andrea Armaroli, Andrea Marini, Fabio Biancalana

We have developed an analytical model based on the perturbation theory to study the optical propagation of two successive solitons in hollow-core photonic crystal fibers filled with Raman-active gases. Based on the time delay between the two solitons, we have found that the trailing soliton dynamics can experience unusual nonlinear phenomena, such as spectral and temporal soliton oscillations and transport toward the leading soliton. The overall dynamics can lead to a spatiotemporal modulation of the refractive index with a uniform temporal period and a uniform or chirped spatial period. (C) 2015 Optical Society of America

Enhancing the radiative emission rate of single molecules by a plasmonic nanoantenna weakly coupled with a dielectric substrate

X. W. Chen, K. G. Lee, H. Eghlidi, Stephan Götzinger, Vahid Sandoghdar

Enhancing the spontaneous emission of single emitters has been an important subject in nano optics in the past decades. For this purpose, plasmonic nanoantennas have been proposed with enhancement factors typically larger than those achievable with optical cavities. However, the intrinsic ohmic losses of plasmonic structures also introduce an additional nonradiative decay channel, reducing the quantum yield. Here we report on experimental studies of a weakly coupled dielectric substrate and a plasmonic nanoantenna for enhancing the radiative decay rate of single terrylene molecules embedded in an ultrathin organic film. We systematically investigate how the refractive index of the dielectric substrate affects the lifetime and the quantum efficiency and show that the coupled structure could moderately enhance the radiative decay rate while maintaining a high quantum efficiency. (C)2015 Optical Society of America

Electronic Properties of Si-H-x Vibrational Modes at Si Waveguide Interface

Muhammad Y. Bashouti, Peyman Yousefi, Juergen Ristein, Silke H. Christiansen

Attenuated total reflectance (ATR) and X-ray photoelectron spectroscopy in suite with Kelvin probe were conjugated to explore the electronic properties of Si-H-x vibrational modes by developing Si waveguide with large dynamic detection range compared with conventional IR. The Si 2p emission and work-function related to the formation and elimination of Si-H-x bonds at Si surfaces are monitored based on the detection of vibrational mode frequencies. A transition between various Si-H-x bonds and thus related vibrational modes is monitored for which effective momentum transfer could be demonstrated. The combination of the aforementioned methods provides for results that permit a model for the kinetics of hydrogen termination of Si surfaces with time and advanced surface characterizing of hybrid-terminated semiconducting solids.

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

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

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

Observation of optical polarization Mobius strips

Thomas Bauer, Peter Banzer, Ebrahim Karimi, Sergej Orlov, Andrea Rubano, Lorenzo Marrucci, Enrico Santamato, Robert W. Boyd, Gerd Leuchs

Mobius strips are three-dimensional geometrical structures, fascinating for their peculiar property of being surfaces with only one "side"-or, more technically, being "nonorientable" surfaces. Despite being easily realized artificially, the spontaneous emergence of these structures in nature is exceedingly rare. Here, we generate Mobius strips of optical polarization by tightly focusing the light beam emerging from a q-plate, a liquid crystal device that modifies the polarization of light in a space-variantmanner. Using a recently developed-method for the three-dimensional nanotomography of optical vector fields, we fully reconstruct the light polarization structure in the focal region, confirming the appearance of Mobius polarization structures. The preparation of such structured light modes may be important for complex light beam engineering and optical micro-and nanofabrication.

Optimal Frames for Polarization State Reconstruction

Matthew R. Foreman, Alberto Favaro, Andrea Aiello

Complete determination of the polarization state of light requires at least four distinct projective measurements of the associated Stokes vector. Stability of state reconstruction, however, hinges on the condition number kappa of the corresponding instrument matrix. Optimization of redundant measurement frames with an arbitrary number of analysis states, m, is considered in this Letter in the sense of minimization of kappa. The minimum achievable kappa is analytically found and shown to be independent of m, except for m = 5 where this minimum is unachievable. Distribution of the optimal analysis states over the Poincare sphere is found to be described by spherical 2 designs, including the Platonic solids as special cases. Higher order polarization properties also play a key role in nonlinear, stochastic, and quantum processes. Optimal measurement schemes for nonlinear measurands of degree t are hence also considered and found to correspond to spherical 2t designs, thereby constituting a generalization of the concept of mutually unbiased bases.

Entangling the Whole by Beam Splitting a Part

Callum Croal, Christian Peuntinger, Vanessa Chille, Christoph Marquardt, Gerd Leuchs, Natalia Korolkova, Ladislav Mista Jr.

A beam splitter is a basic linear optical element appearing in many optics experiments and is frequently used as a continuous-variable entangler transforming a pair of input modes from a separable Gaussian state into an entangled state. However, a beam splitter is a passive operation that can create entanglement from Gaussian states only under certain conditions. One such condition is that the input light is suitably squeezed. We demonstrate, experimentally, that a beam splitter can create entanglement even from modes which do not possess such a squeezing provided that they are correlated to, but not entangled with, a third mode. Specifically, we show that a beam splitter can create three-mode entanglement by acting on two modes of a three-mode fully separable Gaussian state without entangling the two modes themselves. This beam splitter property is a key mechanism behind the performance of the protocol for entanglement distribution by separable states. Moreover, the property also finds application in collaborative quantum dense coding in which decoding of transmitted information is assisted by interference with a mode of the collaborating party.

Self-Catalytic Growth of -Ga2O3 Nanostructures by Chemical Vapor Deposition

Sudheer Kumar, Christian Tessarek, George Sarau, Silke Christiansen, Rajendra Singh

In this work, we have studied the synthesis of single crystalline self-catalyzed beta gallium oxide (-Ga2O3) nanostructures by chemical vapor deposition technique. We have adopted a new approach to grow the nanostructures instead of using conventional foreign metal nano-catalyst based-approaches. The as-grown nanostructures including nanowires and nanosheets (NSHs) were grown on spin-coated Ga2O3 films. The structural studies such as X-ray diffraction, Raman and transmission electron microscope (TEM) investigations on the nanostructures showed monoclinic phase of Ga2O3 and single crystalline structure. Furthermore, high-resolution TEM with a selected area electron diffraction pattern recorded on a single -Ga2O3 nanowire and nanosheet verified their single crystalline nature, having [001] as favorable growth direction. The energy dispersive X-ray spectroscopy-elemental mapping of as-grown nanostructures indicated uniform distribution of Ga and O. Cathodoluminescence imaging and spectrum revealed excellent luminescence characteristics of nanostructures with a broad UV-blue emission band (1.80-4.20eVnm) centered at 2.64eV. This study also highlights the growth mechanism of NSHs. These -Ga2O3 nanostructures have great potential in nanofunctional devices.

Ultrasensitive Silicon Nanowire for Real-World Gas Sensing: Noninvasive Diagnosis of Cancer from Breath Volatolome

Nisreen Shehada, Gerald Broenstrup, Konrads Funka, Silke Christiansen, Marcis Leja, Hossam Haick

We report on an ultrasensitive, molecularly modified silicon nanowire field effect transistor that brings together the lock-and-key and cross-reactive sensing worlds for the diagnosis of (gastric) cancer from exhaled volatolome. The sensor is able to selectively detect volatile organic compounds (VOCs) that are linked with gastric cancer conditions in exhaled breath and to discriminate them from environmental VOCs that exist in exhaled breath samples but do not relate to the gastric cancer per se. Using breath samples collected from actual patients with gastric cancer and from volunteers who do not have cancer, blind analysis validated the ability of the reported sensor to discriminate between gastric cancer and control conditions with >85% accuracy, irrespective of important confounding factors such as tobacco consumption and gender. The reported sensing approach paves the way to use the power of silicon nanowires for simple, inexpensive, portable, and noninvasive diagnosis of cancer and other disease conditions.

Dissipative plasmon solitons in graphene nanodisk arrays

Daria A. Smirnova, Roman E. Noskov, Lev A. Smirnov, Yuri S. Kivshar

We study nonlinear modes in arrays of doped graphene nanodisks with Kerr-type nonlinear response in the presence of an external electric field. We introduce a theoretical model describing the evolution of the nanodisks' polarizations taking into account intrinsic graphene losses and dipole-dipole coupling between the graphene nanodisks. We reveal that this nonlinear system can support discrete dissipative scalar solitons of both longitudinal and transverse polarizations, as well as vector solitons composed of two mutually coupled polarization components. We demonstrate the formation of stable resting and moving localized modes in this discrete model under controlling guidance of the external driving field.

Spectroscopic detection of single Pr3+ ions on the H-3(4)-D-1(2) transition

Emanuel Eichhammer, Tobias Utikal, Stephan Goetzinger, Vahid Sandoghdar

Rare earth ions in crystals exhibit narrow spectral features and hyperfine-split ground states with exceptionally long coherence times. These features make them ideal platforms for quantum information processing in the solid state. Recently, we reported on the first high-resolution spectroscopy of single Pr3+ ions in yttrium orthosilicate nanocrystals via the H-3(4)-P-3(0) transition at a wavelength of 488 nm. Here we show that individual praseodymium ions can also be detected on the more commonly studied H-3(4)-D-1(2) transition at 606 nm. In addition, we present the first measurements of the second-order autocorrelation function, fluorescence lifetime, and emission spectra of single ions in this system as well as their polarization dependencies on both transitions. Furthermore, we demonstrate that by a proper choice of the crystallite, one can obtain narrower spectral lines and, thus, resolve the hyperfine levels of the excited state. We expect our results to make single-ion spectroscopy accessible to a larger scientific community.

Cavity ring-up spectroscopy for ultrafast sensing with optical microresonators

Serge Rosenblum, Yulia Lovsky, Lior Arazi, Frank Vollmer, Barak Dayan

Spectroscopy of whispering-gallery mode microresonators has become a powerful scientific tool, enabling the detection of single viruses, nanoparticles and even single molecules. Yet the demonstrated timescale of these schemes has been limited so far to milliseconds or more. Here we introduce a scheme that is orders of magnitude faster, capable of capturing complete spectral snapshots at nanosecond timescales-cavity ring-up spectroscopy. Based on sharply rising detuned probe pulses, cavity ring-up spectroscopy combines the sensitivity of heterodyne measurements with the highest-possible, transform-limited acquisition rate. As a demonstration, we capture spectra of microtoroid resonators at time intervals as short as 16 ns, directly monitoring submicrosecond dynamics of their optomechanical vibrations, thermorefractive response and Kerr nonlinearity. Cavity ring-up spectroscopy holds promise for the study of fast biological processes such as enzyme kinetics, protein folding and light harvesting, with applications in other fields such as cavity quantum electrodynamics and pulsed optomechanics.

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

Jean-Michel Menard, Philip St. J. Russell

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

Systematic increase of electrocatalytic turnover at nanoporous platinum surfaces prepared by atomic layer deposition

Loic Assaud, Johannes Schumacher, Alexander Tafel, Sebastian Bochmann, Silke Christiansen, Julien Bachmann

We establish a procedure for the fabrication of electrocatalytically active, nanoporous surfaces coated with Pt and exhibiting a high geometric area. Firstly, the mechanism of the surface reactions between platinum(II) acetylacetonate and ozone is investigated by piezoelectric microbalance measurements. The data reveal that ozone oxidizes the metallic Pt surface to an extent which can exceed one monolayer depending on the reaction conditions. Proper reaction parameters yield a self-limited growth in atomic layer deposition (ALD) mode. Secondly, the ALD procedure is applied to porous anodic oxide substrates. The morphology and the crystal structure of the deposits are characterized. The ALD coating results in a continuous layer of Pt nanocrystallites along deep pore walls (aspect ratio 70). Thirdly, the Pt/TiO2 surfaces are shown to be electrochemically active in both acidic and alkaline media, in a way that qualitatively conforms to literature precedents based on Pt. Finally, we apply the anodization and ALD procedure to commercial Ti felts and demonstrate systematically how the electrochemical current density is increased by the large specific surface area and by the presence of the catalyst. Thereby, the catalyst loading, as well as its efficient utilization, can be optimized accurately. The preparative approach demonstrated here can be generalized and applied to the various electrocatalytic reactions of energy conversion devices.

A robust quantum receiver for phase shift keyed signals

C. R. Mueller, Ch Marquardt

The impossibility of perfectly discriminating non-orthogonal quantum states imposes far-reaching consequences both on quantum and classical communication schemes. We propose and numerically analyze an optimized quantum receiver for the discrimination of phase encoded signals. Our scheme outperforms the standard quantum limit and approaches the Helstrom bound for any signal power. The discrimination is performed via an optimized, feedback-mediated displacement prior to a photon counting detector. We provide a detailed analysis of the influence of excess noise and technical imperfections on the average error probability. The results demonstrate the receiver's robustness and show that it can outperform any classical receiver over a wide range of realistic parameters.

Continuous wave terahertz radiation from antennas fabricated on C-12-irradiated semi-insulating GaAs

Prathmesh Deshmukh, M. Mendez-Aller, Abhishek Singh, Sanjoy Pal, S. S. Prabhu, Vandana Nanal, R. G. Pillay, G. H. Doehler, S. Preu

We demonstrate continuous wave (CW) terahertz generation from antennas fabricated on C-12-irradiated semi-insulating (SI) GaAs substrates. The dark current drawn by the antennas fabricated on irradiated substrates is similar to 3 to 4 orders of magnitude lower compared to antennas fabricated on un-irradiated substrates, while the photocurrents decrease by only similar to 1.5 orders of magnitude. This can be attributed to the strong reduction of the carrier lifetime that is 2.5 orders of magnitude, with values around tau(rec) = 0.2 ps. Reduced thermal heating allows for higher bias voltages to the irradiated antenna devices resulting in higher CW terahertz power, just slightly lower than that of low-temperature grown GaAs (LT GaAs) at similar excitation conditions. (C) 2015 Optical Society of America

Carrier-induced refractive index change observed by a whispering gallery mode shift in GaN microrods

C. Tessarek, R. Goldhahn, G. Sarau, M. Heilmann, S. Christiansen

Vertical oriented GaN microrods were grown by metal-organic vapor phase epitaxy with four different n-type carrier concentration sections above 10(19) cm(-3) along the c-axis. In cathodoluminescence investigations carried out on each section of the microrod, whispering gallery modes can be observed due to the hexagonal symmetry. Comparisons of the spectral positions of the modes from each section show the presence of an energy dependent mode shift, which suggest a carrier-induced refractive index change. The shift of the high energy edge of the near band edge emission points out that the band gap parameter in the analytical expression of the refractive index has to be modified. A proper adjustment of the band gap parameter explains the observed whispering gallery mode shift.

Pattern phase diagram for two-dimensional arrays of coupled limit-cycle oscillators

Roland Lauter, Christian Brendel, Steven J. M. Habraken, Florian Marquardt

Arrays of coupled limit-cycle oscillators represent a paradigmatic example for studying synchronization and pattern formation. We find that the full dynamical equations for the phase dynamics of a limit-cycle oscillator array go beyond previously studied Kuramoto-type equations. We analyze the evolution of the phase field in a two-dimensional array and obtain a "phase diagram" for the resulting stationary and nonstationary patterns. Our results are of direct relevance in the context of currently emerging experiments on nano-and optomechanical oscillator arrays, as well as for any array of coupled limit-cycle oscillators that have undergone a Hopf bifurcation. The possible observation in optomechanical arrays is discussed briefly.

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

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

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

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

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

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

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

Wenbin He, Meng Pang, Philip St J. Russell

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

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

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

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

Two-photon spectral amplitude of entangled states resolved in separable Schmidt modes

A. Avella, G. Brida, M. Chekhova, M. Gramegna, A. Shurupov, M. Genovese

The ability to access high dimensionality in Hilbert spaces represents a demanding key-stone for state-of-the-art quantum information. The manipulation of entangled states in continuous variables, wavevector as well frequency, represents a powerful resource in this sense. The number of dimensions of the Hilbert space that can be used in practical information protocols can be determined by the number of Schmidt modes that it is possible to address one by one. In the case of wavevector variables, the Schmidt modes can be losslessly selected using single-mode fibre and a spatial light modulator, but no similar procedure exists for the frequency space. The aim of this work is to present a technique to engineer the spectral properties of biphoton light, emitted via ultrafast spontaneous parametric down conversion, in such a way that the two-photon spectral amplitude (TPSA) contains several non-overlapping Schmidt modes, each of which can be filtered losslessly in frequency variables. Such TPSA manipulation is operated by a fine balancing of parameters like the pump frequency, the shaping of pump pulse spectrum, the dispersion dependence of spontaneous parametric down-conversion crystals as well as their length. Measurements have been performed exploiting the group velocity dispersion induced by the passage of optical fields through dispersive media, operating a frequency-to-time two-dimensional Fourier transform of the TPSA. Exploiting this kind of measurement we experimentally demonstrate the ability to control the Schmidt modes structure in TPSA through the pump spectrum manipulation.

Extremal states for photon number and quadratures as gauges for nonclassicality

Z. Hradil, J. Rehacek, P. de la Hoz, G. Leuchs, L. L. Sanchez-Soto

Rotated quadratures carry the phase-dependent information of the electromagnetic field, so they are somehow conjugate to the photon number. We analyze this noncanonical pair, finding an exact uncertainty relation, as well as a couple of weaker inequalities obtained by relaxing some restrictions of the problem. We also find the intelligent states saturating that relation and complete their characterization by considering extra constraints on the second-order moments of the variables involved. Using these moments, we construct performance measures tailored to diagnose photon-added and Schrodinger-cat-like states, among others.

Dirac solitons in square binary waveguide lattices

Truong X. Tran, Xuan N. Nguyen, Fabio Biancalana

We study optical analogs of two-dimensional (2D) Dirac solitons in square binary waveguide lattices with two different topologies in the presence of Kerr nonlinearity. These 2D solitons turn out to be quite robust. We demonstrate that with the found 2D solitons, the coupled mode equations governing light dynamics in square binary waveguide lattices can be converted into the nonlinear relativistic 2D Dirac equation with the four-component bispinor. This paves the way for using binary waveguide lattices as a classical simulator of quantum nonlinear effects arising from the 2D Dirac equation, something that is thought to be impossible to achieve in conventional (i.e., linear) quantum field theory.

Giant Optical Activity of Quantum Dots, Rods, and Disks with Screw Dislocations

Anvar S. Baimuratov, Ivan D. Rukhlenko, Roman E. Noskov, Pavel Ginzburg, Yurii K. Gun'ko, Alexander V. Baranov, Anatoly V. Fedorov

For centuries mankind has been modifying the optical properties of materials: first, by elaborating the geometry and composition of structures made of materials found in nature, later by structuring the existing materials at a scale smaller than the operating wavelength. Here we suggest an original approach to introduce optical activity in nanostructured materials, by theoretically demonstrating that conventional achiral semiconducting nanocrystals become optically active in the presence of screw dislocations, which can naturally develop during the nanocrystal growth. We show the new properties to emerge due to the dislocation-induced distortion of the crystal lattice and the associated alteration of the nanocrystal's electronic subsystem, which essentially modifies its interaction with external optical fields. The g-factors of intraband transitions in our nanocrystals are found comparable with dissymmetry factors of chiral plasmonic complexes, and exceeding the typical g-factors of chiral molecules by a factor of 1000. Optically active semiconducting nanocrystals-with chiral properties controllable by the nanocrystal dimensions, morphology, composition and blending ratio-will greatly benefit chemistry, biology and medicine by advancing enantiomeric recognition, sensing and resolution of chiral molecules.

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

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

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

Rogue solitons in optical fibers: a dynamical process in a complex energy landscape?

Andrea Armaroli, Claudio Conti, Fabio Biancalana

Nondeterministic giant waves, denoted as rogue, killer, monster, or freak waves, have been reported in many different branches of physics. Their physical interpretation is however still debated: despite massive numerical and experimental evidence, a solid explanation for their spontaneous formation has not been identified yet. Here we propose that rogue waves [more precisely, rogue solitons (RSs)] in optical fibers may actually result from a complex dynamical process very similar to well-known mechanisms such as glass transitions and protein folding. We describe how the interaction among optical solitons produces an energy landscape in a highly dimensional parameter space with multiple quasi-equilibrium points. These configurations have the same statistical distribution of the observed rogue events and are explored during the light dynamics due to soliton collisions, with inelastic mechanisms enhancing the process. Slightly different initial conditions lead to very different dynamics in this complex geometry; a RS turns out to stem from one particularly deep quasi-equilibrium point of the energy landscape in which the system may be transiently trapped during evolution. This explanation will prove to be fruitful to the vast community interested in freak waves. (C) 2015 Optical Society of America.

Classical polarization multipoles: paraxial versus nonparaxial

P. de la Hoz, G. Bjork, H. de Guise, A. B. Klimov, G. Leuchs, L. L. Sanchez-Soto

We discuss the polarization of paraxial and nonparaxial classical light fields by resorting to a multipole expansion of the corresponding polarization matrix. It turns out that only a dipolar term contributes when one considers SU(2) (paraxial) or SU(3) (nonparaxial) as fundamental symmetries. In this latter case, one can alternatively expand in SU(2) multipoles, and then both a dipolar and a quadrupolar component contribute, which explains the richer structure of this nonparaxial instance. These multipoles uniquely determine Wigner functions, in terms of which we examine some intriguing hallmarks arising in this classical scenario.

Raman-induced temporal condensed matter physics in gas-filled photonic crystal fibers

Mohammed F. Saleh, Andrea Armaroli, Truong X. Tran, Andrea Marini, Federico Belli, Amir Abdolvand, Fabio Biancalana

Raman effect in gases can generate an extremely long-living wave of coherence that can lead to the establishment of an almost perfect temporal periodic variation of the medium refractive index. We show theoretically and numerically that the equations, regulate the pulse propagation in hollow-core photonic crystal fibers filled by Raman-active gas, are exactly identical to a classical problem in quantum condensed matter physics - but with the role of space and time reversed - namely an electron in a periodic potential subject to a constant electric field. We are therefore able to infer the existence of Wannier-Stark ladders, Bloch oscillations, and Zener tunneling, phenomena that are normally associated with condensed matter physics, using purely optical means. (C) 2015 Optical Society of America

Modeling the dielectric function of degenerately doped ZnO: Al thin films grown by ALD using physical parameters

Michael Latzel, Manuela Goebelt, Gerald Broenstrup, Cornel Venzago, Sebastian W. Schmitt, George Sarau, Silke H. Christiansen

Transparent conductive thin films are a key building block of modern optoelectronic devices. A promising alternative to expensive indium containing oxides is aluminum doped zinc oxide (AZO). By correlating spectroscopic ellipsometry and photoluminescence, we analyzed the contributions of different optical transitions in AZO grown by atomic layer deposition to a model dielectric function (MDF) over a wide range of photon energies. The derived MDF reflects the effects of the actual band structure and therefore describes the optical properties very accurately. The presented MDF is solely based on physically meaningful parameters in contrast to empirical models like e.g. the widely used Sellmeier equation, but nevertheless real and imaginary parts are expressed as closed-form expressions. We analyzed the influence of the position of the Fermi energy and the Fermi-edge singularity to the different parts of the MDF. This information is relevant for design and simulation of optoelectronic devices and can be determined by analyzing the results from spectroscopic ellipsometry. (C) 2015 Optical Society of America

Towards an optical far-field measurement of higher-order multipole contributions to the scattering response of nanoparticles

Thomas Bauer, Sergej Orlov, Gerd Leuchs, Peter Banzer

We experimentally show an all-optical multipolar decomposition of the lowest-order eigenmodes of a single gold nanoprism using azimuthally and radially polarized cylindrical vector beams. By scanning the particle through these tailored field distributions, the multipolar character of the eigenmodes gets encoded into 2D-scanning intensity maps even for higher-order contributions to the eigenmode that are too weak to be discerned in the direct far-field scattering response. This method enables a detailed optical mode analysis of individual nanoparticles. (C) 2015 AIP Publishing LLC.

Quantum nature of Gaussian discord: Experimental evidence and role of system-environment correlations

Vanessa Chille, Niall Quinn, Christian Peuntinger, Callum Croal, Ladislav Mista Jr., Christoph Marquardt, Gerd Leuchs, Natalia Korolkova

We provide experimental evidence of quantum features in bipartite states classified as entirely classical according to a conventional criterion based on the Glauber P function but possessing nonzero Gaussian quantum discord. Their quantum nature is experimentally revealed by acting locally on one part of the discordant state. We experimentally verify and investigate the effect of discord increase under the action of local loss and link it to the entanglement with the environment. Adding an environmental system purifying the state, we unveil the flow of quantum correlations within a global pure system using the Koashi-Winter inequality. For a discordant state generated by splitting a state in which the initial squeezing is destroyed by random displacements, we demonstrate the recovery of entanglement highlighting the role of system-environment correlations.

From transverse angular momentum to photonic wheels

Andrea Aiello, Peter Banzer, Martin Neugebauer, Gerd Leuchs

Scientists have known for more than a century that light possesses both linear and angular momenta along the direction of propagation. However, only recent advances in optics have led to the notion of spinning electromagnetic fields capable of carrying angular momenta transverse to the direction of motion. Such fields enable numerous applications in nano-optics, biosensing and near-field microscopy, including three-dimensional control over atoms, molecules and nanostructures, and allowing for the realization of chiral nanophotonic interfaces and plasmonic devices. Here, we report on recent developments of optics with light carrying transverse spin. We present both the underlying principles and the latest achievements, and also highlight new capabilities and future applications emerging from this young yet already advanced field of research.

Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostuctures

Sebastian W. Schmitt, George Sarau, Silke Christiansen

Silicon nanowires (SiNWs) attached to a wafer substrate are converted to inversely tapered silicon nanocones (SiNCs). After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference. Furthermore, the reverse taper is responsible for multifold emission peaks in addition to the relatively broad near-infrared (NIR) luminescence spectrum. A thorough numerical mode analysis reveals that unlike a SiNW the inverted SiNC sustains a multitude of leaky whispering gallery modes. The modes are unique to this geometry and they are characterized by a relatively high quality factor (Q similar to 1300) and a low mode volume (0.2 < (lambda/n(eff))(3) < 4). In addition they show a vertical out coupling of the optically excited NIR luminescence with a numerical aperture as low as 0.22. Estimated Purcell factors F-p proportional to Q/V-m of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference. Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.

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

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

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

A large electrochemical setup for the anodization of aluminum towards highly ordered arrays of cylindrical nanopores

Loic Assaud, Sebastian Bochmann, Silke Christiansen, Julien Bachmann

A new electrochemical setup and the associated procedures for growing ordered anodic aluminum oxide pore arrays on large surfaces are presented. The typical size of the samples is 14 x 14 cm(2). The most crucial experimental parameters that allow for the stabilization of the high-field procedures are a very efficient cooling of sample and electrolyte, as well as the initial ramping up of the voltage at an accurately defined rate. The morphology of the cylindrical, parallel alumina pores is similar to those obtained on smaller scales with standard setups. Our setup facilitates the availability of porous anodic alumina as a template system for a number of applications. (C) 2015 AIP Publishing LLC.

A chaotic approach clears up imaging

Harald G. L. Schwefel, Hakan E. Tuereci

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

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

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

Exploiting cellophane birefringence to generate radially and azimuthally polarised vector beams

Johnston Kalwe, Martin Neugebauer, Calvine Ominde, Gerd Leuchs, Geoffrey Rurimo, Peter Banzer

We exploit the birefringence of cellophane to convert a linearly polarised Gaussian beam into either a radially or azimuthally polarised beam. For that, we fabricated a low-cost polarisation mask consisting of four segments of cellophane. The fast axis of each segment is oriented appropriately in order to rotate the polarisation of the incident linearly polarised beam as desired. To ensure the correct operation of the polarisation mask, we tested the polarisation state of the generated beam by measuring the spatial distribution of the Stokes parameters. Such a device is very cost efficient and allows for the generation of cylindrical vector beams of high quality.

Highly Coherent Electron Beam from a Laser-Triggered Tungsten Needle Tip

Dominik Ehberger, Jakob Hammer, Max Eisele, Michael Krueger, Jonathan Noe, Alexander Hoegele, Peter Hommelhoff

We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission triggered by a near-ultraviolet laser with a photon energy of 3.1 eV and compare it to dc-field emission. A carbon nanotube is brought into close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes, we deduce an upper limit of the effective source radius both in laser-triggered and dc-field emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain (0.80 +/- 0.05) nm in laser-triggered and (0.55 +/- 0.02) nm in dc-field emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laser-induced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for time-resolved electron imaging techniques are discussed.

Review of free software tools for image analysis of fluorescence cell micrographs

V. Wiesmann, D. Franz, C. Held, C. Muenzenmayer, R. Palmisano, T. Wittenberg

An increasing number of free software tools have been made available for the evaluation of fluorescence cell micrographs. The main users are biologists and related life scientists with no or little knowledge of image processing. In this review, we give an overview of available tools and guidelines about which tools the users should use to segment fluorescence micrographs. We selected 15 free tools and divided them into stand-alone, Matlab-based, ImageJ-based, free demo versions of commercial tools and data sharing tools. The review consists of two parts: First, we developed a criteria catalogue and rated the tools regarding structural requirements, functionality (flexibility, segmentation and image processing filters) and usability (documentation, data management, usability and visualization). Second, we performed an image processing case study with four representative fluorescence micrograph segmentation tasks with figure-ground and cell separation. The tools display a wide range of functionality and usability. In the image processing case study, we were able to perform figure-ground separation in all micrographs using mainly thresholding. Cell separation was not possible with most of the tools, because cell separation methods are provided only by a subset of the tools and are difficult to parametrize and to use. Most important is that the usability matches the functionality of a tool. To be usable, specialized tools with less functionality need to fulfill less usability criteria, whereas multipurpose tools need a well-structured menu and intuitive graphical user interface.

Squeezed Light from Entangled Nonidentical Emitters via Nanophotonic Environments

Harald R. Haakh, Diego-Martin Cano

We propose a scheme in which broadband nanostructures allow for an enhanced two-photon nonlinearity that generates squeezed light from far-detuned quantum emitters via collective resonance fluorescence. To illustrate the proposal, we consider a pair of two-level emitters detuned by 400 line widths that are coupled by a plasmonic nanosphere. It is shown that the reduced fluctuations of the electromagnetic field arising from the interaction between the emitters provide a means to detect their entanglement. Due to the near-field enhancement in the proposed hybrid systems, these nonclassical effects can be encountered outside both the extremely close separations limiting the observation in free space and narrow frequency bands in high-Q cavities. Our approach permits overcoming the fundamental limitations to the generation of squeezed light from noninteracting single emitters and is more robust against phase decoherence induced by the environment.

When excitons and plasmons meet: Emerging function through synthesis and assembly

Jennifer A. Hollingsworth, Han Htoon, Andrei Piryatinski, Stephan Goetzinger, Vahid Sandoghdar

To meet the challenge of precise nanoscale arrangement of emitter and plasmonic nanoantenna, synthesis and assembly methods continue to evolve in accuracy and reproducibility. This article reviews some of the many strategies being developed for "soft" chemical approaches to precision integration and assembly. We also discuss investigations of the Purcell effect, emission directionality control, and near-unity collection efficiency of photons, emitter emitter coupling, and higher-order emission processes that have been most deeply explored using individual-emitter- (or several-emitter-) nanoantenna pairs fabricated using traditional lithographic methods or dynamically and controllably manipulated using scanning probe methods. Importantly, these results along with theoretical analyses inspire and motivate continued advancements in large-scale synthesis and assembly. We emphasize assembly approaches that have been used to create nanosemiconductor-nanometal hybrids and, in particular, those that have afforded specific plasmonic effects on excitonic properties. We also review direct-synthesis and chemical-linker strategies to creating discrete, though less spatially extended, semiconductor-metal interactions.

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

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

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

Sensing Nanoparticles with a Cantilever-Based Scannable Optical Cavity of Low Finesse and Sub-lambda(3) Volume

Hrishikesh Kelkar, Daqing Wang, Diego Martin-Cano, Bjoern Hoffmann, Silke Christiansen, Stephan Goetzinger, Vahid Sandoghdar

We report on the realization of an open plane-concave Fabry-Perot resonator with a mode volume below lambda(3) at optical frequencies. We discuss some of the less-common features of this microcavity regime and show that the ultrasmall mode volume allows us to detect cavity resonance shifts induced by single nanoparticles even at quality factors as low as 100. Being based on low-reflectivity micromirrors fabricated on a silicon cantilever, our experimental arrangement provides broadband operation, tunability of the cavity resonance, and lateral scanning. These features are interesting for a range of applications including biochemical sensing, modification of photophysics, and optomechanical studies.

Loss-tolerant hybrid measurement test of CHSH inequality with weakly amplified N00N states

Falk Toeppel, Magdalena Stobinska

Although our understanding of Bell's theorem and experimental techniques to test it have improved over the last 40 years, thus far all Bell tests have suffered at least from the detection or the locality loophole. Most photonic Bell tests rely on inefficient discrete-outcome measurements, often provided by photon counting detection. One possible way to close the detection loophole in photonic Bell tests is to involve efficient continuous-variable measurements instead, such as homodyne detection. Here, we propose a test of the Clauser-Horne-Shimony-Holt inequality that applies photon counting and homodyne detection on weakly amplified two-photon N00N states. The scheme suggested is remarkably robust against experimental imperfections and suits the limits of current technology. As amplified quantum states are considered, our work also contributes to the exploration of entangled macroscopic quantum systems. Further, it may constitute an alternative platform for a loophole-free Bell test, which is also important for quantum-technological applications.

Experimental Realization of Quantum Tomography of Photonic Qudits via Symmetric Informationally Complete Positive Operator-Valued Measures

N. Bent, H. Qassim, A. A. Tahir, D. Sych, G. Leuchs, L. L. Sanchez-Soto, E. Karimi, R. W. Boyd

Symmetric informationally complete positive operator-valued measures provide efficient quantum state tomography in any finite dimension. In this work, we implement state tomography using symmetric informationally complete positive operator-valued measures for both pure and mixed photonic qudit states in Hilbert spaces of orbital angular momentum, including spaces whose dimension is not power of a prime. Fidelities of reconstruction within the range of 0.81-0.96 are obtained for both pure and mixed states. These results are relevant to high-dimensional quantum information and computation experiments, especially to those where a complete set of mutually unbiased bases is unknown.

Cylindrically polarized Bessel-Gauss beams

Daena Madhi, Marco Ornigotti, Andrea Aiello

We present a study of radially and azimuthally polarized Bessel-Gauss (BG) beams in both the paraxial and nonparaxial regime. We discuss the validity of the paraxial approximation and the form of the nonparaxial corrections for BG beams. We show that independently on the ratio between the Bessel aperture cone angle theta(0) and the Gaussian beam divergence theta(0), the nonparaxial corrections are alway very small and therefore negligible. The explicit expressions for the nonparaxial vector electric field components are also reported.

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

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

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

Goos-Hanchen and Imbert-Fedorov shifts for paraxial X-waves

Marco Ornigotti, Andrea Aiello, Claudio Conti

We present a theoretical analysis for the Goos-Hanchen and Imbert-Fedorov shifts experienced by an X-wave upon reflection from a dielectric interface. We show that the temporal chirp, as well as the bandwidth of the X-wave, directly affect the spatial shifts in a way that can be experimentally observed, while the angular shifts do not depend on the spectral features of the X-wave. A dependence of the spatial shifts on the spatial structure of the X-wave is also discussed. (C) 2015 Optical Society of America

Encapsulation of silver nanowire networks by atomic layer deposition for indium-free transparent electrodes

Manuela Goebelt, Ralf Keding, Sebastian W. Schmitt, Bjoern Hoffmann, Sara Jaeckle, Michael Latzel, Vuk V. Radmilovic, Velimir R. Radmilovic, Erdmann Spiecker, et al.

We report on the development of a novel nano-composite transparent electrode material to be used in various energy applications e.g. as contacts for solar cells, composed of a wet-chemically synthesized silver nanowire (AgNW) network encapsulated in a transparent conductive oxide (TCO) which was deposited with nano-scale precision by atomic layer deposition (ALD). The AgNWs form a random network on a substrate of choice when being drop casted. ALD encapsulation of AgNWs guarantees a conformal and thickness controlled coating of the wires e.g. by the selected aluminum doped zinc oxide (AZO). Annealing of the AgNWs prior to ALD coating, yield a local sintering of AgNWs at their points of intersection, which improves the conductivity of the composite electrodes by reducing their sheet resistance. To demonstrate the performance of these AgNW/AZO composite transparent electrodes, they were used as a top electrode on wafer-based silicon (Si) - solar cells. A novel combination of scanning electron microscopy and image processing is used to determine the degree of percolation of the AgNWs on large areas of the nano-composite AgNW/AZO electrodes. Our results show that the solar cell with percolated AgNW/AZO electrode show the highest short circuit current density (28 mA/cm(2)) and a series resistance in the same order of magnitude compared to reference solar cells with a thermally evaporated silver grid electrode. The electrode example we chose reveals that the developed AgNW/AZO electrode is a technologically relevant and cheap alternative to conventional solar cell screen printed grid electrodes, which contain similar to 95% more Ag per device area, with a high potential to be further systematically optimized by the presented image processing method. (C) 2015 Elsevier Ltd. All rights reserved.

Quantum theory of an electromagnetic observer: Classically behaving macroscopic systems and the emergence of the classical world in quantum electrodynamics

L. I. Plimak, Misha Ivanov, A. Aiello, S. Stenholm

Quantum electrodynamics under conditions of distinguishability of interactingmatter entities, and of controlled actions and back-actions between them, is considered. Such "mesoscopic quantum electrodynamics" is shown to share its dynamical structure with the classical stochastic electrodynamics. In formal terms, we demonstrate that all general relations of the mesoscopic quantum electrodynamics may be recast in a form lacking Planck's constant. Mesoscopic quantum electrodynamics is therefore subject to "doing quantum electrodynamics while thinking classically," allowing one to substitute essentially classical considerations for quantum ones without any loss in generality. Implications of these results for the quantum measurement theory are discussed.

Interfacing transitions of different alkali atoms and telecom bands using one narrowband photon pair source

Gerhard Schunk, Ulrich Vogl, Dmitry V. Strekalov, Michael Foertsch, Florian Sedlmeir, Harald G. L. Schwefel, Manuela Goebelt, Silke Christiansen, Gerd Leuchs, et al.

Quantum information technology strongly relies on the coupling of optical photons with narrowband quantum systems, such as quantum dots, color centers, and atomic systems. This coupling requires matching the optical wavelength and bandwidth to the desired system, which presents a considerable problem for most available sources of quantum light. Here we demonstrate the coupling of alkali dipole transitions with a tunable source of photon pairs. Our source is based on spontaneous parametric downconversion in a triply resonant whispering gallery mode resonator. For this, we have developed novel wavelength-tuning mechanisms that allow a coarse tuning to either the cesium or rubidium wavelength, with subsequent continuous fine-tuning to the desired transition. As a demonstration of the functionality of the source, we performed a heralded single-photon measurement of the atomic decay. We present a major advance in controlling the spontaneous downconversion process, which makes our bright source of heralded single photons now compatible with a plethora of narrowband resonant systems. (C) 2015 Optical Society of America

Extremal quantum states and their Majorana constellations

G. Bjork, A. B. Klimov, P. de la Hoz, M. Grassl, G. Leuchs, L. L. Sanchez-Soto

The characterization of quantum polarization of light requires knowledge of all the moments of the Stokes variables, which are appropriately encoded in the multipole expansion of the density matrix. We look into the cumulative distribution of those multipoles and work out the corresponding extremal pure states. We find that SU(2) coherent states are maximal to any order whereas the converse case of minimal states (which can be seen as the most quantum ones) is investigated for a diverse range of the number of photons. Taking advantage of the Majorana representation, we recast the problem as that of distributing a number of points uniformly over the surface of the Poincare sphere.

Schmidt modes in the angular spectrum of bright squeezed vacuum

P. Sharapova, A. M. Perez, O. V. Tikhonova, M. V. Chekhova

We investigate both theoretically and experimentally strong correlations in macroscopic (bright) quantum states of light generated via unseeded parametric down-conversion and four-wave mixing. The states generated this way contain only quantum noise, without a classical component, and are referred to as bright squeezed vacuum (BSV). Their important advantage is the multimode structure, which offers a larger capacity for the encoding of quantum information. For the theoretical description of these states and their correlation features we introduce a generalized fully analytical approach, based on the concept of independent collective (Schmidt) modes and valid for the cases of both weak and strong nonlinear interaction. In experiment, we generate states of macroscopic BSV with up to 1010 photons per mode and examine large photon-number spatial correlations that are found to be very well described by our theoretical approach.

Measurements of the Electric Field of Zero-Point Optical Phonons in GaAs Quantum Wells Support the Urbach Rule for Zero-Temperature Lifetime Broadening

Rupak Bhattacharya, Richarj Mondal, Pradip Khatua, Alok Rudra, Eli Kapon, Stefan Malzer, Gottfried Doehler, Bipul Pal, Bhavtosh Bansal

We study a specific type of lifetime broadening resulting in the well-known exponential "Urbach tail" density of states within the energy gap of an insulator. After establishing the frequency and temperature dependence of the Urbach edge in GaAs quantum wells, we show that the broadening due to the zero-point optical phonons is the fundamental limit to the Urbach slope in high-quality samples. In rough analogy with Welton's heuristic interpretation of the Lamb shift, the zero-temperature contribution to the Urbach slope can be thought of as arising from the electric field of the zero-point longitudinal-optical phonons. The value of this electric field is experimentally measured to be 3 kV cm(-1), in excellent agreement with the theoretical estimate.

Junction formation and current transport mechanisms in hybrid n-Si/PEDOT:PSS solar cells

Sara Jaeckle, Matthias Mattiza, Martin Liebhaber, Gerald Broenstrup, Mathias Rommel, Klaus Lips, Silke Christiansen

We investigated hybrid inorganic-organic solar cells combining monocrystalline n-type silicon (n-Si) and a highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). The build-in potential, photo- and dark saturation current at this hybrid interface are monitored for varying n-Si doping concentrations. We corroborate that a high build-in potential forms at the hybrid junction leading to strong inversion of the n-Si surface. By extracting work function and valence band edge of the polymer from ultraviolet photoelectron spectroscopy, a band diagram of the hybrid n-Si/PEDOT:PSS heterojunction is presented. The current-voltage characteristics were analyzed using Schottky and abrupt pn-junction models. The magnitude as well as the dependence of dark saturation current on n-Si doping concentration proves that the transport is governed by diffusion of minority charge carriers in the n-Si and not by thermionic emission of majorities over a Schottky barrier. This leads to a comprehensive explanation of the high observed open-circuit voltages of up to 634 mV connected to high conversion efficiency of almost 14%, even for simple planar device structures without antireflection coating or optimized contacts. The presented work clearly shows that PEDOT: PSS forms a hybrid heterojunction with n-Si behaving similar to a conventional pn-junction and not, like commonly assumed, a Schottky junction.

Quantum uncertainty in the beam width of spatial optical modes

Vanessa Chille, Peter Banzer, Andrea Aiello, Gerd Leuchs, Christoph Marquardt, Nicolas Treps, Claude Fabre

We theoretically investigate the quantum uncertainty in the beam width of transverse optical modes and, for this purpose, define a corresponding quantum operator. Single mode states are studied as well as multimode states with small quantum noise. General relations are derived, and specific examples of different modes and quantum states are examined. For the multimode case, we show that the quantum uncertainty in the beam width can be completely attributed to the amplitude quadrature uncertainty of one specific mode, which is uniquely determined by the field under investigation. This discovery provides us with a strategy for the reduction of the beam width noise by an appropriate choice of the quantum state. (C) 2015 Optical Society of America

Phase retrieval from carrier frequency interferograms: reduction of the impact of space-variant disturbances

J. Schwider, V. Nercissian, K. Mantel

Phase "extraction" by using temporal phase shifting is sensitive to vibrations and drifts, producing systematic phase errors periodic with twice the fringe frequency. This error source may be avoided by evaluating only single carrier frequency interferograms, which makes the procedure immune against vibrations and drifts provided that the integration time is short enough to freeze the fringe pattern. However, the phases extracted from single interferograms in this way often show local irregularities depending on the mean phase of the interference pattern. Such local phase irregularities are caused by local disturbances in the light path like specks and dust particles on the optical components of the interferometer. Moreover, since digitized data are gathered, there is a nonlinear processing step involved which is also responsible for the generation of such irregularities. Here, it is proposed to use a set of suitably combined phase-ramped interferograms to reduce phase dependent irregularities. The proposed averaging technique also reduces edge ringing effects known from Fourier evaluation procedures. Since the imaging optics also contributes to the phase to be measured when tilted wavefronts are used, calibration is mandatory. The calibrated state is only valid if strict rules considering fringe number per diameter as well as the position of the wedge in the interferometer are maintained in the measuring process.

Experimental violation of a Bell-like inequality with optical vortex beams

B. Stoklasa, L. Motka, J. Rehacek, Z. Hradil, L. L. Sanchez-Soto, G. S. Agarwal

Optical beams with topological singularities have a Schmidt decomposition. Hence, they display features typically associated with bipartite quantum systems; in particular, these classical beams can exhibit entanglement. This classical entanglement can be quantified by a Bell inequality formulated in terms of Wigner functions. We experimentally demonstrate the violation of this inequality for Laguerre-Gauss (LG) beams and confirm that the violation increases with increasing orbital angular momentum. Our experimental scheme, which is designed to give directly the parity of the Wigner function, yields negativity at the origin for LG(10) beams, whereas for LG(20) we always get a positive value.

Superoscillations with arbitrary polynomial shape

Ioannis Chremmos, George Fikioris

We present a method for constructing superoscillatory functions the superoscillatory part of which approximates a given polynomial with arbitrarily small error in a fixed interval. These functions are obtained as the product of the polynomial with a sufficiently flat, bandlimited envelope function whose Fourier transform has at least N - 1 continuous derivatives and an Nth derivative of bounded variation, N being the order of the polynomial. Polynomials of arbitrarily high order can be approximated if the Fourier transform of the envelope is smooth, i.e. a bump function.

Ratiometric detection of oligonucleotide stoichiometry on multifunctional gold nanoparticles by whispering gallery mode biosensing

F. C. Wu, Y. Wu, Z. Niu, F. Vollmer

A label-free method is developed to ratiometrically determine the stoichiometry of oligonucleotides attached to the surface of gold nanoparticle (GNP) by whispering gallery mode biosensing. Utilizing this scheme, it is furthermore shown that the stoichiometric ratio of GNP attached oligonucleotide species can be controlled by varying the concentration ratio of thiolated oligonucleotides that are used to modify the GNP.

Growth of GaN Micro- and Nanorods on Graphene-Covered Sapphire: Enabling Conductivity to Semiconductor Nanostructures on Insulating Substrates

Martin Heilmann, George Sarau, Manuela Goebelt, Michael Latzel, Sumesh Sadhujan, Christian Tessarek, Silke Christiansen

The self-catalyzed growth of vertically aligned and hexagonally shaped GaN micro- and nanorods on graphene transferred onto sapphire is achieved through metal organic vapor phase epitaxy. However, a great influence of the underlying substrate is evident, since vertically aligned structures with a regular shape could not be grown on graphene transferred to SiO2. The optical properties of the regular GaN nanorods were investigated by spatially and spectrally resolved cathodoluminescence showing defect related emission only near the interface between the sapphire substrate and nanorods but not from their upper part. Micro-raman spectroscopy confirms that the single-layer graphene remains virtually unchanged in terms of the Raman signal, even after undergoing high temperatures (similar to 1200 degrees C) during nanorod growth. Furthermore, Raman mapping demonstrates that GaN structures predominantly grow on defective parts of graphene, giving new insight into the nucleation and growth mechanism of semiconductors on graphene. To validate the conductivity of graphene, when being attached to the sapphire substrate and after the nanorod growth, current voltage investigations were carried out on single, as-grown, GaN nanorods with a nanoprober in a scanning electron microscope. These measurements demonstrate the viability of graphene as a conductive electrode, for example, as a back contact for GaN nanorods grown on insulating sapphire.

Bright squeezed vacuum: Entanglement of macroscopic light beams

M. V. Chekhova, G. Leuchs, M. Zukowski

We discuss various methods to create macroscopic (bright) entangled light beams. As an example, bright squeezed vacuum is considered in detail. This state of light, obtained via high-gain parametric downconversion, manifests entanglement in both photon numbers and polarization. (C) 2014 Elsevier B.V. All rights reserved.

Time-Reversal-Symmetric Single-Photon Wave Packets for Free-Space Quantum Communication

N. Trautmann, G. Alber, G. S. Agarwal, G. Leuchs

Readout and retrieval processes are proposed for efficient, high-fidelity quantum state transfer between a matter qubit, encoded in the level structure of a single atom or ion, and a photonic qubit, encoded in a time-reversal-symmetric single-photon wave packet. They are based on controlling spontaneous photon emission and absorption of a matter qubit on demand in free space by stimulated Raman adiabatic passage. As these processes do not involve mode selection by high-finesse cavities or photon transport through optical fibers, they offer interesting perspectives as basic building blocks for free-space quantum-communication protocols.

Study of high quality spinel zinc gallate nanowires grown using CVD and ALD techniques

Sudheer Kumar, G. Sarau, C. Tessarek, M. Goebelt, S. Christiansen, R. Singh

High quality single crystalline zinc gallate (ZnGa2O4) nanowires (NWs) were grown using a combination of chemical vapor deposition and atomic layer deposition techniques. Morphological, structural and optical investigations revealed the formation of Ga2O3-ZnO core-shell NWs and their conversion into ZnGa2O4 NWs after annealing via a solid state reaction. This material conversion was systematically confirmed for single NWs by various measurement techniques including scanning and transmission electron microscopy, Raman spectroscopy and voltage-dependent cathodoluminescence. Moreover, a model system based on the obtained results has been provided explaining the formation mechanism of the ZnGa2O4 NWs.

Optical Tracking of Anomalous Diffusion Kinetics in Polymer Microspheres

Matthew R. Foreman, Frank Vollmer

In this Letter we propose the use of whispering gallery mode resonance tracking as a label-free optical means to monitor diffusion kinetics in glassy polymer microspheres. Approximate solutions to the governing diffusion equations are derived for the case of slow relaxation and small Stefan number. Transduction of physical changes in the polymer, including formation of a rubbery layer, swelling, and dissolution, into detectable resonance shifts are described using a perturbative approach. Concrete examples of poly(methyl methacrylate) and polystyrene spheres in water are considered.

New Constructions of Codes for Asymmetric Channels via Concatenation

Markus Grassl, Peter W. Shor, Graeme Smith, John Smolin, Bei Zeng

We present new constructions of codes for asymmetric channels for both binary and nonbinary alphabets, based on methods of generalized code concatenation. For the binary asymmetric channel, our methods construct nonlinear single-error-correcting codes from ternary outer codes. We show that some of the Varshamov-Tenengol'ts-Constantin-Rao codes, a class of binary nonlinear codes for this channel, have a nice structure when viewed as ternary codes. In many cases, our ternary construction yields even better codes. For the nonbinary asymmetric channel, our methods construct linear codes for many lengths and distances which are superior to the linear codes of the same length capable of correcting the same number of symmetric errors.

Functionalization of Silver Nanowires Surface using Ag-C Bonds in a Sequential Reductive Method

Muhammad Y. Bashouti, Sebastian Resch, Juergen Ristein, Mirza Mackovic, Erdmann Spiecker, Siegfried. R. Waldvogel, Silke. H. Christiansen

Silver nanowires (Ag-NW) assembled in interdigitated webs have shown an applicative potential as transparent and conducting electrodes. However, upon integration in practical device designs, the presence of silver oxide, which instantaneously forms on the Ag-NW surfaces in ambient conditions, is unwanted. Here, we report on the functionalization of Ag-NWs with 4-nitrophenyl moieties through A-C bonds using a versatile two step reduction process, i.e., ascorbate reduction combined electrografting. We show that 40% of the Ag atop sites were terminated and provide high surface stability toward oxidation for more than 2 months while keeping the same intrinsic conductivity as in bulk silver.

Recent Advances in Theory and Applications of Electromagnetic Metamaterials

Weiren Zhu, Ivan D. Rukhlenko, Roman E. Noskov, Ronghong Jin, Ji Zhou

Direct laser writing of mu-chips based on hybrid C-Au-Ag nanoparticles for express analysis of hazardous and biological substances

M. Y. Bashouti, A. Manshina, A. Povolotckaia, A. Povolotskiy, A. Kireev, Y. Petrov, M. Mackovic, E. Spiecker, I. Koshevoy, et al.

Micro-chips based on organic-inorganic hybrid nanoparticles (NPs) composed of nanoalloys of gold (Au) and silver (Ag) embedded in an amorphous carbonaceous matrix (C-Au-Ag NPs) were prepared directly on a substrate by the laser-induced deposition (for short: LID) method. The C-Au-Ag NPs show a unique plasmon resonance which enhances Raman scattering of analytes, making the mu-chips suitable to detect ultra-low-volumes (10(-12) liter) and concentrations (10(-9) M) of bio-agents and a hazardous compound. These micro-chips constitute a novel, flexible solid-state device that can be used for applications in point-of-care diagnostics, consumer electronics, homeland security and environmental monitoring.

Quantum squeezing of motion in a mechanical resonator

E. E. Wollman, C. U. Lei, A. J. Weinstein, J. Suh, A. Kronwald, F. Marquardt, A. A. Clerk, K. C. Schwab

According to quantum mechanics, a harmonic oscillator can never be completely at rest. Even in the ground state, its position will always have fluctuations, called the zero-point motion. Although the zero-point fluctuations are unavoidable, they can be manipulated. Using microwave frequency radiation pressure, we have manipulated the thermal fluctuations of a micrometer-scale mechanical resonator to produce a stationary quadrature-squeezed state with a minimum variance of 0.80 times that of the ground state. We also performed phase-sensitive, back-action evading measurements of a thermal state squeezed to 1.09 times the zero-point level. Our results are relevant to the quantum engineering of states of matter at large length scales, the study of decoherence of large quantum systems, and for the realization of ultrasensitive sensing of force and motion.

Nonlinear Radiation Pressure Dynamics in an Optomechanical Crystal

Alex G. Krause, Jeff T. Hill, Max Ludwig, Amir H. Safavi-Naeini, Jasper Chan, Florian Marquardt, Oskar Painter

Utilizing a silicon nanobeam optomechanical crystal, we investigate the attractor diagram arising from the radiation pressure interaction between a localized optical cavity at lambda(c) = 1542 nm and a mechanical resonance at omega(m)/2 pi = 3.72 GHz. At a temperature of T-b approximate to 10 K, highly nonlinear driving of mechanical motion is observed via continuous wave optical pumping. Introduction of a time-dependent (modulated) optical pump is used to steer the system towards an otherwise inaccessible dynamically stable attractor in which mechanical self-oscillation occurs for an optical pump red detuned from the cavity resonance. An analytical model incorporating thermo-optic effects due to optical absorption heating is developed and found to accurately predict the measured device behavior.

Optomechanical Dirac physics

M. Schmidt, V. Peano, F. Marquardt

Recent progress in optomechanical systems may soon allow the realization of optomechanical arrays, i.e. periodic arrangements of interacting optical and vibrational modes. We show that photons and phonons on a honeycomb lattice will produce an optically tunable Dirac-type band structure. Transport in such a system can exhibit transmission through an optically created barrier, similar to Klein tunneling, but with interconversion between light and sound. In addition, edge states at the sample boundaries are dispersive and enable controlled propagation of photon-phonon polaritons.

Effective medium theory for two-dimensional non-magnetic metamaterial lattices up to quadrupole expansions

Ioannis Chremmos, Efthymios Kallos, Melpomeni Giamalaki, Vassilios Yannopapas, Emmanuel Paspalakis

We present a formulation for deriving effective medium properties of infinitely periodic two-dimensional metamaterial lattice structures beyond the conventional static and quasi-static limits. We utilize the multipole expansions, where the polarization currents associated with the supported Bloch modes are expressed via the electric dipole, magnetic dipole, and electric quadrupole moments per unit length. We then propose a method to calculate the Bloch modes based on the lattice geometry and individual unit element structure. The results revert to well-known formulas in the traditional quasistatic limit and are useful for the homogenization of nanorod-type metamaterials which are frequently used in optical applications.

Fabrication and characterization of plasmonic nanocone antennas for strong spontaneous emission enhancement

Bjoern Hoffmann, Simon Vassant, Xue-Wen Chen, Stephan Goetzinger, Vahid Sandoghdar, Silke Christiansen

Plasmonic antennas are attractive nanostructures for a large variety of studies ranging from fundamental aspects of light-matter interactions at the nanoscale to industry-relevant applications such as ultrasensitive sensing, enhanced absorption in solar cells or solar fuel generation. A particularly interesting feature of these antennas is that they can enhance the fluorescence properties of emitters. Theoretical calculations have shown that nanocone antennas provide ideal results, but a high degree of manufacturing precision and control is needed to reach optimal performance. In this study, we report on the fabrication of nanocones with base diameters and heights in the range of 100 nm with variable aspect ratios using focused ion beam milling of sputtered nano-crystalline gold layers. The controlled fabrication process allows us to obtain cones with tailored plasmon resonances. The measured plasmon spectra show very good agreement with finite-difference time-domain calculations. Theoretical investigations predict that these nanocones can enhance the spontaneous emission rate of a quantum emitter by several hundred times while keeping its quantum efficiency above 60%.

Enhanced optical activity and circular dichroism in twisted photonic crystal fiber

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

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

Modified and controllable dispersion interaction in a one-dimensional waveguide geometry

Harald R. Haakh, Stefan Scheel

Dispersion interactions such as the van derWaals interaction between atoms or molecules derive from quantum fluctuations of the electromagnetic field and can be understood as the exchange of virtual photons between the interacting partners. Any modification of the environment in which those photons propagate will thus invariably lead to an alteration of the van der Waals interaction. Here we show how the two-body dispersion interaction inside a cylindrical waveguide can be made to decay asymptotically exponentially and how this effect sensitively depends on the material properties and the length scales of the problem, eventually leading to the possibility of controllable interactions. Further, we discuss the possibility to detect the retarded van der Waals interaction by resonant enhancement of the interaction between Rydberg atoms in the light of long-range potentials due to guided modes.

Effect of rapid thermal annealing on barrier height and 1/f noise of Ni/GaN Schottky barrier diodes

Ashutosh Kumar, M. Latzel, S. Christiansen, V. Kumar, R. Singh

Current-voltage (as a function of temperature), capacitance-voltage, and 1/f noise characteristics of Ni/GaN Schottky barrier diodes (SBDs) as function of rapid thermal annealing (RTA) are studied. It is found that RTA treatments of SBDs at 450 degrees C for 60 s resulted in a significant improvement of ideality factor and Schottky barrier height: the ideality factor decreased from 1.79 to 1.12 and the barrier height increased from 0.94 to 1.13 eV. The spectral power density of current fluctuations in the diode subjected to RTA at 450 degrees C is found to be two orders of magnitude lower as compared to the as-deposited diode. Improved diode characteristics and decreased 1/f noise in RTA treated (450 degrees C/60 s) diode are attributed to reduced level of barrier inhomogeneities at the metal-semiconductor interface and explained within the framework of the spatial inhomogeneity model. (C) 2015 AIP Publishing LLC.

Highly efficient generation of single-mode photon pairs from a crystalline whispering-gallery-mode resonator source

Michael Foertsch, Gerhard Schunk, Josef U. Fuerst, Dmitry Strekalov, Thomas Gerrits, Martin J. Stevens, Florian Sedlmeir, Harald G. L. Schwefel, Sae Woo Nam, et al.

We report a highly efficient source of narrow-band photon pairs based on parametric down-conversion in a crystalline-whispering-gallery-mode resonator. Remarkably, each photon of a pair is detected in a single spatial and temporal mode, as witnessed by Glauber's autocorrelation function. We explore the phase-matching conditions in spherical geometries, and determine the requirements for single-mode operation. Understanding these conditions has allowed us to experimentally demonstrate a single-mode pair-detection efficiency of 1.13 x 10(6) pairs/s per mW pump power per 26.8 MHz bandwidth.

Depolarized guided acoustic wave Brillouin scattering in hollow-core photonic crystal fibers

Wenjia Elser Nee Zhong, Birgit Stiller, Dominique Elser, Bettina Heim, Christoph Marquardt, Gerd Leuchs

By performing quantum-noise-limited optical heterodyne detection, we observe polarization noise in light after propagation through a hollow-core photonic crystal fiber (PCF). We compare the noise spectrum to the one of a standard fiber and find an increase of noise even though the light is mainly transmitted in air in a hollow-core PCF. Combined with our simulation of the acoustic vibrational modes in the hollow-core PCF, we are offering an explanation for the polarization noise with a variation of guided acoustic wave Brillouin scattering (GAWBS). Here, instead of modulating the strain in the fiber core as in a solid core fiber, the acoustic vibrations in hollow-core PCF influence the effective refractive index by modulating the geometry of the photonic crystal structure. This induces polarization noise in the light guided by the photonic crystal structure. (C) 2015 Optical Society of America

Specially shaped Bessel-like self-accelerating beams along predesigned trajectories

Juanying Zhao, I. D. Chremmos, Ze Zhang, Yi Hu, Daohong Song, Peng Zhang, N. K. Efremidis, Zhigang Chen

Over the past several years, spatially shaped self-accelerating beams along different trajectories have been studied extensively. Due to their useful properties such as resistance to diffraction, self-healing, and self-bending even in free space, these beams have attracted great attention with many proposed applications. Interestingly, some of these beams could be designed with controllable spatial profiles and thus propagate along various desired trajectories such as parabolic, snake-like, hyperbolic, hyperbolic secant, three-dimensional spiraling, and even self-propelling trajectories. Experimentally, such beams are realized typically by using a spatial light modulator so as to imprint a desired phase distribution on a Gaussian-like input wave front propagating under paraxial or nonparaxial conditions. In this paper, we provide a brief overview of our recent work on specially shaped self-accelerating beams, including Bessel-like, breathing Bessel-like, and vortex Bessel-like beams. In addition, we propose and demonstrate a new type of dynamical Bessel-like beams that can exhibit not only self-accelerating but also self-propelling during propagation. Both theoretical and experimental results are presented along with a brief discussion of potential applications.

Dielectric Rod Waveguide Antenna as THz Emitter for Photomixing Devices

Alejandro Rivera-Lavado, Sascha Preu, Luis Enrique Garcia-Munoz, Andrey Generalov, Javier Montero-de-Paz, Gottfried Doehler, Dmitri Lioubtchenko, Mario Mendez-Aller, Florian Sedlmeir, et al.

We propose a dielectric rod waveguide antenna (DRW) integrated with a photomixer as a THz emitter. This represents a different approach as opposed to the classical solution of a substrate lens. Main goals are an inexpensive alternative to substrate lenses, reduction of both reflections on the semiconductor-air interface and scattering of terahertz-generated power into the substrate. A radiation pattern measured at 137 GHz is shown as a proof-of-concept. In order to increase radiated power, the improvement of the rod antenna is discussed. Finally, as an application example, evanescent coupling of the DRW into a high index whispering gallery mode resonator is shown.

Quantum Uniqueness

Denis Sych, Gerd Leuchs

Classical physics allows for the existence of pairs of absolutely identical systems. Pairwise application of identical measurements to each of those systems always leads to exactly alike results irrespectively of the choice of measurements. Here we ask a question how the picture looks like in the quantum domain. Surprisingly, we get a counterintuitive outcome. Pairwise application of identical (but a priori unknown) measurements cannot always lead to exactly alike results. We interpret this as quantum uniqueness-a feature that has no classical analog.

Whispering gallery mode sensors

Matthew R. Foreman, Jon D. Swaim, Frank Vollmer

We present a comprehensive overview of sensor technology exploiting optical whispering gallery mode (WGM) resonances. After a short introduction we begin by detailing the fundamental principles and theory of WGMs in optical microcavities and the transduction mechanisms frequently employed for sensing purposes. Key recent theoretical contributions to the modeling and analysis of WGM systems are highlighted. Subsequently we review the state of the art of WGM sensors by outlining efforts made to date to improve current detection limits. Proposals in this vein are numerous and range, for example, from plasmonic enhancements and active cavities to hybrid optomechanical sensors, which are already working in the shot noise limited regime. In parallel to furthering WGM sensitivity, efforts to improve the time resolution are beginning to emerge. We therefore summarize the techniques being pursued in this vein. Ultimately WGM sensors aim for real-world applications, such as measurements of force and temperature, or alternatively gas and biosensing. Each such application is thus reviewed in turn, and important achievements are discussed. Finally, we adopt a more forward-looking perspective and discuss the outlook of WGM sensors within both a physical and biological context and consider how they may yet push the detection envelope further. (C) 2015 Optical Society of America

Higher-order Dirac solitons in binary waveguide arrays

Truong X. Tran, Dung C. Duong

We study optical analogues of higher-order Dirac solitons (HODSs) in binary waveguide arrays. Like higher-order solitons obtained from the well-known nonlinear Schrodinger equation governing the pulse propagation in an optical fiber, these HODSs have amplitude profiles which are numerically shown to be periodic over large propagation distances. At the same time, HODSs possess some unique features. Firstly, the period of a HODS depends on its order parameter. Secondly, the discrete nature in binary waveguide arrays imposes the upper limit on the order parameter of HODSs. Thirdly, the order parameter of HODSs can vary continuously in a certain range. (C) 2015 Elsevier Inc. All rights reserved.

Least-bias state estimation with incomplete unbiased measurements

Jaroslav Rehacek, Zdenek Hradil, Yong Siah Teo, Luis L. Sanchez-Soto, Hui Khoon Ng, Jing Hao Chai, Berthold-Georg Englert

Measuring incomplete sets of mutually unbiased bases constitutes a sensible approach to the tomography of high-dimensional quantum systems. The unbiased nature of these bases optimizes the uncertainty hypervolume. However, imposing unbiasedness on the probabilities for the unmeasured bases does not generally yield the estimator with the largest von Neumann entropy, a popular figure of merit in this context. Furthermore, this imposition typically leads to mock density matrices that are not even positive definite. This provides a strong argument against perfunctory applications of linear estimation strategies. We propose to use instead the physical state estimators that maximize the Shannon entropy of the unmeasured outcomes, which quantifies our lack of knowledge fittingly and gives physically meaningful statistical predictions.

Flying particle sensors in hollow-core photonic crystal fibre

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

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

Microcavity design for low threshold polariton condensation with ultrashort optical pulse excitation

C. Poellmann, U. Leierseder, E. Galopin, A. Lemaitre, A. Amo, J. Bloch, R. Huber, J. -M. Menard

We present a microcavity structure with a shifted photonic stop-band to enable efficient non-resonant injection of a polariton condensate with spectrally broad femtosecond pulses. The concept is demonstrated theoretically and confirmed experimentally for a planar GaAs/AlGaAs multilayer heterostructure pumped with ultrashort near-infrared pulses while photoluminescence is collected to monitor the optically injected polariton density. As the excitation wavelength is scanned, a regime of polariton condensation can be reached in our structure at a consistently lower fluence threshold than in a state-of-the-art conventional microcavity. Our microcavity design improves the polariton injection efficiency by a factor of 4, as compared to a conventional microcavity design, when broad excitation pulses are centered at a wavelength of lambda = 740 nm. Most remarkably, this improvement factor reaches 270 when the excitation wavelength is centered at 750 nm. (c) 2015 AIP Publishing LLC.

Stand-Off Biodetection with Free-Space Coupled Asymmetric Microsphere Cavities

Zachary Ballard, Martin D. Baaske, Frank Vollmer

Asymmetric microsphere resonant cavities (ARCs) allow for free-space coupling to high quality (Q) whispering gallery modes (WGMs) while exhibiting highly directional light emission, enabling WGM resonance measurements in the far-field. These remarkable characteristics make "stand-off" biodetection in which no coupling device is required in near-field contact with the resonator possible. Here we show asymmetric microsphere resonators fabricated from optical fibers which support dynamical tunneling to excite high-Q WGMs, and demonstrate free-space coupling to modes in an aqueous environment. We characterize the directional emission by fluorescence imaging, demonstrate coupled mode effects due to free space coupling by dynamical tunneling, and detect adsorption kinetics of a protein in aqueous solution. Based on our approach, new, more robust WGM biodetection schemes involving microfluidics and in-vivo measurements can be designed.

Direct Schmidt number measurement of high-gain parametric down conversion

I. V. Dyakonov, P. R. Sharapova, T. Sh Iskhakov, G. Leuchs

In this work we estimate the transverse Schmidt number for the bipartite high-gain parametric down conversion state by means of second-order intensity correlation function measurement. Assuming that the number of modes is equal in both beams we determine the Schmidt number considering only one of the subsystems. The obtained results demonstrate that this approach is equally efficient over the whole propagation of the state from the near field to the far field regions of its emitter.

Note on the helicity decomposition of spin and orbital optical currents

Andrea Aiello, M. V. Berry

In the helicity representation, the Poynting vector (current) for a monochromatic optical field, when calculated using either the electric or the magnetic field, separates into right-handed and left-handed contributions, with no cross-helicity contributions. Cross-helicity terms do appear in the orbital and spin contributions to the current. But when the electric and magnetic formulas are averaged ('electric-magnetic democracy'), these terms cancel, restoring the separation into right-handed and left-handed currents for orbital and spin separately.

Modulational instability windows in the nonlinear Schrodinger equation involving higher-order Kerr responses

David Novoa, Daniele Tommasini, Jose A. Novoa-Lopez

We introduce a complete analytical and numerical study of the modulational instability process in a system governed by a canonical nonlinear Schrodinger equation involving local, arbitrary nonlinear responses to the applied field. In particular, our theory accounts for the recently proposed higher-order Kerr nonlinearities, providing very simple analytical criteria for the identification of multiple regimes of stability and instability of plane-wave solutions in such systems. Moreover, we discuss a new parametric regime in the higher-order Kerr response, which allows for the observation of several, alternating stability-instability windows defining a yet unexplored instability landscape.

Optomechanical creation of magnetic fields for photons on a lattice

M. Schmidt, S. Kessler, V. Peano, O. Painter, F. Marquardt

Recently, there has been growing interest in the creation of artificial magnetic fields for uncharged particles, such as cold atoms or photons. These efforts are partly motivated by the resulting desirable features, such as transport along edge states that is robust against backscattering. We analyze how the optomechanical interaction between photons and mechanical vibrations can be used to create artificial magnetic fields for photons on a lattice. The ingredients required are an optomechanical crystal, i. e., a free-standing photonic crystal with localized vibrational and optical modes, and two laser beams with the right pattern of phases. One of the two schemes analyzed here is based on optomechanical modulation of the links between optical modes, while the other is a lattice extension of optomechanical wavelength-conversion setups. We analyze both schemes theoretically and present numerical simulations of the resulting optical spectrum, photon transport in the presence of an artificial Lorentz force, edge states, and the photonic Aharonov Bohm effect. We discuss the requirements for experimental realizations. Finally, we analyze the completely general situation of an optomechanical system subject to an arbitrary optical phase pattern and conclude that it is best described in terms of gauge fields acting in synthetic dimensions. In contrast to existing nonoptomechanical approaches, the schemes analyzed here are very versatile, since they can be controlled fully optically, allowing for time-dependent in situ tunability without the need for individual electrical addressing of localized optical modes. (C) 2015 Optical Society of America