Synchronizing a single-electron shuttle to an external drive

Michael Möckel, Darren R. Southworth, Eva M. Weig, Florian Marquardt

The nanomechanical single-electron shuttle is a resonant system in which a suspended metallic island oscillates between and impacts at two electrodes. This setup holds promise for one-by-one electron transport and the establishment of an absolute current standard. While the charge transported per oscillation by the nanoscale island will be quantized in the Coulomb blockade regime, the frequency of such a shuttle depends sensitively on many parameters, leading to <br>drift and noise. Instead of considering the nonlinearities introduced by the impact events as a nuisance, here we propose to exploit the resulting nonlinear dynamics to realize a highly precise oscillation frequency via synchronization of the shuttle self-oscillations to an external signal. We link the established phenomenological description of synchronization based on the ADLER equation to the microscopic nonlinear dynamics of the electron shuttle by calculating the effective ADLER constant analytically in terms of the microscopic parameters.

High-cooperativity nanofiber laser

Sanli Faez, Pierre Türschmann, Vahid Sandoghdar

Cavity-free efficient coupling between emitters and guided modes is of great<br>current interest for nonlinear quantum optics as well as efficient and scalable<br>quantum information processing. In this work, we extend these activities to the<br>coupling of organic dye molecules to a highly confined mode of a nanofiber,<br>allowing mirrorless and low-threshold laser action in an effective mode volume<br>of less than 100 femtoliters. We model this laser system based on<br>semi-classical rate equations and present an analytic compact form of the laser<br>output intensity. Despite the lack of a cavity structure, we achieve a coupling<br>efficiency of the spontaneous emission to the waveguide mode of 0.07(0.01), in<br>agreement with our calculations. In a further experiment, we also demonstrate<br>the use of a plasmonic nanoparticle as a dispersive output coupler. Our laser<br>architecture is promising for a number of applications in optofluidics and<br>provides a fundamental model system for studying nonresonant feedback<br>stimulated emission.

Shock-induced PT-symmetric potentials in gas-filled photonic-crystal fibers

Mohammed F. Saleh, Andrea Marini, Fabio Biancalana

We have investigated the interaction between a strong soliton and a weak probe with certain configurations that allow optical trapping in gas-filled hollow-core photonic-crystal fibers in the presence of the shock effect. We have shown theoretically and numerically that the shock term can lead to an unbroken parity-time-(PT-) symmetric potential in these kinds of fibers. Time irreversible behavior, a signature feature of the PT symmetry, is also demonstrated numerically. Our results will open different configurations and avenues for observing PT-symmetry breaking in optical fibers, without the need to resort to complex optical systems.

Identifying modes of large whispering-gallery mode resonators from the spectrum and emission pattern

Gerhard Schunk, Josef U. Fuerst, Michael Foertsch, Dmitry V. Strekalov, Ulrich Vogl, Florian Sedlmeir, Harald G. L. Schwefel, Gerd Leuchs, Christoph Marquardt

Identifying the mode numbers in whispering-gallery mode resonators (WGMRs) is important for tailoring them to experimental needs. Here we report on a novel experimental mode analysis technique based on the combination of frequency analysis and far-field imaging for high mode numbers of large WGMRs. The radial mode numbers q and the angular mode numbers p = l-m are identified and labeled via far-field imaging. The polar mode numbers l are determined unambiguously by fitting the frequency differences between individual whispering gallery modes (WGMs). This allows for the accurate determination of the geometry and the refractive index at different temperatures of the WGMR. For future applications in classical and quantum optics, this mode analysis enables one to control the narrow-band phase-matching conditions in nonlinear processes such as second-harmonic generation or parametric down-conversion. (C) 2014 Optical Society of America

Trojan-horse attacks threaten the security of practical quantum cryptography

Nitin Jain, Elena Anisimova, Imran Khan, Vadim Makarov, Christoph Marquardt, Gerd Leuchs

A quantum key distribution (QKD) system may be probed by an eavesdropper Eve by sending in bright light from the quantum channel and analyzing the back-reflections. We propose and experimentally demonstrate a setup for mounting such a Trojan-horse attack. We show it in operation against the quantum cryptosystem Clavis2 from ID Quantique, as a proof-of-principle. With just a few back-reflected photons, Eve discerns Bob's (secret) basis choice, and thus the raw key bit in the Scarani-Acin-Ribordy-Gisin 2004 protocol, with higher than 90% probability. This would clearly breach the security of the cryptosystem. Unfortunately, Eve's bright pulses have a side effect of causing a high level of afterpulsing in Bob's single-photon detectors, resulting in a large quantum bit error rate that effectively protects this system from our attack. However, in a Clavis2-like system equipped with detectors with less-noisy but realistic characteristics, an attack strategy with positive leakage of the key would exist. We confirm this by a numerical simulation. Both the eavesdropping setup and strategy can be generalized to attack most of the current QKD systems, especially if they lack proper safeguards. We also propose countermeasures to prevent such attacks.

Wave-optics description of self-healing mechanism in Bessel beams

Andrea Aiello, Girish S. Agarwal

Bessel beams' great importance in optics lies in that these propagate without spreading and can reconstruct themselves behind an obstruction placed across their path. However, a rigorous wave-optics explanation of the latter property is missing. In this work, we study the reconstruction mechanism by means of a wave-optics description. We obtain expressions for the minimum distance beyond the obstruction at which the beam reconstructs itself, which are in close agreement with the traditional one determined from geometrical optics. Our results show that the physics underlying the self-healing mechanism can be entirely explained in terms of the propagation of plane waves with radial wave vectors lying on a ring. (C) 2014 Optical Society of America

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

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

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

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

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

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

Unpolarized states and hidden polarization

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

We capitalize on a multipolar expansion of the polarization density matrix, in which multipoles appear as successive moments of the Stokes variables. When all the multipoles up to a given order K vanish, we can properly say that the state is Kth-order unpolarized, as it lacks of polarization information to that order. First-order unpolarized states coincide with the corresponding classical ones, whereas unpolarized to any order tally with the quantum notion of fully invariant states. In between these two extreme cases, there is a rich variety of situations that are explored here. The existence of hidden polarization emerges in a natural way in this context.

Study of iron-catalysed growth of beta-Ga2O3 nanowires and their detailed characterization using TEM, Raman and cathodoluminescence techniques

Sudheer Kumar, G. Sarau, C. Tessarek, Muhammad Y. Bashouti, A. Haehnel, S. Christiansen, R. Singh

In this paper, we demonstrate a new catalyst (Fe) to grow single crystalline beta-gallium oxide (beta-Ga2O3) nanowires (NWs) via the vapour-liquid-solid mechanism using the chemical vapour deposition technique. The structural studies of these NWs showed the highly crystalline monoclinic phase of Ga2O3. This was confirmed by detailed scanning transmission electron microscope investigations demonstrating the NW to be single crystalline beta-Ga2O3, growing along the normal of the (1 (1) over bar (1) over bar) plane. We also compared Raman and cathodoluminescence (CL) properties of the as-grown beta-Ga2O3 NWs with a bulk Ga2O3 single crystal grown by the Czochralski method. It was observed that Raman peak positions of a single beta-Ga2O3 NW had a red frequency shift of about 0.3-1.4 cm(-1) as compared to a bulk Ga2O3 single crystal, which was in fact quite small. In addition, the CL measurements of beta-Ga2O3 NWs and the bulk Ga2O3 single crystal exhibited similar spectra, having a strong broad UV-blue emission band and a weak red emission band. Moreover, the structural, morphological and optical properties of Fe-catalysed beta-Ga2O3 NWs were comparable to those of Au-catalysed beta-Ga2O3 NWs.

XPS study of triangular GaN nano/micro-needles grown by MOCVD technique

Mukesh Kumar, Ashish Kumar, S. B. Thapa, S. Christiansen, R. Singh

Triangular GaN nano/micro scale needles (TGN) grown on nickel coated c-plane sapphire substrate with highly dense ensemble of TGN and low dense ensemble of TGN have been investigated in the present work. The observed morphology of these TGN is in the form of triangular faceted needle like structures with average length in the order of fifty micrometres and cross-sections range from 500 nm to 3 mu m near the base of TGN. X-ray diffraction spectra illustrate the wurtzite crystal structure of TGN and better crystalline nature of the highly dense ensemble of TGN. Analysis of the X-ray photoelectron spectroscopy core level spectra shows that surfaces with highly dense ensemble of TGN and low dense ensemble of TGN interact differently with the unintended impurities (such as oxygen and carbon) and these impurities exhibit low reactivity (chemical modifications) to the surface of highly dense ensemble of TGN. (c) 2014 Elsevier B.V. All rights reserved.

Classical entanglement in polarization metrology

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

Quantum approaches relying on entangled photons have been recently proposed to increase the efficiency of optical measurements. We demonstrate here that, surprisingly, the use of classical light with entangled degrees of freedom can also bring outstanding advantages over conventional measurements in polarization metrology. Specifically, we show that radially polarized beams of light allow to perform real-time single-shot Mueller matrix polarimetry. Our results also indicate that quantum optical procedures requiring entanglement without nonlocality can be actually achieved in the classical optics regime.

The Hertz vector revisited: a simple physical picture

Marco Ornigotti, Andrea Aiello

The polarization potentials, also known as Hertz vectors, are useful auxiliary fields that permit the calculation of the fundamental electromagnetic fields in many cases of practical importance. In this article we show that in a vacuum a single Hertz vector written as the product of a scalar potential and a constant vector, naturally arises as consequence of the transversality of the electromagnetic fields. Thus, our treatment shines a new light on the physical meaning of a Hertz potential.

Detection of non-classical space-time correlations with a novel type of single-photon camera

Felix Just, Mykhaylo Filipenko, Andrea Cavanna, Thilo Michel, Thomas Gleixner, Michael Taheri, John Vallerga, Michael Campbell, Timo Tick, et al.

During the last decades, multi-pixel detectors have been developed capable of registering single photons. The newly developed hybrid photon detector camera has a remarkable property that it has not only spatial but also temporal resolution. In this work, we apply this device to the detection of non-classical light from spontaneous parametric down-conversion and use two-photon correlations for the absolute calibration of its quantum efficiency. (C) 2014 Optical Society of America

Taking detection to the limit with optical microcavities: Recent advances presented at the 560. WE Heraeus Seminar

Frank Vollmer, Harald G. L. Schwefel

We provide a review on the applications of whispering gallery mode resonators in sensing, and biosensing in particular. We highlight the most recent developments in this area, which were presented at the 560. WE Heraeus Seminar "Taking Detection to the Limit - Biosensing with Optical Microcavities".

Scanning-aperture trapping and manipulation of single charged nanoparticles

Ji Tae Kim, Susann Spindler, Vahid Sandoghdar

Although trapping and manipulation of small objects have been of interest for a range of applications and many clever techniques have been devised, new methods are still in great demand for handling different materials and geometries. Here, we report on an electrostatic trap that is created in an aqueous medium between the aperture of a nanopipette and a glass substrate without the need for external potentials. After a thorough characterization of the trapping conditions, we show that we can displace or release a particle at will. Furthermore, we demonstrate trapping and manipulation of nanoparticles and lipid vesicles attached to lipid bilayers, paving the way for controlled studies of forces and diffusion associated with biological membranes. We expect the technique to find interesting applications also in other areas such as optonanofluidics and plasmonics.

Optical analog of spontaneous symmetry breaking induced by tachyon condensation in amplifying plasmonic arrays

A. Marini, Tr. X. Tran, S. Roy, S. Longhi, F. Biancalana

We study analytically and numerically an optical analog of tachyon condensation in amplifying plasmonic arrays. Optical propagation is modeled through coupled-mode equations, which in the continuous limit can be converted into a nonlinear one-dimensional Dirac-like equation for fermionic particles with imaginary mass, i.e., fermionic tachyons. We demonstrate that the vacuum state is unstable and acquires an expectation value with broken chiral symmetry, corresponding to the homogeneous nonlinear stationary solution of the system. The quantum field theory analog of this process is the condensation of unstable fermionic tachyons into massive particles. This paves the way for using amplifying plasmonic arrays as a classical laboratory for spontaneous symmetry breaking effects in quantum field theory.

Nanointerferometric amplitude and phase reconstruction of tightly focused vector beams

Thomas Bauer, Sergej Orlov, Ulf Peschel, Peter Banzer, Gerd Leuchs

Highly confined vectorial electromagnetic field distributions are an excellent tool for detailed studies in nano-optics, such as nonlinear microscopy(1), advanced fluorescence imaging(2,3) or nanoplasmonics(4,5). Such field distributions can be generated, for instance, by tight focusing of polarized light beams(6-9). To guarantee high resolution in the investigation of objects with subwavelength dimensions, precise knowledge of the spatial distribution of the exciting vectorial field is of utmost importance. The full-field reconstruction methods presented to date involve, for example, complex near-field techniques(10-13). Here, we demonstrate a simple and straightforward-to-implement measurement scheme and reconstruction algorithm based on the scattering signal of a single spherical nanoparticle as a field probe. We are able to reconstruct the amplitudes and relative phases of the individual focal field components with subwavelength resolution from a single scan measurement without the need for polarization analysis of the scattered light. This scheme has the potential to improve microscopy and nanoscopy techniques.

Suppression and splitting of modulational instability sidebands in periodically tapered optical fibers because of fourth-order dispersion

Andrea Armaroli, Fabio Biancalana

We study the modulational instability induced by periodic variations of group-velocity dispersion in the proximity of the zero dispersion point. Multiple instability peaks originating from parametric resonance coexist with the conventional modulation instability because of fourth-order dispersion, which in turn is suppressed by the oscillations of dispersion. Moreover, isolated unstable regions appear in the space of parameters because of imperfect phase matching. This confirms the dramatic effect of periodic tapering in the control and shaping of MI sidebands in optical fibers. (C) 2014 Optical Society of America

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

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

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

Focus on optomechanics

Ivan Favero, Florian Marquardt

We provide a brief overview of the various topics addressed in this 'focus on' collection on optomechanics.

Optimizing detection limits in whispering gallery mode biosensing

Matthew R. Foreman, Wei-Liang Jin, Frank Vollmer

A theoretical analysis of detection limits in swept-frequency whispering gallery mode biosensing modalities is presented based on application of the Cramer-Rao lower bound. Measurement acuity factors are derived assuming the presence of uncoloured and 1/f Gaussian technical noise. Frequency fluctuations, for example arising from laser jitter or thermorefractive noise, are also considered. Determination of acuity factors for arbitrary coloured noise by means of the asymptotic Fisher information matrix is highlighted. Quantification and comparison of detection sensitivity for both resonance shift and broadening sensing modalities are subsequently given. Optimal cavity and coupling geometries are furthermore identified, whereby it is found that slightly under-coupled cavities outperform critically and over coupled ones. (C) 2014 Optical Society of America

Expanding the genetic code for site-specific labelling of tobacco mosaic virus coat protein and building biotin-functionalized virus-like particles

F. C. Wu, H. Zhang, Q. Zhou, M. Wu, Z. Ballard, Y. Tian, J. Y. Wang, Z. W. Niu, Y. Huang

A method for site-specific and high yield modification of tobacco mosaic virus coat protein (TMVCP) utilizing a genetic code expanding technology and copper free cycloaddition reaction has been established, and biotin-functionalized virus-like particles were built by the self-assembly of the protein monomers.

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

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

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

Conformational distribution of surface-adsorbed fibronectin molecules explored by single molecule localization microscopy

E. Klotzsch, I. Schoen, J. Ries, A. Renn, V. Sandoghdar, V. Vogel

Adsorbed proteins that promote cell adhesion mediate the response of cells to biomaterials and scaffolds. As proteins undergo conformational changes upon surface adsorption, their functional display may be significantly affected by surface chemistry or solution conditions during the adsorption process. A high-resolution localization microscopy technique is extended here to probe the conformation of individual fibronectin (Fn) molecules at the glass-water interface under physiological buffer conditions. To map distances, four available cysteines located on the modules FnIII(7) and FnIII(15) of dimeric Fn were site-specifically labeled with Cy3B, and their relative positions were determined by stepwise photobleaching with nanometer precision. The four labels on single Fn molecules did not show a uniform or linear arrangement. The distances between label positions were distributed asymmetrically around 33 nm with a tail towards higher distances. Exposure of Fn to denaturing solution conditions during adsorption increased the average distances up to 43 nm for 4 M guanidinium HCl, while changing the solution conditions after the adsorption had no effect, indicating that the observed intra-molecular distances are locked-in during the adsorption process. Also surface coatings of different hydrophobicity altered the conformational distribution, shifting label distances from a median of 24 nm on hydrophilic to 49 nm on hydrophobic surfaces. These results further highlight that the conformation of macromolecules at interfaces depends on the adsorption history. While illustrated here for surface adsorbed Fn, the power of localization-based microscopy extends the repertoire of techniques to characterize biomolecules at interfaces.

Experimental realization of an optical antenna designed for collecting 99% of photons from a quantum emitter

X. -L. Chu, T. J. K. Brenner, X. -W. Chen, Y. Ghosh, J. A. Hollingsworth, V. Sandoghdar, Stephan Götzinger

A light source that emits single photons at well-defined times and into a well-defined mode would be a decisive asset for quantum information processing, quantum metrology, and sub-shot-noise detection of absorption. One of the central challenges in the realization of such a deterministic device based on a single quantum emitter concerns the collection of the photons, which are radiated into a 4 pi solid angle. Here, we present the fabrication and characterization of an optical antenna designed to convert the dipolar radiation of an arbitrarily oriented quantum emitter to a directional beam with more than 99% efficiency. Our approach is extremely versatile and can be used for more efficient detection of nanoscopic emitters ranging from semiconductor quantum dots to dye molecules, color centers, or rare-earth ions in various environments. Having addressed the issue of collection efficiency, we also discuss the photophysical limitations of the existing quantum emitters for the realization of a deterministic single-photon source. (C) 2014 Optical Society of America

Spectroscopic detection and state preparation of a single praseodymium ion in a crystal

Tobias Utikal, Emanuel Eichhammer, L. Petersen, Alois Renn, Stephan Götzinger, Vahid Sandoghdar

The narrow optical transitions and long spin coherence times of rare earth ions in crystals make them desirable for a number of applications ranging from solid-state spectroscopy and laser physics to quantum information processing. However, investigations of these features have not been possible at the single-ion level. Here we show that the combination of cryogenic high-resolution laser spectroscopy with optical microscopy allows one to spectrally select individual praseodymium ions in yttrium orthosilicate. Furthermore, this spectral selectivity makes it possible to resolve neighbouring ions with a spatial precision of the order of 10 nm. In addition to elaborating on the essential experimental steps for achieving this long-sought goal, we demonstrate state preparation and read out of the three ground-state hyperfine levels, which are known to have lifetimes of the order of hundred seconds.

Cavity optomechanics

Markus Aspelmeyer, Tobias J. Kippenberg, Florian Marquardt

The field of cavity optomechanics is reviewed. This field explores the interaction between electromagnetic radiation and nanomechanical or micromechanical motion. This review covers the basics of optical cavities and mechanical resonators, their mutual optomechanical interaction mediated by the radiation-pressure force, the large variety of experimental systems which exhibit this interaction, optical measurements of mechanical motion, dynamical backaction amplification and cooling, nonlinear dynamics, multimode optomechanics, and proposals for future cavity-quantum-optomechanics experiments. In addition, the perspectives for fundamental quantum physics and for possible applications of optomechanical devices are described.

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

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

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

Cryogenic Colocalization Microscopy for Nanometer-Distance Measurements

Siegfried Weisenburger, Bo Jing, Dominik Haenni, Luc Reymond, Benjamin Schuler, Alois Renn, Vahid Sandoghdar

The main limiting factor in spatial resolution of localization microscopy is the number of detected photons. Recently we showed that cryogenic measurements improve the photostability of fluorophores, giving access to Angstrom precision in localization of single molecules. Here, we extend this method to colocalize two fluorophores attached to well-defined positions of a double-stranded DNA. By measuring the separations of the fluorophore pairs prepared at different design positions, we verify the feasibility of cryogenic distance measurement with sub-nanometer accuracy. We discuss the important challenges of our method as well as its potential for further improvement and various applications.

Temporal cloaking with accelerating wave packets

Ioannis Chremmos

We theoretically propose a temporal cloaking scheme based on accelerating wave packets. A part of a monochromatic light wave is endowed with a discontinuous nonlinear frequency chirp, so that two opposite accelerating caustics are created in space-time as the different frequency components propagate in the presence of dispersion. The two caustics open a biconvex time gap that contains negligible optical energy, thus concealing the enclosed events. In contrast to previous temporal cloaking schemes, where light propagates successively through two different media with opposite dispersions, accelerating wave packets open and close the cloaked time window continuously in a single dispersive medium. In addition, biconvex time gaps can be tailored into arbitrary shapes and offer a larger suppression of intensity compared with their rhombic counterparts. (C) 2014 Optical Society of America

Optical properties of vertical, tilted and in-plane GaN nanowires on different crystallographic orientations of sapphire

C. Tessarek, S. Figge, A. Gust, M. Heilmann, C. Dieker, E. Spiecker, S. Christiansen

Self-catalysed and self-organized GaN nanowires were grown on c-, a-, m- and r-plane sapphire by metal-organic vapour phase epitaxy. In dependence on the crystallographic orientation of the sapphire substrate, vertical, tilted and in-plane GaN nanowires were achieved. The nanowire orientation is visualized by scanning electron microscopy and analysed by x-ray diffraction. The influence of the sapphire nitridation step on the nanowire formation is investigated. Spatially and spectrally resolved cathodoluminescence studies are carried out on the GaN nanowires to analyse the influence of the GaN nanowire orientation as well as the presence of both N- and Ga-polar sections in a single nanowire on the optical properties.

A comparative study of beta-Ga2O3 nanowires grown on different substrates using CVD technique

Sudheer Kumar, C. Tessarek, S. Christiansen, R. Singh

A comparative study of beta gallium oxide (beta-Ga2O3) nanowires (NWs) grown on different substrates such as silicon (Si), sapphire, and GaN/sapphire has been reported. Field emission scanning electron microscopy (FESEM) results revealed that the beta-Ga2O3 NWs grown on GaN/sapphire substrate were comparatively more aligned than grown on other substrates. The diameter of the NWs varied from 150 to 400 nm, and their length ranging up to tens of micrometers. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) showed the single crystalline monoclinic nature of NWs. The Raman spectra acquired from three samples revealed similar features consisting of two active modes such as mid and high frequency. However, low frequency mode was absent in our results. The cathodoluminescence (CL) spectra of beta-Ga2O3 NWs on different substrates showed a strong broad UV-blue emission band and a weak red emission band in all the samples. Hence, the morphologies and structural properties of the beta-Ga2O3 NWs grown on three substrates showed some observable changes, while their optical properties were quite similar. (C) 2013 Elsevier B. V. All rights reserved.

Bright squeezed-vacuum source with 1.1 spatial mode

A. M. Perez, T. Sh. Iskhakov, P. Sharapova, S. Lemieux, O. V. Tikhonova, M. V. Chekhova, G. Leuchs

Bright squeezed vacuum, a macroscopic nonclassical state of light, can be obtained at the output of a strongly pumped nonseeded traveling-wave optical parametric amplifier (OPA). By constructing the OPA of two consecutive crystals separated by a large distance, we make the squeezed vacuum spatially single-mode without a significant decrease in the brightness or squeezing. (C) 2014 Optical Society of America

Optimized photonic gauge of extreme high vacuum with Petawatt lasers

Angel Paredes, David Novoa, Daniele Tommasini, Hector Mas

One of the latest proposed applications of ultra-intense laser pulses is their possible use to gauge extreme high vacuum by measuring the photon radiation resulting from nonlinear Thomson scattering within a vacuum tube. Here, we provide a complete analysis of the process, computing the expected rates and spectra, both for linear and circular polarizations of the laser pulses, taking into account the effect of the time envelope in a slowly varying envelope approximation. We also design a realistic experimental configuration allowing for the implementation of the idea and compute the corresponding geometric efficiencies. Finally, we develop an optimization procedure for this photonic gauge of extreme high vacuum at high repetition rate Petawatt and multi-Petawatt laser facilities, such as VEGA, JuSPARC and ELI.

Quantum mutual information of an entangled state propagating through a fast-light medium

Jeremy B. Clark, Ryan T. Glasser, Quentin Glorieux, Ulrich Vogl, Tian Li, Kevin M. Jones, Paul D. Lett

It is widely accepted that information cannot travel faster than c, the speed of light in vacuum(1-3). Here, we investigate the behaviour of quantum correlations and information in the presence of dispersion. To do so we send one half of an entangled state of light through a gain-assisted slow-or fast-light medium and detect the transmitted quantum correlations and quantum mutual information(4-6). We show that quantum correlations can be advanced by a small fraction of the correlation time, even in the presence of noise added by phase-insensitive gain. Additionally, although the peak of the quantum mutual information between the modes can be advanced, we find that the degradation of the mutual information due to added noise appears to prevent an advancement of the leading edge. In contrast, we demonstrate a significant delay of both the leading and trailing edges of the mutual information in a slow-light system.

Analysis of parahydrogen polarized spin system in low magnetic fields

P. Tuerschmann, J. Colell, T. Theis, B. Bluemich, S. Appelt

Nuclear magnetic resonance (NMR) spectra of spin systems polarized either thermally or by parahydrogen exhibit strikingly different field dependencies. Thermally polarized spin systems show the well-known roof effect, observed when reducing magnetic field strengths which precludes the independent determination of chemical shift differences and J-coupling constants at low-fields. Quantum mechanical analysis of the NMR spectra with respect to polarization method, pulsed state preparation, and transition probabilities reveals that spectra of parahydrogen polarized systems feature an "inverse roof effect" in the regime where the chemical shift difference delta nu is smaller than J. This inverse roof effect allows for the extraction of both J-coupling and chemical shift information down to very low fields. Based on a two-spin system, the observed non-linear magnetic field dependence of the splitting of spectral lines is predicted. We develop a general solution for the steady state density matrix of a parahydrogen polarized three-spin system including a heteronucleus which allows explaining experimentally observed H-1 spectra. The analysis of three-spin density matrix illustrates two pathways for an efficient polarization transfer from parahydrogen to C-13 nuclei. Examination of the experimental data facilitates the extraction of all relevant NMR parameters using single-scan, high-resolution H-1 and C-13 NMR spectroscopy at low fields at a fraction of the cost associated with cryogenically cooled high-field NMR spectrometers.

Designing Thin Film-Capped Metallic Nanoparticles Configurations for Sensing Applications

Muhammad Y. Bashouti, Adi-Solomon de la Zerda, Dolev Geva, Hossam Haick

Thin film-capped metallic nanoparticles (TFCMNPs) hold big promise for rapid, low-cost, and portable tracing of gas analytes. We show that sensing properties can be controlled by the configuration of the TFCMNPs. To this end, two methods were developed: layer by layer (LbL) and drop-by-drop, i.e., drop casting (DC). The TFCMNP prepared via LbL method was homogeneous and gradually increased in thickness, absorbance, and conductivity relative to TFCMNP prepared via DC method. However, our results indicate that the sensing of TFCMNP devices prepared via DC is significantly higher than that of equivalent LbL devices. These discrepancies can be explained as follows: LbL forms a high dense layer of TFCMNPs without vacancies, and a well-controlled deposition of NPs. The distance between the adjacent NPs is controlled by the capped ligands and the linker molecules making a rigid TFCMNP. Thus, exposing LbL devices to analyte induces a marginal change in the NP-NP distance. However, in DC devices, the analyte induces major change in the NP distances and permittivity due to their lack of connection, making the sensing much more pronounced. The DC and LbL methods used thiol and amine ligands-capped metallic nanoparticles to demonstrate the applicability of the methods to all types of ligands. Our results are of practical importance for integrating TFCMNPs in chemiresistive sensing platforms and for (bio) and chemical sensing applications.

Revealing molecular structure and orientation with Stokes vector resolved second harmonic generation microscopy

Nirmal Mazumder, Chih-Wei Hu, Jianjun Qiu, Matthew R. Foreman, Carlos Macias Romero, Peter Toeroek, Fu-Jen Kao

We report on measurements and characterization of polarization properties of Second Harmonic (SH) signals using a four-channel photon counting based Stokes polarimeter. In this way, the critical polarization parameters can be obtained concurrently without the need of repeated image acquisition. The critical polarization parameters, including the degree of polarization (DOP), the degree of linear polarization (DOLP), and the degree of circular polarization (DOCP), are extracted from the reconstructed Stokes vector based SH images in a pixel-by-pixel manner. The measurements are further extended by varying the polarization states of the incident light and recording the resulting Stokes parameters of the SH signal. In turn this allows the molecular structure and orientation of the samples to be determined. Use of Stokes polarimetry is critical in determination of the full polarization state of light, and enables discrimination of material properties not possible with conventional crossed-polarized detection schemes. The combination of SHG microscopy and Stokes polarimeter hence makes a powerful tool to investigate the structural order of targeted specimens. (C) 2013 Elsevier Inc. All rights reserved.

Sub-kHz lasing of a CaF2 whispering gallery mode resonator stabilized fiber ring laser

M. C. Collodo, F. Sedlmeir, B. Sprenger, S. Svitlov, L. J. Wang, H. G. L. Schwefel

We utilize a high quality calcium fluoride whispering-gallery-mode resonator to passively stabilize a simple erbium doped fiber ring laser with an emission frequency of 196 THz (wavelength 1530 nm) to an instantaneous linewidth below 650 Hz. This corresponds to a relative stability of 3.3 x 10(-12) over 16 mu s. In order to characterize the linewidth we use two identical self-built lasers and a commercial laser to determine the individual lasing linewidth via the three-cornered-hat method. We further show that the lasers are finely tunable throughout the erbium gain region. (C) 2014 Optical Society of America

Dirac soliton stability and interaction in binary waveguide arrays

Truong X. Tran, Xuan N. Nguyen, Dung C. Duong

We analyze the stability of a recently found exact analytical spatial soliton in binary waveguide arrays-an analog of the relativistic Dirac soliton. We demonstrate that this soliton class is very robust. The soliton dynamics and different scenarios of soliton interactions are systematically investigated. (C) 2014 Optical Society of America

Optical Simulation of Neutrino Oscillations in Binary Waveguide Arrays

Andrea Marini, Stefano Longhi, Fabio Biancalana

We theoretically propose and investigate an optical analogue of neutrino oscillations in a pair of vertically displaced binary waveguide arrays with longitudinally modulated effective refractive index. Optical propagation is modeled through coupled-mode equations, which in the continuous limit converge to two coupled Dirac equations for fermionic particles with different mass states, analogously to neutrinos. In addition to simulating neutrino oscillation in the noninteracting regime, our optical setting enables us to explore neutrino interactions in extreme regimes that are expected to play an important role in massive supernova stars. In particular, we predict the quenching of neutrino oscillations and the existence of topological defects, i.e., neutrino solitons, which in our photonic simulator should be observable as excitation of optical gap solitons propagating along the binary arrays at high excitation intensities.

A Simple Comparative Analysis of Exact and Approximate Quantum Error Correction

Carlo Cafaro, Peter van Loock

We present a comparative analysis of exact and approximate quantum error correction by means of simple unabridged analytical computations. For the sake of clarity, using primitive quantum codes, we study the exact and approximate error correction of the two simplest unital (Pauli errors) and nonunital (non-Pauli errors) noise models, respectively. The similarities and differences between the two scenarios are stressed. In addition, the performances of quantum codes quantified by means of the entanglement fidelity for different recovery schemes are taken into consideration in the approximate case. Finally, the role of self-complementarity in approximate quantum error correction is briefly addressed.

Near field of an oscillating electric dipole and cross-polarization of a collimated beam of light: Two sides of the same coin

Andrea Aiello, Marco Ornigotti

We address the question of whether there exists a hidden relationship between the near-field distribution generated by an oscillating electric dipole and the so-called cross-polarization of a collimated beam of light. We find that the answer is affirmative by showing that the complex field distributions occurring in both cases have a common physical origin: the requirement that the electromagnetic fields must be transverse. (C) 2014 American Association of Physics Teachers.

Sharp Switching in Optical Couplers With Variable Coupling Coefficient

Truong X. Tran, Xuan N. Nguyen

We report the switching behavior of a nonlinear optical coupler consisting of two straight waveguides forming a small angle. A very sharp switching can be obtained with these couplers. The switching power can be reduced up to two times compared to conventional couplers. The multiple switching can also happen at two and even more ranges of input powers. Because light switching is based on the optical Kerr effect, ultrafast switching is possible.

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

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

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

Optical nano-structuring in light-sensitive AgCl-Ag waveguide thin films: wavelength effect

Razieh Talebi, Arashmid Nahal, Muhammad Y. Bashouti, Silke H. Christiansen

Irradiation of photosensitive thin films results in the nanostructures formation in the interaction area. Here, we investigate how the formation of nanostructures in photosensitive waveguide AgCl thin films, doped by Ag nanoparticles, can be customized by tuning the wavelength of the incident beam. We found, silver nanoparticles are pushed towards the interference pattern minima created by the interference of the incident beam with the excited TEn-modes of the AgCl-Ag waveguide. The interference pattern determines the grating constant of the resulting spontaneous periodic nanostructures. Also, our studies indicate a strong dependence of the shape and size distribution of the formed Ag nanocoalescences on the wavelength of the incident beam. It also influences on the surface coverage of the sample by the formed silver nanoparticles and on period of the self-organized nano-gratings. It is found, exposure time and intensity of the incident light are the most determinant parameters for the quality and finesse of our nanostructures. More intense incident light with shorter exposure time generates more regular nanostructures with smaller nano-coalescences and, produces gratings with higher diffraction efficiency. At constant intensity longer exposure time produces more complete nanostructures because of optical positive feedback. We observed exposure with longer wavelength produces finer gratings. (C)2014 Optical Society of America

Modulational instability due to cross-phase modulation versus multiple four-wave mixing: the normal dispersion regime

Andrea Armaroli, Stefano Trillo

We investigate the propagation of two parallel polarized pump beams in the framework of a single nonlinear Schrodinger equation in order to thoroughly understand the modulation instability process in the regime of normal group-velocity dispersion. A linear stability analysis based on Floquet theory allows us to account for four-wave mixing and to contrast the results with those arising from incoherent nonlinear coupling due to cross-phase modulation only. Based on the nature of the unstable eigenvectors, we explain why, in the normal dispersion regime, modulation instability is not observed in the scalar configuration. Numerical simulations validate the analysis. (C) 2014 Optical Society of America

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

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

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

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

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

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

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

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

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

Probing and Controlling Autoionization Dynamics with Attosecond Light Pulses in a Strong Dressing Laser Field

Wei-Chun Chu, Toru Morishita, C. D. Lin

We review the theoretical investigations of the autoionzing wave packet excited by an isolated attosecond pulse and dressed by a time-delayed intense laser pulse. The few-level model is described and the applications in photoemission and photoabsorption are given. For the three-level, resonantly coupled system, the main features are explained by the Rabi oscillation modulated in the dressing field. For such a system, by precisely controlling the intensity and the time delay of the dressing pulse, we show the shaping of the attosecond pulse when propagating in a gas medium. A more sophisticated multi-level system with coupling terms involving continuum states is also developed, in which the importance of the continuum-continuum coupling is evaluated with the help of an ab initio calculation.

Three-dimensional photograph of electron tracks through a plastic scintillator

Mykhaylo Filipenko, Timur Iskhakov, Patrick Hufschmidt, Gisela Anton, Michael Campbell, Thomas Gleixner, Gerd Leuchs, Timo Tick, John Vallerga, et al.

The reconstruction of particle trajectories makes it possible to distinguish between different types of charged particles. In high-energy physics, where trajectories are rather long (several meters), large size trackers must be used to achieve sufficient position resolution. However, in low-background experiments like the search for neutrinoless double beta decay, tracks are rather short (some mm to several cm, depending on the detector in use) and three-dimensional trajectories could only be resolved in gaseous time-projection chambers so far. For detectors of a large volume of around one cubic meter (large in the scope of neutrinoless double beta search) and therefore large drift distances (several decimeters to 1 m), this technique is limited by diffusion and repulsion of charge carriers. In this work we present a "proof-of-principle" experiment for a new method of the three-dimensional tracking of charged particles by scintillation light: we used a setup consisting of a scintillator, mirrors, lenses, and a novel imaging device (the hybrid photon detector) in order to image two projections of electron tracks through the scintillator. We took data at the T-22 beamline at DESY with relativistic electrons with a kinetic energy of 5GeV and from this data successfully reconstructed their three-dimensional propagation path in the scintillator. With our setup we achieved a position resolution in the range of 170-248 mu m.

Supercontinuum generation in both frequency and wavenumber domains in nonlinear waveguide arrays

Truong X. Tran, Dung C. Duong, Fabio Biancalana

We study the spatiotemporal effects in waveguide arrays with Kerr and Raman nonlinearities when a short pulse is launched into the system where both dispersive and diffractive resonant radiations can be simultaneously emitted by solitons. We show that it is possible to generate and control the supercontinuum not only in the frequency domain, but also in the wavenumber domain. This work could potentially pave the way for designing unique optical devices that generate spectrally broad supercontinua with a controllable directionality by taking advantage of the combined physics of optical fibers and waveguide arrays.

Self-induced mode mixing of ultraintense lasers in vacuum

Angel Paredes, David Novoa, Daniele Tommasini

We study the effects of the quantum vacuum on the propagation of a Gaussian laser beam in vacuum. By means of a double perturbative expansion in paraxiality and quantum vacuum terms, we provide analytical expressions for the self-induced transverse mode mixing, rotation of polarization, and third harmonic generarion. We discuss the possibility of searching for the self-induced, spatially dependent phase shift of a multipetawatt laser pulse, which may allow the testing of quantum electrodynamics and new physics models, such as Born-Infeld theory and models involving new minicharged or axion-like particles, in parametric regions that have not yet been explored in laboratory experiments.

Single-molecule nucleic acid interactions monitored on a label-free microcavity biosensor platform

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

Biosensing relies on the detection of molecules and their specific interactions. It is therefore highly desirable to develop transducers exhibiting ultimate detection limits. Microcavities are an exemplary candidate technology for demonstrating such a capability in the optical domain and in a label-free fashion. Additional sensitivity gains, achievable by exploiting plasmon resonances, promise biosensing down to the single-molecule level. Here, we introduce a biosensing platform using optical microcavity-based sensors that exhibits single-molecule sensitivity and is selective to specific single binding events. Whispering gallery modes in glass microspheres are used to leverage plasmonic enhancements in gold nanorods for the specific detection of nucleic acid hybridization, down to single 8-mer oligonucleotides. Detection of single intercalating small molecules confirms the observation of single-molecule hybridization. Matched and mismatched strands are discriminated by their interaction kinetics. Our platform allows us to monitor specific molecular interactions transiently, hence mitigating the need for high binding affinity and avoiding permanent binding of target molecules to the receptors. Sensor lifetime is therefore increased, allowing interaction kinetics to be statistically analysed.

All-optical phase-preserving multilevel amplitude regeneration

Tobias Roethlingshoefer, Thomas Richter, Colja Schubert, Georgy Onishchukov, Bernhard Schmauss, Gerd Leuchs

The possibility of all-optical phase-preserving amplitude regeneration for star-8QAM is demonstrated using a modified nonlinear optical loop mirror. Experiments show a reduction in amplitude noise on both amplitude levels simultaneously, considering two different types of signal distortions: deterministic low-frequency amplitude modulation and broadband amplitude noise. Furthermore, using this amplitude regeneration, the robustness against nonlinear phase noise from fiber nonlinearity in a transmission line is increased. The scheme suppresses the conversion of amplitude noise to nonlinear phase noise. This is shown for simultaneous amplitude regeneration of the two amplitude states as well as for amplitude regeneration of the high-power states only. If the transmission is limited by nonlinear phase noise, single-level operation at the more critical higher-power state will benefit because of the wider plateau region. Numerical simulations confirm the experimental results. (C) 2014 Optical Society of America.

Dynamical Casimir-Polder interaction between an atom and surface plasmons

Harald R. Haakh, Carsten Henkel, Salvatore Spagnolo, Lucia Rizzuto, Roberto Passante

We investigate the time-dependent Casimir-Polder potential of a polarizable two-level atom placed near a surface of arbitrary material, after a sudden change in the parameters of the system. Different initial conditions are taken into account. For an initially bare ground-state atom, the time-dependent Casimir-Polder energy reveals how the atom is "being dressed" by virtual, matter-assisted photons. We also study the transient behavior of the Casimir-Polder interaction between the atom and the surface starting from a partially dressed state, after an externally induced change in the atomic level structure or transition dipoles. The Heisenberg equations are solved through an iterative technique for both atomic and field operators in the medium-assisted electromagnetic field quantization scheme. We analyze, in particular, how the time evolution of the interaction energy depends on the optical properties of the surface, in particular on the dispersion relation of surface plasmon polaritons. The physical significance and the limits of validity of the obtained results are discussed in detail.

Direct optical sensing of single unlabelled proteins and super-resolution imaging of their binding sites

Marek Piliarik, Vahid Sandoghdar

Detection of single analyte molecules without the use of any label would improve the sensitivity of current biosensors by orders of magnitude to the ultimate graininess of biological matter. Over two decades, scientists have succeeded in pushing the limits of optical detection to single molecules using fluorescence. However, restrictions in photophysics and labelling protocols make this technique less attractive for biosensing. Recently, mechanisms based on vibrational spectroscopy, photothermal detection, plasmonics and microcavities have been explored for fluorescence-free detection of single biomolecules. Here, we show that interferometric detection of scattering (iSCAT) can achieve this goal in a direct and label-free fashion. In particular, we demonstrate detection of cancer marker proteins in buffer solution and in the presence of other abundant proteins. Furthermore, we present super-resolution imaging of protein binding with nanometer localization precision. The ease of iSCAT instrumentation promises a breakthrough for label-free studies of interactions involving proteins and other small biomolecules.

Polarization Tailored Light Driven Directional Optical Nanobeacon

Martin Neugebauer, Thomas Bauer, Peter Banzer, Gerd Leuchs

We experimentally demonstrate all-optical control of the emission directivity of a dipole-like nanoparticle with spinning dipole moment sitting on the interface to an optical denser medium. The particle itself is excited by a tightly focused polarization tailored light beam under normal incidence. The position dependent local polarization of the focal field allows for tuning the dipole moment via careful positioning of the particle relative to the beam axis. As an application of this scheme, we investigate the polarization dependent coupling to a planar two-dimensional dielectric waveguide.

Quantum error correction and detection: Quantitative analysis of a coherent-state amplitude-damping code

Ricardo Wickert, Peter van Loock

We reexamine a non-Gaussian quantum error-correction code designed to protect optical coherent-state qubits against errors due to an amplitude-damping channel. We improve on a previous result [R. Wickert, N. K. Bernardes, and P. van Loock, Phys. Rev. A 81, 062344 (2010)] by providing a tighter upper bound on the performance attained when considering realistic assumptions, which constrain the operation of the gates employed in the scheme. The quantitative characterization is performed through measures of fidelity and concurrence, the latter obtained by employing the code as an entanglement distillation protocol. We find that, when running the code in fully deterministic error-correction mode, direct transmission can only be beaten for certain combinations of channel and input state parameters. In contrast, in error-detection mode, the usage of higher repetition encodings remains beneficial throughout, however, at the expense of diminishing success probabilities.

Probing photo-carrier collection efficiencies of individual silicon nanowire diodes on a wafer substrate

S. W. Schmitt, G. Broenstrup, G. Shalev, S. K. Srivastava, M. Y. Bashouti, G. H. Doehler, S. H. Christiansen

Vertically aligned silicon nanowire (SiNW) diodes are promising candidates for the integration into various opto-electronic device concepts for e. g. sensing or solar energy conversion. Individual SiNW p-n diodes have intensively been studied, but to date an assessment of their device performance once integrated on a silicon substrate has not been made. We show that using a scanning electron microscope (SEM) equipped with a nano-manipulator and an optical fiber feed-through for tunable (wavelength, power using a tunable laser source) sample illumination, the dark and illuminated current-voltage (I-V) curve of individual SiNW diodes on the substrate wafer can be measured. Surprisingly, the I-V-curve of the serially coupled system composed of SiNW/wafers is accurately described by an equivalent circuit model of a single diode and diode parameters like series and shunting resistivity, diode ideality factor and photocurrent can be retrieved from a fit. We show that the photo-carrier collection efficiency (PCE) of the integrated diode illuminated with variable wavelength and intensity light directly gives insight into the quality of the device design at the nanoscale. We find that the PCE decreases for high light intensities and photocurrent densities, due to the fact that considerable amounts of photo-excited carriers generated within the substrate lead to a decrease in shunting resistivity of the SiNW diode and deteriorate its rectification. The PCE decreases systematically for smaller wavelengths of visible light, showing the possibility of monitoring the effectiveness of the SiNW device surface passivation using the shown measurement technique. The integrated device was pre-characterized using secondary ion mass spectrometry (SIMS), TCAD simulations and electron beam induced current (EBIC) measurements to validate the properties of the characterized material at the single SiNW diode level.

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

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

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

Radial quantum number of Laguerre-Gauss modes

E. Karimi, R. W. Boyd, P. de la Hoz, H. de Guise, J. Rehacek, Z. Hradil, A. Aiello, G. Leuchs, L. L. Sanchez-Soto

We introduce an operator linked with the radial index in the Laguerre-Gauss modes of a two-dimensional harmonic oscillator in cylindrical coordinates. We discuss ladder operators for this variable, and confirm that they obey the commutation relations of the su(1,1) algebra. Using this fact, we examine how basic quantum optical concepts can be recast in terms of radial modes.

Observation of the Geometric Spin Hall Effect of Light

Jan Korger, Andrea Aiello, Vanessa Chille, Peter Banzer, Christoffer Wittmann, Norbert Lindlein, Christoph Marquardt, Gerd Leuchs

The spin Hall effect of light (SHEL) is the photonic analogue of the spin Hall effect occurring for charge carriers in solid-state systems. This intriguing phenomenon manifests itself when a light beam refracts at an air-glass interface (conventional SHEL) or when it is projected onto an oblique plane, the latter effect being known as the geometric SHEL. It amounts to a polarization-dependent displacement perpendicular to the plane of incidence. In this work, we experimentally investigate the geometric SHEL for a light beam transmitted across an oblique polarizer. We find that the spatial intensity distribution of the transmitted beam depends on the incident state of polarization and its centroid undergoes a positional displacement exceeding one wavelength. This novel phenomenon is virtually independent from the material properties of the polarizer and, thus, reveals universal features of spin-orbit coupling.

Incoherent averaging of phase singularities in speckle-shearing interferometry

Klaus Mantel, Vanusch Nercissian, Norbert Lindlein

Interferometric speckle techniques are plagued by the omnipresence of phase singularities, impairing the phase unwrapping process. To reduce the number of phase singularities by physical means, an incoherent averaging of multiple speckle fields may be applied. It turns out, however, that the results may strongly deviate from the expected root N behavior. Using speckle-shearing interferometry as an example, we investigate the mechanism behind the reduction of phase singularities, both by calculations and by computer simulations. Key to an understanding of the reduction mechanism during incoherent averaging is the representation of the physical averaging process in terms of certain vector fields associated with each speckle field. (C) 2014 Optical Society of America

Advanced quantum noise correlations

Ulrich Vogl, Ryan T. Glasser, Jeremy B. Clark, Quentin Glorieux, Tian Li, Neil V. Corzo, Paul D. Lett

We use the quantum correlations of twin beams of light to investigate the fundamental addition of noise when one of the beams propagates through a fastlight medium based on phase-insensitive gain. The experiment is based on two successive four-wave mixing processes in rubidium vapor, which allow for the generation of bright two-mode-squeezed twin beams followed by a controlled advancement while maintaining the shared quantum correlations between the beams. The demonstrated effect allows the study of irreversible decoherence in a medium exhibiting anomalous dispersion, and for the first time shows the advancement of a bright nonclassical state of light. The advancement and corresponding degradation of the quantum correlations are found to be operating near the fundamental quantum limit imposed by using a phase-insensitive amplifier.

Crystalline MgF2 whispering gallery mode resonators for enhanced bulk index sensitivity

R. Zeltner, F. Sedlmeir, G. Leuchs, H. G. L. Schwefel

We report on experiments on refractrometric sensing with crystalline Whispering Gallery Mode (WGM) resonators made of magnesium fluoride, which has a refractive index that is only slightly larger than that of water (Delta n approximate to 0.05). The resulting evanescent field of a WGM resonator placed in an aqueous environment penetrates therefore deep into the surrounding medium, which makes it a promising candidate for sensing applications. We measured a bulk index sensitivity of 1.09 nm/RIU (refractive index unit) in a resonator with a radius of R = 2.91mm and intrinsic Q-factors of more than 10(8) in aqueous environments. Furthermore, we describe the fabrication process of crystalline WGM resonators.

Deep-subwavelength negative-index waveguiding enabled by coupled conformal surface plasmons

R. Quesada, D. Martin-Cano, F. J. Garcia-Vidal, J. Bravo-Abad

In this Letter we introduce a novel route for achieving negative-group-velocity waveguiding at deep-subwavelength scales. Our scheme is based on the strong electromagnetic coupling between two conformal surface plasmon structures. Using symmetry arguments and detailed numerical simulations, we show that the coupled system can be geometrically tailored to yield negative-index dispersion. A high degree of subwavelength modal confinement, of lambda/10 in the transversal dimensions, is also demonstrated. These results can assist in the development of ultrathin surface circuitry for the low-frequency region (microwave and terahertz regimes) of the electromagnetic spectrum. (C) 2014 Optical Society of America

Photon correlations for colloidal nanocrystals and their clusters

A. Shcherbina, G. A. Shcherbina, M. Manceau, S. Vezzoli, L. Carbone, M. De Vittorio, A. Bramati, E. Giacobino, M. V. Chekhova, et al.

of semiconductor "dot-in-rods" and their small clusters are studied by measuring the second-order correlation function with a spatially resolving intensified CCD camera. This measurement allows one to distinguish between a single dot and a cluster and, to a certain extent, to estimate the number of dots in a cluster. A more advanced measurement is proposed, based on higher-order correlations, enabling more accurate determination of the number of dots in a small cluster. Nonclassical features of the light emitted by such a cluster are analyzed. (C) 2014 Optical Society of America

In optical pumping, less can be more

Harald G. L. Schwefel

Cascaded phase-preserving multilevel amplitude regeneration

Tobias Roethlingshoefer, Georgy Onishchukov, Bernhard Schmauss, Gerd Leuchs

The performance of cascaded in-line phase-preserving amplitude regeneration using nonlinear amplifying loop mirrors has been studied in numerical simulations. As an example of a spectrally efficient modulation format with two amplitude states and multiple phase states, the regeneration performance of a star-16QAM format, basically an 8PSK format with two amplitude levels, was evaluated. An increased robustness against amplified spontaneous emission and nonlinear phase noise was observed resulting in a significantly increased transmission distance. (C) 2014 Optical Society of America

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

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

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

Simultaneous measurement of phase and local orientation of linearly polarized light: implementation and measurement results

Sergej Rothau, Christine Kellermann, Vanusch Nercissian, Andreas Berger, Klaus Mantel, Norbert Lindlein

Optical components manipulating both polarization and phase of wave fields find many applications in today's optical systems. With modern lithography methods it is possible to fabricate optical elements with nanostructured surfaces from different materials capable of generating spatially varying, locally linearly polarized-light distributions, tailored to the application in question. Since such elements in general also affect the phase of the light field, the characterization of the function of such elements consists in measuring the phase and the polarization of the generated light, preferably at the same time. Here, we will present first results of an interferometric approach for a simultaneous and spatially resolved measurement of both phase and polarization, as long as the local polarization at any point is linear (e.g., for radially or azimuthally polarized light). (C) 2014 Optical Society of America

Compensation of anisotropy effects in the generation of two-photon light

Andrea Cavanna, Angela M. Perez, Felix Just, Maria V. Chekhova, Gerd Leuchs

We analyse a method to compensate for anisotropy effects in the spatial distribution of parametric down-conversion (PDC) radiation in bulk crystals. In this method, a single nonlinear crystal is replaced by two consecutive crystals with opposite transverse walk-off directions. We implement a simple numerical model to calculate the spatial distribution of intensity and correlations, as well as the Schmidt mode structure, with an account for the anisotropy. Experimental results are presented which prove the validity of both the model and the method. (C) 2014 Optical Society of America

Structure of the sets of mutually unbiased bases with cyclic symmetry

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

Mutually unbiased bases that can be cyclically generated by a single unitary operator are of special interest, for they can be readily implemented in practice. We show that, for a system of qubits, finding such a generator can be cast as the problem of finding a symmetric matrix over the field. 2 equipped with an irreducible characteristic polynomial of a given Fibonacci index. The entanglement structure of the resulting complete sets is determined by two additive matrices of the same size.

Interface investigation of planar hybrid n-Si/PEDOT:PSS solar cells with open circuit voltages up to 645 mV and efficiencies of 12.6 %

Matthias Pietsch, Sara Jaeckle, Silke Christiansen

We have studied interface formation properties of hybrid n-Si/PEDOT:PSS solar cells on planar substrates by varying the silicon substrate doping concentration (N (D)). Final power conversion efficiencies (PCE) of 12.6 % and open circuit voltages (V (oc)) comparable to conventional diffused emitter pn junction solar cells have been achieved. It was observed, that an increase of N (D) leads to an increase of V (oc) with a maximal value of 645 mV, which is, to our knowledge, the highest reported value for n-Si/PEDOT:PSS interfaces. The dependence of the solar cell characteristics on N (D) is analyzed and similarities to minority charge carrier drift-diffusion limited solar cells are presented. The results point out the potential of hybrid n-Si/PEDOT:PSS interfaces to fabricate high performance opto-electronic devices with cost-effective fabrication technologies.

Generation of entangled matter qubits in two opposing parabolic mirrors

N. Trautmann, J. Z. Bernad, M. Sondermann, G. Alber, L. L. Sanchez-Soto, G. Leuchs

We propose a scheme for the remote preparation of entangled matter qubits in free space. For this purpose, a setup of two opposing parabolic mirrors is considered, each one with a single ion trapped at its focus. To get the required entanglement in this extreme multimode scenario, we take advantage of the spontaneous decay, which is usually considered as an apparent nuisance. Using semiclassical methods, we derive an efficient photon-path representation to deal with this problem. We also present a thorough examination of the experimental feasibility of the scheme. The vulnerabilities arising in realistic implementations reduce the success probability, but leave the fidelity of the generated state unaltered. Our proposal thus allows for the generation of high- fidelity entangled matter qubits with high rate.

Efficient algorithm for optimizing data-pattern tomography

L. Motka, B. Stoklasa, J. Rehacek, Z. Hradil, V. Karasek, D. Mogilevtsev, G. Harder, C. Silberhorn, L. L. Sanchez-Soto

We give a detailed account of an efficient search algorithm for the data-pattern tomography proposed by J. Rehacek, D. Mogilevtsev, and Z. Hradil [Phys. Rev. Lett. 105, 010402 (2010)], where the quantum state of a system is reconstructed without a priori knowledge about the measuring setup. The method is especially suited for experiments involving complex detectors, which are difficult to calibrate and characterize. We illustrate the approach with the case study of the homodyne detection of a nonclassical photon state.

Glow discharge techniques in the chemical analysis of photovoltaic materials

Sebastian W. Schmitt, Cornel Venzago, Bjoern Hoffmann, Vladimir Sivakov, Thomas Hofmann, Johann Michler, Silke Christiansen, Gerardo Gamez

Experimental Proof of Concept of Nanoparticle-Assisted STED

Yannick Sonnefraud, Hugo G. Sinclair, Yonatan Sivan, Matthew R. Foreman, Christopher W. Dunsby, Mark A. A. Neil, Paul M. French, Stefan A. Maier

We imaged core-shell nanoparticles, consisting of a dye-doped silica core covered with a layer of gold, with a stimulated emission depletion, fluorescence lifetime imaging (STED-FLIM) microscope. Because of the field enhancement provided by the localized surface plasmon resonance of the gold shell, we demonstrate a reduction of the STED depletion power required to obtain resolution improvement by a factor of 4. This validates the concept of nanoparticle-assisted STED (NP-STED), where hybrid dye-plasmonic nanoparticles are used as labels for STED in order to decrease the depletion powers required for subwavelength imaging.

Photoluminescence analysis of coupling effects: The impact of shunt resistance and temperature

Vasiliki Paraskeva, Constantinos Lazarou, Maria Hadjipanayi, Matthew Norton, Mauro Pravettoni, George E. Georghiou, Martin Heilmann, Silke Christiansen

In multi-junction devices, due to the series connection of junctions, recombination current from the top junctions can be directed to the bottom ones affecting their electrical characteristics. Recently, luminescence coupling effects during External Quantum Efficiency (EQE) measurements at very intense light bias conditions indicated high recombination current flowing towards the bottom junctions of the cells. In an attempt to find the magnitude of coupling current as well as the factors affecting the optical interactions between junctions, excitation and voltage dependent Photoluminescence (PL) measurements of GaInP/GaInAs/Ge have been carried out. An investigation using junctions with different shunt resistances has been conducted to identify the impact of shunts on the coupling current. Furthermore the impact of temperature on the coupling current has been considered. Our results show that a maximum of 2.3% of the recombination current of the top junction is converted to coupling current in the middle junction depending on the devices under examination. The coupling efficiency depends on the shunt resistance of the top junctions as well as on the temperature. Furthermore a physical model of the current limiting junction was built taking into consideration the impact of local ohmic shunts in the solar cell device and used to validate the experimental data taken. (C)2014 Elsevier B.V. All rights reserved.

Generalized Bessel beams with two indices

Marco Ornigotti, Andrea Aiello

We report on a new class of exact solutions of the scalar Helmholtz equation obtained by carefully engineering the form of the angular spectrum of a Bessel beam. We consider in particular the case in which the angular spectrum of such generalized beams has, in the paraxial zone, the same radial structure as Laguerre-Gaussian beams. We investigate the form of these new beams as well as their peculiar propagation properties. (C) 2014 Optical Society of America

The Role of Si during the Growth of GaN Micro- and Nanorods

C. Tessarek, M. Heilmann, E. Butzen, A. Haab, H. Hardtdegen, C. Dieker, E. Spiecker, S. Christiansen

The role of Si during the metal-organic vapor phase epitaxy of GaN rods is investigated. Already a small amount of Si strongly enhances the vertical growth of GaN. Reactive ion etching experiments show that the inner volume of the rod is much more strongly etched than the m-plane surface layer. Transmission electron microscopy and energy dispersive X-ray spectroscopy measurements reveal that Si is predominiantly incorporated in the surface layer of the m-plane sidewall facets of the rods. The formation of a SiN layer prevents growth on and etching of the m-planes and enhances the mobility of atoms promoting vertical growth. Annealing experiments demonstrate the extraordinary thermal resistivity in comparison to undoped GaN rod structures and GaN layers. The subsequent InGaN quantum well growth on the GaN rods reveals the antisurfactant effect of the SiN layer. A model based on the vapor-liquid-solid growth mode is proposed. The results help to understand the role of Si during growth of GaN rod structures to improve the performance of rod based light emitting and electronic devices.

Light bullets in nonlinear waveguide arrays under the influence of dispersion and the Raman effect

Truong X. Tran, Dung C. Duong, Fabio Biancalana

We study the formation and the dynamics of spatially broad light bullets generated in silica waveguide arrays. We show that these bullets are metastable even in the presence of high-order dispersion, coupling dispersion, and the Raman effect and can be approximated by the hyperbolic secant function with a high degree of accuracy. We also investigate the formation of narrow light bullets which are spatially localized in only a few adjacent waveguides and short in time. We reveal that the latter are extremely robust even in the presence of the Raman effect.

Free-carrier-driven spatiotemporal dynamics in amplifying silicon waveguides

Samudra Roy, Andrea Marini, Fabio Biancalana

We theoretically investigate the free-carrier-induced spatiotemporal dynamics of continuous waves in silicon waveguides embedded in an amplifying medium. Optical propagation is governed by a cubic Ginzburg-Landau equation coupled with an ordinary differential equation accounting for the free-carrier dynamics. We find that, owing to free-carrier dispersion, continuous waves are modulationally unstable in both anomalous and normal dispersion regimes and chaotically generate unstable accelerating pulses.

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

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

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

Dissipative optomechanical squeezing of light

Andreas Kronwald, Florian Marquardt, Aashish A. Clerk

We discuss a simple yet surprisingly effective mechanism which allows the generation of squeezed output light from an optomechanical cavity. In contrast to the well known mechanism of 'ponderomotive squeezing', our scheme generates squeezed output light by explicitly using the dissipative nature of the mechanical resonator. We show that our scheme has many advantages over ponderomotive squeezing; in particular, it is far more effective in the good cavity limit commonly used in experiments. Furthermore, the squeezing generated in our approach can be directly used to enhance the intrinsic measurement sensitivity of the optomechanical cavity; one does not have to feed the squeezed light into a separate measurement device. As our scheme is very general, it could also e. g. be implemented using superconducting circuits.

Realization of laterally nondispersing ultrabroadband Airy pulses

Andreas Valdmann, Peeter Piksarv, Heli Valtna-Lukner, Peeter Saari

We present the measurements of the spatiotemporal impulse response of a system creating nondispersing Airy pulses, i.e., ultrabroadband Airy beams whose main lobe size remains constant over propagation. A custom refractive element with a continuous surface profile was used to impose the cubic phase on the input beam. The impulse response of the Airy pulse generator was spatiotemporally characterized by applying a white-light spatial-spectral interferometry setup based on the SEA TADPOLE technique. The results were compared with the theoretical model and previously spatiotemporally characterized Airy pulses generated by a spatial light modulator. (C) 2014 Optical Society of America

Phase regeneration of a star-8QAM signal in a phase-sensitive amplifier with conjugated pumps

B. Stiller, G. Onishchukov, B. Schmauss, G. Leuchs

We demonstrate numerically phase regeneration of a star8QAM signal with two amplitude and four phase states in a phase-sensitive amplifier. In a dual-stage setup, two phase-conjugated idlers are generated in a first stage consisting of two fiber-optic parametric phase-insensitive amplifiers operated in highly nonlinear gain regime. These are used as pumps in the second, phase-sensitive amplification stage which enables efficient phase regeneration via a degenerate four-wave-mixing process. The latter can be operated in two different operation modes: without format conversion or with phase-shifted amplitude levels. In both regimes, we observe high phase-regeneration efficiency for all amplitude levels: the initial phase noise with 0.2 rad standard deviation is reduced by a factor of 5. (C) 2014 Optical Society of America

All-Optical Simultaneous Multilevel Amplitude and Phase Regeneration

Tobias Roethlingshoefer, Georgy Onishchukov, Bernhard Schmauss, Gerd Leuchs

Simultaneous amplitude and phase noise reduction of multiple signal states using a nonlinear amplifying loop mirror with integrated directional phase-sensitive amplifier are presented for the star-eight quadrature amplitude modulation transmission format as an example. The performance of this combined regenerator scheme is compared with that of a cascade of separate phase and amplitude regenerators. It could be shown that an improvement in the error vector magnitude of 4 dB for the high-power states with simultaneous improvement of 5 dB for the low-power states is possible in both cases. Transmission improvement by regeneration is considered for two noise types: 1) amplified spontaneous emission and 2) nonlinear phase noise. Both schemes can either improve the bit error rate by an order of magnitude or enable an increase of the fiber launch power by 3 dB.

Accurate tuning of ordered nanotubular platinum electrodes by galvanic plating

Valentin Roscher, Markus Licklederer, Johannes Schumacher, Grisell Reyes Rios, Bjoern Hoffmann, Silke Christiansen, Julien Bachmann

Platinum nanotubes are created by galvanic deposition inside porous templates. The effects of the electrolyte's ion concentration and pH, of the applied potential and of the deposition duration on the morphology of the tubes are investigated systematically. The system provides a model electrode platform with accurately tunable geometry for the fundamental investigation of electrochemical transformations. For slow electrochemical reactions, we observe a linear increase of the galvanic current with the length of the nanotubes, and therefore with the specific surface area of the electrode. In contrast to this, inherently fast electrochemical transformations are diffusion-limited and give rise to the same current density independently of the geometry. These results delineate a strategy for optimizing the performance of electrochemical energy conversion devices systematically via nanostructuring the electrode surfaces.

Chirped pulse formation dynamics in ultra-long mode-locked fiber lasers

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

By modeling giant chirped pulse formation in ultra-long, normally dispersive, mode-locked fiber lasers, we verify convergence to a steady-state consisting of highly chirped and coherent, nanosecond-scale pulses, which is in good agreement with recent experimental results. Numerical investigation of the transient dynamics reveals the existence of dark soliton-like structures within the envelope of the initial noisy pulse structure. Quasi-stationary dark solitons can persist throughout a large part of the evolution from noise to a stable dissipative soliton solution of the mode-locked laser cavity. (C) 2014 Optical Society of America

Bright integrated photon-pair source for practical passive decoy-state quantum key distribution

S. Krapick, M. S. Stefszky, M. Jachura, B. Brecht, M. Avenhaus, C. Silberhorn

We report on a bright, nondegenerate type-I parametric down-conversion source, which is well suited for passive decoy-state quantum key distribution. We show the photon-number-resolved analysis over a broad range of pump powers and we prove heralded higher-order n-photon states up to n = 4. The inferred photon click statistics exhibit excellent agreements to the theoretical predictions. From our measurement results we conclude that our source meets the requirements to avert photon-number-splitting attacks.

Enhanced nanoparticle detection with liquid droplet resonators

M. R. Foreman, S. Avino, R. Zullo, H. -P. Loock, F. Vollmer, G. Gagliardi

Whispering gallery mode particle sensing experiments are commonly performed with solid resonators, whereby the sensing volume is limited to the weak evanescent tail of the mode near the resonator surface. In this work we discuss in detail the sensitivity enhancements achievable in liquid droplet resonators wherein the stronger internal fields and convenient means of particle delivery can be exploited. Asymptotic formulae are derived for the relative resonance shift, line broadening and mode splitting of TE and TM modes in liquid droplet resonators. As a corollary the relative fraction of internal and external mode energy follows, which is shown to govern achievable sensitivity enhancements of solute concentration measurements in droplet sensors. Experimental measurements of nanoparticle concentration based on whispering gallery mode resonance broadening are also presented.

Fair sampling perspective on an apparent violation of duality

Eliot Bolduc, Jonathan Leach, Filippo M. Miatto, Gerd Leuchs, Robert W. Boyd

In the event in which a quantum mechanical particle can pass from an initial state to a final state along two possible paths, the duality principle states that "the simultaneous observation of wave and particle behavior is prohibited" [Scully MO, Englert B-G, Walther H ( 1991) Nature 351: 111-116]. Whereas wave behavior is associated with the observation of interference fringes, particle behavior generally corresponds to the acquisition of which-path information by means of coupling the paths to a measuring device or part of their environment. In this paper, we show how the consequences of duality change when allowing for biased sampling, that is, post-selected measurements on specific degrees of freedom of the environment of the two-path state. Our work gives insight into a possible mechanism for obtaining simultaneous high which-path information and high-visibility fringes in a single experiment. Further, our results introduce previously unidentified avenues for experimental tests of duality.

Coherent Interaction of Light and Single Molecules in a Dielectric Nanoguide

Sanli Faez, Pierre Tuerschmann, Harald R. Haakh, Stephan Goetzinger, Vahid Sandoghdar

Many of the currently pursued experiments in quantum optics would greatly benefit from a strong interaction between light and matter. Here, we present a simple new scheme for the efficient coupling of single molecules and photons. A glass capillary with a diameter of 600 nm filled with an organic crystal tightly guides the excitation light and provides a maximum spontaneous emission coupling factor (beta) of 18% for the dye molecules doped in the organic crystal. A combination of extinction, fluorescence excitation, and resonance fluorescence spectroscopy with microscopy provides high-resolution spatio-spectral access to a very large number of single molecules in a linear geometry. We discuss strategies for exploring a range of quantum-optical phenomena, including polaritonic interactions in a mesoscopic ensemble of molecules mediated by a single mode of propagating photons.

Large area fabrication of vertical silicon nanowire arrays by silver-assisted single-step chemical etching and their formation kinetics

Sanjay K. Srivastava, Dinesh Kumar, S. W. Schmitt, K. N. Sood, S. H. Christiansen, P. K. Singh

Vertically aligned silicon nanowire (SiNW) arrays have been fabricated over a large area using a silver-assisted single-step electroless wet chemical etching (EWCE) method, which involves the etching of silicon wafers in aqueous hydrofluoric acid (HF) and silver nitrate (AgNO3) solution. A comprehensive systematic investigation on the influence of different parameters, such as the etching time ( up to 15 h), solution temperature (10-80 degrees C), AgNO3 (5-200 mM) and HF ( 2-22 M) concentrations, and properties of the multi-crystalline silicon (mc-Si) wafers, is presented to establish a relationship of these parameters with the SiNW morphology. A linear dependence of the NW length on the etch time is obtained even at higher temperature (10-50 degrees C). The activation energy for the formation of SiNWs on Si( 100) has been found to be equal to similar to 0:51 eV. It has been shown for the first time that the surface area of the Si wafer exposed to the etching solution is an important parameter in determining the etching kinetics in the single-step process. Our results establish that single-step EWCE offers a wide range of parameters by means of which high quality vertical SiNWs can be produced in a very simple and controlled manner. A mechanism for explaining the influence of various parameters on the evolution of the NW structure is discussed. Furthermore, the SiNW arrays have extremely low reflectance ( as low as <3% for Si(100) NWs and <12% for mc-Si NWs) compared to similar to 35% for the polished surface in the 350-1000 nm wavelength range. The remarkably low reflection surface of SiNW arrays has great potential for use as an effective light absorber material in novel photovoltaic architectures, and other optoelectronic and photonic devices.

Improving the Optical Properties of Self-Catalyzed GaN Microrods toward Whispering Gallery Mode Lasing

Christian Tessarek, Robert Roeder, Tom Michalsky, Sebastian Geburt, Helena Franke, Ruediger Schmidt-Grund, Martin Heilmann, Bjoern Hoffmann, Carsten Ronning, et al.

GaN microrods were grown self-catalyzed by a fast and metal-organic vapor phase epitaxy method without microrods with a regular hexagonal cross-section, sharp edges, straight, and smooth sidewall facets act as a microresonator, as seen by the appearance of whispering gallery modes in the yellow defect band range. To improve their optical properties, a reduced Ga precursor flow is required during growth. However, their hexagonal microrod morphology is not maintained under these growth conditions. The approach to start growth a high Ga precursor flow and applying a ramp to a reduced precursor flow yield in significant enhancement of the near band edge emission in the upper part of the microrods. Whispering gallery modes in superposition with stimulated emission of a single whispering gallery mode up to similar to 2 MW/cm(2) and multimodel lasing with a threshold of 2.86 MW/cm(2) from an as-grown microrod under optical excitation at room temperature.

Efficient saturation of an ion in free space

Martin Fischer, Marianne Bader, Robert Maiwald, Andrea Golla, Markus Sondermann, Gerd Leuchs

We report on the demonstration of a lightmatter interface coupling light to a single Yb-174(+) ion in free space. The interface is realized through a parabolic mirror partially surrounding the ion. It transforms a La-guerre- Gaussian beam into a linear dipole wave converging at the mirror's focus. By measuring the non-linear response of the atomic transition, we deduce the power required for reaching an upper-level population of 1/4 to be 692 +/- 20pW at half linewidth detuning from the atomic resonance. Performing this measurement while scanning the ion through the focus provides a map of the focal intensity distribution. From the measured power, we infer a coupling efficiency of 7.2 +/- 0.2% on the linear dipole transition when illuminating from half solid angle, being among the best coupling efficiencies reported for a single atom in free space.

Towards loophole-free Bell inequality test with preselected unsymmetrical singlet states of light

Magdalena Stobinska, Falk Toeppel, Pavel Sekatski, Adam Buraczewski

Can a Bell test with no detection loophole be demonstrated for multiphoton entangled states of light within the current technology? We examine the possibility of a postselection-free Clauser-Horne-Shimony-Holt (CHSH)-Bell inequality test with an unsymmetrical polarization singlet. To that end we employ a preselection procedure which is performed prior to the test. It allows using imperfect (coarse-grained) binary photodetection in the test. We show an example of a preselection scheme which improves violation of the CHSH inequality with the micro-macro polarization singlet produced by the optimal quantum cloning. The preselection is realized by a quantum filter which is believed not to be useful for this purpose.

High-Q MgF2 whispering gallery mode resonators for refractometric sensing in aqueous environment

Florian Sedlmeir, Richard Zeltner, Gerd Leuchs, Harald G. L. Schwefel

We present our experiments on refractometric sensing with ultrahigh-Q, crystalline, birefringent magnesium fluoride (MgF2) whispering gallery mode resonators. The difference to fused silica which is most commonly used for sensing experiments is the small refractive index of MgF2 which is very close to that of water. Compared to fused silica this leads to more than 50% longer evanescent fields and a 4:25 times larger sensitivity. Moreover the birefringence amplifies the sensitivity difference between TM and TE type modes which will enhance sensing experiments based on difference frequency measurements. We estimate the performance of our resonators and compare them with fused silica theoretically and present experimental data showing the interferometrically measured evanescent field decay and the sensitivity of mm-sized MgF2 whispering gallery mode resonators immersed in water. These data show reasonable agreement with the developed theory. Furthermore, we observe stable Q factors in water well above 1 x 10(8). (C) 2014 Optical Society of America

Interference of macroscopic beams on a beam splitter: phase uncertainty converted into photon-number uncertainty

K. Yu Spasibko, F. Toeppel, T. Sh Iskhakov, M. Stobinska, M. V. Chekhova, G. Leuchs

Squeezed-vacuum twin beams, commonly generated through parametric downconversion, are known to have perfect photon-number correlations. According to the Heisenberg principle, this is accompanied by a huge uncertainty in their relative phase. By overlapping bright twin beams on a beam splitter, we convert phase fluctuations into photon-number fluctuations and observe this uncertainty as a typical 'U-shape' of the output photon-number distribution. This effect, although reported for atomic ensembles and giving hope for phase super-resolution, has never been observed for light beams. The shape of the normalized photon-number difference distribution is similar to the one that would be observed for high-order Fock states. It can be also mimicked by classical beams with artificially mixed phase, but without any perspective for phase super-resolution. The probability distribution at the beam splitter output can be used for filtering macroscopic superpositions at the input.

Heterojunction based hybrid silicon nanowire solar cell: surface termination, photoelectron and photoemission spectroscopy study

Muhammad Y. Bashouti, Matthias Pietsch, Gerald Broenstrup, Vladimir Sivakov, Juergen Ristein, Silke Christiansen

Silicon nanowires (SiNWs) combined with a conducting polymer are studied to constitute a hybrid organic/inorganic solar cell. This type of cell shows a particularly high interfacial area between SiNWs and the polymer so that interfacial control and interface optimization are required. For that purpose, we terminated the SiNW surfaces with well selected functional groups (molecules) such as native oxide (hereinafter SiO2-SiNW), hydrogen (hereinafter H-SiNW) and methyl (hereinafter CH3-SiNW). A radial hetero-junction solar cell is formed, and the cell parameters with and without interface control by functionalization with molecules are compared. Electronically, the three surfaces were close to flat-band conditions. The CH3-SiNW, H-SiNW and SiO2-SiNW produced a surface dipole of -0.12, +0.07 and 0.2eV and band bending of 50, 100 and 170meV, respectively. The surface properties of functionalized SiNWs are investigated by photoelectron yield (PY) and photoemission spectroscopy. PY studies on functionalized SiNWs are presented for the first time, and our results show that this type of measurement is an excellent option to carry out interface optimization of NWs for envisaged nano-electronic and photonic applications. The solar cell efficiency is increased dramatically after terminating the surface with CH3 molecules due to the decrease of the defect emission. The differently functionalized SiNW surfaces showed identical absorbance, reflectance and transmission so that a change in PY can be attributed to the Si-C bonds at the surface. This finding permits the design of new solar cell concepts. Copyright (c) 2013 John Wiley & Sons, Ltd.

Confocal polarization imaging in high-numerical-aperture space

C. Macias-Romero, M. R. Foreman, P. R. T. Munro, P. Toeroek

In this work we describe theoretical and experimental physical aspects of high-resolution imaging polarimetry and its application to polarization-multiplexed encoding. We theoretically demonstrate that it is possible to resolve the orientation of two fixed dipole-like emitters placed significantly below the resolution limit if their emission is uncorrelated. Furthermore, we experimentally demonstrate this phenomenon by illuminating closely spaced asymmetric nanopits with unpolarized light and subsequently determining their individual orientation and position from the measured spatial distributions of the azimuth angle of the polarization and degree of polarization, respectively. Reduction of the optical resolution of the imaging system is also shown to only weakly affect resolution obtainable via polarization measurements. (C) 2014 Optical Society of America

Charge Transfer Doping of Silicon

K. J. Rietwyk, Y. Smets, M. Bashouti, S. H. Christiansen, A. Schenk, A. Tadich, M. T. Edmonds, J. Ristein, L. Ley, et al.

We demonstrate a novel doping mechanism of silicon, namely n-type transfer doping by adsorbed organic cobaltocene (CoCp2 *) molecules. The amount of transferred charge as a function of coverage is monitored by following the ensuing band bending via surface sensitive core-level photoelectron spectroscopy. The concomitant loss of electrons in the CoCp2 * adlayer is quantified by the relative intensities of chemically shifted Co2p components in core-level photoelectron spectroscopy which correspond to charged and neutral molecules. Using a previously developed model for transfer doping, the evolution in relative intensities of the two components as a function of coverage has been reproduced successfully. A single, molecule-specific parameter, the negative donor energy of - (0.50 +/- 0.15) eV suffices to describe the self-limiting doping process with a maximum areal density of transferred electrons of 2 x 10(13) cm(-2) in agreement with the measured downward band bending. The advantage of this doping mechanism over conventional doping for nanostructures is addressed.

Generation and subwavelength focusing of longitudinal magnetic fields in a metallized fiber tip

Daniel Ploss, Arian Kriesch, Hannes Pfeifer, Peter Banzer, Ulf Peschel

We demonstrate experimentally and numerically that in fiber tips as they are used in NSOMs azimuthally polarized electrical fields (broken vertical bar E-azi broken vertical bar(2) / broken vertical bar E-tot broken vertical bar(2) approximate to 55% +/- 5% for lambda(0) = 1550 nm), respectively subwavelength confined (FWHM approximate to 450 nm approximate to lambda(0)/3.5) magnetic fields, are generated for a certain tip aperture diameter (d = 1.4 mu m). We attribute the generation of this field distribution in metal-coated fiber tips to symmetry breaking in the bend and subsequent plasmonic mode filtering in the truncated conical taper. (C) 2014 Optical Society of America

Atmospheric continuous-variable quantum communication

B. Heim, C. Peuntinger, N. Killoran, I. Khan, C. Wittmann, Ch Marquardt, G. Leuchs

We present a quantum communication experiment conducted over a point-topoint free-space link of 1.6 km in urban conditions. We study atmospheric influences on the capability of the link to act as a continuous-variable (CV) quantum channel. Continuous polarization states (that contain the signal encoding as well as a local oscillator (LO) in the same spatial mode) are prepared and sent over the link in a polarization multiplexed setting. Both signal and LO undergo the same atmospheric fluctuations. These are intrinsically auto-compensated which removes detrimental influences on the interferometric visibility. At the receiver, we measure the Q-function and interpret the data using the framework of effective entanglement (EE). We compare different state amplitudes and alphabets (two-state and four-state) and determine their optimal working points with respect to the distributed EE. Based on the high entanglement transmission rates achieved, our system indicates the high potential of atmospheric links in the field of CV quantum key distribution.

An entropic analysis of approximate quantum error correction

Carlo Cafaro, Peter van Loock

The concept of entropy and the correct application of the Second Law of thermodynamics are essential in order to understand the reason why quantum error correction is thermodynamically possible and no violation of the Second Law occurs during its execution. We report in this work our first steps towards an entropic analysis extended to approximate quantum error correction (QEC). Special emphasis is devoted to the link among quantum state discrimination (QSD), quantum information gain, and quantum error correction in both the exact and approximate QEC scenarios. (C) 2014 Elsevier B.V. All rights reserved.

Role of Disorder in the Thermodynamics and Atomic Dynamics of Glasses

A. I. Chumakov, G. Monaco, A. Fontana, A. Bosak, R. P. Hermann, D. Bessas, B. Wehinger, W. A. Crichton, M. Krisch, et al.

We measured the density of vibrational states (DOS) and the specific heat of various glassy and crystalline polymorphs of SiO2. The typical (ambient) glass shows a well-known excess of specific heat relative to the typical crystal (alpha-quartz). This, however, holds when comparing a lower-density glass to a higher-density crystal. For glassy and crystalline polymorphs with matched densities, the DOS of the glass appears as the smoothed counterpart of the DOS of the corresponding crystal; it reveals the same number of the excess states relative to the Debye model, the same number of all states in the low-energy region, and it provides the same specific heat. This shows that glasses have higher specific heat than crystals not due to disorder, but because the typical glass has lower density than the typical crystal.

Time-multiplexed measurements of nonclassical light at telecom wavelengths

G. Harder, C. Silberhorn, J. Rehacek, Z. Hradil, L. Motka, B. Stoklasa, L. L. Sanchez-Soto

We report the experimental reconstruction of the statistical properties of an ultrafast pulsed type II parametric down-conversion source in a periodically poled potassium titanyl phosphate waveguide at telecom wavelengths, with almost perfect photon-number correlations. We use a photon-number-resolving time-multiplexed detector based on a fiber-optical setup and a pair of avalanche photodiodes. By resorting to a germane data-pattern tomography, we assess the properties of the nonclassical light states with unprecedented precision.

Single-site-resolved measurement of the current statistics in optical lattices

Stefan Kessler, Florian Marquardt

At present, great effort is spent on the experimental realization of gauge fields for quantum many-body systems in optical lattices. At the same time, the single-site-resolved detection of individual atoms has become a new powerful experimental tool. We discuss a protocol for the single-site-resolved measurement of the current statistics of quantum many-body systems, which makes use of a bichromatic optical superlattice and single-site detection. We illustrate the protocol by a numerical study of the current statistics for interacting bosons in one and two dimensions and discuss the role of the on-site interactions for the current pattern and the ground-state symmetry for small two-dimensional lattices with artificial magnetic fields.

Decoherence in a double-dot Aharonov-Bohm interferometer: Numerical renormalization group study

Bjoern Kubala, David Roosen, Michael Sindel, Walter Hofstetter, Florian Marquardt

Coherence in electronic interferometers is typically believed to be restored fully in the limit of small voltages, frequencies, and temperatures. However, it is crucial to check this essentially perturbative argument by nonperturbative methods. Here we use the numerical renormalization group to study ac transport and decoherence in an experimentally realizable model interferometer, a parallel double quantum dot coupled to a phonon mode. The model allows us to clearly distinguish renormalization effects from decoherence. We discuss finite-frequency transport and confirm the restoration of coherence in the dc limit.

Large Suppression of Quantum Fluctuations of Light from a Single Emitter by an Optical Nanostructure

Diego-Martin Cano, Harald R. Haakh, Karim Murr, Mario Agio

We investigate the reduction of the electromagnetic field fluctuations in resonance fluorescence from a single emitter coupled to an optical nanostructure. We find that such hybrid systems can lead to the creation of squeezed states of light, with quantum fluctuations significantly below the shot-noise level. Moreover, the physical conditions for achieving squeezing are strongly relaxed with respect to an emitter in free space. A high degree of control over squeezed light is feasible both in the far and near fields, opening the pathway to its manipulation and applications on the nanoscale with state-of-the-art setups.

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

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

Tomography by Noise

G. Harder, D. Mogilevtsev, N. Korolkova, Ch Silberhorn

We present an efficient and robust method for the reconstruction of photon number distributions by using solely thermal noise as a probe. The method uses a minimal number of precalibrated quantum devices; only one on-off single-photon detector is sufficient. The feasibility of the method is demonstrated by the experimental inference of single-photon, thermal. and two-photon states. The method is stable to experimental imperfections and provides a direct, user-friendly quantum diagnostics tool.

Approximate quantum error correction for generalized amplitude-damping errors

Carlo Cafaro, Peter van Loock

We present analytic estimates of the performances of various approximate quantum error-correction schemes for the generalized amplitude-damping (GAD) qubit channel. Specifically, we consider both stabilizer and nonadditive quantum codes. The performance of such error-correcting schemes is quantified by means of the entanglement fidelity as a function of the damping probability and the nonzero environmental temperature. The recovery scheme employed throughout our work applies, in principle, to arbitrary quantum codes and is the analog of the perfect Knill-Laflamme recovery scheme adapted to the approximate quantum error-correction framework for the GAD error model. We also analytically recover and/or clarify some previously known numerical results in the limiting case of vanishing temperature of the environment, the well-known traditional amplitude-damping channel. In addition, our study suggests that degenerate stabilizer codes and self-complementary nonadditive codes are especially suitable for the error correction of the GAD noise model. Finally, comparing the properly normalized entanglement fidelities of the best performant stabilizer and nonadditive codes characterized by the same length, we show that nonadditive codes outperform stabilizer codes not only in terms of encoded dimension but also in terms of entanglement fidelity.

Rydberg atoms in hollow-core photonic crystal fibres

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

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

Single-molecule optical spectroscopy

Michel Orrit, Taekjip Ha, Vahid Sandoghdar

Laser Theory for Optomechanics: Limit Cycles in the Quantum Regime

Niels Loerch, Jiang Qian, Aashish Clerk, Florian Marquardt, Klemens Hammerer

Optomechanical systems can exhibit self-sustained limit cycles where the quantum state of the mechanical resonator possesses nonclassical characteristics such as a strongly negative Wigner density, as was shown recently in a numerical study by Qian et al. [Phys. Rev. Lett. 109, 253601 (2012)]. Here, we derive a Fokker-Planck equation describing mechanical limit cycles in the quantum regime that correctly reproduces the numerically observed nonclassical features. The derivation starts from the standard optomechanical master equation and is based on techniques borrowed from the laser theory due to Haake and Lewenstein. We compare our analytical model with numerical solutions of the master equation based on Monte Carlo simulations and find very good agreement over a wide and so far unexplored regime of system parameters. As one main conclusion, we predict negative Wigner functions to be observable even for surprisingly classical parameters, i.e., outside the single-photon strong-coupling regime, for strong cavity drive and rather large limit-cycle amplitudes. The approach taken here provides a natural starting point for further studies of quantum effects in optomechanics.

The key role of off-axis singularities in free-space vortex transmutation

David Novoa, Inigo J. Sola, Miguel Angel Garcia-March, Albert Ferrando

We experimentally demonstrate the generation of off-axis phase singularities in a vortex transmutation process induced by the breaking of rotational symmetry. The process takes place in free space by launching a highly charged vortex, owning full rotational symmetry, into a linear thin diffractive element presenting discrete rotational symmetry. It is shown that off-axis phase singularities follow straight dark rays bifurcating from the symmetry axis. This phenomenon may provide new routes toward the spatial control of multiple phase singularities for applications in atom trapping and particle manipulation.

Interaction of Relativistic Electron-Vortex Beams with Few-Cycle Laser Pulses

Armen G. Hayrapetyan, Oliver Matula, Andrea Aiello, Andrey Surzhykov, Stephan Fritzsche

We study the interaction of relativistic electron-vortex beams (EVBs) with laser light. Exact analytical solutions for this problem are obtained by employing the Dirac-Volkov wave functions to describe the (monoenergetic) distribution of the electrons in vortex beams with well-defined orbital angular momentum. Our new solutions explicitly show that the orbital angular momentum components of the laser field couple to the total angular momentum of the electrons. When the field is switched off, it is shown that the laser-driven EVB coincides with the field-free EVB as reported by Bliokh et al. [Phys. Rev. Lett. 107, 174802 (2011)]. Moreover, we calculate the probability density for finding an electron in the beam profile and demonstrate that the center of the beam is shifted with respect to the center of the field-free EVB.

Tracking Single Particles on Supported Lipid Membranes: Multimobility Diffusion and Nanoscopic Confinement

Chia-Lung Hsieh, Susann Spindler, Jens Ehrig, Vahid Sandoghdar

Supported lipid bilayers have been studied intensively over the past two decades. In this work, we study the diffusion of single gold nanoparticles (GNPs) with diameter of 20 nm attached to GM1 ganglioside or DOPE lipids at different concentrations in supported DOPC bilayers. The indefinite photostability of GNPs combined with the high sensitivity of interferometric scattering microscopy (iSCAT) allows us to achieve 1.9 nm spatial precision at 1 ms temporal resolution, while maintaining long recording times. Our trajectories visualize strong transient confinements within domains as small as 20 nm, and the statistical analysis of the data reveals multiple mobilities and deviations from normal diffusion. We present a detailed analysis of our findings and provide interpretations regarding the effect of the supporting substrate and GM1 clustering. We also comment on the use of high-speed iSCAT for investigating diffusion of lipids, proteins, or viruses in lipid membranes with unprecedented spatial and temporal resolution.

Atomic mercury vapor inside a hollow-core photonic crystal fiber

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

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

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

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

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

Light propagation in conjugated polymer nanowires decoupled from a substrate

Jaeyeon Pyo, Ji Tae Kim, Jewon Yoo, Jung Ho Je

Light-emitting conjugated polymer nanowires are vertically grown and remotely manipulated into a freestanding straight or curved structure in three-dimension. This approach enabled us to eliminate substrate coupling, a critical issue in nanowire photonics in the past decade. We for the first time accomplished characterization of propagation and bending losses of nanowires completely decoupled from a substrate.

Label-free characterization of biomembranes: from structure to dynamics

Alireza Mashaghi, Samaneh Mashaghi, Ilya Reviakine, Ron M. A. Heeren, Vahid Sandoghdar, Mischa Bonn

We review recent progress in the study of the structure and dynamics of phospholipid membranes and associated proteins, using novel label-free analytical tools. We describe these techniques and illustrate them with examples highlighting current capabilities and limitations. Recent advances in applying such techniques to biological and model membranes for biophysical studies and biosensing applications are presented, and future prospects are discussed.

Disentangling the effects of nanoscale structural variations on the light emission wavelength of single nano-emitters: InGaN/GaN multiquantum well nano-LEDs for a case study

George Sarau, Martin Heilmann, Michael Latzel, Silke Christiansen

The scattering in the light emission wavelength of semiconductor nano-emitters assigned to nanoscale variations in strain, thickness, and composition is critical in current and novel nanotechnologies from highly efficient light sources to photovoltaics. Here, we present a correlated experimental and theoretical study of single nanorod light emitting diodes (nano-LEDs) based on InGaN/GaN multiquantum wells to separate the contributions of these intrinsic fluctuations. Cathodoluminescence measurements show that nano-LEDs with identical strain states probed by non-resonant micro-Raman spectroscopy can radiate light at different wavelengths. The deviations in the measured optical transitions agree very well with band profile calculations for quantum well thicknesses of 2.07-2.72 nm and In fractions of 17.5-19.5% tightly enclosing the growth values. The nanorod surface roughness controls the appearance of surface optical phonon modes with direct implications on the design of phonon assisted nano-LED devices. This work establishes a new, simple, and powerful methodology for fundamental understanding as well as quantitative analysis of the strain - light emission relationship and surface-related phenomena in the emerging field of nano-emitters.

Distribution of Squeezed States through an Atmospheric Channel

Christian Peuntinger, Bettina Heim, Christian R. Mueller, Christian Gabriel, Christoph Marquardt, Gerd Leuchs

Continuous variable quantum states of light are used in quantum information protocols and quantum metrology and known to degrade with loss and added noise. We were able to show the distribution of bright polarization squeezed quantum states of light through an urban free-space channel of 1.6 km length. To measure the squeezed states in this extreme environment, we utilize polarization encoding and a postselection protocol that is taking into account classical side information stemming from the distribution of transmission values. The successful distribution of continuous variable squeezed states is accentuated by a quantum state tomography, allowing for determining the purity of the state.

Observation of Optical Undular Bores in Multiple Four-Wave Mixing

J. Fatome, C. Finot, G. Millot, A. Armaroli, S. Trillo

We demonstrate that wave-breaking dramatically affects the dynamics of nonlinear frequency conversion processes that operate in the regime of high efficiency (strong multiple four-wave mixing). In particular, by exploiting an all-optical-fiber platform, we show that input modulations propagating in standard telecom fibers in the regime of weak normal dispersion lead to the formation of undular bores (dispersive shock waves) that mimic the typical behavior of dispersive hydrodynamics exhibited, e.g., by gravity waves and tidal bores. Thanks to the nonpulsed nature of the beat signal employed in our experiment, we are able to clearly observe how the periodic nature of the input modulation forces adjacent undular bores to collide elastically.

Surface angular momentum of light beams

Marco Ornigotti, Andrea Aiello

Traditionally, the angular momentum of light is calculated for "bullet-like" electromagnetic wave packets, although in actual optical experiments "pencil-like" beams of light are more commonly used. The fact that a wave packet is bounded transversely and longitudinally while a beam has, in principle, an infinite extent along the direction of propagation, renders incomplete the textbook calculation of the spin/orbital separation of the angular momentum of a light beam. In this work we demonstrate that a novel, extra surface part must be added in order to preserve the gauge invariance of the optical angular momentum per unit length. The impact of this extra term is quantified by means of two examples: a Laguerre-Gaussian and a Bessel beam, both circularly polarized. (C) 2014 Optical Society of America

Dynamic operation of optical fibres beyond the single-mode regime facilitates the orientation of biological cells

Moritz Kreysing, Dino Ott, Michael J. Schmidberger, Oliver Otto, Mirjam Schuermann, Estela Martin-Badosa, Graeme Whyte, Jochen Guck

The classical purpose of optical fibres is delivery of either optical power, as for welding, or temporal information, as for telecommunication. Maximum performance in both cases is provided by the use of single-mode optical fibres. However, transmitting spatial information, which necessitates higher-order modes, is difficult because their dispersion relation leads to dephasing and a deterioration of the intensity distribution with propagation distance. Here we consciously exploit the fundamental cause of the beam deterioration-the dispersion relation of the underlying vectorial electromagnetic modes-by their selective excitation using adaptive optics. This allows us to produce output beams of high modal purity, which are well defined in three dimensions. The output beam distribution is even robust against significant bending of the fibre. The utility of this approach is exemplified by the controlled rotational manipulation of live cells in a dual-beam fibre-optical trap integrated into a modular lab-on-chip system.

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

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

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

Synthesis of a Covalent Monolayer Sheet by Photochemical Anthracene Dimerization at the Air/Water Interface and its Mechanical Characterization by AFM Indentation

Payam Payamyar, Khaled Kaja, Carlos Ruiz-Vargas, Andreas Stemmer, Daniel J. Murray, Carey J. Johnson, Benjamin T. King, Florian Schiffmann, Joost VandeVondele, et al.

Microlocal approach towards construction of nonreflecting boundary conditions

V. Vaibhav

This paper addresses the problem of construction of non-reflecting boundary condition for certain second-order nonlinear dispersive equations. It is shown that using the concept of microlocality it is possible to relax the requirement of compact support of the initial data. The method is demonstrated for a class of initial data such that outside the computational domain it behaves like a continuous-wave. The generalization is detailed for two existing schemes in the framework of pseudo-differential calculus, namely, Szeftel's method (Szeftel (2006) [1]) and gauge transformation strategy (Antoine et al. (2006) [2]). Efficient numerical implementation is discussed and a comparative performance analysis is presented. The paper also briefly surveys the possibility of extension of the method to higher-dimensional PDEs. (C) 2014 Elsevier Inc. All rights reserved.

Optical analogue of relativistic Dirac solitons in binary waveguide arrays

Truong X. Tran, Stefano Longhi, Fabio Biancalana

We study analytically and numerically an optical analogue of Dirac solitons in binary waveguide arrays in the presence of Kerr non-linearity. Pseudo-relativistic soliton solutions of the coupled-mode equations describing dynamics in the array are analytically derived. We demonstrate that with the found soliton solutions, the coupled mode equations can be converted into the nonlinear relativistic 1D Dirac equation. This paves the way for using binary waveguide arrays as a classical simulator of quantum nonlinear effects arising from the Dirac equation, something that is thought to be impossible to achieve in conventional (i.e. linear) quantum field theory. (C) 2013 Elsevier Inc. All rights reserved.

Geometric spin Hall effect of light in tightly focused polarization-tailored light beams

Martin Neugebauer, Peter Banzer, Thomas Bauer, Sergej Orlov, Norbert Lindlein, Andrea Aiello, Gerd Leuchs

Recently, it was shown that a nonzero transverse angular momentum manifests itself in a polarization-dependent intensity shift of the barycenter of a paraxial light beam [Aiello et al., Phys. Rev. Lett. 103, 100401 (2009)]. The underlying effect is phenomenologically similar to the spin Hall effect of light but does not depend on the specific light-matter interaction and can be interpreted as a purely geometric effect. Thus, it was named the geometric spin Hall effect of light. Here, we experimentally investigate the appearance of this effect in tightly focused vector beams. We use an experimental nanoprobing technique in combination with a reconstruction algorithm to verify the relative shifts of the components of the electric energy density and the shift of the intensity in the focal plane. By that, we experimentally demonstrate the geometric spin Hall effect of light in a highly nonparaxial beam.

Ultrabroadband Airy light bullets

P. Piksarv, A. Valdmann, H. Valtna-Lukner, P. Saari

We present the measurements of the spatiotemporal impulse responses of two optical systems for launching ultrashort Airy pulses, including ultrabroadband nonspreading Airy beams whose main lobe size remains invariantly small over propagation. First, a spatial light modulator and, second, a custom refractive element with continuous surface profile were used to impose the required cubic phase on the input field. A white-light spectral interferometry setup based on the SEA TADPOLE technique was applied for full spatio-temporal characterization of the impulse response with ultrahigh temporal resolution approaching a single cycle of the light wave. The results were compared to the theoretical model.

Probing dipole-forbidden autoionizing states by isolated attosecond pulses

Wei-Chun Chu, Toru Morishita, C. D. Lin

We propose a general technique to retrieve the information of dipole-forbidden resonances in the autoionizing region. In the simulation, a helium atom is pumped by an isolated attosecond pulse in the extreme ultraviolet (EUV) combined with a few-femtosecond laser pulse. The excited wave packet consists of the S-1, P-1, and D-1 states, including the background continua, near the 2s2p(P-1) doubly excited state. The resultant electron spectra with various laser intensities and time delays between the EUV and laser pulses are obtained by a multilevel model and an ab initio time-dependent Schrodinger equation calculation. By taking the ab initio calculation as a "virtual measurement," the dipole-forbidden resonances are characterized by the multilevel model. We found that in contrast to the common assumption, the nonresonant coupling between the continua plays a significant role in the time-delayed electron spectra, which shows the correlation effect between photoelectrons before they leave the core. This technique takes the advantages of ultrashort pulses uniquely and would be a timely test for the current attosecond technology.

Colour and multicolour tuning of InGaN quantum dot based light-emitting diodes

C. Tessarek, S. Figge, D. Hommel

InGaN quantum dots (QDs) formed via spinodal and binodal decomposition are used as an optically active region for light-emitting diodes (LEDs). It is shown that the emission wavelength of the electroluminescence (EL) can be shifted from blue to green by adjusting the deposition time of the InGaN QD layer. A first approach towards a monolithic multicolour emitting diode is presented. Strong difference of the EL on the InGaN QD stacking sequence is observed and is attributed to both low hole concentration and mobility.

Separable Schmidt modes of a nonseparable state

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

Two-photon states entangled in continuous variables such as wave vector or frequency represent a powerful resource for quantum-information protocols in higher-dimensional Hilbert spaces. At the same time, there is a problem of addressing separately the corresponding Schmidt modes. We propose a method of engineering two-photon spectral amplitude in such a way that it contains several nonoverlapping Schmidt modes, each of which can be filtered losslessly. The method is based on spontaneous parametric down-conversion (SPDC) pumped by radiation with a comblike spectrum. There are many ways of producing such a spectrum; here we consider the simplest one, namely, passing the pump beam through a Fabry-Perot interferometer. For the two-photon spectral amplitude (TPSA) to consist of nonoverlapping Schmidt modes, the crystal dispersion dependence, the length of the crystal, the Fabry-Perot free spectral range, and its finesse should satisfy certain conditions. We experimentally demonstrate the control of TPSA through these parameters. We also discuss a possibility to realize a similar situation using cavity-based SPDC.

Vectorial complex-source vortex beams

S. Orlov, P. Banzer

The scalar complex source vortex model is an accurate description of highly focused scalar vortices. We use it to construct a variety of vectorial solutions of Maxwell's equations describing highly focused and variously polarized vector vortex beams accurately. Three different families of optical vector vortex beams are presented and studied in detail. In this model, optical vortices derived within Cartesian symmetry correspond to circularly and linearly polarized highly focused vortex beams in the focus of a high numerical aperture focusing system. In addition, we report on vortical complex-source beams derived within cylindrical and spherical symmetries which exhibit very special and intriguing properties.

Plasmonic kinks and walking solitons in nonlinear lattices of metal nanoparticles

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

We study nonlinear effects in one-dimensional (1D) arrays and two-dimensional (2D) lattices composed of metallic nanoparticles with the nonlinear Kerr-like response and an external driving field. We demonstrate the existence of families of moving solitons in 1D arrays and characterize their properties such as an average drifting velocity. We also analyse the impact of varying external field intensity and frequency on the structure and dynamics of kinks in 2D lattices. In particular, we identify the kinks with positive, negative and zero velocity as well as breathing kinks with a self-oscillating profile.

Probing biomechanical properties with a centrifugal force quartz crystal microbalance

Aaron Webster, Frank Vollmer, Yuki Sato

Application of force on biomolecules has been instrumental in understanding biofunctional behaviour from single molecules to complex collections of cells. Current approaches, for example, those based on atomic force microscopy or magnetic or optical tweezers, are powerful but limited in their applicability as integrated biosensors. Here we describe a new force-based biosensing technique based on the quartz crystal microbalance. By applying centrifugal forces to a sample, we show it is possible to repeatedly and non-destructively interrogate its mechanical properties in situ and in real time. We employ this platform for the studies of micron-sized particles, viscoelastic monolayers of DNA and particles tethered to the quartz crystal microbalance surface by DNA. Our results indicate that, for certain types of samples on quartz crystal balances, application of centrifugal force both enhances sensitivity and reveals additional mechanical and viscoelastic properties.