[
    {
        "text": "Tunable high-index photonic glasses: Materials with extreme photonic properties such as maximum diffuse\nreflectance, high albedo, or tunable band gaps are essential in many current\nand future photonic devices and coatings. While photonic crystals, periodic\nanisotropic structures, are well established, their disordered counterparts,\nphotonic glasses (PGs), are less understood despite their most interesting\nisotropic photonic properties. Here, we introduce a controlled high index model\nPG system. It is made of monodisperse spherical TiO$_2$ colloids to exploit\nstrongly resonant Mie scattering for optimal turbidity. We report spectrally\nresolved combined measurements of turbidity and light energy velocity from\nlarge monolithic crack-free samples. This material class reveals pronounced\nresonances enabled by the possibility to tune both the refractive index of the\nextremely low polydisperse constituents and their radius. All our results are\nrationalized by a model based on the energy coherent potential approximation,\nwhich is free of any fitting parameter. Surprisingly good quantitative\nagreement is found even at high index and elevated packing fraction. This class\nof PGs may be the key to optimized tunable photonic materials and also central\nto understand fundamental questions such as isotropic structural colors, random\nlasing or strong light localization in 3D.",
        "category": "physics_optics"
    },
    {
        "text": "Gaussian-Schell analysis of the transverse spatial properties of\n  high-harmonic beams: High harmonic generation (HHG) is an established means of producing coherent,\nshort wavelength, ultrafast pulses from a compact set-up. Table-top\nhigh-harmonic sources are increasingly being used to image physical and\nbiological systems using emerging techniques such as coherent diffraction\nimaging and ptychography. These novel imaging methods require coherent\nillumination, and it is therefore important to both characterize the spatial\ncoherence of high-harmonic beams and understand the processes which limit this\nproperty. Here we investigate the near- and far-field spatial properties of\nhigh-harmonic radiation generated in a gas cell. The variation with harmonic\norder of the intensity profile, wavefront curvature, and complex coherence\nfactor is measured in the far-field by the SCIMITAR technique. Using the\nGaussian-Schell model, the properties of the harmonic beam in the plane of\ngeneration are deduced. Our results show that the order-dependence of the\nharmonic spatial coherence is consistent with partial coherence induced by both\nvariation of the intensity-dependent dipole phase as well as finite spatial\ncoherence of the driving radiation. These findings are used to suggest ways in\nwhich the coherence of harmonic beams could be increased further, which would\nhave direct benefits to imaging with high-harmonic radiation.",
        "category": "physics_optics"
    },
    {
        "text": "Functional plasmonic nano-circuits with low insertion and propagation\n  losses: We experimentally demonstrate plasmonic nano-circuits operating as\nsub-diffraction directional couplers optically excited with high efficiency\nfrom free-space using optical Yagi-Uda style antennas at \\lambda = 1550 nm. The\noptical Yagi-Uda style antennas are designed to feed channel plasmon waveguides\nwith high efficiency (45 % in coupling, 60 % total emission), narrow angular\ndirectivity (< 40{\\deg}) and low insertion loss. SPP gap waveguides exhibit\npropagation lengths as large as 34 {\\mu}m with adiabatically tuned confinement,\nand are integrated with ultra-compact (5 \\mu m x 10 \\mu m), highly dispersive\ndirectional couplers, which enable 30 dB discrimination over {\\Delta}{\\lambda}\n= 200 nm with only 0.3 dB device loss.",
        "category": "physics_optics"
    },
    {
        "text": "Photonic hooks from Janus microcylinders: Recently, a type of curved light beams, photonic hooks (PHs), was\ntheoretically predicted and experimentally observed. The production of photonic\nhook (PH) is due to the breaking of structural symmetry of a plane-wave\nilluminated microparticle. Herein, we presented and implemented a new approach,\nof utilizing the symmetry-broken of the microparticles in material composition,\nfor the generation of PHs from Janus microcylinders. Finite element method\nbased numerical simulation and energy flow diagram represented theoretical\nanalysis were used to investigate the field distribution characteristics and\nformation mechanism of the PHs. The full width at half-maximum (FWHM) of the PH\n(~0.29$\\lambda$) is smaller than the FWHM of the photonic nanojet\n(~0.35$\\lambda$) formed from a circular microcylinder with the same geometric\nradius. By changing the refractive index contrasts between upper and lower\nhalf-cylinders, or rotating the Janus microcylinder relative to the central\naxis, the shape profiles of the PHs can be efficiently modulated. The\ntunability of the PHs through simple stretching or compression operations, for\nthe Janus microcylinder constituted by one solid inorganic half-cylinder and\nthe other flexible polymer half-cylinder, was studied and discussed as well.",
        "category": "physics_optics"
    },
    {
        "text": "Porous SiO$_2$ coated dielectric metasurface with consistent performance\n  independent of environmental conditions: With the rapid advances of functional dielectric metasurfaces and their\nintegration on on-chip nanophotonic devices, the necessity of metasurfaces\nworking in different environments, especially in biological applications,\narose. However, the metasurfaces' performance is tied to the unit cell's\nefficiency and ultimately the surrounding environment it was designed for, thus\nreducing its applicability if exposed to altering refractive index media. Here,\nwe report a method to increase a metasurface's versatility by covering the\nhigh-index metasurface with a low index porous SiO$_2$ film, protecting the\nmetasurface from environmental changes while keeping the working efficiency\nunchanged. We show, that a covered metasurface retains its functionality even\nwhen exposed to fluidic environments.",
        "category": "physics_optics"
    },
    {
        "text": "Hybrid real- and reciprocal-space full-field imaging with coherent\n  illumination: We present a novel diffractive imaging method that harnesses a low-resolution\nreal-space image to guide the phase retrieval. A computational algorithm is\ndeveloped to utilize such prior knowledge as a real-space constraint in the\niterative phase retrieval procedure. Numerical simulations and proof-of-concept\nexperiments are carried out, demonstrating our method's capability of\nreconstructing high-resolution details that are otherwise inaccessible with\ntraditional phasing algorithms. With the present method, we formulate a\nconceptual design for the coherent imaging experiments at a next-generation\nX-ray light source.",
        "category": "physics_optics"
    },
    {
        "text": "Direct Modulation of Electrically Pumped Coupled Microring Lasers: We demonstrate how the presence of gain-loss contrast between two coupled\nidentical resonators can be used as a new degree of freedom to enhance the\nmodulation frequency response of laser diodes. An electrically pumped microring\nlaser system with a bending radius of 50 {\\mu}m is fabricated on an\nInAlGaAs/InP MQW p-i-n structure. The room temperature continuous wave (CW)\nlaser threshold current of the device is 27 mA. By adjusting the ratio between\nthe injection current levels in the two coupled microrings, our experimental\nresults clearly show a bandwidth improvement by up to 1.63 times the\nfundamental resonant frequency of the individual device. This matches well with\nour rate equation simulation model.",
        "category": "physics_optics"
    },
    {
        "text": "Applying universal scaling laws to identify the best molecular design\n  paradigms for second-order nonlinear optics: We apply scaling and the theory of the fundamental limits of the second-order\nmolecular susceptibility to identify material classes with ultralarge\nnonlinear-optical response. Size effects are removed by normalizing all\nnonlinearities to get intrinsic values so that the scaling behavior of a series\nof molecular homologues can be determined. Several new figures of merit are\nproposed that quantify the desirable properties for molecules that can be\ndesigned by adding a sequence of repeat units, and used in the assessment of\nthe data. Three molecular classes are found. They are characterized by\nsub-scaling, nominal scaling, or super-scaling. Super-scaling homologues most\nefficiently take advantage of increased size. We apply our approach to data\ncurrently available in the literature to identify the best super-scaling\nmolecular paradigms with the aim of identifying desirable traits of new\nmaterials.",
        "category": "physics_optics"
    },
    {
        "text": "Designing of strongly confined short-wave Brillouin phonons in silicon\n  waveguide periodic lattices: We propose a feasible waveguide design optimized for harnessing Stimulated\nBrillouin Scattering with long-lived phonons. The design consists of a fully\nsuspended ridge waveguide surrounded by a 1D phononic crystal that mitigates\nlosses to the substrate while providing the needed homogeneity for the build-up\nof the optomechanical interaction. The coupling factor of these structures was\ncalculated to be 0.54 (W.m)$^{-1}$ for intramodal backward Brillouin scattering\nwith its fundamental TE-like mode and 4.5(W.m)$^{-1}$ for intramodal forward\nBrillouin scattering. The addition of the phononic crystal provides a 30 dB\nattenuation of the mechanical displacement after only five unitary cells,\npossibly leading to a regime where the acoustic losses are only limited by\nfabrication. As a result, the total Brillouin gain, which is proportional to\nthe product of the coupling and acoustic quality factors, is nominally equal to\nthe idealized fully suspended waveguide.",
        "category": "physics_optics"
    },
    {
        "text": "Nonlinear polarization evolution using time-dependent density functional\n  theory: We propose a theoretical and computational approach to investigate temporal\nbehavior of a nonlinear polarization in perturbative regime induced by an\nintense and ultrashort pulsed electric field. First-principles time-dependent\ndensity functional theory is employed to describe the electron dynamics.\nTemporal evolution of third-order nonlinear polarization is extracted from a\nfew calculations of electron dynamics induced by pulsed electric fields with\nthe same time profile but different amplitudes. We discuss characteristic\nfeatures of the nonlinear polarization evolution as well as an extraction of\nnonlinear susceptibilities and time delays by fitting the polarization. We also\ncarry out a decomposition of temporal and spatial changes of the electron\ndensity in power series with respect to the field amplitude. It helps to get\ninsight into the origin of the nonlinear polarization in atomic scale.",
        "category": "physics_optics"
    },
    {
        "text": "Monte Carlo simulation of light scattering in the atmosphere and effect\n  of atmospheric aerosols on the point spread function: We present a Monte Carlo simulation for the scattering of light in the case\nof an isotropic light source. The scattering phase functions are studied\nparticularly in detail to understand how they can affect the multiple light\nscattering in the atmosphere. We show that although aerosols are usually in\nlower density than molecules in the atmosphere, they can have a non-negligible\neffect on the atmospheric point spread function. This effect is especially\nexpected for ground-based detectors when large aerosols are present in the\natmosphere.",
        "category": "physics_optics"
    },
    {
        "text": "Unidirectional Amplification and Shaping of Optical Pulses by Three-Wave\n  Mixing with Negative Phonons: A possibility to greatly enhance frequency-conversion efficiency of\nstimulated Raman scattering is shown by making use of extraordinary properties\nof three-wave mixing of ordinary and backward waves. Such processes are\ncommonly attributed to negative-index plasmonic metamaterials. This work\ndemonstrates the possibility to replace such metamaterials that are very\nchallenging to engineer by readily available crystals which support elastic\nwaves with contra-directed phase and group velocities. The main goal of this\nwork is to investigate specific properties of indicated nonlinear optical\nprocess in short pulse regime and to show that it enables elimination of\nfundamental detrimental effect of fast damping of optical phonons on the\nprocess concerned. Among the applications is the possibility of creation of a\nfamily of unique photonic devices such as unidirectional Raman amplifiers and\nfemtosecond pulse shapers with greatly improved operational properties.",
        "category": "physics_optics"
    },
    {
        "text": "Two-dimensional excitons from twisted light and the fate of the photon's\n  orbital angular momentum: As the bound state of two oppositely charged particles, excitons emerge from\noptically excited semiconductors as the electronic analogue of a hydrogen atom.\nIn the two-dimensional (2D) case, realized either in quantum well systems or\ntruly 2D materials such as transition metal dichalcogenides, the relative\nmotion of an exciton is described by two quantum numbers: the principal quantum\nnumber $n$, and a quantum number $j$ for the angular momentum along the\nperpendicular axis. Conservation of angular momentum demands that only the\n$j=0$ states of the excitons are optically active in a system illuminated by\nplane waves. Here we consider the case for spatially structured light sources,\nspecifically for twisted light beams with non-zero orbital angular momentum per\nphoton. Under the so-called dipole approximation where the spatial variations\nof the light source occur on length scales much larger than the size of the\nsemiconductor's unit cell, we show that the photon (linear and/or angular)\nmomentum is coupled to the center-of-mass (linear and/or angular) momentum of\nthe exciton. Our study establishes that the selection rule for the internal\nstates of the exciton, and thus the exciton spectrum, is independent from the\nspatial structure of the light source.",
        "category": "physics_optics"
    },
    {
        "text": "Digital Holographic Interferometry for Micro-Deformation Analysis of\n  Morpho Butterfly Wing: In this study, we present a detailed analysis of deflections in a butterfly\nwing utilizing digital holographic interferometry. Our methodology revolves\naround an off-axis lensless Fourier holographic setup, and we employ laser\nexcitation to induce deflections in the object. The implementation of a digital\nholographic interferometry setup, tailored for rapid monitoring of\nmicro-deformation, is a central aspect of the research. We offer an overview of\nthe theoretical foundations of this technique, complemented by both\nexperimental and simulated tests aimed at validating our findings.\nSignificantly, our investigation focuses on the detailed analysis of the micro\nstructures found on the wing of the Morpho butterfly. The insights garnered\nfrom our study not only affirm the precision and potential of this methodology\nbut also shed light on promising avenues for further exploration, especially in\nthe domain of high-precision deflection sensing and its diverse applications.",
        "category": "physics_optics"
    },
    {
        "text": "Compressive lensless endoscopy with partial speckle scanning: The lensless endoscope (LE) is a promising device to acquire in vivo images\nat a cellular scale. The tiny size of the probe enables a deep exploration of\nthe tissues. Lensless endoscopy with a multicore fiber (MCF) commonly uses a\nspatial light modulator (SLM) to coherently combine, at the output of the MCF,\nfew hundreds of beamlets into a focus spot. This spot is subsequently scanned\nacross the sample to generate a fluorescent image. We propose here a novel\nscanning scheme, partial speckle scanning (PSS), inspired by compressive\nsensing theory, that avoids the use of an SLM to perform fluorescent imaging in\nLE with reduced acquisition time. Such a strategy avoids photo-bleaching while\nkeeping high reconstruction quality. We develop our approach on two key\nproperties of the LE: (i) the ability to easily generate speckles, and (ii) the\nmemory effect in MCF that allows to use fast scan mirrors to shift light\npatterns. First, we show that speckles are sub-exponential random fields.\nDespite their granular structure, an appropriate choice of the reconstruction\nparameters makes them good candidates to build efficient sensing matrices.\nThen, we numerically validate our approach and apply it on experimental data.\nThe proposed sensing technique outperforms conventional raster scanning: higher\nreconstruction quality is achieved with far fewer observations. For a fixed\nreconstruction quality, our speckle scanning approach is faster than\ncompressive sensing schemes which require to change the speckle pattern for\neach observation.",
        "category": "physics_optics"
    },
    {
        "text": "Longitudinal electromagnetic waves with extremely short wavelength: Electromagnetic waves in vacuum and most materials have transverse\npolarization. Longitudinal electromagnetic waves with electric field parallel\nto wave vector are very rare and appear under special conditions in a limited\nclass of media, for example in plasma. In this work, we study the dispersion\nproperties of an easy-to-manufacture metamaterial consisting of two\nthree-dimensional cubic lattices of connected metallic wires inserted one into\nanother, also known as an interlaced wire medium. It is shown that the\nmetamaterial supports longitudinal waves at extremely wide frequency band from\nvery low frequencies up to the Bragg resonances of the structure. The waves\nfeature unprecedentedly short wavelengths comparable to the period of the\nmaterial. The revealed effects highlight spatially dispersive response of\ninterlaced wire medium and provide a route toward generating electromagnetic\nfields with strong spatial variation.",
        "category": "physics_optics"
    },
    {
        "text": "The spin-flip model of spin-polarized vertical-cavity surface-emitting\n  lasers: asymptotic analysis, numerics, and experiments: The spin-flip model describing optically pumped spin-polarized\nvertical-cavity surface-emitting lasers is considered. The steady-state\nsolutions of the model for elliptically-polarised fields are studied.\nAsymptotic analysis for the existence and stability of the steady-state\nsolutions is developed, particularly in the presence of pump polarisation\nellipticity. The expansion is with respect to small parameters representing the\nellipticity and the difference between the total pump power and the lasing\nthreshold. The analytical results are then confirmed numerically, where it is\nobtained that generally one of the steady-state solutions is stable while the\nother is not. The theoretical results are shown to be in qualitative agreement\nwith the experiments.",
        "category": "physics_optics"
    },
    {
        "text": "Exact analysis of a Veselago lens in the quasi-static regime: The resolution of conventional optical lenses is limited by the wavelength.\nMaterials with negative refractive index have been shown to enable the\ngeneration of an enhanced resolution image where both propagating and\nnon-propagating waves are employed. We analyze such a Veselago lens by\nexploiting some exact one dimensional integral expressions for the quasi-static\nelectric potential of a point charge in that system. Those were recently\nobtained by expanding that potential in the quasi-static eigenfunctions of a\nthree-flat-slabs composite structure. Numerical evaluations of those integrals,\nusing realistic values for physical parameters like the electric permittivities\nof the constituent slabs and their thickness, reveal some surprising effects:\nE.g., the maximum concentration of the electric field occurs not at the\ngeometric optics foci but at the interfaces between the negative permittivity\nslab and the positive permittivity slabs. The analysis provides simple\ncomputational guides for designing such structures in order to achieve enhanced\nresolution of an optical image.",
        "category": "physics_optics"
    },
    {
        "text": "Negative permittivity and permeability of gold nanorods metamaterials in\n  UV- Vis region: In this article, we report the growth of gold nanorods on glass substrates\nand copper nanoparticle thin films by cylindrical direct current magnetron\nsputtering (CDCMS) at room temperature. The grown gold nanorods have short\nlengths of < 20nm and show negative optical parameters in UV-Vis region. So far\nnegative permittivity and permeability were only shown for complex artificial\nstructures. In a case of simple structures like gold nanorods, the negative\noptical parameters were only predicted by simulation methods and considering\nideal structures and they were not yet reported by experimental groups, who has\ngrown or synthesis gold nanorods by physical or chemical methods. The small\nsize of gold nanorods and thickness of our samples compare to other\nexperimental groups could be the reason of negative permittivity and\npermeability in our case. Low loss metamaterials with simultaneously negative\npermittivity and permeability are desired for practical applications in many\noptical devices such as optical switching, waveguides, modulators, and\nplasmonic antenna arrays. The optical properties of the grown gold nanorods\nwere defined by ultraviolet- visible (UV-Vis) spectroscopy and their quality\nwas assessed through multi-technique characterization using transmission\nelectron microscopy (TEM), field emission scanning electron microscopy\n(FE-SEM), X-ray diffraction (XRD), and energy dispersed X-ray (EDX).",
        "category": "physics_optics"
    },
    {
        "text": "All-depth dispersion cancellation in spectral domain optical coherence\n  tomography using numerical intensity correlations: In ultra-high resolution (UHR-) optical coherence tomography (OCT) group\nvelocity dispersion (GVD) must be corrected for in order to approach the\ntheoretical resolution limit. One approach promises not only compensation, but\ncomplete annihilation of even order dispersion effects, and that at all sample\ndepths. This approach has hitherto been demonstrated with an experimentally\ndemanding 'balanced detection' configuration based on using two detectors. We\ndemonstrate intensity correlation (IC) OCT using a conventional spectral domain\n(SD) UHR-OCT system with a single detector. IC-SD-OCT configurations exhibit\ncross term ghost images and a reduced axial range, half of that of conventional\nSD-OCT. We demonstrate that both shortcomings can be removed by applying a\nnovel generic artefact reduction algorithm and using analytic interferograms.\nWe show the superiority of IC-SD-OCT compared to conventional SD-OCT by showing\nhow IC-SD-OCT is able to image spatial structures behind a strongly dispersive\nsilicon wafer. Finally, we question the resolution enhancement of square root 2\nthat IC-SD-OCT is often believed to have compared to SD-OCT. We show that this\nis simply the effect of squaring the reflectivity profile as a natural result\nof processing the product of two intensity spectra instead of a single\nspectrum.",
        "category": "physics_optics"
    },
    {
        "text": "Classical Propagation of Light in Spatio-Temporal Periodic Media: We analyze the propagation of electromagnetic waves in media where the\ndielectric constants undergo rapid temporal periodic modulation. Both spatially\nhomogeneous and periodic media are studied. Fast periodic temporal modulation\nof the dielectric constant of a homogeneous medium leads to existence of\nphotonic band-gap like phenomena. In the presence of both spatial and tem-\nporal periodicity the electromagnetic spectrum is described in a\nfour-dimensional cube, defining an effective Brillouin zone. In the case of\nincommensurability between space and time periodicities, completely dispersed\npoint spectra exist.",
        "category": "physics_optics"
    },
    {
        "text": "Inelastic collision-induced atomic cooling and gain linewidth\n  suppression in He-Ne lasers: He-Ne lasers have been one of the most widely employed optoelectronic\nelements, playing irreplaceable roles in various applications, including\noptical detections, spectroscopy, interferometry, laser processing, and so on.\nFor broad applications that require single-mode operations, the gain linewidth\nneeds to be constrained, which conventionally can be obtained through overall\ngain suppressions. Such an approach inevitably has limited the output power and\nthus restricted further applications that require ultra-high precisions. In\nthis article, we discover that inelastic collisions among He and Ne atoms can\nbe exploited to cool down the Ne atoms, compressing the Doppler broadening and\nconsequently also the gain linewidth, enabling us to further experimentally\ndemonstrate a significantly broadened spectral range of single-mode operation\nwith stable output powers. Our discovery of inelastic collision-induced atomic\ncooling has ultimately overcome the tradeoff between output power and gain\nlinewidth, opening new avenues for both fundamental explorations and disruptive\napplications relying on gaseous laser systems.",
        "category": "physics_optics"
    },
    {
        "text": "Critical coupling to Tamm plasmons: The conditions of critical coupling of light to Tamm plasmons are\ninvestigated with comprehensive numerical simulations, highlighting the\nparameters that maximise absorption of incident light in the metal layer. The\nasymmetric response in reflection and absorption with respect to the direction\nof incidence is discussed, the two cases yielding different optimal coupling\nconditions. These findings are relevant for the design of optimised Tamm\nstructures, particularly in applications such as narrow-band thermal emitters,\nfield-enhanced spectroscopy and refractive-index sensing.",
        "category": "physics_optics"
    },
    {
        "text": "Record Performance of Electrical Injection Sub-wavelength\n  Metallic-Cavity Semiconductor Lasers at Room Temperature: Metallic-Cavity lasers or plasmonic nanolasers of sub-wavelength sizes have\nattracted great attentions in recent years, with the ultimate goal of achieving\ncontinuous wave (CW), room temperature (RT) operation under electrical\ninjection. Despite great efforts, a conclusive and convincing demonstration of\nthis goal has proven challenging. By overcoming several fabrication challenges\nimposed by the stringent requirement of such small scale devices, we were\nfinally able to achieve this ultimate goal. Our metallic nanolaser with a\ncavity volume of 0.67{\\lambda}3 ({\\lambda}=1591 nm) shows a linewidth of 0.5 nm\nat RT, which corresponds to a Q-value of 3182 compared to 235 of the cavity Q,\nthe highest Q under lasing condition for RT CW operation of any sub-wavelength\nlaser. Such record performance provides convincing evidences of the feasibility\nof RT CW metallic nanolasers, thus opening a wide range of practical\npossibilities of novel nanophotonic devices based on metal-semiconductor\nstructures.",
        "category": "physics_optics"
    },
    {
        "text": "Hysteresis phenomena in electron tunneling, induced by surface plasmons: A high spatial resolution surface plasmon near field scanning tunneling\nmicroscope (STM) has been used to study the properties of localized surface\nplasmons (SPO) in so-called hot spots on a gold surface, where the local\nelectromagnetic field is extremely high. A CW semiconductor laser and a\nfemtosecond Ti:Sa laser were used to excite the plasmons and the SPO excited\ntunnel current was used as the detector. When scanning the STM from negative to\npositive bias and reversed, hysteresis in the tunnel signal was found,\nexcluding (or rather minimizing) the role of the presence of a Casimir effect\nin the process. It was found, however, that a multiple image charge induced\ndouble well potential may explain our experimental findings. The stepwise\nbehaviour of the area of the observed hysteresis loops is a new, additional\nindication of the non-classical properties of the SPOs.",
        "category": "physics_optics"
    },
    {
        "text": "Transverse photon spin beyond interfaces: Photons possess spin degree of freedom, corresponding to clockwise and\ncounter clockwise rotating direction of the fields. Photon spin plays an\nimportant role in various applications such as optical communications,\ninformation processing and sensing. In conventional isotropic media, photon\nspin is aligned with the propagation direction of light, obeying spin momentum\nlocking. Interestingly, at certain interfaces, the surface waves decaying away\nfrom the interface possess a photon spin transverse to its propagation, opening\nexciting opportunities for observation of spin dependent unidirectional\nexcitation in confined systems. Here we propose and realize transverse photon\nspin (T-spin) in the interior of a bulk medium, without relying on the presence\nof any interfaces. We show the complete mapping of the T-spin of surface modes\nto that of the bulk modes by introducing the coupling between electric and\nmagnetic responses along orthogonal directions, i.e., the bianisotropy, into\nthe medium. We further discover that an interface formed by two bianisotropic\nmedia of opposite orientations supports edge-dependent propagating modes with\ntunable cutoff frequencies. Our results provide a new platform for manipulating\nthe spin orbit interaction of electromagnetic waves.",
        "category": "physics_optics"
    },
    {
        "text": "Transverse shift of a beam with orbital angular momentum under\n  reflection from a dielectric film: We present the results of numerical analysis of Gauss-Bessel beam reflection\nfrom a dielectric film under different angles of incidence. A transverse shift\nof the beam under the orbital momentum sign change is observed. The value of\nthe shift is independent of the polarization state of the incident beam.",
        "category": "physics_optics"
    },
    {
        "text": "Gradient-Based Optimization of Optical Vortex Beam Emitters: Vortex beams are stable solutions of Maxwell's equations that carry phase\nsingularities and orbital angular momentum, unique properties that give rise to\nmany applications in the basic sciences, optical communications, and quantum\ntechnologies. Scalable integration and fabrication of vortex beam emitters will\nallow these applications to flourish and enable new applications not possible\nwith traditional optics. Here we present a general framework to generate\nintegrated vortex beam emitters using photonic inverse design. We\nexperimentally demonstrate generation of vortex beams with angular momentum\nspanning -3$\\hbar$ to 3$\\hbar$. We show the generality of this design procedure\nby designing a vortex beam multiplexer capable of exciting a custom vortex beam\nfiber. Finally, we produce foundry-fabricated beam emitters with\nwide-bandwidths and high-efficiencies that take advantage of a multi-layer\nheterogeneous integration.",
        "category": "physics_optics"
    },
    {
        "text": "Extraordinary nonlinear optics in ordinary semiconductors: We numerically demonstrate inhibition of absorption, optical transparency,\nand anomalous momentum states of phase locked harmonic pulses in\nsemiconductors, at UV and extreme UV frequencies, in spectral regions where the\ndielectric constant of typical semiconductors is negative. We show that a\ngenerated harmonic signal can propagate through a bulk metallic medium without\nbeing absorbed as a result of a phase locking mechanism between the pump and\nits harmonics. These findings may open new regimes in nonlinear optics and are\nparticularly relevant to the emerging fields of nonlinear negative index\nmeta-materials and nano-plasmonics, especially in the ultrafast pulse regime.",
        "category": "physics_optics"
    },
    {
        "text": "Stable Control of Pulse Speed in Parametric Three-Wave Solitons: We analyze the control of the propagation speed of three wave packets\ninteracting in a medium with quadratic nonlinearity and dispersion. We found\nanalytical expressions for mutually trapped pulses with a common velocity in\nthe form of a three-parameter family of solutions of the three-wave resonant\ninteraction. The stability of these novel parametric solitons is simply related\nto the value of their common group velocity.",
        "category": "physics_optics"
    },
    {
        "text": "Simulation of White Light Generation and Near Light Bullets Using a\n  Novel Numerical Technique: An accurate and efficient simulation has been devised, employing a new\nnumerical technique to simulate the derivative generalised non-linear\nSchr\\\"odinger equation in all three spatial dimensions and time. The simulation\nmodels all pertinent effects such as self-steepening and plasma for the\nnon-linear propagation of ultrafast optical radiation in bulk material.\nSimulation results are compared to published experimental spectral data of an\nexample ytterbium aluminum garnet system at 3.1um radiation and fits to within\na factor of 5. The simulation shows that there is a stability point near the\nend of the 2 mm crystal where a quasi-light bullet (spatial temporal soliton)\nis present. Within this region, the pulse is collimated at a reduced diameter\n(factor of ~2) and there exists a near temporal soliton at the spatial center.\nThe temporal intensity within this stable region is compressed by a factor of\n~4 compared to the input. This study shows that the simulation highlights new\nphysical phenomena based on the interplay of various linear, non-linear and\nplasma effects that go beyond the experiment and is thus integral to achieving\naccurate designs of white light generation systems for optical applications. An\nadaptive error reduction algorithm tailor made for this simulation will also be\npresented in appendix.",
        "category": "physics_optics"
    },
    {
        "text": "Circularly polarized extreme ultraviolet high harmonic generation in\n  graphene: Circularly polarized extreme ultraviolet (XUV) radiation is highly\ninteresting for investigation of chirality-sensitive light-matter interactions.\nRecent breakthroughs have enabled generation of such light sources via high\nharmonic generation (HHG) from rare gases. There is a growing interest in\nextending HHG medium from gases to solids, especially to 2D materials, as they\nhold great promise to develop ultra-compact solid-state photonic devices and\nprovide insights into electronic properties of the materials themselves.\nHowever, HHG in graphene driven by terahertz to mid-infrared fields reported so\nfar only generate low harmonic orders, and furthermore no harmonics driven by\ncircularly polarized lasers. Here, using first-principles simulations within a\ntime-dependent density-functional theory framework, we show that it is possible\nto generate HHG extending to the XUV spectral region in monolayer extended\ngraphene excited by near-infrared lasers. Moreover, we demonstrate that a\nsingle circularly polarized driver is enough to ensure HHG in graphene with\ncircular polarization. The corresponding spectra reflect the six-fold\nrotational symmetry of the graphene crystal. Extending HHG in graphene to the\nXUV spectral regime and realizing circular polarization represent an important\nstep towards the development of novel nanoscale attosecond photonic devices and\nnumerous applications such as spectroscopic investigation and nanoscale imaging\nof ultrafast chiral and spin dynamics in graphene and other 2D materials.",
        "category": "physics_optics"
    },
    {
        "text": "Unsupervised Full-color Cellular Image Reconstruction through Disordered\n  Optical Fiber: Recent years have witnessed the tremendous development of fusing fiber-optic\nimaging with supervised deep learning to enable high-quality imaging of\nhard-to-reach areas. Nevertheless, the supervised deep learning method imposes\nstrict constraints on fiber-optic imaging systems, where the input objects and\nthe fiber outputs have to be collected in pairs. To unleash the full potential\nof fiber-optic imaging, unsupervised image reconstruction is in demand.\nUnfortunately, neither optical fiber bundles nor multimode fibers can achieve a\npoint-to-point transmission of the object with a high sampling density, as is a\nprerequisite for unsupervised image reconstruction. The recently proposed\ndisordered fibers offer a new solution based on the transverse Anderson\nlocalization. Here, we demonstrate unsupervised full-color imaging with a\ncellular resolution through a meter-long disordered fiber in both transmission\nand reflection modes. The unsupervised image reconstruction consists of two\nstages. In the first stage, we perform a pixel-wise standardization on the\nfiber outputs using the statistics of the objects. In the second stage, we\nrecover the fine details of the reconstructions through a generative\nadversarial network. Unsupervised image reconstruction does not need paired\nimages, enabling a much more flexible calibration under various conditions. Our\nnew solution achieves full-color high-fidelity cell imaging within a working\ndistance of at least 4 mm by only collecting the fiber outputs after an initial\ncalibration. High imaging robustness is also demonstrated when the disordered\nfiber is bent with a central angle of 60{\\deg}. Moreover, the cross-domain\ngenerality on unseen objects is shown to be enhanced with a diversified object\nset.",
        "category": "physics_optics"
    },
    {
        "text": "Generalized Jones Calculus for Vortex, Vector, and Vortex-Vector Beam\n  Transformations: The work defines the general form of the Jones vector and establishes the\nJones matrix for polarizers, wave plates, Faraday rotators, Q-plates, and\nspiral phase plates. We establish the generalized Jones calculus for vortex,\nvector, and vortex-vector beam transformations. Systematic formalism presents\nthe theoretical arrangements in the manipulation of the phase and polarization\nof the structured lights. The eigenstates and the time-reversal behaviors of\noptical components and systems are discussed through matrix algebra. The\ngeneralized Jones calculus is then used in the characterization of Sagnac\ninterferometers, and the potential applications are proposed.",
        "category": "physics_optics"
    },
    {
        "text": "Spatially entangled Airy photons: Over the past decade, Airy beams have been the subject of extensive research,\nleading to new physical insights and various applications. In this letter, we\nextend the concept of Airy beams to the quantum domain. We generate entangled\nphotons in a superposition of two-photon Airy states via spontaneous parametric\ndown conversion, pumped by a classical Airy beam. We show that the entangled\nAiry photons preserve the intriguing properties of classical Airy beams, such\nas free acceleration and reduced diffraction, while exhibiting non-classical\nanti-correlations.",
        "category": "physics_optics"
    },
    {
        "text": "Controlling solar radiation forces with graphene in plasmonic\n  metasurface: Controlling and harvesting solar radiation pressure is a significant\nchallenge, however, there are few potential solutions, which are suitable for\nseveral key applications. In this study, an electrically tunable plasmonic\nmetasurface is designed for the visible spectrum. Moreover, the normal and the\ntangential optical forces acting on the metasurface are calculated. Whilst\npresenting high efficiency in the anomalous reflection, the designed active\nmetasurface provides tunability of optical forces acting on the metasurface.\nThe metasurface is composed of tapered silver cells embedded on top of the\ngraphene layer with 20 layers of graphene sheets. Hence, by tuning the Fermi\nlevel of graphene sheets, the transferred momentum to the metasurface can be\ncontrolled. Our results can provide a suitable platform for optical force\ncontrol desired in tunable radiation pressure harvesting, micro vehicles, solar\nsailing and optical tweezers.",
        "category": "physics_optics"
    },
    {
        "text": "Understanding the Coupling Mechanism of Gold Nanostructures by\n  Finite-Difference Time-Domain Method: Gold nanoparticle assemblies show a strong plasmonic response due to the\ncombined effects of the individual nanoparticles' plasmon modes. Increasing the\nnumber of nanoparticles in structured assemblies leads to significant shifts in\nthe optical and physical properties. We use Finite-Difference Time-Domain\n(FDTD) simulations to analyze the electromagnetic response of structurally\nordered gold nanorods in monomer and dimer configurations. The plasmonic\ncoupling between nanorods in monomers or dimers configurations provides a\nunique technique for tuning the spectrum intensity, spatial distribution, and\npolarisation of local electric fields within and surrounding nanostructures.\nOur study shows an exponential coupling behavior when two gold nanorods are\nassembled in end-to-end and side-by-side dimer configurations with a small\nseparation distance. The maximum electric field in the gaps between adjacent\nnanorods in end-to-end dimer configuration describes a more significant\nenhancement factor than the individual gold nanorod. Our FDTD simulation on\ndimer in end-to-end assembly for small separation distance up to ~ 40 nm can\nwell explain the observed experimental growth dynamics of gold nanorods.",
        "category": "physics_optics"
    },
    {
        "text": "Symmetry Classification of Topological Photonic Crystals: In a seminal paper Haldane conjectured that topological phenomena are not\nparticular to quantum systems, and indeed experiments realized unidirectional,\nbackscattering-free edge modes with electromagnetic waves. This raises two\nimmediate questions: (1) Are there other topological effects in electromagnetic\nmedia? And (2) is Haldane's Quantum Hall Effect for light really analogous to\nthe Quantum Hall Effect?\n  We conclusively answer both of these questions by classifying topological\nphotonic crystals according to material (as opposed to crystallographic)\nsymmetries. It turns out there are four topologically distinct types of media,\nof which only one, gyrotropic media, is topologically non-trivial in $d = 2 ,\n3$. That means there are no as-of-yet undiscovered topological effects; in\nparticular, there is no analog of the Quantum Spin Hall Effect in classical\nelectromagnetism. Moreover, at least qualitatively, Haldane's Quantum Hall\nEffect for light is analogous to the Quantum Hall Effect from condensed matter\nphysics as both systems as in the same topological class, class A. Our ideas\nare directly applicable to other classical waves.",
        "category": "physics_optics"
    },
    {
        "text": "A tutorial on the conservation of momentum in photonic time-varying\n  media: Time-varying media break temporal symmetries while preserving spatial\nsymmetries intact. Thus, it represents an excellent conceptual framework to\ninvestigate the fundamental implications of Noether's theorem for the\nelectromagnetic field. At the same time, addressing momentum conservation in\ntime-varying media sheds light on the Abraham-Minkowski debate, where two\nopposing forms of the electromagnetic field momentum are defended. Here, we\npresent a tutorial review on the conservation of momentum in time-varying\nmedia. We demonstrate that the Minkowski momentum is a conserved quantity with\nthree independent approaches of increasing complexity: (i) via the application\nof the boundary conditions for Maxwell equations at a temporal boundary, (ii)\ntesting for constants of motion and deriving conservation laws, and (iii)\napplying temporal and spatial translations within the framework of the\nLagrangian theory of the electromagnetic field. Each approach provides a\ndifferent and complementary insight into the problem.",
        "category": "physics_optics"
    },
    {
        "text": "Analytical vs. Numerical Langevin Description of Noise in Small Lasers: We compare the analytical and numerical predictions of noise in nano- and\nmicrocavity lasers obtained from a rate equation model with stochastic Langevin\nnoise. Strong discrepancies are found between the two approaches and these are\ncritically analyzed and explained on the basis of general considerations and\nthrough the comparison to the numerical predictions of a Stochastic Laser\nSimulator. While the analytical calculations give reliable redictions, the\nnumerical results are entirely incorrect thus unsuitable for predicting the\ndynamics and statistical properties of small lasers.",
        "category": "physics_optics"
    },
    {
        "text": "Drifting Electrons: Nonreciprocal Plasmonics and Thermal Photonics: Light propagates symmetrically in opposite directions in most materials and\nstructures. This fact -- a consequence of the Lorentz reciprocity principle --\nhas tremendous implications for science and technology across the\nelectromagnetic spectrum. Here, we investigate an emerging approach to break\nreciprocity that does not rely on magneto-optical effects or spacetime\nmodulations, but is instead based on biasing a plasmonic material with a direct\nelectric current. Using a 3D Green function formalism and microscopic\nconsiderations, we elucidate the propagation properties of surface\nplasmon-polaritons (SPPs) supported by a generic nonreciprocal platform of this\ntype, revealing some previously overlooked, anomalous, wave-propagation\neffects. We show that SPPs can propagate in the form of steerable, slow-light,\nunidirectional beams associated with inflexion points in the modal dispersion.\nWe also clarify the impact of dissipation (due to collisions and Landau\ndamping) on nonreciprocal effects and shed light on the connections between\ninflexion points, exceptional points at band-edges, and complex modal\ntransitions in leaky-wave structures. We then apply these concepts to the\nimportant area of thermal photonics, and provide the first theoretical\ndemonstration of drift-induced nonreciprocal near-field radiative heat transfer\nbetween two planar bodies. Our findings may open new opportunities toward the\ndevelopment of nonreciprocal magnet-free devices that combine the benefits of\nplasmonics and nonreciprocal photonics for wave-guiding and energy\napplications.",
        "category": "physics_optics"
    },
    {
        "text": "Adapted poling to break the nonlinear efficiency limit in nanophotonic\n  lithium niobate waveguides: Nonlinear frequency mixing is of critical importance in extending the\nwavelength range of optical sources. It is also indispensable for emerging\napplications such as quantum information and photonic signal processing.\nConventional lithium niobate with periodic poling is the most widely used\ndevice for frequency mixing due to the strong second-order nonlinearity. The\nrecent development of nanophotonic lithium niobate waveguides promises\nimprovements of nonlinear efficiencies by orders of magnitude with\nsub-wavelength optical conferment. However, the intrinsic nanoscale\ninhomogeneity in nanophotonic lithium niobate limits the coherent interaction\nlength, leading to low nonlinear efficiencies. Therefore, the performance of\nnanophotonic lithium niobate waveguides is still far behind conventional\ncounterparts. Here, we overcome this limitation and demonstrate ultra-efficient\nsecond order nonlinearity in nanophotonic lithium niobate waveguides\nsignificantly outperforming conventional crystals. This is realized by\ndeveloping the adapted poling approach to eliminate the impact of nanoscale\ninhomogeneity in nanophotonic lithium niobate waveguides. We realize overall\nsecondharmonic efficiency near 10^4 %/W without cavity enhancement, which\nsaturates the theoretical limit. Phase-matching bandwidths and temperature\ntunability are improved through dispersion engineering. The ideal square\ndependence of the nonlinear efficiency on the waveguide length is recovered. We\nalso break the trade-off between the energy conversion ratio and pump power. A\nconversion ratio over 80% is achieved in the single-pass configuration with\npump power as low as 20 mW.",
        "category": "physics_optics"
    },
    {
        "text": "Strain Influence on Optical Absorption of Giant Semiconductor Colloidal\n  Quantum Dots: The lattice mismatch strain field of core/multishell structures with\nspherical symmetry is modeled by a linear continuum elasticity approach. The\neffect of the strain on the energy structure and linear optical absorption in\nlarge core/shell/shell spherical semiconductor quantum dots is analyzed.\nLocalization of the photoexcited carriers induced by coating is found to play\nan important role in explaining the optical stability of large CdSe/CdS/ZnS and\nZnTe/ZnSe/ZnS quantum dots.",
        "category": "physics_optics"
    },
    {
        "text": "Deep imaging inside scattering media through virtual spatiotemporal\n  wavefront shaping: The multiple scattering of light makes materials opaque and obstructs\nimaging. Wavefront shaping can reverse the scattering process, but imaging with\nphysical wavefront shaping has severe deficiencies such as requiring physical\nguidestars, limited within a small isoplanatic patch, restricted to planar\ntargets outside the scattering media, and slow wavefront updates due to the\nhardware. Here, we introduce scattering matrix tomography (SMT): measure the\nhyperspectral scattering matrix of the sample, use it to digitally scan a\nsynthesized confocal spatiotemporal focus and construct a volumetric image of\nthe sample, and then use the synthesized image as many virtual guidestars to\ndigitally optimize the pulse shape, input wavefront, and output wavefront to\ncompensate for aberrations and scattering. The virtual feedback dispenses with\nphysical guidestars and enables hardware-free spatiotemporal wavefront\ncorrections across arbitrarily many isoplanatic patches. We demonstrate SMT\nwith sub-micron diffraction-limited lateral resolution and one-micron\nbandwidth-limited axial resolution at one millimeter beneath ex vivo mouse\nbrain tissue and inside a dense colloid, where all existing imaging methods\nfail due to the overwhelming multiple scattering. SMT translates imaging and\nwavefront shaping into a computational problem. It is noninvasive and\nlabel-free, provides multi-isoplanatic volumetric images inside and outside the\nscattering media, and can be applied to medical imaging, device inspection,\nbiological science, and colloidal physics.",
        "category": "physics_optics"
    },
    {
        "text": "Ghost optical coherence tomography: We demonstrate experimentally ghost optical coherence tomography using a\nbroadband incoherent supercontinuum light source with shot-to-shot random\nspectral fluctuations. The technique is based on ghost imaging in the spectral\ndomain where the object is the spectral interference pattern generated from an\noptical coherence tomography interferometer in which a physical sample is\nplaced. The image of the sample is obtained from the Fourier transform of the\ncorrelation between the spectrally-resolved intensity fluctuations of the\nsupercontinuum and the integrated signal measured at the output of the\ninterferometer. The results are in excellent agreement with measurements\nobtained from a conventional optical coherence tomography system.",
        "category": "physics_optics"
    },
    {
        "text": "Mechanically tunable integrated beamsplitters on a flexible polymer\n  platform: We report the development of a monolithic, mechanically tunable waveguide\nplatform based on the flexible polymer polydimethyl siloxane (PDMS). Such\ndevices preserve single mode guiding across a wide range of linear geometric\ndistortions. This enables the realization of directional couplers with tunable\nsplitting ratios via elastic deformation of the host chip. We fabricated\nseveral devices of this type, and verified their operation over a range of\nwavelengths, with access to the full range of input/output ratios. The low cost\nand relative ease of fabrication of these devices via a modified imprint\nlithographic technique make them an attractive platform for investigation of\nlarge scale optical random walks and related optical phenomena.",
        "category": "physics_optics"
    },
    {
        "text": "Probing rotated Weyl physics on nonlinear lithium niobate-on-insulator\n  chips: Topological photonics, featured by stable topological edge states resistant\nto perturbations, has been utilized to design robust integrated devices. Here,\nwe present a study exploring the intriguing topological rotated Weyl physics in\na 3D parameter space based on quaternary waveguide arrays on lithium\nniobate-on-insulator (LNOI) chips. Unlike previous works that focus on the\nFermi arc surface states of a single Weyl structure, we can experimentally\nconstruct arbitrary interfaces between two Weyl structures whose orientations\ncan be freely rotated in the synthetic parameter space. This intriguing system\nwas difficult to realize in usual 3D Weyl semimetals due to lattice mismatch.\nWe found whether the interface can host gapless topological interface states\n(TISs) or not, is determined by the relative rotational directions of the two\nWeyl structures. In the experiment, we have probed the local characteristics of\nthe TISs through linear optical transmission and nonlinear second harmonic\ngeneration. Our study introduces a novel path to explore topological photonics\non LNOI chips and various applications in integrated nonlinear and quantum\noptics.",
        "category": "physics_optics"
    },
    {
        "text": "Gain without population inversion in V-type systems driven by a\n  frequency-modulated field: We obtain gain of the probe field at multiple frequencies in a closed\nthree-level V-type system using frequency modulated pump field. There is no\nassociated population inversion among the atomic states of the probe\ntransition. We describe both the steady-state and transient dynamics of this\nsystem. Under suitable conditions, the system exhibits large gain\nsimultaneously at series of frequencies far removed from resonance. Moreover,\nthe system can be tailored to exhibit multiple frequency regimes where the\nprobe experiences anomalous dispersion accompanied by negligible\ngain-absorption over a large bandwidth, a desirable feature for obtaining\nsuperluminal propagation of pulses with negligible distortion.",
        "category": "physics_optics"
    },
    {
        "text": "Relevance of Longitudinal Fields of Paraxial Optical Vortices: Longitudinal electromagnetic fields generally become comparable with the\nusually dominant transverse components in strongly-focussed, non-paraxial\nbeams. For optical vortex modes it is highlighted here how their angular\nmomentum properties produce longitudinal fields that in general must be\naccounted for, even within the paraxial regime. First-order longitudinal\ncomponents of quantized Laguerre-Gaussian modes are derived and numerically\nstudied with respect to the paraxial parameter, highlighting light-matter and\nspin-orbit interactions that stem from longitudinal fields of weakly-focussed,\nparaxial beams in free space. New restrictions are cast on the validity of the\nparaxial approximation for optical vortices interacting with atoms, molecules\nand other nanostructures.",
        "category": "physics_optics"
    },
    {
        "text": "Manifestation of the strong quadrupole light-molecule interaction in the\n  SEHR spectra of phenazine and pyrazine: The paper demonstrates possibility of giant enhancement of Surface Enhanced\nHyper Raman Scattering on the base of qualitative consideration of\nelectromagnetic field near some models of rough metal surfaces and of some\nfeatures of the dipole and quadrupole light-molecule interaction, such as it\nwas made in the dipole-quadrupole SERS theory. Consideration of symmetrical\nmolecules allows to obtain selection rules for their SEHR spectra and establish\nsuch regularity as appearance of the lines, caused by totally symmetric\nvibrations, transforming after the unit irreducible representation in molecules\nwith the groups, where the elements of symmetry change z on -z . These lines\nare forbidden in usual HRS spectra. Analysis of literature data on phenazine\nand pyrazine molecules demonstrates that their SEHR spectra can be explained by\nthe SEHRS dipole-quadrupole theory. Analysis of the SEHR spectra of these\nmolecules reveals appearance of strong forbidden bands, caused by vibrations\ntransforming after the unit irreducible representation that strongly confirms\nthis theory which allows to interpret the whole SEHR spectra in detail. The\nresults corroborate this common mechanism of Surface Enhanced optical processes\non molecules adsorbed on rough metal surfaces.",
        "category": "physics_optics"
    },
    {
        "text": "Radiation Properties of One-Dimensional Quasirandom Antenna Arrays Based\n  on Rudin-Shapiro Sequences: The development of exotic new materials, such as metamaterials, has created\nstrong interest within the electromagnetics (EM) community for possible new\nphenomenologies and device applications, with particular attention to\nperiodicity-induced phenomena, such as photonic bandgaps (PBG). Within this\ncontext, motivated by the fairly recent discovery in X-ray crystallography of\n\"quasicrystals,\" whose diffraction patterns display unusual characteristics\nthat are associated with \"aperiodic order,\" we have undertaken a systematic\nstudy of how these exotic effects manifest themselves in the radiation\nproperties of aperiodically-configured antenna arrays. The background for these\nstudies, with promising example configurations, has been reported in a previous\npublication [V. Pierro et al., IEEE Trans. Antennas Propagat., vol. 53, No. 2,\npp. 635-644, Feb. 2005]. In the present paper, we pay attention to various\nconfigurations generated by Rudin-Shapiro (RS) sequences, which constitute one\nof the simplest conceivable examples of deterministic aperiodic geometries\nfeaturing quasirandom (dis)order. After presentation and review of relevant\nbackground material, the radiation properties of one-dimensional RS-based\nantenna arrays are analyzed, followed by illustrative numerical parametric\nstudies to validate the theoretical models. Design parameters and potential\npractical applications are also given attention.",
        "category": "physics_optics"
    },
    {
        "text": "Probing a dissipative phase transition via dynamical optical hysteresis: We experimentally explore the dynamic optical hysteresis of a semiconductor\nmicrocavity as a function of the sweep time. The hysteresis area exhibits a\ndouble power law decay due to the shot noise of the driving laser, which\ntriggers switching between metastable states. Upon increasing the average\nphoton number and approaching the thermodynamic limit, the double power law\nevolves into a single power law. This algebraic behavior characterizes a\ndissipative phase transition. Our findings are in good agreement with\ntheoretical predictions, and the present experimental approach is promising for\nthe exploration of critical phenomena in photonic lattices.",
        "category": "physics_optics"
    },
    {
        "text": "Large-area quantum-spin-Hall waveguide states in a three-layer\n  topological photonic crystal heterostructure: Topological photonic edge states are conventionally formed at the interface\nbetween two domains of topologically trivial and nontrivial photonic crystals.\nRecent works exploiting photonic quantum Hall and quantum valley Hall effects\nhave shown that large-area topological waveguide states could be created in a\nthree-layer topological heterostructure that consists of a finite-width domain\nfeaturing Dirac cone sandwiched between two domains of photonic crystals with\nopposite topological properties. In this work, we show that a new kind of\nlarge-area topological waveguide states could be created employing the photonic\nanalogs of quantum spin Hall effect. Taking the well-used Wu-Hu model in\ntopological photonics as an example, we show that sandwiching a finite-width\ndomain of photonic crystals featuring double Dirac cone between two domains of\nexpanded and shrunken unit cells could lead to the emergence of large-area\ntopological helical waveguide states distributed uniformly in the middle\ndomain. Importantly, we unveil a power-law scaling regarding to the size of the\nbandgap within which the large-area helical states reside as a function of the\nwidth of the middle domain, which implies that these large-area modes in\nprinciple could exist in the middle domain with arbitrary width. Moreover,\npseudospin-momentum locking unidirectional propagations and robustness of these\nlarge-area waveguide modes against sharp bends are explicitly demonstrated. Our\nwork enlarges the photonic systems and platforms that could be utilized for\nlarge-area-mode enabled topologically waveguiding.",
        "category": "physics_optics"
    },
    {
        "text": "Room temperature mid-IR single photon spectral imaging: Spectral imaging and detection of mid-infrared (mid-IR) wavelengths are\nemerging as an enabling technology of great technical and scientific interest;\nprimarily because important chemical compounds display unique and strong mid-IR\nspectral fingerprints revealing valuable chemical information. While modern\nQuantum cascade lasers have evolved as ideal coherent mid-IR excitation\nsources, simple, low noise, room temperature detectors and imaging systems\nstill lag behind. We address this need presenting a novel, field-deployable,\nupconversion system for sensitive, 2-D, mid-IR spectral imaging. Measured room\ntemperature dark noise is 0.2 photons/spatial element/second, which is a\nbillion times below the dark noise level of cryogenically cooled InSb cameras.\nSingle photon imaging and up to 200 x 100 spatial elements resolution is\nobtained reaching record high continuous wave quantum efficiency of about 20 %\nfor polarized incoherent light at 3 \\mum. The proposed method is relevant for\nexisting and new mid-IR applications like gas analysis and medical diagnostics.",
        "category": "physics_optics"
    },
    {
        "text": "Universal method for the synthesis of arbitrary polarization states\n  radiated by a nanoantenna: Optical nanoantennas efficiently convert confined optical energy into\nfree-space radiation. The polarization of the emitted radiation depends mainly\non nanoantenna shape, so it becomes extremely difficult to manipulate it unless\nthe nanostructure is physically altered. Here we demonstrate a simple way to\nsynthetize the polarization of the radiation emitted by a single nanoantenna so\nthat every point on the Poincar\\'e sphere becomes attainable. The nanoantenna\nconsists of a single scatterer created on a dielectric waveguide and fed from\nits both sides so that the polarization of the emitted optical radiation is\ncontrolled by the amplitude and phase of the feeding signals. Our nanoantenna\nis created on a silicon chip using standard top-down nanofabrication tools, but\nthe method is universal and can be applied to other materials, wavelengths and\ntechnologies. This work will open the way towards the synthesis and control of\narbitrary polarization states in nano-optics.",
        "category": "physics_optics"
    },
    {
        "text": "A scanning probe-based pick-and-place procedure for assembly of\n  integrated quantum optical hybrid devices: Integrated quantum optical hybrid devices consist of fundamental constituents\nsuch as single emitters and tailored photonic nanostructures. A reliable\nfabrication method requires the controlled deposition of active nanoparticles\non arbitrary nanostructures with highest precision. Here, we describe an easily\nadaptable technique that employs picking and placing of nanoparticles with an\natomic force microscope combined with a confocal setup. In this way, both the\ntopography and the optical response can be monitored simultaneously before and\nafter the assembly. The technique can be applied to arbitrary particles. Here,\nwe focus on nanodiamonds containing single nitrogen vacancy centers, which are\nparticularly interesting for quantum optical experiments on the single photon\nand single emitter level.",
        "category": "physics_optics"
    },
    {
        "text": "Mid-infrared frequency comb generation via cascaded quadratic\n  nonlinearities in quasi-phase-matched waveguides: We experimentally demonstrate a simple configuration for mid-infrared (MIR)\nfrequency comb generation in quasi-phase-matched lithium niobate waveguides\nusing the cascaded-$\\chi^{(2)}$ nonlinearity. With nanojoule-scale pulses from\nan Er:fiber laser, we observe octave-spanning supercontinuum in the\nnear-infrared with dispersive-wave generation in the 2.5--3 $\\text{\\mu}$m\nregion and intra-pulse difference-frequency generation in the 4--5\n$\\text{\\mu}$m region. By engineering the quasi-phase-matched grating profiles,\ntunable, narrow-band MIR and broadband MIR spectra are both observed in this\ngeometry. Finally, we perform numerical modeling using a nonlinear envelope\nequation, which shows good quantitative agreement with the experiment---and can\nbe used to inform waveguide designs to tailor the MIR frequency combs. Our\nresults identify a path to a simple single-branch approach to mid-infrared\nfrequency comb generation in a compact platform using commercial Er:fiber\ntechnology.",
        "category": "physics_optics"
    },
    {
        "text": "Unidirectional guided-wave-driven metasurfaces for arbitrary wavefront\n  control: Metasurfaces, composed of subwavelength electromagnetic microstructures,\nknown as meta-atoms, are capable of reshaping the wavefronts of incident beams\nin desired manners, making them great candidates for revolutionizing\nconventional optics. However, the requirement for external light excitation and\nthe resonant nature of meta-atoms make it difficult to fully integrate\nmetasurfaces on-chip or to control wavefronts at deep-subwavelength scales.\nHere, we introduce the concept and design of a new class of metasurfaces,\ndriven by unidirectional guided waves, and being capable of arbitrary wavefront\ncontrol based on the unique dispersion properties of unidirectional guided\nwaves rather than resonant meta-atoms. Upon experimentally demonstrating the\nfeasibility and practicality of the unidirectional nature of our designs in the\nmicrowave regime, we numerically validate this new principle through the design\nof several microwave meta-devices using metal-air-gyromagnetic unidirectional\nsurface magnetoplamons, agilely converting unidirectional guided modes into the\nwavefronts of 3D Bessel beams, focused waves, and controllable vortex beams. We\nalso numerically demonstrate sub-diffraction focusing, which is currently\nbeyond the capability of conventional metasurfaces. Furthermore, we directly\nshow how these concepts can be transferred to the terahertz regime, and discuss\ntheir feasibility in the optical domain, too. Based on this nonresonant (that\nis, broadband) mechanism and on standard plasmonic platforms, our metasurfaces\ncan be integrated on-chip, enabling the manipulation of electromagnetic waves\non deep subwavelength scales and over wide frequency ranges, thereby opening up\nnew opportunities for applications in communications, remote sensing, displays,\nand so forth.",
        "category": "physics_optics"
    },
    {
        "text": "Arbitrary optical wavefront shaping via spin-to-orbit coupling: Converting spin angular momentum to orbital angular momentum has been shown\nto be a practical and efficient method for generating optical beams carrying\norbital angular momentum and possessing a space-varying polarized field. Here,\nwe present novel liquid crystal devices for tailoring the wavefront of optical\nbeams through the Pancharatnam-Berry phase concept. We demonstrate the\nversatility of these devices by generating an extensive range of optical beams\nsuch as beams carrying $\\pm200$ units of orbital angular momentum along with\nBessel, Airy and Ince-Gauss beams. We characterize both the phase and the\npolarization properties of the generated beams, confirming our devices'\nperformance.",
        "category": "physics_optics"
    },
    {
        "text": "Anisotropy of x-ray scattering in aligned nanotube structures: Effects of orientational x-ray scattering have been experimentally examined\nin a film of vertically aligned multiwall carbon nanotubes (CNs). Additional\ncontribution to the x-ray fluorescence intensity was revealed at angles close\nto the film normal. Theoretical considerations suggest the intensity\nenhancement to be caused by propagation of C K$_{\\alpha}$-radiation mainly\nalong the channels of CNs.",
        "category": "physics_optics"
    },
    {
        "text": "Design of Highly Efficient Hybrid Si-Au Taper for Dielectric Strip\n  Waveguide to Plasmonic Slot Waveguide Mode Converter: In this paper, we design a dielectric-to-plasmonic slot waveguide mode\nconverter based on the hybrid silicon-gold taper. The effects of mode matching,\nthe effective index matching, and the metallic absorption loss on the\nconversion efficiency are studied. Consequently, a metallic taper-funnel\ncoupler with an overall length of 1.7um is designed to achieve a very high\nconversion efficiency of 93.3% at 1550 nm. The configuration limitations for\nnot allowing this mode converter to achieve a 100% conversion efficiency are\nalso investigated. Such a high-efficiency converter can provide practical\nroutes to realize ultracompact integrated circuits.",
        "category": "physics_optics"
    },
    {
        "text": "Simultaneous vibrational resonance in the amplitude and phase\n  quadratures of an optical field based on Kerr nonlinearity: Vibrational resonance (VR) is a nonlinear phenomenon in which the system\nresponse to a weak signal can be resonantly enhanced by applying a\nhigh-frequency modulation signal with an appropriate amplitude. The majority of\nVR research has focused on amplifying the amplitude or intensity of the system\nresponse to a weak signal, whereas the study of the phase information of system\nresponses in VR remains limited. Here, we investigate the VR phenomena in both\namplitude and phase quadratures of an optical field in a Kerr nonlinear cavity\ndriven by a near-resonant weak signal and a far-detuned modulation signal.\nAnalytical and numerical results demonstrated that the resonant enhancement in\nthe amplitude and phase quadratures of the system response to a weak signal\nsimultaneously occurs as the amplitude of the modulation signal is varied.\nThere is a linear relation between the amplitude and frequency of the\nmodulation signal for achieving an optimal VR effect. Furthermore, we\ngeneralized our study to investigate the quadrature at an arbitrary phase and\ndetermined that the VR enhancement sensitively depends on the phase. Our\nfindings not only broaden the scope of VR research by incorporating phase\ninformation but also introduces an approach for amplifying an optical field by\nmanipulating another optical field.",
        "category": "physics_optics"
    },
    {
        "text": "Slow Light Frequency Reference Cavities -- Proof of Concept for Reducing\n  the Frequency Sensitivity Due to Length Fluctuations: Length changes due to thermo-mechanical noise originating from, for example,\nBrownian motion are a key limiting factor of present day state-of-the-art laser\nfrequency stabilization using Fabry-P\\'erot cavities. We present a\nlaser-frequency stabilization concept using an optical cavity with a strong\nslow-light effect to reduce the impact of cavity length changes on the\nfrequency stability. The resulting noise-reduction factor is proportional to\nthe ratio between the light phase and group velocities in the highly dispersive\ncavity spacer. We experimentally demonstrate a proof-of-principle\nimplementation of this laser-frequency stabilization technique using a\nrare-earth doped crystalline cavity spacer in conjunction with semi-permanent\nspectral tailoring to achieve precise control of the dispersive properties of\nthe cavity. Compared to the same setup in the absence of the slow-light effect\na reduction in frequency sensitivity of four orders of magnitude was achieved.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of long turn-on delay in pulsed quantum cascade lasers: We present an experimental study of the turn-on delay in pulsed mid-infrared\nquantum cascade lasers. We report the unexpectedly long delay time depending on\nthe pumping current, which does not agree with conventional theoretical\npredictions for step-like excitation. Similar behavior has been observed in\nInP- and InAs-based QCLs emitting near 8${\\mu}$m. Numerical simulations\nperformed using a model based on rate equations for excitation by current\npulses with non-zero rise time provide fair agreement with our observations.",
        "category": "physics_optics"
    },
    {
        "text": "Post-compression of picosecond pulses into the few-cycle regime: In this work, we demonstrate post-compression of 1.2 picosecond laser pulses\nto 13 fs via gas-based multi-pass spectral broadening. Our results yield a\nsingle-stage compression factor of about 40 at 200 W in-burst average power and\na total compression factor >90 at reduced power. The employed scheme represents\na route towards compact few-cycle sources driven by industrial-grade Yb:YAG\nlasers at high average power.",
        "category": "physics_optics"
    },
    {
        "text": "Transport of Entanglement: We consider the propagation of two-photon light in a random medium. We show\nthat the Wigner distribution of the two-photon wave function obeys an equation\nthat is analogous to the radiative transport equation for classical light.\nUsing this result, we predict that the entanglement of a photon pair is\ndestroyed with propagation.",
        "category": "physics_optics"
    },
    {
        "text": "Recoil momentum of an atom absorbing light in a gaseous medium and the\n  Abraham-Minkowski debate: We discuss a fundamental question regarding the Abraham-Minkowski debate\nabout the momentum of light in a medium: If an atom in a gas absorbs a photon,\nwhat is the momentum transferred to it? We consider a classical model for the\ninternal degrees of freedom of the absorbing atom, computing the absorbed\nenergy and momentum using the Lorentz force law due to the microscopic\nelectromagnetic fields. Each non-absorbing atom from the gas is treated as a\ndielectric sphere, with the set of atoms forming a linear, dielectric,\nnon-magnetic, and non-absorbing medium with a refractive index $n$ close to\none. Our numerical results indicate that if the atoms are classically\nlocalized, the average absorbed momentum increases with $n$, but is smaller\nthan Minkowski's momentum $np_0$, $p_0$ being the photon momentum in vacuum.\nHowever, experiments performed with Bose-Einstein condensates [Phys. Rev. Lett.\n$\\mathbf{94}$, 170403 (2005)] are consistent with the atom absorbing\nMinkowski's momentum. We argue that there is no contradiction between these\nresults since, in a Bose-Einstein condensate, the atoms are in a quantum state\nspatially superposed in a relatively large volume, forming a ``continuous''\nmedium. In this sense, the experimental verification of an atomic momentum\nrecoil compatible with Minkowski's momentum would be a quantum signature of the\nmedium state.",
        "category": "physics_optics"
    },
    {
        "text": "Optical response of strongly coupled metal nanoparticles in dimer arrays: The optical responses of structured array of noble-metal nanoparticle dimers\nimmersed in a glass matrix are investigated theoretically, motivated by the\nrecent experimental observation of the splitting of the surface plasmon bands\nin silver arrays. To capture the strong electromagnetic coupling between the\ntwo approaching particles in a silver dimer, the spectral representation of the\nmultiple image formula has been used, and a semiclassical description of the\nsilver dielectric function is adopted from the literature. The splitting of\nplasmon resonance band of the incident longitudinal and transverse polarized\nlight is found to be strongly dependent on the particle diameter and their\nseparation. Our results are shown in accord with the recent experimental\nobservation. Moreover, a large redshift for the longitudinal polarization can\nbe reproduced. The reflectivity spectrum is further calculated for a dilute\nsuspension of dimer arrays.",
        "category": "physics_optics"
    },
    {
        "text": "Coherently controlling Raman-induced grating in atomic media: We consider dynamically controllable periodic structures, called Raman\ninduced gratings, in three- and four-level atomic media, resulting from Raman\ninteraction in a standing-wave pump. These gratings are due to periodic spatial\nmodulation of the Raman nonlinearity and fundamentally differ from the ones\nbased on electromagnetically induced transparency. The transmission and\nreflection spectra of such gratings can be simultaneously amplified and\ncontrolled by varying the pump field intensity. It is shown that a transparent\nmedium with periodic spatial modulation of the Raman gain can be opaque near\nthe Raman resonance and yet at the same time it can be a non-linear amplifying\nmirror. We also show that spectral properties of the Raman induced grating can\nbe controlled with the help of an additional weak control field.",
        "category": "physics_optics"
    },
    {
        "text": "Negative and positive refraction are not Lorentz covariant: The refraction of linearly polarized plane waves into a half-space occupied\nby a material moving at constant velocity was studied by directly implementing\nthe Lorentz transformations of electric and magnetic fields. From the\nperspective of a co-moving observer, the moving material was a spatially local,\npseudochiral omega material. Numerical studies revealed that whether or not\nnegative refraction occurrs in the moving material depends upon the speed of\nmovement as well as the angle of incidence and the polarization state of the\nincident plane wave. Furthermore, the phenomenons of negative phase velocity\nand counterposition in the moving material were similarly found not to be\nLorentz covariant; both phenomenons were also found to be sensitive to the\nangle of incidence and the polarization state of the incident plane wave.",
        "category": "physics_optics"
    },
    {
        "text": "Sub-Rayleigh characterization of a binary source by spatially\n  demultiplexed coherent detection: We investigate theoretically coherent detection implemented simultaneously on\na set of mutually orthogonal spatial modes in the image plane as a method to\ncharacterize properties of a composite thermal source below the Rayleigh limit.\nA general relation between the intensity distribution in the source plane and\nthe covariance matrix for the complex field amplitudes measured in the image\nplane is derived. An algorithm to estimate parameters of a two-dimensional\nsymmetric binary source is devised and verified using Monte Carlo simulations\nto provide super-resolving capability for high ratio of signal to detection\nnoise (SNR). Specifically, the separation between two point sources can be\nmeaningfully determined down to $\\textrm{SNR}^{-1/2}$ in the units determined\nby the spatial spread of the transfer function of the imaging system. The\npresented algorithm is shown to make a nearly optimal use of the measured data\nin the sub-Rayleigh region.",
        "category": "physics_optics"
    },
    {
        "text": "Semiconductor-based superlens for sub-wavelength resolution below the\n  dif-fraction limit at extreme ultraviolet frequencies: We theoretically demonstrate negative refraction and sub-wavelength\nresolution below the diffraction limit in the UV and extreme UV ranges using\nsemiconductors. The metal-like re-sponse of typical semiconductors such as GaAs\nor GaP makes it possible to achieve negative refraction and super-guiding in\nresonant semiconductor/dielectric multilayer stacks, similar to what has been\ndemonstrated in metallo-dielectric photonic band gap structures. The\nexploita-tion of this basic property in semiconductors raises the possibility\nof new, yet-untapped ap-plications in the UV and soft x-ray ranges.",
        "category": "physics_optics"
    },
    {
        "text": "Terahertz control in a transmission electron microscope: Ultrafast electron microscopy provides a movie-like access to structural\ndynamics of materials in space and time, but fundamental atomic motions or\nelectron dynamics are, so far, too quick to be resolved. Here we report the\nall-optical control, compression and characterization of electron pulses in a\ntransmission electron microscope by the single optical cycles of\nlaser-generated terahertz light. This concept provides isolated electron pulses\nand merges the spatial resolution of a transmission electron microscope with\nthe temporal resolution that is offered by a single cycle of laser light.\nCentral to these achievements is a perforated parallel-plate metallic waveguide\nin which transverse velocity mismatch and magnetic forces are mitigated by\nelectrically constructive and magnetically destructive interferences of\nincoming and reflected terahertz half-cycles from a displaced waveguide\ntermination. Measurements of spatial chirp via energy-filtered imaging reveal\nflat pulses with no transversal deflection or temporal aberrations at the\nspecimen. We also report the all-optical control of multi-electron states and\ndiscover a substantial two-electron and three-electron anti-correlation in the\ntime domain. These results open up the possibility to visualize atomic and\nelectronic motions together with their quantum correlations on fundamental\ndimensions in space and time.",
        "category": "physics_optics"
    },
    {
        "text": "Near-field interactions and non-universality in speckle patterns\n  produced by a point source in a disordered medium: A point source in a disordered scattering medium generates a speckle pattern\nwith non-universal features, giving rise to the so-called C_0 correlation. We\nanalyze theoretically the relationship between the C_0 correlation and the\nstatistical fluctuations of the local density of states, based on simple\narguments of energy conservation. This derivation leads to a clear physical\ninterpretation of the C_0 correlation. Using exact numerical simulations, we\nshow that C_0 is essentially a correlation resulting from near-field\ninteractions. These interactions are responsible for the non-universality of\nC_0, that confers to this correlation a huge potential for sensing and imaging\nat the subwavelength scale in complex media.",
        "category": "physics_optics"
    },
    {
        "text": "Electrically pumped quantum-dot lasers grown on 300 mm patterned Si\n  photonic wafers: Monolithic integration of quantum dot (QD) gain materials onto Si photonic\nplatforms via direct epitaxial growth is a promising solution for on-chip light\nsources. Recent developments have demonstrated superior device reliability in\nblanket hetero-epitaxy of III-V devices on Si at elevated temperatures. Yet,\nthick, defect management epi designs prevent vertical light coupling from the\ngain region to the Si-on-Insulator (SOI) waveguides. Here, we demonstrate the\nfirst electrically pumped QD lasers grown on a 300 mm patterned (001) Si wafer\nwith a butt-coupled configuration by molecular beam epitaxy (MBE). Unique\ngrowth and fabrication challenges imposed by the template architecture have\nbeen resolved, contributing to continuous wave lasing to 60 {\\deg}C and a\nmaximum double-side output power of 126.6 mW at 20 {\\deg}C with a double-side\nwall plug efficiency of 8.6%. The potential for robust on-chip laser operation\nand efficient low-loss light coupling to Si photonic circuits makes this\nheteroepitaxial integration platform on Si promising for scalable and low-cost\nmass production.",
        "category": "physics_optics"
    },
    {
        "text": "Properties of an adjustable quarter-wave system under condition of\n  multiple beam interference: The polarimetric properties of an adjustable two plate quarter-wave system\nhave been investigated. Multiple beam interference within single wave-plates\nhas been taken into account. It has been shown that different adjustments are\nneeded to produce left-handed and right-handed circular polarized coherent\nlight. Laser light polarization conversion by the systems consisting of two\nbirefringent mica plates has been investigated experimentally. The high-quality\ncircularly polarized light with the intensity-related ellipticity higher than\n0.99 has been produced.",
        "category": "physics_optics"
    },
    {
        "text": "Generation of third-harmonic spin oscillation from strong spin\n  precession induced by terahertz magnetic near fields: The ability to drive a spin system to state far from the equilibrium is\nindispensable for investigating spin structures of antiferromagnets and their\nfunctional nonlinearities for spintronics. While optical methods have been\nconsidered for spin excitation, terahertz (THz) pulses appear to be a more\nconvenient means of direct spin excitation without requiring coupling between\nspins and orbitals or phonons. However, room-temperature responses are usually\nlimited to small deviations from the equilibrium state because of the\nrelatively weak THz magnetic fields in common approaches. Here, we studied the\nmagnetization dynamics in a HoFeO3 crystal at room temperature. A custom-made\nspiral-shaped microstructure was used to locally generate a strong multicycle\nTHz magnetic near field perpendicular to the crystal surface; the maximum\nmagnetic field amplitude of about 2 T was achieved. The observed time-resolved\nchange in the Faraday ellipticity clearly showed second- and third-order\nharmonics of the magnetization oscillation and an asymmetric oscillation\nbehaviour. Not only the ferromagnetic vector M but also the antiferromagnetic\nvector L plays an important role in the nonlinear dynamics of spin systems far\nfrom equilibrium.",
        "category": "physics_optics"
    },
    {
        "text": "A novel pulsed fiber laser: Further study on the bias-pumped\n  gain-switched fiber laser: The bias-pumped gain-switched fiber laser proposed by us is considered a\nnovel pulsed fiber laser based on a new pulsing mechanism. With a certain\nsignal power seeding, synchronization of temporal evolution can be kept between\nthe output signal laser and the pump. The seed laser can be supplied\nconveniently by a CW pump power which is named bias pump power. A pulsed pump\nis responsible for shaping the output pulse. Stable pulsed lasers with tunable\ndurations can be achieved under bias pump combined with pulsed pump. In\naddition, the temporal shape of output pulses can be controllable based on this\nnew pulsing mechanism. Compared with conventional gain-switched fiber laser, a\nmuch simpler pulsed laser design can be provided by this novel pulsed fiber\nlaser because it is no need to add a control unit to realize fast\ngain-switching.",
        "category": "physics_optics"
    },
    {
        "text": "Multi-order interference is generally nonzero: It is demonstrated that the third-order interference, as obtained from\nexplicit solutions of Maxwell's equations for realistic models of three-slit\ndevices, including an idealized version of the three-slit device used in a\nrecent three-slit experiment with light (U. Sinha et al., Science 329, 418\n(2010)), is generally nonzero. The hypothesis that the third-order interference\nshould be zero is shown to be fatally flawed because it requires dropping the\none-to-one correspondence between the symbols in the mathematical theory and\nthe different experimental configurations.",
        "category": "physics_optics"
    },
    {
        "text": "An ultra-broadband electromagnetically indefinite medium formed by\n  aligned carbon nanotubes: Anisotropic materials with different signs of components of the permittivity\ntensor are called indefinite materials. Known realizations of indefinite media\nsuffer of high absorption losses. We show that periodic arrays of parallel\ncarbon nanotubes (CNTs) can behave as a low-loss indefinite medium in the\ninfrared range. We show that a finite-thickness slab of CNTs supports the\npropagation of backward waves with small attenuation in an ultra-broad\nfrequency band. In prospective, CNT arrays can be used for subwavelength\nfocusing and detection, enhancing the radiation efficiency of small sources.",
        "category": "physics_optics"
    },
    {
        "text": "Optically-Nonactive Assorted Helices Array with Interchangeable\n  Magnetic/Electric Resonance: We report here the designing of optically-nonactive metamaterial by\nassembling metallic helices with different chirality. With linearly polarized\nincident light, pure electric or magnetic resonance can be selectively\nrealized, which leads to negative permittivity or negative permeability\naccordingly. Further, we show that pure electric or magnetic resonance can be\ninterchanged at the same frequency band by merely changing the polarization of\nincident light for 90 degrees. This design demonstrates a unique approach to\nconstruct metamaterial.",
        "category": "physics_optics"
    },
    {
        "text": "Simultaneous generation of high power, ultrafast 1D and 2D Airy beams\n  and their frequency doubling characteristics: We report on a simple experimental scheme based on a pair of cylindrical\nlenses (convex and concave) of same focal length and common optical elements\nproducing high power optical beams in 1D and/or 2D Airy intensity profiles with\nlaser polarization as control parameter. Using an ultrafast Yb-fiber laser at\n1064 nm of average power of 5 W in Gaussian spatial profile and pulse-width of\n~180 fs, we have generated 1D and 2D Airy beams at an efficiency of 80% and\n70%, respectively, and pulse width of ~188 fs and ~190 fs, respectively. We\nhave measured the transverse deflection rate of 1D and 2D beams to be ~5.0\n(10^-5) 1/mm and ~2.0 (10^-5) 1/mm, respectively. Simply rotating the\npolarization state of the 1D cubic phase modulated beam in the experiment we\ncan produce 1D and 2D Airy beams on demand. Using a 5 mm long bismuth borate\n(BiB3O6) we have also studied frequency-doubling characteristics of both 1D and\n2D Airy beams. Like 2D Airy beam, the 1D Airy beam also produce\nfrequency-doubled 1D Airy and an additional 1D spatial cubic structure. Like\nthe Gaussian beams, we have observed the focusing dependent conversion\nefficiency for both 1D and 2D Airy beams producing green 1D and 2D Airy beams\nof output powers in excess of 110 mW and 150 mW for 3.4 W and 2.8 W of\nfundamental power respectively.",
        "category": "physics_optics"
    },
    {
        "text": "Left-right asymmetry in an optical nanofiber: Symmetry breaking effect for left- and right-handed circularly polarized\nlight beams propagating in a rotationally symmetric graded-index optical fiber\nis presented. It is shown that a left-right asymmetry manifested as an unequal\ntransmission for opposite circular polarizations occurs due to spin-to-spin\nangular momentum conversion caused by tensor interaction.",
        "category": "physics_optics"
    },
    {
        "text": "High-frequency cavity optomechanics using bulk acoustic phonons: To date, micro- and nano-scale optomechanical systems have enabled many\nproof-of-principle quantum operations through access to high-frequency (GHz)\nphonon modes that are readily cooled to their thermal ground state. However,\nminuscule amounts of absorbed light produce excessive heating that can\njeopardize robust ground state operation within such microstructures. In\ncontrast, we demonstrate an alternative strategy for accessing high-frequency\n($13$ GHz) phonons within macroscopic systems (cm-scale). Counterintuitively,\nwe show that these macroscopic systems, with motional masses that are $>20$\nmillion times larger than those of micro-scale counterparts, offer a\ncomplementary path towards robust quantum operations. Utilizing bulk acoustic\nphonons to mediate resonant coupling between two distinct modes of an optical\ncavity, we demonstrate the ability to perform beam-splitter and entanglement\noperations at MHz rates on an array of phonon modes, opening doors to\napplications ranging from quantum memories and microwave-to-optical conversion\nto high-power laser oscillators.",
        "category": "physics_optics"
    },
    {
        "text": "Looking through walls and around corners with incoherent light:\n  Wide-field real-time imaging through scattering media: Imaging with optical resolution through highly scattering media is a long\nsought-after goal with important applications in deep tissue imaging. Although\nbeing the focus of numerous works, this goal was considered impractical until\nrecently. Adaptive-optics techniques which are effective in correcting weak\nwavefront aberrations, were deemed inadequate for turbid samples, where complex\nspeckle patterns arise and light is scattered to a large number of modes that\ngreatly exceeds the number of degrees of control. This conception changed after\nthe demonstration of focusing coherent light through turbid media by\nwavefront-shaping, using a spatial-light-modulator (SLM). Here we show that\nwavefront-shaping enables widefield real-time imaging through scattering media\nwith both coherent or incoherent illumination, in transmission and reflection.\nIn contrast to the recently introduced schemes for imaging through turbid\nmedia, our technique does not require coherent sources, interferometric\ndetection, raster scanning, or off-line image reconstruction. Our results bring\nwavefront-shaping closer to practical applications, and realize the vision of\nlooking 'through walls' and 'around corners'.",
        "category": "physics_optics"
    },
    {
        "text": "Highly-stable, flexible delivery of microjoule-level ultrafast pulses in\n  vacuumized anti-resonant hollow-core fibers for active synchronization: We demonstrate the stable and flexible light delivery of multi-{\\mu}J,\nsub-200-fs pulses over a ~10-m-long vacuumized anti-resonant hollow-core fiber\n(AR-HCF), which was successfully used for high-performance pulse\nsynchronization. Compared with the pulse train launched into the AR-HCF, the\ntransmitted pulse train out of the fiber exhibits excellent stabilities in\npulse power and spectrum, with pointing stability largely improved. The\nwalk-off between the fiber-delivery and the other free-space-propagation pulse\ntrains, in an open loop, was measured to be <6 fs root-mean-square (RMS) over\n90 minutes, corresponding to a relative optical-path variation of <2x10-7. This\nwalk-off can be further suppressed to ~2 fs RMS simply using an active control\nloop, highlighting the great application potentials of this AR-HCF set-up in\nlarge-scale laser and accelerator facilities.",
        "category": "physics_optics"
    },
    {
        "text": "Modification to the Fresnel formulas for amplified or attenuated\n  internal reflection: The present study addresses the question of total internal reflection of a\nplane wave from an amplifying or attenuating medium. Inspection on the\nexpressions for the modal gain or loss of an asymmetric planar waveguide having\nan amplifying or attenuating cladding has led us to the coefficients for\namplified or attenuated internal reflection. The derived formulas suggest an\ninterpretation that the refracted inhomogeneous plane wave undergoes\namplification or attenuation while it travels a distance equal to the\nGoos-H\\\"anchen shift along the boundary plane before going back into the\nhigh-index medium. Furthermore, the evanescent wave in the low-index medium is\ndiscussed. Unlike the transparent case, the Poynting vector is shown to have a\nnonvanishing time-averaged component perpendicular to the boundary plane.",
        "category": "physics_optics"
    },
    {
        "text": "Nonlinear imaging and 3D-mapping of terahertz fields with Kerr media: We investigate the spatially and temporally resolved four-wave mixing of\nterahertz fields and optical pulses in large band-gap dielectrics, such as\ndiamond. We show that it is possible to perform beam profiling and space-time\nresolved mapping of terahertz fields with sub-wavelength THz resolution by\nencoding the spatial information into an optical signal, which can then be\nrecorded by a standard CCD camera.",
        "category": "physics_optics"
    },
    {
        "text": "Wideband dispersion-free THz waveguide platform: We present an integrated THz spectroscopy and sensing platform featuring low\nloss, vacuum-like dispersion, and strong field confinement in the fundamental\nmode. Its performance was characterized experimentally for frequencies between\n0.1 THz and 1.5 THz. While linear THz spectroscopy and sensing gain mostly from\nlow loss and an extended interaction length, nonlinear THz spectroscopy would\nalso profit from the field enhancement associated to strong mode confinement.\nMoreover, the vacuum-like dispersion allows for a reshaping-free propagation of\nbroadband single- to few-cycle pulses in gas-phase samples or velocity matching\nbetween THz pump and visible to infrared probe pulses. Our platform is based on\na metallic structure and falls in the category of double ridged waveguides. We\ncharacterize essential waveguide properties, for instance, propagation and\nbending losses, but also demonstrate junctions and interferometers, essentially\nbecause those elements are prerequisites for integrated THz waveform synthesis,\nand hence, for coherently controlled linear and nonlinear interactions.",
        "category": "physics_optics"
    },
    {
        "text": "Incorporation of Polar Mellin Transform in a Hybrid Optoelectronic\n  Correlator for Scale & Rotation Invariant Target Recognition: In this paper, we show that our proposed hybrid optoelectronic correlator\n(HOC), which correlates images using spatial light modulators (SLM), detectors\nand field programmable gate arrays (FPGAs), is capable of detecting objects in\na scale and rotation invariant manner, along with the shift invariance feature,\nby incorporating polar mellin transform (PMT). We also illustrate a key\nlimitation of the ideas presented in previous papers on performing PMT and\npresent a solution to circumvent this limitation by cutting out a small circle\nat the center of the Fourier Transform which precedes PMT. Furthermore, we show\nhow to carry out shift, rotation and scale invariant detection of multiple\nmatching objects simultaneously, a process previously thought to be\nincompatible with PMT based correlators. We present results of numerical\nsimulations to validate the concepts.",
        "category": "physics_optics"
    },
    {
        "text": "Homogenization of nonlocal wire metamaterial via a renormalization\n  approach: It is well known that defining a local refractive index for a metamaterial\nrequires that the wavelength be large with respect to the scale of its\nmicroscopic structure (generally the period). However, the converse does not\nhold. There are simple structures, such as the infinite, perfectly conducting\nwire medium, which remain non-local for arbitrarily large wavelength-to-period\nratios. In this work we extend these results to the more realistic and relevant\ncase of finite wire media with finite conductivity. In the quasi-static regime\nthe metamaterial is described by a non-local permittivity which is obtained\nanalytically using a two-scale renormalization approach. Its accuracy is tested\nand confirmed numerically via full vector 3D finite element calculations.\nMoreover, finite wire media exhibit large absorption with small reflection,\nwhile their low fill factor allows considerable freedom to control other\ncharacteristics of the metamaterial such as its mechanical, thermal or chemical\nrobustness.",
        "category": "physics_optics"
    },
    {
        "text": "2-dimensional hyperbolic medium for electrons and photons based on the\n  array of tunnel-coupled graphene nanoribbons: We study the electronic band structure and optical conductivity of an array\nof tunnel-coupled array of graphene nanoribbons. We show that due to the\ncoupling of electronic edge states for the zigzag nanoribbon structure, the\nFermi surface can become a hyperbola similarly to the case of the layered\nmetal-dielectric structures, where the hyperbolic isofrequency contours\noriginate from the coupling of localized surface plasmon polaritons. Moreover,\nwe show that for both types of the ribbon edge, the optical response of the\nstructure can be characterized by a uniaxial conductivity tensor, having\nprincipal components of the different signs. Therefore, the tunnel-coupled\nnanoribbon array can be regarded as a tunable hyperbolic metasurface.",
        "category": "physics_optics"
    },
    {
        "text": "Geometric optics of whispering gallery modes: Quasiclassical approach and geometric optics allow to describe rather\naccurately whispering gallery modes in convex axisymmetric bodies. Using this\napproach we obtain practical formulas for the calculation of eigenfrequencies\nand radiative Q-factors in dielectrical spheroid and compare them with the\nknown solutions for the particular cases and with numerical calculations. We\nshow how geometrical interpretation allows expansion of the method on arbitrary\nshaped axisymmetric bodies.",
        "category": "physics_optics"
    },
    {
        "text": "Measurement of complex supercontinuum light pulses using time domain\n  ptychography: We demonstrate that time-domain ptychography, a recently introduced ultrafast\npulse reconstruction modality, has properties ideally suited for the temporal\ncharacterization of complex light pulses with large time-bandwidth products as\nit achieves temporal resolution on the scale of a single optical cycle using\nlong probe pulses, low sampling rates, and an extremely fast and robust\nalgorithm. In comparison to existing techniques, ptychography minimizes the\ndata to be recorded and processed, and drastically reduces the computational\ntime of the reconstruction. Experimentally we measure the temporal waveform of\nan octave-spanning, 3.5~ps long supercontinuum pulse generated in photonic\ncrystal fiber, resolving features as short as 5.7~fs with sub-fs resolution and\n30~dB dynamic range using 100~fs probe pulses and similarly large delay steps.",
        "category": "physics_optics"
    },
    {
        "text": "Generative Deep Learning Model for a Multi-level Nano-Optic Broadband\n  Power Splitter: We propose a novel Conditional Variational Autoencoder (CVAE) model, enhanced\nwith adversarial censoring and active learning, for the generation of 550 nm\nbroad bandwidth (1250 nm to 1800 nm) power splitters with arbitrary splitting\nratio. The device footprint is 2.25 x 2.25 {\\mu} m2 with a 20 x 20 etched hole\ncombination. It is the first demonstration to apply the CVAE model and the\nadversarial censoring for the photonics problems. We confirm that the optimized\ndevice has an overall performance close to 90% across all bandwidths from 1250\nnm to 1800 nm. To the best of our knowledge, this is the smallest broadband\npower splitter with arbitrary ratio.",
        "category": "physics_optics"
    },
    {
        "text": "Degeneracy of cross sections in scattering of light: We theoretically and numerically prove that under an electromagnetic plane\nwave with linear polarization incident normally to cylindrical passive\nscatterers, a single energy diagram can integrate absorption, scattering, and\nextinction cross sections for arbitrary scattering systems, irrespective of\ninternal configuration, material parameters, and its sizes. For a system with\ndefinite resonant orders, along the corresponding boundary of the energy\ndiagram, not only the magnitudes of scattering coefficients, but also its\nphases, are required the same, corresponding to superabsorption or\nsuperscattering. For systems composed by larger resonant orders, the domain of\nthe energy diagram can completely cover systems with lower one. Hence, systems\nwith different resonant orders can provide the same energy characteristics,\nreflecting the energy degenerate property in light scattering. This degeneracy\nmay relax more degrees of freedom in functional designs, such as energy\nharvesting, imaging, sensing devices. We demonstrate various systems based on\nreal materials to support this finding.",
        "category": "physics_optics"
    },
    {
        "text": "Differential phase reconstruction of microcombs: Measuring microcombs in amplitude and phase provides unique insight into the\nnonlinear cavity dynamics but spectral phase measurements are experimentally\nchallenging. Here, we report a linear heterodyne technique assisted by\nelectro-optic downconversion that enables differential phase measurement of\nsuch spectra with unprecedented sensitivity (-50 dBm) and bandwidth coverage (>\n110 nm in the telecommunications range). We validate the technique with a\nseries of measurements, including single cavity and photonic molecule\nmicrocombs.",
        "category": "physics_optics"
    },
    {
        "text": "Theoretical study on the stimulated Brillouin scattering in a\n  sub-wavelength anisotropic waveguide: Acousto-optical coupling coefficients\n  and effects of transverse anisotropies: A theoretical study on the stimulated Brillouin scattering (SBS) in a\nsub-wavelength anisotropic waveguide is conducted. The optical, photoelastic\nand mechanical anisotropies of the waveguide materials are all taken into\naccount. First, the integral formulae for calculating the acousto-optical\ncoupling coefficients (AOCCs) due to the photoelastic and moving interface\neffects in SBS are extended to an optically anisotropic waveguide. Then, with\nthe extended formulae, the SBSs in an elliptical nanowire with strong\ntransverse anisotropies are simulated. In the simulations, the elastic fields\nare computed with the inclusion of mechanical anisotropy. Observable effects of\nthe strong transverse anisotropies are found in numerical results. Most\nnotably, the SBS gains of some elastic modes are found to be very sensitive to\nthe small misalignment between the waveguide axes and the principal material\naxes. Detailed physical interpretations of this interesting phenomenon are\nprovided. This interesting phenomenon implies an attractive way for more\nsensitive tuning of the SBS gain without significantly changing the phononic\nfrequency.",
        "category": "physics_optics"
    },
    {
        "text": "Acoustically tuneable optical transmission through a subwavelength hole\n  with a bubble: Efficient manipulation of light with sound in subwavelength-sized volumes is\nimportant for applications in photonics, phononics and biophysics, but remains\nelusive. We theoretically demonstrate the control of light with MHz-range\nultrasound in a subwavelength, 300 nm wide water-filled hole with a 100 nm\nradius air bubble. Ultrasound-driven pulsations of the bubble modulate the\neffective refractive index of the hole aperture, which gives rise to spectral\ntuning of light transmission through the hole. This control mechanism opens up\nnovel opportunities for tuneable acousto-optic and optomechanical\nmetamaterials, and all-optical ultrasound transduction.",
        "category": "physics_optics"
    },
    {
        "text": "All-fiber controller of radial polarization using a periodic stress: Our aim is to transpose the polarization control by mechanical stress usually\napplied to single mode fibers, to the (TM01, TE01,HEev21,HEod21) annular mode\nfamily. Nevertheless, the quasi degeneracy of these four modes makes the\nsituation more complex than with the fundamental mode HE11. We propose a simple\ndevice based on periodic perturbation and mode coupling to produce the radially\npolarized TM01 mode or at least one single of the four modes at the extremity\nof an arbitrarily long fiber, the conversion to TM01 mode being achievable by\nclassical crystalline plates.",
        "category": "physics_optics"
    },
    {
        "text": "Resonant Fully dielectric metasurfaces for ultrafast Terahertz pulse\n  generation: Metasurfaces represent a new frontier in materials science paving for\nunprecedented methods of controlling electromagnetic waves, with a range of\napplications spanning from sensing to imaging and communications. For pulsed\nterahertz generation, metasurfaces offer a gateway to tuneable thin emitters\nthat can be utilised for large-area imaging, microscopy and spectroscopy. In\nliterature THz-emitting metasurfaces generally exhibit high absorption, being\nbased either on metals or on semiconductors excited in highly resonant regimes.\nHere we propose the use of a fully dielectric semiconductor exploiting\nmorphology-mediated resonances and inherent quadratic nonlinear response. Our\nsystem exhibits a remarkable 40-fold efficiency enhancement compared to the\nunpatterned at the peak of the optimised wavelength range, demonstrating its\npotential as scalable emitter design.",
        "category": "physics_optics"
    },
    {
        "text": "A highly scalable fully non-blocking silicon photonic switch fabric: Large port count spatial optical switches will facilitate flexible and energy\nefficient data movement in future data communications systems, especially if\nthey are capable of nanosecond-order reconfiguration times. In this work, we\ndemonstrate an 8x8 microring-based silicon photonic switch with software\ncontrolled switching. The proposed switch architecture is modular as it\nassembles multiple identical components with multiplexing/demultiplexing\nfunctionalities. The switch is fully non-blocking, has path independent\ninsertion loss, low crosstalk and is straightforward to control. A scalability\nanalysis shows that this architecture can scale to very large port counts. This\nwork represents the first demonstration of real-time firmware controlled\nswitching with silicon photonics devices integrated at the chip scale.",
        "category": "physics_optics"
    },
    {
        "text": "The thermodynamic dual structure of linear-dissipative driven systems: The spontaneous emergence of dynamical order, such as persistent currents, is\nsometimes argued to require principles beyond the entropy maximization of the\nsecond law of thermodynamics. I show that, for linear dissipation in the\nOnsager regime, current formation can be driven by exactly the Jaynesian\nprinciple of entropy maximization, suitably formulated for extended systems and\nnonequilibrium boundary conditions. The Legendre dual structure of equilibrium\nthermodynamics is also preserved, though it requires the admission of\ncurrent-valued state variables, and their correct incorporation in the entropy.",
        "category": "physics_optics"
    },
    {
        "text": "Ultra-broadband reflectionless Brewster absorber protected by\n  reciprocity: The Brewster's law predicts zero reflection of p-polarization on a dielectric\nsurface at a particular angle. However, when loss is introduced into the\npermittivity of the dielectric, the Brewster condition breaks down and\nreflection unavoidably appears. In this work, we found an exception to this\nlong-standing dilemma by creating a class of nonmagnetic anisotropic\nmetamaterials, where an anomalous Brewster effects with independently tunable\nabsorption and refraction emerges. This loss-independent Brewster effect is\nbestowed by the extra degrees of freedoms introduced by anisotropy and strictly\nprotected by the reciprocity principle. The bandwidth can cover an extremely\nwide spectrum from dc to optical frequencies. Two examples of reflectionless\nBrewster absorbers with different Brewster angles are both demonstrated to\nachieve large absorbance in a wide spectrum via microwave experiments. Our work\nextends the scope of Brewster effect to the horizon of nonmagnetic absorptive\nmaterials, which promises an unprecedented wide bandwidth for reflectionless\nabsorption with high efficiency.",
        "category": "physics_optics"
    },
    {
        "text": "Frequency conversion of structured light: We demonstrate the coherent frequency conversion of structured light, optical\nbeams in which the phase varies in each point of the transverse plane, from the\nnear infrared (803nm) to the visible (527nm). The frequency conversion process\nmakes use of sum-frequency generation in a periodically poled lithium niobate\n(ppLN) crystal with the help of a 1540-nm Gaussian pump beam. We perform\nfar-field intensity measurements of the frequency-converted field, and verify\nthe sought-after transformation of the characteristic intensity and phase\nprofiles for various input modes. The coherence of the frequency-conversion\nprocess is confirmed using a mode-projection technique with a phase mask and a\nsingle-mode fiber. The presented results could be of great relevance to novel\napplications in high-resolution microscopy and quantum information processing.",
        "category": "physics_optics"
    },
    {
        "text": "Recent Advances in Tunable Metasurfaces: Materials, Design and\n  Applications: Metasurfaces, a two-dimensional (2D) form of metamaterials constituted by\nplanar meta-atoms, exhibit exotic abilities to freely tailor electromagnetic\n(EM) waves. Over the past decade, tunable metasurfaces have come to the\nfrontier in the field of nanophotonics, with tremendous effort focused on\ndeveloping and integrating various active materials into metasurfaces. As a\nresult, tunable/reconfigurable metasurfaces with multi-functionalities\ntriggered by various external stimuli have been successfully demonstrated,\nopenings a new avenue to dynamically manipulate and control EM waves for\nphotonic applications in demand. In this review, we first brief the progress of\ntunable metasurfaces development in the last decade and highlight\nrepresentative works from the perspectives of active materials development,\ndesign methodologies and application-driven exploration. Then, we elaborate on\nthe active tuning mechanisms and relevant active materials. Next, we discuss\nrecent achievements in theory as well as machine learning (ML) assisted design\nmethodologies to sustain the development of this field. After that, we\nsummarize and describe typical application areas of the tunable metasurfaces.\nWe conclude this review by analyzing existing challenges and presenting our\nperspectives on future directions and opportunities in this vibrant and\nfast-developing field.",
        "category": "physics_optics"
    },
    {
        "text": "Vortex Laser at Exceptional Point: The optical vortices carrying orbital angular momentum (OAM) are commonly\ngenerated by modulating the available conventional light beam. This article\nshows that a micro-laser operates at the exceptional point (EP) of the\nnon-Hermitian quantum system can directly emit vortex laser with well-defined\nOAM at will. Two gratings (the refractive index modulation and along azimuthal\ndirection and the grating protruding from the micro-ring cavity) modulate the\neigenmode of a micro-ring cavity to be a vortex laser mode. The phase-matching\ncondition ensures that we can tune the OAM of the vortex beam to be arbitrary\norders by changing the grating protruding from the micro-ring cavity while the\nsystem is kept at EP. The results are obtained by analytical analysis and\nconfirmed by 3D full wave simulations.",
        "category": "physics_optics"
    },
    {
        "text": "Geometrodynamics of polarized light: Berry phase and spin Hall effect in\n  a gradient-index medium: We review the geometrical-optics evolution of an electromagnetic wave\npropagating along a curved ray trajectory in a gradient-index dielectric\nmedium. A Coriolis-type term appears in Maxwell equations under transition to\nthe rotating coordinate system accompanying the ray. This term describes the\nspin-orbit coupling of light which consists of (i) the Berry phase responsible\nfor a trajectory-dependent polarization variations and (ii) the spin Hall\neffect representing polarization-dependent trajectory perturbations. These\nmutual phenomena are described within universal geometrical structures\nunderlying the problem and are explained by the dynamics of the intrinsic\nangular momentum carried by the wave. Such close geometro-dynamical\ninterrelations illuminate a dual physical nature of the phenomena.",
        "category": "physics_optics"
    },
    {
        "text": "THz meta-foil - a new photonic material: Seeing sharper or becoming invisible are visions strongly driving the\ndevelopment of THz metamaterials. Strings are a preferred architecture of\nmetamaterials as they extend continuously along one dimension. Here, we\ndemonstrate that laterally interconnecting strings by structural elements that\nare placed in oscillation nodes such as to not quench electromagnetic\nresonances enables manufacturing of self-supported free-standing all-metal\nmetamaterials. Upright S-strings, interconnected by rods, form a space-grid\nwhich we call meta-foil. In this way, we introduce binding between the \"atoms\"\nof the metamaterial, thus doing away with conventional \"frozen-in solutions\"\nlike matrix embedding or thin films on substrates. Meta-foils are locally\nstiff, yet globally flexible. Even bent to cylinders of 1 cm radius, they\nmaintain their spectral response, thus becoming true metamaterials on curved\nsurfaces. Exploiting UV/X-ray lithography and ultimately plastic moulding,\nmeta-foils can be cost-effectively manufactured in large areas and quantities\nto serve as optical elements.",
        "category": "physics_optics"
    },
    {
        "text": "Circular dichroism of cholesteric polymers and the orbital angular\n  momentum of light: We explore experimentally if the light's orbital angular momentum (OAM)\ninteracts with chiral nematic polymer films. Specifically, we measure the\ncircular dichroism of such a material using light beams with different OAM. We\ninvestigate the case of strongly focussed, non-paraxial light beams, where the\nspatial and polarization degrees of freedom are coupled. Within the\nexperimental accuracy, we cannot find any influence of the OAM on the circular\ndichroism of the cholesteric polymer.",
        "category": "physics_optics"
    },
    {
        "text": "Soliton Formation in Whispering-Gallery-Mode Resonators via Input Phase\n  Modulation: We propose a method for soliton formation in whispering-gallery-mode (WGM)\nresonators through input phase modulation. Our numerical simulations of a\nvariant of the Lugiato-Lefever equation suggest that modulating the input phase\nat a frequency equal to the resonator free-spectral-range and at modest\nmodulation depths provides a deterministic route towards soliton formation in\nWGM resonators without undergoing a chaotic phase. We show that the generated\nsolitonic state is sustained when the modulation is turned off adiabatically.\nOur results support parametric seeding as a powerful means of control, besides\ninput pump power and pump-resonance detuning, over frequency comb generation in\nWGM resonators. Our findings also help pave the path towards ultra-short pulse\nformation on a chip.",
        "category": "physics_optics"
    },
    {
        "text": "Classical optics representation of the quantum mechanical translation\n  operator via ABCD matrices: The ABCD matrix formalism describing paraxial propagation of optical beams\nacross linear systems is generalized to arbitrary beam trajectories. As a\nby-product of this study, a one-to-one correspondence between the extended ABCD\nmatrix formalism presented here and the quantum mechanical translation operator\nis established.",
        "category": "physics_optics"
    },
    {
        "text": "Excitation of surface plasmon polaritons guided mode by Rhodamine B\n  molecules doped in PMMA stripe: In this letter we show the inclusion of Rhodamine B molecules (RhB) inside a\ndielectric-loaded surface plasmon waveguide enables for a precise determination\nof its optical characteristics. The principle relies on the coupling of the\nfluorescence emission of the dye to plasmonic waveguided modes allowed in of\nthe structure. Using leakage radiation microscopy in real and reciprocal\nspaces, we measure the propagation constant of the mode and as well as their\nattenuation length.",
        "category": "physics_optics"
    },
    {
        "text": "The Kinetic of the Atomic Relaxation Induced by Laser Noise: We present a theoretical study of strong laser-atom interactions, when the\nlaser field parameters are subjected to random processes. The atom is modelled\nby a two-level and three-level systems, while the statistical fluctuations of\nthe laser field are described by a pre-Gaussian model.",
        "category": "physics_optics"
    },
    {
        "text": "Comment on \"Optical torque on small chiral particles in generic optical\n  fields \": We comment on mistakes and inaccuracies of a paper by Chen et al. concerning\nthe optical torque from generic optical fields on dipolar chiral particles,\ni.e. on those whose scattering is fully described by the first electric,\nmagnetic and magnetoelectric Mie coefficients.",
        "category": "physics_optics"
    },
    {
        "text": "Applications of integrated optical microcombs: Optical microcombs represent a new paradigm for generating laser frequency\ncombs based on compact chip-scale devices, which have underpinned many modern\ntechnological advances for both fundamental science and industrial\napplications. Along with the surge in activity related to optical micro-combs\nin the past decade, their applications have also experienced rapid progress,\nnot only in traditional fields such as frequency synthesis, signal processing,\nand optical communications, but also in new interdisciplinary fields spanning\nthe frontiers of light detection and ranging (LiDAR), astronomical detection,\nneuromorphic computing, and quantum optics. This paper reviews the applications\nof optical microcombs. First, an overview of the devices and methods for\ngenerating optical microcombs is provided, which are categorized into material\nplatforms, device architectures, soliton classes, and driving mechanisms.\nSecond, the broad applications of optical microcombs are systematically\nreviewed, which are categorized into microwave photonics, optical\ncommunications, precision measurements, neuromorphic computing, and quantum\noptics. Finally, the current challenges and future perspectives are discussed.",
        "category": "physics_optics"
    },
    {
        "text": "Low-frequency measurements of electro-optic coefficients of Na:KTP, and\n  RTA: We report the measurement of the r23 and r33 electro-optic coefficients and\nof the z-axis electrical conductivity of the nonlinear optical crystals Na:KTP\nand RTA. We observed a marked decrease of the electro-optic effect in the\nNa:KTP crystals for frequencies below 100 Hz. This effect is absent in RTA.",
        "category": "physics_optics"
    },
    {
        "text": "Optical gain in DNA-DCM for lasing in photonic materials: We present a detailed study of the gain length in an active medium obtained\nby doping of DNA strands with DCM dye molecules. The superior thermal stability\nof the composite and its low quenching, permits to obtain optical gain\ncoefficient larger than 300 cm^-1. We also show that such an active material is\nexcellent for integration into photonic nano-structures, to achieve, for\nexample, efficient random lasing emission, and fluorescent photonic crystals.",
        "category": "physics_optics"
    },
    {
        "text": "Gradient metasurfaces: a review of fundamentals and applications: In the wake of intense research on metamaterials the two-dimensional\nanalogue, known as metasurfaces, has attracted progressively increasing\nattention in recent years due to the ease of fabrication and smaller insertion\nlosses, while enabling an unprecedented control over spatial distributions of\ntransmitted and reflected optical fields. Metasurfaces represent optically thin\nplanar arrays of resonant subwavelength elements that can be arranged in a\nstrictly or quasi periodic fashion, or even in an aperiodic manner, depending\non targeted optical wavefronts to be molded with their help. This paper reviews\na broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised\nto exhibit spatially varying optical responses resulting in spatially varying\namplitudes, phases and polarizations of scattered fields. Starting with\nintroducing the concept of gradient metasurfaces, we present classification of\ndifferent metasurfaces from the viewpoint of their responses, differentiating\nelectrical-dipole, geometric, reflective and Huygens' metasurfaces. The\nfundamental building blocks essential for the realization of metasurfaces are\nthen discussed in order to elucidate the underlying physics of various physical\nrealizations of both plasmonic and purely dielectric metasurfaces. We then\noverview the main applications of gradient metasurfaces, including waveplates,\nflat lenses, spiral phase plates, broadband absorbers, color printing,\nholograms, polarimeters and surface wave couplers. The review is terminated\nwith a short section on recently developed nonlinear metasurfaces, followed by\nthe outlook presenting our view on possible future developments and\nperspectives for future applications.",
        "category": "physics_optics"
    },
    {
        "text": "Fast beam steering with full polarization control using a galvanometric\n  optical scanner and polarization controller: Optical beam steering is a key element in many industrial and scientific\napplications like in material processing, information technologies, medical\nimaging and laser display. Even though galvanometer-based scanners offer\nflexibility, speed and accuracy at a relatively low cost, they still lack the\nnecessary control over the polarization required for certain applications. We\nreport on the development of a polarization steerable system assembled with a\nfiber polarization controller and a galvanometric scanner, both controlled by a\ndigital signal processor board. The system implements control of the\npolarization decoupled from the pointing direction through a feed-forward\ncontrol scheme. This enables to direct optical beams to a desired direction\nwithout affecting its initial polarization state. When considering the full\nworking field of view, we are able to compensate polarization angle errors\nlarger than 0.2 rad, in a temporal window of less than $\\sim 20$ ms. Given the\nunification of components to fully control any polarization state while\nsteering an optical beam, the proposed system is potentially integrable and\nrobust.",
        "category": "physics_optics"
    },
    {
        "text": "Walk-off-induced modulation instability, temporal pattern formation, and\n  frequency comb generation in cavity-enhanced second-harmonic generation: We derive a time-domain mean-field equation to model the full temporal and\nspectral dynamics of light in singly resonant cavity-enhanced second-harmonic\ngeneration systems. We show that the temporal walk-off between the fundamental\nand the second-harmonic fields plays a decisive role under realistic\nconditions, giving rise to rich, previously unidentified nonlinear behaviour.\nThrough linear stability analysis and numerical simulations, we discover a new\nkind of quadratic modulation instability which leads to the formation of\noptical frequency combs and associated time-domain dissipative structures. Our\nnumerical simulations show excellent agreement with recent experimental\nobservations of frequency combs in quadratic nonlinear media [Phys. Rev. A 91,\n063839 (2015)]. Thus, in addition to unveiling a new, experimentally accessible\nregime of nonlinear dynamics, our work enables predictive modeling of frequency\ncomb generation in cavity-enhanced second-harmonic generation systems.",
        "category": "physics_optics"
    },
    {
        "text": "One-dimensional photonic quasicrystals: In this chapter, first we will address principal aspects of 1D\nquasiperiodicity with a particular focus on 1D Fibonacci chains. Further, the\nrest of the chapter will be dedicated to the electromagnetic counterpart of 1D\nFibonacci structures as a relatively simplest case of the large class of\nphotonic quasicrystals.",
        "category": "physics_optics"
    },
    {
        "text": "Suppression of amplitude-to-phase noise conversion in balanced\n  optical-microwave phase detectors: We demonstrate an amplitude-to-phase (AM-PM) conversion coefficient for a\nbalanced optical-microwave phase detector (BOM-PD) of 0.001 rad, corresponding\nto AM-PM induced phase noise 60 dB below the single-sideband relative intensity\nnoise of the laser. This enables us to generate 8 GHz microwave signals from a\ncommercial Er-fibre comb with a single-sideband residual phase noise of -131\ndBc/Hz at 1 Hz offset frequency and -148 dBc/Hz at 1 kHz offset frequency.",
        "category": "physics_optics"
    },
    {
        "text": "Edge and bulk dissipative solitons in modulated PT-symmetric waveguide\n  arrays: We address dissipative soliton formation in modulated PT-symmetric continuous\nwaveguide arrays composed from waveguides with amplifying and absorbing\nsections, whose density gradually increases (due to decreasing waveguide\nseparation) either towards the center of the array or towards its edges. In\nsuch a structure the level of gain/loss at which PT-symmetry gets broken\ndepends on the direction of increase of the waveguide density. Breakup of the\nPT-symmetry occurs when eigenvalues of modes localized in the region, where\nwaveguide density is largest, collide and move into complex plane. In this\nregime of broken symmetry the inclusion of focusing Kerr-type nonlinearity of\nthe material and weak two-photon absorption allows to arrest the growth of\namplitude of amplified modes and may lead to the appearance of stable\nattractors either in the center or at the edge of the waveguide array,\ndepending on the type of array modulation. Such solitons can be stable, they\nacquire specific triangular shapes and notably broaden with increase of\ngain-loss level. Our results illustrate how spatial array modulation that\nbreaks PT-symmetry locally can be used to control specific location of\ndissipative solitons forming in the array.",
        "category": "physics_optics"
    },
    {
        "text": "Surface modes in plasmonic Bragg fibers with negative average\n  permittivity: We investigate surface modes in plasmonic Bragg fibers composed of\nnanostructured coaxial cylindrical metal-dielectric multilayers. We demonstrate\nthat the existence of surface modes is determined by the sign of the spatially\naveraged permittivity of the plasmonic Bragg fiber, $\\bar{% \\varepsilon}$.\nSpecifically, localized surface modes occur at the interface between the\ncylindrical core with $\\bar{\\varepsilon}<0$ and the outermost uniform\ndielectric medium, which is similar to the topologically protected plasmonic\nsurface modes at the interface between two different one-dimensional planar\nmetal-dielectric lattices with opposite signs of the averaged permittivity.\nMoreover, when increasing the number of dielectric-metal rings, the propagation\nconstant of surface modes with different azimuthal mode numbers is approaching\nthat of surface plasmon polaritons formed at the corresponding planar\nmetal/dielectric interface. Robustness of such surface modes of plasmonic Bragg\nfibers is demonstrated too.",
        "category": "physics_optics"
    },
    {
        "text": "Generating far-field orbital angular momenta from near-field optical\n  chirality: We demonstrate that nanostructures carefully designed on both sides of a thin\nsuspended metallic membrane couple light into a chiral near field and transmit\nvortex beams through a central aperture that connects the two sides of the\nmembrane. We show how far-field orbital angular momentum (OAM) indices can be\ntailored through nanostructure designs. We reveal the crucial importance of OAM\nselection rules imposed by the central aperture and derive OAM summation rules\nin perfect agreement with experimental data.",
        "category": "physics_optics"
    },
    {
        "text": "Orbital angular momentum exchange in an optical parametric oscillator: We present a study of orbital angular momentum transfer from pump to\ndown-converted beams in a type-II Optical Parametric Oscillator. Cavity and\nanisotropy effects are investigated and demostrated to play a central role in\nthe transverse mode dynamics. While the idler beam can oscillate in a\nLaguerre-Gauss mode, the crystal birefringence induces an astigmatic effect in\nthe signal beam that prevents the resonance of such mode.",
        "category": "physics_optics"
    },
    {
        "text": "Extended solutions via the trial-orbit method for two-field models: In this work we investigate the presence of defect structures in models\ndescribed by two real scalar fields. The coupling between the two fields is\ninspired on the equations for a multimode laser, and the minimum energy trivial\nconfigurations are shown to be structurely dependent on the parameters of the\nmodels. The trial orbit method is then used and several non-trivial analytical\nsolutions corresponding to topological solitons are obtained.",
        "category": "physics_optics"
    },
    {
        "text": "Light Management in Perovskite Photovoltaic Solar Cells: a perspective: Light Management (LM) is essential for metal-halide perovskite solar cells in\ntheir race for record performance. In this review, criteria on materials,\nprocesses and photonic engineering are established such as to enhance mainly\nthe short circuit current density, towards high energy yields. These criteria\nare used to analyse a large panel of solutions envisaged in the literature for\nsingle junction cells. Moreover, a perspective based on rigorous\nelectromagnetic simulations performed on various comparable structures is\nproposed in order to clarify the conclusions, and to pave the way to further\nperformance enhancement in the case of all-perovskite, two-terminal tandem\ncells.",
        "category": "physics_optics"
    },
    {
        "text": "Deflectometry for specular surfaces: an overview: Deflectometry as a technical approach to assessing reflective surfaces has\nnow existed for almost 40 years. Different aspects and variations of the method\nhave been studied in multiple theses and research articles, and reviews are\nalso becoming available for certain subtopics. Still a field of active\ndevelopment with many unsolved problems, deflectometry now encompasses a large\nvariety of application domains, hardware setup types, and processing workflows\ndesigned for different purposes, and spans a range from qualitative defect\ninspection of large vehicles to precision measurements of microscopic optics.\nOver these years, many exciting developments have accumulated in the underlying\ntheory, in the systems design, and in the implementation specifics. This\ndiversity of topics is difficult to grasp for experts and non-experts alike and\nmay present an obstacle to a wider acceptance of deflectometry as a useful tool\nin other research fields and in the industry.\n  This paper presents an attempt to summarize the status of deflectometry, and\nto map relations between its notable \"spin-off\" branches. The intention of the\npaper is to provide a common communication basis for practitioners and at the\nsame time to offer a convenient entry point for those interested in learning\nand using the method. The list of references is extensive but definitely not\nexhaustive, introducing some prominent trends and established research groups\nin order to facilitate further self-directed exploration by the reader.",
        "category": "physics_optics"
    },
    {
        "text": "Optical combs with a crystalline whispering gallery mode resonator: We report on the experimental demonstration of a tunable monolithic optical\nfrequency comb generator. The device is based on the four-wave mixing in a\ncrystalline calcium fluoride whispering gallery mode resonator. The frequency\nspacing of the comb is given by an integer number of the free spectral range of\nthe resonator. We select the desired number by tuning the pumping laser\nfrequency with respect to the corresponding resonator mode. We also observe\ninteracting optical combs and high-frequency hyperparametric oscillation,\ndepending on the experimental conditions. A potential application of the comb\nfor generating narrowband frequency microwave signals is demonstrated.",
        "category": "physics_optics"
    },
    {
        "text": "Compact antenna for efficient and unidirectional launching and\n  decoupling of surface plasmons: Controlling the launching efficiencies and the directionality of surface\nplasmon polaritons (SPPs) and their decoupling to freely propagating light is a\nmajor goal for the development of plasmonic devices and systems. Here, we\nreport on the design and experimental observation of a highly efficient\nunidirectional surface plasmon launcher composed of eleven subwavelength\ngrooves, each with a distinct depth and width. Our observations show that,\nunder normal illumination by a focused Gaussian beam, unidirectional SPP\nlaunching with an efficiency of at least 52% is achieved experimentally with a\ncompact device of total length smaller than 8 \\mu m. Reciprocally, we report\nthat the same device can efficiently convert SPPs into a highly directive light\nbeam emanating perpendicularly to the sample.",
        "category": "physics_optics"
    },
    {
        "text": "Polarimetry-based analysis of dipolar transitions of single colloidal\n  CdSe/CdS dot-in-rods: We prove experimentally, upon polarization analysis performed on a large\nstatistic of single nanoemitters, that high quality core/shell CdSe/CdS\ndot-in-rods behave as linear dipoles. Moreover, the dipole in-plane and\nout-of-plane orientations could be assessed. We demonstrate in particular that,\ncontrary to expectations, the emitting dipole is not aligned with the elongated\naxis of the dot-in-rod. Besides, the polarimetric measurements prove that the\nexcitation transition cannot be approximated by a single linear dipole,\ncontrary to the emission transition. Finally, we highlight that non-radiative\nchannels of charge carrier recombination do not affect the dipolar nature of\nthe radiative transitions.",
        "category": "physics_optics"
    },
    {
        "text": "Time-Frequency Transfer through a 70 dB Free Space Channel: Towards\n  Satellite-Ground Time Dissemination: Time and frequency transfer lies at the heart of the field of metrology.\nCompared to current microwave dissemination such as GPS, optical domain\ndissemination can provide more than one order of magnitude in terms of higher\naccuracy, which allows for many applications such as the redefinition of the\nsecond, tests of general relativity and fundamental quantum physics, precision\nnavigation and quantum communication. Although optical frequency transfer has\nbeen demonstrated over thousand kilometers fiber lines, intercontinental time\ncomparison and synchronization still requires satellite free space optical time\nand frequency transfer. Quite a few pioneering free space optical time and\nfrequency experiments have been implemented at the distance of tens kilometers\nat ground level. However, there exists no detailed analysis or ground test to\nprove the feasibility of satellite-based optical time-frequency transfer. Here,\nwe analyze the possibility of this system and then provide the first-step\nground test with high channel loss. We demonstrate the optical frequency\ntransfer with an instability of $10^{-18}$ level in 8,000 seconds across a\n16-km free space channel with a loss of up to 70~dB, which is comparable with\nthe loss of a satellite-ground link at medium earth orbit (MEO) and\ngeostationary earth orbit (GEO).",
        "category": "physics_optics"
    },
    {
        "text": "How to calculate the pole expansion of the optical scattering matrix\n  from the resonant states: We present a formulation for the pole expansion of the scattering matrix of\nopen optical resonators, in which the pole contributions are expressed solely\nin terms of the resonant states, their wavenumbers, and their electromagnetic\nfields. Particularly, our approach provides an accurate description of the\noptical scattering matrix without the requirement of a fit for the pole\ncontributions or the restriction to geometries or systems with low Ohmic\nlosses. Hence, it is possible to derive the analytic dependence of the\nscattering matrix on the wavenumber with low computational effort, which allows\nfor avoiding the artificial frequency discretization of conventional\nfrequency-domain solvers of Maxwell's equations and for finding the optical\nfar- and near-field response based on the physically meaningfull resonant\nstates. This is demonstrated for three test systems, including a chiral\narrangement of nanoantennas, for which we calculate the absorption and the\ncircular dichroism.",
        "category": "physics_optics"
    },
    {
        "text": "Anomalous optical coupling between two silicon wires of a slot waveguide\n  in epsilon-near-zero metamaterials: Anomalous optical coupling properties between two silicon wires in a silicon\nslot waveguide embedded in epsilon-near-zero (ENZ) metamaterials are proposed\nand demonstrated. The dependences of optical field enhancement in the slot\nregion and transverse optical force on the slot size and the permittivity of\nsurrounding material are studied in details. It is demonstrated that the\noptical field in the slot region is significantly enhanced due to the giant\nindex contrast at the slot interface between silicon wires and ENZ\nmetamaterials, but the optical mode coupling between silicon wires is greatly\nreduced so that the transverse optical force is suppressed into almost zero.\nMoreover, metal-dielectric multilayer structures are designed to realize ENZ\nmetamaterials in the slot region for achieving the electric field enhancement.",
        "category": "physics_optics"
    },
    {
        "text": "Tunable Hybridization at Mid Zone and Anomalous Bloch-Zener Oscillations\n  in Optical Waveguide Ladders: We have studied the optical oscillation and tunneling of light waves in\noptical waveguide ladders formed by two coupled planar optical waveguide\narrays. For the band structure, a mid-zone gap is formed due to band\nhybridization and its wavenumber position can be tuned throughout the whole\nBrillouin zone, which is different from the Bragg gap. By imposing a gradient\nin the propagation constant in each array, Bloch-Zener oscillation (BZO) is\nrealized with Zener tunneling between the bands occurring at mid zone, which is\ncontrary to the common BZO with tunneling at the center or edge of the\nBrillouin zone. The occurrence of BZO is demonstrated by using the\nfield-evolution analysis. The tunable hybridization at mid zone enhances the\ntunability of BZO in the optical waveguide ladders. This work is of general and\nfundamental importance in understanding the coherent phenomena in lattice\nstructures.",
        "category": "physics_optics"
    },
    {
        "text": "Fault-tolerant and finite-error localization for point emitters within\n  the diffraction limit: We implement an estimator for determining the separation between two\nincoherent point sources. This estimator relies on image inversion\ninterferometry and when used with the appropriate data analytics, it yields an\nestimate of the separation with finite-error, even when the sources come\narbitrarily close together. The experimental results show that the technique\nhas a good tolerance to noise and misalignment, making it an interesting\nconsideration for high resolution instruments.",
        "category": "physics_optics"
    },
    {
        "text": "Antenna enhanced infrared photoinduced force imaging in aqueous\n  environment with super-resolution and hypersensitivity: Tip enhanced IR spectra and imaging have been widely used in cutting-edge\nstudies for the in-depth understanding of the composition, structure and\nfunction of interfaces at the nanoscale. However, molecular monolayer\nsensitivity has only been demonstrated on solid/gas interfaces. In aqueous\nenvironment, the reduced sensitivity due to strong damping of the cantilever\noscillation and background IR absorption extremely limits the practical\napplications of tip enhanced IR nanospectroscopy. Here, we demonstrate\nhypersensitive nanoscale IR spectra and imaging in aqueous environment with the\ncombination of photoinduced force (PiF) microscopy and resonant antennas. The\nhighly confined electromagnetic field inbetween the tip end and antenna\nextremely amplifies the photoinduced force to the detectable level, while the\nexcitation via plasmon internal reflection mode minimizes the environmental\nabsorption. A polydimethylsiloxane (PDMS) layer (~1-2 nm thickness)\nfunctionalized on the AFM tip has been successfully identified in water with\nantennas of different sizes. Sampling volume of ~604 chemical bonds from PDMS\nwas demonstrated with sub-10 nm spatial resolution confirmed by electric (E)\nfield distribution mapping on antennas, which strongly suggests the desired\nrequirements for interfacial spectroscopy. This platform demonstrates for the\nfirst time the application of photoinduced force microscopy in aqueous\nenvironments, providing a brand-new configuration to achieve highly enhanced\nnanoscale IR signals, which is extremely promising for future research of\ninterfaces and nanosystems in aqueous environments.",
        "category": "physics_optics"
    },
    {
        "text": "Quantum well-based waveguide for semiconductor lasers: In this work we study a possibility of waveguide fabrication on the basis of\nactive quantum wells in semiconductor lasers. The efficiency of such a\nwaveguide for an InP structure with In0.53Ga0.47As quantum wells is\ndemonstrated experimentally. An optically-pumped laser on this basis is\nrealized.",
        "category": "physics_optics"
    },
    {
        "text": "A color-switchable ring-shaped random laser in momentum space: A color-switchable random laser is designed through directly coupling random\nlaser with a commercial optical fiber. By using a simple approach of\nselectively coating the random gain layer on the surface of fiber, the red and\nyellow random lasers are respectively achieved with low threshold and good\nemission direction due to the guiding role of optical fibers. Moreover, the\nunique coupling mechanism leads to the random lasing with ring-shape in\nmomentum space, indicating an excellent illuminating source for high-quality\nimaging with an extremely low speckle noise. More importantly, random lasing\nwith different colors can be flexible obtained by simply moving the pump\nposition.",
        "category": "physics_optics"
    },
    {
        "text": "Fiber Bragg Grating Based Thermometry: In recent years there has been considerable interest in developing photonic\ntemperature sensors such as the Fiber Bragg gratings (FBG) as an alternative to\nresistance thermometry. In this study we examine the thermal response of FBGs\nover the temperature range of 233 K to 393 K. We demonstrate, in a hermetically\nsealed dry Argon environment, that FBG devices show a quadratic dependence on\ntemperature with expanded uncertainties (k = 2) of ~500 mK. Our measurements\nindicate that the combined measurement uncertainty is dominated by uncertainty\nin determining the peak center fitting and by thermal aging of polyimide coated\nfibers.",
        "category": "physics_optics"
    },
    {
        "text": "Identifying orbital angular momentum of light in quantum wells: Generation and detection of structured light have recently been the subject\nof intense study, aiming to realize high-capacity optical storage and\ncontinuous-variable quantum technologies. Here, we present a scheme to extract\nthe orbital angular momentum content of Laguerre-Gaussian light beams in a\ndouble-$\\Lambda$ four level system of GaAs/AlGaAs multiple quantum wells.\nArising from a quantum interference term, absorption of a non-vortex probe\nfield depends upon the azimuthal phase of vortex fields so that both magnitude\nand sign of the azimuthal index/indices can be mapped into the absorption\nprofile.",
        "category": "physics_optics"
    },
    {
        "text": "Photonic crystal fibres: mapping Maxwell's equations onto a Schrodinger\n  equation eigenvalue problem: We consider photonic crystal fibres (PCFs) made from arbitrary base materials\nand introduce a short-wavelength approximation which allows for a mapping of\nthe Maxwell's equations onto a dimensionless eigenvalue equations which has the\nform of the Schrodinger equation in quantum mechanics. The mapping allows for\nan entire analytical solution of the dispersion problem which is in qualitative\nagreement with plane-wave simulations of the Maxwell's equations for large-mode\narea PCFs. We offer a new angle on the foundation of the endlessly single-mode\nproperty and show that PCFs are endlessly single mode for a normalized air-hole\ndiameter smaller than ~0.42, independently of the base material. Finally, we\nshow how the group-velocity dispersion relates simply to the geometry of the\nphotonic crystal cladding.",
        "category": "physics_optics"
    },
    {
        "text": "Sagnac Interferometer Enhanced Particle Tracking in Optical Tweezers: A setup is proposed to enhance tracking of very small particles, by using\noptical tweezers embedded within a Sagnac interferometer. The achievable\nsignal-to-noise ratio is shown to be enhanced over that for a standard optical\ntweezers setup. The enhancement factor increases asymptotically as the\ninterferometer visibility approaches 100%, but is capped at a maximum given by\nthe ratio of the trapping field intensity to the detector saturation threshold.\nFor an achievable visibility of 99%, the signal-to-noise ratio is enhanced by a\nfactor of 200, and the minimum trackable particle size is 2.4 times smaller\nthan without the interferometer.",
        "category": "physics_optics"
    },
    {
        "text": "Self-organization, Pattern Formation, Cavity Solitons and Rogue Waves in\n  Singly Resonant Optical Parametric Oscillators: Spatio-temporal dynamics of singly resonant optical parametric oscillators\nwith external seeding displays hexagonal, roll and honeycomb patterns, optical\nturbulence, rogue waves and cavity solitons. We derive appropriate mean-field\nequations with a sinc$^2$ nonlinearity and demonstrate that off-resonance\nseeding is necessary and responsible for the formation of complex spatial\nstructures via self-organization. We compare this model with those derived\nclose to the threshold of signal generation and find that back-conversion of\nsignal and idler photons is responsible for multiple regions of spatio-temporal\nself-organization when increasing the power of the pump field.",
        "category": "physics_optics"
    },
    {
        "text": "Wide bandwidth phase-locked diode laser with an intra-cavity\n  electro-optic modulator: Two extended cavity laser diodes are phase-locked, thanks to an intra-cavity\nelectro-optical modulator. The phase-locked loop bandwidth is on the order of\n10 MHz, which is about twice larger than when the feedback correction is\napplied on the laser current. The phase noise reaches -120 dBrad$^2$/Hz at 10\nkHz. This new scheme reduces the residual laser phase noise, which constitutes\none of the dominant contributions in the sensitivity limit of atom\ninterferometers using two-photon transitions.",
        "category": "physics_optics"
    },
    {
        "text": "Octave Spanning Frequency Comb on a Chip: Optical frequency combs have revolutionized the field of frequency metrology\nwithin the last decade and have become enabling tools for atomic clocks, gas\nsensing and astrophysical spectrometer calibration. The rapidly increasing\nnumber of applications has heightened interest in more compact comb generators.\nOptical microresonator based comb generators bear promise in this regard.\nCritical to their future use as 'frequency markers', is however the absolute\nfrequency stabilization of the optical comb spectrum. A powerful technique for\nthis stabilization is self-referencing, which requires a spectrum that spans a\nfull octave, i.e. a factor of two in frequency. In the case of mode locked\nlasers, overcoming the limited bandwidth has become possible only with the\nadvent of photonic crystal fibres for supercontinuum generation. Here, we\nreport for the first time the generation of an octave-spanning frequency comb\ndirectly from a toroidal microresonator on a silicon chip. The comb spectrum\ncovers the wavelength range from 990 nm to 2170 nm and is retrieved from a\ncontinuous wave laser interacting with the modes of an ultra high Q\nmicroresonator, without relying on external broadening. Full tunability of the\ngenerated frequency comb over a bandwidth exceeding an entire free spectral\nrange is demonstrated. This allows positioning of a frequency comb mode to any\ndesired frequency within the comb bandwidth. The ability to derive octave\nspanning spectra from microresonator comb generators represents a key step\ntowards achieving a radio-frequency to optical link on a chip, which could\nunify the fields of metrology with micro- and nano-photonics and enable\nentirely new devices that bring frequency metrology into a chip scale setting\nfor compact applications such as space based optical clocks.",
        "category": "physics_optics"
    },
    {
        "text": "Analytical mode normalization and resonant state expansion for optical\n  fibers - an efficient tool to model transverse disorder: We adapt the resonant state expansion to optical fibers such as capillary and\nphotonic crystal fibers. As a key requirement of the resonant state expansion\nand any related perturbative approach, we derive the correct analytical\nnormalization for all modes of these fiber structures, including leaky modes\nthat radiate energy perpendicular to the direction of propagation and have\nfields that grow with distance from the fiber core. Based on the normalized\nfiber modes, an eigenvalue equation is derived that allows for calculating the\ninfluence of small and large perturbations such as structural disorder on the\nguiding properties. This is demonstrated for two test systems: a capillary\nfiber and an endlessly single mode fiber.",
        "category": "physics_optics"
    },
    {
        "text": "Subwavelength metal grating metamaterial for polarization selective\n  optical antireflection coating: A metamaterial structure consisting of a one-dimensional metal/air-gap\nsubwavelength grating is investigated for optical antireflection coating on\ngermanium substrate in the infrared regime. For incident light polarized\nperpendicularly to the grating lines, the metamaterial exhibits effective\ndielectric property and Fabry-Perot like plasmon-coupled optical resonance\nresults in complete elimination of reflection and enhancement of transmission.\nIt is found that the subwavelength grating metamaterial antireflection\nstructure does not require a deep subwavelength grating period, which is\nadvantageous for device fabrication. Maximal transmittance of 93.4% with\ncomplete elimination of reflection is seen in the mid-wave infrared range.",
        "category": "physics_optics"
    },
    {
        "text": "Digital holographic microscopy for the evaluation of human sperm\n  structure: The morphology of the sperm head has often been correlated with the outcome\nof in vitro fertilization (IVF), and has been shown to be the sole parameter in\nsemen of value in predicting the success of intracytoplasmic sperm injection\n(ICSI) and intracytoplasmic morphologically selected sperm injection (IMSI). In\nthis paper, we have studied whether Digital Holographic (DH) microscopy may be\nuseful to obtain quantitative data on human sperm head structure and compared\nthis technique to high power digitally enhanced Nomarski microscope. The main\nadvantage of DH is that a high resolution 3-D quantitative sample imaging may\nbe obtained thorugh numerical refocusing at different object planes without any\nmechanical scanning. We show that DH can furnish useful information on the\ndimensions and structure of human spermatozoo, that cannot be revealed by\nconventional phase contrast microscopy. In fact, in this paper DH has been used\nto evaluate volume and indicate precise location of vacuoles, thus suggesting\nits use as an additional useful prognostic quantitative tool in assisted\nreproduction technology (ART).",
        "category": "physics_optics"
    },
    {
        "text": "Deterministic chaos in an ytterbium-doped mode-locked fiber laser: We experimentally study the nonlinear dynamics of a femtosecond ytterbium\ndoped mode-locked fiber laser. With the laser operating in the pulsed regime a\nroute to chaos is presented, starting from stable mode-locking, period two,\nperiod four, chaos and period three regimes. Return maps and bifurcation\ndiagrams were extracted from time series for each regime. The analysis of the\ntime series with the laser operating in the quasi mode-locked regime presents\ndeterministic chaos described by an unidimensional Rossler map. A positive\nLyapunov exponent $\\lambda = 0.14$ confirms the deterministic chaos of the\nsystem. We suggest an explanation about the observed map by relating gain\nsaturation and intra-cavity loss.",
        "category": "physics_optics"
    },
    {
        "text": "Light Reconfigurable Geometric Phase Optical Element with Multi-stable\n  States: We present the design methodology of a light reconfigurable geometric phase\noptical element with multi-stable diffraction efficiency states, enabled by a\nphotoresponsive self-organized chiral liquid crystal. Experimental\ndemonstration shows the device exhibits a broad diffraction efficiency tunable\nrange that can be smoothly modulated under alternate stimulation of ultraviolet\nand green lights. Distinctive to previous designs, the regulation of\ndiffraction efficiency fundamentally stems from the modulation of geometric\nphase together with dynamical phase retardation, and any intermediate\ndiffractive state is memorized. Such multi-stability facilitates applications\nincluding energy-saving all-optical signal processing in classical and quantum\nlevel, and phase hologram for anti-counterfeit.",
        "category": "physics_optics"
    },
    {
        "text": "Continuous 40 GW/cm$^2$ laser intensity in a near-concentric optical\n  cavity: Manipulating free-space electron wave functions with laser fields can bring\nabout new electron-optical elements for transmission electron microscopy. In\nparticular, a Zernike phase plate would enable high-contrast imaging of soft\nmatter, leading to new opportunities in structural biology and materials\nscience. A Zernike plate can be implemented using a tight, intense CW laser\nfocus that shifts the phase of the electron wave by the ponderomotive\npotential. Here, we use a near-concentric cavity to focus 7.5 kW of circulating\nlaser power at 1064 nm into a 7 $\\mu$m waist, setting a record for CW laser\nintensity and establishing a pathway to ponderomotive phase contrast TEM.",
        "category": "physics_optics"
    },
    {
        "text": "Universal imprinting of chirality with chiral light by employing\n  plasmonic metastructures: Chirality, either of light or matter, has proved to be very practical in\nbiosensing and nanophotonics. However, the fundamental understanding of its\ntemporal dynamics still needs to be discovered. A realistic setup for this are\nthe so-called metastructures, since they are optically active and are built\nmassively, hence rendering an immediate potential candidate. Here we propose\nand study the electromagnetic-optical mechanism leading to chiral optical\nimprinting on metastructures. Induced photothermal responses create anisotropic\npermittivity modulations, different for left or right circularly polarized\nlight, leading to temporal-dependent chiral imprinting of hot-spots, namely\nimprinting of chirality. The above effect has not been observed yet, but it is\nwithin reach of modern experimental approaches. The proposed nonlinear\nchiroptical effect is general and should appear in any anisotropic material;\nhowever, we need to design a particular geometry for this effect to be strong.\nThese new chiral time-dependent metastructures may lead to a plethora of\napplications.",
        "category": "physics_optics"
    },
    {
        "text": "Lateral shift of the transmitted light beam through a left-handed slab: It is reported that when a light beam travels through a slab of left-handed\nmedium in the air, the lateral shift of the transmitted beam can be negative as\nwell as positive. The necessary condition for the lateral shift to be positive\nis given. The validity of the stationary-phase approach is demonstrated by\nnumerical simulations for a Gaussian-shaped beam. A restriction to the slab's\nthickness is provided that is necessary for the beam to retain its profile in\nthe traveling. It is shown that the lateral shift of the reflected beam is\nequal to that of the transmitted beam in the symmetric configuration.",
        "category": "physics_optics"
    },
    {
        "text": "Photorefractive writing and probing of anisotropic linear and non-linear\n  lattices: We study experimentally the writing of one- and two-dimensional\nphotorefractive lattices, focusing on the often overlooked transient regime.\nOur measurements agree well with theory, in particular concerning the ratio of\nthe drift to diffusion terms. We then study the transverse dynamics of coherent\nwaves propagating in the lattices, in a few novel and simple configurations.\nFor focused linear waves with broad transverse spectrum, we remark that both\nthe intensity distributions in real space (\"discrete diffraction\") and Fourier\nspace (\"Brillouin zone spectroscopy\") reflect the Bragg planes and band\nstructure. For non-linear waves, we observe modulational instability and\ndiscrete solitons formation in time domain. We discuss also the non-ideal\neffects inherent to the photo-induction technique : anisotropy, residual\nnonlinearity, diffusive term, non-stationarity.",
        "category": "physics_optics"
    },
    {
        "text": "An Upper Bound on the Rate of Information Transfer in Optical Vortex\n  Beams: Light endowed with orbital angular momentum, commonly termed optical vortex\nlight, has an azimuthal phase indexed by the orbital quantum number $l$. In\ncontrast to the two basis states of the optical spin angular momentum, the\ninterest in the information content of optical vortex beams is centred on the\nassumption that $\\lvert{l}\\rangle{}$ forms a countably infinite set of basis\nstates. The recent experimental observation that group velocity is inversely\nproportional to $l$ provides a theoretical basis for a practical measure of\ninformation transfer. This Letter sets an upper bound on that measure.",
        "category": "physics_optics"
    },
    {
        "text": "Propagation of Partially Coherent Light in non-Hermitian Lattices: Band theory for partially coherent light is introduced by using the formalism\nof second-order classical coherence theory under paraxial approximation. It is\ndemonstrated that the cross-spectral density function, describing correlations\nbetween pairs of points in the field, can have bands and gaps and form a\ncorrelation band structure. The propagation of a partially coherent beam in\nnon-Hermitian periodic structures is considered to elucidate the interplay\nbetween the degree of coherence and the gain/loss present in the lattice. We\napply the formalism to study partially coherent Bloch oscillations in lattices\nhaving parity-time symmetry and demonstrate that the oscillations can be\nsustained in such media but they are strongly dependent upon the spatial\ncorrelations of the beam. A transition between breathing and oscillating modes\nis shown to be induced by the degree of spatial coherence.",
        "category": "physics_optics"
    },
    {
        "text": "Mode-selective Single-dipole Excitation and Controlled Routing of Guided\n  Waves in a Multi-mode Topological Waveguide: Topology-linked binary degrees of freedom of guided waves have been used to\nexpand the channel capacity of and to ensure robust transmission through\nphotonic waveguides. However, selectively exciting optical modes associated\nwith the desired degree of freedom is challenging and typically requires\nspatially extended sources or filters. Both approaches are incompatible with\nthe ultimate objective of developing compact mode-selective sources powered by\nsingle emitters. In addition, the implementation of highly desirable\nfunctionalities, such as controllable distribution of guided modes between\nmultiple detectors, becomes challenging in highly-compact devices due to photon\nloss to reflections. Here, we demonstrate that a linearly-polarized dipole-like\nsource can selectively excite a topologically robust edge mode with the desired\nvalley degree of freedom. Reflection-free routing of valley-polarized edge\nmodes into two spatially-separated detectors with reconfigurable splitting\nratios is also presented. An optical implementation of such a source will have\nthe potential to broaden the applications of topological photonic devices.",
        "category": "physics_optics"
    },
    {
        "text": "Randomly spaced phase-only transmission combs for femtosecond pulse\n  shaping: We present a new Randomized Multiple Independent Comb Shaping (RandoMICS)\nalgorithm based on phase-only tailored transmission for ultrashort laser pulse\nreplication. The benefit of this method is satellite-free generation of\nprogrammable laser pulse sequences. The result is achieved by creating a\ntransmission function as a stochastic comb of disjoint segments of optical\nfrequency continuum with numerically optimized segment width distribution. The\nalgorithm is realized by generating a regular aperiodic comb and random\npermutations of its elements. Experimental demonstration is performed with an\nacousto-optic pulse shaper providing broadband multi-window transmission\nfunction with arbitrarily variable widths of the segments. Suppression of\nundesired satellite pulses by the factor of 8 is demonstrated as well as\ngenerating pulse replicas with extended usable delay range compared to\nphase-only pulse shaping with periodic transmission combs.",
        "category": "physics_optics"
    },
    {
        "text": "Nonclassical light in coupled optical systems: anomalous power\n  distribution, Fock space dynamics and supersymmetry: We investigate the dynamics of nonclassical states of light in coupled\noptical structures and we demonstrate a number of intriguing features\nassociated with such arrangements. By diagonalizing the system's Hamiltonian,\nwe show that these geometries can support eigenstates having anomalous optical\nintensity distribution with no classical counterpart. These features may\nprovide new avenues towards manipulating light flow at the quantum level. By\nprojecting the Hamiltonian operator on Hilbert subspaces spanning different\nnumbers of photon excitations, we demonstrate that processes such as coherent\ntransport, state localization and surface Bloch oscillations can take place in\nFock space. Furthermore, we show that Hamiltonian representations of Fock space\nmanifolds differing by one photon obey a discrete supersymmetry relation",
        "category": "physics_optics"
    },
    {
        "text": "Dynamics of particles trapped by dissipative domain walls: In this Letter we study the interactions of the dissipative domain walls with\ndielectric particles. It is shown that particles can be steadily trapped by the\nmoving domain walls. The influence of the ratchet effect on particle trapping\nis considered. It is demonstrated, that the ratchet effect allows to obtain\nhigh accuracy in particle manipulation.",
        "category": "physics_optics"
    },
    {
        "text": "Theoretical description of optofluidic force induction: Optofluidic force induction (OF2i) is an optical nanoparticle\ncharacterization scheme which achieves real-time optical counting with\nsingle-particle sensitivity and high throughput. In a recent paper [\\v{S}imi\\'c\net al., Phys. Rev. Appl. 18, 024056 (2022)], we have demonstrated the working\nprinciple for standardized polystrene nanoparticles, and have developed a\ntheoretical model to analyze the experimental data. In this paper we give a\ndetailed account of the model ingredients including the full working equations,\nprovide additional justification for the assumptions underlying OF2i, and\ndiscuss directions for further developments and future research.",
        "category": "physics_optics"
    },
    {
        "text": "All-Optical Arithmetic and Combinatorial Logic Circuits with High-Q\n  Bacteriorhodopsin Coated Microcavities: We present designs of all-optical computing circuits, namely, half-full\nadder/subtractor, de-multiplexer, multiplexer, and an arithmetic unit, based on\nbacteriorhodopsin (BR) protein coated microcavity switch in a tree\narchitecture. The basic all-optical switch consists of an input infrared (IR)\nlaser beam at 1310 nm in a single mode fiber (SMF-28) switched by a control\npulsed laser beam at 532 nm, which triggers the change in the resonance\ncondition on a silica bead coated with BR between two tapered fibers. We show\nthat fast switching of 50 us can be achieved by injecting a blue laser beam at\n410 nm that helps in truncating the BR photocycle at the M intermediate state.\nRealization of all-optical switch with BR coated microcavity switch has been\ndone experimentally. Based on this basic switch configuration, designs of\nall-optical higher computing circuits have been presented. The design requires\n2n-1 switches to realize n bit computation. The proposed designs require less\nnumber of switches than terahertz optical asymmetric demultiplexer based\ninterferometer designs. The combined advantages of high Q factor, tunability,\ncompactness and low power control signals, with the flexibility of cascading\nswitches to form circuits, makes the designs promising for practical\napplications. The design combines the exceptional sensitivities of BR and\nmicrocavities for realizing low power circuits and networks. The designs are\ngeneral and can be implemented (i) in both fiber-optic and integrated optic\nformats, (ii) with any other coated photosensitive material, or (iii) an\nexternally controlled microresonator switch.",
        "category": "physics_optics"
    },
    {
        "text": "Probability Density Function of Kerr Effect Phase Noise: The probability density function of Kerr effect phase noise, often called the\nGordon-Mollenauer effect, is derived analytically. The Kerr effect phase noise\ncan be accurately modeled as the summation of a Gaussian random variable and a\nnoncentral chi-square random variable with two degrees of freedom. Using the\nreceived intensity to correct for the phase noise, the residual Kerr effect\nphase noise can be modeled as the summation of a Gaussian random variable and\nthe difference of two noncentral chi-square random variables with two degrees\nof freedom. The residual phase noise can be approximated by Gaussian\ndistribution better than the Kerr effect phase noise without correction.",
        "category": "physics_optics"
    },
    {
        "text": "Whispering-gallery-mode based CH3NH3PbBr3 perovskite microrod lasers\n  with high quality factors: Lead halide perovskite based micro- and nano- lasers have been widely studied\nin past two years. Due to their long carrier diffusion length and high external\nquantum efficiency, lead halide perovskites have been considered to have bright\nfuture in optoelectronic devices, especially in the \"green gap\" wavelength\nregion. However, the quality (Q) factors of perovskite lasers are unspectacular\ncompared to conventional microdisk lasers. The record value of full width at\nhalf maximum (FWHM) at threshold is still around 0.22 nm. Herein we synthesized\nsolution-processed, single-crystalline CH3NH3PbBr3 perovskite microrods and\nstudied their lasing actions. In contrast to entirely pumping a microrod on\nsubstrate, we partially excited the microrods that were hanging in the air.\nConsequently, single-mode or few-mode laser emissions have been successfully\nobtained from the whispering-gallery like diamond modes, which are confined by\ntotal internal reflection within the transverse plane. Owning to the better\nlight confinement and high crystal quality, the FWHM at threshold have been\nsignificantly improved. The smallest FWHM at threshold is around 0.1 nm, giving\na Q factor over 5000.",
        "category": "physics_optics"
    },
    {
        "text": "Transformational Plasmon Optics: Transformation optics has recently attracted extensive interest, since it\nprovides a novel design methodology for manipulating light at will. Although\ntransformation optics in principle embraces all forms of electromagnetic\nphenomena on all length scales, so far, much less efforts have been devoted to\nnear-field optical waves, such as surface plasmon polaritons (SPPs). Due to the\ntight confinement and strong field enhancement, SPPs are widely used for\nvarious purposes at the subwavelength scale. Taking advantage of transformation\noptics, here we demonstrate that the confinement as well as propagation of SPPs\ncan be managed in a prescribed manner by careful control of the dielectric\nmaterial properties adjacent to a metal. Since the metal properties are\ncompletely unaltered, it provides a straightforward way for practical\nrealizations. We show that our approach can assist to tightly bound SPPs over a\nbroad wavelength band at uneven and curved surfaces, where SPPs would normally\nsuffer significant scattering losses. In addition, a plasmonic waveguide bend\nand a plasmonic Luneburg lens with practical designs are proposed. It is\nexpected that merging the unprecedented design flexibility based on\ntransformation optics with the unique optical properties of surface modes will\nlead to a host of fascinating near-field optical phenomena and devices.",
        "category": "physics_optics"
    },
    {
        "text": "Electrical Tuning of Phase Change Antennas and Metasurfaces: The success of semiconductor electronics is built on the creation of compact,\nlow-power switching elements that offer routing, logic, and memory functions.\nThe availability of nanoscale optical switches could have a similarly\ntransformative impact on the development of dynamic and programmable\nmetasurfaces, optical neural networks, and quantum information processing.\nPhase change materials are uniquely suited to enable their creation as they\noffer high-speed electrical switching between amorphous and crystalline states\nwith notably different optical properties. Their high refractive index has also\nbeen harnessed to fashion them into compact optical antennas. Here, we take the\nnext important step by realizing electrically-switchable phase change antennas\nand metasurfaces that offer strong, reversible, non-volatile, multi-phase\nswitching and spectral tuning of light scattering in the visible and\nnear-infrared spectral ranges. Their successful implementation relies on a\ncareful joint thermal and optical optimization of the antenna elements that\ncomprise an Ag strip that simultaneously serves as a plasmonic resonator and a\nminiature heating stage.",
        "category": "physics_optics"
    },
    {
        "text": "One more time on the helicity decomposition of spin and orbital optical\n  currents: The helicity representation of the linear momentum density of a light wave is\nwell understood for monochromatic optical fields in both paraxial and\nnon-paraxial regimes of propagation. In this note we generalize such\nrepresentation to nonmonochromatic optical fields. We find that, differently\nfrom the monochromatic case, the linear momentum density, aka the Poynting\nvector divided by $c^2$, does not separate into the sum of right-handed and\nleft-handed terms, even when the so-called electric-magnetic democracy in\nenforced by averaging the electric and magnetic contributions. However, for\nquasimonochromatic light, such a separation is approximately restored after\ntime-averaging. This paper is dedicated to Sir Michael Berry on the occasion of\nhis $80$th birthday.",
        "category": "physics_optics"
    },
    {
        "text": "Atomic layer deposited second order nonlinear optical metamaterial for\n  back-end integration with CMOS-compatible nanophotonic circuitry: We report the fabrication of artificial unidimensional crystals exhibiting an\neffective bulk second-order nonlinearity. The crystals are created by cycling\natomic layer deposition of three dielectric materials such that the resulting\nmetamaterial is non-centrosymmetric in the direction of the deposition.\nCharacterization of the structures by second-harmonic generation Maker-fringe\nmeasurements shows that the main component of their nonlinear susceptibility\ntensor is about 5 pm/V which is comparable to well-established materials and\nmore than an order of magnitude greater than reported for a similar crystal\n[1-Alloatti et al, arXiv:1504.00101[cond-mat.mtrl- sci]]. Our demonstration\nopens new possibilities for second-order nonlinear effects on CMOS-compatible\nnanophotonic platforms.",
        "category": "physics_optics"
    },
    {
        "text": "Angular Momentum of Twisted Radiation from an Electron in Spiral Motion: We theoretically demonstrate for the first time that a single free electron\nin circular/spiral motion emits twisted photons carrying well defined orbital\nangular momentum along the axis of the electron circulation, in adding to spin\nangular momentum. We show that, when the electron velocity is relativistic, the\nradiation field contains harmonic components and the photons of l-th harmonic\ncarry lhbar total angular momentum for each. This work indicates that twisted\nphotons are naturally emitted by free electrons and more ubiquitous in\nlaboratories and in nature than ever been thought.",
        "category": "physics_optics"
    },
    {
        "text": "Tunable Plasmonic Ultrastrong Coupling: Emulating Dicke Physics at Room\n  Temperature: A system of N two-level atoms cooperatively interacting with a photonic field\ncan be described as a single giant atom coupled to the field with interaction\nstrength ~N^0.5. This enhancement, known as Dicke cooperativity in quantum\noptics, has recently become an indispensable element in quantum information\ntechnology based on strong light-matter coupling. Here, we extend the coupling\nbeyond the standard light-matter interaction paradigm, emulating Dicke\ncooperativity in a terahertz metasurface with N meta-atoms. Cooperative\nenhancement manifested in the form of matter-matter coupling, through the\nhybridization of localized surface plasmon resonance in individual meta-atoms\nand surface lattice resonance due to the periodic array of the meta-atoms. By\nvarying the lattice constant of the array, we observe a clear anticrossing\nbehavior, a signature of strong coupling. Furthermore, through engineering of\nthe capacitive split-gap in the meta-atoms, the coupling rate was cooperatively\nenhanced into the ultrastrong coupling regime by a factor of N^0.5. This\nroom-temperature technology serves as a convenient quantum emulator of the\ndynamics of a qubit with a giant dipole moment coherently driven by a single\nbosonic field.",
        "category": "physics_optics"
    },
    {
        "text": "Accurate Single-Ended Measurement of Propagation Delay in Fiber Using\n  Correlation Optical Time Domain Reflectometry: A correlation optical time-domain reflectometry (COTDR) method is presented,\nwhich measures the propagation delay with an accuracy of a few picoseconds.\nThis accuracy is achieved using a test signal data rate of 10 Gbit/s and\nemploying cross-correlation and pulse fitting techniques. In this paper we\nintroduce and evaluate the basic signal processing steps, investigate the\nmeasurement accuracy, and discuss applications for monitoring link delay and\nchromatic dispersion of long fiber spans as well as temperature sensing\napplications.",
        "category": "physics_optics"
    },
    {
        "text": "Optical Neural Network Based on Synthetic Nonlinear Photonic Lattices: We reveal that a synthetic photonic lattice based on coupled optical loops\ncan be utilized as a neural network for processing of optical pulse sequences\nin time domain. As a proof-of-concept, we train the optical system to restore\nan initial shape of the pulse train from the signal distorted due to linear\ndispersion in a fiber-optic link. We also show efficient training of the\noptical network with an intrinsic Kerr-type nonlinearity for the realization of\ntarget nonlinear transmission functions and inference functionality for the\ndiscrimination of different pulse sequences. The theoretical modeling is\nperformed under practical conditions and can guide future experimental\nrealizations.",
        "category": "physics_optics"
    },
    {
        "text": "Parameterized Learning and Distillation with Vortex-encoded Spectral\n  Correlations: Spectral computational methods leverage modal or nonlocal representations of\ndata, and a physically realized approach to spectral computation pertains to\nencoded diffraction. Encoded diffraction offers a hybrid approach that pairs\nanalog wave propagation with digital back-end electronics, however the\nintermediate sensor patterns are correlations rather than linear signal\nweights, which limits the development of robust and efficient downstream\nanalyses. Here, with vortex encoders, we show that the solution for the signal\nfield from sensor intensity adopts the form of polynomial regression, which is\nsubsequently solved with a learned, linear transformation. This result\nestablishes an analytic rationale for a spectral-methods paradigm in physically\nrealized machine learning systems. To demonstrate this paradigm, we quantify\nthe learning that is transferred with an image basis using speckle parameters,\nSingular-Value Decomposition Entropy ($H_{SVD}$) and Speckle-Analogue Density\n(SAD). We show that $H_{SVD}$, a proxy for image complexity, indicates the rate\nat which a model converges. Similarly, SAD, an averaged spatial frequency,\nmarks a threshold for structurally similar reconstruction. With a vortex\nencoder, this approach with parameterized training may be extended to distill\nfeatures. In fact, with images reconstructed with our models, we achieve\nclassification accuracies that rival decade-old, state-of-the-art computer\nalgorithms. This means that the process of learning compressed spectral\ncorrelations distills features to aid image classification, even when the goal\nimages are feature-agnostic speckles. Our work highlights opportunities for\nanalytic and axiom-driven machine-learning designs appropriate for real-time\napplications.",
        "category": "physics_optics"
    },
    {
        "text": "Control of light transmission through opaque scattering media in space\n  and time: We report the first experimental demonstration of combined spatial and\ntemporal control of light trajectories through opaque media. This control is\nachieved by solely manipulating spatial degrees of freedom of the incident\nwavefront. As an application, we demonstrate that the present approach is\ncapable to form bandwidth-limited ultrashort pulses from the otherwise randomly\ntransmitted light with a controllable interaction time of the pulses with the\nmedium. Our approach provides a new tool for fundamental studies of light\npropagation in complex media and has potential for applications for coherent\ncontrol, sensing and imaging in nano- and biophotonics.",
        "category": "physics_optics"
    },
    {
        "text": "Modulation instability induced by periodic power variation in soliton\n  fiber ring lasers: Modulation instability with subsideband generation induced by periodic power\nvariation in soliton fiber ring lasers is reported. We found that different\nwavelength shifts of subsideband generation are related to different periodic\npower variation. The period of power variation and wavelength shifts of\nsubsideband can be changed by altering the linear cavity phase delay. It is\nalso found that the periodic power variation is caused by the interaction\nbetween the nonuniform polarization state of the circulating light and the\npolarizer in the laser cavity.",
        "category": "physics_optics"
    },
    {
        "text": "Non-invasive dynamic or wide-field imaging through opaque layers and\n  around corners: In turbid media, scattering of light scrambles information of the incident\nbeam and represents an obstacle to optical imaging. Noninvasive imaging through\nopaque layers is challenging for dynamic and wide-field objects due to\nunreliable image reconstruction processes. We here propose a new perspective to\nsolve these problems: rather than using the full point-spread-function (PSF),\nthe wave distortions in scattering layers can be characterized with only the\nphase of the optical-transfer-function (OTF, the Fourier transform of PSF),\nwith which diffraction-limit images can be analytically solved. We then develop\na method that exploits the redundant information dynamic objects, and can\nreliably and rapidly recover OTFs' phases within several iterations. It enables\nnot only noninvasive video imaging at 25 ~ 200 Hz of a moving object hidden\ninside turbid media, but also imaging under weak illumination that is\ninaccessible with previous methods. Furthermore, by scanning a localized\nillumination on the object plane, we propose a wide-field imaging approach,\nwith which we demonstrate an application where a photoluminescent sample hidden\nbehind four-layers of opaque polythene films is imaged with a modified\nmulti-photon excitation microscopy setup.",
        "category": "physics_optics"
    },
    {
        "text": "Two-temperature relaxation and melting after absorption of femtosecond\n  laser pulse: The theory and experiments concerned with the electron-ion thermal relaxation\nand melting of overheated crystal lattice constitute the subject of this paper.\nThe physical model includes two-temperature equation of state, many-body\ninteratomic potential, the electron-ion energy exchange, electron thermal\nconductivity, and optical properties of solid, liquid, and two phase\nsolid-liquid mixture. Two-temperature hydrodynamics and molecular dynamics\ncodes are used. An experimental setup with pump-probe technique is used to\nfollow evolution of an irradiated target with a short time step 100 fs between\nthe probe femtosecond laser pulses. Accuracy of measurements of reflection\ncoefficient and phase of reflected probe light are ~1% and $\\sim 1\\un{nm}$,\nrespectively. It is found that,\n  {\\it firstly}, the electron-electron collisions make a minor contribution to\na light absorbtion in solid Al at moderate intensities;\n  {\\it secondly}, the phase shift of a reflected probe results from heating of\nion subsystem and kinetics of melting of Al crystal during $0<t<4\\un{ps},$\nwhere $t$ is time delay between the pump and probe pulses measured from the\nmaximum of the pump;\n  {\\it thirdly} the optical response of Au to a pump shows a marked contrast to\nthat of Al on account of excitation of \\textit{d}-electrons",
        "category": "physics_optics"
    },
    {
        "text": "Optical detection of nano-particle characteristics using coupling to a\n  nano-waveguide: Recently, much research concerning the combination of nano-scale waveguides\nwith nano-crystals and other nano-particles has been reported, because of\npossible applications in the field of quantum information and communication.\nThe most useful and convenient method to verify the nature of such systems is\noptical detection. However, due to the diffraction limit, optical\nidentification of characteristics such as particle type, particle position, etc\nis difficult or impossible. However, if such particles are placed on a\nwaveguide, the coupling of scattered light to the waveguide guided modes can\nreveal the information about the particles. Here we consider how illumination\nwith light of arbitrary polarization can reveal the difference between\nisotropic and non-isotropic nano-particles placed on the surface of an optical\nnanofiber. Specifically, we measure the polarization response function of gold\nnano-rods (GNRs) on an optical nanofiber surface and show that it is\nqualitatively different to that for gold nano-spheres (GNSs). This experimental\ntechnique provides a simple new tool for the optical characterization of hybrid\nnano-optical devices.",
        "category": "physics_optics"
    },
    {
        "text": "Negative refraction with little loss manipulated by the voltage and\n  pulsed laser in double quantum dots: The paper demonstrates that negative refractive index can be achieved via\ntuning the tunneling rate between a double quantum dots(QDs) system by applying\na bias voltage, and a pulsed laser. As the bias voltage being changed, the\nrefraction index can be tunable to negative with the simultaneous negative\npermittivity and permeability. While the varying pulsed laser is applied to the\ndouble QDs system, moreover, the negative refractive index with little loss can\nbe obtained. The flexible manipulation on a solid state system to realize\nnegative refraction may give a new way for experimental research in the future.",
        "category": "physics_optics"
    },
    {
        "text": "Measurement of two independent phase-shifts using coupled parametric\n  amplifiers: In this article, we demonstrate a scheme capable of two-phase measurement,\ni.e. the simultaneous measurement of the two phase-shifts occurring in two\nindependent Mach-Zehnder interferometers using one intensity detector. Our\nscheme utilizes dark-state-enhanced coupled parametric amplifiers in an atomic\nmedium to mix the multiple fields probing the various arms of the\ninterferometers in parallel. The two phase-differences are then encoded in\nseparate continuous-variable parameters in the spectral waveform of the\nparametrically amplified atom-radiated signal field, which can be directly\ndecoupled in a single intensity measurement. Besides resolving two phase\ndifferences in parallel, this method can also be used to increase the channel\ncapacity in optical and quantum communication by the simultaneous use of\nphase-modulation and amplitude-modulation.",
        "category": "physics_optics"
    },
    {
        "text": "Beam shifts and distribution functions: When a beam of light is reflected by a smooth surface its behavior deviates\nfrom geometrical optics predictions. Such deviations are quantified by the\nso-called spatial and angular Goos-Haenchen (GH) and Imbert-Fedorov (IF) shifts\nof the reflected beam. These shifts depend upon the shape of the incident beam,\nits polarization and on the material composition of the reflecting surface. In\nthis article we suggest a novel approach that allows one to unambiguously\nisolate the beam-shape dependent aspects of GH and IF shifts. We show that this\nseparation is possible as a result of some universal features of shifted\ndistribution functions which are presented and discussed.",
        "category": "physics_optics"
    },
    {
        "text": "A general framework for scintillation in nanophotonics: Bombardment of materials by high-energy particles (e.g., electrons, nuclei,\nX- and $\\gamma$-ray photons) often leads to light emission, known generally as\nscintillation. Scintillation is ubiquitous and enjoys widespread applications\nin many areas such as medical imaging, X-ray non-destructive inspection, night\nvision, electron microscopy, and high-energy particle detectors. A large body\nof research focuses on finding new materials optimized for brighter, faster,\nand more controlled scintillation. Here, we develop a fundamentally different\napproach based on integrating nanophotonic structures into scintillators to\nenhance their emission. To start, we develop a unified and ab initio theory of\nnanophotonic scintillators that accounts for the key aspects of scintillation:\nthe energy loss by high-energy particles, as well as the light emission by\nnon-equilibrium electrons in arbitrary nanostructured optical systems. This\ntheoretical framework allows us, for the first time, to experimentally\ndemonstrate nearly an order-of-magnitude enhancement of scintillation, in both\nelectron-induced, and X-ray-induced scintillation. Our theory also allows the\ndiscovery of structures that could eventually achieve several\norders-of-magnitude scintillation enhancement. The framework and results shown\nhere should enable the development of a new class of brighter, faster, and\nhigher-resolution scintillators with tailored and optimized performances - with\nmany potential applications where scintillators are used.",
        "category": "physics_optics"
    },
    {
        "text": "Higher order multipoles in metamaterial homogenization: The higher order multipoles above the electric quadrupole are commonly\nneglected in metamaterial homogenization. We show that they nevertheless can be\nsignificant when second order spatial dispersive effects, such as the magnetic\nresponse, are considered. In this respect, they can be equally important as the\nmagnetization and quadrupole terms, and should not automatically be neglected.",
        "category": "physics_optics"
    },
    {
        "text": "Extrinsic spin angular momentum carried by plasmonic vortex in terahertz\n  cylindrical corrugated waveguide: An electromagnetic surface wave carrying orbital angular momentum (AM) is\ncalled plasmonic vortex that also carries spin AM originating from evanescent\nfield. In this study, spin and orbital AM of plasmonic vortex in a terahertz\ncylindrical corrugated waveguide (CCW) are analyzed. We report the extrinsic\nspin AM of the plasmonic vortex for the first time. The extrinsic spin AM is\nproduced by the rotation of the polarization of the plasmonic vortex whose\nrotation axis is positioned at the vicinity of the CCW wall. The extrinsic spin\nAM depends on the intrinsic orbital AM and radius of CCW. The plasmonic vortex\nwith sufficiently large orbital AM carries large spin AM per photon. This large\nextrinsic spin AM expands degree of freedom of the spin AM and would be applied\nto various terahertz technologies.",
        "category": "physics_optics"
    },
    {
        "text": "High Finesse Cavity with Anapole-Assisted Resonant Subwavelength\n  Particle Mirror: Strong light interaction with a subwavelength object has been a long pursuing\ngoal with difficulties mainly arising from the diffraction limit. We propose a\nhigh finesse cavity with one mirror made of a subwavelength resonant particle\nas a platform to enhance this interaction. High-quality eigenmode solutions are\nobtained for such a highly non-paraxial cavity with a very high field\nconcentration at the particle. The eigenmode solutions interact with the small\nparticle in a more general way than by the electric dipole approximation. With\nthe help of the anapole excitation in the dipole term, the particle is designed\nto scatter like a pure magnetic quadrupole, and in this way, it has anear-unity\nreflectivity when used as a mirror for the strongly focused field of the\neigenmode. Light-matter interactions at the subwavelength scale can be greatly\nenhanced due to the small size of the particle and the high finesse of the\ncavity, which can be potentially interesting for applications in nano optics,\nquantum optomechanics, nonlinear optics, and subwavelength metrology beyond the\nelectric dipole approximation.",
        "category": "physics_optics"
    },
    {
        "text": "The effect of input phase modulation to a phase-sensitive optical\n  amplifier: Many optical applications depend on amplitude modulating optical beams using\ndevices such as acousto-optical modulators (AOMs) or optical choppers. Methods\nto add amplitude modulation (AM) often inadvertently impart phase modulation\n(PM) onto the light as well. While this PM is of no consequence to many\nphase-insensitive applications, phase-sensitive processes can be affected. Here\nwe study the effects of input phase and amplitude modulation on the output of a\nquantum-noise limited phase-sensitive optical amplifier (PSA) realized in hot\n$^{85}$Rb vapor. We investigate the dependence of PM on AOM alignment and\ndemonstrate a novel approach to quantifying PM by using the PSA as a diagnostic\ntool. We then use this method to measure the alignment-dependent PM of an\noptical chopper which arises due to diffraction effects as the chopper blade\npasses through the optical beam.",
        "category": "physics_optics"
    },
    {
        "text": "Behavioral similarity of dissipative solitons in an ultrafast fiber\n  laser: Dissipative solitons are non-dispersive localized waves that exhibit\nintriguing dynamics. In optical systems, the discovery of new soliton dynamics\nand how to accurately manipulate dissipative solitons are still largely\nunexplored and challenging in the ultrafast fiber laser community. Herein, we\nunveil a new type of dissipative soliton behavior in a net-normal-dispersion\nbidirectional ultrafast fiber laser. That is, the bidirectional dissipative\nsolitons will always reveal similar spectral and temporal characteristics\nthrough common gain and loss modulation, even if the transient instability is\ninvolved. The behavioral similarity enables us to accurately design the soliton\npatterns by introducing seed pulses through loss modulation. As a\nproof-of-concept application, the precise and flexible manipulation of\nmulti-soliton patterns is demonstrated. These findings will shed new insights\ninto the complex dissipative soliton dynamics, and also beneficial for\ndesigning ultrafast lasers with desirable soliton patterns for practical\napplications.",
        "category": "physics_optics"
    },
    {
        "text": "Hidden symmetries in plasmonic gratings: Plasmonic gratings constitute a paradigmatic instance of the wide range of\napplications enabled by plasmonics. While subwavelength metal gratings find\napplications in optical biosensing and photovoltaics, atomically thin gratings\nachieved by periodically doping a graphene monolayer perform as metasurfaces\nfor the control of terahertz radiation. In this paper we show how these two\ninstances of plasmonic gratings inherit their spectral properties from an\nunderlying slab with translational symmetry.We develop an analytical formalism\nto accurately derive the mode spectrum of the gratings that provides a great\nphysical insight.",
        "category": "physics_optics"
    },
    {
        "text": "Spatial Coherence and Optical Beam Shifts: A beam of light, reflected at a planar interface, does not follow perfectly\nthe ray optics prediction. Diffractive corrections lead to beam shifts; either\nthe reflected beam is displaced (spatial shift) and/or travels in a different\ndirection (angular shift), as compared to geometric optics. How does the degree\nof spatial coherence of light influence these shifts? Theoretically, this has\nturned out to be a controversial issue. Here we resolve the controversy\nexperimentally; we show that the degree of spatial coherence influences the\nangular beam shifts, while the spatial beam shifts are unaffected.",
        "category": "physics_optics"
    },
    {
        "text": "On-chip, self-detected THz dual-comb spectrometer: We present a directly generated on-chip dual-comb source at THz frequencies.\nThe multi-heterodyne beating signal of two free-running THz quantum cascade\nlaser frequency combs is measured electrically using one of the combs as a\ndetector, fully exploiting the unique characteristics of quantum cascade active\nregions. Up to 30 modes can be detected corresponding to a spectral bandwidth\nof 630 GHz, being the available bandwidth of the dual comb configuration. The\nmulti-heterodyne signal is used to investigate the equidistance of the comb\nmodes showing an accuracy of $10^{-12}$ at the carrier frequency of 2.5 THz.",
        "category": "physics_optics"
    },
    {
        "text": "Integrated O- and C-band Silicon-Lithium Niobate Mach-Zehnder Modulators\n  with 100 GHz Bandwidth, Low Voltage, and Low Loss: Broadband integrated thin-film lithium niobate (TFLN) electro-optic\nmodulators (EOM) are desirable for optical communications and signal processing\nin both the O-band (1310 nm) and C-band (1550 nm). To address these needs, we\ndesign and demonstrate Mach-Zehnder (MZ) EOM devices in a hybrid platform based\non TFLN bonded to foundry-fabricated silicon photonic waveguides. Using a\nsingle silicon lithography step and a single bonding step, we realize MZ EOM\ndevices which cover both wavelength ranges on the same chip. The EOM devices\nachieve 100 GHz EO bandwidth (referenced to 1 GHz) and about 2-3 V$.$cm\nfigure-of-merit ($V_\\pi L$) with low on-chip optical loss in both the O-band\nand C-band.",
        "category": "physics_optics"
    },
    {
        "text": "Mapping the nonlinear optical susceptibility by noncollinear second\n  harmonic generation: We present a method, based on noncollinear second harmonic generation, to\nevaluate the non-zero elements of the nonlinear optical susceptibility. At a\nfixed incidence angle, the generated signal is investigated by varying the\npolarization state of both fundamental beams. The resulting polarization charts\nallows to verify if Kleinman symmetry rules can be applied to a given material\nor to retrieve the absolute value of the nonlinear optical tensor terms, from a\nreference measurement. Experimental measurements obtained from Gallium Nitride\nlayers are reported. The proposed method does not require an angular scan thus\nis useful when the generated signal is strongly affected by sample rotation",
        "category": "physics_optics"
    },
    {
        "text": "Inhomogeneous Gain Saturation in EDF: Experiment and Modeling: Erbium-Doped Fiber Amplifiers can present holes in spectral gain in\nWavelength Division Multiplexing operation. The origin of this inhomogeneous\nsaturation behavior is still a subject of controversy. In this paper we present\nboth an experimental methods and a gain's model. Our experimental method allow\nus to measure the first homogeneous linewidth of the 1.5 $\\mu$m erbium emission\nwith gain spectral hole burning consistently with the other measurement in the\nliterature and the model explains the differences observed in literature\nbetween GSHB and other measurement methods.",
        "category": "physics_optics"
    },
    {
        "text": "Decay dynamics of Localised Surface Plasmons: damping of coherences and\n  populations of the oscillatory plasmon modes: Properties of plasmonic materials are associated with surface plasmons - the\nelectromagnetic excitations coupled to coherent electron charge density\noscillations on a metal/dielectric interface. Although decay of such\noscillations cannot be avoided, there are prospects for controlling plasmon\ndamping dynamics. In spherical metal nanoparticles (MNPs) the basic properties\nof Localized Surface Plasmons (LSPs) can be controlled with their radius. The\npresent paper handles the link between the size-dependent description of LSP\nproperties derived from the dispersion relation based on Maxwell's equations\nand the quantum picture in which MNPs are treated as \"quasi-particles\". Such\npicture, based on the reduced density-matrix of quantum open systems ruled by\nthe master equation in the Lindblad form, enables to distinguish between\ndamping processes of populations and coherences of multipolar plasmon\noscillatory states and to establish the intrinsic relations between the rates\nof these processes, independently of the size of MNP. The impact of the\nradiative and the nonradiative energy dissipation channels is discussed.",
        "category": "physics_optics"
    },
    {
        "text": "Negative Refraction in isotropic achiral and chiral materials: We show that negative refraction in materials can occur at frequencies\n$\\omega$ where the real part of the permittivity $\\veps(\\omega)$ and the real\npart of the permeability $\\mu(\\omega)$ are of different sign, and that light\nwith such frequencies can propagate just as well as light with frequencies\nwhere they are of equal sign. Therefore, in order to have negative refraction\none does not need to be in the \"double negative\" regime. We consider negative\nrefractive index achiral materials using the Drude model, and chiral materials\nusing the Drude-Born-Fedorov model. We find that the time-averaged Poynting\nvector always points along the wave vector, the time-averaged energy flux\ndensity is always positive, and the time-averaged energy density is positive\n(negative) when the refractive index is positive (negative). The phase velocity\nis negative when the real part of the refractive index is negative, and the\ngroup velocity generally changes sign several times as a function of frequency\nnear resonance.",
        "category": "physics_optics"
    },
    {
        "text": "Onset of space-charge effects in strong-field photocurrents from\n  nanometric needle tips: Strong-field photoemission from nanostructures and the associated temporally\nmodulated currents play a key role in the development of ultrafast vacuum\noptoelectronics. Optical light fields could push their operation bandwidth into\nthe petahertz domain. A critical aspect for their functionality in the context\nof applications is the role of charge interactions, including space charge\neffects. Here, we investigated the photoemission and photocurrents from\nnanometric tungsten needle tips exposed to carrier-envelope phase-controlled\nfew-cycle laser fields. We report a characteristic step-wise increase in the\nintensity-rescaled cutoff energies of emitted electrons beyond a certain\nintensity value. By comparison with simulations, we identify this feature as\nthe onset of charge-interaction dominated photoemission dynamics. Our results\nare anticipated to be relevant also for the strong-field photoemission from\nother nanostructures, including photoemission from plasmonic nano-bowtie\nantennas used in carrier-envelope phase-detection and for PHz-scale devices.",
        "category": "physics_optics"
    },
    {
        "text": "Optical anapoles in nanophotonics and meta-optics: Interference of electromagnetic modes supported by subwavelength photonic\nstructures is one of the key concepts that underpins the subwavelength control\nof light in meta-optics. It drives the whole realm of all-dielectric\nMie-resonant nanophotonics with many applications for low-loss nanoscale\noptical antennas, metasurfaces, and metadevices. Specifically, interference of\nthe electric and toroidal dipole moments results in a very peculiar,\nlow-radiating optical state associated with the concept of optical anapole.\nHere, we uncover the physics of multimode interferences and multipolar\ninterplay in nanostructures with an intriguing example of the optical anapole.\nWe review the recently emerged field of anapole electrodynamics explicating its\nrelevance to multipolar nanophotonics, including direct experimental\nobservations, manifestations in nonlinear optics, and rapidly expanding\napplications in nanoantennas, active photonics, and metamaterials.",
        "category": "physics_optics"
    },
    {
        "text": "Switchable thermal antenna by phase transition: We introduce a thermal antenna which can be actively switched by phase\ntransition. The source makes use of periodically patterned vanadium dioxide, a\nmetal-insulator phase transition material which supports a surface\nphonon-polariton (SPP) in the infrared range in its crystalline phase. Using\nelectrodes properly registred with respect to the pattern, the phase transition\nof VO2 can be localy triggered within few microseconds and the SPP can be\ndiffracted making the thermal emission highly directionnal. This switchable\nantenna could find broad applications in the domain of active thermal coatings\nor in those of infrared spectroscopy and sensing.",
        "category": "physics_optics"
    },
    {
        "text": "Control of surface plasmon-polaritons in magnetoelectric\n  heterostructures: The work is devoted to the investigation of the surface plasmon polaritons in\nthe metal - dielectric heterostructures containing materials with\nmagnetoelectric properties. The analysis of various possible configurations\nshows that the magnetoelectric effect influences the plasmonic surface wave\npolarization, localization and dispersion. The essential case of chromium (III)\noxide as a magnetoelectric dielectric is described in detail, revealing the\nswitching between different regimes of magnetoelectric impact on\nplasmon-polaritons. Plasmonic heterostructures with magnetoelectric\nconstituents can be used as a polarization- and dispersion-sensitive instrument\nto reveal the magnetoelectric coupling and besides might act as an optical tool\nfor control of the surface plasmon-polariton dispersion and dynamics via\nexternal electric and magnetic fields. Apart from that, the described effects\ncan be applied to the manipulation of the far-field optical response of\nplasmonic structures. Composite magnetoelectric materials like\n$\\mathrm{BaTiO_3/BiFeO_3}$ that exhibit giant magnetoelectric effect are very\npromising for these purposes.",
        "category": "physics_optics"
    },
    {
        "text": "Radiative heat transfer between metallic nanoparticles: In this letter, we study the radiative heat transfer between two\nnanoparticles in the near field and in the far field. We find that the heat\ntransfer is dominated by the electric dipole-dipole interaction for dielectric\nparticles and by the magnetic dipole-dipole interaction for metallic\nnanoparticles. We introduce polarizabilities formulas valid for arbitrary\nvalues of the skin depth. While the heat transfer mechanism is different for\nmetallic and dielectric nanoparticles, we show that the distance dependence is\nthe same. However, the dependence of the heat flux on the particle radius is\ndifferent.",
        "category": "physics_optics"
    },
    {
        "text": "Optical trapping of high-aspect-ratio NaYF hexagonal prisms for kHz-MHz\n  gravitational wave detectors: We present experimental results on optical trapping of Yb-doped $\\beta-$NaYF\nsub-wavelength-thickness high-aspect-ratio hexagonal prisms with a micron-scale\nradius. The prisms are trapped in vacuum using an optical standing wave,\noriented with the normal vector to their face along the beam propagation\ndirection, and exhibit characteristic modes of three translational and two\ntorsional degrees of freedom. The measured motional spectra are compared with\nnumerical simulations. This plate-like geometry simultaneously enables trapping\nwith low photon-recoil-heating, high mass, and high trap frequency, potentially\nleading to advances in high frequency gravitational wave searches in the\nLevitated Sensor Detector (LSD), currently under construction. The material\nused here has previously been shown to exhibit internal cooling via laser\nrefrigeration when optically trapped and illuminated with light of suitable\nwavelength. Employing such laser refrigeration methods in the context of our\nwork may enable higher trapping intensity thus and higher trap frequencies for\ngravitational wave searches approaching the several hundred kHz range.",
        "category": "physics_optics"
    },
    {
        "text": "Linearly polarized, Q-switched Er-doped fiber laser based on reduced\n  graphene oxide saturable absorber: We demonstrate generation of linearly polarized pulses from a passively\nQ-switched Erbium-doped fiber laser. The cavity was designed using only\npolarization maintaining (PM) fibers and components, resulting in linearly\npolarized output beam with degree of polarization (DOP) at the level of 97.6%.\nReduced graphene oxide (rGO) was used as a saturable absorber for Q-switched\noperation. The laser was capable of delivering 1.85us pulses with 125nJ pulse\nenergy at 115kHz repetition rate.",
        "category": "physics_optics"
    },
    {
        "text": "Nonclassical optical properties of mesoscopic gold: Gold nanostructures have important applications in nanoelectronics,\nnano-optics as well as in precision metrology due to their intriguing\nopto-electronic properties. These properties are governed by the bulk band\nstructure but to some extend are tunable via geometrical resonances. Here we\nshow that the band structure of gold itself exhibits significant size-dependent\nchanges already for mesoscopic critical dimensions below 30 nm. To suppress the\neffects of geometrical resonances and grain boundaries, we prepared atomically\nflat ultrathin films of various thicknesses by utilizing large chemically grown\nsingle-crystalline gold platelets. We experimentally probe thickness-dependent\nchanges of the band structure by means of two-photon photoluminescence and\nobserve a surprising 100-fold increase of the nonlinear signal when the gold\nfilm thickness is reduced below 30 nm allowing us to optically resolve\nsingle-unit-cell steps. The effect is well explained by density functional\ncalculations of the thickness-dependent 2D band structure of gold.",
        "category": "physics_optics"
    },
    {
        "text": "Transverse coherence properties of X-ray beams in third-generation\n  synchrotron radiation sources: This article describes a complete theory of spatial coherence for undulator\nradiation sources. Current estimations of coherence properties often assume\nthat undulator sources are quasi-homogeneous, like thermal sources, and rely on\nthe application of the van Cittert-Zernike theorem for calculating the degree\nof transverse coherence. Such assumption is not adequate when treating third\ngeneration light sources, because the vertical(geometrical) emittance of the\nelectron beam is comparable or even much smaller than the radiation wavelength\nin a very wide spectral interval that spans over four orders of magnitude (from\n0.1 Angstrom up to 10^3 Angstrom). Sometimes, the so-called Gaussian-Schell\nmodel, that is widely used in statistical optics in the description of\npartially-coherent sources, is applied as an alternative to the\nquasi-homogeneous model. However, as we will demonstrate, this model fails to\nproperly describe coherent properties of X-ray beams from non-homogeneous\nundulator sources. As a result, a more rigorous analysis is required. We\npropose a technique, based on statistical optics and Fourier optics, to\nexplicitly calculate the cross-spectral density of an undulator source in the\nmost general case, at any position after the undulator. Our theory, that makes\nconsistent use of dimensionless analysis, allows relatively easy treatment and\nphysical understanding of many asymptotes of the parameter space, together with\ntheir region of applicability. Particular emphasis is given to the asymptotic\nsituation when the horizontal emittance is much larger than the radiation\nwavelength, and the vertical emittance is arbitrary. This case is practically\nrelevant for third generation synchrotron radiation sources.",
        "category": "physics_optics"
    },
    {
        "text": "Highly sensitive multicore fiber accelerometer for low frequency\n  vibration sensing: We report on a compact, highly sensitive all-fiber accelerometer suitable for\nlow frequency and low amplitude vibration sensing. The sensing elements in the\ndevice are two short segments of strongly coupled asymmetric multicore fiber\n(MCF) fusion spliced at 180{\\deg} with respect to each other. Such segments of\nMCF are sandwiched between standard single mode fibers. The reflection spectrum\nof the device exhibits a narrow spectrum whose height and position in\nwavelength changes when it is subjected to vibrations. The interrogation of the\naccelerometer was carried out by a spectrometer and a photodetector to measure\nsimultaneously wavelength shift and light power variations. The device was\nsubjected to a wide range of vibration frequencies, from 1 mHz to 30 Hz, and\naccelerations from 0.76 mg to 29.64 mg, and performed linearly, with a\nsensitivity of 2.213 nW/mg. Therefore, we believe the accelerometer reported\nhere may represent an alternative to existing electronic and optical\naccelerometers, especially for low frequency and amplitude vibrations, thanks\nto its compactness, simplicity, cost-effectiveness, implementation easiness and\nhigh sensitivity.",
        "category": "physics_optics"
    },
    {
        "text": "Quasinormal Modes of Optical Solitons: Quasinormal modes (QNMs) are essential for understanding the stability and\nresonances of open systems, with increasing prominence in black hole physics.\nWe present here the first study of QNMs of optical potentials. We show that\nsolitons can support QNMs, deriving a soliton perturbation equation and giving\nexact analytical expressions for the QNMs of fiber solitons. We discuss the\nboundary conditions in this intrinsically dispersive system and identify novel\nsignatures of dispersion. From here, we discover a new analogy with black holes\nand describe a regime in which the soliton is a robust black hole simulator for\nlight-ring phenomena. Our results invite a range of applications, from the\ndescription of optical pulse propagation with QNMs to the use of\nstate-of-the-art technology from fiber optics to address questions in black\nhole physics, such as QNM spectral instabilities and the role of nonlinearities\nin ringdown.",
        "category": "physics_optics"
    },
    {
        "text": "Thermorefringent noise in crystalline optical materials: Any material in thermal equilibrium exhibits fundamental thermodynamic\nfluctuations of its mechanical and optical properties. Such thermodynamic\nfluctuations of length, elastic constants, and refractive index of amorphous\nmaterials -- like dielectric mirror coatings and substrates -- limit the\nperformance of today's most precise optical instruments. Crystalline materials\nare increasingly employed in optical systems because of their reduced\nmechanical dissipation, which implies a reduction of thermo-mechanical\nfluctuations. However, the anisotropy of the crystalline state implies a\nfundamental source of thermal noise: depolarization induced by thermal\nfluctuations of its birefringence. We establish the theory of this effect,\nelucidate its consequences, discuss its relevance for precision optical\nexperiments with crystalline materials, and hint at the conditions under which\nit can be evaded.",
        "category": "physics_optics"
    },
    {
        "text": "Improvement of the Tm3+:3H4 level lifetime in silica optical fibres by\n  lowering the local phonon energy: The role of some glass network modifiers on the quantum efficiency of the\nnear-infrared fluorescence from the 3H4 level of Tm3+ ion in silica-based doped\nfibres is studied. Modifications of the core composition affect the\nspectroscopic properties of Tm3+ ion. Adding 17.4 mol% of AlO3/2 to the core\nglass caused an increase of the 3H4 level lifetime up to 50 $\\mu$s, 3.6 times\nhigher than in pure silica glass. The quantum efficiency was increased from 2%\nto approximately 8%. On the opposite, 8 mol% of PO5/2 in the core glass made\nthe lifetime decrease downto 9 $\\mu$s. These changes of Tm3+ optical properties\nare assigned to the change of the local phonon energy to which they are\nsubmitted by modifiers located in the vicinity of the doping sites. Some\nqualitative predictions of the maximum achievable quantum efficiency are\npossible using a simple microscopic model to calculate the non-radiative\nde-excitation rates.",
        "category": "physics_optics"
    },
    {
        "text": "Redesigning spectroscopic sensors with programmable photonic circuits: Optical spectroscopic sensors are a powerful tool to reveal light-matter\ninteractions in many fields, such as physics, biology, chemistry, and\nastronomy. Miniaturizing the currently bulky spectrometers has become\nimperative for the wide range of applications that demand in situ or even in\nvitro characterization systems, a field that is growing rapidly. Benchtop\nspectrometers are capable of offering superior resolution and spectral range,\nbut at the expense of requiring a large size. In this paper, we propose a novel\nmethod that redesigns spectroscopic sensors via the use of programmable\nphotonic circuits. Drawing from compressive sensing theory, we start by\ninvestigating the most ideal sampling matrix for a reconstructive spectrometer\nand reveal that a sufficiently large number of sampling channels is a\nprerequisite for both fine resolution and low reconstruction error. This number\nis, however, still considerably smaller than that of the reconstructed spectral\npixels, benefitting from the nature of reconstruction algorithms. We then show\nthat the cascading of a few engineered MZI elements can be readily programmed\nto create an exponentially scalable number of such sampling spectral responses\nover an ultra-broad bandwidth, allowing for ultra-high resolution down to\nsingle-digit picometers without incurring additional hardware costs.\nExperimentally, we implement an on-chip spectrometer with a fully-programmable\n6-stage cascaded MZI structure and demonstrate a < 10 pm resolution with a >\n200 nm bandwidth using only 729 sampling channels. This achieves a\nbandwidth-to-resolution ratio of over 20,000, which is, to our best knowledge,\nabout one order of magnitude greater than any reported miniaturized\nspectrometers to date. We further illustrate that by employing\ndispersion-engineered waveguide components, the device bandwidth can be\nextended to over 400 nm.",
        "category": "physics_optics"
    },
    {
        "text": "Gain-dependent Purcell enhancement, breakdown of Einstein's relations\n  and superradiance in nanolasers: Light emitters in a single-mode nanolaser interact with the same cavity\nfield, that gives rise to polarization correlations which transform the cavity\nmode. Usually these correlations are ignored, however, collective phenomena can\nlead to the distinct sub- and superradiance, whose fully quantum description is\nchallenging. Here, we develop a simple yet rigorous picture of radiative\ntransitions in single-mode nanolasers that accounts for polarization\ncorrelations. We show that the collective behavior of emitters modifies the\nphotonic density of states leading to gain-dependent Purcell enhancement of\nspontaneous emission. Moreover, the stimulated emission rate is dependent on\nboth the photon number and the laser lineshape. As the laser line narrows,\nstimulated emission becomes stronger than predicted by Einstein's relations and\nthe nanolaser reaches the threshold earlier. Finally, we provide concise,\nready-to-use expressions for spontaneous and stimulated emission rates\nseamlessly describing both conventional and superradiant nanolasers.",
        "category": "physics_optics"
    },
    {
        "text": "Controlling phonons and photons at the wavelength-scale: silicon\n  photonics meets silicon phononics: Radio-frequency communication systems have long used bulk- and\nsurface-acoustic-wave devices supporting ultrasonic mechanical waves to\nmanipulate and sense signals. These devices have greatly improved our ability\nto process microwaves by interfacing them to orders-of-magnitude slower and\nlower loss mechanical fields. In parallel, long-distance communications have\nbeen dominated by low-loss infrared optical photons. As electrical signal\nprocessing and transmission approaches physical limits imposed by energy\ndissipation, optical links are now being actively considered for mobile and\ncloud technologies. Thus there is a strong driver for wavelength-scale\nmechanical wave or \"phononic\" circuitry fabricated by scalable semiconductor\nprocesses. With the advent of these circuits, new micro- and nanostructures\nthat combine electrical, optical and mechanical elements have emerged. In these\ndevices, such as optomechanical waveguides and resonators, optical photons and\ngigahertz phonons are ideally matched to one another as both have wavelengths\non the order of micrometers. The development of phononic circuits has thus\nemerged as a vibrant field of research pursued for optical signal processing\nand sensing applications as well as emerging quantum technologies. In this\nreview, we discuss the key physics and figures of merit underpinning this\nfield. We also summarize the state of the art in nanoscale electro- and\noptomechanical systems with a focus on scalable platforms such as silicon.\nFinally, we give perspectives on what these new systems may bring and what\nchallenges they face in the coming years. In particular, we believe hybrid\nelectro- and optomechanical devices incorporating highly coherent and compact\nmechanical elements on a chip have significant untapped potential for\nelectro-optic modulation, quantum microwave-to-optical photon conversion,\nsensing and microwave signal processing.",
        "category": "physics_optics"
    },
    {
        "text": "Infrared carpet cloak designed with uniform silicon grating structure: Through a particularly chosen coordinate transformation, we propose an\noptical carpet cloak that only requires homogeneous anisotropic dielectric\nmaterial. The proposed cloak could be easily imitated and realized by\nalternative layers of isotropic dielectrics. To demonstrate the cloaking\nperformance, we have designed a two-dimensional version that a uniform silicon\ngrating structure fabricated on a silicon-on-insulator wafer could work as an\ninfrared carpet cloak. The cloak has been validated through full wave\nelectromagnetic simulations, and the non-resonance feature also enables a\nbroadband cloaking for wavelengths ranging from 1372 to 2000 nm.",
        "category": "physics_optics"
    },
    {
        "text": "Frequency combs in a microring optical parametric oscillator: We report the soliton frequency comb generation in microring optical\nparametric oscillators operating in the down-conversion regime and with the\nsimultaneous presence of the $\\chi^{(2)}$ and Kerr nonlinearities. The combs\nare studied considering a typical geometry of a bulk LiNbO$_3$ toroidal\nresonator with the normal group velocity dispersion spanning an interval\nbetween the pump and the down-converted signal. We have identified critical\npower signaling a transition between the relatively low pump power\npredominantly $\\chi^{(2)}$ combs and the high pump power ones shaped by the\ncompetition between the $\\chi^{(2)}$ and Kerr nonlinearities.",
        "category": "physics_optics"
    },
    {
        "text": "Achieving Anisotropy in Metamaterials made of Dielectric Cylindrical\n  Rods: We show that anisotropic negative effective dispersion relation can be\nachieved in pure dielectric rod-type metamaterials by turning from the symmetry\nof a square lattice to that of a rectangular one, i.e. by breaking the rotation\nsymmetry of effective homogeneous medium. Theoretical predictions and\nconclusions are verified by both numerical calculations and computer based\nsimulations. The proposed anisotropic metamaterial, is used to construct a\nrefocusing slab-lens and a subdiffraction hyperlens. The all-dielectric origin\nmakes it more straightforward to address loss and scaling, two major issues of\nmetallic structures, thus facilitating future applications in both the\nterahertz and optical range.",
        "category": "physics_optics"
    },
    {
        "text": "Fourier phasing with phase-uncertain mask: Fourier phasing is the problem of retrieving Fourier phase information from\nFourier intensity data. The standard Fourier phase retrieval (without a mask)\nis known to have many solutions which cause the standard phasing algorithms to\nstagnate and produce wrong or inaccurate solutions. In this paper Fourier phase\nretrieval is carried out with the introduction of a randomly fabricated mask in\nmeasurement and reconstruction. Highly probable uniqueness of solution, up to a\nglobal phase, was previously proved with exact knowledge of the mask. Here the\nuniqueness result is extended to the case where only rough information about\nthe mask's phases is assumed. The exponential probability bound for uniqueness\nis given in terms of the uncertainty-to-diversity ratio (UDR) of the unknown\nmask. New phasing algorithms alternating between the object update and the mask\nupdate are systematically tested and demonstrated to have the capability of\nrecovering both the object and the mask (within the object support)\nsimultaneously, consistent with the uniqueness result. Phasing with a\nphase-uncertain mask is shown to be robust with respect to the correlation in\nthe mask as well as the Gaussian and Poisson noises.",
        "category": "physics_optics"
    },
    {
        "text": "Coherent control of acoustic phonons in a silica fiber using a multi-GHz\n  optical frequency comb: Multi-gigahertz mechanical vibrations stemming from interactions between\nlight fields and matter, also known as acoustic phonons, have long been a\nsubject of study. In recent years, specially designed functional devices have\nbeen developed to enhance the light-matter interaction strength, since the\nexcitation of acoustic phonons by a continuous wave laser alone is\ninsufficient. However, with such structure-dependent enhancements, the strength\nof the interaction cannot be aptly and instantly controlled. We propose a new\ntechnique to control the effective interaction strength, which is not via the\nmaterial structure in the spatial domain, as with the above-mentioned specially\ndesigned functional devices, but through the structure of light in the time\ndomain. Here we show the effective excitation and coherent control of acoustic\nphonons in a single-mode fiber using an optical frequency comb by tailoring the\noptical pulse train. We believe this work represents an important step towards\n\"comb-matter interactions.\"",
        "category": "physics_optics"
    },
    {
        "text": "Rotation of polarization of light propagating through a gas of molecular\n  super-rotors: We present a detailed theoretical and experimental study of the rotation of\nthe plane of polarization of light traveling through a gas of fast-spinning\nmolecules. This effect is similar to the polarization drag phenomenon predicted\nby Fermi a century ago and it is a mechanical analog of the Faraday effect. In\nour experiments, molecules were spun up by an optical centrifuge and brought to\nthe super-rotor state that retains its rotation for a relatively long time.\nPolarizability properties of fast-rotating molecules were analyzed considering\nthe rotational Doppler effect and Coriolis forces. We used molecular dynamics\nsimulations to account for intermolecular collisions. We found, both\nexperimentally and theoretically, a nontrivial nonmonotonic time dependence of\nthe polarization rotation angle. This time dependence reflects transfer of the\nangular momentum from rotating molecules to the macroscopic gas flow, which may\nlead to the birth of gas vortices. Moreover, we show that the long-term\nbehavior of the polarization rotation is sensitive to the details of the\nintermolecular potential. Thus, the polarization drag effect appears as a novel\ndiagnostic tool for the characterization of intermolecular interaction\npotentials and studies of collisional processes in gases.",
        "category": "physics_optics"
    },
    {
        "text": "Synthesis and optoelectronic properties of CdSe quantum dots: A series of Cadmium Selenide nanoparticles in the range of 6-12 nm have been\nsynthesized by a wet chemical route that employs Mercaptoethanol as a capping\nagent. These nanoparticles have been characterized by Ultraviolet-visible\n(UV-VIS) absorption, photoluminescence spectroscopy, and X-RAY diffraction\n(XRD). The effect of concentration on the optical spectra has been investigated\nwhereas Mercaptoethanol which is a growth-limiting agent, prevents bulking. The\naugmentation of concentration of Mercaptoethanol decreases the size of Cadmium\nSelenide nanoparticles and shifts the peaks of absorption spectra",
        "category": "physics_optics"
    },
    {
        "text": "Enhancement of valley selective excitation by a linearly polarized\n  two-color laser pulse: Here we proposed the valley selective excitations via a two-color\n(\\ensuremath{\\omega} + \\ensuremath{2\\omega}) laser field, made by superimposing\ntwo linearly polarized pulses at frequencies \\ensuremath{\\omega} and\n\\ensuremath{2\\omega}. We have studied the intensity ratio between a few-cycle\npulse of \\ensuremath{\\omega} and \\ensuremath{2\\omega} laser, and its\nenhancement factor by employing the time-dependent first-principle\ncalculations. The valley polarization depends on the carrier envelope phases\n(CEPs) of pulses and the intensity ratio $I_{\\omega}/I_{2\\omega}$. We found\nthat the two-color field enhances the valley polarization as much as 1.2 times\nlarger than the single-color pulse. The maximum valley asymmetry is achieved\nfor the intensity ratio $I_{\\omega}/I_{2\\omega}$ of 36 with the relative CEP of\n\\ensuremath{\\pi}. In our previous work, we found that the asymmetric vector\npotential induces the valley polarization (Phys. Rev. B 105,115403 (2022)). In\nthis work, we find that the asymmetry of the electric field modulates the\nvalley polarization. Our two-color scheme offers a new path toward the optical\ncontrol of valley pseudospins. \\end{abstract}",
        "category": "physics_optics"
    },
    {
        "text": "A Novel Photonic Material for Designing Arbitrarily Shaped Waveguides in\n  Two Dimensions: We investigate numerically optical properties of novel two-dimensional\nphotonic materials where parallel dielectric rods are randomly placed with the\nrestriction that the distance between rods is larger than a certain value. A\nlarge complete photonic gap (PG) is found when rods have sufficient density and\ndielectric contrast. Our result shows that neither long-range nor short-range\norder is an essential prerequisite to the formation of PGs. A universal\nprinciple is proposed for designing arbitrarily shaped waveguides, where\nwaveguides are fenced with side walls of periodic rods and surrounded by the\nnovel photonic materials. We observe highly efficient transmission of light for\nvarious waveguides. Due to structural uniformity, the novel photonic materials\nare best suited for filling up the outer region of waveguides of arbitrary\nshape and dimension comparable with the wavelength.",
        "category": "physics_optics"
    },
    {
        "text": "Effect of molecular absorption and vibrational modes in polariton\n  assisted photoemission from a layered molecular material: The way molecules absorb, transfer, and emit light can be modified by\ncoupling them to optical cavities. The extent of the modification is often\ndefined by the cavity-molecule coupling strength, which depends on the number\nof coupled molecules. We experimentally and numerically study the evolution of\nphotoemission from a thin layered J-aggregated molecular material strongly\ncoupled to a Fabry-Perot microcavity as a function of the number of coupled\nlayers. We unveil an important difference between the strong coupling\nsignatures obtained from reflection spectroscopy and from polariton assisted\nphotoluminescence. We also study the effect of the vibrational modes supported\nby the molecular material on the polariton assisted emission both for a focused\nlaser beam and for normally incident excitation, for two different excitation\nwavelengths: a laser in resonance with the lower polariton branch, and a laser\nnot in resonance. We found that the Raman scattered photons play an important\nrole in populating the lower polariton branch, especially when the system was\nexcited with a laser in resonance with the lower polariton branch. We also\nfound that the polariton assisted photoemission depends on the extent of\nmodification of the molecular absorption induced by the molecule-cavity\ncoupling.",
        "category": "physics_optics"
    },
    {
        "text": "Adiabatic evolution on a spatial-photonic Ising machine: Combinatorial optimization problems are crucial for widespread applications\nbut remain difficult to solve on a large scale with conventional hardware.\nNovel optical platforms, known as coherent or photonic Ising machines, are\nattracting considerable attention as accelerators on optimization tasks\nformulable as Ising models. Annealing is a well-known technique based on\nadiabatic evolution for finding optimal solutions in classical and quantum\nsystems made by atoms, electrons, or photons. Although various Ising machines\nemploy annealing in some form, adiabatic computing on optical settings has been\nonly partially investigated. Here, we realize the adiabatic evolution of\nfrustrated Ising models with 100 spins programmed by spatial light modulation.\nWe use holographic and optical control to change the spin couplings\nadiabatically, and exploit experimental noise to explore the energy landscape.\nAnnealing enhances the convergence to the Ising ground state and allows to find\nthe problem solution with probability close to unity. Our results demonstrate a\nphotonic scheme for combinatorial optimization in analogy with adiabatic\nquantum algorithms and enforced by optical vector-matrix multiplications and\nscalable photonic technology.",
        "category": "physics_optics"
    },
    {
        "text": "Linear and nonlinear fiber propagation of partially coherent fields\n  exhibiting temporal correlations: Using ultrafast photonic first-order differentiator applied on a partially\ncoherent field, we report the generation of two correlated temporal waveforms\nand study their correlation properties upon linear and nonlinear propagation\nalong the two orthogonal polarization axis of a dispersive optical fiber.\nTemporal correlations are maintained in linear propagation whereas Kerr\nnonlinearity generates anti-correlated temporal intensity patterns for both\npartially and uncorrelated fields. Experiments are in close agreement with the\ntheoretical analysis.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of topological polaritons and photonic magic angles in\n  twisted van der Waals bi-layers: Twisted two-dimensional bi-layers offer exquisite control on the electronic\nbandstructure through the interlayer rotation and coupling, enabling\nmagic-angle flat-band superconductivity and moir\\'e excitons. Here, we\ndemonstrate how analogous principles, combined with large anisotropy, enable\nextreme control and manipulation of the photonic dispersion of phonon\npolaritons (PhPs) in van der Waals (vdW) bi-layers. We experimentally observe\ntunable topological transitions from open (hyperbolic) to closed (elliptic)\ndispersion contours in twisted bi-layered {\\alpha}-MoO3 at photonic magic\nangles, induced by polariton hybridization and robustly controlled by a\ntopological quantity. At these transitions the bilayer dispersion flattens,\nexhibiting low-loss tunable polariton canalization and diffractionless\npropagation with resolution below {\\lambda}0/40. Our findings extend\ntwistronics and moir\\'e physics to nanophotonics and polaritonics, with great\npotential for nano-imaging, nanoscale light propagation, energy transfer and\nquantum applications.",
        "category": "physics_optics"
    },
    {
        "text": "Asymptotic theory of microstructured surfaces: An asymptotic theory for\n  waves guided by diffraction gratings or along microstructured surfaces: An effective surface equation, that encapsulates the detail of a\nmicrostructure, is developed to model microstructured surfaces. The equations\ndeduced accurately reproduce a key feature of surface wave phenomena, created\nby periodic geometry, that are commonly called Rayleigh-Bloch waves, but which\nalso go under other names such as Spoof Surface Plasmon Polaritons in\nphotonics. Several illustrative examples are considered and it is shown that\nthe theory extends to similar waves that propagate along gratings. Line source\nexcitation is considered and an implicit long-scale wavelength is identified\nand compared to full numerical simulations. We also investigate non-periodic\nsituations where a long-scale geometric variation in the structure is\nintroduced and show that localised defect states emerge which the asymptotic\ntheory explains.",
        "category": "physics_optics"
    },
    {
        "text": "CEP-stable soliton-based pulse compression to 4.4 fs and UV generation\n  at 800 kHz repetition rate: We report generation of a femtosecond supercontinuum extending from the\nultraviolet to the near-infrared and detection of its carrier-envelope phase\nvariation by f-to-2f interferometry. The spectrum is generated in a gas-filled\nhollow-core photonic crystal fiber where soliton dynamics allows CEP-stable\nself-compression of OPCPA pump pulses at 800 nm to a duration of 1.7 optical\ncycles, followed by dispersive wave emission. The source provides up to 1\n{\\mu}J of pulse energy at 800 kHz repetition rate resulting in 0.8 W of average\npower, and can be extremely useful for example in strong-field physics,\npump-probe measurements and ultraviolet frequency comb metrology.",
        "category": "physics_optics"
    },
    {
        "text": "Design Principles for Plasmonic Nanoparticle Devices: For all applications of plasmonics to technology it is required to tailor the\nresonance to the optical system in question. This chapter gives an\nunderstanding of the design considerations for nanoparticles needed to tune the\nresonance. First the basic concepts of plasmonics are reviewed with a focus on\nthe physics of nanoparticles. An introduction to the finite element method is\ngiven with emphasis on the suitability of the method to nanoplasmonic device\nsimulation. The effects of nanoparticle shape on the spectral position and\nlineshape of the plasmonic resonance are discussed including retardation and\nsurface curvature effects. The most technologically important plasmonic\nmaterials are assessed for device applicability and the importance of\nsubstrates in light scattering is explained. Finally the application of\nplasmonic nanoparticles to photovoltaic devices is discussed.",
        "category": "physics_optics"
    },
    {
        "text": "A Metalens with Near-Unity Numerical Aperture: The numerical aperture (NA) of a lens determines its ability to focus light\nand its resolving capability. Having a large NA is a very desirable quality for\napplications requiring small light-matter interaction volumes or large angular\ncollections. Traditionally, a large NA lens based on light refraction requires\nprecision bulk optics that ends up being expensive and is thus also a specialty\nitem. In contrast, metasurfaces allow the lens designer to circumvent those\nissues producing high NA lenses in an ultra-flat fashion. However, so far,\nthese have been limited to numerical apertures on the same order of traditional\noptical components, with experimentally reported values of NA <0.9. Here we\ndemonstrate, both numerically and experimentally, a new approach that results\nin a diffraction limited flat lens with a near-unity numerical aperture\n(NA>0.99) and sub-wavelength thickness (~{\\lambda}/3), operating with\nunpolarized light at 715 nm. To demonstrate its imaging capability, the\ndesigned lens is applied in a confocal configuration to map color centers in\nsub-diffractive diamond nanocrystals. This work, based on diffractive elements\nable to efficiently bend light at angles as large as 82{\\deg}, represents a\nstep beyond traditional optical elements and existing flat optics,\ncircumventing the efficiency drop associated to the standard, phase mapping\napproach.",
        "category": "physics_optics"
    },
    {
        "text": "Continuous Ultraviolet to Blue-Green Astrocomb: The characterization of Earth-like exoplanets and precision tests of\ncosmological models using next-generation telescopes such as the ELT will\ndemand precise calibration of astrophysical spectrographs in the visible\nregion, where stellar absorption lines are most abundant. Astrocombs--lasers\nproviding a broadband sequence of ultra-narrow, drift-free, regularly spaced\noptical frequencies on a multi-GHz grid--promise an atomically-traceable,\nversatile calibration scale, but their realization is challenging because of\nthe need for ultra-broadband frequency conversion of mode-locked infrared\nlasers into the blue-green region. Here, we introduce a new concept achieving a\nbroad, continuous spectrum by combining second-harmonic generation and\nsum-frequency-mixing in an aperiodically-poled MgO:PPLN waveguide to generate\ngap-free 390-520 nm light from a 1 GHz Ti:sapphire laser frequency comb. We\nlock a low-dispersion Fabry-Perot etalon to extract a sub-comb of bandwidth\nfrom 392-472 nm with a spacing of 30 GHz, visualizing the thousands of\nresulting comb modes on a high resolution cross-dispersion spectrograph.\nComplementary experimental data and simulations demonstrate the effectiveness\nof the approach for eliminating the spectral gaps present in\nsecond-harmonic-only conversion, in which weaker fundamental frequencies are\nsuppressed by the quadratic \\{chi}^((2)) nonlinearity. Requiring only ~100 pJ\npulse energies, our concept establishes a practical new route to broadband\nUV-visible generation at GHz repetition rates.",
        "category": "physics_optics"
    },
    {
        "text": "Axiparabola: a new tool for high-intensity optics: An axiparabola is a reflective aspherical optics that focuses a light beam\ninto an extended focal line. The light intensity and group velocity profiles\nalong the focus are adjustable through the proper design. The on-axis light\nvelocity can be controlled, for instance, by adding spatio-temporal couplings\nvia chromatic optics on the incoming beam. Therefore the energy deposition\nalong the axis can be either subluminal or superluminal as required in various\napplications. This article first explores how the axiparabola design defines\nits properties in the geometric optics approximation. Then the obtained\ndescription is considered in numerical simulations for two cases of interest\nfor laser-plasma acceleration. We show that the axiparabola can be used either\nto generate a plasma waveguide to overcome diffraction or for driving a\ndephasingless wakefield accelerator.",
        "category": "physics_optics"
    },
    {
        "text": "Substrateless metamaterials at mid-infrared frequencies: We report on the fabrication and mid-infrared transmission properties of\nfree-standing thin metal films, periodically patterned with holes at periods\ndown to 2 microns and area of 3x3 mm2. Square grids were fabricated by electron\nbeam lithography and deep-etching techniques and display substrateless holes,\nwith the metal being supported by a patterned dielectric silicon nitride\nmembrane. The mid-infrared transmission spectra of the substrateless grid\ndisplay extraordinary transmission peaks and resonant absorption lines with a\nQ-factor up to 22. These spectral features are due to the interaction of the\nradiation with surface plasmon modes. The high transmittivity and the negative\nvalue of the dielectric constant at selected frequencies make our substrateless\nstructures ideal candidates for the fabrication of mid-infrared metamaterials.",
        "category": "physics_optics"
    },
    {
        "text": "Fast and energy-efficient non-volatile III-V-on-silicon photonic phase\n  shifter based on memristors: Silicon photonics has evolved from lab research to commercial products in the\npast decade as it plays an increasingly crucial role in data communication for\nnext-generation data centers and high performance computing1. Recently,\nprogrammable silicon photonics has also found new applications in quantum2 and\nclassical 3 information processing. A key component of programmable silicon\nphotonic integrated circuits (PICs) is the phase shifter, traditionally\nrealized via the thermo-optic or plasma dispersion effect which are weak,\nvolatile, and power hungry. A non-volatile phase shifter can circumvent these\nlimitations by requiring zero power to maintain the switched phases. Previously\nnon-volatile phase modulation was achieved via phase-change4 or ferroelectric\nmaterials5, but the switching energy remains high (pico to nano joules) and the\nspeed is slow (micro to milli seconds). Here, we report a non-volatile\nIII-V-on-silicon photonic phase shifter based on HfO2 memristor with sub-pJ\nswitching energy (~400fJ), representing over an order of magnitude improvement\nin energy efficiency compared to the state of the art. The non-volatile phase\nshifter can be switched reversibly using a single 100ns pulse and exhibits an\nexcellent endurance over 800 cycles. This technology can enable future\nenergy-efficient programmable PICs for data centers, optical neural networks,\nand quantum information processing.",
        "category": "physics_optics"
    },
    {
        "text": "High quality factor nanophotonic resonators in bulk rare-earth doped\n  crystals: Numerous bulk crystalline materials exhibit attractive nonlinear and\nluminescent properties for classical and quantum optical applications. A\nchip-scale platform for high quality factor optical nanocavities in these\nmaterials will enable new optoelectronic devices and quantum light-matter\ninterfaces. In this article, photonic crystal nanobeam resonators fabricated\nusing focused ion beam milling in bulk insulators, such as rare-earth doped\nyttrium orthosilicate and yttrium vanadate, are demonstrated. Operation in the\nvisible, near infrared, and telecom wavelengths with quality factors up to\n27,000 and optical mode volumes close to one cubic wavelength is measured.\nThese devices enable new nanolasers, on-chip quantum optical memories, single\nphoton sources, and non-linear devices at low photon numbers based on\nrare-earth ions. The techniques are also applicable to other luminescent\ncenters and crystals.",
        "category": "physics_optics"
    },
    {
        "text": "Effect of the excitation setup in the improved enhancement factor of\n  covered-gold-nanorod-dimer antennas: Devices possessing the ability to sense both electrically and optically\nmolecular targets are of fundamental and technological interest. Towards this\nend, it has been shown that covering the ends of gapped gold-nanorod-dimer\nnanoantennas can improve the enhancement factor (EF) that quantifies the\nnanoantenna efficiency for surface-enhancement Raman spectroscopy (SERS) for an\nincident wave coming from the top of the sample. Here, as the covering breaks\nthe top-bottom symmetry, we investigate the behaviour of the EF for excitation\ncoming from the bottom of the sample. This is relevant in presence of a\nreflecting substrate or due to the placement of the device in a cavity field.\nWe also study the case of a superposition of waves coming from both directions\nin the limit cases in which a node or an antinode of the total incident field\nlies at the center of the gold nanorods. In all these situations we find that\nthe EF of the covered device can continue to be higher than for the uncovered\ncase when the geometrical parameters are tuned to the peak values of the\ncalculated enhancement factor.",
        "category": "physics_optics"
    },
    {
        "text": "On the modeling of thermal and free carrier nonlinearities in Silicon On\n  Insulator microring resonators: The temporal dynamics of integrated silicon resonators has been modeled using\na set of equations coupling the internal energy, the temperature and the free\ncarrier population. Owing to its simplicity, Newton's law of cooling is the\ntraditional choice for describing the thermal evolution of such systems. In\nthis work, we theoretically and experimentally prove that this can be\ninadequate in monolithic planar devices, leading to inaccurate predictions. A\nnew equation, that we train to reproduce the correct temperature behaviour, is\nintroduced to fix the discrepancies with the experimental results. We discuss\nthe limitations and the range of validity of our refined model, identifying\nthose cases where Netwon's law provides, nevertheless, accurate solutions. Our\nmodeling describes the phenomena underlying thermal and free carrier\ninstabilities, and is a valuable tool for the engineering of photonic systems\nwhich relay on resonator dynamical states, such as all optical spiking neural\nnetworks or reservoirs for neuromorphic computing.",
        "category": "physics_optics"
    },
    {
        "text": "Method comparison for simulating non-Gaussian Beams and Diffraction for\n  Precision Interferometry: In the context of simulating precision laser interferometers, we compare via\nseveral examples two wavefront decomposition methods: the Mode Expansion Method\n(MEM) and the Gaussian beam decomposition (GBD) for their precision and\napplicability. To judge the performance of these methods, we define different\ntypes of errors and study their properties. We specify how the two methods can\nbe fairly compared and based on that, the quality of the MEM and GBD are\ncompared in several examples. We test here cases for which analytic results are\navailable, i.e., non-clipped circular and general astigmatic Gaussian beams, as\nwell as clipped circular Gaussian beams, in the near-, far-, and extreme\nfar-field of millions of kilometers occurring in space-gravitational wave\ndetectors. Additionally, we compare the methods for aberrated wavefronts and\nthe interaction with optical components by testing reflections from differently\ncurved mirrors. We find that both methods can be generally used for decomposing\nnon-Gaussian beams. However, which method is more accurate depends on the\noptical system and simulation settings. In the given examples, the MEM more\naccurately describes non-clipped Gaussian beams, while for clipped Gaussian\nbeams and the interaction with surfaces, the GBD is more precise.",
        "category": "physics_optics"
    },
    {
        "text": "Frequency Conversion in a High Q-factor Sapphire Whispering Gallery Mode\n  Resonator due to Paramagnetic Nonlinearity: Nonlinear frequency conversion is a well known and widely exploited family of\neffects in optics, often arising from a Kerr nonlinearity in a crystal medium.\nHere, we report high stability frequency conversion in the microwave regime due\nto a $\\chi^{(3)}$ nonlinearity in sapphire introduced by a dilute concentration\nof paramagnetic spins. First, we produce a high stability comb from two\nmicrowave fields at 12.029 and 12.037 GHz corresponding to two high $Q$-factor\nWhispering Gallery (WG) modes within the Electron Spin Resonance (ESR)\nbandwidth of the Fe$^{3+}$ ion. The resulting comb is generated by a cascaded\nfour-wave mixing effect with a 7.7 MHz repetition rate. Then, by suppressing\nfour-wave mixing by increasing the threshold power, third harmonic generation\nis achieved in a variety of WG modes coupled to various species of paramagnetic\nion within the sapphire.",
        "category": "physics_optics"
    },
    {
        "text": "Electromagnetic Scattering at an Arbitrarily Accelerated Interface: We present a general analytical solution to the problem of electromagnetic\nscattering at a one-dimensional arbitrarily accelerated space-time\nengineered-modulation (ASTEM) interface in the subluminal regime. We show that\nsuch an interface fundamentally produces chirping, whose profile can be\ndesigned according to specifications. This work represents an important step in\nthe development of ASTEM crystals and holds significant potential for\napplications in microwave and optical devices reliant on chirp-based\nfunctionalities.",
        "category": "physics_optics"
    },
    {
        "text": "Super-Planckian thermal emission from a hyperlens: We suggest and theoretically explore a possibility to strongly enhance the\nsteady thermal radiation of a small thermal emitter using an infrared\nhyperlens. The hyperbolic metamaterial of the hyperlens converts emitter's near\nfields into the propagating waves which are efficiently irradiated from the\nhyperlens surface. Thus, with the hyperlens, emitter's spectral radiance goes\nwell beyond the black-body limit for the same emitter in free space. Although\nthe hyperlens can be kept at a much lower temperature than the emitter, the\nwhole structure may radiate, in principle, as efficiently as a black body with\nthe same size as that of the hyperlens and the same temperature as that of the\nemitter. We believe that this study can lead to a breakthrough in radiative\ncooling at microscale, which is crucial for microlasers and\nmicrothermophotovoltaic systems.",
        "category": "physics_optics"
    },
    {
        "text": "Anomalous Resonance Frequency Shift in Liquid Crystal-Loaded THz\n  Metamaterials: Babinet complementary patterns of a spectrally tunable metamaterial\nincorporating a nematic liquid crystal are normally assumed to exhibit the same\ntuning range. Here we show that for a hybrid, terahertz liquid\ncrystal-metamaterial, the sensitivity of its resonances to the variations of\nthe refractive index differs substantially for the two complementary patterns.\nThis is due to a mismatch between the alignment of the liquid crystal and the\ndirection of the local electric field induced in the patterns. Furthermore, and\nmore intriguingly, our experimental data indicate that it is possible to shift\nthe resonance of the positive metamaterial pattern beyond the limit imposed by\nthe alignment mismatch. Our analysis suggests that the observed anomalous\nfrequency shift result from the orientational optical nonlinearity of a nematic\nliquid crystal.",
        "category": "physics_optics"
    },
    {
        "text": "Reciprocity constraints on the matrix of reflection from optically\n  anisotropic surfaces: We derive certain constraints on the reflection matrix for reflection from a\nplane, nonmagnetic, optically anisotropic surface using a reciprocity theorem\nstated long ago by van de Hulst in the context of scattering of polarized\nlight. The constraints are valid for absorbing and chiral media and can be used\nas tools to check the consistency of derived expressions for such matrices in\nterms of the intrinsic parameters of the reflecting medium as illustrated by\nseveral examples.",
        "category": "physics_optics"
    },
    {
        "text": "Frequency stability of a wavelength meter and applications to laser\n  frequency stabilization: Interferometric wavelength meters have attained frequency resolutions down to\nthe MHz range. In particular, Fizeau interferometers, which have no moving\nparts, are becoming a popular tool for laser characterization and\nstabilization. In this article, we characterize such a wavelength meter using\nan ultra-stable laser in terms of relative frequency instability\n$\\sigma_y(\\tau)$ and demonstrate that it can achieve a short-term instability\n$\\sigma_y(1 s) \\approx 2{\\times}10^{-10}$ and a frequency drift of order $10$\nMHz/day. We use this apparatus to demonstrate frequency control of a\nnear-infrared laser, where a frequency instability below $3{\\times}10^{-10}$\nfrom 1 s to 2000 s is achieved. Such performance is for example adequate for\nions trapping and atoms cooling experiments.",
        "category": "physics_optics"
    },
    {
        "text": "Numerical analysis of the emission properties of terahertz\n  photoconductive antenna by finite-difference-time-domain method: The emission properties of terahertz(THz) photoconductive antenna (PCA) have\nbeen numerically studied by three-dimensional finite-difference-time-domain\nmethod based on the full-wave model. The dependence of the THz radiation on\nvarious parameters, such as laser power, bias voltage, substrate's material,\npulse duration of the laser, beam spot's size, dimension of the antenna, were\ncomprehensively simulated and analyzed. This work, on one hand, reveals the\ninternal relationship between the THz radiation of a PCA and the involved\nparameters, so that one can have a better understanding of the PCA. On the\nother hand, it can inspire new PCA's design that aims at improved performance,\nsuch as high radiation power, enhanced optics-to-THz conversion efficiency, and\nbroadband spectrum.",
        "category": "physics_optics"
    },
    {
        "text": "Super-compact universal quantum logic gates with inversedesigned\n  elements: Integrated quantum photonic circuit is a promising platform for the\nrealization of quantum information processing in the future. To achieve the\nlargescale quantum photonic circuits, the applied quantum logic gates should be\nas small as possible for the high-density integration on chips. Here, we report\nthe implementation of super-compact universal quantum logic gates on silicon\nchips by the method of inverse design. In particular, the fabricated\ncontrolled-NOT gate and Hadamard gate are both nearly a vacuum wavelength,\nbeing the smallest optical quantum gates reported up to now. We further design\nthe quantum circuit by cascading these fundamental gates to perform arbitrary\nquantum processing, where the corresponding size is about several orders\nsmaller than that of previous quantum photonic circuits. Our study paves the\nway for the realization of largescale quantum photonic chips with integrated\nsources, and can possess important applications in the field of quantum\ninformation processes.",
        "category": "physics_optics"
    },
    {
        "text": "Tunable Polarization-Multiplexed Achromatic Dielectric Metalens: Tunable metasurfaces provide a compact and efficient strategy for optical\ncomponents that require active wavefront shaping. Varifocal metalens is one of\nthe most important applications. However, the existing tunable metalens rarely\nserves broadband wavelengths restricting their applications in broadband\nimaging and color display due to chromatic aberration. Herein, we demonstrate\nan electrically tunable polarization-multiplexed varifocal achromatic\ndielectric metalens integrated with twisted nematic liquid crystals (TNLCs) in\nthe visible region. The phase profiles at different wavelengths under two\northogonal polarization channels are customized by the particle swarm\noptimization algorithm and optimally matched with the metaunits database to\nachieve polarization-multiplexed dispersion manipulation including achromatic\nperformance. By combining the broadband linear polarization conversion ability\nof TNLC, the tunability of varifocal achromatic metalens is realized by\napplying different voltages. Further, the electrically tunable customized\ndispersion-manipulated metalens and switchable color metaholograms are\ndemonstrated. The proposed devices will accelerate the application of\nmetasurfaces in broadband zoom imaging, AR/VR displays, and spectral detection.",
        "category": "physics_optics"
    },
    {
        "text": "A Raman-type optical frequency comb adiabatically generated in an\n  enhancement cavity: We demonstrate efficient generation of a Raman-type optical frequency comb by\nemploying adiabatic Raman excitation in an enhancement cavity. A broad\nfrequency comb spanning over 130 THz is realized with an excitation power\nreduction exceeding three orders of magnitude compared with a single-pass\nconfiguration.",
        "category": "physics_optics"
    },
    {
        "text": "Ultra-sensitive Light Confinement Driven by Multiple Bound States in the\n  Continuum: We propose a unique framework to study the topological properties of an\noptical bound state in the continuum(BIC). We employ the interactions between\nproximity resonances undergoing avoided resonance crossing in a specialty\noptical microcavity. We utilize a continuous system parameter tuning to induce\ndestructive interference between the resonances with cancelling leakage losses.\nSimilar to the physical insight of Friedrich-Wingten type-BIC, we demonstrate\nthe evolution of an ultra-high quality mode. We report the formation of a\nspecial-BIC line in the system parameter space connecting locations of multiple\nquasi-BICs. Aiming to develop a novel scheme to enhance the performance of\noptical sensing in microcavity, we study the sensitivity of transmission\ncoefficients and quality factor to sense even ultra-small perturbations in the\nsystem configuration.",
        "category": "physics_optics"
    },
    {
        "text": "The photon vortex beam in rotating medium: In this paper we consider the photon vortex beam in the rotating medium,\nwhere the rotating velocity acts as an effective vector potential. Using the\nRiemann-Silberstein vector, we construct the photon wave function. Using the\nMaxwell equations and the first-order Minkowski constitutive relations we get\nthe dynamic equations of photon in moving medium. For the stationary states,\nthe dynamic equations can be written as a Dirac-like equation. We obtain the\napproximate photon vortex beam solutions by the given the medium's different\nvelocity distribution and find the diffracting and nondiffracting\nLaguerre-Gaussian beam solutions in the rotating medium. For the diffracting\nLaguerre-Gaussian beams, we acquire new terms arising from the rotation that\ncan change the Gouy phase, and then accordingly infer the rotation behavior of\nthe photon interference pattern. Furthermore, in our theory we obtain the\nLandau levels structure of transverse photon energy in the nondiffracting\nLaguerre-Gaussian beam solutions.",
        "category": "physics_optics"
    },
    {
        "text": "Propagation dynamics of vortices in Helico-Conical optical beams: We present the dynamics of optical vortices (OVs) that came from the\npropagation of helico-conical optical beam. This dynamics is investigated\nnumerically by tracking the OVs at several distances using rigorous scalar\ndiffraction theory. To ensure that our numerical calculations are correct, we\ncompare the intensity profiles and their corresponding interferograms taken at\ndifferent propagation distances between simulations and experiments. We observe\nthat the peripheral isopolar vortices transport radially inward, toward the\noptical axis along the transverse spatial space as the beam propagates. When\nthe beam has a central vortex, these vortices have significant induced angular\nrates of motion about the optical axis. These propagation dynamics of vortices\ninfluence the internal energy flow and the wave profile reconstruction of the\nbeam, which can be important when deciding their applications.",
        "category": "physics_optics"
    },
    {
        "text": "Slow Light in Metamaterial Waveguides: Metamaterials, which are materials engineered to possess novel optical\nproperties, have been increasingly studied. The ability to fabricate\nmetamaterials has sparked an interest in determining possible applications. We\ninvestigate using a metamaterial for boundary engineering in waveguides.\n  A metamaterial-clad cylindrical waveguide is used to provide confinement for\nan optical signal, thereby increasing the local electromagnetic energy density.\nWe show that metamaterial-clad waveguides have unique optical properties,\nincluding new modes, which we call hybrid modes. These modes have properties of\nboth ordinary guided modes and surface plasmon-polariton modes.\n  We show that for certain metamaterial parameters, the surface\nplasmon-polariton modes of a metamaterial-clad waveguide have less propagation\nloss than those of a metal-clad guide with the same permittivity. This low-loss\nmode is exploited for all-optical control of weak fields. Embedding three-level\n{\\Lambda} atoms in the dielectric core of a metamaterial-clad waveguide allows\nthe use of electromagnetically induced transparency to control an optical\nsignal. Adjusting the pump field alters the group velocity of the signal,\nthereby controllably delaying pulses.\n  Using the low-loss surface mode of a metamaterial-clad guide reduces losses\nby 20% over a metal cladding without sacrificing the group velocity reduction\nor confinement. In addition, we show that losses can be reduced by as much as\n40% with sufficient reduction of the magnetic damping constant of the\nmetamaterial.\n  As this work aims for applications, practical considerations for fabricating\nand testing metamaterial-clad waveguides are discussed. An overview of the\nbenefits and drawbacks for two different dielectric core materials is given.\nAlso, a short discussion of other modes that could be used is given.",
        "category": "physics_optics"
    },
    {
        "text": "Enhancement of radiative processes in nanofibers with embedded plasmonic\n  nanoparticles: Efficient manipulation and long distance transport of single-photons is a key\ncomponent in nanoscale quantum optics. In this letter, we study the emission\nproperties of an individual light emitter placed into a nanofiber and coupled\nto a metallic nanoparticle. We find that plasmonic field enhancement together\nwith the nanofiber optical confinement uniquely and synergistically contribute\nto an overall increase of emission rates as well as quantum yields.",
        "category": "physics_optics"
    },
    {
        "text": "Controlling steady-state second harmonic signal via linear and nonlinear\n  Fano resonances: Nonlinear signal even from a single molecule becomes visible at hot spots of\nplasmonic nanoparticles. In these structures, Fano resonances can control the\nnonlinear response in two ways. \\textit{(i)} A linear Fano resonance can\nenhance the hot spot field, resulting enhanced nonlinear signal. \\textit{(ii)}\nA nonlinear Fano resonance can enhance the nonlinear signal without enhancing\nthe hot spot. In this study, we compare the enhancement of second harmonic\nsignal at the steady-state obtained via these two methods. Since we are\ninterested in the steady-state signal, we adapt a linear enhancement which\nworks at the steady-state. This is different than the dark-hot resonances that\nappears in the transparency window due to enhanced plasmon lifetime.",
        "category": "physics_optics"
    },
    {
        "text": "A projection operator approach for computing the dynamics of AS2S3\n  chalcogenide birefringent photonic crystal fiber coupler: A variety of AS2S3 chalcogenide photonic crystal fiber coupler of special\nproperties are proposed to study the role of birefringence in all optical\ncoupling characteristics based on the projection operator method (POM). The\nequations of motion describing the dynamics of the individual pulse parameters\nthrough x- and y-polarized modes are arrived at by employing POM from the\ncoupled nonlinear Schr\\\"odinger equations. From the pulse parameter dynamics,\nit is observed that the amplitudes of the polarization components are\nsignificantly influenced by the pulse being introduced with different\npolarizing angle even at low input power level. Such a selective polarizing\nangles of the input pulse will provide efficient control over the desired\nsplitting ratio as well as the ability to decide the desired polarization\ncomponent.",
        "category": "physics_optics"
    },
    {
        "text": "Fresnel and Fraunhofer diffraction of a laser Gaussian beam by\n  fork-shaped gratings: Expressions describing the vortex beams, which are generated in a process of\nFresnel diffraction of a Gaussian beam, incident out of waist on a fork-shaped\ngratings of arbitrary integer charge p, and vortex spots in the case of\nFraunhofer diffraction by these gratings are deduced. The common general\ntransmission function of the gratings is defined and specialized for the cases\nof amplitude holograms, binary amplitude gratings, and their phase versions.\nOptical vortex beams, or carriers of phase singularity with charges mp and -mp,\nare the higher negative and positive diffraction order beams. The radial part\nof their wave amplitudes is described by the product of mp-th order\nGauss-doughnut function and a Kummer function, or by the first order\nGauss-doughnut function and a difference of two modified Bessel functions,\nwhose orders do not match the singularity charge value. The wave amplitude and\nthe intensity distributions are discussed for the near and far field, in the\nfocal plane of a convergent lens, as well as the specialization of the results\nwhen the grating charge p=0, i.e. the grating turns from forked into\nrectilinear. The analytical expressions for the vortex radii are also\ndiscussed.",
        "category": "physics_optics"
    },
    {
        "text": "Monte Carlo method for polarized radiative transfer in gradient-index\n  media: Light transfer in gradient-index media generally follows curved ray\ntrajectories, which will cause light beam to converge or diverge during\ntransfer and induce the rotation of polarization ellipse even when the medium\nis transparent. Furthermore, the combined process of scattering and transfer\nalong curved ray path makes the problem more complex. In this paper, a Monte\nCarlo method is presented to simulate polarized radiative transfer in\ngradient-index media that only support planar ray trajectories. The ray\nequation is solved to the second order to address the effect induced by curved\nray trajectories. Three types of test cases are presented to verify the\nperformance of the method, which include transparent medium, Mie scattering\nmedium with assumed gradient index distribution, and Rayleigh scattering with\nrealistic atmosphere refractive index profile. It is demonstrated that the\natmospheric refraction has significant effect for long distance polarized light\ntransfer.",
        "category": "physics_optics"
    },
    {
        "text": "Subwavelength plasmonic kinks in arrays of metallic nanoparticles: We analyze nonlinear effects in optically driven arrays of nonlinear metallic\nnanoparticles. We demonstrate that such plasmonic systems are characterized by\na bistable response, and they can support the propagation of dissipative\nswitching waves (or plasmonic kinks) connecting the states with different\npolarization. We study numerically the properties of such plasmonic kinks which\nare characterized by a subwavelength extent and a tunable velocity.",
        "category": "physics_optics"
    },
    {
        "text": "Mechanism of the ordered particles arrangement in a concentration\n  grating excited in the field of counter-propagating Gaussian beams: In the two-particle approximation, we consider the mechanism for the\nformation of an ordered arrangement of transparent spherical particles of small\nsize in a concentration grating excited by the gradient force of\ncounter-propagating Gaussian laser beams. This mechanism is due to the joint\naction of the transverse gradient force and the Coulomb force arising as a\nresult of dipole-dipole interactions between particles.",
        "category": "physics_optics"
    },
    {
        "text": "Periodic and solitary waves generating in optical fiber amplifiers and\n  fiber lasers with distributed parameters: We study self-similar dynamics of picosecond light pulses generating in\noptical fiber amplifiers and fiber lasers with distributed parameters. A rich\nvariety of periodic and solitary wave solutions are derived for the governing\ngeneralized nonlinear Schr\\\"{o}dinger equation with varying coefficients in the\npresence of gain effect. The constraint on distributed optical fiber parameters\nfor the existence of these wave solutions is presented. The dynamical behaviour\nof those self-similar waves is discussed in a periodic distributed\namplification system. The stability of periodic and solitary wave solutions is\nalso studied numerically by adding white noise. It is proved by using the\nnumerical split-step Fourier method that the profile of these nonlinear\nself-similar waves remains unchanged during evolution.",
        "category": "physics_optics"
    },
    {
        "text": "Acousto-optic holography for micrometer-scale grid patterning of\n  amplified laser pulses with single-pulse accuracy: Many optical systems use acousto-optics for fast steering of light. However,\nacousto-optic (AO) light diffraction permits manipulation and shaping of light\nfields in a much broader sense including holography. While acousto-optic\ndevices operate at incontestable speed, they are one dimensional (1D)\ndiffraction devices depending on analog serial data transfer and as such have a\nmode of use distinguished from common 2D spatial light modulators. Combination\nwith an amplified laser source allows to clock the serial buildup of AO\ndiffraction patterns for avoidance of phase aberration in the output beam due\nto acoustic wave progression. Pseudo 2D holography is established with two\ncrossed AO deflectors for biaxial spatial modulation of the input phase, and\noptionally also of its amplitude. In this configuration, acousto-optics permits\nlight patterning into micrometer-scale 2D point grids with the possibility to\nswitch the pattern between individual laser pulses. We describe algorithms for\nderiving AO holograms for either pure phase modulation or for a co-modulation\nof amplitude. Computationally reconstructed phase holograms show focal patterns\nin presence of side lobes. Side lobes are entirely suppressed by amplitude\nmodulation, though at the expense of reduced optical power transmission.",
        "category": "physics_optics"
    },
    {
        "text": "Modeling an Electrically Driven Graphene-Nanoribbon Laser for Optical\n  Interconnects: Graphene has two very important optical properties of population inversion of\nelectrons, and broadband optical gain. As a result, graphene has potential for\nuse in lasers and amplifiers. In this work, we presented a quantum master model\nand analyzed the properties for the electrically pumped single-AGNR\nvertical-cavity surface-emitting lasers (VCSELs) to investigate the lasing\naction and laser properties for realistic experimental parameters. A\nsemiclassical approximation for the output power and laser linewidth is also\nderived. The laser threshold power was several orders of magnitude lower than\nthat currently achievable with semiconductor microlasers. Our results have\ndemonstrated that a single-AGNR VCSEL can serve as a nanolaser with ultralow\nlasing threshold. Implementation of such a GNR-based VCSEL is especially\npromising for optical interconnection systems since VCSELs emit low optical\npower and single longitudinal mode over a wide wavelength spectral range\nthrough tailoring GNRs.",
        "category": "physics_optics"
    },
    {
        "text": "Flowing cryogenic liquid target for terahertz wave generation: Terahertz wave emission from condensed matter excited by intense laser pulses\nnot only reflects the details in laser-matter interaction but also offers\nbright terahertz wave sources. Flowing liquid targets possess the advantage of\nproviding a fresh area for each laser pulse. To demonstrate a debris-free\ntarget under laser excitation, we investigate the use of liquid nitrogen as a\ntarget. By creating a flowing liquid nitrogen line in the ambient environment,\nwe successfully observe broadband terahertz wave emission under short pulse\nexcitation. Our cryogenic line is able to sustain the excitation of a\nhigh-repetition-rate (1 kHz) laser. The terahertz peak field emitted from\nliquid nitrogen is comparable to that from liquid water, yet a broader\nbandwidth is observed. This demonstration prompts new opportunities in choosing\npotential materials for studying terahertz wave generation process and in\nunderstanding laser-induced ionization in different liquids.",
        "category": "physics_optics"
    },
    {
        "text": "Optimization of sharp and viewing-angle-independent structural color: Structural coloration produces some of the most brilliant colors in nature\nand has many applications. However, the two competing properties of narrow\nbandwidth and broad viewing angle have not been achieved simultaneously in\nprevious studies. Here, we use numerical optimization to discover geometries\nwhere a sharp 7% bandwidth in scattering is achieved, yet the peak wavelength\nvaries less than 1%, and the peak height and peak width vary less than 6% over\nbroad viewing angles (0--90$^\\circ$) under a directional illumination. Our\nmodel system consists of dipole scatterers arranged into several rings;\ninterference among the scattered waves is optimized to yield the\nwavelength-selective and angle-insensitive response. Such designs can be useful\nfor the recently proposed transparent displays that are based on\nwavelength-selective scattering.",
        "category": "physics_optics"
    },
    {
        "text": "Optimal interactions of light with magnetic and electric resonant\n  particles: This work studies the limits of far and near-field electromagnetic response\nof sub-wavelength scatterers, like the unitary limit and of lossless\nscatterers, and the ideal absorption limit of lossy particles. These limit\nbehaviors are described in terms of analytic formulas that approximate finite\nsize effects while rigorously including radiative corrections. This analysis\npredicts the electric and/or magnetic limit responses of both metallic and\ndielectric nanoparticles while quantitatively describing near-field\nenhancements.",
        "category": "physics_optics"
    },
    {
        "text": "Towards On-Chip Integrated Optical Quantum Frequency Combs: Recent development in quantum photonics allowed to start the process of\nbringing photonic-quantum-based systems out of the lab into real world\napplications. As an example, devices for the exchange of a cryptographic key\nsecured by the law of quantum mechanics are currently commercially available.\nIn order to further boost this process, the next step is to migrate the results\nachieved by means of bulky and expensive setups to miniaturized and affordable\ndevices. Integrated quantum photonics is exactly addressing this issue. In this\npaper we briefly review the most recent advancements in the generation of\nquantum states of light (at the core of quantum cryptography and computing) on\nchip. In particular, we focus on optical microcavities, as they can offer a\nsolution to the issue of low efficiency (low number of photons generated)\ntypical of the materials mostly used in integrated platforms. In addition, we\nshow that specifically designed microcavities can also offer further\nadvantages, such as compatibility with existing telecom standard (thus allowing\nto exploit the existing fiber network) and quantum memories (necessary in turns\nto extend the communication distance), as well as longitudinal multimode\ncharacter. This last property (i.e. the increased dimensionality necessary for\ndescribing the quantum state of a photon) is achieved thanks to the generating\nmultiple photon pairs on a frequency comb corresponding to the microcavity\nresonances. Further achievements include the possibility to fully exploit the\npolarization degree of freedom also for integrated devices. These results pave\nthe way to the generation of integrated quantum frequency combs, that in turn\nmay find application as quantum computing platform.",
        "category": "physics_optics"
    },
    {
        "text": "AC-Josephson Effect and Sub-Comb Mode-Locking in a Kerr-Induced\n  Synchronized Cavity Soliton: Kerr-induced synchronization (KIS) [1] involves the capture of a dissipative\nKerr soliton (DKS) microcomb [2] tooth by a reference laser injected into the\nDKS resonator. This phase-locking behavior is described by an Adler equation\nwhose analogous form describes numerous other physical systems [3], such as\nJosephson junctions [4]. We present an AC version of KIS whose behavior is\nsimilar to microwave-driven Josephson junctions, where periodic synchronization\noccurs as so-called Shapiro steps. We demonstrate consistent results in the\nAC-KIS dynamics predicted by the Adler model, Lugiato-Lefever equation, and\nexperimental data from a chip-integrated microresonator system. The (integer)\nShapiro steps in KIS can simply be explained as the sideband created through\nthe reference laser phase modulation triggering the synchronization. Notably,\nour optical system allows for easy tuning of the Adler damping parameter,\nenabling the further observation of fractional-Shapiro steps, where the\nsynchronization happens at a fraction of the driving microwave frequency. Here,\nwe show that the comb tooth is indirectly captured thanks to a four-wave mixing\nBragg-scattering process, leading to sub-comb mode-locking, and we demonstrate\nthis experimentally through noise considerations. Our work opens the door to\nthe study of synchronization phenomena in the context of microresonator\nfrequency combs, synthesis of condensed-matter state analogues with DKSs, and\nthe use of the fractional Shapiro steps for flexible and tunable access to the\nKIS regime.",
        "category": "physics_optics"
    },
    {
        "text": "Control of near-field radiative heat transfer based on anisotropic 2D\n  materials: In this work, we study the near-field radiative heat transfer between two\nsuspended sheets of anisotropic 2D materials. It is found that the radiative\nheat transfer can be enhanced with orders-of-magnitude over the blackbody limit\nfor nanoscale separation. The enhancement is attributed to the excitation of\nanisotropic and hyperbolic plasmonic modes. Meanwhile, a large thermal\nmodulation effect, depending on the twisted angle of principal axes between the\nupper and bottom sheets of anisotropic 2D materials, is revealed. The\nnear-field radiative heat transfer for different concentrations of electron is\ndemonstrated and the role of hyperbolic plasmonic modes is analyzed. Our\nfinding of radiative heat transfer between anisotropic 2D materials may find\npromising applications in thermal nano-devices, such as non-contact thermal\nmodulators, thermal lithography, thermos-photovoltaics, etc.",
        "category": "physics_optics"
    },
    {
        "text": "A Millimeter-Wave Achromatic Half Wave Plate: We have constructed an achromatic half wave plate (AHWP) suitable for the\nmillimeter wavelength band. The AHWP was made from a stack of three sapphire\na-cut birefringent plates with the optical axes of the middle plate rotated by\n50.5 degrees with respect to the aligned axes of the other plates. The measured\nmodulation efficiency of the AHWP at 110 GHz was $96 \\pm 1.5$%. In contrast,\nthe modulation efficiency of a single sapphire plate of the same thickness was\n$43 \\pm 4$%. Both results are in close agreement with theoretical predictions.\nThe modulation efficiency of the AHWP was constant as a function of incidence\nangles between 0 and 15 degrees. We discuss design parameters of an AHWP in the\ncontext of astrophysical broad band polarimetry at the millimeter wavelength\nband.",
        "category": "physics_optics"
    },
    {
        "text": "Spatiotemporal characterization of ultrashort optical vortex pulses: Generation of few-cycle optical vortex pulses is challenging due to the large\nspectral bandwidths, as most vortex generation techniques are designed for\nmonochromatic light. In this work, we use a spiral phase plate to generate\nfew-cycle optical vortices from an ultrafast titanium:sapphire oscillator, and\ncharacterize them in the spatiotemporal domain using a recently introduced\ntechnique based on spatially resolved Fourier transform spectrometry. The\nperformance of this simple approach to the generation of optical vortices is\nanalyzed from a wavelength dependent perspective, as well as in the\nspatiotemporal domain, allowing us to completely characterize ultrashort vortex\npulses in space, frequency, and time.",
        "category": "physics_optics"
    },
    {
        "text": "Multimode Mamyshev Oscillator: We present a spatiotemporally mode-locked Mamyshev oscillator. A wide variety\nof multimode mode-locked states, with varying degrees of spatiotemporal\ncoupling, are observed. We find that some control of the modal content of the\noutput beam is possible through the cavity design. Comparison of simulations to\nexperiments indicates that spatiotemporal mode-locking is enabled by nonlinear\nintermodal interactions and spatial filtering, along with the Mamyshev\nmechanism. This work represents a first exploration of spatiotemporal\nmode-locking in an oscillator with the Mamyshev saturable absorber.",
        "category": "physics_optics"
    },
    {
        "text": "Temporal Fresnel diffraction induced by phase jumps in linear and\n  nonlinear optical fibres: We analytically and numerically study the temporal intensity pattern emerging\nfrom the linear or nonlinear evolutions of a single or double phase jump in an\noptical fiber. The results are interpreted in terms of interferences of the\nwell-known diffractive patterns of a straight edge, strip and slit and a\ncomplete analytical framework is provided in terms of Fresnel integrals for the\ncase of purely dispersive evolution. When Kerr nonlinearity affects the\npropagation, various coherent nonlinear structures emerge according to the\nregime of dispersion.",
        "category": "physics_optics"
    },
    {
        "text": "Omnidirectional total transmission at the interface associated with an\n  anisotropic dielectric-magnetic metamaterial: Based on the Ewald-Oseen extinction theorem, the omnidirectional total\ntransmission of waves incident from vacuum into an anisotropic\ndielectric-magnetic metamaterials is investigated. It is shown that the\nomnidirectional total transmission need not limit at the interface associated\nwith the conventional nonmagnetic anisotropic medium. The recent advent of a\nnew class of anisotropic dielectric-magnetic matermaterial make the\nomnidirectional total transmission become available. It is found that the\ninherent physics underlying the omnidirectional total transmission are\ncollective contributions of the electric and magnetic responses.",
        "category": "physics_optics"
    },
    {
        "text": "Ultra-high endurance silicon photonic memory using vanadium dioxide: Silicon photonics arises as a viable solution to address the stringent\nresource demands of emergent technologies, such as neural networks. Within this\nframework, photonic memories are fundamental building blocks of photonic\nintegrated circuits that have not yet found a standardized solution due to\nseveral trade-off among different metrics such as energy consumption, speed,\nfootprint, or fabrication complexity, to name a few. In particular, a photonic\nmemory exhibiting ultra-high endurance performance (> 10^6 cycles) has been\nelusive to date. Here, we report an ultra-high endurance silicon photonic\nmemory using vanadium dioxide (VO_2) exhibiting a record cyclability of up to\n10^7 cycles without degradation. Moreover, our memory features an ultra-compact\nfootprint below 5 {\\mu}m with potential for nanosecond and picojoule\nprogramming performance. Our silicon photonic memory could find application in\nemerging photonic applications demanding high number of memory updates such as\nphotonic neural networks with in-situ training.",
        "category": "physics_optics"
    },
    {
        "text": "Weak Kerr Nonlinearity Boosts the Performance of Frequency-Multiplexed\n  Photonic Extreme Learning Machines: A Multifaceted Approach: We provide a theoretical, numerical, and experimental investigation of the\nKerr nonlinearity impact on the performance of a frequency-multiplexed Extreme\nLearning Machine (ELM). In such ELM, the neuron signals are encoded in the\nlines of a frequency comb. The Kerr nonlinearity facilitates the randomized\nneuron connections allowing for efficient information mixing. A programmable\nspectral filter applies the output weights. The system operates in a\ncontinuous-wave regime. Even at low input peak powers, the resulting weak Kerr\nnonlinearity is sufficient to significantly boost the performance on several\ntasks. This boost already arises when one uses only the very small Kerr\nnonlinearity present in a 20-meter long erbium-doped fiber amplifier. In\ncontrast, a subsequent propagation in 540 meters of a single-mode fiber\nimproves the performance only slightly, whereas additional information mixing\nwith a phase modulator does not result in a further improvement at all. We\nintroduce a model to show that, in frequency-multiplexed ELMs, the Kerr\nnonlinearity mixes information via four-wave mixing, rather than via self- or\ncross-phase modulation. At low powers, this effect is quartic in the comb-line\namplitudes. Numerical simulations validate our experimental results and\ninterpretation.",
        "category": "physics_optics"
    },
    {
        "text": "Influence of sensor tilts on bio-inspired polarized skylight orientation\n  determination: Inspired by many insects, several polarized skylight orientation\ndetermination approaches have been proposed. However, almost all of these\napproaches always require polarization sensor pointing to the zenith of the sky\ndome. So, the influence of sensor tilts (not point to the sky zenith) on\nbio-inspired polarization orientation determination needs to be analyzed\nurgently. Aiming at this problem, a polarization compass simulation system is\ndesigned based upon solar position model, Rayleigh sky model, and hypothetical\npolarization imager. Then, the error characteristics of four typical\norientation determination approaches are investigated in detail under only\npitch tilt condition, only roll tilt condition, pitch and roll tilts condition\nrespectively. Finally, simulation and field experiments all show that the\norientation errors of four typical approaches are highly consistent when they\nare subjected to tilt interference, in addition, the errors are affected by not\nonly the degree of inclination, but also the solar altitude angle and the\nrelative position between the Sun and polarization sensor. The results of this\npaper can be used to estimate the orientation determination error caused by\nsensor tilts and correct this kind of error.",
        "category": "physics_optics"
    },
    {
        "text": "Goos-Haenchen lateral displacements and angular deviations: When these\n  optical effects offset each other: For optical beams, transmitted by a right angle prism, the Goos-Haenchen\nshift can never be seen as a pure effect. Indeed, the lateral displacement,\ncaused by the total internal reflection, will always be accompanied by angular\ndeviations generated by the transmission through the incoming and outgoing\ninterfaces. This combined effect can be analysed by using the Taylor expansion\nof the Fresnel coefficients. The analytic expression found for the transmitted\nbeam allows to determine the beam parameters, the incidence angles, and the\naxial distance for which lateral displacements are compensated by angular\ndeviations. Proposals to optimize experimental implementations are also briefly\ndiscussed.",
        "category": "physics_optics"
    },
    {
        "text": "Mode-locked ultrashort pulse generation from on-chip normal dispersion\n  microresonators: We describe the generation of stable mode-locked pulse trains from on-chip\nnormal dispersion microresonators. The excitation of hyper-parametric\noscillation is facilitated by the local dispersion disruptions induced by mode\ninteractions. The system is then driven from hyper-parametric oscillation to\nthe mode-locked state with over 200 nm spectral width by controlled pump power\nand detuning. With the continuous-wave driven nonlinearity, the pulses sit on a\npedestal, akin to a cavity soliton. We identify the importance of pump detuning\nand wavelength-dependent quality factors in stabilizing and shaping the pulse\nstructure, to achieve a single pulse inside the cavity. We examine the mode\nlocking dynamics by numerically solving the master equation and provide\nanalytic solutions under appropriate approximations.",
        "category": "physics_optics"
    },
    {
        "text": "Optical coherence properties of planar microcavity emission: An analytical expression for the self coherence function of a microcavity and\npartially coherent source is derived from first principles in terms of the\ncomponent self coherence functions. Excellent agreement between the model and\nexperimental measurements of two Resonant Cavity LEDs (RCLEDs) is evident. The\nvariation of coherence length as a function of numerical aperture is also\ndescribed by the model. This is explained by a microcavity's angular\nsensitivity in filtering out statistical fluctuations of the underlying light\nsource. It is further demonstrated that the variable coherence properties of\nplanar microcavities can be designed by controlling the underlying coherences\nof microcavity and emitter whereby coherence lengths ranging over nearly an\norder of magnitude could be achieved.",
        "category": "physics_optics"
    },
    {
        "text": "Frequency selection by soliton excitation in nondegenerate intracavity\n  downconversion: We show that soliton excitation in intracavity downconversion naturally\nselects a strictly defined frequency difference between the signal and idler\nfields. In particular, this phenomenon implies that if the signal has smaller\nlosses than the idler then its frequency is pulled away from the cavity\nresonance and the idler frequency is pulled towards the resonance and {\\em vice\nversa}. The frequency selection is shown to be closely linked with the relative\nenergy balance between the idler and signal fields.",
        "category": "physics_optics"
    },
    {
        "text": "Spatial distributions of the fields in guided normal modes of two\n  coupled parallel optical nanofibers: We study the cross-sectional profiles and spatial distributions of the fields\nin guided normal modes of two coupled parallel optical nanofibers. We show that\nthe distributions of the components of the field in a guided normal mode of two\nidentical nanofibers are either symmetric or antisymmetric with respect to the\nradial principal axis and the tangential principal axis in the cross-sectional\nplane of the fibers. The symmetry of the magnetic field components with respect\nto the principal axes is opposite to that of the electric field components. We\nshow that, in the case of even $\\mathcal{E}_z$-cosine modes, the electric\nintensity distribution is dominant in the area between the fibers, with a\nsaddle point at the two-fiber center. Meanwhile, in the case of odd\n$\\mathcal{E}_z$-sine modes, the electric intensity distribution at the\ntwo-fiber center attains a local minimum of exactly zero. We find that the\ndifferences between the results of the coupled mode theory and the exact mode\ntheory are large when the fiber separation distance is small and either the\nfiber radius is small or the light wavelength is large. We show that, in the\ncase where the two nanofibers are not identical, the intensity distribution is\nsymmetric about the radial principal axis and asymmetric about the tangential\nprincipal axis.",
        "category": "physics_optics"
    },
    {
        "text": "The energy point of view in plasmonics: The group velocity of a plasmonic guided mode can be written as the ratio of\nthe flux of the Poynting to the integral of the energy density along the\nprofile of the mode. This theorem, linking the way energy propagates in metals\nto the properties of guided modes and Bloch modes in a multilayer, provides a\nunique physical insight in plasmonics. It allows to better understand the link\nbetween the negative permittivity of metals and the wide diversity of exotic\nphenomenon that occur in plasmonics -- like the slowing down of guided modes,\nthe high wavevector and the negative refraction.",
        "category": "physics_optics"
    },
    {
        "text": "Optical and microwave metrology at the 10-18 level with an Er/Yb:glass\n  frequency comb: Optical frequency combs are an essential tool for precision metrology\nexperiments ranging in application from remote spectroscopic sensing of trace\ngases to the characterization and comparison of optical atomic clocks for\nprecision time-keeping and searches for physics beyond the standard model. Here\nwe describe the architecture and fully characterize a telecom-band,\nself-modelocking frequency comb based on a free-space laser with an Er/Yb\nco-doped glass gain medium. The laser provides a robust and cost-effective\nalternative to Er:fiber laser based frequency combs, while offering stability\nand noise performance similar to Ti:sapphire laser systems. Finally, we\ndemonstrate the Er/Yb:glass frequency comb's utility in high-stability\nfrequency synthesis using two ultra-stable optical references at 1157 nm and\n1070 nm and in low-noise photonic microwave generation by dividing these\nreferences to the microwave domain.",
        "category": "physics_optics"
    },
    {
        "text": "Cascade circuit architecture for RF-photonic frequency multiplication\n  with minimum RF energy: A general RF-photonic circuit design for implementing frequency\nmultiplication is proposed. The circuit consists of N cascaded\ndifferentially-driven Mach-Zehnder modulators biased at the minimum\ntransmission point with a progressive RF phase shift of 180/N applied to each\nstage. The frequency multiplication factor obtained is 2N. The novelty of the\ndesign is the reduced input RF energy required in comparison to the\nfunctionally equivalent parallel MZM configuration. Using transfer matrix\nmethod, a N=3 architecture is modeled to obtain frequency sextupling. An\nindustry standard simulation tool is used to verify the architectural concepts\nand analysis. The proposed design requires no optical or electrical filtering\nnor careful adjustment of the modulation index for correct operation. In\naddition, the overall intrinsic conversion efficiency of the N=3 cascade\ncircuit is improved by 5 dB over a parallel MZM circuit. Finite MZM extinction\nand/or phase errors and power imbalances in the electric drive signals are also\ntaken into consideration and their impact on overall performance investigated.\nThe circuit can be integrated in any material platform that offers\nelectro-optic modulators.",
        "category": "physics_optics"
    },
    {
        "text": "A Theoretical Multi-Reflection Method for Analysis of Optomechanical\n  Behavior of the Fabry-Perot Cavity with Moving Boundary Condition: This paper has been withdrawn by the author due to a crucial sign error in\nequation 1.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of radiation torque shot noise on an optically levitated\n  nanodumbbell: According to quantum theory, measurement and backaction are inextricably\nlinked. In optical position measurements, this backaction is known as radiation\npressure shot noise. In analogy, a measurement of the orientation of a\nmechanical rotor must disturb its angular momentum by radiation torque shot\nnoise. In this work, we observe the shot-noise torque fluctuations arising in a\nmeasurement of the angular orientation of an optically levitated nanodumbbell.\nWe feedback cool the dumbbell's rotational motion and investigate its reheating\nbehavior when released from feedback. In high vacuum, the heating rate due to\nradiation torque shot noise dominates over the thermal and technical heating\nrates in the system.",
        "category": "physics_optics"
    },
    {
        "text": "Hybrid silicon-organic Huygens' metasurface for phase modulation: Spatial light modulators have desirable applications in sensing and free\nspace communication because they create an interface between the optical and\nelectronic realms. Electro-optic modulators allow for high-speed intensity\nmanipulation of an electromagnetic wavefront. However, most surfaces of this\nsort pose limitations due to their ability to modulate intensity rather than\nphase. Here we investigate an electro-optic modulator formed from a\nsilicon-organic Huygens' metasurface. In a simulation-based study, we discover\na metasurface design immersed in high-performance electro-optic molecules that\ncan achieve near-full resonant transmission with phase coverage over the full\n2$\\pi$ range. Through the electro-optic effect, we show 140$^\\circ$ (0.79$\\pi$)\nmodulation over a range of -100 to 100 V at 1330 nm while maintaining\nnear-constant transmitted field intensity (between 0.66 and 0.8). These results\npotentiate the fabrication of a high-speed spatial light modulator with the\nresolved parameters.",
        "category": "physics_optics"
    },
    {
        "text": "Weight Bank Addition Photonic Accelerator for Artificial Intelligence: Neural networks powered by artificial intelligence play a pivotal role in\ncurrent estimation and classification applications due to the escalating\ncomputational demands of evolving deep learning systems. The hindrances posed\nby existing computational limitations threaten to impede the further\nprogression of these neural networks. In response to these issues, we propose\nneuromorphic networks founded on photonics that offer superior processing speed\nthan electronic counterparts, thereby enhancing support for real time, three\ndimensional, and virtual reality applications. The weight bank, an integral\ncomponent of these networks has a direct bearing on their overall performance.\nOur study demonstrates the implementation of a weight bank utilizing parallelly\ncascaded micro ring resonators. We present our observations on neuromorphic\nnetworks based on silicon on insulators, where cascaded MRRs play a crucial\nrole in mitigating interchannel and intrachannel cross talk, a persistent issue\nin wavelength division multiplexing systems. Additionally, we design a standard\nsilicon photonic accelerator to perform weight addition. Optimized to offer\nincreased speed and reduced energy consumption, this photonic accelerator\nensures comparable processing power to electronic devices.",
        "category": "physics_optics"
    },
    {
        "text": "Probing optical anapoles with fast electron beams: Optical anapoles are intriguing charge-current distributions characterized by\na strong suppression of electromagnetic radiation. They originate from the\ndestructive interference of the radiation produced by electric and toroidal\nmultipoles. Although anapoles in dielectric structures have been probed and\nmapped with a combination of near- and far-field optical techniques, their\nexcitation using fast electron beams has not been explored so far. Here, we\ntheoretically and experimentally analyze the excitation of optical anapoles in\ntungsten disulfide (WS$_2$) nanodisks using Electron Energy Loss Spectroscopy\n(EELS) in Scanning Transmission Electron Microscopy (STEM). We observe\nprominent dips in the electron energy loss spectra and associate them with the\nexcitation of optical anapoles and anapole-exciton hybrids. We are able to map\nthe anapoles excited in the WS$_2$ nanodisks with subnanometer resolution and\nfind that their excitation can be controlled by placing the electron beam at\ndifferent positions on the nanodisk. Considering current research on the\nanapole phenomenon, we envision EELS in STEM to become a useful tool for\naccessing optical anapoles appearing in a variety of dielectric nanoresonators.",
        "category": "physics_optics"
    },
    {
        "text": "Optimised low-loss multilayers for imaging with sub-wavelength\n  resolution in the visible wavelength range: We optimise the effective skin-depth and resolution of Ag-TiO2, Ag-SrTiO3,\nand Ag-GaP multilayers for imaging with sub-wavelength resolution. In terms of\ntransmission and resolution the optimised multilayers outperform simple designs\nbased on combined use of effective medium theory, impedance matching and\nFabry-Perot resonances. For instance, an optimised Ag-GaP multilayer consisting\nof only 17 layers, operating at the wavelength of 490 nm and having a total\nthickness equal to one wavelength, combines 78% intensity transmission with a\nresolution of 60 nm. It is also shown that use of the effective medium theory\nleads to sub-optimal multilayer designs with respect to the trade-off between\nthe skin depth and resolution already when the period of the structure is on\nthe order of 40 nm or larger.",
        "category": "physics_optics"
    },
    {
        "text": "Polarization-tailored Fano interference in plasmonic crystals: A Mueller\n  matrix model of anisotropic Fano resonance: We present a simple yet elegant Mueller matrix approach for controlling the\nFano interference effect and engineering the resulting asymmetric spectral line\nshape in anisotropic optical system. The approach is founded on a generalized\nmodel of anisotropic Fano resonance, which relates the spectral asymmetry to\ntwo physically meaningful and experimentally accessible parameters of\ninterference, namely, the Fano phase shift and the relative amplitudes of the\ninterfering modes. The differences in these parameters between orthogonal\nlinear polarizations in an anisotropic system are exploited to desirably tune\nthe Fano spectral asymmetry using pre- and post-selection of optimized\npolarization states. Experimental control on the Fano phase and the relative\namplitude parameters and resulting tuning of spectral asymmetry is demonstrated\nin waveguided plasmonic crystals using Mueller matrix-based polarization\nanalysis. The approach enabled tailoring of several exotic regimes of Fano\nresonance including the complete reversal of the spectral asymmetry. The\ndemonstrated control and the ensuing large tunability of Fano resonance in\nanisotropic systems shows potential for Fano resonance-based applications\ninvolving control and manipulation of electromagnetic waves at the nano scale.",
        "category": "physics_optics"
    },
    {
        "text": "Graphene hyperlens for terahertz radiation: We propose a graphene hyperlens for the terahertz (THz) range. We employ and\nnumerically examine a structured graphene-dielectric multilayered stack that is\nan analogue of a metallic wire medium. As an example of the graphene hyperlens\nin action we demonstrate an imaging of two point sources separated with\ndistance $\\lambda_{0}/5$. An advantage of such a hyperlens as compared to a\nmetallic one is the tunability of its properties by changing the chemical\npotential of graphene. We also propose a method to retrieve the hyperbolic\ndispersion, check the effective medium approximation and retrieve the effective\npermittivity tensor.",
        "category": "physics_optics"
    },
    {
        "text": "Photonic thermal diode enabled by surface polariton coupling in\n  nanostructures: A novel photonic thermal diode concept operating in the near field and\ncapitalizing on the temperature-dependence of coupled surface polariton modes\nin nanostructures is proposed. The diode concept utilizes terminals made of the\nsame material supporting surface polariton modes in the infrared, but with\ndissimilar structures. The specific diode design analyzed in this Letter\ninvolves a thin film and a bulk, both made of 3C silicon carbide, separated by\na subwavelength vacuum gap. High rectification efficiency is obtained by tuning\nthe antisymmetric resonant modes of the thin film, resulting from surface\nphonon-polariton coupling, on- and off-resonance with the resonant mode of the\nbulk as a function of the temperature bias direction. Rectification efficiency\nis investigated by varying structural parameters, namely the vacuum gap size,\nthe dielectric function of the substrate onto which the film is coated, and the\nfilm thickness to gap size ratio. Calculations based on fluctuational\nelectrodynamics reveal that high rectification efficiencies in the 80% to 87%\nrange can be maintained in a wide temperature band (~ 700 K to 1000 K). The\nrectification efficiency of the proposed diode concept can be potentially\nfurther enhanced by investigating more complex nanostructures such as gratings\nand multilayered media.",
        "category": "physics_optics"
    },
    {
        "text": "Random laser from engineered nanostructures obtained by surface tension\n  driven lithography: The random laser emission from the functionalized thienyl-S,S-dioxide\nquinquethiophene (T5OCx) in confined patterns with different shapes is\ndemonstrated. Functional patterning of the light emitter organic material in\nwell defined features is obtained by spontaneous molecular self-assembly guided\nby surface tension driven (STD) lithography. Such controlled supramolecular\nnano-aggregates act as scattering centers allowing the fabrication of\none-component organic lasers with no external resonator and with desired shape\nand efficiency. Atomic force microscopy shows that different geometric pattern\nwith different supramolecular organization obtained by the lithographic process\ntailors the coherent emission properties by controlling the distribution and\nthe size of the random scatterers.",
        "category": "physics_optics"
    },
    {
        "text": "Stochastic Pulse Switching in a Degenerate Resonant Optical Medium: Using the idealized integrable Maxwell-Bloch model, we describe random\noptical-pulse polarization switching along an active optical medium in the\nLambda-configuration with disordered occupation numbers of its lower energy\nsub-level pair. The description combines complete integrability and stochastic\ndynamics. For the single-soliton pulse, we derive the statistics of the\nelectric-field polarization ellipse at a given point along the medium in closed\nform. If the average initial population difference of the two lower sub-levels\nvanishes, we show that the pulse polarization will switch intermittently\nbetween the two circular polarizations as it travels along the medium. If this\ndifference does not vanish, the pulse will eventually forever remain in the\ncircular polarization determined by which sub-level is more occupied on\naverage. We also derive the exact expressions for the statistics of the\npolarization-switching dynamics, such as the probability distribution of the\ndistance between two consecutive switches and the percentage of the distance\nalong the medium the pulse spends in the elliptical polarization of a given\norientation in the case of vanishing average initial population difference. We\nfind that the latter distribution is given in terms of the well-known arcsine\nlaw.",
        "category": "physics_optics"
    },
    {
        "text": "Exceptional Point Singularities in Multi-Section DFB Lasers: A laser exhibits both controllable gain and loss and, under proper design\nconditions, is an ideal non-Hermitian system allowing the direct observation\nand engineering of spectral singularities such as exceptional points (EPs). A\ndual section distributed feedback (DFB) quantum cascade laser (QCL) is a\nprototype of such a system, allowing the controlled coupling of a ladder of\ncavity Fabry-Perot (FP) modes to a quarter wave shifted DFB mode. Tuning the\ncoupling strength and the gain difference between these two set of modes\nenables probing the regimes from weak coupling to strong coupling and the\nrobust observation of exceptional point singularities. At these exceptional\npoints, the laser exhibits a sequence of lasing and coherent prefect absorption\ndynamics1,2 when pumped above transparency. Additionally, the pumping scheme\nallows the deliberate lifting of the exceptional point degeneracy. These\nresults show that dual section QCL is a perfect platform to study exceptional\npoints because the coupling parameter and system loss can be tuned in a single\ndevice.",
        "category": "physics_optics"
    },
    {
        "text": "Enhanced beam shifts mediated by Bound States in Continuum: The interaction of light beams with resonant structures has led to the\ndevelopment of various optical platforms for sensing, particle manipulation,\nand strong light-matter interaction. In the current study, we investigate the\nmanifestations of the bound states in continuum (BIC) on the in plane and out\nof plane shifts (referred to as Goos-Hanchen (GH) and Imbert-Fedorov (IF)\nshifts, respectively) of a finite beam with specific polarization incident at\nan arbitrary angle. Based on the angular spectrum decomposition, we develop a\ngeneric formalism for understanding the interaction of the finite beam with an\narbitrary stratified medium with isotropic and homogeneous components. it is\napplied to the case of a Gaussian beam with p and circularly polarized light\nincident on a symmetric structure containing two polar dielectric layers\nseparated by a spacer layer. For p-polarized plane wave incidence one of the\ncoupled Berreman modes of the structure was recently shown to evolve to the\nbound state with infinite localization and diverging quality factor coexisting\nwith the other mode with large radiation leakage (Remesh et al. Optics\nCommunications, 498:127223, 2021). A small deviation from the ideal BIC\nresonance still offers resonances with very high quality factors and these are\nexploited in this study to report giant GH shifts. A notable enhancement in the\nIF shift for circularly polarized light is also shown. Moreover, the reflected\nbeam is shown to undergo distortion leading to a satellite spot. The origin of\nsuch a splitting of the reflected beam is traced to a destructive interference\ndue to the left and right halves of the corresponding spectra.",
        "category": "physics_optics"
    },
    {
        "text": "Three-dimensional light bullets in a Bragg medium with carbon nanotubes: We present a theoretical study of the propagation of three-dimensional\nextremely short electromagnetic pulses (a.k.a. light bullets) through a Bragg\nmedium containing an immersed array of carbon nanotubes. We demonstrate the\npossible stable propagation of such light bullets. In particular, our results\nsuggest these light bullets can carry information about the Bragg medium\nitself.",
        "category": "physics_optics"
    },
    {
        "text": "High-Q plasmonic crystal laser for ultra-sensitive biomolecule detection: Plasmonic lasers provide a paradigm-changing approach for the generation of\ncoherent light at the nanoscale. In addition to the usual properties of\ncoherent radiation, the emission of plasmonic lasers can feature high\nsensitivity to the surrounding environment, which makes this technology\nattractive for developing high-performance and highly-integrated sensing\ndevices. Here, we investigate a plasmonic laser architecture based on a high-Q\nplasmonic crystal consisting of a periodic arrangement of nanoholes on a thin\ngold film cladded with an organic-dye-doped SiO$_2$ gain layer as the gain\nmaterial. We report an extensive full-wave numerical analysis of the device's\nlasing performance and its application as a biochemical sensor, showing that\nthe proposed design features excellent figures of merit for surface sensing\nthat in principle can be over an order of magnitude larger than those of\npreviously reported high-performance plasmonic biosensor architectures.",
        "category": "physics_optics"
    },
    {
        "text": "Photonic glass for high contrast structural color: Non-iridescent structural colors based on disordered arrangement of\nmonodisperse spherical particles, also called photonic glass, show low color\nsaturation due to gradual transition in reflectivity. No significant\nimprovement is usually expected from particles optimization, as the Mie\nresonances are broad for small dielectric particles with moderate refractive\nindex. Moreover, the short range order of a photonic glass alone is also\ninsufficient to cause sharp spectral features. We show here, that the\ncombination of a well-chosen particle geometry with the short range order of a\nphotonic glass has strong synergetic effects. We demonstrate how core-shell\nparticles can be used to obtain a sharp transition in the reflection spectrum\nof photonic glass which is essential to achieve a strong color saturation. The\nFourier transform required for a highly saturated color can be achieved by\nshifting the first zero position of the motif Fourier transform to smaller wave\nnumbers in respect to the peak of the lattice Fourier transform. We show that\nthis can be obtained by choosing a non-monotonous refractive index distribution\nfrom the center of the particle through the shell and into the background\nmaterial. The first-order theoretical predictions are confirmed by numerical\nsimulations.",
        "category": "physics_optics"
    },
    {
        "text": "Master-Slave synchronization of silicon optomechanical nanobeam\n  oscillators by external feedback: The remote synchronization of oscillators is essential for improving the\nperformance, efficiency, and reliability of various systems and technologies,\nranging from everyday telecommunications to cutting-edge scientific research\nand emerging technologies. In this work, we unequivocally demonstrate a\nmaster-slave type of synchronization between two self-sustained optomechanical\ncrystal oscillators that interact solely through an external optical feedback\nstage. Several pieces of experimental evidence rule out the possibility of\nresonant forcing, and, in contrast to previous works, indicate that\nsynchronization is achieved in the regime of natural dynamics suppression. Our\nexperimental results are in agreement with the predictions of a numerical model\ndescribing the specific mechanical lasing dynamics of each oscillator and the\nunidirectional interaction between them. The outcomes of our study pave the way\ntoward the synchronization of clock signals corresponding to far-placed\nprocessing elements in a future synchronous photonic integrated circuit.",
        "category": "physics_optics"
    },
    {
        "text": "Soliton-induced relativistic-scattering and amplification: Solitons are of fundamental importance in photonics due to applications in\noptical data transmission and also as a tool for investigating novel phenomena\nranging from light generation at new frequencies and wave-trapping to rogue\nwaves. Solitons are also relativistic scatterers: they generate\nrefractive-index perturbations moving at the speed of light. Here we found that\nsuch perturbations scatter light in an unusual way: they amplify light by the\nmixing of positive and negative frequencies, as we describe using a first Born\napproximation and numerical simulations. The simplest scenario in which these\neffects may be observed is within the initial stages of optical soliton\npropagation: a steep shock front develops that may efficiently scatter a\nsecond, weaker probe pulse into relatively intense positive and negative\nfrequency modes with amplification at the expense of the soliton. Our results\nshow a novel all-optical amplification scheme that relies on relativistic\nscattering.",
        "category": "physics_optics"
    },
    {
        "text": "Optimizing the electronic control loop of a solid-state ring laser\n  gyroscope: We study in this Letter the dynamical effects of the limited bandwidth of the\ncontrol electronics in a solid-state (Nd-YAG) ring laser gyroscope. We derive a\nstability condition for the rotation-sensing regime in the case of a\nfirst-order control loop, showing that the smallest measurable rotation speed\ndepends directly on the cutoff frequency value. Our experimental measurements\nare in good agreement with this prediction.",
        "category": "physics_optics"
    },
    {
        "text": "Estereoscopio com tela holografica para ver tomografias: A estereoscopia eh uma tecnica que permite a observacao de imagens\ntridimensionais, mas estah sempre associada com o uso de algum equipamento\nespecial para visualizacao, como oculos bicolores ou polarizados. Para o uso em\naplicacoes medicas o emprego de tais equipamentos pode inviabilizar sua\nutilizacao durante procedimentos cirurgicos, por exemplo. Neste trabalho\napresentamos um novo tipo de estereoscopio que utiliza uma tela holografica\npara geracao de imagens tridimensionais sem o uso de qualquer equipamento\nadicional. Apresentamos a descricao do equipamento utilizado e resultados das\nimagens visualizadas.",
        "category": "physics_optics"
    },
    {
        "text": "Light tunneling inhibition and anisotropic diffraction engineering in\n  two-dimensional waveguide arrays: We address two-dimensional waveguide arrays where light tunneling into\nneighboring waveguides may be effectively suppressed by out-of-phase harmonic\nmodulation of the refractive index in neighboring waveguides at suitable\nfrequencies. Genuine two-dimensional features, such as anisotropic diffraction\nengineering, diffraction-free propagation along selected directions in the\ntransverse plane and tunneling inhibition for multi-channel vortices, are shown\nto occur.",
        "category": "physics_optics"
    },
    {
        "text": "Tunable Bloch surface waves devices: This thesis is devoted to develop tunable devices on the base of\none-dimensional photonic crystals (1DPhC) which can sustain Bloch surface waves\n(BSWs). First, we explore the possibilities to control the BSW propagation\ndirection with polarization of incident light. In this case we manufacture\nadditional passive structures such as gratings on the top of the 1DPhC, which\nare working both as a BSW launcher and polarization-controlled wave-splitters.\nWe test this type of launcher in air and in water as an external medium. Then,\nwe demonstrate the tunability of the BSW by adding an active layers into the\nmultilayer stack. Here a crystalline X-cut thin film lithium niobate (TFLN) is\nused to introduce anisotropic properties to the whole 1DPhC. Different ways to\nmanufacture 1D PhCs with LiNbO3 on the top would be described. Finally, we\nexplore the concept of the electro-optically tuned BSW.",
        "category": "physics_optics"
    },
    {
        "text": "Wavefront sensing based on the inverted Hartmann sensor: The classic Hartmann test consists of an array of holes to reconstruct the\nwavefront from the local deviation of each focal spot, and Shack-Hartmann\nsensor improved that with an array of microlenses. This array of microlenses\nimposes practical limitations when the wavefront is not into of visible\nwavelengths, e.g., the fabrication of these. Instead, we propose a\nwavefront-sensing technique using an array of circular obstructions, i.e., the\nHartmann sensor with an inverted Hartmann array. We show that under the same\nconditions the inverted Hartmann sensor and the Shack-Hartmann wavefront sensor\nhave an equivalent spot-map to recover the wavefront. The method might used in\nnon-visible wavelengths and in a wider range of applications.",
        "category": "physics_optics"
    },
    {
        "text": "Two-color optically-addressed spatial light modulator as generic\n  spatio-temporal systems: Nonlinear spatio-temporal systems are the basis for countless physical\nphenomena in such diverse fields as ecology, optics, electronics and\nneuroscience. The canonical approach to unify models originating from different\nfields is the normal form description, which determines the generic dynamical\naspects and different bifurcation scenarios. Realizing different types of\ndynamical systems via one experimental platform that enables continuous\ntransition between normal forms through tuning accessible system parameters is\ntherefore highly relevant. Here, we show that a transmissive,\noptically-addressed spatial light modulator under coherent optical illumination\nand optical feedback coupling allows tuning between pitchfork, transcritical\nand saddle-node bifurcations of steady states. We demonstrate this by\nanalytically deriving the system's normal form equations and confirm these\nresults via extensive numerical simulations. Our model describes a nematic\nliquid crystal device using nano-dimensional dichalcogenide (a-As$_2$S$_3$)\nglassy thin-films as photo sensors and alignment layers, and we use device\nparameters obtained from experimental characterization. Optical coupling, for\nexample using diffraction, holography or integrated unitary maps allow\nimplementing a variety of system topologies of technological relevance for\nneural networks and potentially XY-Hamiltonian models with ultra low energy\nconsumption.",
        "category": "physics_optics"
    },
    {
        "text": "Soliton-induced nonlocal resonances observed through high-intensity\n  tunable spectrally compressed second-harmonic peaks: Experimental data of femtosecond thick-crystal second-harmonic generation\nshows that when tuning away from phase matching, a dominating narrow spectral\npeak appears in the second harmonic that can be tuned over 100's of nm by\nchanging the phase-mismatch parameter. Traditional theory explains this as\nphase matching between a sideband in the broadband pump to its second-harmonic.\nHowever, our experiment is conducted under high input intensities and instead\nshows excellent quantitative agreement with a nonlocal theory describing\ncascaded quadratic nonlinearities. This theory explains the detuned peak as a\nnonlocal resonance that arises due to phase-matching between the pump and a\ndetuned second-harmonic frequency, but where in contrast to the traditional\ntheory the pump is assumed dispersion-free. As a soliton is inherently\ndispersion-free, the agreement between our experiment and the nonlocal theory\nindirectly proves that we have observed a soliton-induced nonlocal resonance.\nThe soliton exists in the self-defocusing regime of the cascaded nonlinear\ninteraction and in the normal dispersion regime of the crystal, and needs high\ninput intensities to become excited.",
        "category": "physics_optics"
    },
    {
        "text": "Exact bidirectional X-wave solutions in fiber Bragg gratings: We find exact solutions describing bidirectional pulses propagating in fiber\nBragg gratings. They are derived by solving the coupled-mode theory equations\nand are expressed in terms of products of modified Bessel functions with\nalgebraic functions. Depending on the values of the two free parameters the\ngeneral bidirectional X-wave solution can also take the form of a\nunidirectional pulse. We analyze the symmetries and the asymptotic properties\nof the solutions and also discuss about additional waveforms that are obtained\nby interference of more than one solutions. Depending on their parameters such\npulses can create a sharp focus with high contrast.",
        "category": "physics_optics"
    },
    {
        "text": "Structured light by discrete-phase orbital angular momentum holograms: Structured light has been created by a myriad of near- and far-field\ntechniques and has found both classical and quantum applications. In the case\nof orbital angular momentum (OAM), continuous spiral phase patterns in dynamic\nor geometric phase are often employed with the phase patterns existing across\nthe entire transverse plane. Here we exploit the uncertainty relation between\nOAM and angle to create structured OAM fields using multilevel OAM holograms.\nWe show theoretically and experimentally that only a multilevel angular phase\ncontour in the near-field is needed to create structured OAM light in the\nfar-field, exploiting the reciprocal nature of angular momentum and angle. We\nuse this approach to demonstrate exotic 3D structured light control to show the\nevolution of the Poynting vector in such fields and to highlight the physics\nunderlying this phenomenon.",
        "category": "physics_optics"
    },
    {
        "text": "Tunable ultra-high-efficiency light absorption of monolayer graphene\n  using critical coupling with guided resonance: We numerically demonstrate a novel monolayer graphene-based perfect\nabsorption multi-layer photonic structure by the mechanism of critical coupling\nwith guided resonance, in which the absorption of graphene can significantly\nclose to 99% at telecommunication wavelengths. The highly efficient absorption\nand spectral selectivity can be obtained with designing structural parameters\nin the near infrared ranges. Compared to previous works, we achieve the\ncomplete absorption of single-atomic-layer graphene in the perfect absorber for\nthe first time, which not only opens up new methods of enhancing the\nlight-graphene interaction, but also makes for practical applications in\nhigh-performance optoelectronic devices, such as modulators and sensors.",
        "category": "physics_optics"
    },
    {
        "text": "Three-dimensional direct laser writing inspired by\n  stimulated-emission-depletion microscopy: Three-dimensional direct laser writing has become a well established,\nversatile, widespread, and even readily commercially available \"workhorse\" of\nnano- and micro-technology. However, its lateral and axial spatial resolution\nis inherently governed by Abbe's diffraction limitation - analogous to optical\nmicroscopy. In microscopy, stimulated-emission-depletion approaches have lately\ncircumvented Abbe's barrier and lateral resolutions down to 5.6 nm using\nvisible light have been achieved. In this paper, after very briefly reviewing\nour previous efforts with respect to translating this success in optical\nmicroscopy to optical lithography, we present our latest results regarding\nresolution improvement in the lateral as well as in the much more relevant\naxial direction. The structures presented in this paper set a new\nresolution-benchmark for next-generation direct-laser-writing optical\nlithography. In particular, we break the lateral and the axial Abbe criterion\nfor the first time.",
        "category": "physics_optics"
    },
    {
        "text": "Reduced motion artifacts and speed improvements in enhanced\n  line-scanning fiber bundle endomicroscopy: Significance: Confocal laser scanning enables optical sectioning in fiber\nbundle endomicroscopy but limits the frame rate. To be able to better explore\ntissue morphology it is useful to stitch sequentially acquired frames into a\nmosaic. However, low frame rates limit the maximum probe translation speed.\nLine-scanning confocal endomicroscopy provides higher frame rates, but residual\nout-of-focus light degrades images. Subtraction based approaches can suppress\nthis residue at the expense of introducing motion artifacts.\n  Aim: To generate high frame rate endomicroscopy images with improved optical\nsectioning, we develop a high-speed subtraction method that only requires the\nacquisition of a single camera frame.\n  Approach: The rolling shutter of a CMOS camera acts as both the aligned and\noffset detector slits required for subtraction-based sectioning enhancement.\nTwo images of the bundle are formed on different regions of the camera,\nallowing both images to be acquired simultaneously.\n  Results: We confirm improved optical sectioning compared to conventional\nline-scanning, particularly far from focus, and show that motion artifacts are\nnot introduced. We demonstrate high-speed mosaicing at frame rates of up to 240\nHz.\n  Conclusion: High-speed acquisition of optically sectioned images using the\nnew subtraction based approach leads to improved mosaicing at high frame rates.",
        "category": "physics_optics"
    },
    {
        "text": "Plasmonic nanoantennas as integrated coherent perfect absorbers on SOI\n  waveguides for modulators and all-optical switches: The performance of plasmonic nanoantenna structures on top of SOI wire\nwaveguides as coherent perfect absorbers for modulators and all-optical\nswitches is explored. The absorption, scattering, reflection and transmission\nspectra of gold and aluminum nanoantenna-loaded waveguides were calculated by\nmeans of 3D finite-difference time-domain simulations for single waves\npropagating along the waveguide, as well as for standing wave scenarios\ncomposed from two counterpropagating waves. The investigated configurations\nshowed losses of roughly 1% and extinction ratios greater than 25 dB for\nmodulator and switching applications, as well as plasmon effects such as strong\nfield enhancement and localization in the nanoantenna region. The proposed\nplasmonic coherent perfect absorbers can be utilized for ultracompact\nall-optical switches in coherent networks as well as modulators and can find\napplications in sensing or in increasing nonlinear effects.",
        "category": "physics_optics"
    },
    {
        "text": "Tight bounds and the role of optical loss in polariton-mediated\n  near-field heat transfer: We introduce an analytical framework for near-field radiative heat transfer\nin bulk plasmonic and polar media. Considering material dispersion, we derive a\nclosed-form expression for the radiative thermal conductance, which\ndisentangles the role of optical loss from other material dispersion\ncharacteristics, such as the spectral width of the Reststrahlen band in polar\ndielectrics, as well as from the temperature. We provide a universal condition\nfor maximizing heat transfer that defines the optimal interplay between a\nmaterial's optical loss and polariton resonance frequency, based on which we\nintroduce tight bounds to near-field heat transfer. With this formalism, one\ncan quantitatively evaluate all polaritonic materials in terms of their\nperformance as near-field thermal emitters.",
        "category": "physics_optics"
    },
    {
        "text": "Remote creation of strong and coherent emissions in air with two-color\n  ultrafast laser pulses: We experimentally demonstrate generation of strong narrow-bandwidth emissions\nwith excellent coherent properties at ~391 nm and ~428 nm from molecular ions\nof nitrogen inside a femtosecond filament in air by an orthogonally polarized\ntwo-color driver field (i. e., 800 nm laser pulse and its second harmonic). The\ndurations of the coherent emissions at 391 nm and 428 nm are measured to be\n~2.4 ps and ~7.8 ps respectively, both of which are much longer than the\nduration of the pump and its second harmonic pulses. Furthermore, the measured\ntemporal decay characteristics of the excited molecular systems suggest an\n\"instantaneous\" population inversion mechanism that may be achieved in\nmolecular nitrogen ions at an ultrafast time scale comparable to the 800 nm\npump pulse.",
        "category": "physics_optics"
    },
    {
        "text": "Ultra-high brilliance multi-MeV $\u03b3$-ray beam from non-linear\n  Thomson scattering: We report on the generation of a narrow divergence ($\\theta\\approx 2.5$\nmrad), multi-MeV ($E_\\text{MAX} = 18$ MeV) and ultra-high brilliance ($\\approx\n2\\times10^{19}$ photons s$^{-1}$ mm$^{-2}$ mrad $^{-2}$ 0.1\\% BW) $\\gamma$-ray\nbeam from the scattering of an ultra-relativistic laser-wakefield accelerated\nelectron beam in the field of a relativistically intense laser (dimensionless\namplitude $a_0\\approx2$). The spectrum of the generated $\\gamma$-ray beam is\nmeasured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear\nevidence of the onset of non-linear Thomson scattering. The photon source has\nthe highest brilliance in the multi-MeV regime ever reported in the literature.",
        "category": "physics_optics"
    },
    {
        "text": "Hyperbolic metamaterial as a tunable near-field spatial filter for the\n  implementation of the active plasmon injection loss compensation scheme: We present how to physically realize the auxiliary source described in the\nrecently introduced active plasmon injection loss compensation scheme for\nenhanced near-field superlensing. Particularly, we show that the\ncharacteristics of the auxiliary source described in the active plasmon\ninjection scheme including tunable narrow-band and selective amplification via\nconvolution can be realized by using a hyperbolic metamaterial functioning as a\nnear-field spatial filter. Besides loss compensation, the proposed near-field\nspatial filter can be useful for real-time high resolution edge detection.",
        "category": "physics_optics"
    },
    {
        "text": "Tuning out disorder-induced localization in nanophotonic cavity arrays: Weakly coupled high-Q nanophotonic cavities are building blocks of slow-light\nwaveguides and other nanophotonic devices. Their functionality critically\ndepends on tuning as resonance frequencies should stay within the bandwidth of\nthe device. Unavoidable disorder leads to random frequency shifts which cause\nlocalization of the light in single cavities. We present a new method to finely\ntune individual resonances of light in a system of coupled nanocavities. We use\nholographic laser-induced heating and address thermal crosstalk between\nnanocavities using a response matrix approach. As a main result we observe a\nsimultaneous anticrossing of 3 nanophotonic resonances, which were initially\nsplit by disorder.",
        "category": "physics_optics"
    },
    {
        "text": "Multimode optical fiber based spectrometers: A standard multimode optical fiber can be used as a general purpose\nspectrometer after calibrating the wavelength dependent speckle patterns\nproduced by interference between the guided modes of the fiber. A transmission\nmatrix was used to store the calibration data and a robust algorithm was\ndeveloped to reconstruct an arbitrary input spectrum in the presence of\nexperimental noise. We demonstrate that a 20 meter long fiber can resolve two\nlaser lines separated by only 8 pm. At the other extreme, we show that a 2\ncentimeter long fiber can measure a broadband continuous spectrum generated\nfrom a supercontinuum source. We investigate the effect of the fiber geometry\non the spectral resolution and bandwidth, and also discuss the additional\nlimitation on the bandwidth imposed by speckle contrast reduction when\nmeasuring dense spectra. Finally, we demonstrate a method to reduce the\nspectrum reconstruction error and increase the bandwidth by separately imaging\nthe speckle patterns of orthogonal polarizations. The multimode fiber\nspectrometer is compact, lightweight, low cost, and provides high resolution\nwith low loss.",
        "category": "physics_optics"
    },
    {
        "text": "Quasi-phase-matching of high-order-harmonic generation using\n  polarization beating in optical waveguides: A scheme for quasi-phase-matching high-harmonic generation is proposed in\nwhich polarization beating within a hollow core birefringent waveguide\nmodulates the generation of harmonics. The evolution of the polarization of a\nlaser pulse propagating in a birefringent waveguide is calculated and is shown\nto periodically modulate the harmonic generation process. The optimum\nconditions for achieving quasi-phase-matching using this scheme are explored\nand the growth of the harmonic intensity as a function of experimental\nparameters is investigated.",
        "category": "physics_optics"
    },
    {
        "text": "Stable high-dimensional weak-light soliton molecules and their active\n  control: Bound states of solitons, alias soliton molecules (SMs), are well known in\none-dimensional (1D) systems, while making stable bound states of\nmultidimensional solitons is a challenging problem because of the underlying\ninstabilities. Here we propose a scheme for the creation of stable (2+1)D and\n(3+1)D optical SMs in a gas of cold Rydberg atoms, in which electromagnetically\ninduced transparency (EIT) is induced by a control laser field. We show that,\nthrough the interplay of the EIT and the strong long-range interaction between\nthe Rydberg atoms, the system gives rise to giant nonlocal Kerr nonlinearity,\nwhich in turn supports stable (2+1)D spatial optical SMs, as well as\nring-shaped soliton necklaces, including rotating ones. They feature a large\nsize, low generation power, and can be efficiently manipulated by tuning the\nnonlocality degree of the Kerr nonlinearity. Stable (3+1)D spatiotemporal\noptical SMs, composed of fundamental or vortex solitons, with low power and\nultraslow propagation velocity, can also be generated in the system. These SMs\ncan be stored and retrieved through the switching off and on of the control\nlaser field. The findings reported here suggest applications to data processing\nand transmission in optical systems.",
        "category": "physics_optics"
    },
    {
        "text": "The role of intraband dynamics in the generation of circularly polarized\n  high harmonics from solids: Recent studies have demonstrated that the polarization states of high\nharmonics from solids can differ from those of the driving pulses. To gain\ninsights on the microscopic origin of this behavior, we perform one-particle\nintraband-only calculations and reproduce some of the most striking\nobservations. For instance, our calculations yield circularly polarized\nharmonics from elliptically polarized pulses that sensitively depend on the\ndriving conditions. Furthermore, we perform experiments on ZnS and find partly\nsimilar characteristics as reported from silicon. Comparison to our\nintraband-only calculations shows reasonable qualitative agreement for a\nbelow-band-gap harmonic. We show that intraband dynamics predict depolarization\neffects for higher field strengths. For harmonics above the band gap, interband\ndynamics become important and the high-harmonic response to elliptical\nexcitation looks systematically different. Our work proposes a method to\ndistinguish between different high-harmonic generation mechanisms and it could\npave the way to compact solid-state high-harmonic sources with controllable\npolarization states.",
        "category": "physics_optics"
    },
    {
        "text": "On Stability of Flat Band Modes in a Rhombic Nonlinear Optical Waveguide\n  Array: The quasi-one-dimensional rhombic array of the waveguides is considered. In\nthe nonlinear case the system of equations describing coupled waves in the\nwaveguides has the solutions that represent the superposition of the flat band\nmodes. The property of stability of these solutions is considered. It was found\nthat the flat band solution is unstable until the power threshold be attained.",
        "category": "physics_optics"
    },
    {
        "text": "Anapole plasmonic meta-atom enabled by inverse design for metamaterials\n  transparency: Anapole states are broadly investigated in nanophotonics for their ability to\nprovide field enhancement and transparency. While low extinction has been\nachieved in dielectric nanoparticles due to the absence of intrinsic losses, in\nthe case of plasmonic nanostructures this is still lacking. Here, we report an\neasy-to-fabricate planar plasmonic nanostructure found via topology\noptimization, which exhibits an anapole state with close-to-ideal\ncharacteristics in the visible regime including weak absorption, high\nnear-field enhancment, and strong suppression of scattering. The nanonantenna\ncan act as an individual meta-atom because, due to low inter-coupling, it\npreserves its optical response even when used in highly packed metasurfaces and\nmetamaterials. The low losses are due to the optimized topology which provides\nconcentration of the field outside the structure with minimum penetration\ninside the metal. Compared to anapole states in dielectric structures, the\naccessibility of the volume of enhanced field is suitable for sensing\napplications. Anapole states are typically interpreted as the result of the\ninterference between electric and toroidal dipole moments. Here and based on a\nnovel approach, in the context of secondary multipole analysis, we introduce\nthe concept of anapole state without using the contribution of toroidal dipole\nmoments. The article provides new insight into anapoles in plasmonic\nnanostructures and ways to achieve them, while remarking the power of topology\noptimization to unlock designs with novel functionalities.",
        "category": "physics_optics"
    },
    {
        "text": "Twisted Light Transmission over 143 kilometers: Spatial modes of light can potentially carry a vast amount of information,\nmaking them promising candidates for both classical and quantum communication.\nHowever, the distribution of such modes over large distances remains difficult.\nIntermodal coupling complicates their use with common fibers, while free-space\ntransmission is thought to be strongly influenced by atmospheric turbulence.\nHere we show the transmission of orbital angular momentum modes of light over a\ndistance of 143 kilometers between two Canary Islands, which is 50 times\ngreater than the maximum distance achieved previously. As a demonstration of\nthe transmission quality, we use superpositions of these modes to encode a\nshort message. At the receiver, an artificial neural network is used for\ndistinguishing between the different twisted light superpositions. The\nalgorithm is able to identify different mode superpositions with an accuracy of\nmore than 80% up to the third mode order, and decode the transmitted message\nwith an error rate of 8.33%. Using our data, we estimate that the distribution\nof orbital angular momentum entanglement over more than 100 kilometers of free\nspace is feasible. Moreover, the quality of our free-space link can be further\nimproved by the use of state-of-the-art adaptive optics systems.",
        "category": "physics_optics"
    },
    {
        "text": "Analysis of shot noise in the detection of ultrashort optical pulse\n  trains: We present a frequency domain model of shot noise in the photodetection of\nultrashort optical pulse trains using a time-varying analysis. Shot\nnoise-limited photocurrent power spectral densities, signal-to-noise\nexpressions, and shot noise spectral correlations are derived that explicitly\ninclude the finite response of the photodetector. It is shown that the strength\nof the spectral correlations in the shot noise depends on the optical pulse\nwidth, and that these correlations can create orders-of-magnitude imbalance\nbetween the shot noise-limited amplitude and phase noise of photonically\ngenerated microwave carriers. It is also shown that only by accounting for\nspectral correlations can shot noise be equated with the fundamental quantum\nlimit in the detection of optical pulse-to-pulse timing jitter.",
        "category": "physics_optics"
    },
    {
        "text": "Auxiliary Optomechanical Tools for 3D Cell Manipulation: Advances in laser and optoelectronic technologies brought the general concept\nof optomechanical manipulation to the level of standard biophysical tools,\npaving ways towards controlled experiments and measurements of tiny mechanical\nforces. Recent developments in direct laser writing (DLW), enabled the\nrealization of new types of micron-scale optomechanical tools, capable of\nperforming designated functions. Here we further develop the concept of\nDLW-fabricated optomechanically-driven tools and demonstrate full-3D\nmanipulation capabilities over biological objects. In particular, we resolved a\nlong-standing problem of out-of-plane rotation in a pure liquid, which was\ndemonstrated on a living cell, clamped between a pair of forks, designed for\nefficient manipulation with holographic optical tweezers. The demonstrated\nconcept paves new ways towards realization of flexible tools for performing\non-demand functions over biological objects, such as cell tomography and\nsurgery to name just few.",
        "category": "physics_optics"
    },
    {
        "text": "Experimental and numerical analysis of the chromatic dispersion\n  dependence upon the actual profile of small core microstructured fibres: The chromatic dispersion curve of the fundamental mode in small core\nmicrostructured fibres (SCMF) is both calculated using a Finite Element Method\n(FEM) and measured with a low coherence interferometric method. The great\nsensitivity of the chromatic dispersion to variations of the geometrical\nparameters of SCMFs (the pitch and the diameter) is pointed out. An excellent\nagreement is obtained between the numerical and the experimental results over a\nhalf micrometer spectral bandwidth [1.1 $\\mu$m-1.6 $\\mu$m].",
        "category": "physics_optics"
    },
    {
        "text": "Circular displacement current induced anomalous magneto-optical effects\n  in high index Mie resonators: Dielectric Mie nanoresonators showing strong light-matter interaction at the\nnanoscale may enable new functionality in photonic devices. Recently, strong\nmagneto-optical effects have been observed in magneto-optical nanophotonic\ndevices due to the electromagnetic field localization. However, most reports so\nfar have been focused on the enhancement of conventional magneto-optical\neffects. Here, we report the observation of circular displacement current\ninduced anomalous magneto-optical effects in high-index-contrast\nSi/Ce:YIG/YIG/SiO2 Mie resonators. In particular, giant modulation of light\nintensity in transverse magnetic configuration up to 6.4 % under s-polarized\nincidence appears, which is non-existent in planar magneto-optical thin films.\nApart from that, we observe a large rotation of transmitted light polarization\nin the longitudinal magnetic configuration under near normal incidence\nconditions, which is two orders of magnitude higher than for a planar\nmagneto-optical thin film. These phenomena are essentially originated from the\nunique circular displacement current when exciting the magnetic resonance modes\nin the Mie resonators, which changes the incident electric field direction\nlocally. Our work indicates an uncharted territory of light polarization\ncontrol based on the complex modal profiles in all-dielectric magneto-optical\nMie resonators and metasurfaces, which opens the door for versatile control of\nlight propagation by magnetization for a variety of applications in vectoral\nmagnetic field and biosensing, free space non-reciprocal photonic devices,\nmagneto-optical imaging and optomagnetic memories.",
        "category": "physics_optics"
    },
    {
        "text": "Tuning of resonant doublets in coupled optical cavities: The mode profile of a coupled optical cavity often exhibits a resonant\ndoublet, which arises from the strong coupling between its sub-cavities.\nTraditional readout methods rely on setting fields of different frequencies to\nbe resonant in either sub-cavity, which is challenging in the case of strong\ncoupling. In this regime, the coupled cavity behaves as a single resonator, and\na field must be resonant in all its parts. Consequently, specialized sensing\nschemes are necessary to control strongly coupled cavities. To address this\nissue, we propose a novel technique for the relative measurement of the degrees\nof freedom of a strongly coupled cavity. Our approach enables simultaneous\nfrequency stabilization and fine-tuning of frequency splitting in the resonant\ndoublet. Overall, our proposed technique offers a promising solution to control\nthe properties of coupled cavities, facilitating advanced applications in the\nfields of gravitational-wave detection, quantum cavity optomechanics, and other\nrelated areas.",
        "category": "physics_optics"
    },
    {
        "text": "Self-organized compound pattern and pulsation of dissipative solitons in\n  a passively mode-locked fiber laser: We experimentally observe soliton self-organization and pulsation in a\npassively mode-locked fiber laser. The optomechanical interaction in the\noptical fiber is key to the formation of equidistant soliton bunches. These\nsolitons simultaneously undergo a pulsation process with a period corresponding\nto tens of the cavity round trip time. Using the dispersive Fourier\ntransformation technique, we find that the Kelly sidebands in the shot-to-shot\nspectra appear periodically, synchronizing with the pulsation.",
        "category": "physics_optics"
    },
    {
        "text": "Answering some questions about structured illumination microscopy: Structured illumination microscopy (SIM) provides images of fluorescent\nobjects at an enhanced resolution greater than that of conventional\nepifluorescence wide-field microscopy. Initially demonstrated in 1999 to\nenhance the lateral resolution two-fold, it has since been extended to enhance\naxial resolution two-fold (2008), applied to live-cell imaging (2009) and\ncombined with myriad other techniques, including interferometric detection\n(2008), confocal microscopy (2010) and light sheet illumination (2012). Despite\nthese impressive developments, SIM remains, perhaps, the most poorly understood\n`super-resolution' method. In this article, we provide answers to the 13\nquestions regarding SIM proposed by Prakash et al., along with answers to a\nfurther three questions. After providing a general overview of the technique\nand its developments, we explain why SIM as normally used is still\ndiffraction-limited. We then highlight the necessity for a non-polynomial, and\nnot just non-linear, response to the illuminating light in order to make SIM a\ntrue, diffraction-unlimited, super-resolution technique. In addition, we\npresent a derivation of a real-space SIM reconstruction approach that can be\nused to process conventional SIM and image scanning microscopy (ISM) data and\nextended to process data with quasi-arbitrary illumination patterns. Finally,\nwe provide a simple bibliometric analysis of SIM development over the past two\ndecades and provide a short outlook on potential future work.",
        "category": "physics_optics"
    },
    {
        "text": "Self-organized nonlinear gratings for ultrafast nanophotonics: Modern nonlinear optical materials allow light of one wavelength be\nefficiently converted into light at another wavelength. However, designing\nnonlinear optical materials to operate with ultrashort pulses is difficult,\nbecause it is necessary to match both the phase velocities and group velocities\nof the light. Here we show that self-organized nonlinear gratings can be formed\nwith femtosecond pulses propagating through nanophotonic waveguides, providing\nsimultaneous group-velocity matching and quasi-phase-matching for second\nharmonic generation. We record the first direct microscopy images of\nphoto-induced nonlinear gratings, and demonstrate how these waveguides enable\nsimultaneous $\\chi^{(2)}$ and $\\chi^{(3)}$ nonlinear processes, which we\nutilize to stabilize a laser frequency comb. Finally, we derive the equations\nthat govern self-organized grating formation for femtosecond pulses and explain\nthe crucial role of group-velocity matching. In the future, such nanophotonic\nwaveguides could enable scalable, reconfigurable nonlinear optical systems.",
        "category": "physics_optics"
    },
    {
        "text": "Two-photon assisted clock comparison to picosecond precision: We have experimentally demonstrated a clock comparison scheme utilizing\ntime-correlated photon pairs generated from the spontaneous parametric down\nconversion process of a laser pumped beta-barium borate crystal. The\ncoincidence of two-photon events are analyzed by the cross correlation of the\ntwo time stamp sequences. Combining the coarse and fine part of the time\ndifferences at different resolutions, a 64 ps precision for clock\nsynchronization has been realized. We also investigate the effects of hardware\ndevices used in the system on the precision of clock comparison. The results\nindicate that the detector's time jitter and the background noise will degrade\nthe system performance. With this method, comparison and synchronization of two\nremote clocks could be implemented with a precision at the level of a few tens\nof picoseconds.",
        "category": "physics_optics"
    },
    {
        "text": "Optomechanical vibration of a nanofilm arising from luminescence-induced\n  optical force: Optical force, which is generated by the exchange of momentum between light\nand matter, has been applied in a wide range of fields including molecular\nbiology, photochemistry, and optomechanics as a technique to manipulate small\nobjects for atomic and micro-sized regimes. So far, the main approach for\nvarious optical manipulations has been the geometric design of the irradiated\nlight, such as optical vortices and localized surface plasmons. On the other\nhand, luminescence from materials should also act as optical force, which we\ncall luminescence-induced optical force (LiOF). LiOF occurs by designing an\nanisotropic dielectric environment surrounding an isotropic emitter. In this\npaper, as a model, we assumed a Fabry-Perot cavity structure in which a\nluminescent nanofilm is placed parallel to a metallic mirror. Then, we\ntheoretically calculated the LiOF and revealed that the LiOF could drive the\nvibrational motion of the film. This mechanism will provide new insights into\ndeveloping unconventional optomechanics.",
        "category": "physics_optics"
    },
    {
        "text": "Exploiting the optical quadratic nonlinearity of zincblende\n  semiconductors for guided-wave terahertz generation: a material comparison: We present a detailed analysis and comparison of dielectric waveguides made\nof CdTe, GaP, GaAs and InP for modal phase matched optical difference frequency\ngeneration (DFG) in the terahertz domain. From the form of the DFG equations,\nwe derived the definition of a very general figure of merit (FOM). In turn,\nthis FOM enabled us to compare different configurations, by taking into account\nlinear and nonlinear susceptibility dispersion, terahertz absorption, and a\nrigorous evaluation of the waveguide modes properties. The most efficient\nwaveguides found with this procedure are predicted to approach the quantum\nefficiency limit with input optical power in the order of kWs.",
        "category": "physics_optics"
    },
    {
        "text": "Extreme nonlinear optical enhancement in chalcogenide glass fibers with\n  deep-subwavelength metallic nanowires: A nanostructured chalcogenide-metal optical fiber is proposed. This hybrid\nnanofiber is embedded with a periodic array of triangular-shaped\ndeep-subwavelength metallic nanowires set up in a bowtie configuration. Our\nsimulations show that the proposed nanostructured fiber supports a guided\ncollective plasmonic mode enabling both subwavelength field confinement and\nextreme nonlinear light- matter interactions, much larger than a bare\nchalcogenide nanowire of comparable diameter. This is all achieved with less\nthan 3% by volume of metal content.",
        "category": "physics_optics"
    },
    {
        "text": "Resolving the temporal dynamics of mode-locked laser with single-shot\n  time-microscope: Mode-locked lasers, which produce ultrashort pulses in the picosecond and\nfemtosecond range, have enabled some of the most precise measurements. However,\ndespite significant recent progress, resolving the temporal behavior of their\nshort pulses is still a challenge. State-of-the-art oscilloscopes with tens of\npicosecond resolution prevent time-resolved observations in mode-locked lasers\nand limit the real-time pulse evolution tracking of ultrafast lasers. Here,\nusing the time-lens technique with a Raman amplifier, we implement an ultrafast\nsingle-shot time-microscope (TM) with a high temporal magnification factor of\n355 and a time measurement window of 1 millisecond that contains ~1.8*10^4\nconsecutive pulses. We use this TM to characterize the temporal evolution of\nmode-locked lasers and reveal a temporal sideway oscillation (winding)\nbehavior, a previously unobserved feature of lasers in both theory and\nexperiment. Our experimental observations confirm that the winding behavior is\nan essential feature in the operation of mode-locked lasers. We theoretically\nand experimentally found that the winding characteristic evolution originates\nfrom gain-induced fluctuations for relatively high gain energies, while\nQ-switched modulations being the main cause for lower energies. Our findings\nbased on advanced real-time measurements open up new insights into ultrafast\nand transient optics and may impact future laser designs, modern ultrafast\ndiagnostics, and influence progress in nonlinear optics in general.",
        "category": "physics_optics"
    },
    {
        "text": "Spin-orbit interactions in optically active materials: We investigate the inherent influence of light polarization on the intensity\ndistribution in anisotropic media undergoing a local inhomogeneous rotation of\nthe principal axes. Whereas in general such configuration implies a complicated\ninteraction between geometric and dynamic phase, we show that, in a medium\nshowing an inhomogeneous circular birefringence, the geometric phase vanishes.\nDue to the spin-orbit interaction, the two circular polarizations perceive\nreversed spatial distribution of the dynamic phase. Based upon this effect,\npolarization-selective lens, waveguides and beam deflectors are proposed.",
        "category": "physics_optics"
    },
    {
        "text": "Giant nonlinearity of carbon nanotubes in a photonic metamaterial: Metamaterials, artificial media structured on the subwavelength scale offer a\nrich paradigm for developing unique photonic functionalities ranging from\nnegative index of refraction and directionally asymmetric transmission to\nslowing light. Here we demonstrate that a combination of carbon nanotubes with\na photonic metamaterial offers a new paradigm for the development of nonlinear\nmedia with exceptionally strong ultrafast nonlinear response invaluable in\nphotonic applications. It is underpinned by strong coupling between weakly\nradiating Fano-type resonant plasmonic modes and the excitonic response of\nsingle-walled semiconductor carbon nanotubes. Using a \"combinatorial\" approach\nto material discovery we show that the optical response of such a composite\nsystem can be tailored and optimized by metamaterial design.",
        "category": "physics_optics"
    },
    {
        "text": "Squeezed Light Induced Two-photon Absorption Fluorescence of Fluorescein\n  Biomarkers: Two-photon absorption (TPA) fluorescence of biomarkers has been decisive in\nadvancing the fields of biosensing and deep-tissue in vivo imaging of live\nspecimens. However, due to the extremely small TPA cross section and the\nquadratic dependence on the input photon flux, extremely high peak-intensity\npulsed lasers are imperative, which can result in significant photo- and\nthermal-damage. Previous works on entangled TPA (ETPA) with spontaneous\nparametric down-conversion (SPDC) light sources found a linear dependence on\nthe input photon-pair flux, but are limited by low optical powers, along with a\nvery broad spectrum. We report that by using a high-flux squeezed light source\nfor TPA, a fluorescence enhancement of 47 is achieved in fluorescein biomarkers\nas compared to classical TPA. Moreover, a polynomial behavior of the TPA rate\nis observed in the DCM laser dye.",
        "category": "physics_optics"
    },
    {
        "text": "Lattice Induced Transparency in Metasurfaces: Lattice modes are intrinsic to the periodic structures and their occurrence\ncan be easily tuned and controlled by changing the lattice constant of the\nstructural array. Previous studies have revealed excitation of sharp absorption\nresonances due to lattice mode coupling with the plasmonic resonances. Here, we\nreport the first experimental observation of a lattice induced transparency\n(LIT) by coupling the first order lattice mode (FOLM) to the structural\nresonance of a metamaterial resonator at terahertz frequencies. The observed\nsharp transparency is a result of the destructive interference between the\nbright mode and the FOLM mediated dark mode. As the FOLM is swept across the\nmetamaterial resonance, the transparency band undergoes large change in its\nbandwidth and resonance position. Besides controlling the transparency\nbehaviour, LIT also shows a huge enhancement in the Q-factor and record high\ngroup delay of 28 ps, which could be pivotal in ultrasensitive sensing and slow\nlight device applications.",
        "category": "physics_optics"
    },
    {
        "text": "Bio-inspired Compact, High-resolution Snapshot Hyperspectral Imaging\n  System with 3D Printed Glass Lightguide Array: To address the major challenges to obtain high spatial resolution in snapshot\nhyperspectral imaging, 3D printed glass lightguide array has been developed to\nsample the intermediate image in high spatial resolution and redistribute the\npixels in the output end to achieve high spectral resolution. Curved 3D printed\nlightguide array can significantly simplify the snapshot hyperspectral imaging\nsystem, achieve better imaging performance, and reduce the system complexity\nand cost. We have developed two-photon polymerization process to print glass\nlightguide array, and demonstrated the system performance with biological\nsamples. This new snapshot technology will catalyze new hyperspectral imaging\nsystem development and open doors for new applications from UV to IR.",
        "category": "physics_optics"
    },
    {
        "text": "Ultra Low-Power All-Optical Switching: Using analytical modeling and detailed numerical simulations, we investigate\nproperties of hybrid systems of Photonic Crystal micro-cavities which\nincorporate a highly non-linear Ultra Slow Light medium. We demonstrate that\nsuch systems, while being miniature in size (order wavelength), and integrable,\ncould enable ultra-fast non-linear all-optical switching at single photon\nenergy levels.",
        "category": "physics_optics"
    },
    {
        "text": "Computing matrix inversion with optical networks: With this paper we bring about a discussion on the computing potential of\ncomplex optical networks and provide experimental demonstration that an optical\nfiber network can be used as an analog processor to calculate matrix inversion.\nA 3x3 matrix is inverted as a proof-of-concept demonstration using a fiber\nnetwork containing three nodes and operating at telecomm wavelength. For an NxN\nmatrix, the overall solving time (including setting time of the matrix elements\nand calculation time of inversion) scales as O(N^2), whereas matrix inversion\nby most advanced computer algorithms requires ~O(N^2.37) computational time.\nFor well-conditioned matrices, the error of the inversion performed optically\nis found to be less than 3%, limited by the accuracy of measurement equipment.",
        "category": "physics_optics"
    },
    {
        "text": "Spin-to-orbital angular momentum conversion in focusing, scattering, and\n  imaging systems: We present a general theory of spin-to-orbital angular momentum (AM)\nconversion of light in focusing, scattering, and imaging optical systems. Our\ntheory employs universal geometric transformations of nonparaxial optical\nfields in such systems and allows for direct calculation and comparison of the\nAM conversion efficiency in different physical settings. Observations of the AM\nconversions using local intensity distributions and far-field polarimetric\nmeasurements are discussed.",
        "category": "physics_optics"
    },
    {
        "text": "Ultra-wideband THz/IR Metamaterial Absorber based on Doped Silicon: Metamaterial-based absorbers have been extensively investigated in the\nterahertz (THz) range with ever increasing performances. In this paper, we\npropose an all-dielectric THz absorber based on doped silicon. The unit cell\nconsists of a silicon cross resonator with an internal cross-shaped air cavity.\nNumerical results suggest that the proposed absorber can operate from THz to\nmid-infrared, having an average power absorption of >95% between 0.6 and 10\nTHz. Experimental results using THz time-domain spectroscopy show a good\nagreement with simulations. The underlying mechanisms for broadband absorptions\nare attributed to the combined effects of multiple cavities modes formed by\nsilicon resonators and bulk absorption in the substrate, as confirmed by\nsimulated field patterns. This ultra-wideband absorption is polarization\ninsensitive and can operate across a wide range of the incident angle. The\nproposed absorber can be readily integrated into silicon-based platforms and is\nexpected to be used in sensing, imaging, energy harvesting and wireless\ncommunications systems.",
        "category": "physics_optics"
    },
    {
        "text": "Three-Dimensional Organic Microlasers with Low Lasing Thresholds\n  Fabricated by Multiphoton Lithography: Cuboid-shaped organic microcavities containing a pyrromethene laser dye and\nsupported upon a photonic crystal have been investigated as an approach to\nreducing the lasing threshold of the cavities. Multiphoton lithography\nfacilitated fabrication of the cuboid cavities directly on the substrate or on\nthe decoupling structure, while similar structures were fabricated on the\nsubstrate by UV lithography for comparison. Significant reduction of the lasing\nthreshold by a factor of ~30 has been observed for cavities supported by the\nphotonic crystal relative to those fabricated on the substrate. The lasing mode\nspectra of the cuboid microresonators provide strong evidence showing that the\nlasing modes are localized in the horizontal plane, with the shape of an\ninscribed diamond.",
        "category": "physics_optics"
    },
    {
        "text": "Transverse orbital angular momentum of spatiotemporal optical vortices: Spatiotemporal optical vortices (STOVs) are electromagnetic wave packets that\ntransport a phase line singularity perpendicular to their propagation\ndirection. We address the problem of the transverse orbital angular momentum\n(OAM) ``per photon\" actually transported by STOVs propagating in free space or\nnon-dispersive media, the most frequent experimental situation. Unlike\nlongitudinal vortices in monochromatic light beams, STOVs do not carry any net\ntransverse OAM about a fixed transverse axis crossing its center. However,\nSTOVs transport an intrinsic transverse OAM per photon about a moving,\ntransverse axis through its center, and an opposite extrinsic transverse OAM.\nTheir applications would thus preclude setting particles at rest into rotation,\nbut STOVs could transmit their intrinsic transverse OAM to photons of other\nwaves. The intrinsic transverse OAM per photon of an elliptically symmetric\nSTOV of frequency $\\omega_0$ and topological charge $l$ is $\\gamma\nl/2\\omega_0$, where $\\gamma$ is the STOV ellipticity. Thus circularly symmetric\nSTOVs ($\\gamma=1$) carry half the intrinsic longitudinal OAM of circularly\nsymmetric monochromatic light beams with a vortex of the same $l$ and\n$\\omega_0$. We show that the formula $(\\gamma+1/\\gamma)l/2\\omega_0$ for the\nintrinsic transverse OAM in Phys. Rev. A 107, L031501 (2023) yields infinite\nvalues and is not conserved on propagation for a particular STOV. When STOVs\nlose their elliptical symmetry upon propagation, they preserve the intrinsic\ntransverse OAM $\\gamma l/2\\omega_0$ despite the phase singularity may split,\nthe split singularities may disappear, or even change the sign of their\ntopological charges.",
        "category": "physics_optics"
    },
    {
        "text": "Hyperuniformity and wave localization in pinwheel scattering arrays: We investigate the structural and spectral properties of deterministic\naperiodic arrays designed from the statistically isotropic pinwheel tiling. By\nstudying the scaling of the cumulative integral of its structure factor in\ncombination with higher-order structural correlation analysis we conclude that\npinwheel arrays belong to the weakly hyperuniformity class. Moreover, by\nsolving the multiple scattering problem for electric point dipoles using the\nrigorous Green's matrix theory, we demonstrate a clear transition from\ndiffusive transport to localization behavior. This is shown by studying the\nThouless number as a function of the scattering strength and the spectral\nstatistics of the scattering resonances. Surprisingly, despite the absence of\nsharp diffraction peaks, clear spectral gaps are discovered in the density of\nstates of pinwheel arrays that manifest a distinctive long-range order.\nFurthermore, the level spacing statistics at large optical density exhibits a\nsharp transition from level repulsion to the Poisson behavior, consistently\nwith the onset of the wave localization regime. Our findings reveal the\nimportance of hyperuniform aperiodic structures with statistically isotropic\nk-space for the engineering of enhanced light-matter interaction and\nlocalization properties.",
        "category": "physics_optics"
    },
    {
        "text": "Parity-Time Symmetry meets Photonics: A New Twist in non-Hermitian\n  Optics: In the past decade, the concept of parity-time ($\\mathcal{PT}$) symmetry,\noriginally introduced in non-Hermitian extensions of quantum mechanical\ntheories, has come into thinking of photonics, providing a fertile ground for\nstudying, observing, and utilizing some of the peculiar aspects of\n$\\mathcal{PT}$ symmetry in optics. Together with related concepts of\nnon-Hermitian physics of open quantum systems, such as non-Hermitian\ndegeneracies (exceptional points) and spectral singularities, $\\mathcal{PT}$\nsymmetry represents one among the most fruitful ideas introduced in optics in\nthe past few years. Judicious tailoring of optical gain and loss in integrated\nphotonic structures has emerged as a new paradigm in shaping the flow of light\nin unprecedented ways, with major applications encompassing laser science and\ntechnology, optical sensing, and optical material engineering. In this\nperspective, I review some of the main achievements and emerging areas of\n$\\mathcal{PT}$-symmetric and non-Hermtian photonics, and provide an outline of\nchallenges and directions for future research in one of the fastest growing\nresearch area of photonics.",
        "category": "physics_optics"
    },
    {
        "text": "Concealing arbitrary objects remotely with multi-folded transformation\n  optics: An invisibility cloak that can hide an arbitrary object external to the cloak\nitself has not been devised before. In this Letter, we introduce a novel way to\ndesign a remote cloaking device that makes any object located at a certain\ndistance invisible. This is accomplished using multi-folded transformation\noptics to remotely generate a hidden region around the object that no field can\npenetrate and that does not disturb the far-field scattering electromagnetic\nfield. As a result, any object in the hidden region can stay in position or\nmove freely within that region and remain invisible. Our idea is further\nextended in order to design a remote illusion optics that can transform any\narbitrary object into another one. Unlike other cloaking methods, this method\nwould require no knowledge of the details of the object itself. The proposed\nmulti-folded transformation optics will be crucial in the design of remote\ndevices in a variety of contexts.",
        "category": "physics_optics"
    },
    {
        "text": "Direct observation of laser guided corona discharges: Laser based lightning control holds a promising way to solve the problem of\nthe long standing disaster of lightning strikes. But it is a challenging\nproject due to insufficient understanding of the interaction between laser\nplasma channel and high voltage electric filed. In this work, a direct\nobservation of laser guided corona discharge is reported. The high voltage\ncorona discharge can be guided along laser plasma filament, and enhanced\nthrough the interaction with laser filaments. The fluorescence lifetime of\nlaser filament guided corona discharge was measured to be several microseconds,\nwhich is 3 orders of magnitude longer than the fluorescence lifetime of laser\nfilaments. This could be advantageous towards laser assisted leader development\nin the atmosphere.",
        "category": "physics_optics"
    },
    {
        "text": "Enhancing or suppressing spin Hall effect of light in layered\n  nanostructures: The spin Hall effect (SHE) of light in layered nanostructures is investigated\ntheoretically in this paper. A general propagation model describing the\nspin-dependent transverse splitting in the SHE of light is established from the\nviewpoint of classical electrodynamics. We show that the transverse\ndisplacement of wave-packet centroid can be tuned to either a negative or a\npositive value, or even zero, by just adjusting the structure parameters,\nsuggesting that the SHE of light in layered nanostructures can be enhanced or\nsuppressed in a desired way. The inherent secret behind this interesting\nphenomenon is the optical Fabry-Perot resonance in the layered nanostructure.\nWe believe that these findings will open the possibility for developing new\nnano-photonic devices.",
        "category": "physics_optics"
    },
    {
        "text": "The Poynting vector field singularities: Effects of symmetry and its\n  violation: The phenomenological theory revealing the generic effects of the problem\nsymmetry, its violation, and energy conservation law on the singularities of\nthe Poynting vector field is presented. The bifurcation scenario of their\nformation (annihilation) under variations of the problem parameters is\nelucidated. The results describe the singularities in scattering a linearly\npolarized plane electromagnetic wave. However, they are valid for any\nconfiguration of the incident beam at its scattering by a subwavelength\nparticle. The author shows that topological changes in the pattern of the\nPoynting vector field occur through a finite number of pitchfork bifurcations.\nIt means that the patterns are topologically stable under variations of the\nproblem parameter(s) that lie between the bifurcation values. The latter\nensures that the discussed topological properties of the problem are robust to\nweak symmetry violation, which is inevitable in any actual experiment. The\ngeneral consideration is illustrated by a detailed study of singularities in\nscattering by an infinite right circular germanium cylinder. The results open\nthe possibility of fitting and controlling radiation patterns on subwavelength\nscales important for various nanotechnologies.",
        "category": "physics_optics"
    },
    {
        "text": "Experimental observation of robust surface states on photonic crystals\n  possessing single and double Weyl points: We designed and fabricated a time-reversal invariant Weyl photonic crystal\nthat possesses single Weyl nodes with topological charge of 1 and double Weyl\nnodes with a higher topological charge of 2. Using numerical simulations and\nmicrowave experiment, nontrivial band gaps with nonzero Chern numbers for a\nfixed kz was demonstrated. The robustness of the surface state between the Weyl\nphotonic crystal and PEC against kz-conserving scattering was experimentally\nobserved.",
        "category": "physics_optics"
    },
    {
        "text": "Efficient inverse design of large-area metasurfaces for incoherent light: Incoherent light is ubiquitous, yet designing optical devices that can handle\nits random nature is very challenging, since directly averaging over many\nincoherent incident beams can require a huge number of scattering calculations.\nWe show how to instead solve this problem with a reciprocity technique which\nleads to three orders of magnitude speedup: one Maxwell solve (using any\nnumerical technique) instead of thousands. This improvement enables us to\nperform efficient inverse design, large scale optimization of the metasurface\nfor applications such as light collimators and concentrators. We show the\nimpact of the angular distribution of incident light on the resulting\nperformance, and show especially promising designs for the case of \"annular\"\nbeams distributed only over nonzero angles.",
        "category": "physics_optics"
    },
    {
        "text": "Spectral Shaping with Integrated Self-Coupled Sagnac Loop Reflectors: We propose and theoretically investigate integrated photonic filters based on\ncoupled Sagnac loop reflectors (SLRs) formed by a self-coupled wire waveguide.\nBy tailoring coherent mode interference in the device, three different filter\nfunctions are achieved, including Fano-like resonances, wavelength\ninterleaving, and varied resonance mode splitting. For each function, the\nimpact of device structural parameters is analyzed to facilitate optimized\nperformance. Our results theoretically verify the proposed device as a compact\nmulti-functional integrated photonic filter for flexible spectral shaping.",
        "category": "physics_optics"
    },
    {
        "text": "Hybrid guided space-time optical modes in unpatterned films: Light can be confined transversely and delivered axially in a waveguide.\nHowever, waveguides are lossy static structures whose modal characteristics are\nfundamentally determined by the boundary conditions, and thus cannot be readily\nchanged post-fabrication. Here we show that unpatterned planar optical films\ncan be exploited for low-loss two-dimensional waveguiding by using `space-time'\nwave packets, which are the unique family of one-dimensional\npropagation-invariant pulsed optical beams. We observe `hybrid guided'\nspace-time modes that are index-guided in one transverse dimension in the film\nand localized along the unbounded transverse dimension via the intrinsic\nspatio-temporal structure of the field. We demonstrate that these field\nconfigurations enable overriding the boundary conditions by varying\npost-fabrication the group index of the fundamental mode in a 2-$\\mu$m-thick,\n25-mm-long silica film, which is achieved by modifying the field's\nspatio-temporal structure along the unbounded dimension. Tunability of the\ngroup index over an unprecedented range from 1.26 to 1.77 around the\nplanar-waveguide value of 1.47 is verified - while maintaining a spectrally\nflat zero-dispersion profile. Our work paves the way to to the utilization of\nspace-time wave packets in on-chip photonic platforms, and may enable new\nphase-matching strategies that circumvent the restrictions due to intrinsic\nmaterial properties.",
        "category": "physics_optics"
    },
    {
        "text": "Reentrant delocalization transition in one-dimensional photonic\n  quasicrystals: Waves propagating in certain one-dimensional quasiperiodic lattices are known\nto exhibit a sharp localization transition. We theoretically predict and\nexperimentally observe that the localization of light in one-dimensional\nphotonic quasicrystals may be followed by a second delocalization transition\nfor some states on increasing quasiperiodic modulation strength - an example of\na reentrant transition. We further propose that this phenomenon can be\nqualitatively captured by a dimerized tight-binding model with long-range\ncouplings.",
        "category": "physics_optics"
    },
    {
        "text": "Trapping atoms in the evanescent field of laser written wave guides: We analyze evanescent fields of laser written waveguides and show that they\ncan be used to trap atoms close to the surface of an integrated optical atom\nchip. In contrast to subwavelength nanofibres it is generally not possible to\ncreate a stable trapping potential using only the fundamental modes. This is\nwhy we create a stable trapping potential by using two different laser colors,\nsuch that the waveguide supports two modes for the blue detuned laser, while\nfor the red detuned light the waveguide has only a single mode. In particular,\nwe study such a two-color trap for Cesium atoms, and calculate both the\npotential and losses for the set of parameters that are within experimental\nreach. We also optimize system parameters in order to minimize trap losses due\nto photon scattering and tunneling to the surface.",
        "category": "physics_optics"
    },
    {
        "text": "Transparent dielectric metasurfaces for mode modulation and spatial\n  multiplexing: Expanding the use of physical degrees of freedom to employ spatial\nmultiplexing of data in optical communication is considered the most disruptive\nand effective solution to meet the capacity demand of the growing information\ntraffic. Development of space-division-multiplexing methods stimulated research\non spatial modulation, detection and processing of data, attracting interest\nfrom various fields of science. A passive all-dielectric metasurface with\nnear-unity transmission is used to engineer spatial mode profiles, potentially\nof arbitrary complexity. The broadband response of the metasurface covers S, C,\nand L bands of fibre communications. Unlike conventional phase plates, the\nmetasurface allows for both phase and polarization conversion, providing full\nflexibility for mode engineering. We employ the metasurface for both mode\nmodulation and mode multiplexing in free-space optical communication, and\ndemonstrate that it is capable of mode multiplexing with an extinction ratio in\nexcess of 20 dB over the whole C-band with negligible penalty even for 100 Gb/s\nDP-QPSK signals. These results merge two seemingly different fields, optical\ncommunication and metamaterials, and suggest a novel approach for ultimate\nminiaturisation of mode multiplexers and advanced LiFi technologies.",
        "category": "physics_optics"
    },
    {
        "text": "Reformulated Fourier Modal Method with improved near field computations: In this paper we propose a new formulation of the Fourier Modal Method based\non an alternative treatment of interface conditions allowing us to overcome the\neffect of the Gibbs phenomenon. Explicit consideration of the interface\nconditions for the discontinuous part of the field leads to an equation for the\neigenvalue problem, which can be written in an inversion-free form. The results\nof the method are in good agreement with the results for the classical approach\nbased on the Li factorization rules both for dielectric and metallic gratings.\nMoreover, the developed method allows calculating the near field much more\naccurately, and may find its applications in sensing and nonlinear optics.",
        "category": "physics_optics"
    },
    {
        "text": "High Efficiency Terahertz Generation in a Multi-Stage System: We describe a robust system for laser-driven narrowband terahertz generation\nwith high conversion efficiency in periodically poled Lithium Niobate (PPLN).\nIn the multi-stage terahertz generation system, the pump pulse is recycled\nafter each PPLN stage for further terahertz generation. By out-coupling the\nterahertz radiation generated in each stage, extra absorption is circumvented\nand effective interaction length is increased. The separation of the terahertz\nand optical pulses at each stage is accomplished by an appropriately designed\nout-coupler. To evaluate the proposed architecture, the governing 2-D coupled\nwave equations in a cylindrically symmetric geometry are numerically solved\nusing the finite difference method. Compared to the 1-D calculation which\ncannot capture the self-focusing and diffraction effects, our 2-D numerical\nmethod captures the effects of difference frequency generation, self-phase\nmodulation, self-focusing, beam diffraction, dispersion, and terahertz\nabsorption. We found that the terahertz generation efficiency can be greatly\nenhanced by compensating the dispersion of the pump pulse after each stage.\nWith a two-stage system, we predict the generation of a $17.6$ mJ terahertz\npulse with total conversion efficiency $\\eta_{\\text{total}}=1.6\\%$ at $0.3$ THz\nusing a 1.1 J pump laser with a two-line spectrum centered at 1 $\\mu$m. The\ngeneration efficiency of each stage is above $0.8\\%$ with the out-coupling\nefficiencies above $93.0\\%$.",
        "category": "physics_optics"
    },
    {
        "text": "Single-laser feedback cooling of optomechanical resonators: Measurement-based control has emerged as an important technique to prepare\nmechanical resonators in pure quantum states for applications in quantum\ninformation processing and quantum sensing. Conventionally this has required\ntwo separate channels, one for probing the motion and another one acting back\non the resonator. In this work, we analyze and experimentally demonstrate a\ntechnique of single-laser feedback cooling, where one laser is used for both\nprobing and controlling the mechanical motion. We show using an analytical\nmodel and experiments that feedback cooling is feasible in this mode as long as\ncertain stability requirements are fulfilled. Our results demonstrate that, in\naddition to being more experimentally feasible construction, the interference\neffects of the single-laser feedback can actually be used to enhance cooling at\nsome parameter regimes.",
        "category": "physics_optics"
    },
    {
        "text": "Synchronization of two DFB lasers using frequency-shifted feedback for\n  microwave photonics: The phase synchronization of two semiconductor DFB lasers submitted to both\nfrequency-shifted cross-injection and self-feedback is investigated, both\nnumerically and experimentally. A dedicated setup based on monolithic DFB\nlasers permits to effectively lock the frequency difference of the two lasers\non an external reference over a broad range of parameters. In agreement with a\ndelayed rate equation model, it is shown that the sensitivity of the locking to\nthe optical feedback phases is mitigated by using strong cross-injection, thus\npaving the way to applications for microwave photonics. Other synchronization\ndynamics can also be observed, such as bounded-phase oscillations.",
        "category": "physics_optics"
    },
    {
        "text": "Energy-efficient tunable silicon photonic micro-resonator with graphene\n  transparent nano-heaters: Thermally-tuning silicon micro-cavities are versatile and beneficial elements\nin low-cost large-scale photonic integrated circuits (PICs). Traditional metal\nheaters used for thermal tuning in silicon micro-cavities usually need a thick\nSiO2 upper-cladding layer, which will introduce some disadvantages including\nlow response speed, low heating efficiency, low achievable temperature and\ncomplicated fabrication processes. In this paper, we propose and experimentally\ndemonstrate thermally-tuning silicon micro-disk resonators by introducing\ngraphene transparent nano-heaters, which contacts the silicon core directly\nwithout any isolator layer. This makes the graphene transparent nano-heater\npotentially to have excellent performances in terms of the heating efficiency,\nthe temporal response and the achievable temperature. It is also shown that the\ngraphene nano-heater is convenient to be used in ultrasmall photonic integrated\ndevices due to the single-atom thickness and excellent flexibility of graphene.\nBoth experiments and simulations imply that the present graphene transparent\nnano-heater is promising for thermally-tuning nanophotonic integrated devices\nfor e.g. optical modulating, optical filtering/switching, etc.",
        "category": "physics_optics"
    },
    {
        "text": "From higher-order Kerr nonlinearities to quantitative modeling of 3rd\n  and 5th harmonic generation in argon: The recent measurement of negative higher-order Kerr effect (HOKE) terms in\ngases has given rise to a controversial debate, fed by its impact on short\nlaser pulse propagation. By comparing the experimentally measured yield of the\nthird and fifth harmonics, with both an analytical and a full comprehensive\nnumerical propagation model, we confirm the absolute and relative values of the\nreported HOKE indices.",
        "category": "physics_optics"
    },
    {
        "text": "Fluorescence enhancement in topologically optimized gallium phosphide\n  all-dielectric nanoantennas: Nanoantennas capable of large fluorescence enhancement with minimal\nabsorption are crucial for future optical technologies from single-photon\nsources to biosensing. Efficient dielectric nanoantennas have been designed,\nhowever, evaluating their performance at the individual emitter level is\nchallenging due to the complexity of combining high-resolution nanofabrication,\nspectroscopy and nanoscale positioning of the emitter. Here, we study the\nfluorescence enhancement in infinity-shaped gallium phosphide (GaP)\nnanoantennas based on a topologically optimized design. Using fluorescence\ncorrelation spectroscopy (FCS), we probe the nanoantennas enhancement factor\nand observed an average of 63-fold fluorescence brightness enhancement with a\nmaximum of 93-fold for dye molecules in nanogaps between 20 nm and 50 nm. The\nexperimentally determined fluorescence enhancement of the nanoantennas was\nconfirmed by numerical simulations of the local density of optical states\n(LDOS). Furthermore, we show that beyond design optimisation of dielectric\nnanoantennas, increased performances can be achieved via tailoring of\nnanoantenna fabrication.",
        "category": "physics_optics"
    },
    {
        "text": "Voigt waves in homogenized particulate composites based on isotropic\n  dielectric components: Homogenized composite materials (HCMs) can support a singular form of optical\npropagation, known as Voigt wave propagation, while their component materials\ndo not. This phenomenon was investigated for biaxial HCMs arising from\nnondissipative isotropic dielectric component materials. The biaxiality of\nthese HCMs stems from the oriented spheroidal shapes of the particles which\nmake up the component materials. An extended version of the Bruggeman\nhomogenization formalism was used to investigate the influence of component\nparticle orientation, shape and size, as well as volume fraction of the\ncomponent materials, upon Voigt wave propagation. Our numerical studies\nrevealed that the directions in which Voigt waves propagate is highly sensitive\nto the orientation of the component particles and to the volume fraction of the\ncomponent materials, but less sensitive to the shape of the component particles\nand less sensitive still to the size of the component particles. Furthermore,\nwhether or not such an HCM supports Voigt wave propagation at all is critically\ndependent upon the size of the component particles and, in certain cases, upon\nthe volume fraction of the component materials.",
        "category": "physics_optics"
    },
    {
        "text": "Spatial Coherence of Synchrotron Radiation: Theory and measurement of spatial coherence of synchrotron radiation beams\nare briefly reviewed. Emphasis is given to simple relationships between\nelectron beam characteristics and far field properties of the light beam.",
        "category": "physics_optics"
    },
    {
        "text": "Low-loss, silicon integrated, aluminum nitride photonic circuits and\n  their use for electro-optic signal processing: Photonic miniaturization requires seamless integration of linear and\nnonlinear optical components to achieve passive and active functions\nsimultaneously. Among the available material systems, silicon photonics holds\nimmense promise for optical signal processing and on-chip optical networks.\nHowever, silicon is limited to wavelengths above 1100 nm and does not provide\nthe desired lowest order optical nonlinearity for active signal processing.\nHere we report the integration of aluminum nitride (AlN) films on silicon\nsubstrates to bring active functionalities to chip-scale photonics. Using\nCMOS-compatible sputtered thin films we fabricate AlN-on-insulator waveguides\nthat exhibit low propagation loss (0.6 dB/cm). Exploiting AlN's inherent\nPockels effect we demonstrate electro-optic modulation up to 4.5 Gb/s with very\nlow energy consumption (down to 10 fJ/bit). The ultra-wide transparency window\nof AlN devices also enables high speed modulation at visible wavelengths. Our\nlow cost, wideband, carrier-free photonic circuits hold promise for ultra-low\npower and high speed signal processing at the microprocessor chip level.",
        "category": "physics_optics"
    },
    {
        "text": "Miniature optical planar camera based on a wide-angle metasurface\n  doublet corrected for monochromatic aberrations: Optical metasurfaces are two-dimensional arrays of nano-scatterers that\nmodify optical wavefronts at subwavelength spatial resolution. They are poised\nto revolutionize optics by enabling complex low-cost systems where multiple\nmetasurfaces are lithographically stacked and integrated with electronics. For\nimaging applications, metasurface stacks can perform sophisticated image\ncorrections and can be directly integrated with image sensors. Here, we\ndemonstrate this concept with a miniature flat camera integrating a monolithic\nmetasurface lens doublet corrected for monochromatic aberrations, and an image\nsensor. The doublet lens, which acts as a fisheye photographic objective, has a\nsmall $f$-number of 0.9, an angle-of-view larger than\n60$^\\circ\\times$60$^\\circ$, and operates at 850 nm wavelength with 70% focusing\nefficiency. The camera exhibits nearly diffraction-limited image quality, which\nindicates the potential of this technology in the development of optical\nsystems for microscopy, photography, and computer vision.",
        "category": "physics_optics"
    },
    {
        "text": "Discrete Approximation of Topologically Protected Modes in\n  Magneto-Optical Media: Topologically protected waves in the linearly polarized Maxwell's equations\nwith gyrotropic, magneto-optic media were studied a decade ago both\ncomputationally and experimentally. This paper develops a robust tight-binding\nmodel for this system that makes careful use of Wannier function\nrepresentations. The model provides very good approximations to the underlying\nband structure. When solved on a semi-infinite strip, it produces exponentially\nlocalized edge modes whose corresponding eigenvalues span the frequency band\ngaps. A set of coupled differential equations are derived which allows one to\nfind how the electromagnetic field propagates unidirectionally, without\nbackscatter from defects. Furthermore, the discrete model predicts\ntopologically protected edge modes with nontrivial Chern number which are\nconsistent with direct simulation.",
        "category": "physics_optics"
    },
    {
        "text": "Plasmonic structure integrated single-photon detector configurations to\n  improve absorptance and polarization contrast: Configurations capable of maximizing both absorptance and polarization\ncontrast were determined for 1550 nm polarized light illumination of different\nplasmonic structure integrated superconducting nanowire single-photon detectors\n(SNSPDs) consisting of p=264 nm and P=792 nm periodic niobium-nitride (NbN)\npatterns on silica substrate. Global NbN absorptance maxima appear in case of\np/s-polarized light illumination in S/P-orientation (gamma=90 azimuthal angle)\nand the highest polarization contrast is attained in S-orientation of all\ndevices. Common nanophotonical origin of absorptance enhancement is collective\nresonance on nano-cavity-gratings with different profiles, which is promoted by\ncoupling between localized modes in quarter wavelength MIM nano-cavities and\nlaterally synchronized Brewster-Zenneck-type surface waves in integrated SNSPDs\npossessing a three-quarter-wavelength-scaled periodicity. The spectral\nsensitivity and dispersion characteristics reveal that device design specific\noptimal configurations exist.",
        "category": "physics_optics"
    },
    {
        "text": "Cavity optomechanical mass sensor in water with sub-femtogram resolution: Sub-femtogram resolution of an in-liquid cavity optomechanical mass sensor\nbased on the twin-microbottle glass resonator is demonstrated. An evaluation of\nthe frequency stability using an optomechanical phase-locked loop reveals that\nthis cavity optomechanical sensor has the highest mass resolution of\n$(7.0\\times2.0)\\times 10^{-16}$ g in water, which is four orders of magnitude\nbetter than that in our first-generation setup [Sci. Adv. 8, eabq2502 (2022)].\nThis highly sensitive mass sensor provides a free-access optomechanical probe\nin liquid and could thus be extended to a wide variety of in-situ chemical and\nbiological metrology applications.",
        "category": "physics_optics"
    },
    {
        "text": "Symmetry analysis and multipole classification of eigenmodes in\n  electromagnetic resonators for engineering their optical properties: The resonator is one of the main building blocks of a plethora of photonic\nand microwave devices from nanolasers to compact biosensors and magnetic\nresonance scanners. The symmetry of the resonators is tightly related to their\nmode structure and multipole content which determines the linear and non-linear\nresponse of the resonator. Here, we develop the algorithm for the\nclassification of eigenmodes in resonators of the simplest shapes depending on\ntheir symmetry group. For each type of mode, we find its multipole content. As\nan illustrative example, we apply the developed formalism to the analysis of\ndielectric triangular prism and demonstrate the formation of high-Q resonances\noriginated due to suppression of the scattering through the main multipole\nchannel. The developed approach one to engineer, predict, and explain\nscattering phenomena and optical properties of resonators and meta-atmos basing\nonly on their symmetry without the need for numerical simulations and it can be\nused for the design of new photonic and microwave devices.",
        "category": "physics_optics"
    },
    {
        "text": "Spatial Filter with Volume Gratings for High-peak-power Multistage Laser\n  Amplifiers: The regular spatial filters comprised of lens and pinhole are essential\ncomponent in high power laser systems, such as lasers for inertial confinement\nfusion, nonlinear optical technology and directed-energy weapon. On the other\nhand the pinhole is treated as a bottleneck of high power laser due to harmful\nplasma created by the focusing beam. In this paper we present a spatial filter\nbased on angular selectivity of Bragg diffraction grating to avoid the harmful\nfocusing effect in the traditional pinhole filter. A spatial filter consisted\nof volume phase gratings in two-pass amplifier cavity were reported.\nTwo-dimensional filter was proposed by using single Pi-phase-shifted Bragg\ngrating, numerical simulation results shown that its angular spectrum bandwidth\ncan be less than 160urad. The angular selectivity of photo-thermo-refractive\nglass and RUGATE film filters, construction stability, thermal stability and\nthe effects of misalignments of gratings on the diffraction efficiencies under\nhigh-pulse-energy laser operating condition are discussed. Keywords: spatial\nfilter, pinhole spatial filter, RUGATE filter, angular selectivity of volume\nphase grating, Pi-phase-shifted Bragg grating, high-energy pulsed laser,\nmulti-pass laser amplifier",
        "category": "physics_optics"
    },
    {
        "text": "Wide-field 3D nanoscopy on chip through large and tunable\n  spatial-frequency-shift effect: Linear super-resolution microscopy via synthesis aperture approach permits\nfast acquisition because of its wide-field implementations, however, it has\nbeen limited in resolution because a missing spatial-frequency band occurs when\ntrying to use a shift magnitude surpassing the cutoff frequency of the\ndetection system beyond a factor of two, which causes ghosting to appear. Here,\nwe propose a method of chip-based 3D nanoscopy through large and tunable\nspatial-frequency-shift effect, capable of covering full extent of the\nspatial-frequency component within a wide passband. The missing of\nspatial-frequency can be effectively solved by developing a\nspatial-frequency-shift actively tuning approach through wave vector\nmanipulation and operation of optical modes propagating along multiple\nazimuthal directions on a waveguide chip to interfere. In addition, the method\nincludes a chip-based sectioning capability, which is enabled by saturated\nabsorption of fluorophores. By introducing ultra-large propagation effective\nrefractive index, nanoscale resolution is possible, without sacrificing the\ntemporal resolution and the field-of-view. Imaging on GaP waveguide material\ndemonstrates a lateral resolution of lamda/10, which is 5.4 folds above Abbe\ndiffraction limit, and an axial resolution of lamda/19 using 0.9 NA detection\nobjective. Simulation with an assumed propagation effective refractive index of\n10 demonstrates a lateral resolution of lamda/22, in which the huge gap between\nthe directly shifted and the zero-order components is completely filled to\nensure the deep-subwavelength resolvability. It means that, a fast wide-field\n3D deep-subdiffraction visualization could be realized using a standard\nmicroscope by adding a mass-producible and cost-effective\nspatial-frequency-shift illumination chip.",
        "category": "physics_optics"
    },
    {
        "text": "Simultaneous amplitude and phase modulation for wide-field nonlinear\n  microscopy applications: In wide-field nonlinear microscopy, wavefront modulation by means of\nphase-only spatial light modulators (SLMs) allows achieving simultaneous\ntwo-photon excitation and fluorescence emission from specific\nregion-of-interests (ROIs) of biological specimens. This is basically\naccomplished at the illumination path of the microscope by the reconstruction\nof computer generated holograms (CGHs) onto the sample plane. However, as\ntwo-photon absorption (TPA) is inherently an intensity-square dependent process\nand iterative Fourier transform algorithms (IFTAs) can only approximate the\nillumination of selected ROIs with the reconstructed CGHs, both signal\nacquisition and/or image formation can be largely affected by the spatial\nirregularities of the illumination patterns. In addition, the speckle\nassociated with the superposition of coherent light at the selected ROIs\nprevents illumination strategies based on CGHs to be successfully used for\nlarge-area (more than 50x50 $\\mu$m2) excitation tasks. To overcome these\nlimitations, we propose an alternative complex illumination method (CIM) able\nto generate simultaneous nonlinear excitation of large-area ROIs with full\ncontrol over the amplitude and phase of the optical wavefront. We\nexperimentally demonstrate spatially uniform illumination, as well as\nstructured illumination with user-defined intensity levels onto micrometric but\nlarge-area ROIs. Furthermore, a proof-of-concept experiment on wide-field\nsecond harmonic generation (SHG) is provided. We believe that the proposed CIM\ncould find applications in wide-field nonlinear microscopy, particularly for\nspeed up signal acquisition time or improve two-photon image formation.",
        "category": "physics_optics"
    },
    {
        "text": "Controlling the intensity of light in large areas at the interfaces of a\n  scattering medium: The recent advent of wave-shaping methods has demonstrated the focusing of\nlight through and inside even the most strongly scattering materials. Typically\nin wavefront shaping, light is focused in an area with the size of one speckle\nspot. It has been shown that the intensity is not only increased in the target\nspeckle spot, but also in an area outside the optimized speckle spot.\nConsequently, the total transmission is enhanced, even though only the\nintensity in a single speckle spot is controlled. Here, we experimentally study\nhow the intensity enhancement on both interfaces of a scattering medium depends\non the optimization area on the transmission side. We observe that as the\noptimization radius increases, the enhancement of the total transmitted\nintensity increases. We find a concomitant decrease of the total reflected\nintensity, which implies an energy redistribution between transmission and\nreflection channels. In addition, we find a qualitative evidence of a\nlong-range reflection-transmission correlation. Our result is useful for\nefficient light harvesting in solar cells, multi-channel quantum secure\ncommunications, imaging, and complex beam delivery through a scattering medium.",
        "category": "physics_optics"
    },
    {
        "text": "High-speed generation of vector beams through random spatial\n  multiplexing: Complex vector modes have become topical of late due to their fascinating\nproperties and the many applications they have found across a broad variety of\nresearch fields. Even though such modes can be generated in a wide variety of\nways, digital holography stands out as one of the most flexible and versatile.\nAlong this line, Digital Micromirror Devices (DMDs) have gain popularity in\nrecent time due to their high refresh rates, which allows the generation of\nvector modes at kHz rates. Nonetheless, most techniques are limited either by\nthe diversity of vector modes that can be generated or by the speed at which\nthey can be switched. Here we propose a technique based on the concept of\nrandom encoding, which allows the generation of arbitrary vector beams at\nspeeds limited only by the refresh rate of the DMD. Our technique will be of\ngreat relevance in research fields such as optical communications, laser\nmaterial processing and optical manipulation, amongst others.",
        "category": "physics_optics"
    },
    {
        "text": "Multi-Wavelength Photonic Neuromorphic Computing for Intra and\n  Inter-Channel Distortion Compensations in WDM Optical Communication Systems: DSP (digital signal processing) has been widely applied in optical\ncommunication systems to mitigate signal distortions and has become one of the\nkey technologies that have sustained data traffic growth over the past decade.\nHowever, the strict energy budget of application-specific integrated\ncircuit-based DSP chips has prevented the deployment of some powerful but\ncomputationally costly DSP algorithms. As a result, fiber nonlinearity-induced\nsignal distortions impede fiber communications systems, especially in\nwavelength-division multiplexed (WDM) transmission systems. To solve these\nchallenges, photonics hardware (i.e., photonic neural networks) promises to\nbreak performance limitations in electronics and gain advantages in bandwidth,\nlatency, and power consumption in solving intellectual tasks that are\nunreachable by conventional digital electronic platforms. This work proposes a\nphotonic recurrent neural network (RNN) capable of simultaneously resolving\ndispersion and both intra and inter-channel fiber nonlinearities in multiple\nWDM channels in the photonic domain, for the first time to our best knowledge.\nFurthermore, our photonic RNN can directly process optical WDM signals in the\nphotonic domain, avoiding prohibitive energy consumption and speed overhead in\nanalog to digital converters (ADC). We demonstrate in simulation that our\nphotonic RNN can process multiple WDM channels simultaneously and achieve a\nreduced bit error rate compared to typical DSP algorithms for all WDM channels\nin a pulse-amplitude modulation 4-level (PAM4) transmission system, thanks to\nits unique capability to address inter-channel fiber nonlinearities. In\naddition to signal quality performance, the proposed system also promises to\nsignificantly reduce the power consumption and the latency compared to the\nstate-of-the-art DSP chips, according to our power and latency analysis.",
        "category": "physics_optics"
    },
    {
        "text": "Spin angular momentum and optical chirality of Poincar\u00e9 vector vortex\n  beams: The optical chirality and spin angular momentum of structured scalar vortex\nbeams has been intensively studied in recent years. The pseudoscalar\ntopological charge $\\ell$ of these beams is responsible for their unique\nproperties. Constructed from a superposition of scalar vortex beams with\ntopological charges $\\ell_\\text{A}$ and $\\ell_\\text{B}$, cylindrical vector\nvortex beams are higher-order Poincar\\'e modes which possess a spatially\ninhomogeneous polarization distribution. Here we highlight the highly\ntailorable and exotic spatial distributions of the optical spin and chirality\ndensities of these higher-order structured beams under both paraxial (weak\nfocusing) and non-paraxial (tight focusing) conditions. Our analytical theory\ncan yield the spin angular momentum and optical chirality of each point on any\nhigher-order or hybrid-order Poincar\\'e sphere. It is shown that the tunable\nPancharatnam topological charge $\\ell_{\\text{P}} = (\\ell_\\text{A} +\n\\ell_\\text{B})/2$ and polarization index $m = (\\ell_\\text{B} -\\ell_\\text{A})/2$\nof the vector vortex beam plays a decisive role in customizing their spin and\nchirality spatial distributions. We also provide the correct analytical\nequations to describe a focused, non-paraxial scalar Bessel beam.",
        "category": "physics_optics"
    },
    {
        "text": "Physical-layer key distribution using synchronous complex dynamics of\n  DBR semiconductor lasers: Common-signal-induced synchronization of semiconductor lasers with optical\nfeedback inspired a promising physical key distribution with\ninformation-theoretic security and potential in high rate. A significant\nchallenge is the requirement to shorten the synchronization recovery time for\nincreasing key rate without sacrificing operation parameter space for security.\nHere, open-loop synchronization of wavelength-tunable multi-section distributed\nBragg reflector (DBR) lasers is proposed as a solution for physical-layer key\ndistribution. Experiments show that the synchronization is sensitive to two\noperation parameters, i.e., currents of grating section and phase section.\nFurthermore, fast wavelength-shift keying synchronization can be achieved by\ndirect modulation on one of the two currents. The synchronization recovery time\nis shortened by one order of magnitude compared to close-loop synchronization.\nAn experimental implementation is demonstrated with a final key rate of 5.98\nMbit/s over 160 km optical fiber distance. It is thus believed that\nfast-tunable multi-section semiconductor lasers opens a new avenue of high-rate\nphysical-layer key distribution using laser synchronization.",
        "category": "physics_optics"
    },
    {
        "text": "Faster light with competing absorption and gain: We experimentally investigate the propagation of optical pulses through a\nfast-light medium with competing absorption and gain. The combination of strong\nabsorption and optical amplification in a potassium-based four-wave mixing\nprocess results in pulse peak advancements up to $88\\%$ of the input pulse\nwidth, more than $35 \\times$ that which is achievable without competing\nabsorption. We show that the enhancement occurs even when the total gain of the\nfour-wave mixer is unity, thereby rendering the medium transparent. By varying\nthe pulse width, we observe a transition between fast and slow light, and show\nthat fast light is optimized for large pulse widths.",
        "category": "physics_optics"
    },
    {
        "text": "Transverse radiation force in a tailored optical fiber: We show, by means of simple model calculations, how a weak laser beam sent\nthrough an optical fiber exerts a transverse radiation force if there is an\nazimuthal asymmetry present in the fiber such that one side has a slightly\ndifferent refractive index than the other. The refractive index difference\n$\\Delta n$ needs only to be very small, of order $10^{-3}$, in order to produce\nan appreciable transverse displacement of order 10 microns. We argue that the\neffect has probably already been seen in a recent experiment of She et al.\n[Phys. Rev. Lett. 101, 243601 (2008)], and we discuss correspondence between\nthese observations and the theory presented. The effect could be used to bend\noptical fibers in a predictable and controlled manner and we propose that it\ncould be useful for micron-scale devices.",
        "category": "physics_optics"
    },
    {
        "text": "Dispersion engineered high-Q silicon Nitride Ring-Resonators via Atomic\n  Layer Deposition: We demonstrate dispersion engineering of integrated silicon nitride based\nring resonators through conformal coating with hafnium dioxide deposited on top\nof the structures via atomic layer deposition (ALD). Both, magnitude and\nbandwidth of anomalous dispersion can be significantly increased. All results\nare confirmed by high resolution frequency-comb-assisted-diode-laser\nspectroscopy and are in very good agreement with the simulated modification of\nthe mode spectrum.",
        "category": "physics_optics"
    },
    {
        "text": "Incorporation of erbium ions into thin-film lithium niobate integrated\n  photonics: As an active material with favorable linear and nonlinear optical properties,\nthin-film lithium niobate has demonstrated its potential in integrated\nphotonics. Integration with rare-earth ions, which are promising candidates for\nquantum memories and transducers, will enrich the system with new applications\nin quantum information processing. Here, we investigate the optical properties\nat 1.5 micron wavelengths of rare-earth ions (Er$^{3+}$) implanted in thin-film\nlithium niobate waveguides and micro-ring resonators. Optical quality factors\nnear a million after post annealing show that ion implantation damage can be\nsuccessfully repaired. The transition linewidth and fluorescence lifetime of\nerbium ions are characterized, revealing values comparable to bulk-doped\ncrystals. The ion-cavity coupling is observed through a Purcell enhanced\nfluorescence, from which a Purcell factor of ~3.8 is extracted. This platform\nis compatible with top-down lithography processes and leads to a scalable path\nfor controlling spin-photon interfaces in photonic circuits.",
        "category": "physics_optics"
    },
    {
        "text": "Egocentric physics : just about Mie: We show that the physics of anapole excitations can be accurately described\nin terms of a resonant state expansion formulation of standard Mie theory\nwithout recourse to Cartesian coordinate based `toroidal' currents that have\npreviously been used to describe this phenomenon. In this purely Mie theory\nframework, the anapole behavior arises as a result of a Fano-type interference\neffect between different quasi-normal modes of the scatterer that effectively\neliminate the scattered field in the associated multipole order.",
        "category": "physics_optics"
    },
    {
        "text": "Robust mode-locking in a hybrid ultrafast laser based on nonlinear\n  multimodal interference: We experimentally demonstrate the realization of a\nhalf-polarization-maintaining (half-PM) fiber laser, in which mode-locking is\nprovided by a reflective multimode-interference saturable absorber (SA). In the\nspecially designed SA, linearly polarized light is coupled into a 15-cm-long\ngraded-index multimode fiber (GIMF) through the PM fiber, and then reflected\nback to the PM structure through a mirror pigtailed with a single-mode fiber\n(SMF). The modulation depth and saturation peak power are measured to be 1.5%\nand 0.6 W, respectively. The proposed SA device is incorporated into a novel\nhalf-PM erbium-doped fiber oscillator, which generates soliton pulses with 409\nfs temporal duration at a 33.3 MHz repetition rate. The proposed fiber laser is\ncompared with a conventional non-PM fiber laser mode-locked by nonlinear\npolarization evolution (NPE) in terms of optical properties such as spectral\nbandwidth, pulse duration, and stability performance. Short- and long-time\nstability tests and superior noise performance corroborate robust mode-locking\nin this setup.",
        "category": "physics_optics"
    },
    {
        "text": "Clarifying the impact of dual optical feedback on semiconductor lasers\n  through analysis of the effective feedback phase: Time-delayed optical feedback is known to trigger a wide variety of complex\ndynamical behavior in semiconductor lasers. Adding a second optical feedback\nloop is naturally expected to further increase the complexity of the system and\nits dynamics, but due to interference between the two feedback arms it was also\nquickly identified as a way to improve the laser stability. While these two\naspects have already been investigated, the influence of the feedback phases,\ni.e. sub-wavelength changes in the mirror positions, on the laser behavior\nstill remains to be thoroughly studied, despite indications that this parameter\ncould have a significant impact. Here, we analyze the effect of the feedback\nphase on the laser stability in a dual-feedback configuration. We show an\nincreased sensitivity of the laser system to feedback phase changes when two\nfeedback loops are present, and clarify the interplay between the frequency\nshift induced by the feedback and the interferometric effect between the two\nfeedback arms.",
        "category": "physics_optics"
    },
    {
        "text": "Quasi-two-dimensional optomechanical crystals with a complete phononic\n  bandgap: A fully planar two-dimensional optomechanical crystal formed in a silicon\nmicrochip is used to create a structure devoid of phonons in the GHz frequency\nrange. A nanoscale photonic crystal cavity is placed inside the phononic\nbandgap crystal in order to probe the properties of the localized acoustic\nmodes. By studying the trends in mechanical damping, mode density, and\noptomechanical coupling strength of the acoustic resonances over an array of\nstructures with varying geometric properties, clear evidence of a complete\nphononic bandgap is shown.",
        "category": "physics_optics"
    },
    {
        "text": "Counter-propagating frequency mixing with Terahertz waves in diamond: Frequency conversion by means of Kerr-nonlinearity is one of the most common\nand exploited nonlinear optical processes in the UV, visible, IR and Mid-IR\nspectral regions. Here we show that wave mixing of an optical field and a\nTerahertz wave can be achieved in diamond, resulting in the frequency\nconversion of the THz radiation either by sum- or difference-frequency\ngeneration. In the latter case, we show that this process is phase-matched and\nmost efficient in a counter-propagating geometry.",
        "category": "physics_optics"
    },
    {
        "text": "Photocurrents in Bi2Se3: bulk versus surface, and injection versus shift\n  currents: Optical injection and detection of charge currents can complement\nconventional transport and photoemission measurements without the necessity of\ninvasive contact that may disturb the system being examined. This is a\nparticular concern for the surface states of a topological insulator. In this\nwork one- and two-color sources of photocurrents are examined in epitaxial,\nthin films of Bi2Se3. We demonstrate that optical excitation and terahertz\ndetection simultaneously captures one- and two- color photocurrent\ncontributions, as previously not required in other material systems. A method\nis devised to isolate the two components, and in doing so each can be related\nto surface or bulk excitations through symmetry. This strategy allows surface\nstates to be examined in a model system, where they have independently been\nverified with angle-resolved photoemission spectroscopy.",
        "category": "physics_optics"
    },
    {
        "text": "Edge states in dynamical superlattices: We address edge states and rich localization regimes available in the\none-dimensional (1D) dynamically modulated superlattices, both theoretically\nand numerically. In contrast to conventional lattices with straight waveguides,\nthe quasi-energy band of infinite modulated superlattice is periodic not only\nin the transverse Bloch momentum, but it also changes periodically with\nincrease of the coupling strength between waveguides. Due to collapse of\nquasi-energy bands dynamical superlattices admit known dynamical localization\neffect. If, however, such a lattice is truncated, periodic longitudinal\nmodulation leads to appearance of specific edge states that exist within\ncertain periodically spaced intervals of coupling constants. We discuss unusual\ntransport properties of such truncated superlattices and illustrate different\nexcitation regimes and enhanced robustness of edge states in them, that is\nassociated with topology of the quasi-energy band.",
        "category": "physics_optics"
    },
    {
        "text": "The anisotropic photorefractive effect in lithium sulfo-phosphate glass\n  system doped with nickel ions: In this work, nonlinear optical (NLO) studies of nickel oxide doped (ranging\nfrom 0.2 to 1.0 mol%) Li2SO4-MgO-P2O5 glasses are reported. A combination of\nfemtosecond (fs) laser, as a pumping light source and a high-accuracy\npolarimeter with low power probing laser, is used to investigate the\nlight-induced optical anisotropy (OA) in these glass materials. The\nlight-induced birefringence (LIB) exhibits slow relaxation tendency up to about\n10 s suggesting on anisotropic photorefractive effect evidently dominated fast\nKerr effect. This behavior is evaluated in the light of other results reported\nrecently and is explained by the glass polymerization mechanisms. The\nphotorefractive birefringence increases with increase of the quantity of NiO up\nto 0.8 mol% and it is attributed to the enhanced degree of depolymerization of\nthe glass network due to the hike in the concentration of Ni2+ ions that occupy\noctahedral (Oh) positions. Further increase of NiO content (>0.8 mol%) causes,\nhowever, a certain decrease of the photorefractive birefringence. Notable\nchange in concentration trend is interpreted by rising of phonon losses due to\nincreasing portion of the nickel ions occupying tetrahedral (Th) sites that\nfacilitates the polymerization of the glass network. The doped glasses with the\nNiO content of about 0.8 mol% may be considered as optimal in the sense of\nphotorefractive efficiency. Relevant samples exhibit largest magnitudes of the\nphotorefractive birefringence and appear to be favorable for potential\napplications.",
        "category": "physics_optics"
    },
    {
        "text": "Towards compact phase-matched and waveguided nonlinear optics in\n  atomically layered semiconductors: Nonlinear frequency conversion provides essential tools for light generation,\nphoton entanglement, and manipulation. Transition metal dichalcogenides (TMDs)\npossess huge nonlinear susceptibilities and 3R-stacked TMD crystals further\ncombine broken inversion symmetry and aligned layering, representing ideal\ncandidates to boost the nonlinear optical gain with minimal footprint. Here, we\nreport on the efficient frequency conversion of 3R-MoS2, revealing the\nevolution of its exceptional second-order nonlinear processes along the\nordinary (in-plane) and extraordinary (out-of-plane) directions. By measuring\nsecond harmonic generation (SHG) of 3R-MoS2 with various thickness - from\nmonolayer (~0.65 nm) to bulk (~1 {\\mu}m) - we present the first measurement of\nthe in-plane SHG coherence length (~530 nm) at 1520 nm and achieve record\nnonlinear optical enhancement from a van der Waals material, >10^4 stronger\nthan a monolayer. It is found that 3R-MoS2 slabs exhibit similar conversion\nefficiencies of lithium niobate, but within propagation lengths >100-fold\nshorter at telecom wavelengths. Furthermore, along the extraordinary axis, we\nachieve broadly tunable SHG from 3R-MoS2 in a waveguide geometry, revealing the\ncoherence length in such structure for the first time. We characterize the full\nrefractive index spectrum and quantify both birefringence components in\nanisotropic 3R-MoS2 crystals with near-field nano-imaging. Empowered with these\ndata we assess the intrinsic limits of the conversion efficiency and nonlinear\noptical processes in 3R-MoS2 attainable in waveguide geometries. Our analysis\nhighlights the potential of 3R-stacked TMDs for integrated photonics, providing\ncritical parameters for designing highly efficient on-chip nonlinear optical\ndevices including periodically poled structures, resonators, compact optical\nparametric oscillators and amplifiers, and optical quantum circuits.",
        "category": "physics_optics"
    },
    {
        "text": "Autler-Townes splitting via frequency upconversion at ultra-low power\n  levels in cold $^{87}$Rb atoms using an optical nanofiber: The tight confinement of the evanescent light field around the waist of an\noptical nanofiber makes it a suitable tool for studying nonlinear optics in\natomic media. Here, we use an optical nanofiber embedded in a cloud of\nlaser-cooled 87Rb for near-infrared frequency upconversion via a resonant\ntwo-photon process. Sub-nW powers of the two-photon beams, at 780 nm and 776\nnm, co-propagate through the optical nanofiber and generation of 420 nm photons\nis observed. A measurement of the Autler-Townes splitting provides a direct\nmeasurement of the Rabi frequency of the 780 nm transition. Through this\nmethod, dephasings of the system can be studied. In this work, the optical\nnanofiber is used as an excitation and detection tool simultaneously, and it\nhighlights some of the advantages of using fully fibered systems for nonlinear\noptics with atoms.",
        "category": "physics_optics"
    },
    {
        "text": "High-efficiency and high-power single-frequency fiber laser at 1.6 um\n  based on cascaded energy-transfer pumping: In this paper, a technique combing cascaded energy-transfer pumping (CEP)\nmethod and master-oscillator power-amplifier (MOPA) configuration is proposed\nfor power scaling of 1.6-um-band single-frequency fiber lasers (SFFLs), where\nthe Er3+ ion has a limited gain. The CEP technique is fulfilled by coupling a\nprimary signal light at 1.6 um and a C-band auxiliary laser. The numerical\nmodel of the fiber amplifier with the CEP technique reveals that the energy\ntransfer process involves the pump competition and the in-band particle\ntransition between the signal and auxiliary lights. Moreover, for the signal\nemission, the population density in the upper level is enhanced and the\neffective population inversion is achieved due to the CEP. A single-frequency\nMOPA laser at 1603 nm with an output power of 52.6 W is obtained\nexperimentally. Besides, a slope efficiency of 30.4% is improved by more than\n10% through the CEP technique. Both the output power and slope efficiency are\nby far the highest for 1.6-um-band SFFLs. Meanwhile, a laser linewidth of 5.2\nkHz and a polarization-extinction ratio of ~18 dB are obtained at the maximum\noutput power. The proposed technique provides an optional method of increasing\nthe slope efficiency and power scaling for fiber lasers operating at L-band.",
        "category": "physics_optics"
    },
    {
        "text": "The Talbot Effect: The Talbot effect, also referred to as self-imaging or lensless imaging, was\noriginally discovered in the 1830's by Henry Fox Talbot. Over the years,\nvarious investigators have found different aspects of this phenomenon, and a\ntheory of the Talbot effect capable of explaining the various observations\nbased on the classical theory of diffraction has emerged. Unfortunately, many\nof the standard Optics textbooks do not discuss the Talbot effect. The goal of\nthe present paper is to bring to the reader's attention the essential features\nas well as an elementary explanation of this wonderful phenomenon.",
        "category": "physics_optics"
    },
    {
        "text": "Statistics for surface modes of nanoparticles with shape fluctuations: We develop a numerical method for approximating the surface modes of\nsphere-like nanoparticles in the quasi-static limit, based on an expansion of\n(the angular part of) the potentials into spherical harmonics. Comparisons of\nthe results obtained in this manner with exact solutions and with a\nperturbation ansatz prove that the scheme is accurate if the shape deviations\nfrom a sphere are not too large. The method allows fast calculations for large\nnumbers of particles, and thus to obtain statistics for nanoparticles with\nrandom shape fluctuations. As an application we present some statistics for the\ndistribution of resonances, polariziabilities, and dipole axes for particles\nwith random perturbations.",
        "category": "physics_optics"
    },
    {
        "text": "Multidimensional topological strings by curved potentials: Simultaneous\n  realization of mobility edge and topological protection: By considering a cigar-shaped trapping potential elongated in a proper\ncurvilinear coordinate, we discover a new form of wave localization which\narises from the interplay of geometry and topological protection. The potential\nis modulated in its shape such that local curvature introduces a trapping\npotential. The curvature varies along the trap curvilinear axis encodes a\ntopological Harper modulation. The varying geometry maps our system in a\none-dimensional Andre-Aubry-Harper grating. We show that a mobility edge exists\nand topologically protected states arises. These states are extremely robust\nwith respect to disorder in shape of the string. The results may be relevant\nfor localization phenomena in Bose-Einstein condensates, optical fibers and\nwaveguides, and new laser devices, but also for fundamental studies on string\ntheory. Taking into account that the one-dimensional modulation mimic the\nexistence of a additional dimensions, our system is the first example of\nphysically realizable five-dimensional string.",
        "category": "physics_optics"
    },
    {
        "text": "Electrically interfaced Brillouin-active waveguide for multi-domain\n  transduction: New strategies to convert signals between optical and microwave domains could\nplay a pivotal role in advancing both classical and quantum technologies.\nThrough recent studies, electro-optomechanical systems have been used to\nimplement microwave-to-optical conversion using resonant optical systems,\nresulting in transduction over limited optical bandwidth. Here, we present an\noptomechanical waveguide system with an integrated piezoelectric transducer\nthat produces electro-optomechanical transduction over a wide optical bandwidth\nthrough coupling to a continuum of optical modes. Efficient electromechanical\nand optomechanical coupling within this system enables bidirectional\noptical-to-microwave conversion with a quantum efficiency of up to $-$54.16 dB.\nWhen electrically driven, this system produces a low voltage acousto-optic\nphase modulation over a wide ($>$100 nm) wavelength range. Through\noptical-to-microwave conversion, we show that the amplitude-preserving nature\ninherent to forward Brillouin scattering is intriguing and has the potential to\nenable new schemes for microwave photonic signal processing. We use these\nproperties to demonstrate a multi-channel microwave photonic filter by\ntransmitting an optical signal through a series of electro-optomechanical\nwaveguide segments having distinct resonance frequencies. Building on these\ndemonstrations, such electro-optomechanical systems could bring flexible\nstrategies for modulation, channelization, and spectrum analysis in microwave\nphotonics.",
        "category": "physics_optics"
    },
    {
        "text": "Size and host-medium effects on topologically protected surface states\n  in bi-anisotropic 3D optical waveguides: We study the optical properties of bi-anisotropic optical waveguides with\nnontrivial topological structure in wavevector space, placed in an ordinary\ndielectric matrix. We derive an exact analytical description of the eigenmodes\nof the systems with arbitrary parameters that allows us to investigate\ntopologically protected surface states (TPSS) in details. In particular, we\nfind that the TPSS on the waveguides would disappear (1) if their radius is\nsmaller than a critical radius due to the dimensional quantization of azimuthal\nwavenumber, and also (2) if the permittivity of the host-medium exceeds a\ncritical value. Interestingly, we also find that the TPSS in the waveguides\nhave negative refraction for some geometries. We have found a TPSS phase\ndiagram that will pave the way for development of the topological waveguides\nfor optical interconnects and devices.",
        "category": "physics_optics"
    },
    {
        "text": "Spatially and polarization resolved plasmon mediated transmission\n  through continuous metal films: The experimental demonstration and characterization is made of the\nplasmon-mediated resonant transmission through an embedded undulated continuous\nthin metal film under normal incidence. 1D undulations are shown to enable a\nspatially resolved polarisation filtering whereas 2D undulations lead to\nspatially resolved, polarization independent transmission. Whereas the needed\nsubmicron microstructure lends itself in principle to CD-like low-cost mass\nreplication by means of injection moulding and embossing, the present paper\ndemonstrates the expected transmission effects on experimental models based on\nmetal-coated photoresist gratings. The spectral and angular dependence in the\nneighbourhood of resonance are investigated and the question of the excess\nlosses exhibited by surface plasmons is discussed",
        "category": "physics_optics"
    },
    {
        "text": "Energy flow structuring in the focused field: We propose an iterative method of energy flow shaping in the focal region\nwith the amplitude, phase and polarization modulation of incident light. By\nusing an iterative optimization based on the diffraction calculation with help\nof the fast Fourier transform, we can tailor the polarization and phase\nstructure in the focal plane. By appropriate design of the polarization and\nphase gradients, arbitrary energy flow including spin and orbital parts can be\ndesigned and tailored independently. The capability of energy flow structuring\nis demonstrated by the measurement of the Stokes parameters and\nself-interference pattern. This provides a novel method to control the\nvectorial feature of the focal volume.",
        "category": "physics_optics"
    },
    {
        "text": "Asymmetric conical diffraction in dislocated edge-centered square\n  lattices: We investigate linear and nonlinear evolution dynamics of light beams\npropagating along a dislocated edge-centered square lattice. The band structure\nand Brillouin zones of this novel lattice are analyzed analytically and\nnumerically. Asymmetric Dirac cones as well as the corresponding Bloch modes of\nthe lattice are obtained. By adopting the tight-binding approximation, we give\nan explanation of the asymmetry of Dirac cones. By utilizing the appropriate\nBloch modes, linear and nonlinear asymmetric conical diffraction is\ndemonstrated. We find that both the focusing and defocusing nonlinearities can\nenhance the asymmetry of the conical diffraction.",
        "category": "physics_optics"
    },
    {
        "text": "Towards Hosting Bound State in Continuum in Specialty Optical\n  Microcavity: We exploit the interaction between supported proximity resonances in an open\nnon-uniformly pumped optical microcavity to host a Bound State in Continuum\n(BIC). Using the modeling of the S-matrix, we study the coupling between the\ninteracting states forming a BIC. We report the divergence of Quality Factor\n(Q-factor) of one of the interacting states, numerically demonstrated with the\nappropriate tuning of the spatial variation of unequal gain-loss within the\nsystem whose stability has been discussed in terms of the Petermann factor at\nthe point of BIC. Such high lifetime has been marked as a signature of BIC.",
        "category": "physics_optics"
    },
    {
        "text": "High-Q impurity photon states bounded by a photonic-band-pseudogap in an\n  optically-thick photonic-crystal slab: We show that, taking a two-dimensional photonic-crystal slab system as an\nexample, surprisingly high quality factors (Q) over 10^5 are achievable, even\nin the absence of a rigorous photonic-band-gap. We find that the density of\nin-plane Bloch modes can be controlled by creating additional photon feedback\nfrom a finite-size photonic-crystal boundary that serves as a low-Q resonator.\nThis mechanism enables significant reduction in the coupling strength between\nthe bound state and the extended Bloch modes by more than a factor of 40.",
        "category": "physics_optics"
    },
    {
        "text": "Analytical expressions for the longitudinal evolution of nondiffracting\n  pulses truncated by finite apertures: In this paper, starting from some general and plausible assumptions based on\ngeometrical optics and on a common feature of the truncated Bessel beams, a\nheuristic derivation is presented of very simple analytical expressions,\ncapable of describing the longitudinal (on-axis) evolution of axially-symmetric\nnondiffracting pulses when truncated by finite apertures. We apply our\nanalytical formulation to several situations involving subluminal, luminal or\nsuperluminal localized pulses and compare the results with those obtained by\nnumerical simulations of the Rayleigh-Sommerfeld diffraction integrals. The\nresults are in excellent agreement. The present approach can be very useful,\nbecause it can yield, in general, closed analytical expressions, avoiding the\nneed of time-consuming numerical simulations, and also because such expressions\nprovide a powerful tool for exploring several important properties of the\ntruncated localized pulses, as their depth of fields, the longitudinal pulse\nbehavior, the decaying rates, and so on.",
        "category": "physics_optics"
    },
    {
        "text": "The Huygens principle for a uniaxial dielectric-magnetic medium with\n  gyrotropic-like magnetoelectric properties: The dyadic Green functions for a uniaxial dielectric-magnetic medium,\ntogether with a reversible field transformation, were implemented to derive a\nformulation of the Huygens principle appropriate to a uniaxial\ndielectric-magnetic medium with gyrotropic-like magnetoelectric properties.",
        "category": "physics_optics"
    },
    {
        "text": "Numerical Simulation of Radiative Transfer of Electromagnetic Angular\n  Momentum: We present numerical simulations of light emitted by a source and scattered\nby surrounding electric dipoles with Zeeman splitting. We calculate the leakage\nof electromagnetic angular momentum to infinity.",
        "category": "physics_optics"
    },
    {
        "text": "Sensing with THz metamaterial absorbers: Metamaterial perfect absorbers from microwaves to optical part of the\nelectromagnetic spectrum has been intensely studied for its ability to absorb\nelectromagnetic radiation. Perfect absorption of light by metamaterials have\nopened up new opportunities for application oriented functionalities such as\nefficient sensors and emitters. We present an absorber based sensing scheme at\nthe terahertz frequencies and discuss optimized designs to achieve high\nfrequency and amplitude sensitivities. The major advantage of a perfect\nmetamaterial absorber as a sensor is the sensitive shift in the absorber\nresonance frequency along with the sharp change in the amplitude of the\nresonance due to strong interaction of the analyte with the electric and the\nmagnetic fields at resonant perfect absorption frequency. We compare the\nsensing performance of the perfect metamaterial absorber with its complementary\nstructural design and planar metasurface with identical structure. The best FoM\nvalues obtained for the absorber sensor here is 2.67 which we found to be\nsignificantly higher than the identical planar metamaterial resonator design.\nWe further show that the sensitivity of the sensor depends on the analyte\nthickness with the best sensitivity values obtained for thicknesses approaching\n{\\lambda}/4n, with {\\lambda} being the free space resonance wavelength and n\nbeing the refractive index of the analyte. Application of metamaterial\nabsorbers as sensors in the terahertz spectral domain would be of tremendous\nsignificance due to several materials having unique spectral signature at the\nterahertz frequencies.",
        "category": "physics_optics"
    },
    {
        "text": "Compounding meta-atoms into meta-molecules with hybrid artificial\n  intelligence techniques: Molecules composed of atoms exhibit properties not inherent to their\nconstituent atoms. Similarly, meta-molecules consisting of multiple meta-atoms\npossess emerging features that the meta-atoms themselves do not possess.\nMetasurfaces composed of meta-molecules with spatially variant building blocks,\nsuch as gradient metasurfaces, are drawing substantial attention due to their\nunconventional controllability of the amplitude, phase, and frequency of light.\nHowever, the intricate mechanisms and the large degrees of freedom of the\nmulti-element systems impede an effective strategy for the design and\noptimization of meta-molecules. Here, we propose a hybrid artificial\nintelligence-based framework consolidating compositional pattern-producing\nnetworks and cooperative coevolution to resolve the inverse design of\nmeta-molecules in metasurfaces. The framework breaks the design of the\nmeta-molecules into separate designs of meta-atoms, and independently solves\nthe smaller design tasks of the meta-atoms through deep learning and\nevolutionary algorithms. We leverage the proposed framework to design metallic\nmeta-molecules for arbitrary manipulation of the polarization and wavefront of\nlight. Moreover, the efficacy and reliability of the design strategy are\nconfirmed through experimental validations. This framework reveals a promising\ncandidate approach to expedite the design of large-scale metasurfaces in a\nlabor-saving, systematic manner.",
        "category": "physics_optics"
    },
    {
        "text": "Spectral singularities and threshold gain of a slab laser under\n  illumination of a focused Gaussian beam: We study spectral singularities of an infinite planar slab of homogeneous\noptically active material in focus of a thin lens under illumination of a\nGaussian beam. We describe the field distribution of the Gaussian beam under\nthis configuration as a plane wave propagated near the optical axis which its\nphase and amplitude vary with distance from center of the beam. Based on this\napproximation, we carry out the transfer matrix for the slab. We explore the\nconsequences of this configuration on determining the threshold gain of the\nactive medium and tuning the resonance frequencies related to spectral\nsingularities. We show that the spectral singularities and the threshold gain\nbesides that vary with distance from center of the Gaussian beam, also they\nchange with relative aperture of the focusing lens. As a result, using a thin\nlens with higher relative aperture, the spectral singularities corresponding\nresonances shift to the higher frequencies (lower wavelengths). Numerical\nresults confirm the theoretical findings",
        "category": "physics_optics"
    },
    {
        "text": "Remarkable optics of short-pitch deformed helix ferroelectric liquid\n  crystals: symmetries, exceptional points and polarization-resolved angular\n  patterns: In order to explore electric-field-induced transformations of polarization\nsingularities in the polarization-resolved angular (conoscopic) patterns\nemerging after deformed helix ferroelectric liquid crystal (DHFLC) cells with\nsubwavelength helix pitch, we combine the transfer matrix formalism with the\nresults for the effective dielectric tensor of biaxial FLCs evaluated using an\nimproved technique of averaging over distorted helical structures. Within the\nframework of the transfer matrix method, we deduce a number of symmetry\nrelations and show that the symmetry axis of L lines (curves of linear\npolarization) is directed along the major in-plane optical axis which rotates\nunder the action of the electric field. When the angle between this axis and\nthe polarization plane of incident linearly polarized light is above its\ncritical value, the C points (points of circular polarization) appear in the\nform of symmetrically arranged chains of densely packed star-monstar pairs. We\nalso emphasize the role of phase singularities of a different kind and discuss\nthe enhanced electro-optic response of DHFLCs near the exceptional point where\nthe condition of zero-field isotropy is fulfilled.",
        "category": "physics_optics"
    },
    {
        "text": "Exact solution of the Landau-Lifshitz equation in a plane wave: The Landau-Lifshitz form of the Lorentz-Abraham-Dirac equation in the\npresence of a plane wave of arbitrary shape and polarization is solved exactly\nand in closed form. The explicit solution is presented in the particular,\nparadigmatic cases of a constant crossed field and of a monochromatic wave with\ncircular and with linear polarization.",
        "category": "physics_optics"
    },
    {
        "text": "Asymmetric parametric amplification in nonlinear left-handed\n  transmission lines: We study parametric amplification in nonlinear left-handed transmission\nlines, which serve as model systems for nonlinear negative index metamaterials.\nWe experimentally demonstrate amplification of a weak pump signal in three\nregimes: with the signal in the left-handed band, with the signal in the stop\nband, and with the signal at a defect frequency. In particular, we demonstrate\nthe amplification of the incident wave by up to 15dB in the left-handed regime.",
        "category": "physics_optics"
    },
    {
        "text": "Toy model of harmonic and sum frequency generation in 2D nanostructures: Optical nonlinearities of matter are often associated with the response of\nindividual atoms. Here, using a toy oscillator model, we show that in the\nconfined geometry of a two-dimensional dielectric nanoparticle a collective\nnonlinear response of the atomic array can arise from the Coulomb interactions\nof the bound optical electrons, even if the individual atoms exhibit no\nnonlinearity. We determine the multipole contributions to the nonlinear\nresponse of nanoparticles and demonstrate that the odd order and even order\nnonlinear electric dipole moments scale with the area and perimeter of the\nnanoparticle, respectively.",
        "category": "physics_optics"
    },
    {
        "text": "Fully-resonant, tunable, monolithic frequency conversion as a coherent\n  UVA source: We demonstrate a monolithic frequency converter incorporating up to four\ntuning degrees of freedom, three temperature and one strain, allowing resonance\nof pump and generated wavelengths simultaneous with optimal phase-matching.\nWith a Rb-doped periodically-poled potassium titanyl phosphate (KTP)\nimplementation, we demonstrate efficient continuous-wave second harmonic\ngeneration from 795 nm to 397 nm, with low-power efficiency of 72%/W and\nhigh-power slope efficiency of 4.5%. The measured performance shows good\nagreement with theoretical modeling of the device. We measure optical\nbistability effects, and show how they can be used to improve the stability of\nthe output against pump frequency and amplitude variations.",
        "category": "physics_optics"
    },
    {
        "text": "Photonic integrated processor for structured light detection and\n  distinction: Integrated photonic devices have become pivotal elements across most research\nfields that involve light-based applications. A particularly versatile category\nof this technology are programmable photonic integrated processors, which are\nbeing employed in an increasing variety of applications, like communication or\nphotonic computing. Such processors accurately control on-chip light within\nmeshes of programmable optical gates. Free-space optics applications can\nutilize this technology by using appropriate on-chip interfaces to couple\ndistributions of light to the photonic chip. This enables, for example, access\nto the spatial properties of free-space light, particularly to phase\ndistributions, which is usually challenging and requires either specialized\ndevices or additional components. Here we discuss and show the detection of\namplitude and phase of structured higher-order light beams using a multipurpose\nphotonic processor. Our device provides measurements of amplitude and phase\ndistributions which can be used to, e.g., directly distinguish light's orbital\nangular momentum without the need for further elements interacting with the\nfree-space light. Paving a way towards more convenient and intuitive phase\nmeasurements of structured light, we envision applications in a wide range of\nfields, specifically in microscopy or communications where the spatial\ndistributions of lights properties are important.",
        "category": "physics_optics"
    },
    {
        "text": "Trace formula for chaotic dielectric resonators tested with microwave\n  experiments: We measured the resonance spectra of two stadium-shaped dielectric microwave\nresonators and tested a semiclassical trace formula for chaotic dielectric\nresonators proposed by Bogomolny et al. [Phys. Rev. E 78, 056202 (2008)]. We\nfound good qualitative agreement between the experimental data and the\npredictions of the trace formula. Deviations could be attributed to missing\nresonances in the measured spectra in accordance with previous experiments\n[Phys. Rev. E 81, 066215 (2010)]. The investigation of the numerical length\nspectrum showed good qualitative and reasonable quantitative agreement with the\ntrace formula. It demonstrated, however, the need for higher-order corrections\nof the trace formula. The application of a curvature correction to the Fresnel\nreflection coefficients entering the trace formula yielded better agreement,\nbut deviations remained, indicating the necessity of further investigations.",
        "category": "physics_optics"
    },
    {
        "text": "A Stern-Gerlach experiment with light: separating photons by spin with\n  the method of A. Fresnel: In 1822 A. Fresnel described an experiment to separate a beam of light into\nits right- and left- circular polarization components using chiral interfaces.\nFresnel's experiment combined three crystalline quartz prisms of alternating\nhandedness to achieve a visible macroscopic separation between the two circular\ncomponents. Such quartz polyprisms were rather popular optical components in\nXIXth century but today remain as very little known optical devices. This work\nshows the analogy between Fresnel's experiment and Stern-Gerlach experiment\nfrom quantum mechanics since both experiments produce selective deflection of\nparticles (photons in case of Fresnel's method) according to their spin angular\nmomentum. We have studied a historical quartz polyprism with eight chiral\ninterfaces producing a large spatial separation of light by spin. We have also\nconstructed a modified Fresnel biprism to produce smaller separations and we\nhave illustrated the process of weak measurement for light. The polarimetric\nanalysis of a Fresnel polyprism reveals that it acts as a spin angular momentum\nanalyzer.",
        "category": "physics_optics"
    },
    {
        "text": "Small footprint nano-mechanical plasmonic phase modulators: The authors' recent Nature Photonics article titled \"Compact Nano-Mechanical\nPlasmonic Phase Modulators\" [1] is reviewed which reports a new phase\nmodulation principle with experimental demonstration of a 23 {\\mu}m long\nnon-resonant modulator having 1.5 {\\pi} rad range with 1.7 dB excess loss at\n780 nm. Analysis showed that by decreasing all dimensions, a low loss,\nultra-compact {\\pi} rad phase modulator is possible. Application of this type\nof nano-mechanical modulator in a miniature 2 x 2 switch is suggested and an\noptical design numerically validated. The footprint of the switch is 0.5 {\\mu}m\nx 2.5 {\\mu}m.",
        "category": "physics_optics"
    },
    {
        "text": "Cylindrically Polarized Nondiffracting Optical Pulses: We extend the concept of radially and azimuthally polarized optical beams to\nthe polychromatic domain by introducing cylindrically polarized nondiffracting\noptical pulses. In particular, we discuss in detail the case of cylindrically\npolarized X-waves, both in the paraxial and nonparaxial regime. The explicit\nexpressions for the electric and magnetic fields of cylindrically polarized\nX-waves is also reported.",
        "category": "physics_optics"
    },
    {
        "text": "Geometric Photonic Spin Hall Effect with Metapolarization: We develop a geometric photonic spin Hall effect (PSHE) which manifests as\nspin-dependent shift in momentum space. It originates from an effective\nspace-variant Pancharatnam-Berry (PB) phase created by artificially engineering\nthe polarization distribution of the incident light. Unlikely the previously\nreported PSHE involving the light-matter interaction, the resulting\nspin-dependent splitting in the geometric PSHE is purely geometrically depend\nupon the polarization distribution of light which can be tailored by assembling\nits circular polarization basis with suitably magnitude and phase. This\nmetapolarization idea enables us to manipulate the geometric PSHE by suitably\ntailoring the polarization geometry of light. Our scheme provides great\nflexibility in the design of various polarization geometry and\npolarization-dependent application, and can be extrapolated to other physical\nsystem, such as electron beam or atom beam, with the similar spin-orbit\ncoupling underlying.",
        "category": "physics_optics"
    },
    {
        "text": "Broadband Epsilon-Near-Zero Metamaterials with Step-Like\n  Metal-Dielectric Multilayer Structures: The concept of the broadband epsilon-near-zero meta-atom consisting of\nlayered stacks with specified metallic filling ratio and thickness is proposed\nbased on the Bergman spectral representation of the effective permittivity. The\nstep-like metal-dielectric multilayer structures are designed to achieve\nrealistic broadband epsilon-near-zero meta-atoms in optical frequency range.\nThese meta-atoms can be integrated as building blocks for unconventional\noptical components with exotic electromagnetic properties over a wide frequency\nrange, such as the demonstrated broadband directional emission and phase front\nshaping.",
        "category": "physics_optics"
    },
    {
        "text": "High-Harmonic Generation from Monolayer and Bilayer Graphene: High-harmonic generation (HHG) in solids is an emerging method to probe\nultrafast electron dynamics in solids at attosecond timescale. In this work, we\nstudy HHG from monolayer and bilayer graphene. Bilayer graphenes with AA and AB\nstacking are considered in this work. It is found that the monolayer and\nbilayer graphenes exhibit significantly different harmonic spectra. The\ndifference in the spectra is attributed to the interlayer coupling between the\ntwo layers. Also, the intraband and interband contributions to the total\nharmonic spectrum play a significant role. Moreover, interesting polarization\nand ellipticity dependence are noticed in total harmonic spectrum for monolayer\nand bilayer graphene.",
        "category": "physics_optics"
    },
    {
        "text": "Adaptive dual-comb spectroscopy in the green region: Dual-comb spectroscopy is extended to the visible spectral range with a\nset-up based on two frequency-doubled femtosecond ytterbium-doped fiber lasers.\nThe dense rovibronic spectrum of iodine around 19240 cm-1 is recorded within 12\nms at Doppler-limited resolution with a simple scheme that only uses\nfree-running femtosecond lasers.",
        "category": "physics_optics"
    },
    {
        "text": "Overlay Alignment Using Two Photonic Crystals: In this paper we proposed a novel overlay alignment method using two sets of\nidentical photonic crystals (PhCs). In this method the reflection or\ntransmission spectrum of the two overlaid photonic crystals is measured to help\nwafer tilt, yaw rotation, and translation aligning. The initial testing results\nwith two 1D photonic crystals and analysis of the alignment accuracy are\npresented. This method is particularly useful in building photonic crystal\nstacks with nanoimprint lithography (NIL).",
        "category": "physics_optics"
    },
    {
        "text": "Chirality of plasmonic metasurfaces with rectangular holes: Chiral response is of tremendous importance to many fields, such as\nanalytical chemistry, polarization manipulation and biological sensing. Here, a\nchiral metasurface based on rectangular holes is systematically investigated.\nThe results show that the chirality is closely related to the size and the\norientation of resonance unit. It is found that the period of the structure is\nalways smaller than the wavelength at which chirality appears, which will\nprovide a good basis for the design of chiral structures. More importantly, the\nCD is highly sensitive to the orientation of resonance unit. By adjusting the\nrotation angle, it is not only possible to invert the CD, but also to change\nthe symmetry of the structure to realize the regulation of chirality. The\nchirality can be significantly enhanced in the proposed structure, and the\nmaximum of circular dichroism (CD) can reach 0.76. To better understand the\nphysical mechanism, the distributions of electric field for LCP and RCP waves\nare also discussed as well. This work will not only deepen the understanding of\nchiral metasurfaces, but also provide guidance for the design of similar chiral\nstructures.",
        "category": "physics_optics"
    },
    {
        "text": "Label-Free Nanoscopy with Contact Microlenses: Super-Resolution\n  Mechanisms and Limitations: Despite all the success with developing super-resolution imaging techniques,\nthe Abbe limit poses a severe fundamental restriction on the resolution of\nfar-field imaging systems based on diffraction of light. Imaging with contact\nmicrolenses, such as microspheres or microfibers, can increase the resolution\nby a factor of two beyond the Abbe limit. The theoretical mechanisms of these\nmethods are debated in the literature. In this work, we focus on the recently\nexpressed idea that optical coupling between closely spaced nanoscale objects\ncan lead to the formation of the modes that drastically impact the imaging\nproperties. These coupling effects emerge in nanoplasmonic or nanocavity\nclusters, photonic molecules, or various arrays under resonant excitation\nconditions. The coherent nature of imaging processes is key to understanding\ntheir physical mechanisms. We used a cluster of point dipoles, as a simple\nmodel system, to study and compare the consequences of coherent and incoherent\nimaging. Using finite difference time domain modeling, we show that the\ncoherent images are full of artefacts. The out-of-phase oscillations produce\nzero-intensity points that can be observed with practically unlimited\nresolution (determined by the noise). We showed that depending on the phase\ndistribution, the nanoplasmonic cluster can appear with the arbitrary shape,\nand such images were obtained experimentally.",
        "category": "physics_optics"
    },
    {
        "text": "Light propagation in nanorod arrays: We study propagation of TM- and TE-polarized light in two-dimensional arrays\nof silver nanorods of various diameters in a gelatin background. We calculate\nthe transmittance, reflectance and absorption of arranged and disordered\nnanorod arrays and compare the exact numerical results with the predictions of\nthe Maxwell-Garnett effective-medium theory. We show that interactions between\nnanorods, multipole contributions and formations of photonic gaps affect\nstrongly the transmittance spectra that cannot be accounted for in terms of the\nconventional effective-medium theory. We also demonstrate and explain the\ndegradation of the transmittance in arrays with randomly located rods as well\nas weak influence of their fluctuating diameter. For TM modes we outline the\nimportance of skin-effect, which causes the full reflection of the incoming\nlight. We then illustrate the possibility of using periodic arrays of nanorods\nas high-quality polarizers.",
        "category": "physics_optics"
    },
    {
        "text": "The analogy between optical beam shifts and quantum weak measurements: We describe how the notion of optical beam shifts (including the spatial and\nangular Goos-H\\\"anchen shift and Imbert-Federov shift) can be understood as a\nclassical analogue of a quantum measurement of the polarization state of a\nparaxial beam by its transverse amplitude distribution. Under this scheme,\ncomplex quantum weak values are interpreted as spatial and angular shifts of\npolarized scalar components of the reflected beam. This connection leads us to\npredict an extra spatial shift for beams with a radially-varying phase\ndependence.",
        "category": "physics_optics"
    },
    {
        "text": "Ray-wave correspondence in the nonlinear description of stadium-cavity\n  lasers: We show that the solution of fully nonlinear lasing equations for stadium\ncavities exhibits a highly directional emission pattern. This directionality\ncan be well explained by a ray-dynamical model, where the dominant ray-escape\ndynamics is governed by the unstable manifolds of the unstable short periodic\norbits for the stadium cavity. Investigating the cold-cavity modes relevant for\nthe lasing, we found that all of the high-Q modes have the emission\ndirectionality corresponding to that of the ray-dynamical model.",
        "category": "physics_optics"
    },
    {
        "text": "Supercritical-Xenon-Filled Hollow-Core Photonic Bandgap Fiber as a\n  Raman-Free, Dispersion-Controllable Nonlinear Optical Medium: We propose supercritical xenon in a hollow-core photonic bandgap fiber as a\nhighly nonlinear medium, and demonstrate 200 nm control over the guidance\nwindow of the fiber as the xenon goes through its supercritical phase\ntransition. The large optical polarizability and monoatomic nature of xenon are\npredicted to allow large optical nonlinearity, on the same order as that of\nfused silica, while maintaining greatly reduced Raman scattering, offering\nbenefits for nonlinear and quantum optics.",
        "category": "physics_optics"
    },
    {
        "text": "High Modulation Efficiency and Large Bandwidth Thin-Film Lithium Niobate\n  Modulator for Visible Light: We experimentally demonstrate a visible light thin-film lithium niobate\nmodulator at 532 nm. The waveguides feature a propagation loss of 2.2 dB/mm\nwhile a grating for fiber interface has a coupling loss of 5 dB. Our\ndemonstrated modulator represents a low voltage-length product of 1.1 V*cm and\na large bandwidth beyond 30 GHz.",
        "category": "physics_optics"
    },
    {
        "text": "Cloaking and anamorphism for light and mass diffusion: We first review classical results on cloaking and mirage effects for\nelectromagnetic waves. We then show that transformation optics allows the\nmasking of objects or produces mirages in diffusive regimes. In order to\nachieve this, we consider the equation for diffusive photon density in\ntransformed coordinates, which is valid for diffusive light in scattering\nmedia. More precisely, generalizing transformations for star domains introduced\nin [Diatta and Guenneau, J. Opt. 13, 024012, 2011] for matter waves, we\nnumerically demonstrate that infinite conducting objects of different shapes\nscatter diffusive light in exactly the same way. We also propose a design of\nexternal light-diffusion cloak with spatially varying sign-shifting parameters\nthat hides a finite size scatterer outside the cloak. We next analyse\nnon-physical parameter in the transformed Fick's equation derived in [Guenneau\nand Puvirajesinghe, R. Soc. Interface 10, 20130106, 2013], and propose to use a\nnon-linear transform that overcomes this problem. We finally investigate other\nform invariant transformed diffusion-like equations in the time domain, and\ntouch upon conformal mappings and non-Euclidean cloaking applied to diffusion\nprocesses.",
        "category": "physics_optics"
    },
    {
        "text": "Monolithic Integration of Embedded III-V Lasers on SOI: Silicon photonic integration has gained great success in many application\nfields owing to the excellent optical device properties and complementary\nmetal-oxide semiconductor (CMOS) compatibility. Realizing monolithic\nintegration of III-V lasers and silicon photonic components on single silicon\nwafer is recognized as a long-standing obstacle for ultra-dense photonic\nintegration, which can provide considerable economical, energy efficient and\nfoundry-scalable on-chip light sources, that has not been reported yet. Here,\nwe demonstrate embedded InAs/GaAs quantum dot (QD) lasers directly grown on\ntrenched silicon-on-insulator (SOI) substrate, enabling monolithic integration\nwith butt-coupled silicon waveguides. By utilizing the patterned grating\nstructures inside pre-defined SOI trenches and unique epitaxial method via\nmolecular beam epitaxy (MBE), high-performance embedded InAs QD lasers with\nout-coupled silicon waveguide are achieved on such template. By resolving the\nepitaxy and fabrication challenges in such monolithic integrated architecture,\nembedded III-V lasers on SOI with continuous-wave lasing up to 85 oC are\nobtained. The maximum output power of 6.8 mW can be measured from the end tip\nof the butt-coupled silicon waveguides, with estimated coupling efficiency of\napproximately -7.35 dB. The results presented here provide a scalable and\nlow-cost epitaxial method for realization of on-chip light sources directly\ncoupling to the silicon photonic components for future high-density photonic\nintegration.",
        "category": "physics_optics"
    },
    {
        "text": "Parity-Time Symmetry and Exceptional points: A Tutorial: The physics of systems that cannot be described by a Hermitian Hamiltonian,\nhas been attracting a great deal of attention in recent years, motivated by\ntheir nontrivial responses and by a plethora of applications for sensing,\nlasing, energy transfer/harvesting, topology and quantum networks.\nElectromagnetics is an inherently non-Hermitian research area because all\nmaterials are lossy, loss and gain distributions can be controlled with various\nmechanisms, and the underlying systems are open to radiation. Therefore, the\nrecent developments in non-Hermitian physics offer exciting opportunities for a\nbroad range of basic research and engineering applications relevant to the\nantennas and propagation community. In this work, we offer a tutorial geared at\nintroducing the unusual electromagnetic phenomena emerging in non-Hermitian\nsystems, with particular emphasis on a sub-class of these systems that obey\nparity-time (PT) symmetry. We discuss the basic concepts behind this topic and\nexplore their implications for various phenomena. We first discuss the basic\nfeatures of P, T and PT operators applied to electromagnetic and quantum\nmechanical phenomena. We then discuss the exotic response of PT-symmetric\nelectromagnetic structures and their opportunities, with particular attention\nto singularities, known as exceptional points, emerging in these systems, and\ntheir unusual scattering response.",
        "category": "physics_optics"
    },
    {
        "text": "Reducing of phase retrieval errors in Fourier analysis of 2-dimensional\n  digital model interferograms: In order to measure the radial displacements of facets on surface of a\ngrowing spherical Cu_{2-\\delta}Se crystal with sub-nanometer resolution, we\nhave investigated the reliability and accuracy of standard method of Fourier\nanalysis of fringes obtained applying digital laser interferometry method.\nGuided by the realistic experimental parameters (density and orientation of\nfringes), starting from 2-dimensional model interferograms and using\nunconventional custom designed Gaussian filtering window and unwrapping\nprocedure of the retrieved phase, we have demonstrated that for considerable\nportion of parameter space the non-negligible inherent phase retrieval error is\npresent solely due to non-integer number of fringes within the digitally\nrecorded image (using CCD camera). Our results indicate the range of\nexperimentally adjustable parameters for which the generated error is\nacceptably small. We also introduce a modification of the (last part) of the\nusual phase retrieval algorithm which significantly reduces the error in the\ncase of small fringe density.",
        "category": "physics_optics"
    },
    {
        "text": "Nonlinear Wavepacket Dynamics in Proximity to a Stationary Inflection\n  Point: A stationary inflection point (SIP) in the Bloch dispersion relation of a\nperiodic waveguide is an exceptional point degeneracy where three Bloch\neigenmodes coalesce forming the so-called frozen mode with a divergent\namplitude and vanishing group velocity of its propagating component. We have\ndeveloped a theoretical framework to study the time evolution of wavepackets\ncentered at an SIP. Analysis of the evolution of statistical moments\ndistribution of linear pulses shows a strong deviation from the conventional\nballistic wavepacket dynamics in dispersive media. The presence of nonlinear\ninteractions dramatically changes the situation, resulting in a mostly\nballistic propagation of nonlinear wavepackets with the speed and even the\ndirection of propagation essentially dependent on the wavepacket amplitude.\nSuch a behavior is unique to nonlinear wavepackets centered at an SIP and can\nbe used for the realization of a novel family of beam power routers for\nclassical waves.",
        "category": "physics_optics"
    },
    {
        "text": "Deterministic generation of parametrically driven dissipative Kerr\n  soliton: We theoretically study the nature of parametrically driven dissipative Kerr\nsoliton (PD-DKS) in a doubly resonant degenerate micro-optical parametric\noscillator (DR-D{\\mu}OPO) with the cooperation of \\c{hi}(2) and \\c{hi}(3)\nnonlinearities. Lifting the assumption of close-to-zero group velocity mismatch\n(GVM) that requires extensive dispersion engineering, we show that there is a\nthreshold GVM above which single PD-DKS in DR-D{\\mu}OPO can be generated\ndeterministically. We find that the exact PD-DKS generation dynamics can be\ndivided into two distinctive regimes depending on the phase matching condition.\nIn both regimes, the perturbative effective third-order nonlinearity resulting\nfrom the cascaded quadratic process is responsible for the soliton annihilation\nand the deterministic single PD-DKS generation. We also develop the\nexperimental design guidelines for accessing such deterministic single PD-DKS\nstate. The working principle can be applied to different material platforms as\na competitive ultrashort pulse and broadband frequency comb source architecture\nat the mid-infrared spectral range.",
        "category": "physics_optics"
    },
    {
        "text": "Rhythmic soliton interactions for integrated dual-microcomb spectroscopy: Rotation symmetry of microresonators supports the generation of phase-locked\ncounter-propagating (CP) solitons that can potentially miniaturize dual-comb\nsystems. Realization of these dual-comb compatible solitons in photonic\nintegrated circuits remains a challenge. Here, we synthesized such CP solitons\nin an integrated silicon nitride microresonator and observed forced soliton\noscillation due to rhythmic, time-varying soliton interactions. The\ninteractions result in seconds mutual-coherence passively. Temporal motion in\nthe soliton streams is discerned by measuring a quadratic-scaling frequency\nnoise peaks and an inverse quadratic-scaling microcomb sidebands. By generating\na CP soliton trimer to have two synchronized solitons in one of the orbiting\ndirections, we resolve the incapability of measuring two unsynchronized CP\nsoliton dimer pulses by optical cross-correlation, and show CP solitons undergo\ncomplex motion trajectory. We further prove that precise dual-comb spectroscopy\nwith an acquisition time as short as 0.6 $\\mu$s is feasible using these\nsolitons, although the temporal motion limits the dynamic range. Besides\nrevealing soliton interactions with different group velocities, our work\npropels the realization of photonic integrated dual-comb spectrometers with\nhigh passive coherence.",
        "category": "physics_optics"
    },
    {
        "text": "Phase-locked array of quantum cascade lasers with an integrated Talbot\n  cavity: We show a phase-locked array of three quantum cascade lasers with an\nintegrated Talbot cavity at one side of the laser array. The coupling scheme is\ncalled diffraction coupling. By controlling the length of Talbot to be a\nquarter of Talbot distance (Zt/4), in-phase mode operation can be selected. The\nin-phase operation shows great modal stability under different injection\ncurrents, from the threshold current to the full power current. The far-field\nradiation pattern of the in-phase operation contains three lobes, one central\nmaximum lobe and two side lobes. The interval between adjacent lobes is about\n10.5{\\deg}. The output power is about 1.5 times that of a single-ridge laser.\nFurther studies should be taken to achieve better beam performance and reduce\noptical losses brought by the integrated Talbot cavity.",
        "category": "physics_optics"
    },
    {
        "text": "Resonance vector mode locking: A mode locked fibre laser as a source of ultra-stable pulse train has\nrevolutionised a wide range of fundamental and applied research areas by\noffering high peak powers, high repetition rates, femtosecond range pulse\nwidths and a narrow linewidth. However, further progress in linewidth narrowing\nseems to be limited by the complexity of the carrier-envelope phase control.\nHere for the first time we demonstrate experimentally and theoretically a new\nmechanism of resonance vector self-mode locking where tuning in-cavity\nbirefringence leads to excitation of the longitudinal modes sidebands\naccompanied by the resonance phase locking of sidebands with the adjacent\nlongitudinal modes. An additional resonance with acoustic phonons provides the\nrepetition rate tunability and linewidth narrowing down to Hz range that\ndrastically reduces the complexity of the carrier-envelope phase control and so\nwill open the way to advance lasers in the context of applications in\nmetrology, spectroscopy, microwave photonics, astronomy, and\ntelecommunications.",
        "category": "physics_optics"
    },
    {
        "text": "Plasmon-exciton Interactions in Gold-WSe2 Multilayer structures: Van der Waals materials such as thin films of transition-metal\ndichalcogenides (TMDCs) manifest strongly bound exciton states in the visible\nspectrum at ambient conditions that provide an ideal platform for\nexciton-photon couplings. Utilizing semiconducting TMDCs in the form of\nmultilayer structure combined with metals can increase significantly the\nlight-matter interaction. In this way, the interaction between excitons and\nsurface-plasmon polaritons emerge as a platform for transferring the\nelectromagnetic energy at confined modal volumes. Here, we theoretically\ninvestigate how moving electrons can be used as a probe of hybrid\nexciton-plasmon polaritons of gold-WSe2 multilayers within the context of\nelectron energy-loss spectroscopy (EELS) and cathodoluminescence (CL)\nspectroscopy. Interestingly, and in contrast to WSe2 slab waveguides where\nquasi-propagating photonic modes interact with only exciton A, in gold-WSe2\nmultilayer, exciton A and exciton B can both strongly interact with surface\nplasmon polaritons. Hence, we observe CL emission suppression at excitonic or\nplasmonic peaks, which reveals the energy transfer between excitons and\nplasmons in the form of nonradiating guided waves. Our work provides a\nsystematic study for deeper understanding of the effect of the configuration\nand the thickness of layers on the photonic and plasmonic modes and hence on\nthe strength of the coupling between excitons and surface-plasmon polaritons.\nOur findings pay the way for designing efficient photodetectors, sensitive\nsensors, and light emitting devices based on metal/semiconducting hybrid\nmaterials.",
        "category": "physics_optics"
    },
    {
        "text": "Digital Holography at Shot Noise Level: By a proper arrangement of a digital holography setup, that combines off-axis\ngeometry with phase-shifting recording conditions, it is possible to reach the\ntheoretical shot noise limit, in real-time experiments.We studied this limit,\nand we show that it corresponds to 1 photo-electron per pixel within the whole\nframe sequence that is used to reconstruct the holographic image. We also show\nthat Monte Carlo noise synthesis onto holograms measured at high illumination\nlevels enables accurate representation of the experimental holograms measured\nat very weak illumination levels. An experimental validation of these results\nis done.",
        "category": "physics_optics"
    },
    {
        "text": "Dynamic particle enhancement in limited-view optoacoustic tomography: Limited-view artefacts are commonly present in optoacoustic tomography\nimages, mainly due to practical geometrical and physical constraints imposed by\nthe imaging systems as well as limited light penetration into large optically\nopaque samples. Herein, a new approach termed dynamic particle-enhanced\noptoacoustic tomography (DPOT) is proposed for improving image contrast and\nvisibility of optoacoustic images under limited-view scenarios. The method is\nbased on the non- linear combination of a temporal sequence of tomographic\nreconstructions representing sparsely distributed moving particles. We\ndemonstrate experimental performance by dynamically imaging the flow of\nsuspended microspheres in three dimensions, which shows promise for DPOT\napplicability in angiographic imaging in living organisms.",
        "category": "physics_optics"
    },
    {
        "text": "Quantum photonics in triangular-cross-section nanodevices in silicon\n  carbide: Silicon carbide is evolving as a prominent solid-state platform for the\nrealization of quantum information processing hardware. Angle-etched\nnanodevices are emerging as a solution to photonic integration in bulk\nsubstrates where color centers are best defined. We model triangular\ncross-section waveguides and photonic crystal cavities using Finite-Difference\nTime-Domain and Finite-Difference Eigensolver approaches. We analyze optimal\ncolor center positioning within the modes of these devices and provide\nestimates on achievable Purcell enhancement in nanocavities with applications\nin quantum communications. Using open quantum system modeling, we explore\nemitter-cavity interactions of multiple non-identical color centers coupled to\nboth a single cavity and a photonic crystal molecule in SiC. We observe\npolariton and subradiant state formation in the cavity-protected regime of\ncavity quantum electrodynamics applicable in quantum simulation.",
        "category": "physics_optics"
    },
    {
        "text": "Universal phase relation between longitudinal and transverse fields\n  observed in focused terahertz beams: We directly observe longitudinal electromagnetic fields in focused freely\npropagating terahertz beams of radial and linear polarization. In accordance\nwith theory, the longitudinal fields are phase shifted by a value of pi/2 with\nrespect to the transverse field components. This behavior is found for all\nfrequency components of single cycle THz radiation pulses. Additionally we show\nthat the longitudinal field of a radially polarized THz beam has a smaller spot\nsize as compared to the transverse field of a linearly polarized beam, that is\nfocused under the same conditions.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of Ultrafast Interfacial Exciton Formation and Recombination\n  in Graphene/MoS2 Heterostructure: In this study,we combined time-resolved terahertz spectroscopy along with\ntransient absorption spectroscopy to revisit the interlayer non-equilibrium\ncarrier dynamics in largely lateral size Gr/MoS2 heterostructure fabricated\nwith chemical vapor deposition method. Our experimental results reveal that,\nwith photon-energy below the A-exciton of MoS2 monolayer, hot electrons\ntransfer from graphene to MoS2 takes place in time scale of less than 0.5 ps,\nresulting in ultrafast formation of interfacial exciton in the heterostructure,\nsubsequently, recombination relaxation of the interfacial exciton occurs in\ntime scale of ~18 ps. A new model considering carrier heating and photogating\neffect in graphene is proposed to estimate the amount of carrier transfer in\nthe heterostructure, which shows a good agreement with experimental result.\nMoreover, when the photon-energy is on-resonance with the A-exciton of MoS2,\nphotogenerated holes in MoS2 are transferred to graphene layer within 0.5 ps,\nleading to the formation of interfacial exciton, the subsequent\nphotoconductivity (PC) relaxation of graphene and bleaching recovery of\nA-exciton in MoS2 take place around ~10 ps time scale, ascribing to the\ninterfacial exciton recombination. The faster recombination time of interfacial\nexciton with on-resonance excitation could come from the reduced interface\nbarrier caused by bandgap renormalization effect. Our study provides deep\ninsight into the understanding of interfacial charge transfer as well as the\nrelaxation dynamics in graphene-based heterostructures, which are promising for\nthe applications of graphene-based optoelectronic devices.",
        "category": "physics_optics"
    },
    {
        "text": "Chaotic mode-competition dynamics in a multimode semiconductor laser\n  with optical feedback and injection: Photonic computing is attracting increasing interest to accelerate\ninformation processing in machine learning applications. The mode-competition\ndynamics of multimode semiconductor lasers is useful for solving the\nmulti-armed bandit problem in reinforcement learning for computing\napplications. In this study, we numerically evaluate the chaotic\nmode-competition dynamics in a multimode semiconductor laser with optical\nfeedback and injection. We observe the chaotic mode-competition dynamics among\nthe longitudinal modes and control them by injecting an external optical signal\ninto one of the longitudinal modes. We define the dominant mode as the mode\nwith the maximum intensity; the dominant-mode ratio for the injected mode\nincreases as the optical injection strength increases. We find that the\ncharacteristics of the dominant mode ratio in terms of the optical injection\nstrength are different among the modes owing to the different optical feedback\nphases. We propose a control technique for the characteristics of the dominant\nmode ratio by precisely tuning the initial optical frequency detuning between\nthe optical injection signal and injected mode. We also evaluate the\nrelationship between the region for the large dominant mode ratio and injection\nlocking range. The region for the large dominant mode ratio does not correspond\nto the injection-locking range. This discrepancy results from the complex\nmode-competition dynamics in multimode semiconductor lasers with both optical\nfeedback and injection. This control technique of chaotic mode-competition\ndynamics in multimode lasers is promising for applications in reinforcement\nlearning and reservoir computing as photonic artificial intelligence.",
        "category": "physics_optics"
    },
    {
        "text": "Band dynamics accompanied by bound states in the continuum at the\n  third-order $\u0393$ point in leaky-mode photonic lattices: Bound states in the continuum (BICs) and Fano resonances in planar photonic\nlattices, including metasurfaces and photonic crystal slabs, have been studied\nextensively in recent years. Typically, the BICs and Fano resonances are\nassociated with the second stop bands open at the second-order Gamma ($\\Gamma$)\npoint. This paper address the fundamental properties of the fourth stop band\naccompanied by BICs at the third-order $\\Gamma$ point in one-dimensional (1D)\nleaky-mode photonic lattices. At the fourth stop band, one band edge mode\nsuffers radiation loss, thereby generating a Fano resonance, while the other\nband edge mode becomes a nonleaky BIC. The fourth stop band is controlled\nprimarily by the Bragg processes associated with the first, second, and fourth\nFourier harmonic components of the periodic dielectric constant modulation. The\ninterplay between these three major processes closes the fourth band gap and\ninduces a band flip whereby the leaky and BIC edges transit across the fourth\nband gap. At the fourth stop band, a new type of BIC is formed owing to the\ndestructive interplay between the first and second Fourier harmonics. When the\nfourth band gap closes with strongly enhanced $Q$ factors, Dirac cone\ndispersions can appear at the third-order $\\Gamma$ point. Our results\ndemonstrate a method for manipulating electromagnetic waves by utilizing the\nhigh-$Q$ Bloch modes at the fourth stop band.",
        "category": "physics_optics"
    },
    {
        "text": "Scattering optimization of photonic cluster: from minimal to maximal\n  reflectivity: The optimization of the light scattered by photonic cluster made of small\nparticles is studied with the help of the local perturbation method and special\noptimization algorithm. It was shown that photonic cluster can be optimized in\na such a way that its reflectivity will be increased or decreased by several\norders of magnitude for selected wavelength and direction.",
        "category": "physics_optics"
    },
    {
        "text": "Extraordinary coupled spin and chirality of electromagnetic guided waves: Optical spin and chirality play key roles in the interaction between light\nand chiral materials. The propagating guided modes by optical waveguides hardly\nproduce optical chirality because of the broken symmetry between electric and\nmagnetic components. Here for the first time we discover that the directional\ncoupling of spatially guided modes can create polarization helicity, and thus\ngenerate optical spin and chirality. They are originated from the intrinsic\nphase retardation of {\\pi}/2 between two directionally coupled spatial modes.We\nshow that these near-field coupled spin and chirality manifests as\nodd-symmetric density distribution in the cross-section of symmetric\nwaveguides, and especially this odd-symmetric distribution is intrinsically\nlocked to coupling paths regardless of the propagation direction of light.\nFurthermore, such odd-symmetric chirality could produce a considerable chiral\ngradient force that can be used to separate chiral nanoparticles. These\nextraordinary electromagnetic properties may further prosper spin photonics and\nchiral light-matter interaction on nanophotonic platforms.",
        "category": "physics_optics"
    },
    {
        "text": "Toggling Near-field Directionality via Polarization Control of Surface\n  Waves: Directional excitation of guidance modes is central to many applications\nranging from light harvesting, optical information processing to quantum\noptical technology. Of paramount interest, especially, the active control of\nnear-field directionality provides a new paradigm for the real-time on-chip\nmanipulation of light. Here we find that for a given dipolar source, its\nnear-field directionality can be toggled efficiently via tailoring the\npolarization of surface waves that are excited, for example, via tuning the\nchemical potential of graphene in a graphene-metasurface waveguide. This\nfinding enables a feasible scheme for the active near-field directionality.\nCounterintuitively, we reveal that this scheme can transform a circular\nelectric/magnetic dipole into a Huygens dipole in the near-field coupling.\nMoreover, for Janus dipoles, this scheme enables us to actively flip their\nnear-field coupling and non-coupling faces.",
        "category": "physics_optics"
    },
    {
        "text": "Fast and optimal broad-band Stokes/Mueller polarimeter design by the use\n  of a genetic algorithm: A fast multichannel Stokes/Mueller polarimeter with no mechanically moving\nparts has been designed to have close to optimal performance from 430-2000 nm\nby applying a genetic algorithm. Stokes (Mueller) polarimeters are\ncharacterized by their ability to analyze the full Stokes (Mueller) vector\n(matrix) of the incident light. This ability is characterized by the condition\nnumber, $\\kappa$, which directly influences the measurement noise in\npolarimetric measurements. Due to the spectral dependence of the retardance in\nbirefringent materials, it is not trivial to design a polarimeter using\ndispersive components. We present here both a method to do this optimization\nusing a genetic algorithm, as well as simulation results. Our results include\nfast, broad-band polarimeter designs for spectrographic use, based on 2 and 3\nFerroelectric Liquid Crystals, whose material properties are taken from\nmeasured values. The results promise to reduce the measurement noise\nsignificantly over previous designs, up to a factor of 4.5 for a Mueller\npolarimeter, in addition to extending the spectral range.",
        "category": "physics_optics"
    },
    {
        "text": "Period-doubling bifurcation in spatiotemporal mode-locked lasers: Period-doubling bifurcation is a universal dynamic of nonlinear systems,\nwhich has been extensively investigated in laser systems with a single\ntransverse mode. This study presents an experimental observation and\ntheoretical investigation of the period-doubling bifurcation in spatiotemporal\nmode-locked (STML) multimode fiber lasers. In the period-doubling state, it is\nobserved that the pulse train modulation varies with the transverse-mode, and\nthe output beam profile fluctuates rapidly and periodically. The numerical\nsimulations conducted in this study are in good agreement with the experimental\nobservations. Furthermore, a simple iterative model is proposed by considering\nthe mode-dependent saturable absorption effect, and the experimental results\ncan be qualitatively interpreted by this model. Based on these results,\nspatiotemporal saturable absorption is believed to be the key factor for the\nunique spatiotemporal characteristic of period-doubling bifurcation in\nmultimode lasers. This study contributes to the understanding of the complex\nspatiotemporal dynamics in STML multimode lasers and to the discovery of novel\ndynamics in high-dimensional nonlinear systems.",
        "category": "physics_optics"
    },
    {
        "text": "Fast control of the reflection of a ferroelectric by an extremely short\n  pulse: We propose a new type of optical switch based on a ferroelectric. It is based\non the gap which exists for waves propagating from a dielectric to a\nferroelectric material. This gap depends on the polarization of the\nferroelectric. We show that it can be shifted by a control electromagnetic\npulse so that the material becomes transparent. This device would shift much\nfaster than the relaxation time of the ferroelectric (1 nano s). Estimates are\ngiven for a real material.",
        "category": "physics_optics"
    },
    {
        "text": "High-Q two-dimensional lithium niobate photonic crystal slab\n  nanoresonators: Lithium niobate (LN), known as \"silicon of photonics,\" exhibits outstanding\nmaterial characteristics with great potential for broad applications. Enhancing\nlight-matter interaction in the nanoscopic scale would result in intriguing\ndevice characteristics that enable revealing new physical phenomena and\nrealizing novel functionalities inaccessible by conventional means. High-Q two\ndimensional (2D) photonic crystal (PhC) slab nanoresonators are particularly\nsuitable for this purpose, which, however, remains open challenge to be\nrealized on the lithium niobate platform. Here we take an important step\ntowards this direction, demonstrating 2D LN PhC slab nanoresonators with\noptical Q as high as $3.51 \\times 10^5$, about three orders of magnitude higher\nthan other 2D LN PhC structures reported to date. The high optical quality,\ntight mode confinement, together with pure polarization characteristics of the\ndevices enable us to reveal peculiar anisotropy of photorefraction quenching\nand unique anisotropic thermo-optic nonlinear response, which have never been\nreported before. They also allow us to observe third harmonic generation for\nthe first time in on-chip LN nanophotonic devices, and strong\norientation-dependent generation of second harmonic. The demonstrated high-Q 2D\nLN PhC nanoresonators not only offer an excellent device platform for the\nexploration of extreme nonlinear and quantum optics at single-photon and\nfew-photon level, but also open up a great avenue towards future development of\nlarge-scale integrated LN photonic circuits for energy efficient nonlinear\nphotonic and electro-optic signal processing.",
        "category": "physics_optics"
    },
    {
        "text": "On negative higher-order Kerr effect and filamentation: As a contribution to the ongoing controversy about the role of higher-order\nKerr effect (HOKE) in laser filamentation, we first provide thorough details\nabout the protocol that has been employed to infer the HOKE indices from the\nexperiment. Next, we discuss potential sources of artifact in the experimental\nmeasurements of these terms and show that neither the value of the observed\nbirefringence, nor its inversion, nor the intensity at which it is observed,\nappear to be flawed. Furthermore, we argue that, independently on our values,\nthe principle of including HOKE is straightforward. Due to the different\ntemporal and spectral dynamics, the respective efficiency of defocusing by the\nplasma and by the HOKE is expected to depend substantially on both incident\nwavelength and pulse duration. The discussion should therefore focus on\ndefining the conditions where each filamentation regime dominates.",
        "category": "physics_optics"
    },
    {
        "text": "Coherent broadband mid-infrared supercontinuum generation in As2Se3\n  photonic crystal fiber: The generation of fully coherent broadband mid-infrared (MIR) supercontinuum\n(SC) from 2.3 um to 8.3 um is demonstrated by using a 4.1 um pump and an As2Se3\nphotonic crystal fiber (PCF).By introducing the random quantum noise and the\npower instability on the input pulse and by numerically implementing the\nYoung,s double slits experiment, we examine the coherence properties across the\nSC spectrum. It is found that the coherence of this MIR SC source depends\nstrongly on the input pulse duration, the peak power, the power stability, and\nthe zero-dispersion wavelength (ZDW) of the As2Se3 PCF.The optimal conditions\nfor the MIR SC with a maximal coherent bandwidth are identified.",
        "category": "physics_optics"
    },
    {
        "text": "Experimental demonstration of multi-watt CW supercontinuum tailoring in\n  photonic crystal fibers: We demonstrate experimentally that the spectral broadening of CW\nsupercontinuum can be controlled by using photonic crystal fibers with two\nzero-dispersion wavelengths pumped by an Yb fiber laser at 1064 nm. The\nspectrum is bounded by two dispersive waves whose spectral location depends on\nthe two zero-dispersion wavelengths of the fiber. The bandwidth of the\ngenerated spectrum and the spectral power density may thus be tailored for\nparticular applications, such as high-resolution optical coherence tomography\nor optical spectroscopy.",
        "category": "physics_optics"
    },
    {
        "text": "Theoretical method for generating regular spatiotemporal pulsed-beam\n  with controlled transverse-spatiotemporal dispersion: Herein we theoretically report a method that generates a\ntransverse-spatiotemporal dispersion (T-STD), which is distinct from previous\nspatial, temporal, and longitudinal-spatiotemporal optics dispersions. By\nmodulating T-STD, two not yet reported spatiotemporally structured beams\n(STSBs), i.e., the honeycomb beam and the picket-fence beam, can be generated\nin the space-time domain. The generated STSBs have novel and tunable periodic\ndistributions. T-STD, STSB and their inner physical relationship are analyzed\nand introduced. We believe that this method might open a path towards new\noptical beams for potential applications, such as ultrafast optical fabrication\nand detection.",
        "category": "physics_optics"
    },
    {
        "text": "Inherent Limit to Coherent Phonon Generation under Non-Resonant Light\n  Field Driving: Coherent manipulation of quasi-particles is a crucial method to realize\nultrafast switching of the relating macroscopic order. In this letter, we\nstudied coherent phonon generation under strong light field which allows to\ninduce non-perturbative nonlinear optical phenomena in solids. The efficiency\nof coherent phonon generation starts to saturate and deviate from the\npronounced linear power dependence when the light intensity goes into the\nnon-perturbative regime. We propose a novel theoretical model based on Floquet\npicture and show that the saturation is due to a limitation of the driving\nforce inherent in non-resonant driving of the electronic system in the\nnon-perturbative regime.",
        "category": "physics_optics"
    },
    {
        "text": "Refraction laws for two-dimensional plasmons: Despite numerous applications of two-dimensional plasmons for electromagnetic\nenergy manipulation at the nanoscale, their quantitative refraction and\nreflection laws (analogs of Fresnel formulas in optics) have not yet been\nestablished. This fact can be traced down to the strong non-locality of\nequations governing the 2d plasmon propagation. Here, we tackle this difficulty\nby direct solution of plasmon scattering problem with Wiener-Hopf technique. We\nobtain the reflection and transmission coefficients for 2d plasmons at the\ndiscontinuity of 2d conductivity at arbitrary incidence angle, for both gated\nand non-gated 2d systems. At a certain incidence angle, the absolute\nreflectivity has a pronounced dip reaching zero for gated plasmons. The dip is\nassociated with wave passage causing no dynamic charge accumulation at the\nboundary. For all incidence angles, the reflection has a non-trivial phase\ndifferent from zero and $\\pi$.",
        "category": "physics_optics"
    },
    {
        "text": "Time-resolved shadowgraphs of transient plasma induced by\n  spatiotemporally focused femtosecond laser pulses in fused silica glass: We report on experimental observations of formation and evolution of\ntransient plasma produced in fused silica glass with spatiotemporally focused\n(STF) femtosecond laser pulses using a pump-probe shadow imaging technique.\nSurprisingly, the observation shows that the track of the plasma is\nsignificantly curved, which is attributed to an asymmetric density distribution\nof the transient plasma produced in the focal volume caused by the pulse front\ntilt of the STF laser field.",
        "category": "physics_optics"
    },
    {
        "text": "Measuring Thermal Load in Fiber Amplifiers in the Presence of\n  Transversal Mode Instabilities: We report on detailed in-situ distributed temperature measurements inside a\nhigh power fiber amplifier. The deducted thermal load and the TMI-threshold of\na commercial LMA fiber with 25 micron core and 400 micron cladding was measured\nat various seed wavelengths. By matching these results with detailed\nsimulations we show that photodarkening has a negligible impact on the thermal\nload and, therefore, on the TMI threshold in this fiber.",
        "category": "physics_optics"
    },
    {
        "text": "Spin-orbit coupling within tightly focused circularly polarized\n  spatiotemporal vortex wavepacket: Spin-orbital coupling and interaction as intrinsic light fields\ncharacteristics have been extensively studied. Previous studies involve the\nspin angular momentum (SAM) carried by circular polarization and orbital\nangular momentum (OAM) associated with a spiral phase wavefront within the beam\ncross section, where both the SAM and OAM are in parallel with the propagation\ndirection. In this work, we study a new type of spin-orbital coupling between\nthe longitudinal SAM and the transverse OAM carried by a spatiotemporal optical\nvortex (STOV) wavepacket under tight focusing condition. Intricate\nspatiotemporal phase singularity structures are formed when a circularly\npolarized STOV wavepacket is tightly focused by a high numerical aperture\nobjective lens. For the transversely polarized components, phase singularity\norientation can be significantly tilted away from the transverse direction\ntowards the optical axis due to the coupling between longitudinal SAM and\ntransverse OAM. The connection between the amount of rotation and the temporal\nwidth of the wavepacket is revealed. More interestingly, spatiotemporal phase\nsingularity structure with a continuous evolution from longitudinal to\ntransverse orientation through the wavepacket is observed for the\nlongitudinally polarized component. These exotic spin-orbit coupling phenomena\nare expected to render new effects and functions when they are exploited in\nlight matter interactions.",
        "category": "physics_optics"
    },
    {
        "text": "Haptic Manipulation of Microspheres Using Optical Tweezers Under the\n  Guidance of Artificial Force Fields: Using optical tweezers and a haptic device, microspheres having diameters\nranging from 3 to 4 um (floating in a fluid solution) are manipulated in order\nto form patterns of coupled optical microresonators by assembling the spheres\nvia chemical binding. For this purpose, biotin-coated microspheres trapped by a\nlaser beam are steered and chemically attached to an immobilized\nstreptavidin-coated sphere (i.e. anchor sphere) one by one using an XYZ piezo\nscanner controlled by a haptic device. The positions of all spheres in the\nscene are detected using a CCD camera and a collision-free path for each\nmanipulated sphere is generated using the potential field approach. The forces\nacting on the manipulated particle due to the viscosity of the fluid and the\nartificial potential field are scaled and displayed to the user through the\nhaptic device for better guidance and control during steering. In addition, a\nvirtual fixture is implemented such that the desired angle of approach and\nstrength are achieved during the binding phase. Our experimental studies in\nvirtual and real environments with 8 human subjects show that haptic feedback\nsignificantly improves the user performance by reducing the task completion\ntime, the number of undesired collisions during steering, and the positional\nerrors during binding. To our knowledge, this is the first time that a haptic\ndevice is coupled with OT to guide the user during an optical manipulation task\ninvolving steering and assembly of microspheres to construct a coupled\nmicroresonator.",
        "category": "physics_optics"
    },
    {
        "text": "\"Phase Transitions\" in small systems: why standard threshold definitions\n  fail for nanolasers: Since the development of micro- and nanolasers, the question of laser\nthreshold has been subject to debate. Different definitions have been used to\ntry and establish its occurrence, often encountering major obstacles. We\nexamine a set of common physical definitions which we apply to measurements\ntaken in a micro-VCSEL. Their predictions not only clearly disagree, pointing\nto different pump values at which the laser should cross threshold, but they\nalso correspond to autocorrelation values which demonstrate very low field\ncoherence. A topological analysis of the rate equations, with average\nspontaneous emission added to the lasing mode, clearly identifies the\ncontradictions and explains the origin of the discrepancies. Additional\nconsiderations help understanding the failure of the approach and highlight the\npath towards a unique and general definition of threshold in all lasers,\nirrespective of their sizes. A critical scrutiny of the assumptions made in the\nrate equations with spontaneous emission illustrates their strength and\nweaknesses and better defines the bounds within which their predictions hold.\nWe remark in the conclusions how the main results of this paper could hold for\nother small systems.",
        "category": "physics_optics"
    },
    {
        "text": "Mode conversion and orbital angular momentum transfer among multiple\n  modes by helical gratings: The coupling feature of helical gratings (HGs) has been studied on flexible\nconversion between two orbital angular momentum (OAM) modes by both the\ntransverse and longitudinal modulation of HGs. Apart from one to one OAM\nexchange, HGs can achieve OAM transfer among multiple modes, provided both the\nhelix and phase matching conditions are well satisfied for successive coupling.\nIt is contributed from the fold number of modulation fringes of HGs and the\ndifference multiplication of propagation constants between two of the\nsuccessive modes. Based on the coupled mode theory, we investigate the mode\nconversion and OAM transfer among three modes using a 3 fold HGs in a ring core\nfiber, and numerically simulate it by transmission matrix. Our work about the\nmultiple modes transferring can be regarded as an extension of mode coupling\nfor optical gratings, and might flourish the mode conversion, as well as OAM\nmanipulation in optics fields.",
        "category": "physics_optics"
    },
    {
        "text": "Spatio-temporal optical vortices: We present the first experimental evidence, supported by theory and\nsimulation, of spatiotemporal optical vortices (STOVs). Quantized STOVs are a\nfundamental element of the nonlinear collapse and subsequent propagation of\nshort optical pulses in material media. A STOV consists of a ring-shaped null\nin the electromagnetic field about which the phase is spiral, forming a dynamic\ntorus which is concentric with and tracks the propagating pulse. Depending on\nthe sign of the material dispersion, the local electromagnetic energy flow is\nsaddle or spiral about the STOV. STOVs are born and evolve conserving\ntopological charge; they can be simultaneously created in pairs with opposite\nwindings, or generated from a point null. Our results, here obtained for\noptical pulse collapse and filamentation in air, are generalizable to broad\nclass of nonlinearly propagating waves.",
        "category": "physics_optics"
    },
    {
        "text": "High-Efficiency Second Harmonic Generation of Low-Temporal-Coherent\n  Light Pulse: The nonlinear frequency conversion of low-temporal-coherent light holds a\nvariety of applications and has attracted considerable interest. However, its\nphysical mechanism remains relatively unexplored, and the conversion efficiency\nand bandwidth are extremely insufficient. Here, considering the instantaneous\nbroadband characteristic, we establish a model of second harmonic generation\n(SHG) of low-temporal-coherent pulse, and reveal its differences from the\ncoherent conditions. It is found that the second harmonic (SH) of\nlow-temporal-coherent light is produced by not only the degenerate SH processes\nbut also crossed sum-frequency processes. On the basis of this, we propose a\nmethod for realizing low-temporal-coherent SHG with high efficiency and broad\nbandwidth, and experimentally demonstrate a conversion efficiency up to 70%\nwith a bandwidth of 3.1 THz (2.9 nm centered at 528 nm). To the best of our\nknowledge, this is the highest efficiency and broadest bandwidth of\nlow-temporal-coherent SHG, and its efficiency is almost the same with that of\nthe narrowband coherent condition. Furthermore, the spectral evolution\ncharacteristics of the broadband low-temporal-coherent pulse in SHG process are\nrevealed in experiments, that the SH power spectral density (PSD) is\nproportional to the self-convolution of the fundamental wave PSD, which is\ngreatly different from that of the coherent process. Our research opens a door\nfor the study of the low-coherent nonlinear optical processes.",
        "category": "physics_optics"
    },
    {
        "text": "Strong circular dichroism in twisted single-ring hollow-core photonic\n  crystal fiber: We report a series of experimental, analytical and numerical studies\ndemonstrating strong circular dichroism in helically twisted hollow-core\nsingle-ring photonic crystal fiber (SR-PCF), formed by spinning the preform\nduring fiber drawing. In the SR-PCFs studied, the hollow core is surrounded by\na single ring of non-touching capillaries. Coupling between these capillaries\nresults in the formation of helical Bloch modes carrying orbital angular\nmomentum. In the twisted fiber, strong circular birefringence appears in the\nring, so that when a core mode with a certain circular polarization state (say\nLC) phase-matches to the ring, the other (RC) is strongly dephased. If in\naddition the orbital angular momentum is the same in core and ring, and the\npolarization states are non-orthogonal (e.g., slightly elliptical), the LC core\nmode will experience high loss while the RC mode is efficiently transmitted.\nThe result is a single-circular-polarization SR-PCF that acts as a circular\npolarizer over a certain wavelength range. Such fibers have many potential\napplications, for example, for generating circularly polarized light in\ngas-filled SR-PCF and realizing polarizing elements in the deep and vacuum\nultraviolet.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of optical vortices in momentum space: Vortex, the winding of a vector field in two dimensions, has its core the\nfield singularity and its topological charge defined by the quantized winding\nangle of the vector field. Vortices are one of the most fundamental topological\nexcitations in nature, widely known in hair whorls as the winding of hair\nstrings, in fluid dynamics as the winding of velocities, in angular-momentum\nbeams as the winding of phase angle and in superconductors and superfluids as\nthe winding of order parameters. Nevertheless, vortices have hardly been\nobserved other than those in the real space. Although band degeneracies, such\nas Dirac cones, can be viewed as momentum-space vortices in their mathematical\nstructures, there lacks a well-defined physical observable whose winding number\nis an arbitrary signed integer. Here, we experimentally observed momentum-space\nvortices as the winding of far-field polarization vectors in the Brillouin zone\n(BZ) of periodic plasmonic structures. Using a home-made polarization-resolved\nmomentum-space imaging spectroscopy, we completely map out the dispersion,\nlifetime and polarization of all radiative states at the visible wavelengths.\nThe momentum space vortices were experimentally identified by their winding\npatterns in the polarization-resolved iso-frequency contours and their\ndiverging radiative quality factors. Such polarization vortices can exist\nrobustly on any periodic systems of vectorial fields, while they are not\ncaptured by the existing topological band theory developed for scaler fields.\nThis work opens up a promising avenue for exploring topological photonics in\nthe momentum space, studying bound states in continuum (BICs), as well as for\nrendering and steering vector beams and designing high-Q plasmonic resonances.",
        "category": "physics_optics"
    },
    {
        "text": "Covariant theory of light in a dispersive medium: The relativistic theory of the time- and position-dependent energy and\nmomentum densities of light in glasses and other low-loss dispersive media,\nwhere different wavelengths of light propagate at different phase velocities,\nhas remained a largely unsolved challenge until now. This is astonishing in\nview of the excellent theoretical understanding of Maxwell's equations and the\nabundant experimental measurements of optical phenomena in dispersive media.\nThe challenge is related to the complexity of the interference patterns of\npartial waves and to the coupling of the field and medium dynamics by the\noptical force. In this work, we use the mass-polariton theory of light [Phys.\nRev. A 96, 063834 (2017)] to derive the stress-energy-momentum (SEM) tensors of\nthe field and the dispersive medium. Our starting point, the fundamental local\nconservation laws of energy and momentum, is close to that of a recent\ntheoretical work on light in dispersive media by Philbin [Phys. Rev. A 83,\n013823 (2011)], which however, excludes the power-conversion and force density\nsource terms describing the coupling between the field and the medium. In the\ngeneral inertial frame, we present the SEM tensors in terms of Lorentz scalars,\nfour-vectors, and field tensors that reflect in a transparent way the Lorentz\ncovariance of the theory. The SEM tensors of the field and the medium are\nsymmetric, form-invariant, and in full accordance with the covariance principle\nof the special theory of relativity. The SEM tensor of the coupled field-medium\nstate of light also has zero four-divergence. Therefore, light in a dispersive\nmedium has well-defined four-momentum and rest frame. The volume integrals of\nthe total energy and momentum densities of light agree with the model of\nmass-polariton quasiparticles having a nonzero rest mass. The predictions of\nour work are directly accessible to experiments.",
        "category": "physics_optics"
    },
    {
        "text": "Complete multipolar description of reflection and transmission across a\n  metasurface for perfect absorption of light: Relating the electromagnetic scattering and absorption properties of an\nindividual particle to the reflection and transmission coefficients of a\ntwo-dimensional material composed of these particles is a crucial concept that\nhas driven both fundamental and applied physics. It is at the heart of both the\ncharacterization of material properties as well as the phase and amplitude\nengineering of a wave. Here we propose a multipolar description of the\nreflection and transmission coefficients across a monolayer of particles using\na vector spherical harmonic decomposition. This enables us to provide a\ngeneralized condition for perfect absorption which occurs when both the\nso-called \\textit{generalized Kerker condition} is reached and when the\nsuperposition of odd and even multipoles reaches a critical value. Using these\nconditions, we are able to propose two very different designs of\ntwo-dimensional materials that perfectly absorb a plane electromagnetic wave\nunder normal incidence. One is an infinite array of silica microspheres that\noperates at mid-infrared frequencies, while the other is an infinite array of\ngermanium nano-clusters that operates at visible frequencies. Both systems\noperate in a deeply multipolar regime. Our findings are important to the\nmetamaterials and metasurfaces communities who design materials mainly\nrestricted to the dipolar behavior of individual resonators, as well as the\nself-assembly and nanochemistry communities who separate the individual\nparticle synthesis from the materials assembly.",
        "category": "physics_optics"
    },
    {
        "text": "Investigation of the Kerr-lens Mode Locking Ability of Cr:ZnSe\n  Solid-State Laser: The theoretical calculation for nonlinear refractive index in Cr: ZnSe -\nactive medium predicts the strong defocusing cascaded second-order nonlinearity\nwithin 2000 - 3000 nm spectral range. On the basis of this result the optimal\ncavity configuration for Kerr-lens mode locking is proposed that allows to\nachieve a sub-100 fs pulse duration. The numerical simulations testify about\nstrong destabilizing processes in the laser resulting from a strong self-phase\nmodulation. The stabilization of the ultrashort pulse generation is possible\ndue to spectral filtering that increases the pulse duration up to 300 fs.",
        "category": "physics_optics"
    },
    {
        "text": "Broadband field-enhancement in epsilon-near-zero photonic gap antennas: In recent years, the large electric field enhancement and tight spatial\nconfinement supported by the so-called epsilon-near-zero (ENZ) mode have\nattracted significant attention for the realization of efficient nonlinear\noptical devices. Here, we experimentally demonstrate a new type of antenna,\ntermed an ENZ photonic gap antenna (PGA), which consists of a dielectric pillar\nwithin which a thin slab of indium tin oxide (ITO) material is embedded. In ENZ\nPGAs, hybrid dielectric-ENZ modes emerge from strong coupling between the\ndielectric antenna modes and the ENZ bulk plasmon resonance. These hybrid modes\nefficiently couple to free space and allow for large enhancements of the\nincident electric field over nearly an octave bandwidth, without the stringent\nlateral nanofabrication requirements required by conventional plasmonic or\ndielectric nanoantennas. The linear response of single ENZ PGAs is probed using\ndark field scattering and interpreted using a simple coupled oscillator\nframework. Third harmonic generation is used to probe the field enhancement and\nlarge enhancements are observed in the THG efficiency over a broad spectral\nrange. This proof of concept demonstrates the potential of ENZ PGAs, which we\nhave previously shown can support electric field enhancements of up to\n100--200X, which is comparable with those of the best plasmonic antennas.",
        "category": "physics_optics"
    },
    {
        "text": "Homogeneous optical cloak constructed with uniform layered structures: The prospect of rendering objects invisible has intrigued researchers for\ncenturies. Transformation optics based invisibility cloak design is now\nbringing this goal from science fictions to reality and has already been\ndemonstrated experimentally in microwave and optical frequencies. However, the\nmajority of the invisibility cloaks reported so far have a spatially varying\nrefractive index which requires complicated design processes. Besides, the size\nof the hidden object is usually small relative to that of the cloak device.\nHere we report the experimental realization of a homogenous invisibility cloak\nwith a uniform silicon grating structure. The design strategy eliminates the\nneed for spatial variation of the material index, and in terms of size it\nallows for a very large obstacle/cloak ratio. A broadband invisibility behavior\nhas been verified at near-infrared frequencies, opening up new oppotunities for\nusing uniform layered medium to realize invisibility at any frequency ranges,\nwhere high-quality dielectrics are available.",
        "category": "physics_optics"
    },
    {
        "text": "Adaptive optics for multifocal plane microscopy: Multifocal plane microscopy (MUM) allows three dimensional objects to be\nimaged in a single camera frame. Our approach uses dual orthogonal diffraction\nphase gratings with a quadratic distortion of the lines to apply defocus to the\nfirst diffraction orders which, when paired with a relay lens, allows for 9\nfocal planes to be imaged on a single camera chip. This approach requires a\nstrong signal level to ensure sufficient intensity in the diffracted light, but\nhas the advantage of being compact and straightforward to implement. As the\nmicroscope begins to focus deeper into the sample, aberrations caused by\nrefractive index mismatch and inhomogeneity in the sample's media have an\nadverse effect on the signal's quality. In this paper, we investigate the image\nquality improvement brought by applying adaptive optics (AO) to multifocal\nplane microscopy. A single correction device (an 8x8 deformable mirror (DM)) is\ncombined with an image-based AO control strategy to perform the correction of\noptical aberrations. We compare full end-to-end modelling results using an\nestablished numerical modelling system adapted for microscopy to laboratory\nresults both on a test sample and on a number of biological samples. Finally,\nwe will demonstrate that combining AO and MUM, we are able to improve the image\nquality of biological samples and provide a good correction throughout the\nvolume of the biological sample.",
        "category": "physics_optics"
    },
    {
        "text": "Hybrid Epsilon-Near-Zero Modes of Photonic Gap Antennas: We demonstrate that in photonic gap antennas composed of an epsilon-near-zero\n(ENZ) layer embedded within a high-index dielectric, hybrid modes emerge from\nthe strong coupling between the ENZ thin film and the photonic modes of the\ndielectric antenna. These hybrid modes show giant electric field enhancements,\nlarge enhancements of the far-field spontaneous emission rate and a\nunidirectional radiation response. We analyze both parent and hybrid modes\nusing quasinormal mode theory and find that the hybridization can be well\nunderstood using a coupled oscillator model. Under plane wave illumination,\nhybrid ENZ antennas can concentrate light with an electric field amplitude\n$\\sim$100 times higher than that of the incident wave, which places them on par\nwith the best plasmonic antennas. In addition, the far-field spontaneous\nemission rate of a dipole embedded at the antenna hotspot reaches up to\n$\\sim$2300 that in free space, with nearly perfect unidirectional emission.",
        "category": "physics_optics"
    },
    {
        "text": "A bird's-eye view of nonlinear-optical processes: unification through\n  scale invariance: The Schr\\\"{o}dinger equation has the property that when changing the length\nscale by $\\vec{r} \\to \\epsilon \\vec{r}$ and the energy scale by $E \\to E /\n\\epsilon^2$, the shape of the wavefunction remains unchanged. The same\nre-scaling leaves the intrinsic hyperpolarizability (as well as higher-order\nhyperpolarizabilities) unchanged. As such, the intrinsic hyperpolarizability is\nthe best quantity for comparing molecules since it re-normalizes for trivial\ndifferences that are due to molecular size and energy gap. Similarly, the\nintrinsic hyperpolarizability is invariant to changes in the number of\nelectrons. In this paper, we review the concept of scale invariance and how it\ncan be applied to better understand the nonlinear optical response, which can\nbe used to develop new paradigms for it's optimization.",
        "category": "physics_optics"
    },
    {
        "text": "Overcoming the acoustic diffraction limit in photoacoustic imaging by\n  localization of flowing absorbers: The resolution of photoacoustic imaging deep inside scattering media is\nlimited by the acoustic diffraction limit. In this work, taking inspiration\nfrom super-resolution imaging techniques developed to beat the optical\ndiffraction limit, we demonstrate that the localization of individual optical\nabsorbers can provide super-resolution photoacoustic imaging well beyond the\nacoustic diffraction limit. As a proof-of-principle experiment, photoacoustic\ncross-sectional images of microfluidic channels were obtained with a 15 MHz\nlinear CMUT array while absorbing beads were flown through the channels. The\nlocalization of individual absorbers allowed to obtain super-resolved\ncross-sectional image of the channels, by reconstructing both the channel width\nand position with an accuracy better than $\\lambda/10$. Given the discrete\nnature of endogenous absorbers such as red blood cells, or that of exogenous\nparticular contrast agents, localization is a promising approach to push the\ncurrent resolution limits of photoacoustic imaging.",
        "category": "physics_optics"
    },
    {
        "text": "Lossless Suppression and Enhancement of Soliton Self-Frequency Shifts: Soliton self-frequency shifts (SSFS) can be suppressed in optical fibers\nthrough spectral recoil, but this process leads to losses through continuous\ntransfer of energy to a dispersive wave. We demonstrate a novel way to alter\nthe strength of SSFS in photonic crystal fibers through a frequency-dependent\nnonlinear parameter $\\gamma(\\omega)$. Our numerical simulations show both\nsuppression and enhancement of SSFS depending on the sign of nonlinear slope. A\nlarge enough positive value of this slope can lead to total suppression of\nSSFS, without spectral recoil and without energy transfer to a resonant\ndispersive wave. Numerical simulations are supported by mathematical\npredictions based on the moment method.",
        "category": "physics_optics"
    },
    {
        "text": "Noise analysis in outdoor dynamic speckle measurement: Dynamic speckle method is an effective tool for estimation of speed of\nprocesses. Speed distribution is encoded in a map built by statistical\npointwise processing of time-correlated speckle patterns. For industrial\ninspection,the outdoor noisy measurement is required. The paper analyzes\nefficiency of the dynamic speckle method in the presence of environmental noise\nas phase fluctuations due to lack of vibration isolation and shot noise due to\nambient light. Usage of normalized estimates for the case of non-uniform laser\nillumination is studied. Feasibility of the outdoor measurement has been proven\nby numerical simulationsof noisy image capture and real experiments with test\nobjects. Good agreement has been demonstrated both in simulation and experiment\nbetween the ground truth map and the maps extracted from noisy data.",
        "category": "physics_optics"
    },
    {
        "text": "Hysteresis assisted narrowband resonances in a chain of nonlinear\n  plasmonic arrays: The plasmonic structures exhibiting narrowband resonances (NBR) are of a\ngreat interest for various applications. We propose to use hysteresis behavior\nin a 1D system of nonlinear nanoresonators in order to achieve the NRB; the\nnonlinearity is provided by saturation of a two-level quantum system coupled\nwith the nanoresonators (nanolaser/spaser configuration). Quantum Dots (QD)\nwere assumed as quantum systems; their numerical parameters have been adopted\nfor estimations. Role of the loss compensation on the quality of the NBR is\nshown for below (under compensation) and above threshold (generating spasers)\noperation modes. Amplitude and phase detection schemes of the prospective\nexperimental realization are compared using the developed model. Possible\nsensor oriented applications of the proposed system are discussed.",
        "category": "physics_optics"
    },
    {
        "text": "Accurate and Efficient Modeling of the Transverse Mode Instability in\n  High Energy Laser Amplifiers: We study the transverse mode instability (TMI) in the limit where a single\nhigher-order mode (HOM) is present. We demonstrate that when the beat length\nbetween the fundamental mode and the HOM is small compared to the length scales\non which the pump amplitude and the optical mode amplitudes vary, TMI is a\nthree-wave mixing process in which the two optical modes beat with the\nphase-matched component of the index of refraction that is induced by the\nthermal grating. This limit is the usual limit in applications, and in this\nlimit TMI is identified as a stimulated thermal Rayleigh scattering (STRS)\nprocess. We demonstrate that a phase-matched model that is based on the\nthree-wave mixing equations can have a large computational advantage over\ncurrent coupled mode methods that must use longitudinal step sizes that are\nsmall compared to the beat length.",
        "category": "physics_optics"
    },
    {
        "text": "Spectral characterization of a frequency comb based on cascaded\n  quadratic nonlinearities inside an optical parametric oscillator: We present an experimental study of optical frequency comb generation based\non cascaded quadratic nonlinearities inside a continuous-wave-pumped optical\nparametric oscillator. We demonstrate comb states which produce\nnarrow-linewidth intermode beat note signals, and we verify the mode spacing\nuniformity of the comb at the Hz level. We also show that spectral quality of\nthe comb can be improved by modulating the parametric gain at a frequency that\ncorresponds to the comb mode spacing. We have reached a high average output\npower of over 4 W in the near-infrared region, at ~2 {\\mu}m.",
        "category": "physics_optics"
    },
    {
        "text": "Two-photon absorption in potassium niobate: We report measurements of thermal self-locking of a Fabry-Perot cavity\ncontaining a potassium niobate (KNbO3) crystal. We develop a method to\ndetermine linear and nonlinear optical absorption coefficients in intracavity\ncrystals by detailed analysis of the transmission lineshapes. These lineshapes\nare typical of optical bistability in thermally loaded cavities. For our\ncrystal, we determine the one-photon absorption coefficient at 846 nm to be\n(0.0034 \\pm 0.0022) per m and the two-photon absorption coefficient at 846 nm\nto be (3.2 \\pm 0.5) \\times 10^{-11} m/W and the one-photon absorption\ncoefficient at 423 nm to be (13 \\pm 2) per m. We also address the issue of\nblue-light-induced-infrared-absorption (BLIIRA), and determine a coefficient\nfor this excited state absorption process. Our method is particularly well\nsuited to bulk absorption measurements where absorption is small compared to\nscattering. We also report new measurements of the temperature dependence of\nthe index of refraction at 846 nm, and compare to values in the literature.",
        "category": "physics_optics"
    },
    {
        "text": "The transition from the ballistic to the diffusive regime in a turbid\n  medium: By varying the absorption coefficient and width of an intralipid- India ink\nsolution in a quasi one-dimensional experiment, the transition between the\nballistic and the diffusive regimes is investigated. The medium's attenuation\ncoefficient changes abruptly between two different values within a single\nmean-free-path. This problem is analyzed both experimentally and theoretically,\nand it is demonstrated that the transition location depends on the scattering\ncoefficient as well as on the measuring solid angle.",
        "category": "physics_optics"
    },
    {
        "text": "Polariton properties in bigyrotropic medium: The spectra of polaritons in bigyrotropic medium are received. The ion and\nelectron polarizations of medium under influence of high-frequency\nelectromagnetic and external magnetostatic field are considered. It is shown\nthat by varying of the external magnetostatic field one can control polariton\nspectrum and velocity of polaritons.",
        "category": "physics_optics"
    },
    {
        "text": "Dispersion management of a nonlinear amplifying loop mirror-based\n  Erbium-doped fiber laser: We report an investigation of dispersion management of an all-polarization\nmaintaining Er-fiber oscillator mode-locked via nonlinear amplification loop\nmirror in a figure-nine cavity configuration with two output ports. The\nperformance of the laser was investigated within the net cavity dispersion\nranging from -0.034 ps2 to +0.006 ps2. We show that the spectral and temporal\nphase of the pulses at both figure-nine outputs have clearly different\ncharacteristics. One of the laser outputs provides pulses with significantly\nbetter quality; nonetheless, the rejection output also offers ultrashort pulses\nwith broad spectra. Pulses as short as 79 fs with an energy of 83 pJ were\ngenerated directly from the laser in the near-zero dispersion regime.",
        "category": "physics_optics"
    },
    {
        "text": "Bridging the Mid-Infrared-to-Telecom Gap with Silicon Nanophotonic\n  Spectral Translation: Expanding far beyond traditional applications in optical interconnects at\ntelecommunications wavelengths, the silicon nanophotonic integrated circuit\nplatform has recently proven its merits for working with mid-infrared (mid-IR)\noptical signals in the 2-8 {\\mu}m range. Mid-IR integrated optical systems are\ncapable of addressing applications including industrial process and\nenvironmental monitoring, threat detection, medical diagnostics, and free-space\ncommunication. Rapid progress has led to the demonstration of various silicon\ncomponents designed for the on-chip processing of mid-IR signals, including\nwaveguides, vertical grating couplers, microcavities, and electrooptic\nmodulators. Even so, a notable obstacle to the continued advancement of\nchip-scale systems is imposed by the narrow-bandgap semiconductors, such as\nInSb and HgCdTe, traditionally used to convert mid-IR photons to electrical\ncurrents. The cryogenic or multi-stage thermo-electric cooling required to\nsuppress dark current noise, exponentially dependent upon the ratio Eg/kT, can\nlimit the development of small, low-power, and low-cost integrated optical\nsystems for the mid-IR. However, if the mid-IR optical signal could be\nspectrally translated to shorter wavelengths, for example within the\nnear-infrared telecom band, photodetectors using wider bandgap semiconductors\nsuch as InGaAs or Ge could be used to eliminate prohibitive cooling\nrequirements. Moreover, telecom band detectors typically perform with higher\ndetectivity and faster response times when compared with their mid-IR\ncounterparts. Here we address these challenges with a silicon-integrated\napproach to spectral translation, by employing efficient four-wave mixing (FWM)\nand large optical parametric gain in silicon nanophotonic wires.",
        "category": "physics_optics"
    },
    {
        "text": "Frequency-Modulation Mode-Locked Laser with GHz Spectral Width Tunable\n  in the 2-3 um Region: A narrow-bandwidth actively mode-locked laser using a Cr:ZnS gain medium has\nbeen successfully demonstrated. A free-space electro-optic phase modulator is\nemployed in the solid-state laser resonator to achieve frequency-modulation\n(FM) mode-locking, which achieves a narrow spectral width of ~1 GHz and a pulse\nduration of ~500 ps over a wide tuning range of 1947-2445 nm. The operation\nfrequency of the modulator determines the repetition rate of the mode-locked\npulse train and can stabilize it to millihertz-level without any additional\nfeedback loop systems. We also study the theoretical expression of pulse\nduration and spectral width in a FM mode-locking in a laser cavity that\ncontains considerable group-delay dispersion. The results indicates that larger\nintracavity dispersion can only stabilize the laser operation by avoiding mode\nswitching, but also narrow the spectral width and increase the pulse duration.\nThe proposed laser features a narrow spectral width at a desired mid-infrared\nwavelength and a comb-like spectral structure with stabilized longitudinal mode\nspacing, providing a powerful tool for sensing and control of molecules.",
        "category": "physics_optics"
    },
    {
        "text": "Experimental observations of bright dissipative Kerr cavity solitons and\n  their collapsed snaking in a driven resonator with normal dispersion: Driven Kerr nonlinear optical resonators can sustain localized structures\nknown as dissipative Kerr cavity solitons, which have recently attracted\nsignificant attention as the temporal counterparts of microresonator optical\nfrequency combs. Whilst conventional wisdom asserts that bright cavity solitons\ncan only exist in the region of anomalous dispersion, recent theoretical\nstudies have predicted that higher-order dispersion can fundamentally alter the\nsituation, enabling bright localized structures even under conditions of normal\ndispersion driving. Here we demonstrate a flexible optical fibre ring resonator\nplatform that offers unprecedented control over dispersion conditions, and we\nreport on the first experimental observations of bright localized structures\nthat are fundamentally enabled by higher-order dispersion. In broad agreement\nwith past theoretical predictions, we find that several distinct bright\nstructures can co-exist for the same parameters, and we observe experimental\nevidence of their collapsed snaking bifurcation structure. In addition to\nenabling direct experimental verifications of a number of theoretical\npredictions, we show that the ability to judiciously control the dispersion\nconditions offers a novel route for ultrashort pulse generation: the bright\nstructures observed in our work correspond to pulses of light as short as\n220~fs -- the record for a passive all-fibre ring resonator. We envisage that\nour work will stimulate further fundamental studies on the impact of\nhigher-order dispersion on Kerr cavity dynamics, as well as guide the\ndevelopment of novel ultrashort pulse sources and dispersion-engineered\nmicroresonator frequency combs.",
        "category": "physics_optics"
    },
    {
        "text": "Boosted second-harmonic generation in the LiNbO$\\mathrm{_3}$ metasurface\n  governed by high-Q guided resonances and bound states in the continuum: To date, second-harmonic generation (SHG) at nanoscale has been concentrated\non employing high-refractive-index nanostructures, owing to the strong field\nconfinement at deep subwavelength scales based on optically resonant effects.\nHowever, low-index nanostructures generally exhibit weaker resonant effects and\nlower field confinement. To address this issue, by harnessing the large\nnonlinearity of LiNbO$\\mathrm{_3}$, we propose a novel approach to employ\nguided resonances and bound states in the continuum (BICs) with a\nLiNbO$\\mathrm{_3}$ metasurface consisting of a LiNbO$\\mathrm{_3}$ disk array\nsitting on a LiNbO3 thin film. Such a system can transform the guided modes\nsupported by LiNbO$\\mathrm{_3}$ thin film into high-quality guided resonances\nwhich can be excited directly under plane-wave illumination. Importantly, we\nfurther demonstrate strong field confinement inside LiNbO3 thin film with\ntailorable Q-factor by realising a Friedrich-Wintgen BIC. Such a unique mode\nengineering enables a record-high SHG efficiency of 5\\% under a pump intensity\nas low as 0.4 $\\mathrm{MW/cm^{2}}$. Moreover, we reveal the influence of\nnonlinear resonances and cross-coupling on the SHG by showing the anomalous SHG\nand efficiency tuning with the rotation of the crystal axis. Our work offers a\nnew route to constructing enhanced SHG based on high-Q guided resonances and\nBICs, including low-index and high-index nonlinear materials.",
        "category": "physics_optics"
    },
    {
        "text": "Background-free 3D nanometric localisation and sub-nm asymmetry\n  detection of single plasmonic nanoparticles by four-wave mixing\n  interferometry with optical vortices: Single nanoparticle tracking using optical microscopy is a powerful technique\nwith many applications in biology, chemistry and material sciences. Despite\nsignificant advances, localising objects with nanometric position accuracy in a\nscattering environment remains challenging. Applied methods to achieve contrast\nare dominantly fluorescence based, with fundamental limits in the emitted\nphoton fluxes arising from the excited-state lifetime as well as\nphotobleaching. Furthermore, every localisation method reported to date\nrequires signal acquisition from multiple spatial points, with consequent speed\nlimitations. Here, we show a new four-wave mixing interferometry technique,\nwhereby the position of a single non-fluorescing gold nanoparticle is\ndetermined with better than 20 nm accuracy in plane and 1 nm axially from rapid\nsingle-point acquisition measurements by exploiting optical vortices. The\ntechnique is also uniquely sensitive to particle asymmetries of only 0.5%\naspect ratio, corresponding to a single atomic layer of gold, as well as\nparticle orientation, and the detection is background-free even inside\nbiological cells. This method opens new ways of of unraveling single-particle\ntrafficking within complex 3D architectures.",
        "category": "physics_optics"
    },
    {
        "text": "All-Digital Stokes Polarimetry with a Digital Micro-mirror Device: Stokes polarimetry is widely used to extract the polarisation structure of\noptical fields, typically from six measurements, although it can be extracted\nfrom only four. To measure the required intensities, most approaches are based\non optical polarisation components. In this work, we present an all-digital\napproach that enables a rapid measure of all four intensities without any\nmoving components. Our method employs a Polarisation Grating (PG) to\nsimultaneously project the incoming mode into left- and right-circular\npolarised states, followed by a polarisation-insensitive Digital Micromirror\nDevice (DMD), which digitally introduces a phase retardance for the acquisition\nof the remaining two polarisation states. We demonstrate how this technique can\nbe applied to measuring the SoP, vectorness and intra-modal phase of optical\nfields, without any moving components and shows excellent agreement with\ntheory, illustrating fast, real-time polarimetry.",
        "category": "physics_optics"
    },
    {
        "text": "Metamaterials and the mathematical Science of invisibility: In this chapter, we review some recent developments in the field of\nphotonics: cloaking, whereby an object becomes invisible to an observer, and\nmirages, whereby an object looks like another one (say, of a different shape).\nSuch optical illusions are made possible thanks to the advent of metamaterials,\nwhich are new kinds of composites designed using the concept of\ntransformational optics. Theoretical concepts introduced here are illustrated\nby finite element computations.",
        "category": "physics_optics"
    },
    {
        "text": "Four-photon interference: a realizable experiment to demonstrate\n  violation of EPR postulates for perfect correlations: Bell's theorem reveals contradictions between the predictions of quantum\nmechanics and the EPR postulates for a pair of particles only in situations\ninvolving imperfect statistical correlations. However, with three or more\nparticles, contradictions emerge even for perfect correlations. We describe an\nexperiment which can be realized in the laboratory, using four-photon entangled\nstates generated by parametric down-conversion, to demonstrate this\ncontradiction at the level of perfect correlations.",
        "category": "physics_optics"
    },
    {
        "text": "Developing Microwave Photonic Temperature Sensors: In recent years there has been considerable interest in exploiting the\ntemperature dependence of sapphire whispering gallery mode frequency to develop\na mechanically stable, high accuracy temperature sensor. Disk-resonator-based\ndevices have been demonstrated to measure temperature with .01 K or better\naccuracy in the temperature range of 273 K to 373 K. Here we have utilized\nautomated data acquisition and processing to rapidly evaluate a\nmechanically-stabilized sapphire whispering gallery mode resonator based on a\nhollow cylinder configuration.",
        "category": "physics_optics"
    },
    {
        "text": "Soliton electro-optic effects in paraelectrics: The combination of charge separation induced by the formation of a single\nphotorefractive screening soliton and an applied external bias field in a\nparaelectric is shown to lead to a family of useful electro-optic guiding\npatterns and properties.",
        "category": "physics_optics"
    },
    {
        "text": "Attosecond transient absorption of a bound wave packet coupled to a\n  smooth continuum: We investigate the possibility to use transient absorption of a coherent\nbound electron wave packet in hydrogen as an attosecond pulse characterization\ntechnique. In recent work we have shown that photoionization of such a coherent\nbound electron wave packet opens up for pulse characterization with\nunprecedented temporal accuracy --- independent of the atomic structure ---\nwith maximal photoemission at all kinetic energies given a wave packet with\nzero relative phase [Pabst and Dahlstr\\\"om, Phys. Rev. A, 94, 13411 (2016)].\nHere, we perform numerical propagation of the time-dependent Schr\\\"odinger\nequation and analytical calculations based on perturbation theory to show that\nthe energy-resolved maximal absorption of photons from the attosecond pulse\ndoes not uniquely occur at zero relative phase of the initial wave packet.\nInstead, maximal absorption occurs at different relative wave packet phases,\ndistributed as a non-monotonous function with a smooth $-\\pi/2$ shift across\nthe central photon energy (given a Fourier-limited Gaussian pulse). Similar\nresults are found also in helium. Our finding is surprising because it implies\nthat the energy-resolved photoelectrons are not mapped one-to-one with the\nenergy-resolved absorbed photons of the attosecond pulse.",
        "category": "physics_optics"
    },
    {
        "text": "Ultrafast tilting of the dispersion of a photonic crystal and adiabatic\n  spectral compression of light pulses: We demonstrate, by theory and experiment, the ultrafast tilting of the\ndispersion curve of a photonic-crystal waveguide following the absorption of a\nfemtosecond pump pulse. By shaping the pump-beam cross section with a\nnanometric shadow mask, different waveguide eigenmodes acquire different\nspatial overlap with the perturbing pump, leading to a local flattening of the\ndispersion by up to 11 %. We find that such partial mode perturbation can be\nused to adiabatically compress the spectrum of a light pulse traveling through\nthe waveguide.",
        "category": "physics_optics"
    },
    {
        "text": "Dynamics of femtosecond synthesized coronary profile laser beams\n  filamentation in air: Multiple filamentation in air of high-power ultrashort laser radiation with\ntransverse intensity profile resembling a \"corona\" composed by incoherent\ncombining of several annularly distributed independent top-hat sub-beams is\ntheoretically studied. Through the numerical solution of time-averaged\nnonlinear Schrodinger equation, we study the spatio-angular dynamics of\nsynthesized near-infrared \"corona-beam\" (CB) along the optical path by varying\nthe number and power of the beamlets (corona-spikes). For the first time to our\nknowledge, the evident advances in the multiple filamentation region\nmanipulating of synthesized CB are demonstrated. Particularly, by adjusting the\nnumber and aperture of the constituting sub-beams it makes possible to\nsignificantly delay the CB filamentation onset distance and increase the\nfilamentation length in air. In addition, at the post-filamentation stage of\nfemtosecond pulse propagation under certain conditions the synthesized\ncorona-beams exhibit significantly lower angular divergence of its most intense\npart (post-filamentation light channel) compared to the beams with regular\nuni-modal intensity profiles (Gaussian, plateau-like) that provides enhancing\nof laser power delivered to the receiver over the atmospheric links.",
        "category": "physics_optics"
    },
    {
        "text": "A chip-scale second-harmonic source via injection-locked all-optical\n  poling: Second-harmonic generation allows for coherently bridging distant regions of\nthe optical spectrum, with applications ranging from laser technology to\nself-referencing of frequency combs. However, accessing the nonlinear response\nof a medium typically requires high-power bulk sources, specific nonlinear\ncrystals, and complex optical setups, hindering the path toward large-scale\nintegration. Here we address all of these issues by engineering a chip-scale\nsecond-harmonic (SH) source based on the frequency doubling of a semiconductor\nlaser self-injection-locked to a silicon nitride microresonator. The\ninjection-locking mechanism, combined with a high-Q microresonator, results in\nan ultra-narrow intrinsic linewidth at the fundamental harmonic frequency as\nsmall as 57 Hz. Owing to the extreme resonant field enhancement,\nquasi-phase-matched second-order nonlinearity is photoinduced through the\ncoherent photogalvanic effect and the high coherence is mapped on the generated\nSH field. We show how such optical poling technique can be engineered to\nprovide efficient SH generation across the whole C and L telecom bands, in a\nreconfigurable fashion, overcoming the need for poling electrodes. Our device\noperates with milliwatt-level pumping and outputs SH power exceeding 2 mW, for\nan efficiency as high as 280%/W under electrical driving. Our findings suggest\nthat standalone, highly-coherent, and efficient SH sources can be integrated in\ncurrent silicon nitride photonics, unlocking the potential of $\\chi^{(2)}$\nprocesses in the next generation of integrated photonic devices.",
        "category": "physics_optics"
    },
    {
        "text": "Monolithic Ultrahigh-Q Lithium Niobate Microring Resonator: We demonstrate an ultralow loss monolithic integrated lithium niobate\nphotonic platform consisting of dry-etched subwavelength waveguides. We show\nmicroring resonators with a quality factor of 10$^7$ and waveguides with\npropagation loss as low as 2.7 dB/m.",
        "category": "physics_optics"
    },
    {
        "text": "The 3D Lima\u00e7on: Properties and Applications: We perform electromagnetic wave simulations of fully three-dimensional\noptical Lima\\c{c}on-microcavities, one basis for their future applications in\nmicrolasers and photonic devices. The analysis of the three-dimensional modes\nand far-fields reveals an increase of the quality factors as compared to the\ntwo-dimensional case. The structure of the far-field in the third dimension\nshows pronounced maxima in the emission directionality inclined to the\nresonator plane which may be exploited for coupling the resonator modes to the\nenvironment. This triggers ideas for technical applications, like the suggested\nsensor that can detect small changes in the environment based on changes in the\nemission profile.",
        "category": "physics_optics"
    },
    {
        "text": "Spatiotemporal evolutions of similariton pulses in multimode fibers with\n  Raman amplification: This letter is to pave the way towards the demonstration of spatiotemporal\nsimilariton pulses evolution in passive multimode fibers with Raman\namplification. We present numerically these issues in a graded-index and\nstep-index multimode fibers and provide a first look at the complex\nspatiotemporal dynamics of similariton pulses. The results showed that the\nsimilariton pulses can be generated in both multimode fibers. The temporal and\nthe spectral evolution of the pulses can be characterized as parabolic shapes\nwith linear chirp and kW peak power. By compressed these, high energy\nfemtoseconds pulses can be obtained starting initial picoseconds pulses.\nSpatial beam profile could be preserved in both multimode fibers under the\npredominantly excitation of the fundamental mode. Specifically, Raman\namplification and similariton pulses generation in graded-index multimode fiber\nimproves the spatial beam cleaning process under the different initial\ncondition.",
        "category": "physics_optics"
    },
    {
        "text": "Broadband Optical Serrodyne Frequency Shifting: We demonstrate serrodyne frequency shifting of light from 200 MHz to 1.2 GHz\nwith an efficiency of better than 60 percent. The frequency shift is imparted\nby an electro-optic phase modulator driven by a high-frequency, high-fidelity\nsawtooth waveform that is passively generated by a commercially available\nNon-Linear Transmission Line (NLTL). We also implement a push-pull\nconfiguration using two serrodyne-driven phase modulators allowing for\ncontinuous tuning between -1.6 GHz and +1.6 GHz. Compared to competing\ntechnologies, this technique is simple and robust, and offers the largest\navailable tuning range in this frequency band.",
        "category": "physics_optics"
    },
    {
        "text": "Nonlocal Optical Response of Particle Plasmons in Single Gold Nanorods: Particle plasmons in metal nanoparticles have primarily been investigated\nthrough the use of local optical response approximations. However, as\nnanoparticle size approaches the average distance of electrons to the metal\nsurface, mesoscopic effects such as size-dependent plasmon linewidth broadening\nand resonance energy blue shifts are expected to become observable. In this\nwork, we compared the experimental spectral characteristics with simulated\nvalues obtained using a generalized nonlocal optical response theory-based\nlocal analogue model. Our results show that the nonlocal plasmon damping\neffects in single nanoparticles are less significant compared to those observed\nin plasmon-coupled systems. Moreover, our study demonstrates that\nsingle-particle dark-field spectroscopy is an effective tool for investigating\nthe nonlocal optical response of particle plasmons in single nanoparticles.\nThese results have important implications for the rational design of novel\nnanophotonic devices.",
        "category": "physics_optics"
    },
    {
        "text": "Intensity correlation speckles as a technique for removing Doppler\n  broadening: A method involving intensity correlation measurements is described, which\nallows for the complete removal of Doppler broadening in the emission of\nelectromagnetic radiation from far-away sources that are inaccessible to\nconventional Doppler-free measurements. The technique, relying on a correction\nto g(2) of order N-1, probes the separation between neighboring spectral lines\nand is also applicable to the elimination of broadening due to collisions (N is\nthe number of emitting particles and g(2) is the second-order field correlation\nfunction). Possible applications include a determination of cosmological\nparameters from red shifts of gravitationally-lensed quasars.",
        "category": "physics_optics"
    },
    {
        "text": "Strong bulk photovoltaic effect and second-harmonic generation in\n  two-dimensional selenium and tellurium: Few-layer selenium and tellurium films have been recently prepared, and they\nprovide a new platform to explore novel properties of two-dimensional (2D)\nelemental materials. In this work, we have performed a systematic\nfirst-principles study on the electronic, linear, and nonlinear optical (NLO)\nproperties of atomically thin selenium and tellurium films within the\ndensity-functional theory with the generalized gradient approximation plus\nscissors correction using the band gaps from the relativistic hybrid\nHeyd-Scuseria-Ernzerhof functional calculations. Interestingly, we find that\nfew-layer Se and Te possess large second-harmonic generation (SHG), linear\nelectro-optic effect, and bulk photovoltaic effect. In particular, trilayer\n(TL) Te possesses large SHG coefficient, being more than 65 times larger than\nthat of GaN, a widely used NLO material. Bilayer (BL) Te has huge static SHG\ncoefficient, being more than 100 times larger than that of GaN. Furthermore,\nmonolayer (ML) Se possesses large SHG coefficient. Moreover, we predict that TL\nTe exhibits strong bulk photovoltaic effect (BPVE), being greater than that of\nGeS, a polar system with the largest BPVE found so far. Although the shift\ncurrent conductivities of bulk and 2D Se are comparable, the shift current\nconductivities of TL Te are five times larger than those of bulk Te. Finally,\nan analysis of the calculated electronic band structures indicates that the\nstrong NLO responses of 2D Se and Te materials are primarily derived from their\nlow-dimensional structures with high anisotropy, directional covalent bonding,\nlone-pair electrons, and relatively small band gaps. These findings provide a\npractical strategy to search for excellent NLO and BPVE materials.",
        "category": "physics_optics"
    },
    {
        "text": "Mode-locked Yb-doped fiber laser emitting broadband pulses at ultra-low\n  repetition rates: We report on an environmentally stable, Yb-doped, all-normal dispersion,\nmode-locked fibre laser that is capable of creating broadband pulses with\nultra-low repetition rates. Specifically, through careful positioning of fibre\nsections in an all-PM-fibre cavity mode-locked with a nonlinear amplifying loop\nmirror, we achieve stable pulse trains with repetition rates as low as 506 kHz.\nThe pulses have several nanojules of energy and are compressible down to\nultrashort (< 500 fs) durations.",
        "category": "physics_optics"
    },
    {
        "text": "Calculated threshold of supratransmission phenomena in waveguide arrays\n  with saturable nonlinearity: In this work, we consider a semi-infinite discrete nonlinear Schr\\\"odinger\nequation with saturable nonlinearity driven at one edge by a driving force. The\nequation models the dynamics of coupled photorefractive waveguide arrays. It\nhas been reported that when the frequency of the driving force is in the\nforbidden band, energy can be transmitted along the lattices provided that the\ndriving amplitude is above a critical value. This nonlinear tunneling is called\nsupratransmission. Here, we explain the source of supratransmission using\ngeometric illustrations. Approximations to the critical amplitude for\nsupratransmission are presented as well.",
        "category": "physics_optics"
    },
    {
        "text": "Isotropic Magnetic Purcell Effect: Manipulating the spontaneous emission rate of optical emitters with\nall-dielectric nanoparticles benefits from their low-loss nature and thus\nprovides relatively large extrinsic quantum yield. However, such Purcell effect\ngreatly depends on the orientation of the dipole emitter. Here, we introduce\nthe concept of isotropic magnetic Purcell effect with Purcell factors about 300\nand large extrinsic quantum yield (more than 80%) for a magnetic dipole emitter\nof arbitrary orientation in an asymmetric silicon nanocavity. The extrinsic\nquantum yield can be even boosted up to nearly 100% by utilizing a GaP\nnanocavity. Isotropy of the Purcell factor is manifested via the\norientation-independent emission of the magnetic dipole source. This isotropic\nPurcell effect is robust against small displacement of emitter on the order of\n10 nm, releasing the requirement of precise alignment in experiments.",
        "category": "physics_optics"
    },
    {
        "text": "Multiple-beam Propagation in an Anderson Localized Optical Fiber: We investigate the simultaneous propagation of multiple beams in a disordered\nAnderson localized optical fiber. The profiles of each beam fall off\nexponentially, enabling multiple channels at high-density. We examine the\ninfluence of fiber bends on the movement of the beam positions, which we refer\nto as drift. We investigate the extent of the drift of localized beams induced\nby macro-bending and show that it is possible to design Anderson localized\noptical fibers that can be used for practical beam-multiplexing applications.",
        "category": "physics_optics"
    },
    {
        "text": "Comb mode converters based on sampled helical gratings: Helical gratings (HGs) have an achievement of flexible mode conversion for\nfibre guided orbital angular momentum (OAM) modes. Sampled reflection HGs can\nrealise the generation and conversion of OAM mode with comb spectra. They can\nbe used to simultaneously filter wavelengths and convert modes, and thus may be\napplied to the hybrid multiplexing technology with wavelength division\nmultiplexing and mode division multiplexing.",
        "category": "physics_optics"
    },
    {
        "text": "Linearization of a dual-parallel Mach-Zehnder modulator using optical\n  carrier band processing: The linearization of a microwave photonic link based on a dual-parallel\nMach-Zehnder modulator is theoretically described and experimentally\ndemonstrated. Up to four different radio frequency tones are considered in the\nstudy, which allow us to provide a complete mathematical description of all\nthird-order distortion terms that arise at the photodetector. Simulations show\nthat a complete linearization is obtained by properly tuning the DC bias\nvoltages and processing the optical carrier and. As a result, a suppression of\n17 dBm is experimentally obtained for the third-order distortion terms, as well\nas a SDFR improvement of 3 dB. The proposed linearization method enables the\nsimultaneous modulation of four different signals without the need of\nadditional radio frequency components, which is desirable to its implementation\nin integrated optics and makes it suitable for several applications in\nmicrowave photonics.",
        "category": "physics_optics"
    },
    {
        "text": "Solving integral equations in free-space with inverse-designed ultrathin\n  optical metagratings: As standard microelectronic technology approaches fundamental limitations in\nspeed and power consumption, novel computing strategies are strongly needed.\nAnalog optical computing enables processing large amounts of data at a\nnegligible energy cost and high speeds. Based on these principles, ultrathin\noptical metasurfaces have been recently explored to process large images in\nreal-time, in particular for edge detection. By incorporating feedback, it has\nalso been recently shown that metamaterials can be tailored to solve complex\nmathematical problems in the analog domain, although these efforts have so far\nbeen limited to guided-wave systems and bulky setups. Here, we present an\nultrathin Si metasurface-based platform for analog computing that is able to\nsolve Fredholm integral equations of the second kind using free-space visible\nradiation. A Si-based metagrating was inverse-designed to implement the\nscattering matrix synthesizing a prescribed Kernel corresponding to the\nmathematical problem of interest. Next, a semi-transparent mirror was\nincorporated into the sample to provide adequate feedback and thus perform the\nrequired Neumann series, solving the corresponding equation in the analog\ndomain at the speed of light. Visible wavelength operation enables a highly\ncompact, ultrathin device that can be interrogated from free-space, implying\nhigh processing speeds and the possibility of on-chip integration.",
        "category": "physics_optics"
    },
    {
        "text": "Integrated optomechanical arrays of two high reflectivity SiN membranes: Multi-element cavity optomechanics constitutes a direction to observe novel\neffects with mechanical resonators. Several exciting ideas include\nsuperradiance, increased optomechanical coupling, and quantum effects between\ndistinct mechanical modes among others. Realizing these experiments has so far\nbeen difficult, because of the need for extremely precise positioning of the\nelements relative to one another due to the high-reflectivity required for each\nelement. Here we overcome this challenge and present the fabrication of\nmonolithic arrays of two highly reflective mechanical resonators in a single\nchip. We characterize the optical spectra and losses of these 200 $\\mu$m-long\nFabry-P\\'{e}rot interferometers, measuring finesse values of up to 220. In\naddition, we observe an enhancement of the coupling rate between the cavity\nfield and the mechanical center-of-mass mode compared to the single membrane\ncase. Further enhancements in coupling with these devices are predicted,\npotentially reaching the single-photon strong coupling regime, giving these\nintegrated structures an exciting prospect for future multi-mode quantum\nexperiments.",
        "category": "physics_optics"
    },
    {
        "text": "Topological dragging of solitons: We put forward properties of solitons supported by optical lattices featuring\ntopological dislocations, and show that solitons experience attractive and\nrepulsive forces around the dislocations. Suitable arrangements of dislocations\nare even found to form soliton traps, and the properties of such solitons are\nshown to crucially depend on the trap topology. The uncovered phenomenon opens\na new concept for soliton control and manipulation, e.g., in disk-shaped\nBose-Einstein condensates.",
        "category": "physics_optics"
    },
    {
        "text": "Realization of an Economical Polymer Optical Fiber Demultiplexer: Polymer Optical Fiber (POF) can be and are being used in various fields of\napplications. Two of the main fields are the automotive and the home\nentertainment sector. The POF can be applied in several different optical\ncommunication systems as automotive multi-media busses or in-house Ethernet\nsystems.\n  The requirements of bandwidth are increasing very fast in these sectors and\ntherefore solutions that satisfy these demands are of high actuality. One\nsolution is to use the wavelength division multiplexing (WDM) technique. Here,\nseveral different wavelengths can carry information over one POF fiber. All\nwavelengths that are transmitted over the fiber, must be separated at the\nreceiver to regain and redirect the information channels. These separators are\nso-called Demultiplexers.\n  There are several systems available on the market, which are all afflicted\nwith certain disadvantages. But all these solutions have one main disadvantage,\nthey are all too expensive for most of the applications mentioned above. So the\ngoal of this study is to develop an economical Demultiplexer for WDM\ntransmission over POF.\n  The main idea is to separate the chromatic light in its monochromatic\ncomponents with the help of a prism with low reciprocal dispersive power. The\nprism and the other assemblies, which are needed to adjust the optical path,\nshould be manufactured in injection molding technique. This manufacturing\ntechnique is a very simple and economical way to produce a mass production\napplicable Demultiplexer for POF.",
        "category": "physics_optics"
    },
    {
        "text": "Digital in-line holography in thick optical systems: application to\n  visualization in pipes: In this paper we apply digital in-line holography to image opaque objects\nthrough a thick plano-concave pipe. Opaque fibers and opaque particles are\nconsidered}. Analytical expression of the intensity distribution in the CCD\nsensor plane is derived using generalized Fresnel transform. \\textbf{The\nproposed model has the ability to deal with various pipe shape and thickness\nand compensates for the lack of versatility of classical DIH models. Holograms\nobtained with a 12 mm thick plano-concave pipe are then reconstructed using\nfractional Fourier transform (FRFT).} This method allows us to get rid of\nastigmatism. Numerical and experimental results are presented.",
        "category": "physics_optics"
    },
    {
        "text": "Exceptional points in a topological photonic system: Exceptional points as branch singularities describe peculiar degeneracies of\nnon-Hermitian systems that do not obey energy conservation. This work shows\nthat exceptional points can emerge in a topological photonic system, for\nexample, the topological photonic waveguide coupled with two degenerate\ncounter-propagation topological whispering gallery modes. Such a photonic\narchitecture is designed by crystal-symmetry-protected topological photonic\ninsulators based on air rods in conventional dielectric materials. The relevant\nexceptional point reveals the breaking of the parity-time symmetry, reflected\nby the change of the transmission-dip number in the optical transmission\nspectra of the system. Achieving exceptional points in topological photonic\nsystems possibly opens a new avenue toward robust optical devices with\nexceptional-point-based unique properties and functionalities.",
        "category": "physics_optics"
    },
    {
        "text": "Room-temperature waveguide-coupled silicon single-photon avalanche\n  diodes: Single photon detection is important for a wide range of low-light\napplications, including quantum information processing, spectroscopy, and light\ndetection and ranging (LiDAR). A key challenge in these applications has been\nto integrate single-photon detection capability into photonic circuits for the\nrealization of complex photonic microsystems. Short-wavelength ($\\lambda$ < 1.1\n$\\mu$m) integrated photonics platforms that use silicon (Si) as photodetectors\noffer the opportunity to achieve single-photon avalanche diodes (SPADs) that\noperate at or near room temperature. Here, we report the first\nwaveguide-coupled Si SPAD. The device is monolithically integrated in a Si\nphotonic platform and operates in the visible spectrum. The device exhibited a\nsingle photon detection efficiency of > 6% for wavelengths of 488 nm and 532 nm\nwith an excess voltage less than 20% of the breakdown voltage. The dark count\nrate was below 100 kHz at room temperature, with the possibility of improving\nby approximately 35% by reducing the temperature to -5$^{\\circ}$C.",
        "category": "physics_optics"
    },
    {
        "text": "Generation and analysis of fractional Hankel-Bessel vortex beams via\n  computational holography: In this work, the experimental optical generation of the fractional\nHankel-Bessel vortex beams were investigated through the holographic technique\nby means of computer generated holograms (CGHs) and reproduced in a spatial\nlight modulator (SLMs). The intensity and phase profiles were simulated and the\nexperimental results are presented in this work, as well as their propagation\nalong the z-axis is demonstrated. The experimental results are in accordance\nwith the theoretical predictions described in the theory and literature. These\nresults presents excellent perspectives of this optical vortex and potential\napplications in optical manipulation, optical microscopy and optics\ncommunications, optical metrology, among others.",
        "category": "physics_optics"
    },
    {
        "text": "Asymmetries of azimuthal photon distributions in non-linear Compton\n  scattering in ultra-short intense laser pulses: Non-linear Compton scattering in ultra-short intense laser pulses is\ndiscussed with the focus on angular distributions of the emitted photon energy.\nThis is an observable which is accessible easily experimentally. Asymmetries of\nthe azimuthal distributions are predicted for both linear and circular\npolarization. We present a systematic survey of the influence of the laser\nintensity, the carrier envelope phase and the laser polarization on the\nemission spectra for single-cycle and few-cycle laser pulses. For linear\npolarization, the dominant direction of the emission changes from a\nperpendicular pattern with respect to the laser polarization at low-intensity\nto a dominantly parallel emission for high-intensity laser pulses.",
        "category": "physics_optics"
    },
    {
        "text": "Ab initio self-consistent laser theory and random lasers: We review our recent work leading to steady-state solutions of the\nsemiclassical (Maxwell-Bloch) equations of a laser. These are coupled\nnon-linear partial differential equations in space and time which have\npreviously been solved either by fully time-dependent numerical simulations or\nby using major approximations which neglect non-linear modal interactions\nand/or the openness of the laser system. We have found a time-independent\ntechnique for determining these stationary solutions which can treat lasers of\narbitrary complexity and degree of openness. Our method has been shown to agree\nwith time-dependent numerical solutions to high accuracy and has been applied\nto find the electric field patterns (lasing modes) of random lasers, which lack\na laser cavity and are so strongly damped that the linear system has no\ndetectable resonances. Our work provides a link between an important non-linear\nwave system and the field of quantum/wave chaos in linear systems.",
        "category": "physics_optics"
    },
    {
        "text": "An omnidirectional retroreflector based on the transmutation of\n  dielectric singularities: In the field of transformation optics, metamaterials mimic the effect of\ncoordinate transformations on electromagnetic waves, creating the illusion that\nthe waves are propagating through a virtual space. Transforming space by\nappropriately designed materials makes devices possible that have been deemed\nimpossible. In particular, transformation optics has led to the demonstration\nof invisibility cloaking for microwaves, surface plasmons and infrared light.\nHere we report the achievement of another \"impossible task\". We implement, for\nmicrowaves, a device that would normally require a dielectric singularity, an\ninfinity in the refractive index. We transmute a singularity in virtual space\ninto a mere topological defect in a real metamaterial. In particular, we\ndemonstrate an omnidirectional retroreflector, a device for faithfully\nreflecting images and for creating high visibility, from all directions. Our\nmethod is robust, potentially broadband and similar techniques could be applied\nfor visible light.",
        "category": "physics_optics"
    },
    {
        "text": "Harmonic Generation with Phase Synchronism: We establish how the intensities of the higher harmonics that arise when a\nphotoelectron recombines with a parent ion depend functionally on the\nparameters of the laser wave and atomic medium, and estimate the limiting\nvalues of these parameters that are needed to observe the phase synchronization\neffect.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of superluminal signaling of terahertz pulses: Superluminal tunneling of light through a barrier has attracted broad\ninterest in the last several decades. Despite the observation of such phenomena\nin various systems, it has been under intensive debate whether the transmitted\nlight truly carry the information of the original pulse. Here we report\nobservation of anomalous time response for terahertz electromagnetic pulses\npassing through thin metal films, with the pulse shape of the transmitted beam\nfaithfully resembling that of the incident beam. A causal theoretical analysis\nis developed to explain the experiments, though the theory of Special\nRelativity may confront a challenge in this exceptional circumstance. These\nfindings may facilitate future applications in high-speed optical communication\nor signal transmission, and may reshape our fundamental understanding about the\ntunneling of light.",
        "category": "physics_optics"
    },
    {
        "text": "Emerging chiral optics from chiral interfaces: Twisted atomic bilayers are emerging platforms for manipulating chiral\nlight-matter interaction at the extreme nanoscale, due to their inherent\nmagnetoelectric responses induced by the finite twist angle and quantum\ninterlayer coupling between the atomic layers. Recent studies have reported the\ndirect correspondence between twisted atomic bilayers and chiral metasurfaces,\nwhich features a chiral surface conductivity, in addition to the electric and\nmagnetic surface conductivities. However, far-field chiral optics in light of\nthese consitututive conductivities remains unexplored. Within the framework of\nthe full Maxwell equations, we find that the chiral surface conductivity can be\nexploited to realize perfect polarization transformation between linearly\npolarized light. Remarkably, such an exotic chiral phenomenon can occur either\nfor the reflected or transmitted light.",
        "category": "physics_optics"
    },
    {
        "text": "Strong Exciton-Photon Coupling in Large Area MoSe$_2$ and WSe$_2$\n  Heterostructures Fabricated from Two-Dimensional Materials Grown by Chemical\n  Vapor Deposition: Two-dimensional semiconducting transition metal dichalcogenides embedded in\noptical microcavities in the strong exciton-photon coupling regime may lead to\npromising applications in spin and valley addressable polaritonic logic gates\nand circuits. One significant obstacle for their realization is the inherent\nlack of scalability associated with the mechanical exfoliation commonly used\nfor fabrication of two-dimensional materials and their heterostructures.\nChemical vapor deposition offers an alternative scalable fabrication method for\nboth monolayer semiconductors and other two-dimensional materials, such as\nhexagonal boron nitride. Observation of the strong light-matter coupling in\nchemical vapor grown transition metal dichalcogenides has been demonstrated so\nfar in a handful of experiments with monolayer molybdenum disulfide and\ntungsten disulfide. Here we instead demonstrate the strong exciton-photon\ncoupling in microcavities comprising large area transition metal dichalcogenide\n/ hexagonal boron nitride heterostructures made from chemical vapor deposition\ngrown molybdenum diselenide and tungsten diselenide encapsulated on one or both\nsides in continuous few-layer boron nitride films also grown by chemical vapor\ndeposition. These transition metal dichalcogenide / hexagonal boron nitride\nheterostructures show high optical quality comparable with mechanically\nexfoliated samples, allowing operation in the strong coupling regime in a wide\nrange of temperatures down to 4 Kelvin in tunable and monolithic microcavities,\nand demonstrating the possibility to successfully develop large area transition\nmetal dichalcogenide based polariton devices.",
        "category": "physics_optics"
    },
    {
        "text": "Perfect absorption in GaAs metasurfaces by degenerate critical coupling: Enhancing absorption in optically thin semiconductors is the key in the\ndevelopment of high-performance optical and optoelectronic devices. In this\npaper, we resort to the concept of degenerate critical coupling and design an\nultra-thin semiconductor absorber composed of free-standing GaAs nanocylinder\nmetasurfaces in the near infrared. The numerical results show that perfect\nabsorption can be achieved through overlapping two Mie modes with opposite\nsymmetry, with each mode contributing a theoretical maximum of 50% in their\nrespective critical coupling state. The absorption also shows the\npolarization-independent and angle-insensitive robustness. This work, together\nwith the design concept, opens up great opportunities for the realization of\nhigh-efficiency metasurface devices, including optical emitters, modulators,\ndetectors, and sensors.",
        "category": "physics_optics"
    },
    {
        "text": "Lifting the Bandwidth Limit of Optical Homodyne Measurement: Homodyne measurement is a corner-stone of quantum optics. It measures the\nfundamental variables of quantum electrodynamics - the quadratures of light,\nwhich represent the cosine-wave and sine-wave components of an optical field\nand constitute the quantum optical analog of position and momentum. Yet,\nstandard homodyne, which is used to measure the quadrature information, suffers\nfrom a severe bandwidth limitation: While the bandwidth of optical states can\neasily span many THz, standard homodyne detection is inherently limited to the\nelectrically accessible, MHz to GHz range, leaving a dramatic gap between the\nrelevant optical phenomena and the measurement capability. We demonstrate a\nfully parallel optical homodyne measurement across an arbitrary optical\nbandwidth, effectively lifting this bandwidth limitation completely. Using\noptical parametric amplification, which amplifies one quadrature while\nattenuating the other, we measure two-mode quadrature squeezing of 1.7dB below\nthe vacuum level simultaneously across a bandwidth of 55THz, using just one\nlocal-oscillator - the pump. As opposed to standard homodyne, our measurement\nis highly robust to detection inefficiency, and was obtained with $>50\\%$ loss\nin the detection channel. This broadband parametric homodyne measurement opens\na wide window for parallel processing of quantum information.",
        "category": "physics_optics"
    },
    {
        "text": "Pattern fluctuations and revivals in complex structured light from\n  unitary transformation: Astigmatic unitary transformations allow for the adiabatic connections of all\nfeasible states of paraxial Gaussian beams on the same modal sphere, i.e.,\nHermite-Laguerre-Gaussian (HLG) modes. Here, we present a comprehensive\ninvestigation into the unitary modal evolution of complex structured Gaussian\nbeams, comprised by HLG modes from disparate modal spheres, via astigmatic\ntransformation. The non-synchronized higher-order geometric phases in cyclic\ntransformations originates pattern fluctuations in the superposition state of\nthese HLG modes, as well as possible pattern revivals in transformations with\nspecific geodesic loops. Using Ince-Gaussian modes as an illustrative example,\nwe systematically analyze and experimentally corroborate the beamforming\nmechanism behind the pattern evolution. Our results outline a generic modal\nconversion theory of structured Gaussian beams via astigmatic unitary\ntransformation, offering a new approach for shaping spatial modal structure.\nThese findings may inspire a wide variety of applications based on structured\nlight.",
        "category": "physics_optics"
    },
    {
        "text": "The Quest for the Ideal Scintillator for Hybrid Phototubes: In this paper we present the results of extensive studies of scintillators\nfor hybrid phototubes with luminescent screens. The results of the developments\nof such phototubes with a variety of scintillators are presented. New\nscintillator materials for such kind of application are discussed. The\nrequirements for scintillators to use in such hybrid phototubes are formulated.\nIt is shown that very fast and highly efficient inorganic scintillators like\nZnO:Ga will be ideal scintillators for such kind of application.",
        "category": "physics_optics"
    },
    {
        "text": "Measuring a piecewise constant axion field in classical electrodynamics: In order to settle the problem of the \"Post constraint\" in material media, we\nconsider the propagation of a plane electromagnetic wave in a medium with a\npiecewise constant axion field. Although a constant axion field does not affect\nthe wave propagation in a homogeneous medium, we show that the reflection and\ntransmission of a wave at an interface between the two media is sensitive to\nthe difference of the axion values. This observation can be used to determine\nexperimentally the axion piece in matter despite the fact that a constant axion\nvalue does not contribute to the Maxwell equations.",
        "category": "physics_optics"
    },
    {
        "text": "Antiresonances and Ultrafast Resonances in a Twin Photonic Oscillator: We consider the properties of the small-signal modulation response of\nsymmetry-breaking phase-locked states of twin coupled semiconductor lasers. The\nextended stability and the varying asymmetry of these modes allows for the\nintroduction of a rich set of interesting modulation response features, such as\nsharp resonances and anti-resonance as well as efficient modulation at very\nhigh frequencies exceeding the free running relaxation frequencies by orders of\nmagnitude.",
        "category": "physics_optics"
    },
    {
        "text": "Optical Magnetism: from Red to Blue: A family of coupled nano-strips with varying dimensions is fabricated to\nobtain optical magnetic responses across the whole visible spectrum, from red\nto blue. The proposed approach provides one with a universal building block and\na general recipe for producing controllable optical magnetism for various\npractical implementations.",
        "category": "physics_optics"
    },
    {
        "text": "Spatial quadratic solitons guided by narrow layers of a nonlinear\n  material: We report analytical solutions for spatial solitons supported by layers of a\nquadratically nonlinear material embedded into a linear planar waveguide. A\nfull set of symmetric, asymmetric, and antisymmetric modes pinned to a\nsymmetric pair of the nonlinear layers is obtained. The solutions describe a\nbifurcation of the subcritical type, which accounts for the transition from the\nsymmetric to asymmetric modes. The antisymmetric states (which do not undergo\nthe bifurcation) are completely stable (the stability of the solitons pinned to\nthe embedded layers is tested by means of numerical simulations). Exact\nsolutions are also found for nonlinear layers embedded into a nonlinear\nwaveguide, including the case when the uniform and localized nonlinearities\nhave opposite signs (competing nonlinearities). For the layers embedded into\nthe nonlinear medium, stability properties are explained by comparison to the\nrespective cascading limit.",
        "category": "physics_optics"
    },
    {
        "text": "Self-Pulsing in driven-dissipative photonic Bose-Hubbard dimers: We experimentally investigate the nonlinear dynamics of two coupled fiber\nring resonators, coherently driven by a single laser beam. We comprehensively\nexplore the optical switching arising when scanning the detuning of the\nundriven cavity, and show how the driven cavity detuning dramatically changes\nthe resulting hysteresis cycle. By driving the photonic dimer\nout-of-equilibrium, we observe the occurrence of stable self-switching\noscillations near avoided resonance crossings. All results agree well with the\ndriven-dissipative Bose-Hubbard dimer model in the weakly coupled regime.",
        "category": "physics_optics"
    },
    {
        "text": "Robust Fourier ptychographic microscopy via a physics-based defocusing\n  strategy for calibrating angle-varied LED illumination: Fourier ptychographic microscopy (FPM) is a recently developed computational\nimaging technique for wide-field, high-resolution microscopy with a high\nspace-bandwidth product. It integrates the concepts of synthetic aperture and\nphase retrieval to surpass the resolution limit imposed by the employed\nobjective lens. In the FPM framework, the position of each sub-spectrum needs\nto be accurately known to ensure the success of the phase retrieval process.\nDifferent from the conventional methods with mechanical adjustment or\ndata-driven optimization strategies, here we report a physics-based defocusing\nstrategy for correcting large-scale positional deviation of the LED\nillumination in FPM. Based on a subpixel image registration process with a\ndefocused object, we can directly infer the illumination parameters including\nthe lateral offsets of the light source, the in-plane rotation angle of the LED\narray, and the distance between the sample and the LED board. The feasibility\nand effectiveness of our method are validated with both simulation study and\nexperiments. We show that the reported strategy can obtain high-quality\nreconstruction of both the complex object and pupil even the LED array is\nrandomly placed under the sample with both unknown lateral offsets and\nrotations. As such, it enables the development of robust FPM systems by\nreducing the requirement on fine mechanical adjustment and data-driven\ncorrection in the construction process.",
        "category": "physics_optics"
    },
    {
        "text": "Applicability of Gaussian Noise Approximation for Optically\n  Pre-Amplified DPSK Receivers with Balanced Detection: This letter presents a comparison of exact probability density function with\nthe Gaussian noise approximation in optically pre-amplified DPSK receivers with\noptical Mach-Zehnder interferometer demodulation (MZI) and balanced detection,\nincluding the impact of phase noise. It is found that the Gaussian noise\napproximation significantly over-estimates ASE-ASE beat noise in DPSK receivers\nwith balanced detection particularly when phase noise is negligible, compared\nto IM/DD receivers, ASE- amplified spontaneous emission. However, the Gaussian\nnoise approximation is still applicable for DPSK receivers with balanced\ndetection and the measured 3-dB advantage is predicted by the Gaussian noise\ndistribution.",
        "category": "physics_optics"
    },
    {
        "text": "Experimental optical trapping of microparticles with higher order Frozen\n  Waves: In this work, we optically trapping microparticles with higher order Frozen\nWave using holographic optical tweezers. Frozen Waves are diffraction resistant\noptical beams, obtained by superposing copropagating Bessel beams with the same\nfrequency and order, obtaining efficient modeling of its shape. Based on this,\nwe developed a holographic optical tweezers system for the generation of Frozen\nWaves and with this, it was possible to create traps in a stable way for the\ntrapping and guiding of the microparticles in the transverse plane. The\nexperimental results show that it is possible to obtain excellent stability\ncondition for optical trapping using higher order Frozen Waves. These results\nindicate that the Frozen Waves is promising for optical trapping and guiding of\nparticles, which may be useful in various application such as biological\nresearch, atomic physics and optical manipulations using structured light with\norbital angular momentum.",
        "category": "physics_optics"
    },
    {
        "text": "Identifying signature Zernike modes for efficient light delivery through\n  brain tissue: Recent progress in neuroscience to image and investigate brain function has\nbeen made possible by impressive developments in optogenetic and opto-molecular\ntools. Such research requires advances in optical techniques for the delivery\nof light through brain tissue with high spatial resolution. The tissue causes\ndistortions of the wavefront of the incoming light which broadens the focus,\nthereby reducing the intensity and resolution especially in techniques\nrequiring focal illumination. Adaptive wavefront correction has been\ndemonstrated to compensate for these distortions. However, in many situations\niterative derivation of the corrective wavefront introduces time constraints\nthat limit its usefulness when used to probe living cells. Here we demonstrate\na direct and fast technique by working with a small set of Zernike modes and\ndemonstrate that corrections derived a priori can lead to significant\nimprovement of the focus. We verify this idea by the electrical response of\nwhole-cell patched neurons following two-photon photolysis of caged\nneurotransmitters along its dendrites.In particular, we find that the\norganization of the neuropile in the cortical region of rat brain slicesprovide\nsufficient a priori information to preselect an effective wavefront correction.",
        "category": "physics_optics"
    },
    {
        "text": "Broadband Single-Mode Hybrid Photonic Crystal Waveguides for Terahertz\n  Integration on a Chip: A novel terahertz hybrid waveguide chip consisting of silicon photonic\ncrystals sandwiched in parallel gold plates is developed. Both simulation and\nexperimental results demonstrate that the hybrid waveguide offers a wide\nsingle-mode transmission window with low group velocity dispersion and low\nloss. This compact, substrate-free terahertz chip would play a significant role\nin broadband, dense-integrated, multi-functional terahertz systems.",
        "category": "physics_optics"
    },
    {
        "text": "On electromagnetic surface waves supported by an isotropic chiral\n  material: Electromagnetic surface waves supported by an isotropic chiral material were\ninvestigated via the associated canonical boundary-value problem. Specifically,\ntwo scenarios were considered: surface waves guided by the planar interface of\nan isotropic chiral material and (a) an isotropic dielectric material and (b) a\nuniaxial dielectric (or plasmonic) material. Both plasmonic and non-plasmonic\nachiral partnering materials were investigated. In scenario (a) surface waves\nakin to surface-plasmon-polariton (SPP) waves were excited, while in scenario\n(b) surface waves akin to Dyakonov waves and akin to SPP waves were excited.\nFor numerical studies, an isotropic chiral material capable of simultaneously\nsupporting attenuation and amplification of plane waves, depending upon\ncircular polarization state, was used. The amplitude of the SPP-like waves\ncould be either amplified or attenuated, depending upon the relative\npermittivity of the isotropic dielectric partnering material for scenario (a),\nor depending upon the direction of propagation relative to the optic axis of\nthe uniaxial dielectric partnering material for scenario (b).",
        "category": "physics_optics"
    },
    {
        "text": "PT-symmetric cross injection dual optoelectronic oscillator: An optoelectronic oscillator (OEO) is a time delay oscillator (TDO) that uses\nphotonics technology to provide the long delay required to generate pristine\nmicrowave carriers. Parity-time (PT) symmetry concepts applied to an OEO offer\nthe potential to achieve combined low phase noise and high sidemode\nsuppression. A TDO composed of a pair of identical ring resonators coupled by a\n2x2 coupler is modelled, and the coupler transmission matrix required for the\noscillator to be PT- symmetric is derived. In a first configuration, the\ncoupler is interpreted as the composition of a gain/loss block and a\nMach-Zehnder interferometer (MZI) block. In practice, there are excess losses\nthat must be compensated by a special dual amplifier with saturation\ncharacteristics compatible with PT- symmetry. The PT- symmetry phase transition\ndetermined by the gain/loss and the MZI differential phase parameters is found\nto be global and not local in its effect on modes. This is resolved by placing\na short delay line within one arm of the MZI resulting in a frequency dependent\nand hence local mode-selective PT- symmetry phase transition. In addition, it\nis demonstrated that the first configuration may be transformed into a second\nbut equivalent configuration as a cross-injection dual TDO with imbalanced\ndelays. The local PT- symmetry phase transition may then be understood in terms\nof the Vernier effect. Advantageously, the second configuration enables the\nspecial dual amplifier to be replaced by a pair of matched but otherwise\nindependent amplifiers. Thereby, the second configuration is amenable to\npractical implementation as a dual OEO using standard RF-photonic and\nRF-electronic components. The theory is validated by complex envelope model\nsimulations using Simulink and phase model analytic results evaluated using\nMATLAB. There is excellent agreement between the theoretical and simulation\nresults.",
        "category": "physics_optics"
    },
    {
        "text": "A Thermal-Photovoltaic Device Based on Thermally Enhanced\n  Photoluminescence: Single-junction photovoltaic cells are considered to be efficient solar\nenergy converters, but even ideal cells cannot exceed the their fundamental\nthermodynamic efficiency limit, first analysed by Shockley and Queisser (SQ).\nFor moderated irradiation levels, the efficiency limit ranges between 30%-40%.\nThe efficiency loss is, to a great extent, due to the inherent heat-dissipation\naccompanying the process of electro-chemical potential generation. Concepts\nsuch as solar thermo-photovoltaics (STPV) and thermo-photonics4 aim to harness\nthis dissipated heat, yet exceeding the SQ limit has not been achieved, mainly\ndue to the very high operating temperatures needed. Recently, we demonstrated\nthat in high-temperature endothermic-photoluminescence (PL), the photon rate is\nconserved with temperature increase, while each photon is blue shifted. We also\ndemonstrated how endothermic-PL generates orders of magnitude more\nenergetic-photons than thermal emission at similar temperatures. These new\nfindings show that endothermic-PL is an ideal optical heat-pump. Here, we\npropose and thermodynamically analyse a novel device based on Thermally\nEnhanced Photo Luminescence (TEPL). In such a device, solar radiation is\nharvested by a low-bandgap PL material. In addition to the PL excitation, the\notherwise lost heat raises the temperature and allows the TEPL emission to be\ncoupled to a higher bandgap solar cell. The excessive thermal energy is then\nconverted to electrical work at high voltage and enhanced efficiency. Our\nresults show that such a TEPL based device can reach theoretical maximal\nefficiencies of 70%, as high as in STPV, while the significantly lowered\noperating temperatures are below 1000C. In addition to the theoretical\nanalysis, we experimentally demonstrated enhanced photo-current in TEPL device\npumped by sub-bandgap radiation. This opens the way for a new direction in\nphotovoltaics.",
        "category": "physics_optics"
    },
    {
        "text": "A Femtosecond Magnetic Circular Dichroism Spectrometer: We describe the development of a broadband magneto-optical spectrometer with\nfemtosecond temporal resolution. The absorption spectrometer is based on a\nwhite-light supercontinuum (ca. 320 - 750 nm) using shot-to-shot temporal and\nspectral referencing at 1 kHz. Static and transient absorption spectra using\ncircularly polarised light are collected in a magnetic field. The difference\nspectra with respect to the external field direction give the static and\ntransient magneto-optical Faraday rotation (magnetic optical rotary dispersion)\nand ellipticity (magnetic circular dichroism) spectra. An achromatic\nquarter-wave plate is used and the impact of the deviation from ideal\nretardance on the spectra is discussed. Results from solution-based and\nthin-film samples are used to demonstrate the performance and wide\napplicability of the instrument. The sensitivities for the static and\ntime-resolved data were found to be 5 and 0.4 mdeg, respectively. The method\npresents a simple way to measure magneto-optical spectra using a transient\nabsorption spectrometer and an electromagnet.",
        "category": "physics_optics"
    },
    {
        "text": "Solitons in nonlinear lattices: This article offers a comprehensive survey of results obtained for solitons\nand complex nonlinear wave patterns supported by purely nonlinear lattices\n(NLs), which represent a spatially periodic modulation of the local strength\nand sign of the nonlinearity, and their combinations with linear lattices. A\nmajority of the results obtained, thus far, in this field and reviewed in this\narticle are theoretical. Nevertheless, relevant experimental settings are\nsurveyed too, with emphasis on perspectives for implementation of the\ntheoretical predictions in the experiment. Physical systems discussed in the\nreview belong to the realms of nonlinear optics (including artificial optical\nmedia, such as photonic crystals, and plasmonics) and Bose-Einstein\ncondensation (BEC). The solitons are considered in one, two, and three\ndimensions (1D, 2D, and 3D). Basic properties of the solitons presented in the\nreview are their existence, stability, and mobility. Although the field is\nstill far from completion, general conclusions can be drawn. In particular, a\nnovel fundamental property of 1D solitons, which does not occur in the absence\nof NLs, is a finite threshold value of the soliton norm, necessary for their\nexistence. In multidimensional settings, the stability of solitons supported by\nthe spatial modulation of the nonlinearity is a truly challenging problem, for\nthe theoretical and experimental studies alike. In both the 1D and 2D cases,\nthe mechanism which creates solitons in NLs is principally different from its\ncounterpart in linear lattices, as the solitons are created directly, rather\nthan bifurcating from Bloch modes of linear lattices.",
        "category": "physics_optics"
    },
    {
        "text": "Role of edge inclination in optical microdisk resonator for label-free\n  sensing: In this paper we report on the measurement and modelling of enhanced optical\nrefractometric sensors based on whispering-gallery-modes. The devices under\ntest are optical microresonators made of silicon nitride on silicon oxide. In\nour approach, these microresonators are vertically coupled to a buried\nwaveguide with the aim of creating integrated and cost-effective devices. The\noptimization analysis is a delicate balance of resonance quality factor and\nevanescent field overlap with the sorrounding environment to analyze. By\nnumerical simulations we show that the microdisk thickness is critical to yield\nhigh figure of merit for the sensor, while edge inclination is less important.\nWe also show that figures of merit as high as 1600/RIU are feasible.",
        "category": "physics_optics"
    },
    {
        "text": "Measurement of Optical Orbital and Spin Angular Momentum: Implications\n  for Photon Angular Momentum: The expression for the total angular momentum carried by a laser optical\nvortex beam, splits, in the paraxial approximation, into two terms which seem\nto represent orbital and spin angular momentum respectively. There are,\nhowever, two very different competing versions of the formula for the spin\nangular momentum, one based on the use of the Poynting vector, as in classical\nelectrodynamics, the other related to the canonical expression for the angular\nmomentum which occurs in Quantum Electrodynamic. I analyze the possibility that\na sufficiently sensitive optical measurement could decide which of these\ncorresponds to the actual physical angular momentum carried by the beam.",
        "category": "physics_optics"
    },
    {
        "text": "Revealing the spin optics of conics: Ellipse and hyperbola are two well-known curves in mathematics with numerous\napplications in various fields, but their properties and inherent differences\nin spin optics are less understood. Here, we investigate the peculiar optical\nspin properties of the two curves and establish a connection between their foci\nand the spin states of incident light. We show that the optical spin Hall\neffect is the intrinsic optical spin property of ellipse, where photons with\ndifferent spin states can be exactly separated to each of its two foci. While a\nhyperbola exhibits optical spin-selective effect, where only photons with one\nparticular spin state can be accumulated at its foci. These properties are then\nexperimentally demonstrated in near field by arranging nanoslits in conic\nshape. Based on the spin properties of the curves, we design spin-based\nplasmonic devices with various functionalities. Our results reveal the\nintrinsic optical spin properties behind conic curves and provide a route for\ndesigning spin-based plasmonic device.",
        "category": "physics_optics"
    },
    {
        "text": "Photon statistics of amplified spontaneous emission: Developments in quantum technologies lead to new applications that require\nradiation sources with specific photon statistics. A widely used Poissonian\nstatistics are easily produced by lasers; however, some applications require\nsuper- or sub-Poissonian statistics. Statistical properties of a light source\nare characterized by the second-order coherence function g^(2)(0). This\nfunction distinguishes stimulated radiation of lasers with g^(2)(0)=1 from\nlight of other sources. For example, g^(2)(0)=2 for black-body radiation, and\ng^(2)(0)=0 for single-photon emission. One of the applications requiring\nsuper-Poissonian statistics (g^(2)(0)>1) is ghost imaging with thermal light.\nGhost imaging also requires light with a narrow linewidth and high intensity.\nCurrently, rather expensive and inefficient light sources are used for this\npurpose. In the last year, a superluminescent diode based on amplified\nspontaneous emission (ASE) has been considered as a new light source for ghost\nimaging. Even though ASE has been widely studied, its photon statistics has not\nbeen settled - there are neither reliable theoretical estimates of the\nsecond-order coherence function nor unambiguous experimental data. Our computer\nsimulation clearly establishes that coherence properties of light produced by\nASE are similar to that of a thermal source with g^(2)(0)=2 independent of pump\npower. This result manifests the fundamental difference between ASE and laser\nradiation.",
        "category": "physics_optics"
    },
    {
        "text": "Index-leveling for forced-flow turbulent face-cooling of laser\n  amplifiers: Direct laser slab face-cooling by a fluid crossing the main and pump laser\nbeams is an important method to reach high average laser powers. However, the\nflow regime is usually maintained at low Reynolds numbers, to prevent the onset\nof turbulence features in the flow, that would degrade wavefront quality. We\nshow here how bringing the fluid temperature to the thermo-optical null point,\nclose to the water/ice transition in the case of water, allows one to mitigate\nthe optical consequences of hydrodynamic instabilities, by bleaching optically\nthe temperature inhomogeneities within the flow. This optical process, dubbed\n'index-leveling', opens the door to a highly efficient forced-flow, weakly\nturbulent face-cooling regime that should be instrumental to boost the kilowatt\ncapabilities of next generation high-power lasers.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of subluminal twisted light in vacuum: comment: Our analysis based on analytical and numerical calculations leads to\nconclusion that the promising results obtained in [F. Bouchard, J. Harris, H.\nMand, R. W. Boyd, and E. Karimi, Optica 3, 351, 2016] are questionable in\nseveral respects.",
        "category": "physics_optics"
    },
    {
        "text": "Superradiance Startup at Finite Temperatures of the Electromagnetic\n  Field: We use quantum-electrodynamical approach to study the initial stage of Dicke\nsuperradiance from a system of two-level atoms. Applying the zeroth-order\nMagnus approximation, we obtain the expression for the mean number of quanta\nemitted in the presence of thermal fluctuations of the electromagnetic field.",
        "category": "physics_optics"
    },
    {
        "text": "Ultra-low transmission loss (7.7 dB/km at 750 nm) inhibited-coupling\n  guiding hollow-core photonic crystal fibers with a single ring of tubular\n  lattice cladding: The advent of photonic bandgap (PBG) guiding hollow- core photonic crystal\nfiber (HC-PCF) sparked the hope of guiding light with attenuation below the\nfundamental silica Rayleigh scattering limit (SRSL) of conventional step-index\nfibers. Unfortunately, the combination of the strong core-cladding\noptical-overlap, the surface roughness at the silica cladding struts and the\npresence of interface-modes limited the lowest reported transmission-loss to\n1.2 dB/km at 1550 nm. This hope is recently revived by the introduction of\nhypocycloid core- contour (i.e. negative curvature) in inhibited-coupling (IC)\nguiding HC-PCF, and the reduction of their confinement loss to a level that\nmakes them serious contenders for light transmission below the SRSL in UV-\nVIS-NIR spectral range. Here, we report on several IC guiding HC-PCFs with a\nhypocycloid core-contour and a cladding structure made of a single ring from a\ntubular lattice. The fibers guide in the UV-VIS and NIR, and among which we\nlist one with a record transmission loss of 7.7 dB/km at ~750 nm (only a factor\n~2 above the SRSL), and a second with an ultra-broad fundamental-band with loss\nin the range of 10-20 dB/km spanning from 600 to 1200 nm. Both fibers present\nnear-single mode guidance and very low bend loss sensitivity. The results show\nthat the limit in the transmission is set by confinement loss for wavelengths\nlonger than ~1 {\\mu}m and by surface- roughness for shorter wavelengths, thus\nindicating that transmission loss well below the SRSL in the visible is\npossible with a surface roughness reduction and would open the possibility of\nthe first UV low-loss light- guidance.",
        "category": "physics_optics"
    },
    {
        "text": "Highly efficient terahertz generation using 3D Dirac semimetals: We show that 3D Dirac semimetals are promising candidates for highly\nefficient optical-to-terahertz conversion due to their extreme optical\nnonlinearities. In particular, we predict that the conversion efficiency of\nCd3As2 exceeds typical materials like LiNbo3 by >5000 times over nanoscale\npropagation distances. Our studies show that even when no restrictions are\nplaced on propagation distance, Cd3As2 still outperforms LiNbo3 in efficiency\nby >10 times. Our results indicate that by tuning the Fermi energy, Pauli\nblocking can be leveraged to realize a step-like efficiency increase in the\noptical-to-terahertz conversion process. We find that large optical to\nterahertz conversion efficiencies persist over a wide range of input\nfrequencies, input field strengths, Fermi energies, and temperatures. Our\nresults could pave the way to the development of ultrathin-film terahertz\nsources for compact terahertz technologies.",
        "category": "physics_optics"
    },
    {
        "text": "A Method for Increasing the Resolution of Optical Detectors: In addition to the optical aberrations, the magnitude of the optical cells of\ndetectors is one of the most important parameters, which restricts the\nresolution of detectors. Adaptive optic and the methods of reducing the\naberrations are often used to increase the resolution of optical systems. In\nthe best situation the image structures, which are larger than the magnitude of\nthe cells, are detected and the finer structures are removed. In this paper, a\nnew method for increasing the resolution of images is presented. In this\nmethod, the cells will scan the images by a piezoelectric crystal. The\npiezoelectric crystal moves the cells in n identical steps. In each step, cells\nmove 1/n of the cell magnitude and the data are recorded. Finally, data are\nanalyzed and the structure of images is reconstructed. In this method, the\nstructure of images is n times finer than the cells magnitude.",
        "category": "physics_optics"
    },
    {
        "text": "Mechanism and modulation of terahertz generation from a semimetal -\n  graphite: Semi-metals might offer a stronger interaction and a better confinement for\nterahertz wave than semiconductors, while preserve tunability. Particularly,\ngraphene-based materials are envisioned as terahertz modulators, filters and\nultra-broadband sources. However, the understanding of terahertz generation\nfrom those materials is still not clear, thus limits us recognizing the\npotential and improving device performances. Graphite, the mother material of\ngraphene and a typical bulk semi-metal, is a good system to study semi-metals\nand graphene-based materials. Here we experimentally modulate and maximize the\nterahertz signal from graphite surface, thus reveal the mechanism - surface\nfield driving photon induced carriers into transient current to radiate\nterahertz wave. We also discuss the differences between graphite and\nsemiconductors; particularly graphite shows no temperature dependency from room\ntemperature to 80C. Above knowledge will help us understand terahertz\ngenerations, achieve maximum output and electric modulation, in semi-metal or\ngraphene based devices.",
        "category": "physics_optics"
    },
    {
        "text": "Mode-locking based on a zero-area pulse formation in a laser with a\n  coherent absorber: We observe experimentally a mode-locking in a continuous narrow-band tunable\ndye laser with molecular iodine absorber cells, which transitions have large\nphase relaxation time T2. We show that the mode-locking arises due to coherent\ninteraction of light with the absorbing medium leading to Rabi oscillations, so\nthat zero-area (0{\\pi}-) pulses in the absorber are formed. Such mode-locking\nregime is different to most typical passive modelocking mechanisms where\nsaturation plays the main role.",
        "category": "physics_optics"
    },
    {
        "text": "Modulation control and spectral shaping of optical fiber supercontinuum\n  generation in the picosecond regime: Numerical simulations are used to study how fiber supercontinuum generation\nseeded by picosecond pulses can be actively controlled through the use of input\npulse modulation. By carrying out multiple simulations in the presence of\nnoise, we show how tailored supercontinuum Spectra with increased bandwidth and\nimproved stability can be generated using an input envelope modulation of\nappropriate frequency and depth. The results are discussed in terms of the\nnon-linear propagation dynamics and pump depletion.",
        "category": "physics_optics"
    },
    {
        "text": "Microwave electrometry with multi-photon coherence in Rydberg atoms: A scheme for measurement of microwave (MW) electric field is proposed via\nmulti-photon coherence in Rydberg atoms. It is based on the three-photon\nelectromagnetically induced absorption (TPEIA) spectrum. In this process, the\nmulti-photon produces a narrow absorption peak, which has a larger magnitude\nthan the electromagnetically induced transparency (EIT) peak under the same\nconditions. The TPEIA peak is sensitive to MW fields, and can be used to\nmeasure MW electric field strength. It is interesting to find that the\nmagnitude of TPEIA peaks shows a linear relationship with the MW field\nstrength. The simulation results show that the minimum detectable strength of\nthe MW fields is about 1/10 that based on an common EIT effect, and the probe\nsensitivity is improved by about 4 times. Furthermore, the MW sensing based on\nthree-photon coherence shows a broad tunability, and the scheme may be useful\nfor designing novel MW sensing devices.",
        "category": "physics_optics"
    },
    {
        "text": "Plasmonic Induced Potential in Metal-Semiconductor Composits: We utilize experimental tools such as conductive atomic force microscopy and\nelectrostatic force microscopy on photo-excited arrays of gold nanoparticles on\nindium tin oxide substrate. The nanoparticles are partially covered by a thin\nsemiconductor polymer. The change in the current and potential profiles of the\ncomposite metal-semiconductor sample after excitation at plasmonic resonance\nfrequency of metallic nanoparticles is analyzed.",
        "category": "physics_optics"
    },
    {
        "text": "Bound States in the Continuum in Multipolar Lattices: We develop a theory of bound states in the continuum (BICs) in multipolar\nlattices -- periodic arrays of resonant multipoles. We predict that BICs are\ncompletely robust to changes in lattice parameters remaining pinned to specific\ndirections in the $k$-space. The lack of radiation for BICs in such structures\nis protected by the symmetry of multipoles forming the lattice. We also show\nthat some multipolar lattices can host BICs forming a continuous line in the\n$k$-space and such BICs carry zero topological charge. The developed approach\nsets a direct fundamental relation between the topological charge of BIC and\nthe asymptotic behavior of the Q-factor in its vicinity. We believe that our\ntheory is a significant step towards gaining deeper insight into the physics of\nBICs and the engineering of high-Q states in all-dielectric metasurfaces.",
        "category": "physics_optics"
    },
    {
        "text": "All-dielectric metasurface cylindrical lens: Conventional optical components have been proposed to realize high-quality\nline focusing with uniform intensity distribution such as cylindrical lenses,\nsegmented wedge-arrays, or a combination of prisms and spherical mirrors.\nHowever, numerous factors such as the manufacturing tolerances of conventional\nlenses or the need for precise alignment of the lenses cause wavefront\naberrations that impact the performance of optical systems. These\naforementioned limitations of conventional optical components affect the\nuniformity of the intensity distribution. Here, we numerically and\nexperimentally demonstrate an integrable planar all-dielectric cylindrical lens\nfor uniform line focusing. The lens has a NA of 0.247 and a measured uniformity\nof 0.92% at 800 nm.",
        "category": "physics_optics"
    },
    {
        "text": "Progress of the Volume FEL (VFEL) experiments in millimeter range: Use of non one-dimensional distributed feedback in VFEL gives possibility of\nfrequency tuning in wide range. In present work dependence of lasing process on\nthe angle between resonant diffraction grating grooves and direction of\nelectron beam velocity is discussed.",
        "category": "physics_optics"
    },
    {
        "text": "Temporal Walk-off Induced Dissipative Quadratic Solitons: A plethora of applications have recently motivated extensive efforts on the\ngeneration of low noise Kerr solitons and coherent frequency combs in various\nplatforms ranging from fiber to whispering gallery and integrated microscale\nresonators. However, the Kerr (cubic) nonlinearity is inherently weak, and in\ncontrast, strong quadratic nonlinearity in optical resonators is expected to\nprovide an alternative means for soliton formation with promising potential.\nHere, we demonstrate the formation of a dissipative quadratic soliton via\nnon-stationary optical parametric amplification in the presence of significant\ntemporal walk-off between pump and signal leading to half-harmonic generation\naccompanied by a substantial pulse compression (exceeding a factor of 40) at\nlow pump pulse energies ($\\sim$ 4 picojoules). The bright quadratic soliton\nforms in a low-finesse cavity in both normal and anomalous dispersion regimes,\nwhich is in stark contrast with bright Kerr solitons. We present a route to\nsignificantly improve the performance of the demonstrated quadratic soliton\nwhen extended to an integrated nonlinear platform to realize highly-efficient\nextreme pulse compression leading to the formation of few-cycle soliton pulses\nstarting from ultra-low energy picosecond scale pump pulses that are widely\ntunable from ultra-violet to mid-infrared spectral regimes.",
        "category": "physics_optics"
    },
    {
        "text": "Active Loss Engineering in Vanadium Dioxide Based BIC Metasurfaces: Metasurfaces have unlocked significant advancements across photonics, yet\ntheir efficient active control remains challenging. The active materials\nrequired often lack continuous tunability, exhibit inadequate refractive index\n(RI) changes, or suffer from high losses. These aspects pose an inherent\nlimitation for resonance-shifting based switching: when RI changes are small,\nthe resulting shift is also minor. Conversely, high RI changes typically come\nwith high intrinsic losses necessitating broad modes because narrow ones cannot\ntolerate such losses. Therefore, larger spectral shifts are required to\neffectively detune the modes. This paper introduces a novel active metasurface\napproach that converts the constraint of high intrinsic losses into a\nbeneficial feature. This is achieved by controlling the losses in a hybrid\nvanadium dioxide (VO$_{2}$) - silicon metasurface, supporting\nsymmetry-protected bound states in the continuum (BICs) within the infrared\nspectrum. By leveraging the temperature-controlled losses in VO$_{2}$ and\ncombining them with the inherent far-field-coupling tunability of BICs, we gain\nunprecedented precision in independently controlling both the radiative and\nnonradiative losses of the resonant system. Our dual-control mechanism allows\nus to optimize our metasurfaces and we experimentally demonstrate quality\nfactors above 200, a maximum reflectance amplitude of 90%, a relative switching\ncontrast of 78%, and continuous tuning from under- to over-coupling within the\ninfrared spectral range. This study provides a foundation for experimentally\nand technologically simple, fine-tunable, active metasurfaces for applications\nranging from molecular sensors to filters and optical modulators.",
        "category": "physics_optics"
    },
    {
        "text": "Resonant-state expansion applied to three-dimensional open optical\n  systems: A complete set of static modes: We present two alternative complete sets of static modes of a homogeneous\ndielectric sphere, for their use in the resonant-state expansion (RSE), a\nrigorous perturbative method in electrodynamics. Physically, these modes are\nneeded to correctly describe the static electric field of a charge\nredistribution within the optical system due to a perturbation of the\npermittivity. We demonstrate the convergence of the RSE towards the exact\nresult for a perturbation describing a size reduction of the basis sphere. We\nthen revisit the quarter-sphere perturbation treated in [Doost {\\it et al.},\nPhys. Rev. A {\\bf 90}, 013834 (2014)], where only a single static mode per each\nangular momentum was introduced, and show that using a complete set of static\nmodes leads to a small, though non-negligible correction of the RSE result,\nimproving the agreement with finite-element simulations. As another example of\napplying the RSE with a complete set of static modes, we calculate the resonant\nstates of a dielectric cylinder, also comparing the result with a\nfinite-element simulation.",
        "category": "physics_optics"
    },
    {
        "text": "Frequency comb based four-wave-mixing spectroscopy: We experimentally demonstrate four-wave-mixing spectroscopy using frequency\ncombs. The experiment uses a geometry where excitation pulses and\nfour-wave-mixing signals generated by a sample co-propagate. We separate them\nin the radio frequency domain by heterodyne detection with a local oscillator\ncomb that has a different repetition frequency.",
        "category": "physics_optics"
    },
    {
        "text": "Resonant Critical Coupling of Surface Lattice Resonances with\n  Fluorescent Absorptive Thin Film: Surface lattice resonance supported on nanoparticle arrays is a promising\ncandidate in enhancing fluorescent effects in both absorption and emission. The\noptical enhancement provided by surface lattice resonance is primarily through\nthe light confinement beyond the diffraction limit, where the nanoparticle\narrays can enhance light-matter interaction for increased absorption as well as\nproviding more local density of states for enhanced spontaneous emission. In\nthis work, we optimize the in-coupling efficiency to the fluorescent molecules\nby finding the conditions to maximize the absorption, also known as the\ncritical coupling condition. We studied the transmission characteristics and\nthe fluorescent emission of a $TiO_2$ nanoparticle array embedded in an\nindex-matching layer with fluorescent dye at various concentrations. A modified\ncoupled-mode theory that describes the nanoparticle array was then derived and\nverified by numerical simulations. With the analytical model, we analyzed the\nexperimental measurements and discovered the condition to critically couple\nlight into the fluorescent dye, which is demonstrated as the strongest\nemission. This study presents a useful guide for designing efficient energy\ntransfer from excitation beam to the emitters, which maximizes the external\nconversion efficiency.",
        "category": "physics_optics"
    },
    {
        "text": "Synchronously controlled optical modes in the transmittance and\n  reflectance spectra of multilayer photonic structure with dual-frequency\n  nematic liquid crystal: A method for the electrically controlled synchronous mode tuning in the\ntransmittance and reflectance spectra of a photonic structure consisting of an\nasymmetric dielectric Fabry-Perot microcavity and an ultrathin metallic film\nhas been proposed. The excitation of a broadband Tamm plasmon-polariton at the\nmetal-Bragg mirror interface is accompanied by the two phenomena opposing one\nanother. Due to the strong absorption induced by the metal, a sufficient\nrejection level in reflection of the off-resonant radiation has been obtained\nin a wide spectral range, which almost coincides with the photonic band gap,\nwhile at the cavity mode frequencies the structure becomes transparent for the\nreflected radiation. This leads to the appearance of a series of narrow\nresonance peaks in the reflectance spectra that coincide in frequency with the\ntransmittance peaks. Based on the structural transformations in the\ndual-frequency nematic liquid crystal used as a photonic structure defect, the\ntuning of the modes corresponding to the extraordinary waves to both the short-\nand long-wavelength spectral ranges has been implemented.",
        "category": "physics_optics"
    },
    {
        "text": "Amplified optomechanics in a unidirectional ring cavity: We investigate optomechanical forces on a nearly lossless scatterer, such as\nan atom pumped far off-resonance or amicromirror, inside an optical ring\ncavity. Our model introduces two additional features to the cavity: an isolator\nis used to prevent circulation and resonant enhancement of the pump laser field\nand thus to avoid saturation of or damage to the scatterer, and an optical\namplifier is used to enhance the effective $Q$-factor of the counterpropagating\nmode and thus to increase the velocity-dependent forces by amplifying the\nback-scattered light. We calculate friction forces, momentum diffusion, and\nsteady-state temperatures to demonstrate the advantages of the proposed setup.",
        "category": "physics_optics"
    },
    {
        "text": "Broadband three-mode converter and multiplexer based on cascaded\n  symmetric Y-junctions and subwavelength engineered MMI and phase shifters: Mode-division multiplexing has emerged as a promising route for increasing\ntransmission capacity while maintaining the same level of on-chip integration.\nDespite the large number of on-chip mode converters and multiplexers reported\nfor the silicon-on-insulator platform, scaling the number of multiplexed modes\nis still a critical challenge. In this paper, we present a novel three-mode\narchitecture based on multimode interference couplers, passive phase shifters\nand cascaded symmetric Y-junctions. This architecture can readily operate up to\nthe third-order mode by including a single switchable phase shifter. Moreover,\nwe exploit subwavelength grating metamaterials to overcome bandwidth\nlimitations of multimode interference couplers and phase shifters, resulting in\na simulated bandwidth of 161 nm with insertion loss and crosstalk below 1.18 dB\nand -20 dB, respectively.",
        "category": "physics_optics"
    },
    {
        "text": "Inverse Design of Nanophotonic Devices using Dynamic Binarization: The complexity of applications addressed with photonic integrated circuits is\nsteadily rising and poses increasingly challenging demands on individual\ncomponent functionality, performance and footprint. Inverse design methods have\nrecently shown great promise to address these demands using fully automated\ndesign procedures that enable access to non-intuitive device layouts beyond\nconventional nanophotonic design concepts. Here we present a dynamic\nbinarization method for the objective-first algorithm that lies at the core of\nthe currently most successful inverse design algorithms. Our results\ndemonstrate significant performance advantages over previous implementations of\nobjective first algorithms, which we show for a fundamental TE00 to TE20\nwaveguide mode converter both in simulation and in experiments with fabricated\ndevices.",
        "category": "physics_optics"
    },
    {
        "text": "Impact of Si nanocrystals in a-SiOx<Er> in C-Band emission for\n  applications in resonators structures: Si nanocrystals (Si-NC) in a-SiOx<Er> were created by high temperature\nannealing. Si-NC samples have large emission in a broadband region, 700nm to\n1000nm. Annealing temperature, annealing time, substrate type, and erbium\nconcentration is studied to allow emission at 1550 nm forsamples with erbium.\nEmission in the C-Band region is largely reduced by the presence of Si-NC. This\nreduction may be due to less efficient energy transfer processes from the\nnanocrystals than from the amorphous matrix to the Er3+ ions, perhaps due to\nthe formation of more centro-symmetric Er3+ sites at the nanocrystal surfaces\nor to very different optimal erbium concentrations between amorphous and\ncrystallized samples.",
        "category": "physics_optics"
    },
    {
        "text": "Shapeshifting diffractive optical devices: In optical devices like diffraction gratings and Fresnel lenses, light\nwavefront is engineered through the structuring of device surface morphology,\nwithin thicknesses comparable to the light wavelength. Fabrication of such\ndiffractive optical elements involves highly accurate multi-step lithographic\nprocesses that in fact set into stone both the device morphology and optical\nfunctionality. In this work, we introduce shapeshifting diffractive optical\nelements directly written on an erasable photoresist. We first develop a\nlithographic configuration that allows writing/erasing cycles of aligned\noptical elements directly in the light path. Then, we show the realization of\ncomplex diffractive gratings with arbitrary combinations of grating vectors.\nFinally, we demonstrate a shapeshifting diffractive lens that is reconfigured\nin the light-path in order to change the imaging parameters of an optical\nsystem.",
        "category": "physics_optics"
    },
    {
        "text": "Dipole-dipole interaction in random electromagnetic fields: We demonstrate that a non-vanishing interaction force exists between pairs of\ninduced dipoles in random, statistically stationary electromagnetic field. This\nnew type of optical binding force leads to long-range interaction between\ndipolar particles even when placed in spatially incoherent fields. We also\ndiscuss several unique features of dipole-dipole interaction in spatially\nincoherent Gaussian fields.",
        "category": "physics_optics"
    },
    {
        "text": "Big Crunch-based omnidirectional light concentrators: Omnidirectional light concentration remains an unsolved problem despite such\nimportant practical applications as design of efficient mobile photovoltaic\ncells. Optical black hole designs developed recently offer partial solution to\nthis problem. However, even these solutions are not truly omnidirectional since\nthey do not exhibit a horizon, and at large enough incidence angles light may\nbe trapped into quasi-stationary orbits around such imperfect optical black\nholes. Here we propose and realize experimentally another gravity-inspired\ndesign of a broadband omnidirectional light concentrator based on the\ncosmological Big Crunch solutions. By mimicking the Big Crunch spacetime via\ncorresponding effective optical metric we make sure that every photon world\nline terminates in a single point.",
        "category": "physics_optics"
    },
    {
        "text": "Carrier density driven lasing dynamics in ZnO nanowires: We report on the temporal lasing dynamics of high quality ZnO nanowires using\ntime-resolved micro-photoluminescence technique. The temperature dependence of\nthe lasing characteristics and of the corresponding decay constants demonstrate\nthe formation of an electron-hole plasma to be the underlying gain mechanism in\nthe considered temperature range from 10 K to 300 K. We found that the\ntemperature dependent emission onset-time ($t_{\\text{on}}$) strongly depends on\nthe excitation power and becomes smallest in the lasing regime, with values\nbelow 5 ps. Furthermore, the observed red shift of the dominating lasing modes\nin time is qualitatively discussed in terms of the carrier density induced\nchange of the refractive index dispersion after the excitation laser pulse.\nThis theory is supported by extending an existing model for the calculation of\nthe carrier density dependent complex refractive index for different\ntemperatures. This model coincides with the experimental observations and\nreliably describes the evolution of the refractive index after the excitation\nlaser pulse.",
        "category": "physics_optics"
    },
    {
        "text": "Limit of light coupling strength in solar cells: We introduce a limit for the strength of coupling light into the modes of\nsolar cells. This limit depends on both a cell's thickness and its modal\nproperties. For a cell with refractive index n and thickness d, we obtain a\nmaximal coupling rate of 2c*sqrt(n^2-1)/d where c is speed of light. Our method\ncan be used in the design of solar cells and in calculating their efficiency\nlimits; besides, it can be applied to a broad variety of resonant phenomena and\ndevices.",
        "category": "physics_optics"
    },
    {
        "text": "Tunability of Spin-Dependent Secondary Topological Interface States\n  Induced in an Optical Complex Superlattice: The past decade has witnessed a booming development of topological photonics,\nwhich revolutionizes the methodology for controlling the behavior of light. A\ngigantic achievement is to engineer robust confined modes localized at\ninterfaces between topologically distinct regions, where the optical context\ncan trigger exotic topological phenomena exclusive to photons. Here, we provide\nan experimentally flexible approach to engineering topologically induced\ninterface states in the visible regime via a unique design of complex\nsuperlattice formed by connecting two component superlattices of distinguished\ntopological phases. Assisted by the intrinsic pseudospin degree due to the\nsplitting between TM and TE polarized modes, we attain a precise manipulation\nof the spin-dependent topological interface states that can manifest themselves\nstraightforwardly through transmission spectra. More specifically, since these\ntopological localized modes stem from the hybridization of artificial photonic\norbitals that are of topological origin as well, they are deemed as a novel\ntopological effect and thus named as the secondary topological interface\nstates. Our work develops an innovative and productive strategy to tune\ntopologically protected localized modes, based on which various applications\nsuch as selective local enhancement can be exploited.",
        "category": "physics_optics"
    },
    {
        "text": "Helicity, Spin, and Infra-zilch of Light: a Lorentz Covariant\n  Formulation: In this paper, a novel conserved Lorentz covariant tensor, termed the\nhelicity tensor, is introduced in Maxwell theory. The conservation of the\nhelicity tensor expresses the conservation laws contained in the helicity\narray, introduced by Cameron et al., including helicity, spin, and the\nspin-flux or infra-zilch. The Lorentz covariance of the helicity tensor is in\ncontrast to previous formulations of the helicity hierarchy of conservation\nlaws, which required the non-Lorentz covariant transverse gauge. The helicity\ntensor is shown to arise as a Noether current for a variational symmetry of a\nduality-symmetric Lagrangian for Maxwell theory. This symmetry transformation\ngeneralizes the duality symmetry and includes the symmetry underlying the\nconservation of the spin part of the angular momentum.",
        "category": "physics_optics"
    },
    {
        "text": "Radiative heat transfer in a parallelogram shaped cavity: An exact analytical description of the internal radiative field inside an\nemitting-absorbing gray semi-transparent medium enclosed in a two-dimensional\nparallelogram cavity is proposed. The expressions of the incident radiation and\nthe radiative flux field are angularly and spatially discretized with a double\nGauss quadrature, and the temperature field is obtained by using an iterative\nprocess. Some numerical solutions are tabulated and graphically presented as\nthe benchmark solutions. Temperature and two components of the radiative flux\nare finally sketched on the whole domain. It is shown that the proposed method\ngives perfectly smooth results.",
        "category": "physics_optics"
    },
    {
        "text": "Resonant chiral effects in nonlinear dielectric metasurfaces: We study the resonant enhancement of linear and nonlinear chiroptical effects\nin asymmetric silicon metasurfaces supporting multipolar Mie resonances and\nquasi-bound states in the continuum (quasi-BICs). We demonstrate theoretically\nand observe in experiment the pronounced linear circular dichroism at the\nquasi-BIC resonances. We further find that both local field enhancement and\nthird-harmonic signal are large for Mie resonances and some quasi-BIC modes. We\nexplain the selectivity of the nonlinear enhancement by employing the concept\nof critical coupling being more favorable for the modes with moderately large\nradiative quality factors ($Q$ factors). We demonstrate experimentally strong\nnonlinear chiroptical response associated with high efficiency of the\nthird-harmonic generation and large nonlinear circular dichroism varying from\n$+0.918\\pm0.049$ to $-0.771\\pm0.004$ for the samples with different\nasymmetries. We believe our results suggest a general strategy for engineering\nnonlinear chiroptical response in dielectric resonant metasurfaces.",
        "category": "physics_optics"
    },
    {
        "text": "Enhanced high-order harmonic generation from periodic potentials in\n  inhomogeneous laser fields: We theoretically study high-order harmonic generation (HHG) from solid-phase\nsystems in spatially inhomogeneous strong laser fields originated by resonant\nplasmons within a metallic nanostructure. The intensity of the second plateau\nin HHG may be enhanced by two-three orders and be comparable with the intensity\nof the first plateau. This is due to bigger transition probabilities to higher\nconduction bands. It provides us a practical way to increase the yields of HHG\nwith laser intensity below the damage threshold. It presents a promising way to\ntriple the range of HHG spectra in experimental measurements. It also allows to\ngenerate intense isolated attosecond pulse from solids driven by few-cycle\nlaser fields.",
        "category": "physics_optics"
    },
    {
        "text": "Laser engineering of biomimetic surfaces: The exciting properties of micro- and nano-patterned surfaces found in\nnatural species hide a virtually endless potential of technological ideas,\nopening new opportunities for innovation and exploitation in materials science\nand engineering. Due to the diversity of biomimetic surface functionalities,\ninspirations from natural surfaces are interesting for a broad range of\napplications in engineering, including phenomena of adhesion, friction, wear,\nlubrication, wetting phenomena, self-cleaning, antifouling, antibacterial\nphenomena, thermoregulation and optics. Lasers are increasingly proving to be\npromising tools for the precise and controlled structuring of materials at\nmicro- and nano-scales. When ultrashort-pulsed lasers are used, the optimal\ninterplay between laser and material parameters enables structuring down to the\nnanometer scale. Besides this, a unique aspect of laser processing technology\nis the possibility for material modifications at multiple (hierarchical) length\nscales, leading to the complex biomimetic micro- and nano-scale patterns, while\nadding a new dimension to structure optimization. This article reviews the\ncurrent state of the art of laser processing methodologies, which are being\nused for the fabrication of bioinspired artificial surfaces to realize\nextraordinary wetting, optical, mechanical, and biological-active properties\nfor numerous applications. The innovative aspect of laser functionalized\nbiomimetic surfaces for a wide variety of current and future applications is\nparticularly demonstrated and discussed. The article concludes with\nillustrating the wealth of arising possibilities and the number of new laser\nmicro/nano fabrication approaches for obtaining complex high-resolution\nfeatures, which prescribe a future where control of structures and subsequent\nfunctionalities are beyond our current imagination.",
        "category": "physics_optics"
    },
    {
        "text": "Flat-band localization and self-collimation of light in photonic\n  crystals: We investigate the optical properties of a photonic crystal composed of a\nquasi-one-dimensional flat-band lattice array through finite-difference\ntime-domain simulations. The photonic bands contain flat bands (FBs) at\nspecific frequencies, which correspond to compact localized states as a\nconsequence of destructive interference. The FBs are shown to be nondispersive\nalong the $\\Gamma\\rightarrow X$ line, but dispersive along the\n$\\Gamma\\rightarrow Y$ line. The FB localization of light in a single direction\nonly results in a self-collimation of light propagation throughout the photonic\ncrystal at the FB frequency.",
        "category": "physics_optics"
    },
    {
        "text": "Nonradiating Photonics with Resonant Dielectric Nanostructures: Nonradiating sources of energy have traditionally been studied in quantum\nmechanics and astrophysics, while receiving a very little attention in the\nphotonics community. This situation has changed recently due to a number of\npioneering theoretical studies and remarkable experimental demonstrations of\nthe exotic states of light in dielectric resonant photonic structures and\nmetasurfaces, with the possibility to localize efficiently the electromagnetic\nfields of high intensities within small volumes of matter. These recent\nadvances underpin novel concepts in nanophotonics, and provide a promising\npathway to overcome the problem of losses usually associated with metals and\nplasmonic materials for the efficient control of the light-matter interaction\nat the nanoscale. This review paper provides the general background and several\nsnapshots of the recent results in this young yet prominent research field,\nfocusing on two types of nonradiating states of light that both have been\nrecently at the center of many studies in all-dielectric resonant meta-optics\nand metasurfaces: optical {\\em anapoles} and photonic {\\em bound states in the\ncontinuum}. We discuss a brief history of these states in optics, their\nunderlying physics and manifestations, and also emphasize their differences and\nsimilarities. We also review some applications of such novel photonic states in\nboth linear and nonlinear optics for the nanoscale field enhancement, a design\nof novel dielectric structures with high-$Q$ resonances, nonlinear wave mixing\nand enhanced harmonic generation, as well as advanced concepts for lasing and\noptical neural networks.",
        "category": "physics_optics"
    },
    {
        "text": "Optical sorting and detection of sub-micron objects in a motional\n  standing wave: An extended interference pattern close to surface may result in both a\ntransmissive or evanescent surface fields for large area manipulation of\ntrapped particles. The affinity of differing particle sizes to a moving\nstanding wave light pattern allows us to hold and deliver them in a\nbi-directional manner and importantly demonstrate experimentally particle\nsorting in the sub-micron region. This is performed without the need of fluid\nflow (static sorting). Theoretical calculations experimentally confirm that\ncertain sizes of colloidal particles thermally hop more easily between\nneighboring traps. A new generic method is also presented for particle position\ndetection in an extended periodic light pattern and applied to characterization\nof optical traps and particle behavior",
        "category": "physics_optics"
    },
    {
        "text": "Terahertz Fano-like resonators based on free-standing metallic wire\n  woven meshes: Most periodic terahertz (THz) structures need a substrate to support; thus,\nadditional absorption occurs, resulting in a low quality (Q) factor.\nFree-standing structures that do not require any holder or substrate show high\nlevels of flexibility and stretchability and hence are well-suited for THz\napplications. In this work, a free-standing THz metal structure consisting of\nmetallic wire woven meshes is proposed and demonstrated. Experimental and\nnumerical results exhibit that this metallic mesh achieves a sharp Fano-like\nresonance dip, which has not been found in previous studies. Investigation\nresults indicate that the high Q Fano-like resonance dip comes from the\nsingle-layer metal bent wire because of its bending effect. The resonance field\nlongitudinally covers the input and output end faces due to the large field\nvolume of the woven meshes and benefits from near-field sensing applications.",
        "category": "physics_optics"
    },
    {
        "text": "Versatile high-speed confocal microscopy using a single laser beam: We present a new flexible high speed laser scanning confocal microscope and\nits extension by an astigmatism particle tracking device (APTV). Many standard\nconfocal microscopes use either a single laser beam to scan the sample at\nrelatively low overall frame rate, or many laser beam to simultaneously scan\nthe sample and achieve a high overall frame rate. Single-laser-beam confocal\nmicroscope often use a point detector to acquire the image. To achieve high\noverall frame rates, we use, next to the standard 2D probe scanning unit, a\nsecond 2D scan unit projecting the image directly on a 2D CCD-sensor (re-scan\nconfiguration). Using only a single laser beam eliminates cross-talk and leads\nto an imaging quality that is independent of the frame rate with a lateral\nresolution of 0.235\\unit{\\mu m}. The design described here is suitable for high\nframe rate, i.e., for frame rates well above video rate (full frame) up to a\nline rate of 32kHz. The dwell time of the laser focus on any spot in the sample\n(122ns) is significantly shorter than in standard confocal microscopes (in the\norder of milli or microseconds). This short dwell time reduces phototoxicity\nand bleaching of fluorescent molecules. The new design opens further\nflexibility and facilitates coupling to other optical methods. The setup can\neasily be extended by an APTV device to measure three dimensional dynamics\nwhile being able to show high resolution confocal structures. Thus one can use\nthe high resolution confocal information synchronized with an APTV dataset.",
        "category": "physics_optics"
    },
    {
        "text": "High-order (N=4-6) multi-photon absorption and mid-infrared Kerr\n  nonlinearity in GaP, ZnSe, GaSe, and ZGP crystals: We report a study of high-order multi-photon absorption, nonlinear\nrefraction, and their anisotropy in four notable mid-infrared \\c{hi}(2)\ncrystals: GaP, ZnSe, GaSe and ZGP using the Z- scan method and 2.35-{\\mu}m\nfemtosecond pulses with peak intensity in excess of 200 GW/cm2. We found that\nthe nonlinear absorption obeys a perturbation model with multi-photon\nabsorption (MPA) orders from N = 4 to 6, in good agreement with the bandgaps of\nthe crystals. A study of the role of free carrier absorption, performed by\nvarying the pulse duration between 30 and 70 fs while maintaining a constant\npeak intensity showed that at our intensity levels, free carriers generated in\nthe process of MPA absorb much stronger than would be expected from their\nlinear absorption cross section. Possible mechanisms include high-field\neffects, such as intra-valley scattering in the conduction band and absorption\nto higher lying bands. Nonlinear refractive indices were measured using (i)\nclosed aperture Z-scan and (ii) spectral broadening due to self-phase\nmodulation, both methods agreeing well with each other.",
        "category": "physics_optics"
    },
    {
        "text": "Nonlinear coupled-mode theory for periodic plasmonic waveguides and\n  metamaterials with loss and gain: We derive general coupled-mode equations describing the nonlinear interaction\nof electromagnetic modes in media with loss and gain. Our approach is\nrigorously based on the Lorentz reciprocity theorem, and it can be applied to a\nbroad range of metal-dielectric photonic structures, including plasmonic\nwaveguides and metamaterials. We verify that our general results agree with the\nprevious analysis of particular cases, and predict novel effects on self- and\ncross-phase modulation in multi-layer nonlinear fishnet metamaterials.",
        "category": "physics_optics"
    },
    {
        "text": "Diode-laser-pumped continuous-wave mid-infrared optical parametric\n  oscillator: We report a singly resonant continuous-wave optical parametric oscillator\n(SRO), which is pumped by a semiconductor laser system and operates in the\nmid-infrared region (2.9 to 3.6 {\\mu}m). Fast tuning of the mid-infrared output\nwavelength by more than 200 nm is demonstrated by scanning the pump laser\nwavelength through only 1.25 nm, without any need to adjust the SRO settings.\nThe exceptionally large tuning range is obtained by choosing a pump wavelength\n(780 nm) that corresponds to a turning point of the group velocity mismatch\nbetween the phase-matched signal and idler waves of the SRO.",
        "category": "physics_optics"
    },
    {
        "text": "Resonant mode approximation of the scattering matrix of photonic crystal\n  slabs near several Wood-Rayleigh anomalies: The resonant mode approximation of the scattering matrix is considered for\ncalculating the optical properties of multilayered periodic structures within\nthe formalism of the Fourier-modal method for two diffraction thresholds in\nclose proximity of the spectral-angular range of interest. The developed\napproximation opens up possibilities for the fast calculation of the scattering\nmatrix of these structures when describing the integral characteristics of\nspectra and dispersion curves containing high-Q resonances, such as bound\nstates in the continuum.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of Brewster's effect for transverse-electric electromagnetic\n  waves in metamaterials: Experiment and theory: We have experimentally realized Brewster's effect for transverse-electric\nwaves with metamaterials. In dielectric media, Brewster's no-reflection effect\narises only for transverse-magnetic waves. However, it has been theoretically\npredicted that Brewster's effect arises for TE waves under the condition that\nthe relative permeability r is not equal to unity. We have designed an array of\nsplit-ring resonators as a metamaterial with mu_r 1 using a finite-difference\ntime-domain method. The reflection measurements were carried out in a 3-GHz\nregion and the disappearance of reflected waves at a particular incident angle\nwas confirmed.",
        "category": "physics_optics"
    },
    {
        "text": "Scattering properties of PT-symmetric chiral metamaterials: The combination of gain and loss in optical systems that respect parity-time\n(PT)-symmetry has pointed recently to a variety of novel optical phenomena and\npossibilities. Many of them can be realized by combining the PT-symmetry\nconcepts with metamaterials. Here we investigate the case of chiral\nmetamaterials, showing that combination of chiral metamaterials with\nPT-symmetric gain-loss enables a very rich variety of phenomena and\nfunctionalities. Examining a simple one-dimensional chiral PT-symmetric system,\nwe show that with normally incident waves the PT-symmetric and the\nchirality-related characteristics can be tuned independently and superimposed\nalmost at will. On the other hand, under oblique incidence, chirality affects\nall the PT-related characteristics, leading also to novel and uncommon wave\npropagation features, such as asymmetric transmission and asymmetric optical\nactivity and ellipticity. All these features are highly controllable both by\nchirality and by the angle of incidence, making PT-symmetric chiral\nmetamaterials valuable in a large range of polarization-control-seeking\napplications.",
        "category": "physics_optics"
    },
    {
        "text": "Closed-form solutions and scaling laws for Kerr frequency combs: A single closed-form analytical solution of the driven nonlinear\nSchr\\\"{o}dinger equation is developed, reproducing a large class of the\nbehaviors in Kerr-comb systems, including bright-solitons, dark-solitons, and a\nlarge class of periodic wavetrains. From this analytical framework, a Kerr-comb\narea theorem and a pump-detuning relation are developed, providing new insights\ninto soliton- and wavetrain-based combs along with concrete design guidelines\nfor both. This new area theorem reveals significant deviation from the\nconventional soliton area theorem, which is crucial to understanding cavity\nsolitons in certain limits. Moreover, these closed-form solutions represent the\nfirst step towards an analytical framework for wavetrain formation, and reveal\nnew parameter regimes for enhanced Kerr-comb performance.",
        "category": "physics_optics"
    },
    {
        "text": "Generation of arbitrary cylindrical vector beams on the higher order\n  Poincare sphere: We propose and experimentally demonstrate a novel interferometric approach to\ngenerate arbitrary cylindrical vector beams on the higher order Poincare\nsphere. Our scheme is implemented by collinear superposition of two orthogonal\ncircular polarizations with opposite topological charges. By modifying the\namplitude and phase factors of the two beams, respectively, any desired vector\nbeams on the higher order Poincare sphere with high tunability can be acquired.\nOur research provides a convenient way to evolve the polarization states in any\npath on the high order Poincare sphere.",
        "category": "physics_optics"
    },
    {
        "text": "Flux trajectory analysis of Airy-type beams: Airy beams are solutions to the paraxial Helmholtz equation known for\nexhibiting shape invariance along their self-accelerated propagation in free\nspace. These two properties are associated with the fact that they are not\nsquare integrable, that is, they carry infinite energy. To circumvent this\ndrawback, families of so-called finite-energy Airy-type beams have been\nproposed in the literature and, in some cases, also implemented in the\nlaboratory. Here an analysis of the propagation of this type of structured\nlight beams is presented from a flux trajectory perspective with the purpose to\nbetter understand the mechanisms that make infinite and finite energy beams to\nexhibit different behaviors. As it is shown, while the foremost part of the\nbeam can be clearly and unambiguously associated with the well-known\naccelerating term, the rear part of the beam corresponds to a nearly\nhomogeneous distribution of flow trajectories, particularly for large\npropagation distances. This is shown to be related with an effective transfer\nof trajectories between adjacent lobes (gradually, from the fore part of the\nbeam to its rear part), which leads to smearing out the transverse flow along\nthe rear part of the beam. This is sharp contrast with the situation found in\nideal Airy beams, where trajectories belonging to a given lobe of the intensity\ndistribution remain the same all along the propagation. The analysis is\nsupplemented with an also trajectory-based description of Young's experiment\nperformed with finite-energy Airy beams in order to provide a dynamical\nunderstanding of the autofocusing phenomenon observed with circular Airy beams.",
        "category": "physics_optics"
    },
    {
        "text": "Probing the ultrafast gain and refractive index dynamics of a VECSEL: Typically, strong gain saturation and gain dynamics play a crucial role in\nsemiconductor laser mode-locking. While there have been several investigations\nof the ultrafast gain dynamics in vertical-external-cavity surface-emitting\nlasers (VECSELs), little is known about the associated refractive index\nchanges. Yet, such refractive index changes do not only have a profound impact\non the pulse formation process leading to self-phase modulation, which needs to\nbe compensated by dispersion, but they are also of particular relevance for\nassessing the feasibility of Kerr-lens mode-locking of VECSELs. Here, we\nmeasure both refractive index as well as gain dynamics of a VECSEL chip using\nthe ultrafast beam deflection method. We find that, in contrast to the gain\ndynamics, the refractive index dynamics is dominated by an instantaneous\n($\\sim$100~fs) and a very slow component ($\\sim$100~ps). The time-resolved\nmeasurement of nonlinear refraction allows us to predict a pulse-length\ndependent, effective nonlinear refractive index $n_{2,eff}$, which is shown to\nbe negative and in the order of $10^{-16}$ $m^2/W$ for short pulse lengths\n($\\sim$100~fs) . It becomes positive for large excitation fluences and large\npulse lengths (few ps). These results agree with some previous reports of\nself-mode-locked VECSELs for which the cavity design and pulse properties\ndetermine sign and strength of the nonlinear refractive index when assuming\nKerr-lens mode-locking.",
        "category": "physics_optics"
    },
    {
        "text": "Generation of 1.5-octave intense infrared pulses by nonlinear\n  interactions in DAST crystal: Infrared pulses with large spectral width extending from 1.2 to 3.4 um are\ngenerated in the organic crystal DAST (4-N,\nN-dimethylamino-4-N-methylstilbazolium tosylate). The input pulse has a central\nwavelength of 1.5 um and 65 fs duration. With 2.8 mJ input energy we obtained\nup to 700 uJ in the broadened spectrum. The output can be easily scaled up in\nenergy by increasing the crystal size together with the energy and the beam\nsize of the pump. The ultra-broad spectrum is ascribed to cascaded second order\nprocesses mediated by the exceptionally large effective chi2 nonlinearity of\nDAST, but the shape of the spectrum indicates that a delayed chi3 process may\nalso be involved. Numerical simulations reproduce the experimental results\nqualitatively and provide an insight in the mechanisms underlying the\nasymmetric spectral broadening.",
        "category": "physics_optics"
    },
    {
        "text": "Pulse chirp increasing pulse compression followed by positive resonant\n  radiation in fibers: Pulse self-compression followed by the generation of resonant radiation is a\nwell known phenomenon in non-linear optics. Resonant radiation is important as\nit allows for efficient and tunable wavelength conversion. We vary the chirp of\nthe initial pulse and find in simulations and experiments that a small positive\nchirp enhances the pulse compression and strongly increases the generation of\nresonant radiation. This result corroborates previously published simulation\nresults indicating an improved degree of pulse compression for a small positive\nchirp [1]. It also demonstrates how pulse evolution can be studied without\ncutting back the fiber.",
        "category": "physics_optics"
    },
    {
        "text": "Engineering of orbital angular momentum supermodes in coupled optical\n  waveguides: In this work we demonstrate the existence of orbital angular momentum (OAM)\nbright and dark supermodes in a three-evanescently coupled cylindrical\nwaveguides system. Bright and dark supermodes are characterized by their\ncoupling and decoupling from one of the waveguides, respectively. In addition,\nwe demonstrate that complex couplings between modes of different waveguides\nappear naturally due to the characteristic spiral phase-front of OAM modes in\ntwo-dimensional configurations where the waveguides are arranged forming a\ntriangle. Finally, by adding dissipation to the waveguide uncoupled to the dark\nsupermode, we are able to filter it out, allowing for the design of OAM mode\nclonners and inverters.",
        "category": "physics_optics"
    },
    {
        "text": "Generation of arbitrary complex quasi-non-diffracting optical patterns: Due to their unique ability to maintain an intensity distribution upon\npropagation, non-diffracting light fields are used extensively in various areas\nof science, including optical tweezers, nonlinear optics and quantum optics, in\napplications where complex transverse field distributions are required.\nHowever, the number and type of rigorously non-diffracting beams is severely\nlimited because their symmetry is dictated by one of the coordinate system\nwhere the Helmholtz equation governing beam propagation is separable. Here, we\ndemonstrate a powerful technique that allows the generation of a rich variety\nof quasi-non-diffracting optical beams featuring nearly arbitrary intensity\ndistributions in the transverse plane. These can be readily engineered via\nmodifications of the angular spectrum of the beam in order to meet the\nrequirements of particular applications. Such beams are not rigorously\nnon-diffracting but they maintain their shape over large distances, which may\nbe tuned by varying the width of the angular spectrum. We report the generation\nof unique spiral patterns and patterns involving arbitrary combinations of\ntruncated harmonic, Bessel, Mathieu, or parabolic beams occupying different\nspatial domains. Optical trapping experiments illustrate the opto-mechanical\nproperties of such beams.",
        "category": "physics_optics"
    },
    {
        "text": "Phase-controlled Fano resonance by the nanoscale optomechanics: Observation of the Fano line shapes is essential to understand properties of\nthe Fano resonance in different physical systems. We explore a tunable Fano\nresonance by tuning the phase shift in a Mach-Zehnder interferometer (MZI)\nbased on a single-mode nano-optomechanical cavity. The Fano resonance is\nresulted from the optomechanically induced transparency caused by a\nnano-mechanical resonator and can be tuned by applying an optomechanical MZI.\nBy tuning the phase shift in one arm of the MZI, we can observe the\nperiodically varying line shapes of the Fano resonance, which represents an\nelaborate manipulation of the Fano resonance in the nanoscale optomechanics.",
        "category": "physics_optics"
    },
    {
        "text": "Far-field characterization of the thermal dynamics in lasing\n  microspheres: This work reports the dynamical thermal behavior of lasing microspheres\nplaced on a dielectric substrate while they are homogeneously heated-up by the\ntop-pump laser used to excite the active medium. The lasing modes are collected\nin the far-field and their temporal spectral traces show characteristic\nlifetimes of about 2 ms. The latter values scale with the microsphere radius\nand are independent of the pump power in the studied range. Finite-Element\nMethod simulations reproduce the experimental results, revealing that the\nthermal dynamics is dominated by the heat dissipated towards the substrate\nthrough the medium surrounding the contact point. The characteristic system\nscale regarding thermal transport is of few hundreds of nanometers, thus\nenabling an effective toy model for investigating heat conduction in\nnon-continuum gaseous media and near-field radiative energy transfer.",
        "category": "physics_optics"
    },
    {
        "text": "All-optical nonlinear activation function based on stimulated Brillouin\n  scattering: Photonic neural networks have demonstrated their potential over the past\ndecades, but have not yet reached the full extent of their capabilities. One\nreason for this lies in an essential component - the nonlinear activation\nfunction, which ensures that the neural network can perform the required\narbitrary nonlinear transformation. The desired all-optical nonlinear\nactivation function is difficult to realize, and as a result, most of the\nreported photonic neural networks rely on opto-electronic activation functions.\nUsually, the sacrifices made are the unique advantages of photonics, such as\nresource-efficient coherent and frequency-multiplexed information encoding. In\naddition, opto-electronic activation functions normally limit the photonic\nneural network depth by adding insertion losses. Here, we experimentally\ndemonstrate an in-fiber photonic nonlinear activation function based on\nstimulated Brillouin scattering. Our design is coherent and frequency\nselective, making it suitable for multi-frequency neural networks. The\noptoacoustic activation function can be tuned continuously and all-optically\nbetween a variety of activation functions such as LeakyReLU, Sigmoid, and\nQuadratic. In addition, our design amplifies the input signal with gain as high\nas $20\\,\\mathrm{dB}$, compensating for insertion losses on the fly, and thus\npaving the way for deep optical neural networks.",
        "category": "physics_optics"
    },
    {
        "text": "Spatially multiplexed picosecond pulse-train generations through\n  simultaneous intra-modal four wave mixing and inter-modal cross-phase\n  modulation: We report on the experimental generation of spatially multiplexed picosecond\n40-GHz pulse trains at telecommunication wavelengths by simultaneous\nintra-modal multiple four wave mixing and intermodal cross-phase modulation in\nkm-long bi-modal and 6-LP-mode graded-index few-mode fibers. More precisely, an\ninitial beat-signal injected into the fundamental mode is first nonlinearly\ncompressed into well-separated pulses by means of an intra-modal multiple\nfour-wave mixing process, while several group-velocity matched continuous-wave\nprobe signals are injected into higher-order modes in such a way to develop\nsimilar pulsed profile thanks to an intermodal cross-phase modulation\ninteraction. Specifically, by simultaneously exciting three higher-order modes\n(LP11, LP02 and LP31) of a 6-LP-mode fiber along group-velocity matched\nwavelengths with the fundamental mode, four spatially multiplexed 40-GHz\npicosecond pulse-trains are generated at selective wavelengths with negligible\ncross-talks between all the modes.",
        "category": "physics_optics"
    },
    {
        "text": "Photonic crystal fiber for high resolution lensless in-line holographic\n  microscopy: We propose to use high numerical aperture single mode optical fibers like\nphotonic crystal fiber for lensless in-line holographic microscopy. Highly\ndivergent beam helps to overcome the spatial sampling limitation of the image\nsensor. In this paper, a submicron lateral resolution has been demonstrated,\nwith an imaging sensor of pixel pitch 1.12 micrometer and a photonic crystal\nfiber of mode field diameter 1.8 micrometer. In earlier methods of single-shot\nlensless imaging, submicron resolution has been obtained at very small working\ndistance and field of view. The proposed method improves the resolution without\ncompromising the working distance. A working distance of (but not limited to)\n~1.7 mm with a field of View ~1.4 mm has been demonstrated.",
        "category": "physics_optics"
    },
    {
        "text": "Moment-generating function method used to accurately evaluate the impact\n  of the linearized optical noise amplified by EDFAs: In a nonlinear optical fiber communication (OFC) system with signal power\nmuch stronger than noise power, the noise field in the fiber can be described\nby linearized noise equation (LNE). In this case, the noise impact on the\nsystem performance can be evaluated by moment-generating function (MGF) method.\nMany published MGF calculations were based on the LNE using continuous wave\n(CW) approximation, where the modulated signal needs to be artificially\nsimplified as an unmodulated signal. Results thus obtained should be treated\ncarefully. More reliable results can be obtained by improving the CW-based LNE\nwith the accurate LNE proposed by Holzlohner et al in Ref. [1]. In this work we\nshow that, for the case of linearized noise amplified by EDFAs, its MGF can be\ncalculated by obtaining the noise propagation information directly from the\naccurate LNE. Our results agree well with the experimental data of multi-span\nDPSK systems.",
        "category": "physics_optics"
    },
    {
        "text": "High-frequency and high-quality silicon carbide optomechanical\n  microresonators: Silicon carbide (SiC) exhibits excellent material properties attractive for\nbroad applications. We demonstrate the first SiC optomechanical microresonators\nthat integrate high mechanical frequency, high mechanical quality, and high\noptical quality into a single device. The radial-breathing mechanical mode has\na mechanical frequency up to 1.69 GHz with a mechanical Q around 5500 in\natmosphere, which corresponds to a mechanical f-Q product as high as 9.47x10^12\nHz. The strong optomechanical coupling allows us to efficiently excite and\nprobe the coherent mechanical oscillation by optical waves. The demonstrated\ndevices, in combination with the superior thermal property, chemical inertness,\nand defect characteristics of SiC, show great potential for applications in\nmetrology, sensing, and quantum photonics, particularly in harsh environments\nthat are challenging for other device platforms.",
        "category": "physics_optics"
    },
    {
        "text": "Kirchhoff's metasurfaces: Thermo-optical properties of the nanodisc and metal hole array plasmonic\nperfect absorber (PPA) metasurfaces were designed and characterised at\nmidinfrared wavelengths. Both, light emitter and detector systems are highly\nthought after for the future sensor networks in the internet-of-things for\nvarious spectral domains. Reciprocity of the absorbance and emittance is shown\nexperimentally, i.e., the PPAs are following Kirchhoff's law where the patterns\nexhibiting a strong optical absorption were found enhanced thermal radiation.\nDesign principles and scaling for photo-thermal conversion are discussed. The\nhighest efficiency of light-to-heat and heat-to-radiation were obtained for the\nAu-Si-Au structures.",
        "category": "physics_optics"
    },
    {
        "text": "Three-wave mixing with three incoming waves: Signal-Idler Coherent\n  Cancellation and Gain Enhancement in a Parametric Amplifier: Coherent, purely-dispersive three-wave mixing systems in optics and\nsuperconducting microwave circuits can be operated as parametric amplifiers,\ngenerating from a pump wave at one frequency amplified signal and idler waves\nat lower frequencies. Here we demonstrate the reciprocal process using a\nJosephson amplifier in which coherently imposed signal and idler beams\nup-convert to the pump frequency. For signal and idler beams strong enough to\nsignificantly deplete the pump, we show that this reciprocal process (\"coherent\ncancellation\") leads to large, phase-sensitive modulation and even enhancement\nof the amplifier gain, in good agreement with theoretical predictions.",
        "category": "physics_optics"
    },
    {
        "text": "Saturable absorption of free-electron laser radiation by graphite near\n  the carbon K-edge: The interaction of intense light with matter gives rise to competing\nnonlinear responses that can dynamically change material properties. Prominent\nexamples are saturable absorption (SA) and two-photon absorption (TPA), which\ndynamically increase and decrease the transmission of a sample depending on\npulse intensity, respectively. The availability of intense soft X-ray pulses\nfrom free-electron lasers (FEL) has led to observations of SA and TPA in\nseparate experiments, leaving open questions about the possible interplay\nbetween and relative strength of the two phenomena. Here, we systematically\nstudy both phenomena in one experiment by exposing graphite films to soft X-ray\nFEL pulses of varying intensity, with the FEL energy tuned to match carbon 1s\nto $\\pi^*$ or 1s to $\\sigma^*$ transitions. It is observed for lower\nintensities that the nonlinear contribution to the absorption is dominated by\nSA attributed to ground-state depletion; for larger intensities ($>10^{14}$\nW/cm$^2$), TPA becomes more dominant. The relative strengths of the two\nphenomena depend in turn on the specific transition driven by the X-ray pulse.\nBoth observations are consistent with our real-time electronic structure\ncalculations. Our results reveal the competing contributions of distinct\nnonlinear material responses to spectroscopic signals measured in the X-ray\nregime, demonstrating an approach of general utility for interpreting FEL\nspectroscopies.",
        "category": "physics_optics"
    },
    {
        "text": "3-D Metamaterials: Trends on Applied Designs, Computational Methods and\n  Fabrication Techniques: Metamaterials are artificially engineered devices that go beyond the\nproperties of conventional materials in nature. Metamaterials allow the\ncreation of negative refractive indexes, light trapping with epsilon-near-zero\ncompounds, bandgap selection, superconductivity phenomena, non-Hermitian\nresponses and, more generally, to manipulate the propagation of electromagnetic\nand acoustic waves. In the past, low computational resources and the lack of\nproper manufacturing techniques have limited the attention to 1-D and 2-D\nmetamaterials. However, the true potential of metamaterials will be ultimately\nreached in 3-D configurations, when the degrees of freedom associated to the\npropagating direction are finally exploited in design. This is expected to lead\nto a new era in metamaterial field, from which future high-speed and\nlow-latency communication networks can benefit. Here, a comprehensive overview\nof the past, present and future trends related to 3-D metamaterial devices is\npresented, focusing on efficient computational methods, innovative designs and\nfunctional manufacturing techniques.",
        "category": "physics_optics"
    },
    {
        "text": "Tighter spots of light with superposed orbital angular momentum beams: The possibility of focusing light to an ever tighter spot has important\nimplications for many applications and fields of optics research, such as\nnano-optics and plasmonics, laser-scanning microscopy, optical data storage and\nmany more. The size of lateral features of the field at the focus depends on\nseveral parameters, including the numerical aperture of the focusing system,\nbut also the wavelength and polarization, phase and intensity distribution of\nthe input beam. Here, we study the smallest achievable focal feature sizes of\ncoherent superpositions of two co-propagating beams carrying opposite orbital\nangular momentum. We investigate the feature sizes for this class of beams not\nonly in the scalar limit, but also use a fully vectorial treatment to discuss\nthe case of tight focusing. Both our numerical simulations and our experimental\nresults confirm that lateral feature sizes considerably smaller than those of a\ntightly focused Gaussian light beam can be observed. These findings may pave\nthe way for improving the resolution of imaging systems or may find\napplications in nano-optics experiments.",
        "category": "physics_optics"
    },
    {
        "text": "Electrical-Field Distributions in Waveguide Arrays - Exact and\n  Approximate: Five methods of calculating electrical field distributions in one dimensional\nwave-guide arrays are reviewed. We analytically solve the scalar Helmholtz\nEquation and, based on the computed Bloch functions and associated bands of\npropagation constants, generate the exact field distribution maps. For the\napproximated slowly varying envelope equation we show that the base Bloch\nfunctions are identical to those in the exact case, and study the differences\nin the bands of propagation constants. We demonstrate that by selecting the\nreference refractive index value, it is possible to minimize the error in\npropagation constants of any desired band. For the distributions calculated by\nthe coupled mode theory, we reveal the similarity and differences of the band\nmade of eigenvalues of the coupled mode equations matrix when compared to the\nfirst band of propagation constants found by the exact solution. Analysis of\ntwo numeric beam propagation methods shows that the relative accuracy of the\ncalculated field distributions of each of these methods depends on excitation\nconditions. The presented analysis of the slowly varying envelope equation\nprovides guide lines for selecting the value of the reference refractive index\nto be incorporated in these numeric methods where an analytic solution is\ndifficult to work out or in the frequently occurring cases where an analytic\nsolution does not exist at all.",
        "category": "physics_optics"
    },
    {
        "text": "Silicon Photonics: The Inside Story: The electronic chip industry embodies the height of technological\nsophistication and economics of scale. Fabricating inexpensive photonic\ncomponents by leveraging this mighty manufacturing infrastructure has fueled\nintense interest in silicon photonics. If it can be done economically and in an\nenergy efficient manner, empowering silicon with optical functionality will\nbring optical communications to the realm of computers where limitations of\nmetallic interconnects are threatening the industry's future. The field is\nmaking stunning progress and stands to have a bright future, as long as the\ncommunity recognizes the real challenges, and maintains an open mind with\nrespect to its applications. This talk will review recent 'game changing'\ndevelopments and discuss promising applications beyond data communication. It\nwill conclude with recent observation of extreme-value statistical behavior in\nsilicon photonics, a powerful example of how scientific discoveries can\nunexpectedly emerge in the course of technology development.",
        "category": "physics_optics"
    },
    {
        "text": "Photonic crystal nanofiber using an external grating: We implement a photonic crystal nanofiber device by reversibly combining an\noptical nanofiber and a nanofabricated grating. Using the finite-difference\ntime-domain method, we design the system for minimal optical loss while\ntailoring the resonant wavelength and bandwidth of the device. Experimentally\nwe demonstrate that the combined system shows a strong photonic stop-band in\ngood agreement with numerical predictions. The resulting device may be used to\nrealize strong light-matter coupling near to the nanofiber surface.",
        "category": "physics_optics"
    },
    {
        "text": "Describing meta-atoms using the exact higher-order polarizability\n  tensors: In nanophotonics, multipole framework has become an indispensable theoretical\ntool for analyzing subwavelength meta-atoms and their radiation properties.\nThis work presents higher-order exact dynamic polarizability (alpha) tensors,\nwhich can fully represent anisotropic meta-atoms with higher-order multipole\ntransitions. By using the irreducible exact Cartesian multipoles and field\ncomponents as the basis, the exact alpha-tensor rigorously reflects symmetry\ninformation of particles including reciprocity. In addition, the exact\nalpha-tensor can be obtained from T-matrix simply using basis transformation.\nFinally, we show that description of meta-atoms using alpha-tensors\nincorporated with multiple-scattering theory vastly extends the applicability\nof the multipole framework in nanophotonics, allowing accurate and efficient\ndepiction of complicated, random, multi-scale systems.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of nonlinear fractal higher-order topological insulator: Higher-order topological insulators (HOTIs) are unique materials hosting\ntopologically protected states, whose dimensionality is at least by a factor of\n2 lower than that of the bulk. Topological states in such insulators may be\nstrongly confined in their corners that leads to considerable enhancement of\nnonlinear processes involving such states. However, all nonlinear HOTIs\ndemonstrated so far were built on periodic bulk lattice materials. Here we\ndemonstrate first \\textit{nonlinear photonic} HOTI with the fractal origin.\nDespite their fractional effective dimensionality, the HOTIs constructed here\non two different types of the Sierpi\\'nski gasket waveguide arrays, may support\ntopological corner states for unexpectedly wide range of coupling strengths,\neven in parameter regions where conventional HOTIs become trivial. We\ndemonstrate thresholdless solitons bifurcating from corner states in nonlinear\nfractal HOTIs and show that their localization can be efficiently controlled by\nthe input beam power. We observe sharp differences in nonlinear light\nlocalization on outer and multiple inner corners and edges representative for\nthese fractal materials. Our findings not only represent a new paradigm for\nnonlinear topological insulators, but also open new avenues for potential\napplications of fractal materials to control the light flow.",
        "category": "physics_optics"
    },
    {
        "text": "Controllable atomic collision in a tight optical dipole trap: Single atoms are interesting candidates for studying quantum optics and\nquantum information processing. Recently, trapping and manipulation of single\natoms using tight optical dipole traps have generated considerable interest.\nHere we report an experimental investigation of the dynamics of atoms in a\nmodified optical dipole trap with a backward propagating dipole trap beam,\nwhere a change in the two-atom collision rate by six times has been achieved.\nThe theoretical model presented gives a prediction of high probabilities of\nfew-atom loading rates under proper experimental conditions. This work provides\nan alternative approach to the control of the few-atom dynamics in a dipole\ntrap and the study of the collective quantum optical effects of a few atoms.",
        "category": "physics_optics"
    },
    {
        "text": "Integrated lithium niobate photonic millimeter-wave radar: Millimeter-wave (mmWave,>30 GHz) radars are the key enabler in the coming 6G\nera for high-resolution sensing and detection of targets. Photonic radar\nprovides an effective approach to overcome the limitations of electronic radars\nthanks to the high frequency, broad bandwidth, and excellent reconfigurability\nof photonic systems. However, conventional photonic radars are mostly realized\nin tabletop systems composed of bulky discrete components, whereas the more\ncompact integrated photonic radars are difficult to reach the mmWave bands due\nto the unsatisfactory bandwidths and signal integrity of the underlining\nelectro-optic modulators. Here, we overcome these challenges and demonstrate a\ncentimeter-resolution integrated photonic radar operating in the mmWave V band\n(40-50 GHz) based on a 4-inch wafer-scale thin-film lithium niobate (TFLN)\ntechnology. The fabricated TFLN mmWave photonic integrated circuit consists of\na first electro-optic modulator capable of generating a broadband linear\nfrequency modulated mmWave radar waveform through optical frequency\nmultiplication of a low-frequency input signal, and a second electro-optic\nmodulator responsible for frequency de-chirp of the received reflected echo\nwave, therefore greatly relieving the bandwidth requirements for the\nanalog-to-digital converter in the receiver. Thanks to the absence of optical\nand electrical filters in the system, our integrated photonic mmWave radar\nfeatures continuous on-demand tunability of the center frequency and bandwidth,\ncurrently only limited by the bandwidths of electrical amplifiers. We achieve\nmulti-target ranging with a resolution of 1.50 cm and velocity measurement with\na resolution of 0.067 m/s. Furthermore, we construct an inverse synthetic\naperture radar (ISAR) and successfully demonstrate the imaging of targets with\nvarious shapes and postures with a two-dimensional resolution of 1.50 cm * 1.06\ncm.",
        "category": "physics_optics"
    },
    {
        "text": "Evolution of Raman G and G'(2D) Modes in Folded Graphene Layers: Bernal- and non-Bernal-stacked graphene layers have been systematically\nstudied by Raman imaging and spectroscopy. Two dominant Raman modes, G and G'\n(or 2D) of folded graphene layers exhibit three types of spectral features when\ninterlayer lattice mismatches, defined by a rotational angle varies. Among\nthese folded graphene layers, the most interesting one is the folded graphene\nlayers that present an extremely strong G mode enhanced by a twist-induced Van\nHove singularity. The evolution of Raman G and G' modes of such folded graphene\nlayers are probed by changing the excitation photon energies. For the first\ntime, doublet splitting of the G' mode in folded double-layer (1 + 1) and of\nthe G mode in folded tetra-layer (2 + 2) graphene are clearly observed and\ndiscussed. The G' mode splitting in folded double-layer graphene is attributed\nto the coexistence of inner and outer scattering processes and the trigonal\nwarping effect as well as further downwards bending of the inner dispersion\nbranch at visible excitation energy. While the two peaks of the G mode in\nfolded tetra-layer graphene are assigned to Raman-active mode (E2g) and lattice\nmismatch activated infrared-active mode (E1u), which is further verified by the\ntemperature-dependent Raman measurements. Our study provides a summary and\nthorough understanding of Raman spectra of Bernal- and non-Bernal-stacked\ngraphene layers and further demonstrates the versatility of Raman spectroscopy\nfor exploiting electronic band structures of graphene layers.",
        "category": "physics_optics"
    },
    {
        "text": "Tunable nonlinear coherent perfect absorption with epsilon-near-zero\n  plasmonic waveguides: We propose a scheme to realize nonlinear coherent perfect absorption (CPA) at\nthe nanoscale using epsilon-near-zero (ENZ) plasmonic waveguides. The general\nconditions to achieve CPA in a linear ENZ plasmonic waveguide are analyzed and\npresented. The proposed ENZ waveguides support an effective ENZ response at\ntheir cut-off frequency, where the CPA effect occurs under the illumination of\ntwo counter-propagating plane waves with equal amplitudes and appropriate phase\ndistributions. In addition, the strong and uniform field enhancement inside the\nnanochannels of the waveguides at the ENZ resonance can efficiently boost Kerr\nnonlinearities, resulting in a new all-optical switching intensity-dependent\nCPA phenomenon which can be tunable with ultrafast speed. The proposed\nfree-standing ENZ structures combine third-order nonlinear functionality with\nstanding wave CPA interference effects in a nanoscale plasmonic configuration,\nthus, leading to a novel degree of tunable light-matter interactions achieved\nin subwavelength regions. Our findings provide a new platform to efficiently\nexcite nonlinear phenomena at the nanoscale and design tunable coherent perfect\nabsorbers.",
        "category": "physics_optics"
    },
    {
        "text": "Polarization-control of absorption of virtual dressed-states in helium: The extreme ultraviolet absorption spectrum of an atom is strongly modified\nin the presence of a synchronized intense infrared field. In this work we\ndemonstrate control of the absorption properties of helium atoms dressed by an\ninfrared pulse by changing the relative polarization of the infrared and\nextreme ultraviolet fields. Light-induced features associated with the dressed\n$1s2s$, $1s3s$ and $1s3d$ states, referred to as $2s^{+}$, $3s^{\\pm}$ and\n$3d^{\\pm}$ light induced states, are shown to be strongly modified or even\neliminated when the relative polarization is rotated. The experimental results\nagree well with calculations based on the solution of the time-dependent\nSchr\\\"{o}dinger equation using a restricted excitation model that allows\nefficient treatment of the three dimensional problem. We also present an\nanalysis of the light induced states based on Floquet theory, which allows for\na simple explanation of their properties. Our results open a new route to\ncreating controllable superpositions of dipole allowed and non-dipole allowed\nstates in atoms and molecules.",
        "category": "physics_optics"
    },
    {
        "text": "Symmetry-based analytical solutions to the \u03c7^{(2)} nonlinear\n  directional coupler: In general the ubiquitous \\chi^{(2)} nonlinear directional coupler, where\nnonlinearity and evanescent coupling are intertwined, is nonintegrable. We\nrigorously demonstrate that matching excitation to the even or odd fundamental\nsupermodes yields dynamical analytical solutions for any phase matching in a\nsymmetric coupler. We analyze second harmonic generation and optical parametric\namplification regimes and study the influence of fundamental fields parity and\npower on the operation of the device. These fundamental solutions are useful to\ndevelop applications in classical and quantum fields such as all-optical\nmodulation of light and quantum-states engineering.",
        "category": "physics_optics"
    },
    {
        "text": "A systematic approach for designing zero-DGD coupled multi-core optical\n  fibers: An analytical method is presented for designing N-coupled multi-core fibers\nwith zero differential group delay. This approach effectively reduces the\nproblem to a system of N-1 algebraic equations involving the associated\ncoupling coefficients and propagation constants as obtained from coupled mode\ntheory. Once the parameters of one of the cores are specified, the roots of the\nresulting N-1 equations can then be used to determine the characteristics of\nthe remaining waveguide elements. Using this technique, a number of pertinent\ngeometrical configurations are investigated in order to minimize intermodal\ndispersion.",
        "category": "physics_optics"
    },
    {
        "text": "Fermionic time-reversal symmetry in a photonic topological insulator: Much of the recent enthusiasm directed towards topological insulators as a\nnew state of matter is motivated by their hallmark feature of protected chiral\nedge states. In fermionic systems, Kramers degeneracy gives rise to these\nentities in the presence of time-reversal symmetry (TRS). In contrast, bosonic\nsystems obeying TRS are generally assumed to be fundamentally precluded from\nsupporting edge states. In this work, we dispel this perception and\nexperimentally demonstrate counter-propagating chiral states at the edge of a\ntime-reversal-symmetric photonic waveguide structure. The pivotal step in our\napproach is encoding the effective spin of the propagating states as a degree\nof freedom of the underlying waveguide lattice, such that our photonic\ntopological insulator is characterised by a $\\mathbb{Z}_2$-type invariant. Our\nfindings allow for fermionic properties to be harnessed in bosonic systems,\nthereby opening new avenues for topological physics in photonics as well as\nacoustics, mechanics and even matter waves.",
        "category": "physics_optics"
    },
    {
        "text": "Photonics for artificial intelligence and neuromorphic computing: Research in photonic computing has flourished due to the proliferation of\noptoelectronic components on photonic integration platforms. Photonic\nintegrated circuits have enabled ultrafast artificial neural networks,\nproviding a framework for a new class of information processing machines.\nAlgorithms running on such hardware have the potential to address the growing\ndemand for machine learning and artificial intelligence, in areas such as\nmedical diagnosis, telecommunications, and high-performance and scientific\ncomputing. In parallel, the development of neuromorphic electronics has\nhighlighted challenges in that domain, in particular, related to processor\nlatency. Neuromorphic photonics offers sub-nanosecond latencies, providing a\ncomplementary opportunity to extend the domain of artificial intelligence.\nHere, we review recent advances in integrated photonic neuromorphic systems,\ndiscuss current and future challenges, and outline the advances in science and\ntechnology needed to meet those challenges.",
        "category": "physics_optics"
    },
    {
        "text": "Exact localization length for s-polarized electromagnetic waves incident\n  at the critical angle on a randomly-stratified dielectric medium: The interplay between Anderson localization and total internal reflection of\nelectromagnetic waves incident near the critical angle on randomly-stratified\ndielectric media is investigated theoretically. Using an exact analytical\nformula for the localization length for the Schr\\\"odinger equation with a\nGaussian $\\delta$-correlated random potential in one dimension, we show that\nwhen the incident angle is equal to the critical angle, the localization length\nfor an incident $s$ wave of wavelength $\\lambda$ is directly proportional to\n$\\lambda^{4/3}$ throughout the entire range of the wavelength, for any value of\nthe disorder strength. This result is different from that of a recent study\nreporting that the localization length at the critical incident angle for a\nbinary multilayer system with random thickness variations is proportional to\n$\\lambda$ in the large $\\lambda$ region. We also discuss the characteristic\nbehaviors of the localization length or the tunneling decay length for all\nother incident angles. Our results are confirmed by an independent numerical\ncalculation based on the invariant imbedding method.",
        "category": "physics_optics"
    },
    {
        "text": "An experiment on the shifts of reflected C-lines: An experiment is described that tests theoretical predictions on how C-lines\nincident obliquely on a surface behave on reflection. C-lines in a polarised\nwave are the analogues of the optical vortices carried by a complex scalar\nwave, which is the usual model for describing light and other electromagnetic\nwaves. The centre of a laser beam that carries a (degenerate) C-line is shifted\non reflection by the well-known Goos-H\\\"anchen and Imbert-Fedorov effects, but\nthe C-line itself splits into two, both of which are shifted longitudinally and\nlaterally; their shifts are different from that of the beam centre. To maximise\nthe effect to be measured, internal reflection in a glass prism close to the\ncritical angle was used. In a simple situation like this two recently published\nindependent theories of C-line reflection overlap and it is shown that their\npredictions are identical. The measured differences in the lateral shifts of\nthe two reflected C-lines are compared with theoretical expectations over a\nrange of incidence angles.",
        "category": "physics_optics"
    },
    {
        "text": "Photonic spin Hall effect in bilayer graphene Moir\u00e9 superlattices: The formation of a superstructure - with a related Moir\\'e pattern - plays a\ncrucial role in the extraordinary optical and electronic properties of twisted\nbilayer graphene, including the recently observed unconventional\nsuperconductivity. Here we put forward a novel, interdisciplinary approach to\ndetermine the Moir\\'e angle in twisted bilayer graphene based on the photonic\nspin Hall effect. We show that the photonic spin Hall effect exhibits clear\nfingerprints of the underlying Moir\\'e pattern, and the associated light beam\nshifts are well beyond current experimental sensitivities in the near-infrared\nand visible ranges. By discovering the dependence of the frequency position of\nthe maximal photonic spin Hall effect shift on the Moir\\'e angle, we argue that\nthe latter could be unequivocally accessed via all-optical far-field\nmeasurements. We also disclose that, when combined with the Goos-H\\\"anchen\neffect, the spin Hall effect of light enables the complete determination of the\nelectronic conductivity of the bilayer. Altogether our findings demonstrate\nthat sub-wavelength spin-orbit interactions of light provide a unprecedented\ntoolset for investigating optoelectronic properties of multilayer\ntwo-dimensional van der Waals materials.",
        "category": "physics_optics"
    },
    {
        "text": "Data transmission in long-range dielectric-loaded surface plasmon\n  polariton waveguides: In this paper we report successful transmission of 10 Gbit/s on-off-keying\n(OOK) modulated signal through the LR-DLSPPWs with almost negligible\ndegradation of the data flow consistency",
        "category": "physics_optics"
    },
    {
        "text": "Transparency in a periodic chain of quantum emitters strongly coupled to\n  a waveguide: We demonstrate the emergence of transparent behavior in a chain of\nperiodically spaced non-identical quantums emitters coupled to a waveguide, in\nthe special case when the inter-atomic separation is a half-integral multiple\nof the resonant wavelength, i.e. $kL$ is an integral multiple of $\\pi$, with\n$k$ being the spatial frequency and $L$ the spatial periodicity. When equal but\nopposite frequency detunings are assigned in pairs to a system of even number\nof atoms, perfect transmission ensues. When the chain size is odd, a similar\nassignment leads to the disappearance of collective effects as the odd atom\ndetermines the spectral behavior. We also manifest the robustness of these\nfeatures against dissipative effects and show, how the spectral behavior hinges\nsignificantly on the relative detunings between the atoms as compared to the\ndecay rate. A key distinction from the phenomenon of Electromagnetically\nInduced Transparency (EIT) is that in the waveguide case, the presence of an\nintrinsic waveguide mediated phase coupling between the atoms strongly affects\nthe transport properties. Furthermore, while reciprocity in single-photon\ntransport does not generally hold due to the phase coupling, we observe an\ninteresting exception for $kL = n\\pi$ at which the waveguide demonstrates\nreciprocal behavior with regard to both the transmission and reflection\ncoefficients.",
        "category": "physics_optics"
    },
    {
        "text": "Backward waves in a grounded bilayer slab containing double-negative\n  (DNG) and double-positive (DPS) metamaterials: Simple dispersion relations for the guided modes in a grounded DNG/DPS\nbilayer slab are given in terms of normalized parameters. Relations\ncorresponding to the grounded single-layer DNG slab are refound as specific\ncases. Numerical examples are given showing dispersion curves of the lower\norder modes and the respective total normalized power carried on the\npropagation direction. Snapshots obtained by the finite-difference time-domain\nmethod are provided showing the electromagnetic field inside the grounded\nDNG/DPS bilayer slabs. Since an important characteristic of the guided modes in\nthe slab containing a DNG layer is the existence of a turning point (TP) at\nwhich the power carried by each mode of order m>0 equals zero and changes the\nsign, we present implicit relations at the TP for the normalized parameters of\nthe guided modes in the grounded DNG/DPS and DNG slabs. We show that a thin DPS\nlayer coating on the grounded DNG slab produces a shift of the TP on the\ndispersion curve.",
        "category": "physics_optics"
    },
    {
        "text": "Shot noise reduced terahertz detection via spectrally post-filtered\n  electro-optic sampling: In ultrabroadband terahertz electro-optic sampling, spectral filtering of the\ngate pulse can strongly reduce the quantum noise while the signal level is only\nweakly affected. The concept is tested for phase-matched electro-optic\ndetection of field transients centered at 45 THz with 12-fs near-infrared gate\npulses in AgGaS2. Our new approach increases the experimental signal-to-noise\nratio by a factor of 3 compared to standard electro-optic sampling. Under\ncertain conditions an improvement factor larger than 5 is predicted by our\ntheoretical analysis.",
        "category": "physics_optics"
    },
    {
        "text": "Systematic design study of all-optical delay line based on Brillouin\n  scattering enhanced cascade coupled ring resonators: We present a technique to improve the slow-light performance of a\nside-coupled spaced sequence of resonators (SCISSOR) combined with a stimulated\nBrillouin scattering (SBS) gain medium in optical fiber. We evaluate device\nperformance of SCISSOR-only and SCISSOR + SBS systems for different numbers of\ncascaded resonators from 1 to 70 using two different data fidelity metrics\nincluding eye-opening and mutual information. A practical system design is\ndemonstrated by analyzing its performance in terms of fractional delay, power\ntransmission, and data fidelity. We observe that the results from the two\nmetrics are in good agreement. Based on system optimization under practical\nresource and fidelity constraints, the SCISSOR consisting of 70 cascaded\nresonators provides a fractional delay of 8 with 22 dB attenuation at a signal\nbit rate of 10 Gbps. The combined optimal SCISSOR (with 70 resonators) + SBS\nsystem provides a improved fractional delay up to 17 with unit power\ntransmission under the same constraints.",
        "category": "physics_optics"
    },
    {
        "text": "Few-cycle Surface Plasmon Polariton Generation by Rotating Wavefront\n  Pulses: A concept for the efficient generation of surface plasmon polaritons (SPPs)\nwith a duration of very few cycles is presented. The scheme is based on grating\ncoupling and laser pulses with wavefront rotation (WFR), so that the resonance\ncondition for SPP excitation is satisfied only for a time window shorter than\nthe driving pulse. The feasibility and robustness of the technique is\ninvestigated by means of simulations with realistic parameters. In optimal\nconditions, we find that a $29.5$~fs pulse with $800$~nm wavelength can excite\na $3.8$~fs SPP ($\\sim 1.4$ laser cycles) with a peak field amplitude $2.7$\ntimes the peak value for the laser pulse.",
        "category": "physics_optics"
    },
    {
        "text": "Multi-mode Dynamics of Terahertz Quantum Cascade Lasers: spontaneous and\n  actively induced generation of dense and harmonic coherent regimes: We present an extended study concerning the dynamics of dense and harmonic\ncoherent regimes in quantum cascade lasers (QCLs) in a Fabry--Perot (FP)\nconfiguration emitting in the terahertz (THz) spectral region. Firstly, we\nstudy the device in free running operation, reproducing the main features of\nthe self-generated of optical frequency combs (OFCs) and harmonic frequency\ncombs (HFCs) in this spectral range, commenting on the points of difference\nfrom the mid-infrared region, and finding excellent agreement with the most\nrecent experimental evidences. Then, we analyze the THz-QCL dynamics under\nradiofrequency (RF) injection, with a focus on the effect of the modulation of\nthe current on the degree of locking of the system, and we perform a systematic\ninvestigation aimed to provide a procedure for the generation of train of\npulses with short duration and high contrast. Furthermore, we extend our study\nto the generation of sequences of pulses with repetition frequency which is a\nmultiple of the free-spectral range (FSR) of the laser cavity, reproducing\nharmonic mode-locking (HML) of the laser, a method which has been recently\ndemontrated in experiments with THz-QCLs, and the results of which are\nparticulary promising for a large stream of applications, ranging from optical\ncommunication to imaging.",
        "category": "physics_optics"
    },
    {
        "text": "Exploiting lens aberrations to create electron vortex beams: A model for a new electron vortex beam production method is proposed and\nexperimentally demonstrated. The technique calls on the controlled manipulation\nof the degrees of freedom of the lens aberrations to achieve a helical phase\nfront. These degrees of freedom are accessible by using the corrector lenses of\na transmission electron microscope. The vortex beam is produced through a\nparticular alignment of these lenses into a specifically designed astigmatic\nstate and applying an annular aperture in the condensor plane. Experimental\nresults are found to be in good agreement with simulations.",
        "category": "physics_optics"
    },
    {
        "text": "Radiation pressure backaction on a hexagonal boron nitride\n  nanomechanical resonator: Hexagonal boron nitride (hBN) is a van der Waals material with excellent\nmechanical properties hosting quantum emitters and optically active spin\ndefects, several of them being sensitive to strain. Establishing optomechanical\ncontrol of hBN will enable hybrid quantum devices that combine the spin degree\nof freedom with the cavity optomechanical toolbox. In this letter, we report\nthe first observation of radiation pressure backaction at telecom wavelengths\nwith a hBN drum-head mechanical resonator. The thermomechanical motion of the\nresonator is coupled to the optical mode of a high finesse fiber-based\nFabry-P\\'erot microcavity in a membrane-in-the-middle configuration. We are\nable to resolve the optical spring effect and optomechanical damping with a\nsingle photon coupling strength of $g_0/2\\pi = 1200$ Hz. Our results pave the\nway for tailoring the mechanical properties of hBN resonators with light.",
        "category": "physics_optics"
    },
    {
        "text": "Dirac points in helically structured 1D photonic crystals: We reported about observation of Dirac points in a helically structured 1D\nphotonic crystals, moreover, both as in the presence of longitudinal magnetic\nfield as its absence. We obtained analytical formulas for Dirac points\nfrequencies and the analytical dispersion relations for wave vectors.",
        "category": "physics_optics"
    },
    {
        "text": "Ultra-broadband supercontinuum generation in gas-filled photonic-crystal\n  fibers: The epsilon-near-zero regime: In this Letter, we show theoretically that the nonlinear photoionisation\nprocess of a noble gas inside a hollow-core photonic crystal fibre can be\nexploited in obtaining broadband supercontinuum generation via pumping close to\nthe mid-infrared regime. The interplay between the Kerr and photoionisation\nnonlinearities is strongly enhanced in this regime. Photoionisation\ncontinuously modifies the medium dispersion, in which the refractive index\nstarts to significantly decrease and approach the epsilon-near-zero regime.\nSubsequently, the self-phase modulation induced by the Kerr effect is boosted\nbecause of the accompanied slow-light effect. As a result of this interplay, an\noutput spectrum that comprises of a broadband light with multiple\ndispersive-wave emission is obtained.",
        "category": "physics_optics"
    },
    {
        "text": "Multiplane Quantitative Phase Imaging Using a Wavelength-Multiplexed\n  Diffractive Optical Processor: Quantitative phase imaging (QPI) is a label-free technique that provides\noptical path length information for transparent specimens, finding utility in\nbiology, materials science, and engineering. Here, we present quantitative\nphase imaging of a 3D stack of phase-only objects using a\nwavelength-multiplexed diffractive optical processor. Utilizing multiple\nspatially engineered diffractive layers trained through deep learning, this\ndiffractive processor can transform the phase distributions of multiple 2D\nobjects at various axial positions into intensity patterns, each encoded at a\nunique wavelength channel. These wavelength-multiplexed patterns are projected\nonto a single field-of-view (FOV) at the output plane of the diffractive\nprocessor, enabling the capture of quantitative phase distributions of input\nobjects located at different axial planes using an intensity-only image sensor.\nBased on numerical simulations, we show that our diffractive processor could\nsimultaneously achieve all-optical quantitative phase imaging across several\ndistinct axial planes at the input by scanning the illumination wavelength. A\nproof-of-concept experiment with a 3D-fabricated diffractive processor further\nvalidated our approach, showcasing successful imaging of two distinct phase\nobjects at different axial positions by scanning the illumination wavelength in\nthe terahertz spectrum. Diffractive network-based multiplane QPI designs can\nopen up new avenues for compact on-chip phase imaging and sensing devices.",
        "category": "physics_optics"
    },
    {
        "text": "Optical phase mining by adjustable spatial differentiator: Phase is a fundamental resource for optical imaging but cannot be directly\nobserved with intensity measurements. The existing methods to quantify a phase\ndistribution rely on complex devices and structures. Here we experimentally\ndemonstrate a phase mining method based on so-called adjustable spatial\ndifferentiation, just generally by analyzing the polarization in light\nreflection on a single planar dielectric interface. With introducing an\nadjustable bias, we create a virtual light source to render the measured images\nwith a shadow-cast effect. We further successfully recover the phase\ndistribution of a transparent object from the virtual shadowed images. Without\nany dependence on resonance or material dispersion, this method directly stems\nfrom the intrinsic properties of light and can be generally extended to a board\nfrequency range.",
        "category": "physics_optics"
    },
    {
        "text": "Hollow Gaussian beam generation through nonlinear interaction of photons\n  with orbital-angular-momemtum: Hollow Gaussian beams (HGB) are a special class of doughnut shaped beams that\ndo not carry orbital angular momentum (OAM). Such beams have a wide range of\napplications in many fields including atomic optics, bio-photonics, atmospheric\nscience, and plasma physics. Till date, these beams have been generated using\nlinear optical elements. Here, we show a new way of generating HGBs by\nthree-wave mixing in a nonlinear crystal. Based on nonlinear interaction of\nphotons having OAM and conservation of OAM in nonlinear processes, we\nexperimentally generated ultrafast HGBs of order as high as 6 and power >180 mW\nat 355 nm. This generic concept can be extended to any wavelength, timescales\n(continuous-wave and ultrafast) and any orders. We show that the removal of\nazimuthal phase of vortices does not produce Gaussian beam. We also propose a\nnew and only method to characterize the order of the HGBs.",
        "category": "physics_optics"
    },
    {
        "text": "Subwavelength gratings for OVDs - From local interactions to using\n  light-transport: In the past 30 years, subwavelength gratings have been developed and produced\nas highly secured Diffractive Optical Variable Image Devices (DOVIDs). They\nallowed new distinct optical effects and dramatically lowered DOVIDs\ncounterfeiting. In particular, subwalength gratings coated with a high\nrefractive index dielectric are well-known and mass-produced to secure\ndocuments, such as the Diffractive Identification Devices (DIDs). These\nsubmicronic gratings are called Zero Order Devices or Filters (ZOD, ZOF) or\ndiffractive microstructures designed for Zero-order read-out. Similar\nstructures are called Resonant Waveguide Gratings (RWG) or Resonant Leaky Mode\nWaveguides, when optimized for different purposes. A study using time-resolved\noptical simulations can demonstrate and quantify how light is coupled and\npropagation in DIDs structure when observed across the gratings (in collinear\nincidence). The leaky resonant modes of the RWG are playing a significant role\nin the appearance of DIDs in collinear incidence, which has practical impacts\non the engineering of such security elements. From the learning of such\nnon-local light interactions in DIDs, new optically variable effects can be\ndesigned and produced by using light transport in RWG. As an example,\nmulti-frequency RWGs can be designed to couple light for different colors and\npolarizations and to outcouple them away from the specular configuration\ntypical to the Zero Order Devices. As a further step, we present new structures\nable to couple light in the substrates or in selected layers of security\ndocuments or labels, embling waveguiding light in these layers. These optical\ncouplers enable selective light transport at a macroscopic scale using total\ninternal reflection in the volume of the substrates, while being compatible\nwith current roll-to-roll methods.",
        "category": "physics_optics"
    },
    {
        "text": "Self-Regulated Transport in Photonic Crystals with Phase-Changing\n  Defects: Phase changing materials (PCM) are widely used for optical data recording,\nsensing, all-optical switching, and optical limiting. Our focus here is on the\ncase when the change in the transmission characteristics of the optical\nmaterial is caused by the input light itself. Specifically, the light-induced\nheating triggers the phase transition in the PCM. In this paper, using a\nnumerical example, we demonstrate that incorporating the PCM in a photonic\nstructure can lead to a dramatic modification of the effects of light-induced\nphase transition, as compared to a stand-alone sample of the same PCM. Our\nfocus is on short pulses. We discuss some possible applications of such\nphase-changing photonic structures for optical sensing and limiting.",
        "category": "physics_optics"
    },
    {
        "text": "First-Principles Threshold Calculation of Photonic Crystal\n  Surface-Emitting Lasers Using Rigorous Coupled Wave Analysis: We show that the threshold of a photonic crystal surface-emitting laser can\nbe calculated from first-principles by the method of rigorous coupled wave\nanalysis (RCWA), which has been widely used to simulate the response spectra of\npassive periodic structures. Here, the scattering matrix (S-matrix) of a\nsurface-emitting laser structure with added gain is calculated on the complex\nfrequency plane using RCWA, and the lasing threshold is determined by the value\nof gain for which the pole of the S-matrix reaches the real axis. This approach\ncan be used for surface emitting laser struc tures in general, and is\nparticularly useful for the surface emitting laser systems with complex\nin-plane structures.",
        "category": "physics_optics"
    },
    {
        "text": "Design and Characterization of Q-enhanced Silicon Nitride Racetrack\n  Micro-Resonators: Q-enhanced racetrack micro-resonators for the silicon nitride photonics\nintegration platform have been designed, fabricated and characterized. The\nproposed geometries permit to mitigate the impact of radiation loss at curved\nwaveguides, one of the major limitations of silicon nitride circuits, therefore\nproviding an increase of the intrinsic Q factor of micro-resonators when\ncompared with the conventional structures with the same bent radii. The schemes\nput forward in this work permit a reduction of the size of the devices that has\na direct impact on the integration scale in this platform. When used in the\ncurved sections of waveguides routing optical signals within an integrated\nphotonic circuit, these geometries provide a reduction of the radiation loss\nand permit the use of smaller bent radii and to increase the circuit\nintegration density.",
        "category": "physics_optics"
    },
    {
        "text": "Shrinking-shifting and Amplifying-shifting Device Using Transformation\n  Optics: Based on transformation optics (TO), this paper uses geometric divisions and\nlinear coordinate transformations to design shrinking-shifting - and reshaping,\nand amplifying-shifting - and reshaping devices. The proposed devices can\nreshape the sizes and locations of the wrapped-objects inside the core-region.\nThe shrinking-shifting device shrinks the larger object into a smaller one and\nshifts it to different location, whereas the shrinking-reshaping device can\ngenerate a smaller-size image with different shape located at different\nlocation. In contrast to previously designed shrinking devices, the real object\nwrapped inside the proposed core-region and the transformed object contains the\nsame material properties, and the location-shifting is another feature. Here,\nthe shifting-region is located inside the physical-space boundaries to achieve\nthe non-negative, homogeneous, and anisotropic material properties of the\nproposed device, which are easier for real implementations. Thus, we further\nverified this concept with the amplifying-shifting and -reshaping devices for\nvisually transformation of smaller object into bigger one placed at different\nlocation and position. We also applied active scatterer to further validate the\nworking functionality of proposed devices. In addition, the proposed devices\nbehave like the concentrator and or rotator effect in the absence of any\nscatterer. Our findings highlight the role of TO, suggesting directions for\nfuture research on bi-functional devices that will be useful for shrinking and\namplifying devices, illusion optics, camouflage, and object protection etc.\n  Keywords: amplifying, reshaping, shrinking, transformation optics,\ninvisibility cloaks",
        "category": "physics_optics"
    },
    {
        "text": "Monopole Antimonopole Instability in Non-Hermitian Coupled Waveguides: A non-Hermitian coupled waveguide system with periodically varying\nparameters, in which the Berry curvature is analogous to a hyperbolic magnetic\nmonopole or antimonopole, is investigated. It is shown to have a purely\nimaginary Berry connection, and is consequently influenced by a geometric\nmultiplier. It is possible for this multiplier to induce net gain or loss in\nthe system, corresponding to the existence of the antimonopole or monopole in\nparameter space, respectively. For the right choice of parameters, the system\nwill display an apparent non-adiabatic change in behaviour, which implies a\nswitch between the dominant eigenstate in the waveguides, leading to a change\nin parameter space analogous to a charge reversal of the hyperbolic magnetic\nmonopole.",
        "category": "physics_optics"
    },
    {
        "text": "Programmable unitary spatial modes manipulation: Free space propagation and conventional optical systems such as lenses and\nmirrors all perform spatial unitary transforms. However, the subset of\ntransforms available through these conventional systems is limited in scope. We\npresent here a unitary programmable mode converter (UPMC) capable of performing\nany spatial unitary transform of the light field. It is based on a succession\nof reflections on programmable deformable mirrors and free space propagation.\nWe first show theoretically that a UPMC without limitations on resources can\nperform perfectly any transform. We then build an experimental implementation\nof the UPMC and show that, even when limited to three reflections on an array\nof 12 pixels, the UPMC is capable of performing single mode tranforms with an\nefficiency greater than 80% for the first 4 modes of the TEM basis.",
        "category": "physics_optics"
    },
    {
        "text": "Tunable multi-color coherent light generation in single MgO: PPLN bulk\n  crystal: We report a theoretical design analysis of domain-engineered periodically\npoled lithium niobate (PPLN) for wavelength conversion of near-infrared sources\nto generate coherent light in the visible spectral range. Our analysis on the\nspectral outputs show that with a proper design of the quasi phase matching\n(QPM) periods, tunable, multiple nonlinear optical processes can be\nsimultaneously phase matched in a single segmented crystal. We show that a\nthree-segment single PPLN crystal has potential to generate violet (432 nm),\nblue (490 nm) and orange (600 nm) wavelengths by simultaneous sum frequency and\nsecond harmonic generation processes. Such a design scheme has promising\npotential for a compact, robust and tunable multi-colored visible light source\nwhich can find various applications such as in biomedicine, high-density\noptical data storage and laser based color displays.",
        "category": "physics_optics"
    },
    {
        "text": "Broadband second harmonic generation in whispering gallery mode\n  resonators: Optical frequency conversion processes in nonlinear materials are limited in\nwavelength by the accessible phase matching and the required high pump powers.\nIn this letter, we report a novel broadband phase matching (PM) technique in\nhigh quality factor (Q) whispering gallery mode (WGM) resonators made of\nbirefringent crystalline materials. This technique relies on two interacting\nWGMs, one with constant and the other with spatially oscillating phase\nvelocity. Thus, phase matching occurs cyclically. The technique can be\nimplemented with a WGM resonator with its disk plane parallel to the optic axis\nof the crystal. With a single beta barium borate (BBO) resonator in that\nconfiguration, we experimentally demonstrated efficient second harmonic\ngeneration (SHG) to harmonic wavelengths from 780 nm in the near infrared to\n317 nm in the ultraviolet (UV). The observed SHG conversion efficiency is as\nhigh as 4.6% (mW)-1. This broadband PM technique opens a new way for nonlinear\noptics applications in WGM resonators. This work is also the first reported\ncontinuous wave UV generation by direct SHG in a WGM resonator",
        "category": "physics_optics"
    },
    {
        "text": "Phase Space Engineering in Optical Microcavities I: Preserving\n  near-field uniformity while inducing far-field directionality: Optical microcavities have received much attention over the last decade from\ndifferent research fields ranging from fundamental issues of cavity QED to\nspecific applications such as microlasers and bio-sensors. A major issue in the\nlatter applications is the difficulty to obtain directional emission of light\nin the far-field while keeping high energy densities inside the cavity (i.e.\nhigh quality factor). To improve our understanding of these systems, we have\nstudied the annular cavity (a dielectric disk with a circular hole), where the\ndistance cavity-hole centers, d, is used as a parameter to alter the properties\nof cavity resonances. We present results showing how one can affect the\ndirectionality of the far-field while preserving the uniformity (hence the\nquality factor) of the near-field simply by increasing the value of d.\nInterestingly, the transition between a uniform near- and far-field to a\nuniform near- and directional far-field is rather abrupt. We can explain this\nbehavior quite nicely with a simple model, supported by full numerical\ncalculations, and we predict that the effect will also be found in a large\nclass of eigenmodes of the cavity.",
        "category": "physics_optics"
    },
    {
        "text": "Scalably manufactured high-index atomic layer-polymer hybrid\n  metasurfaces for high-efficiency virtual reality metaoptics in the visible: Metalenses, which exhibit superior light-modulating performance with\nsub-micrometer-scale thicknesses, are suitable alternatives to conventional\nbulky refractive lenses. However, fabrication limitations, such as a high cost,\nlow throughput, and small patterning area, hinder their mass production. Here,\nwe demonstrate the mass production of low-cost, high-throughput, and\nlarge-aperture visible metalenses using an argon fluoride immersion scanner and\nwafer-scale nanoimprint lithography. Once a 12-inch master stamp is imprinted,\nhundreds of centimeter-scale metalenses can be fabricated. To enhance light\nconfinement, the printed metasurface is thinly coated with a high-index film,\nresulting in drastic increase of conversion efficiency. As a proof of concept,\na prototype of a virtual reality device with ultralow thickness is demonstrated\nwith the fabricated metalens.",
        "category": "physics_optics"
    },
    {
        "text": "Iterative, backscatter-analysis algorithms for increasing transmission\n  and focusing light through highly-scattering random media: Scattering hinders the passage of light through random media and consequently\nlimits the usefulness of optical techniques for sensing and imaging. Thus,\nmethods for increasing the transmission of light through such random media are\nof interest. Against this backdrop, recent theoretical and experimental\nadvances have suggested the existence of a few highly transmitting\neigen-wavefronts with transmission coefficients close to one in strongly\nbackscattering random media.\n  Here, we numerically analyze this phenomenon in 2-D with fully spectrally\naccurate simulators and provide rigorous numerical evidence confirming the\nexistence of these highly transmitting eigen-wavefronts in random media with\nperiodic boundary conditions that is composed of hundreds of thousands of\nnon-absorbing scatterers.\n  Motivated by bio-imaging applications where it is not possible to measure the\ntransmitted fields, we develop physically realizable algorithms for increasing\nthe transmission through such random media using backscatter analysis. We show\nvia numerical simulations that the algorithms converge rapidly, yielding a\nnear-optimum wavefront in just a few iterations. We also develop an algorithm\nthat combines the knowledge of these highly transmitting eigen-wavefronts\nobtained from backscatter analysis, with intensity measurements at a point to\nproduce a near-optimal focus with significantly fewer measurements than a\nmethod that does not utilize this information.",
        "category": "physics_optics"
    },
    {
        "text": "Computational multifocal microscopy: Despite recent advances, high performance single-shot 3D microscopy remains\nan elusive task. By introducing designed diffractive optical elements (DOEs),\none is capable of converting a microscope into a 3D \"kaleidoscope\", in which\ncase the snapshot image consists of an array of tiles and each tile focuses on\ndifferent depths. However, the acquired multifocal microscopic (MFM) image\nsuffers from multiple sources of degradation, which prevents MFM from further\napplications. We propose a unifying computational framework which simplifies\nthe imaging system and achieves 3D reconstruction via computation. Our optical\nconfiguration omits chromatic correction grating and redesigns the multifocal\ngrating to enlarge the tracking area. Our proposed setup features only one\nsingle grating in addition to a regular microscope. The aberration correction,\nalong with Poisson and background denoising, are incorporated in our\ndeconvolution-based fully-automated algorithm, which requires no empirical\nparameter-tuning. In experiments, we achieve the spatial resolutions of\n$0.35$um (lateral) and $0.5$um (axial), which are comparable to the resolution\nthat can be achieved with confocal deconvolution microscopy. We demonstrate a\n3D video of moving bacteria recorded at $25$ frames per second using our\nproposed computational multifocal microscopy technique.",
        "category": "physics_optics"
    },
    {
        "text": "Stability of optimal-wave-front-sample coupling under sample translation\n  and rotation: The method of wavefront shaping to control optical properties of opaque media\nis a promising technique for authentication applications. One of the main\nchallenges of this technique is the sensitivity of the wavefront-sample\ncoupling to translation and/or rotation. To better understand how translation\nand rotation affect the wavefront- sample coupling we perform experiments in\nwhich we first optimize reflection from an opaque surface--to obtain an optimal\nwavefront--and then translate or rotate the surface and measure the new\nreflected intensity pattern. By using the correlation between the optimized and\ntranslated or rotated patterns we determine how sensitive the wavefront-sample\ncoupling is. These experiments are performed for different\nspatial-light-modulator (SLM) bin sizes, beam-spot sizes, and nanoparticle\nconcentrations. We find that all three parameters affect the different\npositional changes, implying that an optimization scheme can be used to\nmaximize the stability of the wavefront-sample coupling. We also develop a\nmodel to simulate sample translation or rotation and its effect on the coupling\nstability, with the simulation results being qualitatively consistent with\nexperiment.",
        "category": "physics_optics"
    },
    {
        "text": "Optimization of radiation pressure in dielectric nanowaveguides: Stimulated Brillouin scattering (SBS) processes have been allowing important\ntechnological breakthroughs in integrated photonics and nano-optomechanics, by\nexploiting light-sound (photon-phonon) interactions at the nanoscale. These\nnonlinear processes are created by two main effects: radiation pressure and\nelectrostriction; however, the former is the predominant one in\nhigh-index-contrast nanowaveguides. In this letter, we derive a simple set of\nanalytical expressions that can be used for optimizing the radiation pressure\non the waveguide boundaries, for any optical mode, polarization, and\nwavelength. We observe a very strong influence of the waveguide geometric\nparameters on the optimal radiation pressure value. Furthermore, we explain how\nthe existence of such optimal geometric dimensions is physically related to the\nminimization of the electromagnetic momentum flow in the propagation direction.\nThis work provides a novel and robust method, yet simple, to optimize the\nradiation pressure in dielectric nanowaveguides, which may be of great\nrelevance for designing integrated photonic-phononic devices.",
        "category": "physics_optics"
    },
    {
        "text": "Macroscopic Magneto-Chiroptical Metasurfaces: Nanophotonic chiral antennas exhibit orders of magnitude higher circular\ndichroism (CD) compared to molecular systems. Merging magnetism and structural\nchirality at the nanometric level allows for the efficient magnetic control of\nthe dichroic response, bringing exciting new prospects to active nanophotonic\ndevices and magnetochirality. Here we devise macroscale enantiomeric\nmagnetophotonic metasurfaces of plasmon and ferromagnetic spiral antennas.\nMixed 2D- and 3D- chiral nanoantennas induce large CD response, where we\nidentify reciprocal and non-reciprocal contributions. The simultaneous\nchiroptical and magneto-optical response in a wide spectral range with these\nmetasurfaces delivers an attractive platform for the study of magnetochirality\nat the nanoscale. Exploring further this type of magnetophotonic metasurfaces\nallows the realization of high-sensitivity chiral sensors and prompts the\ndesign of novel macroscopic optical devices operating with polarized light.",
        "category": "physics_optics"
    },
    {
        "text": "A Completely Covariant Approach to Transformation Optics: We show that the Plebanski based approach to transformation optics overlooks\nsome subtleties in the electrodynamics of moving dielectrics that restricts its\napplicability to a certain class of transformations. An alternative, completely\ncovariant, approach is developed that is more generally applicable and provides\na clearer picture of transformation optics.",
        "category": "physics_optics"
    },
    {
        "text": "Stability conditions for one-dimensional optical solitons in\n  cubic-quintic-septimal media: Conditions for stable propagation of one-dimensional bright spatial solitons\nin media exhibiting optical nonlinearities up to the seventh-order are\ninvestigated. The results show well-defined stability regions even when all the\nnonlinear terms are focusing. Conditions for onset of the supercritical\ncollapse of the optical beam are identified too. A variational approximation is\nused to predict dependence of the soliton propagation constant on the norm, and\nrespective stability regions are identified using the Vakhitov-Kolokolov\ncriterion. Analytical results obtained by means of the variational\napproximation are corroborated by numerical simulations of the\ncubic-quintic-septimal nonlinear Schroedinger equation.",
        "category": "physics_optics"
    },
    {
        "text": "Quantum emitter coupled to plasmonic nanotriangle: Spatially dependent\n  emission and thermal mapping: Herein we report on our studies of radiative and non-radiative interaction\nbetween an individual quantum emitter and an anisotropic plasmonic\nnanostructure: a gold nanotriangle. Our theoretical and three-dimensional\nelectromagnetic simulation studies highlight an interesting connection between:\ndipole-orientation of the quantum emitter, anisotropy of the plasmonic\nnanostructure and, radiative and non-radiative energy transfer processes\nbetween the emitter and the plasmonic geometry. For the out of plane\norientation of quantum emitter, the total decay rate and non-radiative decay\nrate was found to be maximum, showing radiation extraction efficiency of 0.678.\nAlso the radiative decay rate was greater for the same orientation, and showed\na pronounced spatial dependence with respect to the nanotriangle. Our study has\ndirect implication on two aspects: designing nanoparticle optical antennas to\ncontrol emission from individual atoms and molecules and geometrical control of\nquenching of emission into plasmonic decay channels.",
        "category": "physics_optics"
    },
    {
        "text": "Intermodal Four-Wave-Mixing and Parametric Amplification in km-long\n  Fibers: We theoretically and numerically investigate intermodal four-wave-mixing in\nkm-long fibers, where random birefringence fluctuations are present along the\nfiber length. We identify several distinct regimes that depend on the relative\nmagnitude between the length scale of the random fluctuations and the\nbeat-lengths of the interacting quasi-degenerate modes. In addition, we analyze\nthe impact of polarization mode-dispersion and we demonstrate that random\nvariations of the core radius, which are typically encountered during the\ndrawing stage of the fiber, can represent the major source of bandwidth\nimpairment. These results set a boundary on the limits of validity of the\nclassical Manakov model and may be useful for the design of multimode\nparametric amplifiers and wavelength converters, as well as for the analysis of\nnonlinear impairments in long-haul spatial division multiplexed transmission.",
        "category": "physics_optics"
    },
    {
        "text": "Suppression of Nonlinear Optical Rogue Wave Formation Using\n  Polarization-Structured Beams: A nonlinear self-focusing material can amplify random small-amplitude phase\nmodulations present in an optical beam, leading to the formation of amplitude\nsingularities commonly referred to as optical caustics. By imposing\npolarization structuring on the beam, we demonstrate the suppression of\namplitude singularities caused by nonlinear self-phase modulation. Our results\nare the first to indicate that polarization-structured beams can suppress\nnonlinear caustic formation in a saturable self-focusing medium and add to the\ngrowing understanding of catastrophic self-focusing effects in beams containing\npolarization structure.",
        "category": "physics_optics"
    },
    {
        "text": "Coherence retrieval using trace regularization: The mutual intensity and its equivalent phase-space representations quantify\nan optical field's state of coherence and are important tools in the study of\nlight propagation and dynamics, but they can only be estimated indirectly from\nmeasurements through a process called coherence retrieval, otherwise known as\nphase-space tomography. As practical considerations often rule out the\navailability of a complete set of measurements, coherence retrieval is usually\na challenging high-dimensional ill-posed inverse problem. In this paper, we\npropose a trace-regularized optimization model for coherence retrieval and a\nprovably-convergent adaptive accelerated proximal gradient algorithm for\nsolving the resulting problem. Applying our model and algorithm to both\nsimulated and experimental data, we demonstrate an improvement in\nreconstruction quality over previous models as well as an increase in\nconvergence speed compared to existing first-order methods.",
        "category": "physics_optics"
    },
    {
        "text": "Engineering Photon Statistics of Spatial Light Modes: The nature of light sources is defined by the statistical fluctuations of the\nelectromagnetic field. As such, the photon statistics of light sources are\ntypically associated with distinct emitters. Here, we demonstrate the\npossibility of producing light beams with various photon statistics through the\nspatial modulation of coherent light. This is achieved by the sequential\nencoding of controllable Kolmogorov phase screens in a digital micromirror\ndevice. Interestingly, the flexibility of our scheme allows for the arbitrary\nshaping of spatial light modes with engineered photon statistics at different\nspatial positions. The performance of our scheme is assessed through the\nphoton-number-resolving characterization of different families of spatial light\nmodes with engineered photon statistics. We believe that the possibility of\ncontrolling the photon fluctuations of the light field at arbitrary spatial\nlocations has important implications for quantum spectroscopy, sensing, and\nimaging.",
        "category": "physics_optics"
    },
    {
        "text": "Power oscillations induced by the relative Goos-Haenchen phase: By using an optical interferometer composed of a dielectric laser\nellipsometer, to change the optical response of transverse electric and\nmagnetic incident radiation, and two polarisers, to trigger the interference\npattern induced by the relative Goos-Haenchen phase, we show under which\nconditions it is possible to optimize the laser power oscillations induced by\nthe relative phase difference between orthogonal polarised states. The\nGoos-Haenchen interference can then be used to sense rotation, to test optical\ncomponents, and to simulate quarter and half wave plates.",
        "category": "physics_optics"
    },
    {
        "text": "Energy deposition in air by moderately focused femtosecond laser\n  filaments: Filamentation of high-power femtosecond laser pulses in air is accompanied by\na fairly strong release of optical energy into the propagation medium due to\nlaser-induced ionization of air molecules and production of an underdense\nplasma of charged species. We present the results of our laboratory experiments\nand numerical simulations aimed to the estimation of energy deposition amount\nby laser filament upon propagation in air depending on the conditions of\nspatial focusing, pulse energy, and radiation wavelength. For the first time to\nour knowledge, our study reveals a more than 50% decrease in the filament\nenergy deposited in air in the range of moderate numerical aperture values,\napproximately from 0.003 to 0.007, at the carrier wavelengths of 740 nm and 470\nnm. We attribute such a considerable reduction in the laser pulse energy\nrelease for femtosecond plasma to the competing effects of Kerr self-focusing\nand geometric divergence of focused laser pulse.",
        "category": "physics_optics"
    },
    {
        "text": "Optical frequency metrology with a Rb-stabilized ring-cavity resonator\n  -- Study of cavity-dispersion errors: We have developed a technique to measure the absolute frequencies of optical\ntransitions by using an evacuated Rb-stabilized ring-cavity resonator as a\ntransfer cavity. We study possible wavelength-dependent errors due to\ndispersion at the cavity mirrors by measuring the frequency of the same\ntransition in the $D_2$ line of Cs at three cavity lengths. We find no\ndiscernable change in values within our error of 30 kHz. Our values are\nconsistent with measurements using the frequency-comb technique and have\nsimilar accuracy.",
        "category": "physics_optics"
    },
    {
        "text": "Er doped oxide nanoparticles in silica based optical fibres: Erbium doped materials are of great interest in optical telecommunications\ndue to the Er3+ intra-4f emission at 1.54 ?m. Erbium doped fibre amplifiers\n(EDFA) were developed in silica glass because of the low losses at this\nwavelength and the reliability of this glass. Developments of new rare earth\ndoped fibre amplifiers aim to control their spectroscopic properties including\nshape and width of the gain curve and optical quantum efficiency. Standard\nsilica glass modifiers, such as aluminium, result in very good properties in\ncurrent EDFA. However, for more drastic spectroscopic changes, more important\nmodifications of the rare earth ions local environment are required. To address\nthis aim, we present a fibre fabrication route creating rare earth doped\ncalcia?silica or calcia?phosphosilica nanoparticles embedded in silica glass.\nBy adding alkaline earth elements such as calcium, in low concentration, one\ncan obtain a glass with an immisci- bility gap so that phase separation occurs\nwith an appropriate heat treatment. We investigated the role of two elements:\ncalcium and phosphorus (a standard silica modifier). Scanning electron\nmicroscopy shows that nanoparticles are only observed when calcium is\nincorporated. The size of the particles is determined to be around 50 nm in\npreform samples. The nature of these particles depends on phosphorus content:\nwithout P, electron diffraction shows that the particles are amorphous whilst\nthey are partially crystalline when phosphorus is added. In addition through\nuse of energy dispersive x-ray techniques, we have shown that erbium ions are\nlocated in the nanoparticles.",
        "category": "physics_optics"
    },
    {
        "text": "Cavity Soliton Laser based on mutually coupled semiconductor\n  microresonators: We report on experimental observation of localized structures in two mutually\ncoupled broad-areahttp://hal.archives-ouvertes.fr/images/calendar.gif\nsemiconductor resonators. These structures coexist with a dark homogeneous\nbackground and they have the same properties as cavity solitons without\nrequiring the presence of a driving beam into the system. They can be switched\nindividually on and off by means of a local addressing beam.",
        "category": "physics_optics"
    },
    {
        "text": "Bounded modes to the rescue of optical transmission: This paper presents a brief survey of the evolution of knowledge about\ndiffraction gratings. After recalling some basic facts, historically and\nphysically, we introduce the concept of Wood anomalies. Next, we present some\nrecent works in order to introduce the role of bounded modes in transmission\ngratings. The consequences of these recent results are then introduced. This\npaper is a secondary publication, published in Europhysics News (EPN 38, 3\n(2007) 27-31). In the present version, some additional notes have been added\nwith related references.",
        "category": "physics_optics"
    },
    {
        "text": "Eigenmodes of aberrated systems: the tilted lens: When light is passed through aberrated optical systems, the resulting\ndegradation in amplitude and phase has deleterious effects, for example, on\nresolution in imaging, spot sizes in focussing, and the beam quality factor of\nthe output beam. Traditionally this is either pre- or post-corrected by\nadaptive optics or phase conjugation. Here we consider the medium as a complex\nchannel and search for the eigenmodes of the channel, the modes that propagate\nthrough this system without alteration. We employ a quantum-inspired approach\nand apply it to the tilted lens as our example channel, a highly astigmatic\nsystem that is routined used as a desired distortion inducer to measure orbital\nangular momentum. We find the eigenmodes analytically, show their robustness in\na practical experiment, and outline how this approach may be extended to\narbitrary astigmatic systems.",
        "category": "physics_optics"
    },
    {
        "text": "Fano resonances in antennas: General control over radiation patterns: The concepts of many optical devices are based on the fundamental physical\nphenomena such as resonances. One of the commonly used devices is an\nelectromagnetic antenna that converts localized energy into freely propagating\nradiation and vise versa, offering unique capabilities for controlling\nelectromagnetic radiation. Here we propose a concept for controlling the\nintensity and directionality of electromagnetic wave scattering in\nradio-frequency and optical antennas based on the physics of Fano resonances.\nWe develop an analytical theory of spatial Fano resonances in antennas that\ndescribes switching of the radiation pattern between the forward and backward\ndirections, and confirm our theory with both numerical calculations and\nmicrowave experiments. Our approach bridges the concepts of conventional radio\nantennas and photonic nanoantennas, and it provides a paradigm for the design\nof wireless optical devices with various functionalities and architectures.",
        "category": "physics_optics"
    },
    {
        "text": "Phase retrieval based on shaped incoherent sources: The current ghost imaging phase reconstruction schemes require either complex\noptical systems, Fourier transform steps, or iterative algorithms, which may\nincrease the difficulty of system design, cause phase retrieval error or take\ntoo much time. To address this problem, we propose a five-step phase-shifting\nmethod in which no complex optical systems, Fourier transform steps, or\niterative algorithms are needed. With five designed incoherent sources, one can\nobtain five different corresponding ghost imaging patterns, then the phase\ninformation of the object can be calculated from those five speckle patterns.\nThe applicability of this theoretical proposal is demonstrated via numerical\nsimulations with two kinds of complicated objects, and the results illustrate\nthe phase information of the complicated object can be reconstructed\nsuccessfully and quantitatively.",
        "category": "physics_optics"
    },
    {
        "text": "Binary sampling ghost imaging: add random noise to fight quantization\n  caused image quality decline: When the sampling data of ghost imaging is recorded with less bits, i.e.,\nexperiencing quantization, decline of image quality is observed. The less bits\nused, the worse image one gets. Dithering, which adds suitable random noise to\nthe raw data before quantization, is proved to be capable of compensating image\nquality decline effectively, even for the extreme binary sampling case. A brief\nexplanation and parameter optimization of dithering are given.",
        "category": "physics_optics"
    },
    {
        "text": "Temperature-Dependent Group Delay of Photonic-Bandgap Hollow-Core Fiber\n  Tuned by Surface-Mode Coupling: Surface modes (SM) are highly spatially localized modes existing at the\ncore-cladding interface of photonic-bandgap hollow-core fiber (PBG-HCF). When\ncoupling with SM, the air modes (AM) in the core would suffer a higher loss\ndespite being spectrally within the cladding photonic bandgap, and would be\nhighly dispersive around the avoided crossing (anti-crossing) wavelength. In\nthis paper, we numerically demonstrate that such avoided crossings can play an\nimportant role in the tuning of the temperature dependence of group delay of AM\nof PBG-HCF. At higher temperatures, both the thermal-optic effect and thermal\nexpansion contribute to the redshift of avoided crossing wavelength, giving\nrise to a temperature dependence of the AM dispersion. Numerical simulations\nshow that the redshift of avoided crossing can significantly tune the thermal\ncoefficient of delay (TCD) of PBG-HCF from -400 ps/km/K to 400 ps/km/K,\napproximately -120 ppm/K to 120 ppm/K. In comparison with the known tuning\nmechanism by the thermal-induced redshift of photonic bandgap [Fokoua et al.,\nOptica 4, 659, 2017], the tuning of TCD by SM coupling presents a much broader\ntuning range and higher efficiency. Our finding would provide a new route to\ndesign PBG-HCF for propagation time sensitive applications.",
        "category": "physics_optics"
    },
    {
        "text": "Reconstruction of hidden 3D shapes using diffuse reflections: We analyze multi-bounce propagation of light in an unknown hidden volume and\ndemonstrate that the reflected light contains sufficient information to recover\nthe 3D structure of the hidden scene. We formulate the forward and inverse\ntheory of secondary and tertiary scattering reflection using ideas from energy\nfront propagation and tomography. We show that using careful choice of\napproximations, such as Fresnel approximation, greatly simplifies this problem\nand the inversion can be achieved via a backpropagation process. We provide a\ntheoretical analysis of the invertibility, uniqueness and choices of\nspace-time-angle dimensions using synthetic examples. We show that a 2D streak\ncamera can be used to discover and reconstruct hidden geometry. Using a 1D high\nspeed time of flight camera, we show that our method can be used recover 3D\nshapes of objects \"around the corner\".",
        "category": "physics_optics"
    },
    {
        "text": "Sub-diffraction light propagation in fibers with anisotropic dielectric\n  cores: We present a detailed study of light propagation in waveguides with\nanisotropic metamaterial cores. We demonstrate that in contrast to conventional\noptical fibers, our structures support free-space-like propagating modes even\nwhen the waveguide radius is much smaller than the wavelength. We develop\nanalytical formalism to describe mode structure and propagation in strongly\nanisotropic systems and study the effects related to waveguide boundaries and\nmaterial composition.",
        "category": "physics_optics"
    },
    {
        "text": "A reinterpretation of the metamaterial perfect absorber: We develop a simple treatment of a metamaterial perfect absorber (MPA) based\non grating theory. We analytically prove that the condition of MPA requires the\nexistence of two currents, which are nearly out of phase and have almost\nidentical amplitude, akin to a magnetic dipole. Furthermore, we show that\nnon-zero-order Bragg modes within the MPA may consume electromagnetic energy\nsignificantly.",
        "category": "physics_optics"
    },
    {
        "text": "Asymptotically Fault-Tolerant Programmable Photonics: Component errors limit the scaling of programmable coherent photonic\ncircuits. These errors arise because the standard tunable photonic coupler --\nthe Mach-Zehnder interferometer (MZI) -- cannot be perfectly programmed to the\ncross state. Here, we introduce two modified circuit architectures that\novercome this limitation: (1) a 3-splitter MZI mesh for generic errors, and (2)\na broadband MZI+Crossing design for correlated errors. Because these designs\nallow for perfect realization of the cross state, the matrix fidelity no longer\ndecreases with mesh size, allowing scaling to arbitrarily large meshes. The\nproposed architectures support progressive self-configuration, are more compact\nthan previous MZI-doubling schemes, and do not require additional phase\nshifters. This eliminates a major obstacle to the development of\nvery-large-scale linear photonic circuits.",
        "category": "physics_optics"
    },
    {
        "text": "Theoretical Study of Plasmonic Lasing in Junctions with many Molecules: We calculate the quantum state of the plasmon field excited by an ensemble of\nmolecular emitters, which are driven by exchange of electrons with metallic\nnano-particle electrodes. Assuming identical emitters that are coupled\ncollectively to the plasmon mode but are otherwise subject to independent\nrelaxation channels, we show that symmetry constraints on the total system\ndensity matrix imply a drastic reduction in the numerical complexity. For\n$N_{\\text{m}}$ three-level molecules we may thus represent the density matrix\nby a number of terms scaling as $(N_{\\rm m}+8)!/(8!N_{\\rm m}!)$ instead of\n$9^{N_{\\text{m}}}$, and this allows exact simulations of up to\n$N_{\\text{m}}=10$ molecules. Our simulations demonstrate that many emitters\ncompensate strong plasmon damping and lead to the population of high plasmon\nnumber states and a narrowed linewidth of the plasmon field. For large\n$N_{\\text{m}}$, our exact results are reproduced by an approximate approach\nbased on the plasmon reduced density matrix. With this approach, we have\nextended the simulations to more than $50$ molecules and shown that the plasmon\nnumber state population follows a Poisson-like distribution. An alternative\napproach based on nonlinear rate equations for the molecular state populations\nand the mean plasmon number also reproduce the main lasing characteristics of\nthe system.",
        "category": "physics_optics"
    },
    {
        "text": "Ultraviolet Mie resonances in computationally discovered boron phosphide\n  nanoparticles: Controlling ultraviolet light at the nanoscale using optical Mie resonances\nholds great promise for a diverse set of applications, such as lithography,\nsterilization, and biospectroscopy. However, Mie resonances hosted by\ndielectric nanoantennas are difficult to realize at ultraviolet wavelengths due\nto the lack of both suitable materials and fabrication methods. Here, we\nsystematically search for improved materials by computing the frequency\ndependent optical permittivity of 338 binary semiconductors and insulators from\nfirst principles, and evaluate their potential performance as high refractive\nindex materials using Mie theory. Our analysis reveals several interesting\ncandidate materials among which boron phosphide (BP) appears particularly\npromising. We then prepare BP nanoparticles and demonstrate that they support\nMie resonances at visible and ultraviolet wavelengths using both far-field\noptical measurements and near-field electron energy-loss spectroscopy. We also\npresent a laser reshaping method to realize spherical Mie-resonant BP\nnanoparticles. With a refractive index above 3 and low absorption losses, BP\nnanostructures advance Mie optics to the ultraviolet.",
        "category": "physics_optics"
    },
    {
        "text": "(3+1)D-printed adiabatic 1-to-N broadband couplers: We report single-mode 3D optical couplers leveraging adiabatic power transfer\ntowards up to 4 output ports. We use the CMOS compatible additive (3+1)D\n\\emph{flash}-TPP printing for fast and scalable fabrication. Coupling optical\nlosses of such devices are reduced below $\\sim$~0.06~dB by tailoring the\ncoupling and waveguides geometry, and we demonstrate almost octave-spanning\nbroadband functionality from 520~nm to 980~nm.",
        "category": "physics_optics"
    },
    {
        "text": "Fiber guided mode analyses and conversion using superposed helical\n  gratings: Optical fibers can support various modal forms, including vector modes,\nlinear polarization (LP) modes, and orbital angular momentum (OAM) modes, etc.\nThe modal correlation among these modes is investigated via Jones matrix,\nassociated with polarization and helical phase corresponding to spin angular\nmomentum (SAM) and OAM of light, respectively. We can generate different modal\nforms by adopting superposed helical gratings (SHGs) with opposite helix\norientations. Detailed analysis and discussion on mode conversion is given as\nfor mode coupling in optical fibers with both low and high contrast index,\nrespectively. Our study may deepen the understanding for various fiber guided\nmodes and mode conversion among them via fiber gratings",
        "category": "physics_optics"
    },
    {
        "text": "Deciphering exciton-generation processes in quantum-dot\n  electroluminescence: Electroluminescence (EL) of colloidal nanocrystals promises a new generation\nof high-performance and solution-processable light-emitting diodes (LEDs). The\noperation of nanocrystal-based LEDs relies on the recombination of\nelectrically-generated excitons. However, a fundamental question, i.e, how\nexcitons are electrically generated in individual nanocrystals, remains\nunanswered. Here, we reveal a molecular mechanism of sequential electron-hole\ninjection for the exciton generation in nanocrystal-based EL devices. To\ndecipher the corresponding elementary processes, we develop electrically-pumped\nsingle-nanocrystal spectroscopy. While hole injection into neutral quantum dots\n(QDs) is generally-considered to be inefficient, we find that the intermediate\nnegatively-charged state of QD triggers confinement-enhanced Coulomb\ninteractions, which simultaneously accelerate hole injection and hinder\nexcessive electron injection. In-situ/operando spectroscopy on state-of-the-art\nQD-LEDs demonstrate that exciton generation at the ensemble level is consistent\nwith the charge-confinement-enabled sequential electron-hole injection\nmechanism revealed at the single-nanocrystal level. Our findings provide a\nuniversal mechanism for enhancing charge balance in nanocrystal-based EL\ndevices.",
        "category": "physics_optics"
    },
    {
        "text": "Transfer matrix method for four-flux radiative transfer: We develop a transfer matrix formalism for four-flux radiative transfer\nmodels, which is ideally suited for studying transport through multiple\nscattering layers. The model, derived for spherical particles within the\ndiffusion approximation, predicts the specular and diffuse reflection and\ntransmission of multilayer composite films for diffuse or collimated incidence.\nThe model shows remarkable agreement with numerical Monte Carlo simulations for\na range of absorption and film thicknesses, as well as for an example\nmultilayer slab.",
        "category": "physics_optics"
    },
    {
        "text": "Strain-controlled high harmonic generation with Dirac fermions in\n  silicene: Two-dimensional (2D) materials with zero band gap exhibit remarkable\nelectronic properties with wide tunability. High harmonic generation (HHG) in\nsuch materials offers unique platforms to develop novel optoelectronic devices\nat nanoscale, as well as to investigate strong-field and ultrafast nonlinear\nbehaviour of massless Dirac fermions. However, control of HHG by modulating\nelectronic structure of materials remains largerly unexplored to date. Here we\nreport controllable HHG by tuning the electronic structures via mechanical\nengineering. Using an \\textit{ab initio} approach based on time-dependent\ndensity-functional theory (TDDFT), we show that the HHG process is sensitive to\nthe modulation of band structures of monolayer silicene while preserving the\nDirac cones under biaxial and uniaxial strains, which can lead to significant\nenhancement of harmonic intensity up to an order of magnitude. With the\nadditional advantage of silicene in compatibility and integration into the\ncurrent silicon-based electronic industry, this study may open a new avenue to\ndevelop efficient solid-state optoelectronic nano-devices, and provide a\nvaluable tool to understand the strong-field and mechanically induced ultrafast\nnonlinear response of Dirac carriers in 2D materials.",
        "category": "physics_optics"
    },
    {
        "text": "Accurate Self-Configuration of Rectangular Multiport Interferometers: Multiport interferometers based on integrated beamsplitter meshes are widely\nused in photonic technologies. While the rectangular mesh is favored for its\ncompactness and uniformity, its geometry resists conventional\nself-configuration approaches, which are essential to programming large meshes\nin the presence of fabrication error. Here, we present a new configuration\nalgorithm, related to the $2\\times 2$ block decomposition of a unitary matrix,\nthat overcomes this limitation. Our proposed algorithm is robust to errors,\nrequires no prior knowledge of the process variations, and relies only on\nexternal sources and detectors. We show that self-configuration using this\ntechnique reduces the effect of fabrication errors by the same quadratic factor\nobserved in triangular meshes. This relaxes a significant limit to the size of\nmultiport interferometers, removing a major roadblock to the scaling of optical\nquantum and machine-learning hardware.",
        "category": "physics_optics"
    },
    {
        "text": "Anderson Localization with Second Quantized Fields: Quantum Statistical\n  Aspects: We report a theoretical study of Anderson localization of nonclassical light\nwith emphasis on the quantum statistical aspects of localized light. We\ndemonstrate, from the variance in mean intensity of localized light, as well as\nsite-to-site correlations, that the localized light carries signatures of\nquantum statistics of input light. For comparison, we also present results for\ninput light with coherent field statistics and thermal field statistics. Our\nresults show that there is an enhancement in fluctuations of localized light\ndue to the medium's disorder. We also find superbunching of the localized\nlight, which may be useful for enhancing the interaction between radiation and\nmatter. Another important consequence of sub-Poissonian statistics of the\nincoming light is to quench the total fluctuations at the output. Finally, we\ncompare the effects of Gaussian and Rectangular distributions for the disorder,\nand show that Gaussian disorder accelerates the localization of light.",
        "category": "physics_optics"
    },
    {
        "text": "On the origin of the optical nonlinearity of a gallium-silica interface: Simultaneous measurements of the intensity and phase of a probe wave\nreflected from an interface between silica and elemental alpha-gallium reveal\nits very strong optical nonlinearity, affecting both these parameters of the\nreflected wave. The data corroborate with a non-thermal mechanism of optical\nresponse which assumes appearance of a homogeneous highly metallic layer, only\na few nanometer thick, between the silica and bulk alpha-gallium.",
        "category": "physics_optics"
    },
    {
        "text": "Corner reflectors: fractal analysis and integrated single-photon sources: In this work, the properties of the radiation emitted by a corner reflector\nwith an electric dipole feeder are analyzed in the optical domain where the\ndistance between the dipole and the corner apex can be large in terms of the\noptical wavelength. A comprehensive study of the fractal properties of the\nradiated intensity patterns is presented. The use of this setup for the\nrealization of single-photon sources in photonic integrated circuits is also\nput forward and a detailed study of the emission properties of the device and\nits optimal configurations is presented.",
        "category": "physics_optics"
    },
    {
        "text": "The Double Jones Birefringence in Magneto-electric Medium: In this paper, the Maxwell's equations for the tensorial magneto-electric\n(ME) medium have been solved which in fact is the extension of anisotropic\nnonmagnetic medium. All of the dielectric permittivity, magnetic permeability\nand the ME tensors are considered. The transverse polarization is shown\nexplicitly and the propagation of electromagnetic wave in the ME medium is\nfound to have the Double Jones Birefringence. We also find the condition of\nD'yakonov surface wave for magneto-isotropic but with ME anisotropic medium.\nEspecially when the incident angle is $\\frac{\\pi}{4}$, it may be measurable in\nprinciple.",
        "category": "physics_optics"
    },
    {
        "text": "Fourier Transform Model for All-Order PMD Compensation based on a\n  Coupled-Mode Equation Solution using the First Born Approximation: We present a Fourier transform methodology for all-order polarization mode\ndispersion (PMD) analysis, based on the first Born approximation to the\ncoupled-mode equation solution. Our method predicts wavelength-dependent PMD\neffects and allows design of filters for their mitigation.",
        "category": "physics_optics"
    },
    {
        "text": "Plasmonic materials for energy: from physics to applications: Physical mechanisms unique to plasmonic materials, which can be exploited for\nthe existing and emerging applications of plasmonics for renewable energy\ntechnologies, are reviewed. The hybrid nature of surface plasmon (SP) modes -\npropagating surface plasmon polaritons (SPPs) and localized surface plasmons\n(LSPs) - as collective photon-electron oscillations makes them attractive\ncandidates for energy applications. High density of optical states in the\nvicinity of plasmonic structures enhances light absorption and emission,\nenables localized heating, and drives near-field heat exchange between hot and\ncold surfaces. SP modes channel the energy of absorbed photons directly to the\nfree electrons, and the generated hot electrons can be utilized in\nthermoelectric, photovoltaic and photo-catalytic platforms. Advantages and\ndisadvantages of using plasmonics over conventional technologies for solar\nenergy and waste heat harvesting are discussed, and areas where plasmonics is\nexpected to lead to performance improvements not achievable by other methods\nare identified.",
        "category": "physics_optics"
    },
    {
        "text": "Re-sampling of inline holographic images for improved reconstruction\n  resolution: Digital holographic microscopy based on Gabor in-line holography is a\nwell-known method to reconstruct both the amplitude and phase of small objects.\nTo reconstruct the image of an object from its hologram, obtained under\nillumination by monochromatic scalar waves, numerical calculations of Fresnel\nintegrals are required. To improve spatial resolution in the resulting\nreconstruction, we re-sample the holographic data before application of the\nreconstruction algorithm. This procedure amounts to inverting an interpolated\nFresnel diffraction image to recover the object. The advantage of this method\nis demonstrated on experimental data, for the case of visible-light Gabor\nholography of a resolution grid and a gnat wing.",
        "category": "physics_optics"
    },
    {
        "text": "Thermodynamic theory of highly multimoded nonlinear optical systems: The quest for ever higher information capacities has brought about a\nrenaissance in multimode optical waveguide systems. This resurgence of interest\nhas recently initiated a flurry of activities in nonlinear multimode fiber\noptics. The sheer complexity emerging from the presence of a multitude of\nnonlinearly interacting modes has led not only to new opportunities in\nobserving a host of novel optical effects that are otherwise impossible in\nsingle-mode settings, but also to new theoretical challenges in understanding\ntheir collective dynamics. In this Article, we present a consistent\nthermodynamical framework capable of describing in a universal fashion the\nexceedingly intricate behavior of such nonlinear highly multimoded photonic\nconfigurations at thermal equilibrium. By introducing pertinent extensive\nvariables, we derive new equations of state and show that both the 'internal\nenergy' and optical power in many-mode arrangements always flow in such a way\nso as to satisfy the second law of thermodynamics. The laws governing\nisentropic processes are derived and the prospect for realizing Carnot-like\ncycles is also presented. In addition to shedding light on fundamental issues,\nour work may pave the way towards a new generation of high power multimode\noptical structures and could have ramifications in other many-state nonlinear\nsystems, ranging from Bose-Einstein condensates to optomechanics.",
        "category": "physics_optics"
    },
    {
        "text": "Complex-amplitude Fourier single-pixel imaging via coherent structured\n  illumination: We propose a method of complex-amplitude Fourier single-pixel imaging (CFSI)\nwith coherent structured illumination to acquire both the amplitude and phase\nof an object. In the proposed method, an object is illustrated by a series of\ncoherent structured light fields which are generated by a phase-only spatial\nlight modulator, the complex Fourier spectrum of the object can be acquired\nsequentially by a single-pixel photodetector. Then the desired\ncomplex-amplitude image can be retrieved directly by applying an inverse\nFourier transform. We experimentally implemented this CFSI with several\ndifferent types of objects. The experimental results show that the proposed\nmethod provides a promising complex-amplitude imaging approach with high\nquality and a stable configuration. Thus, it might find broad applications in\noptical metrology and biomedical science.",
        "category": "physics_optics"
    },
    {
        "text": "Highly-tunable efficient second-harmonic generation in a lithium niobate\n  nanophotonic waveguide: Highly-tunable coherent light generation is crucial for many important\nphotonic applications. Second-harmonic generation (SHG) is a dominant approach\nfor this purpose, which, however, exhibits a trade-off between the conversion\nefficiency and the wavelength tunability in a conventional nonlinear platform.\nRecent development of the integrated lithium niobate (LN) technology makes it\npossible to achieve a large wavelength tuning while maintaining a high\nconversion efficiency. Here we report on-chip SHG that simultaneously achieves\na large tunability and a high conversion efficiency inside a single device. We\nutilize the unique strong thermo-optic birefringence of LN to achieve flexible\ntemperature tuning of type-I inter-modal phase matching. We experimentally\ndemonstrate spectral tuning with a tuning slope of 0.84 nm/K for a telecom-band\npump, and a nonlinear conversion efficiency of 4.7% W$^{-1}$, in a LN\nnanophotonic waveguide only 8~mm long. Our device shows great promise for\nefficient on-chip wavelength conversion to produce highly-tunable coherent\nvisible light for broad applications, while taking advantage of the mature and\ncost-effective telecom laser technology.",
        "category": "physics_optics"
    },
    {
        "text": "Use of Dynamical Undulator Mechanism to Produce Short Wavelength\n  Radiation in Volume FEL (VFEL): VFEL lasing in system with dynamical undulator is described. In this system\nradiation of long wavelength creates the undulator for lasing on shorter\nwavelength. Two diffraction gratings with different spatial periods form VFEL\nresonator. The grating with longer period pumps the resonator with long\nwavelength radiation to provide necessary amplitude of undulator field. The\ngrating with shorter period makes mode selection for short wavelength\nradiation. Lasing of such a system in terahertz frequency range is discussed.",
        "category": "physics_optics"
    },
    {
        "text": "Vacuum as a hyperbolic metamaterial: As demonstrated by Chernodub, vacuum in a strong magnetic field behaves as a\nperiodic Abrikosov vortex lattice in a type-II superconductor. We investigate\nelectromagnetic behavior of vacuum in this state. Since superconductivity is\nrealized along the axis of magnetic field only, strong anisotropy of the vacuum\ndielectric tensor is observed. The diagonal components of the tensor are\npositive in the x and y directions perpendicular to the magnetic field, and\nnegative in the z direction along the field. As a result, vacuum behaves as a\nhyperbolic metamaterial medium. If the magnetic field is constant, low\nfrequency extraordinary photons experience this medium as a (3+1) Minkowski\nspacetime in which the role of time is played by the spatial z coordinate.\nSpatial variations of the magnetic field curve this effective spacetime, and\nmay lead to formation of \"event horizons\", which are analogous to\nelectromagnetic black holes in hyperbolic metamaterials. We also note that\nhyperbolic metamaterials behave as diffractionless \"perfect lenses\". Since\nlarge enough magnetic fields probably had arisen in the course of evolution of\nearly Universe, the demonstrated hyperbolic behavior of early vacuum may have\nimprints in the large scale structure of the present-day Universe.",
        "category": "physics_optics"
    },
    {
        "text": "Increasing the imaging capabilities of multimode fibers by exploiting\n  the properties of highly scattering media: We present a novel design that exploits the focusing properties of scattering\nmedia to increase the resolution and the working distance of multimode fiber\nbased imaging devices. Placing a highly scattering medium in front of the\ndistal tip of the multimode fiber enables the formation of smaller sized foci\nat increased working distances away from the fiber tip. We perform a parametric\nstudy of the effect of the working distance and the separation between the\nfiber and the scattering medium on the focus size. We experimentally\ndemonstrate submicron focused spots as far away as 800{\\mu}m with 532nm light.",
        "category": "physics_optics"
    },
    {
        "text": "Optomechanical Metamaterials: Dirac polaritons, Gauge fields, and\n  Instabilities: Freestanding photonic crystals can be used to trap both light and mechanical\nvibrations. These \"optomechanical crystal\" structures have already been\nexperimentally demonstrated to yield strong coupling between a photon mode and\na phonon mode, co-localized at a single defect site. Future devices may feature\na regular superlattice of such defects, turning them into \"optomechanical\narrays\". In this letter we predict that tailoring the optomechanical band\nstructure of such arrays can be used to implement Dirac physics of photons and\nphonons, to create a photonic gauge field via mechanical vibrations, and to\nobserve a novel optomechanical instability.",
        "category": "physics_optics"
    },
    {
        "text": "Chiral scatterers designed by Bayesian optimization: The helicity or chirality of scattered light is strongly linked to the dual\nsymmetry of the scatterer. The latter depends on chiral materials or on\nscatterers which are not superimposable with their mirror image. This\ninherently yields asymmetric structures of various shapes with many degrees of\nfreedom. In order to explore these high dimensional parameter spaces, numerical\nsimulations and especially optimization strategies are a valueable tool. Here,\nwe optimize the emission of chiral line sources in two-dimensional dimer setups\nusing Bayesian optimization. We deduce relevant objective functions from recent\ntheoretical findings for chiral electromagnetic fields and employ rigorous\nsimulations of Maxwell's equations.",
        "category": "physics_optics"
    },
    {
        "text": "An integrated optical device for frequency conversion across the full\n  telecom C-band spectrum: High-density communication through optical fiber is made possible by\nWavelength Division Multiplexing, which is the simultaneous transmission of\nmany discrete signals at different optical frequencies. Vast quantities of data\nmay be transmitted without interference using this scheme but flexible routing\nof these signals requires an electronic middle step, carrying a cost in\nlatency. We present a technique for frequency conversion across the entire WDM\nspectrum with a single device, which removes this latency cost. Using an\noptical waveguide in lithium niobate and two infrared pump beams, we show how\nto maximize conversion efficiency between arbitrary frequencies by analyzing\nthe role of dispersion in cascaded nonlinear processes. The technique is\npresented generally and may be applied to any suitable nonlinear material or\nplatform, and to classical or quantum optical signals.",
        "category": "physics_optics"
    },
    {
        "text": "Machine learning predicts extreme events in ultrashort pulse lasers: In this paper we present a nonlinear autoregressive neural network with a\nhidden layer of 50 neurons, three delays and one output layer that accurately\nis capable of predict the appearence of extreme events in a Kerr lens mode\nlocking Ti:Sapphire laser with ultrashort pulses. Extreme events are produced\nin the context of a chaotic atractor and with chirped pulses. The prediction of\nthis neural network works well with experimental and theoretical time series of\namplitude of laser pulses. When fed with experimental time series we have\n95.45\\% of hits and 6.67\\% of false positives while using theoretical time\nseries the network predicts 100\\% of extreme events but the false positive rise\nto 23.33\\%.",
        "category": "physics_optics"
    },
    {
        "text": "Anti-PT-symmetry-enhanced interconversion between microwave and optical\n  fields: The intrinsic dissipation of systems into a shared reservoir introduces\ncoherence between two systems, enabling anti-Parity-Time (anti-PT) symmetry. In\nthis paper, we propose an anti-PT symmetric converter, consisting of a\nmicrowave cavity coupled dissipatively to a ferromagnetic sphere, which\nsupports significant improvements in the conversion efficiency when compared to\ncoherently coupled setups. In particular, when only the ferrite sample is\ndriven, the strong coherence induced by the vacuum of the mediating channel\nleads to much stronger enhancements in the intended conversion. The enhancement\nis an inalienable artifact of the emergence of a long-lived, dark mode\nassociated with a quasi-real singularity of the hybrid system. In addition, we\nobserve considerable asymmetry in the efficiencies of microwave-to-optical and\noptical-to-microwave conversions, in spite of the symmetrical structure of the\ntrilinear optomagnonic coupling stimulating both the transduction phenomena.\nThe nonreciprocity stems from the intrinsic asymmetry in the couplings of the\nmicrowave and optical fields to the cavity-magnon network as well as the phase\ncoupling entailed by the spatial separation.",
        "category": "physics_optics"
    },
    {
        "text": "First-principle calculation of solar cell efficiency under incoherent\n  illumination: Because of the temporal incoherence of sunlight, solar cells efficiency\nshould depend on the degree of coherence of the incident light. However,\nnumerical computation methods, which are used to optimize these devices,\nfundamentally consider fully coherent light. Hereafter, we show that the\nincoherent efficiency of solar cells can be easily analytically calculated. The\nincoherent efficiency is simply derived from the coherent one thanks to a\nconvolution product with a function characterizing the incoherent light. Our\napproach is neither heuristic nor empiric but is deduced from first-principle,\ni.e. Maxwell's equations. Usually, in order to reproduce the incoherent\nbehavior, statistical methods requiring a high number of numerical simulations\nare used. With our method, such approaches are not required. Our results are\ncompared with those from previous works and good agreement is found.",
        "category": "physics_optics"
    },
    {
        "text": "Attenuation of slow Metal-Insulator-Metal plasmonic waveguides, from\n  Joule absorption to roughness-induced backscattering: By combining analytical and numerical approaches, we theoretically\ninvestigate the effect of fabrication imperfections, e.g. roughness at metal\ninterfaces, on nanometer metal-insulator-metal waveguides supporting slow\ngap-plasmon modes. Realistic devices with vapor deposition- and\nchemically-grown metal films are considered. We obtain quantitative predictions\nfor the attenuations induced by absorption and by backscattering, and\nanalytically derive how both attenuations scale with respect to the group\nvelocity. Depending on the material parameters and fabrication quality,\nroughness-induced backscattering is find to be a significant additional source\nof attenuation for small group velocities, which is often neglected in the\nliterature.",
        "category": "physics_optics"
    },
    {
        "text": "Improved performance of traveling wave directional coupler modulator\n  based on electro-optic polymer: Polymer based electro-optic modulators have shown great potentials in high\nfrequency analog optical links. Existing commercial LiNibO3 Mach-Zehnder\nmodulators have intrinsic drawbacks in linearity to provide high fidelity\ncommunication. In this paper, we present the design, fabrication and\ncharacterization of a traveling wave directional coupler modulator based on\nelectro-optic polymer, which is able to provide high linearity, high speed, and\nlow optical insertion loss. A silver ground electrode is used to reduce\nwaveguide sidewall roughness due to the scattering of UV light in\nphotolithography process in addition to suppressing the RF loss. A 1-to-2\nmulti-mode interference 3dB-splitter, a photobleached refractive index taper\nand a quasi-vertical taper are used to reduce the optical insertion loss of the\ndevice. The symmetric waveguide structure of the MMI-fed directional coupler is\nintrinsically bias-free, and the modulation is obtained at the 3-dB point\nregardless of the ambient temperature. By achieving low RF loss, characteristic\nimpedance matching with 50{\\Omega} load, and excellent velocity matching\nbetween the RF wave and the optical wave, a travelling wave electrode is\ndesigned to function up to 62.5GHz. Domain-inversion poling with push-pull\nconfiguration is applied using alternating pulses on a 2-section\ndirectional-coupler to achieve a spurious free dynamic range of 110dB/Hz2/3.\nThe 3-dB electrical bandwidth of device is measured to be 10GHz.",
        "category": "physics_optics"
    },
    {
        "text": "Revisiting the Polarization Mode Dispersion in Fixed Modulus Model for\n  Random Birefringence: Sometime ago, Galtarossa et al presented some approximate analytical results\nfor the Polarization Mode Dispersion (PMD) assuming a Fixed Modulus Model for\nthe random birefringence. We solve the model exactly and present some new\nanalytical and numerical results.",
        "category": "physics_optics"
    },
    {
        "text": "Frequency dependence of mode coupling gain in Yb doped fiber amplifiers\n  due to stimulated thermal Rayleigh scattering: Using a numerical model we study the frequency dependence of mode coupling\ngain due to stimulated thermal Rayleigh scattering in step index, Yb doped,\nfiber amplifiers. The frequency at the gain peak is shown to vary with core\nsize, doping size, population saturation, thermal lensing, fiber coiling,\ndirection of pumping, photodarkening, and pump noise spectra. The predicted\nfrequencies are compared with measured values whenever possible.",
        "category": "physics_optics"
    },
    {
        "text": "Debye representation of dispersive focused waves: We report on a matrix-based diffraction integral that evaluates the focal\nfield of any diffraction-limited axisymmetric complex system. This diffraction\nformula is a generalization of the Debye integral applied to apertured focused\nbeams, which may be accommodated to broadband problems. Longitudinal chromatic\naberration may limit the convenience of the Debye formulation and,\nadditionally, spatial boundaries of validity around the focal point are\nprovided. Fresnel number is reformulated in order to guarantee that the focal\nregion is entirely into the region of validity of the Debye approximation when\nthe Fresnel number of the focusing geometry largely exceeds unity. We have\napplied the matrix-based Debye integral to several examples. Concretely, we\npresent an optical system for beam focusing with strong angular dispersion and\nfree of longitudinal chromatic aberration. This simple formalism leaves an open\ndoor for analysis and design of focused beams with arbitrary angular\ndispersion. Our results are valid for ultrashort pulsed and polychromatic\nincoherent sources.",
        "category": "physics_optics"
    },
    {
        "text": "Single Photon Two-Level Atom Interactions in 1-D Dielectric Waveguide:\n  Quantum Mechanical Formalism and Applications: In this paper, we propose an effective model including a macroscopic\nHamiltonian to describe the interactions between a two-level atom and scattered\nlight in a 1-D dielectric waveguide. The proposed formalism allows us to\nincorporate the effect of changing optical media inside the continuum while\ndemonstrating a non-classical derivation of Fresnel Law. We obtain the\ntransport characteristics of the two-level system, explore its high-Q\nbandreject filter property and discuss the implications of radiative and\nnon-radiative dissipation. In addition, we apply our formalism to a modified\nFabry-P\\'{e}rot interferometer and show the variation in its spontaneous\nemission characteristics with changing interferometer length. Finally, we\nconclude with further remarks on the link between the waveguide and cavity\nquantum electrodynamics.",
        "category": "physics_optics"
    },
    {
        "text": "Three-dimensional metamaterials with an ultra-high effective refractive\n  index over broad bandwidth: The authors introduce a general mechanism, based on electrostatic and\nmagnetostatic considerations, for designing three-dimensional isotopic\nmetamaterials that possess an enhanced refractive index over an extremely large\nfrequency range. The mechanism allows nearly independent control of effective\nelectric permittivity and magnetic permeability without the use of resonant\nelements.",
        "category": "physics_optics"
    },
    {
        "text": "Optical rogue wave in random distributed feedback fiber laser: The famous demonstration of optical rogue wave (RW)-rarely and unexpectedly\nevent with extremely high intensity-had opened a flourishing time for temporal\nstatistic investigation as a powerful tool to reveal the fundamental physics in\ndifferent laser scenarios. However, up to now, optical RW behavior with\ntemporally localized structure has yet not been presented in random fiber laser\n(RFL) characterized with mirrorless open cavity, whose feedback arises from\ndistinctive distributed multiple scattering. Here, thanks to the participation\nof sustained and crucial stimulated Brillouin scattering (SBS) process,\nexperimental explorations of optical RW are done in the highly-skewed transient\nintensity of an incoherently-pumped standard-telecom-fiber-constructed RFL.\nFurthermore, threshold-like beating peak behavior can also been resolved in the\nradiofrequency spectroscopy. Bringing the concept of optical RW to RFL domain\nwithout fixed cavity may greatly extend our comprehension of the rich and\ncomplex kinetics such as photon propagation and localization in disordered\namplifying media with multiple scattering.",
        "category": "physics_optics"
    },
    {
        "text": "Nonlinear waves in a positive-negative coupled waveguide zigzag array: We consider the coupled electromagnetic waves propagating in a waveguide\narray, which consists of alternating waveguides of positive and negative\nrefraction indexes. Due to zigzag configuration there are interactions between\nboth nearest and next nearest neighboring waveguides exist. It is shown that\nthere is a stop band in the spectrum of linear waves. The system of evolution\nequations for coupled waves has the steady state solution describing the\nelectromagnetic pulse running in the array. Numerical simulation demonstrates\nrobustness of these solitary waves.",
        "category": "physics_optics"
    },
    {
        "text": "Long-distance thermal temporal ghost imaging over optical fibers: A thermal ghost imaging scheme between two distant parties is proposed and\nexperimentally demonstrated over long-distance optical fibers. In the scheme,\nthe weak thermal light is split into two paths. Photons in one path are\nspatially diffused according to their frequencies by a spatial dispersion\ncomponent, then illuminate the object and record its spatial transmission\ninformation. Photons in the other path are temporally diffused by a temporal\ndispersion component. By the coincidence measurement between photons of two\npaths, the object can be imaged in a way of ghost imaging, based on the\nfrequency correlation between photons in the two paths. In the experiment, the\nweak thermal light source is prepared by the spontaneous four-wave mixing in a\nsilicon waveguide. The temporal dispersion is introduced by single mode fibers\nof 50 km, which also could be looked as a fiber link. Experimental results show\nthat this scheme can be realized over long-distance optical fibers.",
        "category": "physics_optics"
    },
    {
        "text": "Opaque perfect lenses: The response of the ``perfect lens'', consisting of a slab of lossless\nmaterial of thickness $d$ with $\\epsilon=\\mu=-1$ at one frequency $\\omega_0$ is\ninvestigated. It is shown that as time progresses the lens becomes increasingly\nopaque to any physical TM line dipole source located a distance $d_0<d/2$ from\nthe lens and which has been turned on at time $t=0$. Here a physical source is\ndefined as one which supplies a bounded amount of energy per unit time. In fact\nthe lens cloaks the source so that it is not visible from behind the lens\neither. For sources which are turned on exponentially slowly there is an exact\ncorrespondence between the response of the perfect lens in the long time\nconstant limit and the response of lossy lenses in the low loss limit. Contrary\nto the usual picture where the field intensity has a minimum at the front\ninterface we find that the field diverges to infinity there in the long time\nconstant limit.",
        "category": "physics_optics"
    },
    {
        "text": "The photonic wheel: demonstration of a state of light with purely\n  transverse angular momentum: The concept of angular momentum is ubiquitous to many areas of physics. In\nclassical mechanics, a system may possess an angular momentum which can be\neither transverse (e.g., in a spinning wheel) or longitudinal (e.g., for a\nfluidic vortex) to the direction of motion. Photons, however, are well-known to\nexhibit intrinsic angular momentum which is longitudinal only: the spin angular\nmomentum defining the beam polarization and the orbital angular momentum\nassociated with a spiraling phase front. Here we show that it is possible to\ngenerate a novel state of light that contains purely transverse angular\nmomentum, the analogue of a spinning mechanical wheel. We use an optical\nnano-probing technique to experimentally demonstrate its occurrence in our\nsetup. Such a state of light can provide additional rotational degree of\nfreedom in optical tweezers and optical manipulation.",
        "category": "physics_optics"
    },
    {
        "text": "Second-order nonlinear mixing in planar photonic crystal microcavities: Second-harmonic and sum-frequency mixing phenomena associated with\n3D-localized photonic modes are studied in InP-based planar photonic crystal\nmicrocavities excited by short-pulse radiation near 1550 nm. Three-missing-hole\nmicrocavities that support two closely-spaced modes exhibit rich second-order\nscattering spectra that reflect intra- and inter-mode mixing via the bulk InP\nchi(2) during ring-down after excitation by the broadband, resonant pulse.\nSimultaneous excitation with a non-resonant source results in tunable\nsecond-order radiation from the microcavity.",
        "category": "physics_optics"
    },
    {
        "text": "Side face excited microstructured fibers for photonic integrated\n  circuits formations: In the paper, we propose the new technology for mass production of the\nphotonic crystal devices and all-optical integrated circuits. We carry out the\nbrief analysis of positive and negative sides paying attention to recent\nadvances in silicon fibers fabrication which is crucial moment in the proposed\ntechnology.",
        "category": "physics_optics"
    },
    {
        "text": "Optical diode based on the one-way light-speed anisotropy: We report that a triangular Fabry-Perot resonator filled with a parity-odd\nlinear anisotropic medium exhibiting the one-way light speed anisotropy acts as\na perfect diode. A Linear crystal such as the nematic liquid crystals whose\nmolecular structures break parity can exhibit the one-way light speed\nanisotropy. The one-way light speed anisotropy also can be induced in a\nnon-linear medium in the presence of constant electric and magnetic field\nstrengths.",
        "category": "physics_optics"
    },
    {
        "text": "Energy currents in the field formed by waves with different frequencies: The behavior of instantaneous and averaged vectors of the Poynting vector\ntransverse component for the resulting field formed as a superposition of waves\nwith different frequencies and different polarizations is considered. Results\nof computer simulation and experimental data are obtained for the superposition\nof frequently differed vortices with different and the same signs of\ntopological charges. It is shown that the captured particle rotates due to the\norbital momentum and substantial increase of particle rotation speed in the\ncase of polychromatic radiation as opposed to monochromatic is observed.",
        "category": "physics_optics"
    },
    {
        "text": "Synthesizing gas-filled fiber Raman lines enables access to the\n  molecular fingerprint region: The synthesis of multiple narrow optical spectral lines, precisely and\nindependently tuned across the near- to mid-infrared (IR) region, is a pivotal\nresearch area that enables selective and real-time detection of trace gas\nspecies within complex gas mixtures. However, existing methods for developing\nsuch light sources suffer from limited flexibility and very low pulse energy,\nparticularly in the mid-IR domain. Here, we introduce a new concept based on\nthe gas-filled anti-resonant hollow-core fiber (ARHCF) technology that enables\nthe synthesis of multiple independently tunable spectral lines with high pulse\nenergy of >1 {\\mu}J and a few nanoseconds pulse width in the near- and mid-IR\nregion. The number and wavelengths of the generated spectral lines can be\ndynamically reconfigured. A proof-of-concept laser beam synthesized of two\nnarrow spectral lines at 3.99 {\\mu}m and 4.25 {\\mu}m wavelengths is\ndemonstrated and combined with photoacoustic (PA) modality for real-time SO2\nand CO2 detection. The proposed concept also constitutes a promising way for IR\nmultispectral microscopic imaging.",
        "category": "physics_optics"
    },
    {
        "text": "Fast Design of Plasmonic Metasurfaces Enabled by Deep Learning: Metasurfaces is an emerging field that enables the manipulation of light by\nan ultra-thin structure composed of sub-wavelength antennae and fulfills an\nimportant requirement for miniaturized optical elements. Finding a new design\nfor a metasurface or optimizing an existing design for a desired functionality\nis a computationally expensive and time consuming process as it is based on an\niterative process of trial and error. We propose a deep learning (DL)\narchitecture dubbed bidirectional autoencoder for nanophotonic metasurface\ndesign via a template search methodology. In contrast with the earlier\napproaches based on DL, our methodology addresses optimization in the space of\nmultiple metasurface topologies instead of just one, in order to tackle the one\nto many mapping problem of inverse design. We demonstrate the creation of a\nGeometry and Parameter Space Library (GPSL) of metasurface designs with their\ncorresponding optical response using our DL model. This GPSL acts as a\nuniversal design and response space for the optimization. As an example\napplication, we use our methodology to design a multi-band gap-plasmon based\nhalf-wave plate metasurface. Through this example, we demonstrate the power of\nour technique in addressing the non-uniqueness problem of common inverse\ndesign. Our network converges aptly to multiple metasurface topologies for the\ndesired optical response with a low mean absolute error between desired optical\nresponse and the optical response of topologies searched. Our proposed\ntechnique would enable fast and accurate design and optimization of various\nkinds of metasurfaces with different functionalities.",
        "category": "physics_optics"
    },
    {
        "text": "All-optical coherent control of vacuum Rabi oscillations: When an atom strongly couples to a cavity, it can undergo coherent vacuum\nRabi oscillations. Controlling these oscillatory dynamics quickly relative to\nthe vacuum Rabi frequency enables remarkable capabilities such as Fock state\ngeneration and deterministic synthesis of quantum states of light, as\ndemonstrated using microwave frequency devices. At optical frequencies,\nhowever, dynamical control of single-atom vacuum Rabi oscillations remains\nchallenging. Here, we demonstrate coherent transfer of optical frequency\nexcitation between a single quantum dot and a cavity by controlling vacuum Rabi\noscillations. We utilize a photonic molecule to simultaneously attain strong\ncoupling and a cavity-enhanced AC Stark shift. The Stark shift modulates the\ndetuning between the two systems on picosecond timescales, faster than the\nvacuum Rabi frequency. We demonstrate the ability to add and remove excitation\nfrom the cavity, and perform coherent control of light-matter states. These\nresults enable ultra-fast control of atom-cavity interactions in a nanophotonic\ndevice platform.",
        "category": "physics_optics"
    },
    {
        "text": "Ultrafast optical control over spin and momentum in solids: The coupling of laser light to matter can exert sub-cycle coherent control\nover material properties, with optically induced currents and magnetism shown\nto be controllable on ultrafast femtosecond time scales. Here, by employing\nlaser light consisting of both linear and circular pulses, we show that charge\nof specified spin and crystal momentum can be created with precision throughout\nthe first Brillouin zone. Our hybrid pulses induce in a controlled way both\nadiabatic intraband motion as well as vertical interband excitation between\nvalence and conduction bands, and require only a gapped spin split valley\nstructure for their implementation. This scenario is commonly found in the 2d\nsemi-conductors, and we demonstrate our approach with monolayer WSe$_2$. We\nthus establish a route from laser light to local control over excitations in\nreciprocal space, opening the way to the preparation of momenta specified\nexcited states at ultrafast time scales.",
        "category": "physics_optics"
    },
    {
        "text": "Azimuthal modulation instability, breathers and solitons in ring-core\n  optical fibers: We numerically investigate azimuthal modulation instability in an optical\nfiber supporting orbital angular momentum modes only, i.e. a vortex fiber, by\nmeans of the scalar multimode unidirectional pulse propagation equation. We\ndemonstrate that the nonlinear stage of azimuthal modulation instability taking\nplace in such a ring-core fiber, with anomalous rotation group-velocity\ndispersion, can be simply described by analytical breather solutions of the\ncorresponding nonlinear Schr\\\"odinger equation. Azimuthal soliton dynamics and\nnonlinear compression are also unveiled as well as specific spatial rotating\nfeatures as a function of the topological charge involved. Our results open a\nnew route for studying transverse nonlinear waves in optical fibers and for\nmanipulating orbital angular momentum states.",
        "category": "physics_optics"
    },
    {
        "text": "Two-flux tunable Aharonov-Bohm caging in a photonic lattice: We study the Aharonov-Bohm caging effect in a one-dimensional lattice of\ntheta-shaped units defining a chain of interconnected plaquettes, each one\nthreaded by two synthetic flux lines. In the proposed system, light trapping\nresults from the destructive interference of waves propagating along three\narms, this implies that the caging effect is tunable and it can be controlled\nby changing the tunnel couplings $J$. These features reflect on the diffraction\npattern allowing to establish a clear connection between the lattice topology\nand the resulting AB interference.",
        "category": "physics_optics"
    },
    {
        "text": "Experimental realisation of $\\mathcal{PT}$-symmetric flat bands: The capability to temporarily arrest the propagation of optical signals is\none of the main challenges hampering the ever more widespread use of light in\nrapid long-distance transmission as well as all-optical on-chip signal\nprocessing or computations. To this end, flat-band structures are of particular\ninterest, since their hallmark compact eigenstates do not only allow for the\nlocalization of wave packets, but importantly also protect their transverse\nprofile from deterioration without the need for additional diffraction\nmanagement. In this work, we experimentally demonstrate that, far from being a\nnuisance to be compensated, judiciously tailored loss distributions can in fact\nbe the key ingredient in synthesizing such flat bands in non-Hermitian\nenvironments. We probe their emergence in the vicinity of an exceptional point\nand directly observe the associated compact localised modes that can be excited\nat arbitrary positions of the periodic lattice.",
        "category": "physics_optics"
    },
    {
        "text": "Dichroism for Orbital Angular Momentum using Stimulated Parametric Down\n  Conversion: We theoretically analyze stimulated parametric down conversion as a means to\nproduce dichroism based on the orbital angular momentum (OAM) of an incident\nsignal field. The nonlinear interaction is shown to provide differential gain\nbetween signal states of differing OAM, the peak gain occurring at half the OAM\nof the pump field.",
        "category": "physics_optics"
    },
    {
        "text": "Light Emission by Free Electrons in Photonic Time-Crystals: Photonic time-crystals (PTCs) are spatially-homogeneous media whose\nelectromagnetic (EM) susceptibility varies periodically in time, causing\ntemporal reflections and refractions for any wave propagating within the\nmedium. The time-reflected and time-refracted waves interfere, giving rise to\nFloquet modes with momentum bands separated by gaps (rather than energy bands\nand gaps, as in photonic crystals). Here, we show that a free electron moving\nin a PTC spontaneously emits radiation, and, when associated with momentum-gap\nmodes, the radiation of the electron is exponentially amplified by the\nmodulation of the refractive index. Moreover, under strong electron-photon\ncoupling, the quantum formulation reveals that the spontaneous emission into\nthe PTC bandgap experiences destructive quantum interference with the emission\nof the electron into the PTC band modes, leading to suppression of the\ninterdependent emission. Free-electron physics in PTCs offers a new platform\nfor studying a plethora of exciting phenomena such as radiating dipoles moving\nat relativistic speeds and highly efficient quantum interactions with free\nelectrons. Radiation emission in PTCs and its non-resonant nature hold the\npromise for constructing new particle detectors with adjustable sensitivity and\nhighly tunable laser sources, ranging from terahertz to the x-ray regime,\ndrawing their power from the modulation.",
        "category": "physics_optics"
    },
    {
        "text": "Wavelength dependence of reversible photodegradation of disperse orange\n  11 dye-doped PMMA thin films: Using transmittance imaging microscopy we measure the wavelength dependence\nof reversible photodegradation in disperse orange 11 (DO11) dye-doped\n(poly)methyl-methacrylate (PMMA). The reversible and irreversible inverse\nquantum efficiencies (IQEs) are found to be constant over the spectral region\ninvestigated, with the average reversible IQE being $\\overline{B}_\\alpha= 8.70\n(\\pm 0.38)\\times 10^5$ and the average irreversible IQE being\n$\\overline{B}_\\epsilon= 1.396 (\\pm 0.031)\\times 10^8$. The large difference\nbetween the IQEs is hypothesized to be due to the reversible decay channel\nbeing a direct decay mechanism of the dye, while the irreversible decay channel\nis an indirect mechanism, with the dye first absorbing light, then heating the\nsurrounding environment causing polymer chain scission and cross linking.\nAdditionally, the DO11/PMMA's irreversible IQE is found to be among the largest\nof those reported for organic dyes, implying that the system is highly\nphotostable. We also find that the recovery rate is independent of wavelength,\nwith a value of $\\overline{\\beta}=3.88(\\pm 0.47) \\times 10^{-3}$ min$^{-1}$.\nThese results are consistent with the correlated chromophore domain model of\nreversible photodegradation.",
        "category": "physics_optics"
    },
    {
        "text": "Efficient low-power terahertz generation via on-chip triply-resonant\n  nonlinear frequency mixing: Achieving efficient terahertz (THz) generation using compact turn-key sources\noperating at room temperature and modest power levels represents one of the\ncritical challeges that must be overcome to realize truly practical\napplications based on THz. Up to now, the most efficient approaches to THz\ngeneration at room temperature -- relying mainly on optical rectification\nschemes -- require intricate phase-matching set-ups and powerful lasers. Here\nwe show how the unique light-confining properties of triply-resonant photonic\nresonators can be tailored to enable dramatic enhancements of the conversion\nefficiency of THz generation via nonlinear frequency down-conversion processes.\nWe predict that this approach can be used to reduce up to three orders of\nmagnitude the pump powers required to reach quantum-limited conversion\nefficiency of THz generation in nonlinear optical material systems.\nFurthermore, we propose a realistic design readily accesible experimentally,\nboth for fabrication and demonstration of optimal THz conversion efficiency at\nsub-W power levels.",
        "category": "physics_optics"
    },
    {
        "text": "Ultraviolet photonic crystal laser: We fabricated two dimensional photonic crystal structures in zinc oxide films\nwith focused ion beam etching. Lasing is realized in the near ultraviolet\nfrequency at room temperature under optical pumping. From the measurement of\nlasing frequency and spatial profile of the lasing modes, as well as the\nphotonic band structure calculation, we conclude that lasing occurs in the\nstrongly localized defect modes near the edges of photonic band gap. These\ndefect modes originate from the structure disorder unintentionally introduced\nduring the fabrication process.",
        "category": "physics_optics"
    },
    {
        "text": "Separating Pathways in Double-Quantum Optical Spectroscopy Reveals\n  Excitonic Interactions: Techniques for coherent multidimensional optical spectroscopy have been\ndeveloped and utilised to understand many different processes, including energy\ntransfer in photosynthesis and many-body effects in semiconductor\nnanostructures. Double-quantum 2D spectroscopy is one variation that has been\nparticularly useful for understanding many-body effects. In condensed matter\nsystems, however, there are often many competing signal pathways, which can\nmake it difficult to isolate different contributions and retrieve quantitative\ninformation. Here, a means of separating overlapping pathways while maintaining\nthe fidelity of the relevant peak/s is demonstrated. This selective approach is\nused to isolate the double-quantum signal from a mixed two exciton state in a\nsemiconductor quantum well. The removal of overlapping peaks allows analysis of\nthe relevant peak-shape and thus details of interactions with the environment\nand other carriers to be revealed. An alternative pulse ordering identifies a\ndouble-quantum state associated only with GaAs defects, the signature of which\nhas previously been confused with other interaction induced effects. The\nexperimental approach described here provides access to otherwise hidden\ndetails of excitonic interactions and demonstrates that the manner in which the\ndouble-quantum coherence is generated can be important and provide an\nadditional control to help understand the many-body physics in complex systems.",
        "category": "physics_optics"
    },
    {
        "text": "Asynchronous Locking in Metamaterials of Fluids of Light and Sound: Phonons, the quanta of vibrations, are very important for the equilibrium and\ndynamical properties of matter. GHz coherent phonons can also interact with and\nact as interconnects in a wide range of quantum systems. Harnessing and\ntailoring their coupling to opto-electronic excitations thus becomes highly\nrelevant for engineered materials for quantum technologies. With this\nperspective we introduce polaromechanical metamaterials, two-dimensional arrays\nof $\\mu$m-size zero-dimensional traps confining light-matter polariton fluids\nand GHz phonons. A strong exciton-mediated polariton-phonon interaction\ndetermines the inter-site polariton coupling with remarkable consequences for\nthe dynamics. When locally perturbed by optical excitation, polaritons respond\nby locking the energy detuning between neighbor sites at integer multiples of\nthe phonon energy, evidencing synchronization involving the polariton and\nphonon fields. These results open the path for the coherent control of quantum\nlight fluids with hypersound in a scalable platform.",
        "category": "physics_optics"
    },
    {
        "text": "Photo-induced second-order nonlinearity in stoichiometric silicon\n  nitride waveguides: We report the observation of second-harmonic generation in stoichiometric\nsilicon nitride waveguides grown via low-pressure chemical vapour deposition.\nQuasi-rectangular waveguides with a large cross section were used, with a\nheight of 1 {\\mu}m and various different widths, from 0.6 to 1.2 {\\mu}m, and\nwith various lengths from 22 to 74 mm. Using a mode-locked laser delivering\n6-ps pulses at 1064 nm wavelength with a repetition rate of 20 MHz, 15% of the\nincoming power was coupled through the waveguide, making maximum average powers\nof up to 15 mW available in the waveguide. Second-harmonic output was observed\nwith a delay of minutes to several hours after the initial turn-on of pump\nradiation, showing a fast growth rate between 10$^{-4}$ to 10$^{-2}$ s$^{-1}$,\nwith the shortest delay and highest growth rate at the highest input power.\nAfter this first, initial build-up, the second-harmonic became generated\ninstantly with each new turn-on of the pump laser power. Phase matching was\nfound to be present independent of the used waveguide width, although the\nlatter changes the fundamental and second-harmonic phase velocities. We address\nthe presence of a second-order nonlinearity and phase matching, involving an\ninitial, power-dependent build-up, to the coherent photogalvanic effect. The\neffect, via the third-order nonlinearity and multiphoton absorption leads to a\nspatially patterned charge separation, which generates a spatially periodic,\nsemi-permanent, DC-field-induced second-order susceptibility with a period that\nis appropriate for quasi-phase matching. The maximum measured second-harmonic\nconversion efficiency amounts to 0.4% in a waveguide with 0.9 x 1 {\\mu}m$^2$\ncross section and 36 mm length, corresponding to 53 {\\mu}W at 532 nm with 13 mW\nof IR input coupled into the waveguide. The according $\\chi^{(2)}$ amounts to\n3.7 pm/V, as retrieved from the measured conversion efficiency.",
        "category": "physics_optics"
    },
    {
        "text": "Theory of light reflection and transmission by a plasmonic nanocomposite\n  slab: Emergence of broadband perfect absorption: A theory of light reflection and transmission by an optically thin\nnanocomposite slab which contains randomly distributed metal nanoparticles\n(NPs) is developed. The underlying model takes into account the reflection of\nlight scattered by NPs from the slab boundaries, enhanced decay of localized\nsurface plasmons in dense NP arrays and light scattering at the slab surface --\nthe factors which are beyond the scope of the Maxwell Garnett approximation. It\nis demonstrated that the first two effects lead to broadband perfect absorption\nobserved in such nanocomposites, whereas the last one is responsible for its\nomnidirectional character and polarization insensitivity. These findings open\nup new possibilities for engineering broadband perfect absorption in plasmonic\nnanocomposites.",
        "category": "physics_optics"
    },
    {
        "text": "Defect-induced nonlinearity in 2D nanoparticles: Optical nonlinearity depends on symmetry and symmetries vanish in the\npresence of defects. Vaccancy defects in centrosymmetric crystals and thin\nfilms are a well-known source of even-order optical nonlinearity, e.g. causing\nsecond harmonic generation. The emerging ability to manipulate defects in\ntwo-dimensional materials and nanoparticles provides an opportunity for\nengineering of optical nonlinearity. Here, we demonstrate the effect of defects\non the nonlinear optical response of two-dimensional dielectric nanoparticles.\nUsing a toy model, where bound optical electrons of linear atoms are coupled by\nnonlinear Coulomb interactions, we model defect-induced nonlinearity. We find\nthat defects at particle edges contribute strongly to even-order optical\nnonlinearity and that unique nonlinear signatures of different defect states\ncould provide the smallest conceivable QR-codes and extremely high density\noptical data storage, in principle approaching 1 bit per atom.",
        "category": "physics_optics"
    },
    {
        "text": "Towards Engineering Intrinsic Linewidths and Line-Broadening in\n  Perovskite Nanoplatelets: Perovskite nanoplatelets possess extremely narrow absorption and emission\nlinewidths, which are crucial characteristics for many optical applications.\nHowever, their underlying intrinsic and extrinsic line-broadening mechanisms\nare poorly understood. Here, we apply multi-dimensional coherent spectroscopy\nto determine the homogeneous line-broadening of colloidal perovskite\nnanoplatelet ensembles. We demonstrate control of not only their intrinsic\nlinewidths, but also control of various broadening mechanisms by tuning the\nplatelet geometry. Remarkably, we find that decreasing nanoplatelet thickness\nby a single polyhedral layer results in a 2-fold reduction of the inhomogeneous\nlinewidth and a 3-fold reduction of the intrinsic homogeneous linewidth to the\nsub-meV regime. In addition, our measurements suggest homogeneously broadened\nexciton resonances in 3-layer (but not necessarily 4-layer) nanoplatelets at\nroom-temperature.",
        "category": "physics_optics"
    },
    {
        "text": "Electrical control of single-photon emission in highly-charged\n  individual colloidal quantum dots: Electron transfer to an individual quantum dot promotes the formation of\ncharged excitons with enhanced recombination pathways and reduced lifetimes.\nExcitons with only one or two extra charges have been observed and exploited\nfor very efficient lasing or single quantum dot LEDs. Here, by room-temperature\ntime-resolved experiments on individual giant-shell CdSe/CdS quantum dots, we\nshow the electrochemical formation of highly charged excitons containing more\nthan twelve electrons and one hole. We report the control over intensity\nblinking, along with a deterministic manipulation of quantum dot photodynamics,\nwith an observed 210-fold increase of the decay rate, accompanied by 12-fold\ndecrease of the emission intensity, while preserving single-photon emission\ncharacteristics. These results pave the way for deterministic control over the\ncharge state, and room-temperature decay-rate engineering for colloidal quantum\ndot-based classical and quantum communication technologies.",
        "category": "physics_optics"
    },
    {
        "text": "Non-Hermitian Photonics based on Charge-Parity Symmetry: Parity-time ($\\mathcal{PT}$) symmetry, originally conceived for non-Hermitian\nopen quantum systems, has opened an excitingly new avenue for the coherent\ncontrol of light. By tailoring optical gain and loss in integrated photonic\nstructures, $\\mathcal{PT}$ symmetric non-Hermitian photonics has found\napplications in many fields ranging from single mode lasing to novel\ntopological matters. Here we propose a new paradigm towards non-Hermitian\nphotonics based on the charge-parity ($\\mathcal{CP}$) symmetry that has the\npotential to control the flow of light in an unprecedented way. In particular,\nwe consider continuous dielectric chiral materials, where the charge\nconjugation and parity symmetries are broken individually, but preserved\njointly. Surprisingly, the phase transition between real and imaginary spectra\nacross the exceptional point is accompanied by a dramatic change of the\nphotonic band topology from dielectric to hyperbolic. We showcase broad\napplications of $\\mathcal{CP}$ symmetric photonics such as all-angle\npolarization-dependent negative refraction materials, enhanced spontaneous\nemission for laser engineering, and non-Hermitian topological photonics. The\n$\\mathcal{CP}$ symmetry opens an unexplored pathway for studying non-Hermitian\nphotonics without optical gain/loss by connecting two previously distinct\nmaterial properties: chirality and hyperbolicity, therefore providing a\npowerful tool for engineering many promising applications in photonics and\nother related fields.",
        "category": "physics_optics"
    },
    {
        "text": "Orientation of Nd$^{3+}$ dipoles in yttrium aluminum garnet: A simple\n  yet accurate model: We report an experimental study of the 1064nm transition dipoles in neodymium\ndoped yttrium aluminum garnet (Nd-YAG) by measuring the coupling constant\nbetween two orthogonal modes of a laser cavity for different cuts of the YAG\ngain crystal. We propose a theoretical model in which the transition dipoles,\nslightly elliptic, are oriented along the crystallographic axes. Our\nexperimental measurements show a very good quantitative agreement with this\nmodel, and predict a dipole ellipticity between 2% and 3%. This work provides\nan experimental evidence for the simple description in which transition dipoles\nand crystallographic axes are collinear in Nd-YAG (with an accuracy better than\n1 deg), a point that has been discussed for years.",
        "category": "physics_optics"
    },
    {
        "text": "Spontaneous Rotational Symmetry Breaking in a Kramers Two-Level System: Here, I develop a model for a two-level system that respects the\ntime-reversal symmetry of the atom Hamiltonian and the Kramers theorem. The\ntwo-level system is formed by two Kramers pairs of excited and ground states.\nIt is shown that due to the spin-orbit interaction it is in general impossible\nto find a basis of atomic states for which the crossed transition dipole moment\nvanishes. The parametric electric polarizability of the Kramers two-level\nsystem for a definite ground-state is generically nonreciprocal. I apply the\ndeveloped formalism to study Casimir-Polder forces and torques when the\ntwo-level system is placed nearby either a reciprocal or a nonreciprocal\nsubstrate. In particular, I investigate the stable equilibrium orientation of\nthe two-level system when both the atom and the reciprocal substrate have\nsymmetry of revolution about some axis. Surprisingly, it is found that when\nchiral-type dipole transitions are dominant the stable ground state is not the\none in which the symmetry axes of the atom and substrate are aligned. The\nreason is that the rotational symmetry may be spontaneously broken by the\nquantum vacuum fluctuations, so that the ground state has less symmetry than\nthe system itself.",
        "category": "physics_optics"
    },
    {
        "text": "Plasmonic Antennas Hybridized with Dielectric Waveguides: For the purpose of using plasmonics in an integrated scheme where single\nemitters can be probed efficiently, we experimentally and theoretically study\nthe scattering properties of single nano-rod gold antennas as well as antenna\narrays placed on one-dimensional dielectric silicon nitride waveguides. Using\nreal space and Fourier microscopy correlated with waveguide transmission\nmeasurements, we quantify the spectral properties, absolute strength and\ndirectivity of scattering. The scattering processes can be well understood in\nthe framework of the physics of dipolar objects placed on a planar layered\nenvironment with a waveguiding layer. We use the single plasmonic structures on\ntop of the waveguide as dipolar building blocks for new types of antennas where\nthe waveguide enhances the coupling between antenna elements. We report on\nwaveguide hybridized Yagi-Uda antennas which show directionality in\nout-coupling of guided modes as well as directionality for in-coupling into the\nwaveguide of localized excitations positioned at the feed element. These\nmeasurements together with simulations demonstrate that this system is ideal as\na platform for plasmon quantum optics schemes as well as for fluorescence\nlab-on-chip applications.",
        "category": "physics_optics"
    },
    {
        "text": "Particle acceleration in sub-cycle optical cells: A single laser pulse with spot size smaller than half its wavelength ($w_0 <\n\\lambda/2$) can provide a net energy gain to ultra-relativistic particles. In\nthis paper, we discuss the properties of an optical cell consisting of $N$\nsub-cycle pulses that propagate in the direction perpendicular to the electron\nmotion. We show that the energy gain produced by the cell is proportional to\n$N$ and it is sizable even for $\\mathcal{O}(1\\mathrm{~TW})$ pulses.",
        "category": "physics_optics"
    },
    {
        "text": "From amorphous speckle pattern to reconfigurable Bessel beam via\n  wavefront shaping: Bessel beams are non-diffracting light structures, which can be produced with\nsimple tabletop optical elements such as axicon lenses or ring spatial filters\nand coherent laser beams. One of their main characteristic is that Bessel beams\nmaintain their spatial characteristics after meters of propagation. In this\npaper we demonstrate a system and method for generating Bessel beams from\namorphous speckle patterns, exploiting adaptive optimization by a spatial light\nmodulator. These speckles are generated by light modes transmitted through a\nscattering curtain and selected by a ring shaped filter. With the proposed\nstrategy it is possible to produce at user defined positions, reconfigurable,\nnon-diffracting Bessel beams through a disordered medium.",
        "category": "physics_optics"
    },
    {
        "text": "Direct Spectro-Temporal Characterization of Femtosecond\n  Extreme-Ultraviolet Pulses: We propose a method for a straightforward characterization of the temporal\nshape of femtosecond pulses in the extreme-ultraviolet/soft X-ray spectral\nregion. The approach is based on the presence of a significant linear frequency\nchirp in the pulse. This allows to establish an homothetic relation between the\npulse spectrum and its temporal profile. The theoretical approach is\nreminiscent of the one employed by Fraunhofer for describing far-field\ndiffraction. As an application, we consider the case of a seeded free-electron\nlaser (FEL). Theory is successfully benchmarked with numerical simulations and\nwith experimental data collected on the FERMI@Elettra FEL. The proposed method\nprovides FEL users with an on-line, shot-to-shot spectro-temporal diagnostic\nfor time-resolved experiments.",
        "category": "physics_optics"
    },
    {
        "text": "Extended depth of field of diffraction limited imaging system using\n  spatial light modulator based intensity compensated polarization coded\n  aperture: Reducing the aperture size is a conventional technique to obtain enhanced\nimage resolution in optics but it is obscured by depleting illumination.\nPolarization coded apertures (PCAs) can be employed to circumvent this critical\nartifact. We experimentally demonstrate intensity compensated polarization\nencrypted apertures, which are designed using the polarization modulation\ncharacteristics of LC-SLM. PCAs are not limited by the aperture size and hence\nfar-field point spread function (PSF) can be more conveniently recorded using\nthese PCAs. We experimentally validate that Depth of field (DOF) of a\ndiffraction-limited lens and axial intensity of binary Fresnel zone plate\n(BFZP) is enhanced using PCAs with nominal intensity loss.",
        "category": "physics_optics"
    },
    {
        "text": "Micro-optics fabrication using blurred tomography: We demonstrate the fabrication of millimeter-sized optical components using\ntomographic volumetric additive manufacturing (VAM). By purposely blurring the\nwriting beams through the use of a large etendue source, the layer-like\nartifacts called striations are eliminated enabling the rapid and direct\nfabrication of smooth surfaces. We call this method blurred tomography, and\ndemonstrate its capability by printing a plano-convex optical lens with\ncomparable imaging performance to that of a commercially-available glass lens.\nFurthermore, due to the intrinsic freeform design nature of VAM, we demonstrate\nthe double-sided fabrication of a biconvex microlens array, and for the first\ntime demonstrate overprinting of a lens onto an optical fiber using this\nprinting modality. This approach to VAM will pave the way for low-cost,\nrapid-prototyping of freeform optical components.",
        "category": "physics_optics"
    },
    {
        "text": "Wave-vector and polarization dependence of conical refraction: We experimentally address the wave-vector and polarization dependence of the\ninternal conical refraction phenomenon by demonstrating that an input light\nbeam of elliptical transverse profile refracts into two beams after passing\nalong one of the optic axes of a biaxial crystal, i.e. it exhibits double\nrefraction instead of refracting conically. Such double refraction is\ninvestigated by the independent rotation of a linear polarizer and a\ncylindrical lens. Expressions to describe the position and the intensity\npattern of the refracted beams are presented and applied to predict the\nintensity pattern for an axicon beam propagating along the optic axis of a\nbiaxial crystal.",
        "category": "physics_optics"
    },
    {
        "text": "Modeling Dispersive Coupling and Losses of Localized Optical and\n  Mechanical Modes in Optomechanical Crystals: Periodically structured materials can sustain both optical and mechanical\nexcitations which are tailored by the geometry. Here we analyze the properties\nof dispersively coupled planar photonic and phononic crystals: optomechanical\ncrystals. In particular, the properties of co-resonant optical and mechanical\ncavities in quasi-1D (patterned nanobeam) and quasi-2D (patterned membrane)\ngeometries are studied. It is shown that the mechanical Q and optomechanical\ncoupling in these structures can vary by many orders of magnitude with modest\nchanges in geometry. An intuitive picture is developed based upon a\nperturbation theory for shifting material boundaries that allows the\noptomechanical properties to be designed and optimized. Several designs are\npresented with mechanical frequency ~ 1-10 GHz, optical Q-factor Qo > 10^7,\nmotional masses meff 100 femtograms, optomechanical coupling length LOM < 5\nmicrons, and a radiation-limited mechanical Q-factor Qm > 10^7.",
        "category": "physics_optics"
    },
    {
        "text": "Time Translation Symmetry Breaking in an Isolated Spin-Orbit-Coupled\n  Fluid of Light: We study the interplay between intrinsic spin-orbit coupling and nonlinear\nphoton-photon interactions in a nonparaxial, elliptically polarized fluid of\nlight propagating in a bulk Kerr medium. We find that in situations where the\nnonlinear interactions induce birefringence, i.e., a polarization-dependent\nnonlinear refractive index, their interplay with spin-orbit coupling results in\nan interference between the two polarization components of the fluid traveling\nat different wave vectors, which entails the breaking of translation symmetry\nalong the propagation direction. This phenomenon leads to a Floquet band\nstructure in the Bogoliubov spectrum of the fluid, and to characteristic\noscillations of its intensity correlations. We characterize these oscillations\nin detail and point out their exponential growth at large propagation\ndistances, revealing the presence of parametric resonances.",
        "category": "physics_optics"
    },
    {
        "text": "Ultra-low threshold CW Triply Resonant OPO in the near infrared using\n  Periodically Poled Lithium Niobate: We have operated a CW triply resonant OPO using a PPLN crystal pumped by a\nNd:YAG laser at 1.06 micron and generating signal and idler modes in the 2-2.3\nmicron range. The OPO was operated stably in single mode operation over large\nperiods of time with a pump threshold as low as 500 microwatts.",
        "category": "physics_optics"
    },
    {
        "text": "Dynamic 3D shape measurement based on the phase-shifting moir\u00e9\n  algorithm: In order to increase the efficiency of phase retrieval,Wang proposed a\nhigh-speed moire phase retrieval method.But it is used only to measure the tiny\nobject. In view of the limitation of Wang method,we proposed a dynamic\nthree-dimensional (3D) measurement based on the phase-shifting moire\nalgorithm.First, four sinusoidal fringe patterns with a pi/2 phase-shift are\nprojected on the reference plane and acquired four deformed fringe patterns of\nthe reference plane in advance. Then only single-shot deformed fringe pattern\nof the tested object is captured in measurement process.Four moire fringe\npatterns can be obtained by numerical multiplication between the the AC\ncomponent of the object pattern and the AC components of the reference patterns\nrespectively. The four low-frequency components corresponding to the moire\nfringe patterns are calculated by the complex encoding FT (Fourier transform)\n,spectrum filtering and inverse FT.Thus the wrapped phase of the object can be\ndetermined in the tangent form from the four phase-shifting moire fringe\npatterns using the four-step phase shifting algorithm.The continuous phase\ndistribution can be obtained by the conventional unwrapping algorithm. Finally,\nexperiments were conducted to prove the validity and feasibility of the\nproposed method. The results are analyzed and compared with those of Wang\nmethod, demonstrating that our method not only can expand the measurement\nscope, but also can improve accuracy.",
        "category": "physics_optics"
    },
    {
        "text": "In-plane hyperbolic polariton tuners in terahertz and long-wave infrared\n  regimes: Development of terahertz (THz) and long-wave infrared (LWIR) technologies is\nmainly bottlenecked by the limited intrinsic response of traditional materials.\nHyperbolic phonon polaritons (HPhPs) of van der Waals semiconductors couple\nstrongly with THz and LWIR radiation. However, the mismatch of photon-polariton\nmomentum makes far-field excitation of HPhPs challenging. Here, we propose an\nIn-Plane Hyperbolic Polariton Tuner that is based on patterning van der Waals\nsemiconductors, here {\\alpha}-MoO3, into ribbon arrays. We demonstrate that\nsuch tuners respond directly to far-field excitation and give rise to LWIR and\nTHz resonances with high quality factors up to 300, which are strongly\ndependent on in-plane hyperbolic polariton of the patterned {\\alpha}-MoO3. We\nfurther show that with this tuner, intensity regulation of reflected and\ntransmitted electromagnetic waves, as well as their wavelength and polarization\nselection can be achieved. This is important to development of THz and LWIR\nminiaturized devices.",
        "category": "physics_optics"
    },
    {
        "text": "Surface solitons at interfaces of arrays with spatially-modulated\n  nonlinearity: We address the properties of two-dimensional surface solitons supported by\nthe interface of a waveguide array whose nonlinearity is periodically\nmodulated. When the nonlinearity strength reaches its minima at the points\nwhere the linear refractive index attains its maxima, we found that nonlinear\nsurface waves exist and can be made stable only within a limited band of input\nenergy flows, and for lattice depths exceeding a lower threshold.",
        "category": "physics_optics"
    },
    {
        "text": "A silicon integrated microwave photonic beamformer: Optical beamforming networks (OBFNs) based on optical true time delay lines\n(OTTDLs) are well-known as the promising candidate to solve the bandwidth\nlimitation of traditional electronic phased array antennas (PAAs) due to beam\nsquinting. Here we report the first monolithic 1x8 microwave photonic\nbeamformer based on switchable OTTDLs on the silicon-on-insulator platform. The\nchip consists of a modulator, an eight-channel OBFN, and 8 photodetectors,\nwhich includes hundreds of active and passive components in total. It has a\nwide operating bandwidth from 8 to 18 GHz, which is almost two orders larger\nthan that of electronic PAAs. The beam can be steered to 31 distinguishable\nangles in the range of -75.51{\\deg} to 75.64{\\deg} based on the beam pattern\ncalculation with the measured RF response. The response time for beam steering\nis 56 {\\mu}s. These results represent a significant step towards the\nrealization of integrated microwave photonic beamformers that can satisfy\ncompact size and low power consumption requirements for the future radar and\nwireless communication systems.",
        "category": "physics_optics"
    },
    {
        "text": "A compact resonant \u03a0-shaped photoacoustic cell with low window\n  background for gas sensing: A resonant photoacoustic cell capable of detecting the traces of gases at an\namplitude-modulation regime is represented. The cell is designed so as to\nminimize the window background for the cell operation at a selected acoustic\nresonance. A compact prototype cell (the volume of acoustic cavity of ~ 0.2\ncm^3, total cell weight of 3.5 g) adapted to the narrow diffraction-limited\nbeam of near-infrared laser is produced and examined experimentally. The\nnoise-associated measurement error and laser-initiated signals are studied as\nfunctions of modulation frequency. The background signal and useful response to\nlight absorption by the gas are analyzed in measurements of absorption for\nammonia traces in nitrogen flow with the help of a pigtailed DFB laser diode\noperated near a wavelength of 1.53 um. The performance of absorption detection\nand gas-leak sensing for the prototype operated at the second longitudinal\nacoustic resonance (the resonance frequency of ~ 4.38 kHz, Q-factor of ~ 13.9)\nis estimated. The noise-equivalent absorption normalized to laser-beam power\nand detection bandwidth is ~ 1.44x10^{-9} cm^{-1} W Hz^{-1/2}. The amplitude of\nthe window-background signal is equivalent to an absorption coefficient of ~\n2.82x10^{-7} cm^{-1}.",
        "category": "physics_optics"
    },
    {
        "text": "Optical multipolar torque in structured electromagnetic fields: on\n  `helicity gradient' torque, quadrupolar torque and the spin of field gradient: Structured light mechanically interacts with matter via optical forces and\ntorques. The optical torque is traditionally calculated via the flux of total\nangular momentum (AM) into a volume enclosing an object. In [Phys. Rev. A 92,\n043843 (2015)] a powerful method was suggested to calculate optical torque\nseparately from the flux of the spin and the orbital parts of optical AM,\nproviding useful physical insight. However, the method predicted a new type of\ndipolar torque dependent on the gradient of the helicity density of the optical\nbeam, inconsistent with prior torque calculations. In this work we intend to\nclarify this discrepancy and clear up the confusion. We re-derive, from first\nprinciples and with detailed derivations, both the traditional dipolar total\ntorque using total AM flux, and the spin and orbital torque components based on\nthe corresponding AM contributions, ensuring that their sum agrees with the\ntotal torque. We also test our derived analytical expressions against numerical\nintegration, with exact agreement. We find that `helicity gradient' torque\nterms indeed exist in the spin and orbital components separately, but we\npresent corrected prefactors, such that upon adding them, they cancel out, and\nthe `helicity gradient' term vanishes from the total dipolar torque,\nreconciling literature results. We also derive the analytical expression of the\nquadrupolar torque, showing that it is proportional to the spin of the EM field\ngradient, rather than the local EM field spin, as sometimes wrongly assumed in\nthe literature. We provide examples of counter-intuitive situations where the\nspin of the EM field gradient behaves very differently to the local EM spin.\nNaively using the local EM field spin leads to wrong predictions of the torque\non large particles with strong contributions of quadrupole and higher-order\nmultipoles, especially in a structured incident field.",
        "category": "physics_optics"
    },
    {
        "text": "A platform for electrically pumped polariton simulators and topological\n  lasers: Two-dimensional electronic materials such as graphene and transition metal\ndichalgenides feature unique electrical and optical properties due to the\nconspirative effect of band structure, orbital coupling, and crystal symmetry.\nSynthetic matter, as accomplished by artificial lattice arrangements of cold\natoms, molecules, electron patterning, and optical cavities, has emerged to\nprovide manifold intriguing frameworks to likewise realize such scenarios.\nExciton-polaritons have recently been added to the list of promising candidates\nfor the emulation of system Hamiltonians on a semiconductor platform, offering\nversatile tools to engineer the potential landscape and to access the\nnon-linear electro-optical regime. In this work, we introduce an electronically\ndriven square and honeycomb lattice of exciton-polaritons, paving the way\ntowards real world devices based on polariton lattices for on-chip\napplications. Our platform exhibits laser-like emission from high-symmetry\npoints under direct current injection, hinting at the prospect of electrically\ndriven polariton lasers with possibly topologically non-trivial properties.",
        "category": "physics_optics"
    },
    {
        "text": "Simple structured illumination microscope setup with high acquisition\n  speed by using a spatial light modulator: We describe a two-beam interference structured illumination fluorescence\nmicroscope. The novelty of the presented system lies in its simplicity. A\nprogrammable electro-optical spatial light modulator in an intermediate image\nplane enables precise and rapid control of the excitation pattern in the\nspecimen. The contrast of the projected light pattern is strongly influenced by\nthe polarization state of the light entering the high NA objective. To achieve\nhigh contrast, we use a segmented polarizer. Furthermore, a mask with six holes\nblocks unwanted components in the spatial frequency spectrum of the\nillumination grating. Both these passive components serve their purpose in a\nsimpler and almost as efficient way as active components. We demonstrate a\nlateral resolution of 114.2 +- 9.5 nm at a frame rate of 7.6 fps per\nreconstructed 2D slice.",
        "category": "physics_optics"
    },
    {
        "text": "Experimental demonstration of fiber-accessible metal nanoparticle\n  plasmon waveguides for planar energy guiding and sensing: Experimental evidence of mode-selective evanescent power coupling at\ntelecommunication frequencies with efficiencies up to 75 % from a tapered\noptical fiber to a carefully designed metal nanoparticle plasmon waveguide is\npresented. The waveguide consists of a two-dimensional square lattice of\nlithographically defined Au nanoparticles on an optically thin silicon\nmembrane. The dispersion and attenuation properties of the waveguide are\nanalyzed using the fiber taper. The high efficiency of power transfer into\nthese waveguides solves the coupling problem between conventional optics and\nplasmonic devices and could lead to the development of highly efficient\nplasmonic sensors and optical switches.",
        "category": "physics_optics"
    },
    {
        "text": "Imaging objects through scattering layers and around corners by\n  retrieval of the scattered point spread function: We demonstrate a high-speed method to image objects through a thin scattering\nmedium and around a corner. The method employs a reference object of known\nshape to retrieve the speckle-like point spread function of the scatterer. We\nextract the point spread function of the scatterer from a dynamic scene that\nincludes a static reference object, and use this to image the dynamic objects.\nSharp images are reconstructed from the transmission through a diffuser and\nfrom reflection off a rough surface. The sharp and clean reconstructed images\nfrom single shot data exemplify the robustness of the method.",
        "category": "physics_optics"
    },
    {
        "text": "Visualizing plasmon-exciton polaritons at the nanoscale using electron\n  microscopy: Polaritons are compositional light-matter quasiparticles that have recently\nenabled remarkable breakthroughs in quantum and nonlinear optics, as well as in\nmaterial science. Despite the enormous progress, however, a direct\nnanometer-scale visualization of polaritons has remained an open challenge.\nHere, we demonstrate that plasmon-exciton polaritons, or plexcitons, generated\nby a hybrid system composed of an individual silver nanoparticle and a\nfew-layer transition metal dichalcogenide can be spectroscopically mapped with\nnanometer spatial resolution using electron energy loss spectroscopy in a\nscanning transmission electron microscope. Our experiments reveal important\ninsights about the coupling process, which have not been reported so far. These\ninclude nanoscale variation of Rabi splitting and plasmon-exciton detuning, as\nwell as absorption-dominated extinction signals, which in turn provide the\nultimate evidence for the plasmon-exciton hybridization in the strong coupling\nregime. These findings pioneer new possibilities for in-depth studies of\npolariton-related phenomena with nanometer spatial resolution.",
        "category": "physics_optics"
    },
    {
        "text": "Switch between the types of the symmetry breaking bifurcation in\n  optically induced photorefractive rotational double-well potential: We study the possibility of switching the types of symmetry breaking\nbifurcation (SBB) in the cylinder shell waveguide with helical double-well\npotential along propagation direction. This model is described by the\none-dimensional nonlinear Schr\\\"{o}dinger (NLS) equation. The symmetry- and\nantisymmetry-breakings can be caused by increasing the applied voltage onto the\nwaveguide in the self-focusing and -defocusing cases, respectively. In the\nself-focusing case, the type of SBB can be switched from supercritical to\nsubcritical. While in the self-defocusing case, the type of SBB can not be\nswitched because only one type of SBB is found.",
        "category": "physics_optics"
    },
    {
        "text": "Colloidal superlattices for unnaturally high-index metamaterials at\n  broadband optical frequencies: The recent advance in the assembly of metallic nanoparticles (NPs) has\nenabled sophisticated engineering of unprecedented light-matter interaction at\nthe optical domain. In this work, I expand the design flexibility of NP optical\nmetamaterial to push the upper limit of accessible refractive index to the\nunnaturally high regime. The precise control over the geometrical parameters of\nNP superlattice monolayer conferred the dramatic increase in electric resonance\nand related effective permittivity far beyond the naturally accessible regime.\nSimultaneously, effective permeability, another key factor to achieving high\nrefractive index, was effectively suppressed by reducing the thickness of NPs.\nBy establishing this design rule, I have achieved unnaturally high refractive\nindex (15.7 at the electric resonance and 7.3 at the quasi-static limit) at\nbroadband optical frequencies (100 THz ~ 300 THz). I also combined this NP\nmetamaterial with graphene to electrically control the high refractive index\nover the broad optical frequencies.",
        "category": "physics_optics"
    },
    {
        "text": "Interface states in polariton topological insulators: We address linear and nonlinear topological interface states in polariton\ncondensates excited at the interface of the honeycomb and Lieb arrays of\nmicrocavity pillars in the presence of spin-orbit coupling and Zeeman splitting\nin the external magnetic field. Such interface states appear only in total\nenergy gaps of the composite structure when parameters of the honeycomb and\nLieb arrays are selected such that some topological gaps in the spectrum of one\nof the arrays overlap with topological or nontopological gaps in the spectrum\nof the other array. This is in contrast to conventional edge states at the\ninterface of periodic topological and uniform trivial insulators, whose\nbehavior is determined exclusively by the spectrum of the topological medium.\nThe number of emerging interface states is determined by the difference of the\nChern numbers of the overlapping gaps. Illustrative examples with one or two\ncoexisting unidirectional interface states are provided. The representative\nfeature of the system is the possibility of wide tuning of the concentration of\npower of the interface states between two limiting cases when practically all\npower is concentrated either in the Lieb or the honeycomb array. Localization\nof the interface states and their penetration depth into arrays drastically\nvary with Bloch momentum or upon modification of the amplitude of the interface\nstate in the nonlinear regime. We illustrate topological protection of the\ninterface states manifested in the absence of backscattering on interface\ndefects, and study their modulation instability in the nonlinear regime. The\nlatter leads to formation of quasisolitons whose penetration into different\narrays also depends on Bloch momentum. In addition, we discuss the impact of\nlosses and coherent pump leading to bistability of the interface states.",
        "category": "physics_optics"
    },
    {
        "text": "A Stimulated Raman Loss spectrometer for metrological studies of\n  quadrupole lines of hydrogen isotopologues: We discuss layout and performance of a high-resolution Stimulated Raman Loss\nspectrometer that has been newly developed for accurate studies of spectral\nlineshapes and line center frequencies of hydrogen isotopologues and in general\nof Raman active transitions. Thanks to the frequency comb calibration of the\ndetuning between pump and Stokes lasers and to an active alignment of the two\nbeams, the frequency accuracy is well below 100 kHz. Over the vertical axis the\nspectrometer benefits from shot-noise limited detection, signal enhancement via\nmultipass cell, active flattening of the spectral baseline and measurement\ntimes of few seconds over spectral spans larger than 10 GHz. Under these\nconditions an efficient averaging of Raman spectra is possible over long\nmeasurement times with minimal distortion of spectral lineshapes. By changing\nthe pump laser, transitions can be covered in a very broad frequency span, from\n50 to 5000 $\\mathrm{cm^{-1}}$, including both vibrational and rotational bands.\nThe spectrometer has been developed for studies of fundamental and collisional\nphysics of hydrogen isotopologues and has been recently applied to the\nmetrology of the Q(1) 1-0 line of $\\mathrm{H_2}$.",
        "category": "physics_optics"
    },
    {
        "text": "Surface waves in photonic crystal slabs: Photonic crystals with a finite size can support surface modes when\nappropriately terminated. We calculate the dispersion curves of surface modes\nfor different terminations using the plane wave expansion method. These\nnon-radiative surface modes can be excited with the help of attenuated total\nreflection technique. We did experiments and simulations to trace the surface\nband curve, both in good agreement with the numerical calculations.",
        "category": "physics_optics"
    },
    {
        "text": "Slow light with a swept-frequency source: We introduce a new concept for stimulated-Brillouin-scattering-based slow\nlight in optical fibers that is applicable for broadly-tunable frequency-swept\nsources. It allows slow light to be achieved, in principle, over the entire\ntransparency window of the optical fiber. We demonstrate a slow light delay of\n10 ns at 1550 nm using a 10-m-long photonic crystal fiber with a source sweep\nrate of 0.4 MHz/ns and a pump power of 200 mW. We also show that there exists a\nmaximal delay obtainable by this method, which is set by the SBS threshold,\nindependent of sweep rate. For our fiber with optimum length, this maximum\ndelay is ~38 ns, obtained for a pump power of 760 mW.",
        "category": "physics_optics"
    },
    {
        "text": "Strong Modulation of Infrared Light using Graphene Integration with\n  Plasmonic Fano-Resonant Metasurfaces: Plasmonic metasurfaces represent a promising platform for enhancing\nlight-matter interaction. Active control of the optical response of\nmetasurfaces is desirable for applications such as beam-steering, modulators\nand switches, biochemical sensors, and compact optoelectronic devices. Here we\nuse a plasmonic metasurface with two Fano resonances to enhance the interaction\nof infrared light with electrically controllable single layer graphene. It is\nexperimentally shown that the narrow spectral width of these resonances,\ncombined with strong light/graphene coupling, enables reflectivity modulation\nby nearly an order of magnitude leading to a modulation depth as large as 90%.\n. Numerical simulations demonstrate the possibility of strong active modulation\nof the phase of the reflected light while keeping the reflectivity nearly\nconstant, thereby paving the way to tunable infrared lensing and beam steering",
        "category": "physics_optics"
    },
    {
        "text": "Engineered discreteness enables observation and control of chimera-like\n  states in a system with local coupling: Chimera states -- named after the mythical beast with a lion's head, a goat's\nbody, and a dragon's tail -- correspond to spatiotemporal patterns\ncharacterised by the coexistence of coherent and incoherent domains in coupled\nsystems. They were first identified in 2002 in theoretical studies of spatially\nextended networks of Stuart-Landau oscillators, and have been subject to\nextensive theoretical and experimental research ever since. While initially\nconsidered peculiar to networks with weak nonlocal coupling, recent theoretical\nstudies have predicted that chimera-like states can emerge even in systems with\npurely local coupling. Here we report on the first experimental observations of\nchimera-like states in a system with local coupling -- a coherently-driven Kerr\nnonlinear optical resonator. We show that artificially engineered discreteness\n-- realised by suitably modulating the coherent driving field -- allows for the\nnonlinear localisation of spatiotemporal complexity, and we demonstrate\nunprecedented control over the existence, characteristics, and dynamics of the\nresulting chimera-like states. Moreover, using ultrafast time lens imaging, we\nresolve the chimeras' picosecond-scale internal structure in real time.",
        "category": "physics_optics"
    },
    {
        "text": "Bound states in the continuum and high-Q resonances supported by a\n  dielectric ridge on a slab waveguide: We investigate the diffraction of guided modes of a dielectric slab waveguide\non a simple integrated structure consisting of a single dielectric ridge on the\nsurface of the waveguide. Numerical simulations based on aperiodic rigorous\ncoupled-wave analysis demonstrate the existence of sharp resonant features and\nbound states in the continuum (BICs) in the reflectance and the transmittance\nspectra occurring at oblique incidence of a TE-polarized guided mode on the\nridge. Using the effective index method, we explain the resonances by the\nexcitation of the cross-polarized modes of the ridge. The formation of the BICs\nis confirmed using a theoretical model based on the coupled-wave theory. The\nmodel suggests that the BICs occur due to coupling of quasi-TE and quasi-TM\nmodes of the structure. Simple analytical expressions for the angle of\nincidence and the ridge width predicting the location of the BICs are obtained.\nThe existence of high-Q resonances and BICs makes the considered integrated\nstructure promising for filtering, sensing, transformation of optical signals,\nand enhancing nonlinear light-matter interactions.",
        "category": "physics_optics"
    },
    {
        "text": "3D Source Localization and Polarimetry using High Numerical Aperture\n  Imaging with Rotating PSF: Rotating-PSF imaging via spiral phase engineering can localize point sources\nover large focal depths in a snapshot mode. This letter presents a full\nvector-field analysis of the rotating-PSF imager that quantifies the PSF\nsignature of the polarization state of the imaging light. For sufficiently high\nimage-space numerical apertures, there can be significant wave-polarization\ndependent contributions to the overall PSF, which would allow one to jointly\nlocalize and sense the polarization state of light emitted by point sources in\na 3D field.",
        "category": "physics_optics"
    },
    {
        "text": "Periodic transmission peaks in non-periodic disordered one-dimensional\n  photonic structures: A better understanding of the optical properties of a device structure\ncharacterized by a random arrangement of materials with different dielectric\nproperties at a length scale comparable to the wavelength of light is crucial\nfor the realization of new optical and optoelectronic devices. Here we have\nstudied the light transmission of disordered photonic structures made with two\nand three different materials, characterized by the same optical thickness. In\ntheir transmission spectra a formation of peaks, with a transmission of up to\n75%, is evident. The spectral position of such peaks is very regular, which is\na result of the constraint that all layers have the same optical thickness.\nThis gives rise to a manifold of applications such as new types of bandpass\nfilters and resonators for distributed feedback lasers.",
        "category": "physics_optics"
    },
    {
        "text": "High spatial frequency laser induced periodic surface structure\n  formation in germanium by mid-IR femtosecond pulses: Formation of high spatial frequency laser induced periodic surface structures\n(HSFL) in germanium by femtosecond mid-IR pulses with wavelengths between\n$\\lambda=2.0$ and $3.6 \\; \\mathrm{\\mu m}$ was studied with varying angle of\nincidence and polarization. The period of these structures varied from\n$\\lambda/3$ to $\\lambda/8$. A modified surface-scattering model including Drude\nexcitation and the optical Kerr effect explains spatial period scaling of HSFL\nacross the mid-IR wavelengths. Transmission electron microscopy (TEM) shows the\npresence of a $30 \\; \\mathrm{n m}$ amorphous layer above the structure of\ncrystalline germanium. Various mechanisms including two photon absorption and\ndefect-induced amorphization are discussed as probable causes for the formation\nof this layer.",
        "category": "physics_optics"
    },
    {
        "text": "All-Optical Delay of Images using Slow Light: Two-dimensional images carried by optical pulses (2 ns) are delayed by up to\n10 ns in a 10 cm cesium vapor cell. By interfering the delayed images with a\nlocal oscillator, the transverse phase and amplitude profiles of the images are\nshown to be preserved. It is further shown that delayed images can be well\npreserved even at very low light levels, where each pulse contains on average\nless than one photon.",
        "category": "physics_optics"
    },
    {
        "text": "Circular dichroism in high-harmonic generation in achiral nanostructures\n  under vortex beam irradiation: In this study, we investigate the nonlinear optical phenomena emerging from\nthe interaction of vortex beams with achiral nanoparticles, leading to the\nobservation of nonlinear circular dichroism in the high-harmonic generation.\nDespite the achiral symmetry of the nanoparticles, the interplay between the\nvector properties of the light, the symmetry of the nanoparticles, and the\nsymmetry of the crystalline lattice of the nanoparticle material leads to\ncircular dichroism in the nonlinear regime. We derive a formula that describes\nthe conditions for the {appearance} of circular dichroism across all possible\nscenarios, taking into account all the symmetries. We also show that the\nabsolute value of the incident beam's orbital angular momentum does not play a\nmajor role. We believe that this work provides important insights that can help\nin improvement the design process of chiral sensors, making them more versatile\nand effective.",
        "category": "physics_optics"
    },
    {
        "text": "Guided mode meta-optics: Metasurface-dressed nanophotonic waveguides for\n  arbitrary designer mode couplers and on-chip OAM emitters with configurable\n  topological charge: Metasurfaces have achieved fruitful results in tailoring complexing light\nfields in free space. However, a systematic investigation on applying the\nconcept of meta-optics to completely control waveguide modes is still elusive.\nHere we present a comprehensive catalog capable of selectively and exclusively\nexcite almost arbitrary high-order waveguide modes of interest, leveraging\nsilicon metasurface-patterned silicon nitride waveguides. By simultaneously\nengineering the phase-matched gradient of the metasurface and the vectorial\nspatial modal overlap between the nanoantenna near-field and target waveguide\nmode for excitation, either single or multiple high-order modes are\nsuccessfully launched with high purity reaching 98% and broad bandwidth.\nMoreover, on-chip twisted light generators are also theoretically demonstrated\nwith configurable OAM topological charge \\ell from -3 to +2, serving as a\ncomprehensive framework for metasurface-enabled guided mode optics and\nmotivating further applications such as versatile integrated couplers,\ndemultiplexers, and mode-division multiplexing-based communication systems.",
        "category": "physics_optics"
    },
    {
        "text": "Simplified single-shot supercontinuum spectral interferometry: We have experimentally demonstrated a simplified method for performing\nsingle-shot supercontinuum spectral interferometry (SSSI) that does not require\npre-characterization of the probe pulse. The method, originally proposed by D.\nT. Vu, D. Jang, and K. Y. Kim, uses a genetic algorithm (GA) and as few as two\ntime-delayed pump-probe shots to retrieve the pump-induced phase shift on the\nprobe [Opt. Express 26, 20572 (2018)]. We show that the GA is able to\nsuccessfully retrieve the transient modulations on the probe, and that the\nerror in the retrieved modulation decreases dramatically with the number of\nshots used. In addition, we propose and demonstrate a practical method that\nallows SSSI to be done with a single pump-probe shot (again, without the need\nfor pre-characterization of the probe). This simplified method can prove to be\nimmensely useful when performing SSSI with a low-repetition-rate laser source.",
        "category": "physics_optics"
    },
    {
        "text": "Broadband large-ellipticity harmonic generation with polar molecule: We investigate the polarization properties of high harmonic generation from\npolar molecules with a linearly polarized field. It is found that elliptically\npolarized harmonics are observed in a wide spectral range from the plateau to\nthe cutoff. Further analyses show that the nonsymmetric structure of the\nhighest occupied molecular orbital is the origin of ellipticity of the\nharmonics. The results provide a method for generation of large-ellipticity XUV\npulses, which will benefit the application of HHG as a tool of detection in\nmaterials and biology science.",
        "category": "physics_optics"
    },
    {
        "text": "Engineering the light coupling between metalens and photonic crystal\n  resonators for robust on-chip microsystems: We designed an on-chip transformative optic system of a metalens-photonic\ncrystal resonator metasystem on a foundry compatible silicon photonic platform.\nBy adjusting the on-chip metalens' focusing length and mode dimension, the\ninsertion loss between the metalens and the photonic crystal resonator and\nwaveguide structures are minimized through mode-matching. The micro-system does\nnot involve any single mode silicon nanowire waveguide, and thus mechanically\nrobust without any oxide claddings. The proposed microsystem is ideal for\nminiaturized chemical and biosensors operating in air or solution environment.",
        "category": "physics_optics"
    },
    {
        "text": "Theory of Optical Rectification Effect in Metallic Thin Film with\n  Periodic Modulation: We conducted theoretical and numerical investigations of the optical\nrectification (OR) effect in metallic structures with periodic modulation. A\nnew formulation of the OR effect is presented, and the mechanism by which the\nOR effect is generated, which has been a controversial issue in previous\nstudies, is clarified. We reveal that the OR effect is strongly enhanced by a\ncombination of spatial variation of the metallic structure and local electric\nfield enhancement. Our theory was numerically evaluated and agreed fairly well\nwith experiment.",
        "category": "physics_optics"
    },
    {
        "text": "Microsphere kinematics from the polarization of tightly focused\n  nonseparable light: Recently, it was shown that vector beams can be utilized for fast kinematic\nsensing via measurements of their global polarization state [Optica 2(10), 864\n(2015)]. The method relies on correlations between the spatial and polarization\ndegrees of freedom of the illuminating field which result from its nonseparable\nmode structure. Here, we extend the method to the nonparaxial regime. We study\nexperimentally and theoretically the far-field polarization state generated by\nthe scattering of a dielectric microsphere in a tightly focused vector beam as\na function of the particle position. Using polarization measurements only, we\ndemonstrate position sensing of a Mie particle in three dimensions. Our work\nextends the concept of back focal plane interferometry and highlights the\npotential of polarization analysis in optical tweezers employing structured\nlight.",
        "category": "physics_optics"
    },
    {
        "text": "Asymptotic approximations for the plasmon resonances of nearly touching\n  spheres: Excitation of surface-plasmon resonances of closely spaced nanometallic\nstructures is a key technique used in nanoplasmonics to control light on\nsubwavelength scales and generate highly confined electric-field hotspots. In\nthis paper we develop asymptotic approximations in the near-contact limit for\nthe entire set of surface-plasmon modes associated with the prototypical sphere\ndimer geometry. Starting from the quasi-static plasmonic eigenvalue problem, we\nemploy the method of matched asymptotic expansions between a gap region, where\nthe boundaries are approximately paraboloidal, pole regions within the spheres\nand close to the gap, and a particle-scale region where the spheres appear to\ntouch at leading order. For those modes that are strongly localised to the gap,\nrelating the gap and pole regions gives a set of effective eigenvalue problems\nformulated over a half space representing one of the poles. We solve these\nproblems using integral transforms, finding asymptotic approximations, singular\nin the dimensionless gap width, for the eigenvalues and eigenfunctions. In the\nspecial case of modes that are both axisymmetric and odd about the plane\nbisecting the gap, where matching with the outer region introduces a\nlogarithmic dependence upon the dimensionless gap width, our analysis follows\n[O. Schnitzer, \\textit{Physical Review B}, \\textbf{92} 235428 2015]. We also\nanalyse the so-called anomalous family of even modes, characterised by field\ndistributions excluded from the gap. We demonstrate excellent agreement between\nour asymptotic formulae and exact calculations.",
        "category": "physics_optics"
    },
    {
        "text": "Two-color flat-top solitonic pulses in $\u03c7^{(2)}$ optical\n  microresonators via second harmonic generation: We studied numerically the generation of the coherent frequency combs at\nsecond harmonic generation in $\\chi^{(2)}$ microresonators via conventional\nfrequency scan method. It was demonstrated for the first time that under\nparticular conditions it is possible to generate two-color flat-top solitonic\npulses, platicons, using pump amplitude modulation or controllable mode\ninteraction approach, if the signs of the group velocity coefficients at pump\nfrequency and its second harmonic are opposite but absolute values of these\ncoefficients are rather close. It was revealed that platicons may be observed\non both sides of the linear microresonator resonance (at positive, as well as\nnegative pump frequency detunings). For the efficient platicon excitation, one\nneeds simultaneous accurate matching of both microresonator free spectral\nranges and resonant eigenfrequencies. Platicon generation processes were\nsimulated numerically, excitation conditions and platicon generation domains\nwere found for different generation methods, and the properties of generated\nplaticons were studied for the different combinations of the medium parameters.",
        "category": "physics_optics"
    },
    {
        "text": "Ionization-induced Susceptibility by Nearly-free Electrons in Gases\n  Influenced by the Coulomb Potential: In the present paper we study the influence of the Coulomb potential on the\nreal and imaginary parts of the plasma-induced susceptibility in a photoionized\ngas. We show that the real part of the susceptibility is more than one order of\nmagnitude larger due to the action of a Coulomb potential. Surprisingly, the\nlong-range Coulomb potential of the atomic core leads to an additional\ncontribution to the imaginary part of the susceptibility which has no\ncounterpart in the case of a short-range potential. We demonstrate that the\norigin of this behavior are electrons in states very close to the continuum\n(nearly-free electrons), and analyze the dependence of the susceptibility on\nthe intensity and wavelengths.",
        "category": "physics_optics"
    },
    {
        "text": "Direct Observation of the Faraday Rotation Using Radially-Polarized\n  Twisted Light: A novel experimental technique for the realisation of the optical Faraday\neffect using Laguerre- Gaussian (LG) light is described. The experiment employs\na zero-order vortex half-wave retarder to generate a radially or\nazimuthally-polarised LG doughnut beam. The light emerging from the retarder\nthen passes through a linear polariser, which gives rise to two intensity\nlobes, with the orientation of the intensity gap between the two lobes pointing\nparallel (perpendicular) to the polarization direction of the radially\n(azimuthally) polarised beam. To complete the Faraday set up, the light\ntraverses a material subject to a magnetic field, before passing through a\nfinal linear polariser, which results in a visible rotation of the lobes\npattern. This technique exhibits the Faraday effect readily visually, without\nfurther elaborate steps to detect changes in the light intensity. The degree of\nrotation of the plane of polarisation is determined directly by the visibly\nclear change in the orientation of the intensity gap between the lobes.",
        "category": "physics_optics"
    },
    {
        "text": "Bragg-Grating Enhanced Narrow-Band Spontaneous Parametric Down\n  Conversion: We propose a new method to narrow the linewidth of entangled photons\ngenerated from spontaneous parametric down conversion (SPDC). An internal Bragg\ngrating is incorporated onto a nonlinear optical crystal waveguide. We study\ntheoretically the spectral characteristics of SPDC under two Bragg grating\nstructures. We show that using the Bragg grating with a midway pi-phase\nshifter, it is a promising way to generate narrow-line entangled photons.",
        "category": "physics_optics"
    },
    {
        "text": "Classical Approaches to Chiral Polaritonics: We provide a theoretical framework based on classical electromagnetism, to\ndescribe optical properties of Fabry-P\\'erot cavities, filled with multilayered\nand linear chiral materials. We find a formal link between transfer-matrix,\nscattering-matrix and Green-function approaches to compute the\npolarization-dependent optical transmission, and cavity-modified circular\ndichroism signals. We show how general symmetries like Lorentz reciprocity and\ntime-reversal symmetry constrain the modelling of such cavities. We apply this\napproach to investigate numerically and analytically the properties of various\nFabry-P\\'erot cavities, made of either metallic or helicity-preserving\ndielectric photonic crystal mirrors. In the latter case, we analyze the onset\nof chiral cavity-polaritons in terms of partial helicity-preservation of\nelectromagnetic waves reflected at the mirrors interfaces. Our approach is\nrelevant for designing innovative Fabry-P\\'erot cavities for chiral-sensing,\nand for probing cavity-modified stereochemistry.",
        "category": "physics_optics"
    },
    {
        "text": "Input-output relations for a 3-port grating coupled Fabry-Perot cavity: We analyze an optical 3-port reflection grating by means of a scattering\nmatrix formalism. Amplitude and phase relations between the 3 ports, i.e. the 3\norders of diffraction are derived. Such a grating can be used as an\nall-reflective, low-loss coupler to Fabry-Perot cavities. We derive the input\noutput relations of a 3-port grating coupled cavity and find distinct\nproperties not present in 2-port coupled cavities. The cavity relations further\nreveal that the 3-port coupler can be designed such that the additional cavity\nport interferes destructively. In this case the all-reflective, low-loss,\nsingle-ended Fabry-Perot cavity becomes equivalent to a standard transmissive,\n2-port coupled cavity.",
        "category": "physics_optics"
    },
    {
        "text": "Spatiotemporal Isotropic-to-Anisotropic Meta-Atoms: Metamaterials and metasurfaces are designed by periodically arranged\nsubwavelength geometries, allowing a tailored manipulation of the\nelectromagnetic response of matter. Here, we exploit temporal variations of\npermittivity inside subwavelength geometries to propose the concept of\nspatiotemporal meta-atoms having time-dependent properties. We exploit\nisotropic-to-anisotropic temporal boundaries within spatially subwavelength\nregions where their permittivity is rapidly changed in time. In so doing, it is\nshown how resulting scattered waves travel in directions that are different\nfrom the direction of the impinging wave, and depend on the values of the\nchosen anisotropic permittivity tensor. To provide a full physical insight of\ntheir performance, multiple scenarios are studied numerically such as the\neffect of using different values of permittivity tensor, different geometries\nof the spatiotemporal meta-atom and time duration of the induced\nisotropic-to-anisotropic temporal boundary. The intrinsic asymmetric response\nof the proposed spatiotemporal meta-atoms is also studied demonstrating, both\ntheoretically and numerically, its potential for an at-will manipulation of\nscattered waves in real time. These results may open new paradigms for\ncontrolling wave-matter interactions and may pave the way for the next\ngeneration of metamaterials and metasurfaces by unleashing their potential\nusing four-dimensional (4D) unit cells.",
        "category": "physics_optics"
    },
    {
        "text": "Unidirectional vortex waveguides and multistable vortex pairs in\n  polariton condensates: Vortices carrying quantized topological charges have potential application in\ninformation processing. In this work, we investigate vortex carriers and\nwaveguides in microcavity polariton condensates, nonresonantly excited by a\nhomogeneous pump with intensity grooves. An intensity groove with a ring shape\nin the pump gives rise to dark-ring states of the condensate with a $\\pi$-phase\njump, akin to dark solitons. The dark-ring states can be destroyed by a\nstronger density of the surrounding condensate and reduce into\nvortex-antivortex pairs. Multiple vortex-pair states are found to be stable in\nthe same dark ring of the pump. When the pump ring is broader, higher-order\ndark states with multiple $\\pi$-phase jumps can be obtained and interestingly\nthey can be used to construct vortex waveguides. If a single vortex is\nimprinted in such waveguides, it can travel in a particular direction, showing\none-way transportation. In other words, an imprinted vortex with a certain\ncharge in a specifically designed higher-order dark state is only allowed to\npropagate unidirectionally.",
        "category": "physics_optics"
    },
    {
        "text": "Symmetry selective Dynamic Casimir Effect in One-Dimensional Photonic\n  Crystals: Real photon pairs can be created in a dynamic cavity with periodically\nmodulated refractive index of the constituent media or oscillating boundaries.\nThis effect is called Dynamic Casimir effect (DCE), which represents one of the\nmost amazing predictions of quantum field theory. Here, we investigate DCE in a\ndynamic one-dimensional photonic crystal system with both temporal and spatial\nmodulation of the refractive index profile. Such a system can resonantly\ngenerate photons at driving frequencies equal to even or odd integer times of\nthat of the fundamental cavity mode governed by the symmetry of the spatial\nmodulation. We further observe interesting spectral and scaling behaviors for\nphotons excited at the band edge. Our discovery introduces a new degree of\nfreedom to enhance the efficiency of DCE.",
        "category": "physics_optics"
    },
    {
        "text": "Photon-efficient optical tweezers via wavefront shaping: Optical tweezers enable non-contact trapping of micro-scale objects using\nlight. Despite their widespread use, it is currently not known how tightly it\nis possible to three-dimensionally trap micro-particles with a given photon\nbudget. Reaching this elusive limit would enable maximally-stiff particle\ntrapping for precision measurements on the nanoscale, and photon-efficient\ntweezing of light-sensitive objects. Here we solve this problem by customising\na trapping light field to suit a specific particle, with the aim of\nsimultaneously optimising trap stiffness in all three dimensions. Initially\ntaking a theoretical approach, we develop an efficient multi-parameter\noptimisation routine to design bespoke optical traps for a wide range of\nmicro-particles. We show that the confinement volume of micro-spheres held in\nthese sculpted traps can be reduced by one-to-two orders-of-magnitude in\ncomparison to a conventional optical tweezer of the same power. We go on to\nconduct proof-of-principle experiments, and use a wavefront shaping inspired\nstrategy to suppress the Brownian fluctuations of optically trapped\nmicro-spheres in every direction concurrently, thus demonstrating\norder-of-magnitude reductions in their confinement volumes. Our work paves the\nway towards the fundamental limits of optical control over the mesoscopic\nrealm.",
        "category": "physics_optics"
    },
    {
        "text": "Invariant quantities of a Mueller matrix under rotation and retarder\n  transformations: Mueller matrices are defined with respect to appropriate Cartesian reference\nframes for the representation of the states of polarization of the input and\noutput electromagnetic waves. The polarimetric quantities that are invariant\nunder rotations of the said reference frames about the respective directions of\npropagation (rotation transformations) provide particularly interesting\nphysical information. Moreover, certain properties are also invariant with\nrespect to the action of birefringent devices located at both sides of the\nmedium under consideration (retarder transformations). The polarimetric\nproperties that remain invariant under rotation and retarder transformations\nare analyzed and interpreted, providing significant parameterizations of\nMueller matrices in terms of meaningful physical quantities.",
        "category": "physics_optics"
    },
    {
        "text": "Suspended core subwavelength fibers: practical designs for the low-loss\n  terahertz guidance: In this work we report two designs of subwavelength fibers packaged for\npractical terahertz wave guiding. We describe fabrication, modeling and\ncharacterization of microstructured polymer fibers featuring a\nsubwavelength-size core suspended in the middle of a large porous outer\ncladding. This design allows convenient handling of the subwavelength fibers\nwithout distorting their modal profile. Additionally, the air-tight porous\ncladding serves as a natural enclosure for the fiber core, thus avoiding the\nneed for a bulky external enclosure for humidity-purged atmosphere. Fibers of 5\nmm and 3 mm in outer diameters with a 150 \\mu m suspended solid core and a 900\n\\mu m suspended porous core respectively, were obtained by utilizing a\ncombination of drilling and stacking techniques. Characterization of the fiber\noptical properties and the near-field imaging of the guided modes were\nperformed using a terahertz near-field microscopy setup. Near-field imaging of\nthe modal profiles at the fiber output confirmed the effectively single-mode\nbehavior of such waveguides. The suspended core fibers exhibit broadband\ntransmission from 0.10 THz to 0.27 THz (larger core), and from 0.25 THz to 0.51\nTHz (smaller core). Due to the large fraction of power that is guided in the\nholey cladding, fiber propagation losses as low as 0.02 cm-1 are demonstrated.\nLow-loss guidance combined with the core isolated from environmental\nperturbations make these all-dielectric fibers suitable for practical terahertz\nimaging and sensing applications.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of large spontaneous emission rate enhancement of quantum\n  dots in a broken-symmetry slow-light waveguide: Quantum states of light and matter can be manipulated on the nanoscale to\nprovide a technological resource for aiding the implementation of scalable\nphotonic quantum technologies [1-3]. Experimental progress relies on the\nquality and efficiency of the coupling between photons and internal states of\nquantum emitters [4-6]. Here we demonstrate a nanophotonic waveguide platform\nwith embedded quantum dots (QDs) that enables both Purcell-enhanced emission\nand strong chiral coupling. The design uses slow-light effects in a glide-plane\nphotonic crystal waveguide with QD tuning to match the emission frequency to\nthe slow-light region. Simulations were used to map the chirality and Purcell\nenhancement depending on the position of a dipole emitter relative to the air\nholes. The highest Purcell factors and chirality occur in separate regions, but\nthere is still a significant area where high values of both can be obtained.\nBased on this, we first demonstrate a record large radiative decay rate of 17\nns^-1 (60 ps lifetime) corresponding to a 20 fold Purcell enhancement. This was\nachieved by electric-field tuning of the QD to the slow-light region and\nquasi-resonant phonon-sideband excitation. We then demonstrate a 5 fold Purcell\nenhancement for a dot with high degree of chiral coupling to waveguide modes,\nsubstantially surpassing all previous measurements. Together these demonstrate\nthe excellent prospects for using QDs in scalable implementations of on-chip\nspin-photonics relying on chiral quantum optics.",
        "category": "physics_optics"
    },
    {
        "text": "Electrical Charge Control of h-BN Single Photon Sources: Colour centres of hexagonal boron nitride (h-BN) have been discovered as\npromising and practical single photon sources due to their high brightness and\nnarrow spectral linewidth at room-temperature. In order to realize h-BN based\nphotonic quantum communications, the ability to electrically activate the\nsingle photon fluorescence using an external electric field is crucial. In this\nwork, we show the electrical switching of the photoluminescence from h-BN\nquantum emitters, enabled by the controllable electron transfer from the nearby\ncharge reservoir. By tuning the Fermi level of graphene next to the h-BN\ndefects, we observed luminescence brightening of a quantum emitter upon the\napplication of a voltage due to the direct charge state manipulation. In\naddition, the correlation measurement of the single photon sources with the\ngraphene's Raman spectroscopy allows us to extract the exact charge transition\nlevel of quantum emitters, providing the information on the crystallographic\nnature of the defect structure. With the complete on-off switching of emission\nintensity of h-BN quantum emitters using a voltage, our result paves the way\nfor the van der Waals colour centre based photonic quantum information\nprocessing, cryptography and memory applications.",
        "category": "physics_optics"
    },
    {
        "text": "Quantum gravity simulation by non-paraxial nonlinear optics: We show that an analog of the physics at the Planck scale can be found in the\npropagation of tightly focused laser beams. Various equations that occur in\ngeneralized quantum mechanics are formally identical to those describing the\nnonlinear nonlocal propagation of nonparaxial laser beams. The analysis\nincludes a generalized uncertainty principle and shows that the nonlinear\nfocusing of a light beam with dimensions comparable to the wavelength\ncorresponds to the spontaneous excitation of the so-called maximally localized\nstates. The approach, driven by the ideas of the quantum gravity physics,\nallows one to predict the existence of self-trapped subwavelength solitary\nwaves for both focusing and defocusing nonlinearities, and opens the way to\nlaboratory simulations of phenomena that have been considered to be\ninaccessible.",
        "category": "physics_optics"
    },
    {
        "text": "Comparison of electromagnetic field solvers for the 3D analysis of\n  plasmonic nano antennas: Plasmonic nano antennas are highly attractive at optical frequencies due to\ntheir strong resonances - even when their size is smaller than the wavelength -\nand because of their potential of extreme field enhancement. Such antennas may\nbe applied for sensing of biological nano particles as well as for single\nmolecule detection. Because of considerable material losses and strong\ndispersion of metals at optical frequencies, the numerical analysis of\nplasmonic antennas is very demanding. An additional difficulty is caused when\nvery narrow gaps between nano particles are utilized for increasing the field\nenhancement. In this paper we discuss the main difficulties of time domain\nsolvers, namely FDTD and FVTD and we compare various frequency domain solvers,\nnamely the commercial FEM packages JCMsuite, Comsol, HFSS,and Microwave Studio\nwith the semi-analytic MMP code that may be used as a reference due to its fast\nconvergence and high accuracy.",
        "category": "physics_optics"
    },
    {
        "text": "Opto-thermal dynamics in whispering-gallery microresonators: Optical whispering-gallery-mode microresonators with ultrahigh quality\nfactors and small mode volumes have played an important role in modern physics.\nThey have been demonstrated as a diverse platform for a wide range of photonics\napplications, such as nonlinear optics, optomechanics, quantum optics, and\ninformation processing. Thermal behaviors induced by power buildup in\nresonators or environmental perturbations are ubiquitous in high-quality-factor\nwhispering-gallery-mode resonators and have played an important role in their\noperation for various applications. Here in this review, we discuss the\nmechanisms of laser field induced thermal nonlinear effects, including thermal\nbistability and thermal oscillation. With the help of the thermal bistability\neffect, optothermal spectroscopy and optical non-reciprocity have been\ndemonstrated. On the other hand, by tuning the temperature of the environment,\nthe resonant mode frequency will shift, which could also be used for thermal\nsensing/tuning applications. Thermal locking technique and thermal imaging\nmechanisms are discussed briefly. Last, we review some techniques to realize\nthermal stability in a high-quality-factor resonator system.",
        "category": "physics_optics"
    },
    {
        "text": "Observation of Dirac cones and room temperature polariton lasing in an\n  organic honeycomb lattice: Artificial one- and two-dimensional lattices have emerged as a powerful\nplatform for the emulation of lattice Hamiltonians, the fundamental study of\ncollective many-body effects as well as phenomena arising from non-trivial\ntopology. Exciton-polaritons, bosonic part-light and part-matter\nquasiparticles, combine pronounced nonlinearities with the possibility of\non-chip implementation. In this context, organic semiconductors, hosting\nultra-stable Frenkel excitons, embedded in a microcavity have proven to be\nversatile contenders for the study of nonlinear many-body physics and bosonic\ncondensation, which also enable deployment at ambient conditions. Here, we\nimplement a well-controlled, high-quality optical lattice, hosting light-matter\nquasi-particles. The realized polariton graphene presents with excellent cavity\nquality factors, showing distinct signatures of Dirac cone and flatband\ndispersions as well as polariton lasing at room temperature. This is realized\nby filling coupled dielectric microcavities with the fluorescent protein\nmCherry. We demonstrate the emergence of a coherent polariton condensate at\nambient conditions, profiting from coupling conditions as precise and\ncontrollable as in state-of-the-art inorganic semiconductor-based systems,\nwithout limitations due to e.g. lattice matching in epitaxial growth. This\nprogress allows straightforward extension to more complex systems, such as the\nstudy of topological phenomena in two-dimensional lattices including\ntopological lasers and non-Hermitian optics.",
        "category": "physics_optics"
    },
    {
        "text": "Six-pack off-axis holography: We present a new holographic concept, named six-pack holography (6PH), in\nwhich we compress six off-axis holograms into a single multiplexed off-axis\nhologram without loss of magnification or resolution. The multiplexed hologram\ncontains straight off-axis fringes with six different orientations, and can be\ngenerated optically or digitally. We show that since the six different complex\nwavefronts do not overlap in the spatial frequency domain, they can be fully\nreconstructed. 6PH allows more than 50% improvement in the spatial bandwidth\nconsumption when compared to the best multiplexing method proposed so far. We\nexpect the 6PH concept to be useful for a variety of applications, such as\nfield-of-view multiplexing, wavelength multiplexing, temporal multiplexing,\nmultiplexing for super-resolution imaging, and others.",
        "category": "physics_optics"
    },
    {
        "text": "Hybrid Diffractive Optics Design via Hardware-in-the-Loop Methodology\n  for Achromatic Extended-Depth-of-Field Imaging: End-to-end optimization of diffractive optical elements (DOEs) profile\nthrough a digital differentiable model combined with computational imaging have\ngained an increasing attention in emerging applications due to the compactness\nof resultant physical setups. Despite recent works have shown the potential of\nthis methodology to design optics, its performance in physical setups is still\nlimited and affected by manufacturing artifacts of DOE, mismatch between\nsimulated and resultant experimental point spread functions, and calibration\nerrors. Additionally, the computational burden of the digital differentiable\nmodel to effectively design the DOE is increasing, thus limiting the size of\nthe DOE that can be designed. To overcome the above mentioned limitations, the\nbroadband imaging system with phase-only spatial light modulator (SLM) as an\nencoded DOE is proposed and developed in this paper. A co-design of the SLM\nphase pattern and image reconstruction algorithm is produced following the\nend-to-end strategy, using for optimization a convolutional neural network\nequipped with quantitative and qualitative loss functions. The optics of the\nimaging system is hybrid consisting of SLM as DOE and refractive lens. SLM\nphase-pattern is optimized by applying the Hardware-in-the-loop technique,\nwhich helps to eliminate the mismatch between numerical modeling and physical\nreality of image formation as light propagation is not numerically modeled but\nis physically done. In our experiments, the hybrid optics is implemented by the\noptical projection of the SLM phase-pattern on a lens plane for a depth range\n0.4-1.9m. Comparison with compound multi-lens optics such as Sony A7 III and\niPhone Xs Max cameras show that the proposed system is advanced in all-in-focus\nsharp imaging.",
        "category": "physics_optics"
    },
    {
        "text": "Cavity piezo-mechanics for superconducting-nanophotonic quantum\n  interface: Hybrid quantum systems are essential for the realization of distributed\nquantum networks. In particular, piezo-mechanics operating at typical\nsuperconducting qubit frequencies features low thermal excitations, and offers\nan appealing platform to bridge superconducting quantum processors and optical\ntelecommunication channels. However, integrating superconducting and\noptomechanical elements at cryogenic temperatures with sufficiently strong\ninteractions remains a tremendous challenge. Here, we report an integrated\nsuperconducting cavity piezo-optomechanical platform where 10-GHz phonons are\nresonantly coupled with photons in a superconducting and a nanophotonic\ncavities at the same time. Benefited from the achieved large piezo-mechanical\ncooperativity ($C_\\mathrm{em}\\sim7$) and the enhanced optomechanical coupling\nboosted by a pulsed optical pump, we demonstrate coherent interactions at\ncryogenic temperatures via the observation of efficient microwave-optical\nphoton conversion. This hybrid interface makes a substantial step towards\nquantum communication at large scale, as well as novel explorations in\nmicrowave-optical photon entanglement and quantum sensing mediated by gigahertz\nphonons.",
        "category": "physics_optics"
    },
    {
        "text": "Characterizing cylindrical particles upon local measurements of two\n  Stokes parameters: Researchers routinely characterize optical samples by computing the\nscattering cross-section. However, the experimental determination of this\nmagnitude requires the measurement and integration of the components of the\nscattered field in all directions. Here, we propose a method to determine the\nscattering cross-section and global polarization state of radiation through\nmeasurements of two Stokes parameters at an angle of choice in far-field. The\nmethod applies to cylindrically symmetric samples whose optical response is\nwell-described by a single multipolar order j. Moreover, the formalism is\napplicable for a wide range of different illuminations, and it only requires\nthe use of a single camera and conventional wave plates. Our findings\nsignificantly reduce the complexity of routine characterization measurements\nfor cylindrical samples in optical laboratories.",
        "category": "physics_optics"
    },
    {
        "text": "Optical disassembly of cellular clusters by tunable tug-of-war tweezers: Bacterial biofilms underlie many persistent infections, posing major hurdles\nin antibiotic treatment. Here, we design and demonstrate tug-of-war optical\ntweezers that can facilitate assessment of cell-cell adhesion - a key\ncontributing factor to biofilm development, thanks to the combined actions of\noptical scattering and gradient forces. With a customized optical landscape\ndistinct from that of conventional tweezers, not only can such tug-of-war\ntweezers stably trap and stretch a rod-shaped bacterium in the observing plane,\nbut, more importantly, they can also impose a tunable lateral force that pulls\napart cellular clusters without any tethering or mechanical movement. As a\nproof of principle, we examined a Sinorhizobium meliloti strain that forms\nrobust biofilms and found that the strength of intercellular adhesion depends\non the growth medium. This technique may herald new photonic tools for optical\nmanipulation and biofilm study, as well as other biological applications.",
        "category": "physics_optics"
    },
    {
        "text": "Optomechanical coupling between two optical cavities: cooling of a\n  micro-mirror and parametric normal mode splitting: We propose a technique aimed at cooling a harmonically oscillating mirror\nmechanically coupled to another vibrating mirror to its quantum mechanical\nground state. Our method involves optmechanical coupling between two optical\ncavities. We show that the cooling can be controlled by the mechanical coupling\nstrength between the two movable mirrors, the phase difference between the\nmechanical modes of the two oscillating mirrors and the photon number in each\ncavity. We also show that both mechanical and optical cooling can be achieved\nby transferring energy from one cavity to the other. We also analyze the\noccurrence of normal-mode splitting (NMS). We find that a hybridization of the\ntwo oscillating mirrors with the fluctuations of the two driving optical fields\noccurs and leads to a splitting of the mechanical and optical fluctuation\nspectra.",
        "category": "physics_optics"
    },
    {
        "text": "Electrically tunable second harmonic generation in atomically thin ReS2: Electrical tuning of second-order nonlinearity in optical materials is\nattractive to strengthen and expand the functionalities of nonlinear optical\ntechnologies, though its implementation remains elusive. Here, we report the\nelectrically tunable second-order nonlinearity in atomically thin ReS2 flakes\nbenefiting from their distorted 1T crystal structure and interlayer charge\ntransfer. Enabled by the efficient electrostatic control of the\nfew-atomic-layer ReS2, we show that second harmonic generation (SHG) can be\ninduced in odd-number-layered ReS2 flakes which are centrosymmetric and thus\nwithout intrinsic SHG. Moreover, the SHG can be precisely modulated by the\nelectric field, reversibly switching from almost zero to an amplitude more than\none order of magnitude stronger than that of the monolayer MoS2. For the\neven-number-layered ReS2 flakes with the intrinsic SHG, the external electric\nfield could be leveraged to enhance the SHG. We further perform the\nfirst-principles calculations which suggest that the modification of in-plane\nsecond-order hyperpolarizability by the redistributed interlayer-transferring\ncharges in the distorted 1T crystal structure underlies the electrically\ntunable SHG in ReS2. With its active SHG tunability while using the facile\nelectrostatic control, our work may further expand the nonlinear optoelectronic\nfunctions of two-dimensional materials for developing electrically controllable\nnonlinear optoelectronic devices.",
        "category": "physics_optics"
    },
    {
        "text": "Performance of Polarization-based Stereoscopy Screens: The screen is a key part of stereoscopic display systems using polarization\nto separate the different channels for each eye. The system crosstalk,\ncharacterizing the imperfection of the screen in terms of preserving the\npolarization of the incoming signal, and the scattering rate, characterizing\nthe ability of the screen to deliver the incoming light to the viewers,\ndetermine the image quality of the system. Both values will depend on the\nviewing angle. In this work we measure the performance of three silver screens\nand three rear-projection screens. Additionally, we measure the surface texture\nof the screens using white-light interferometry. While part of our optical\nresults can be explained by the surface roughness, more work is needed to\nunderstand the optical properties of the screens from a microscopic model.",
        "category": "physics_optics"
    },
    {
        "text": "Interpretable inverse design of particle spectral emissivity using\n  machine learning: We examine the optical properties of a system of nano and micro particles of\nvarying size, shape, and material (including metals and dielectrics, and\nsub-wavelength and super-wavelength regimes). Training data is generated by\nnumerically solving Maxwel Equations. We then use a combination of decision\ntree and random forest models to solve both the forward problem (particle\ndesign in, optical properties out) and inverse problem (desired optical\nproperties in, range of particle designs out). We show that on even\ncomparatively sparse datasets these machine learning models solve both the\nforward and inverse problems with excellent accuracy and 4 to 8 orders of\nmagnitude faster than traditional methods. A single trained model is capable of\nhandling the full diversity of our dataset, producing a variety of different\ncandidate particle designs to solve an inverse problem. The interpretability of\nour models confirms that dielectric particles emit and absorb electromagnetic\nradiation volumetrically, while metallic particles interaction with light is\ndominated by surface modes. This work demonstrates the possibility for\napproachable and interpretable machine learning models to be used for rapid\nforward and inverse design of devices that span a broad and diverse parameter\nspace.",
        "category": "physics_optics"
    },
    {
        "text": "Hybrid Silicon Photonic-Lithium Niobate Electro-Optic Mach-Zehnder\n  Modulator Beyond 100 GHz Bandwidth: Electro-optic modulation, the imprinting of a radio-frequency (RF) waveform\non an optical carrier, is one of the most important photonics functions, being\ncrucial for high-bandwidth signal generation, optical switching, waveform\nshaping, data communications, ultrafast measurements, sampling, timing and\nranging, and RF photonics. Although silicon (Si) photonic electro-optic\nmodulators (EOMs) can be fabricated using wafer-scale technology compatible\nwith the semiconductor industry, such devices do not exceed an electrical 3-dB\nbandwidth of about 50 GHz, whereas many applications require higher RF\nfrequencies. Bulk Lithium Niobate (LN) and etched LN modulators can scale to\nhigher bandwidths, but are not integrated with the Si photonics fabrication\nprocess adopted widely over the last decade. As an alternative, an\nultra-high-bandwidth Mach-Zehnder EOM based on Si photonics is shown, made\nusing conventional lithography and wafer-scale fabrication, bonded to an\nunpatterned LN thin film. This hybrid LN-Si MZM achieves beyond 100 GHz 3-dB\nelectrical bandwidth. Our design integrates silicon photonics light\ninput/output and optical components, including directional couplers, low-radius\nbends, and path-length difference segments, realized in a foundry Si photonics\nprocess. The use of a simple low-temperature (200C) back-end integration\nprocess to bond a postage-stamp-sized piece of LN where desired, and achieving\nlight routing into and out of LN to harness its electro-optic property without\nany etching or patterning of the LN film, may be broadly-useful strategies for\nadvanced integrated opto-electronic microchips.",
        "category": "physics_optics"
    },
    {
        "text": "Bianisotropic Metasurfaces: Ultra-thin Surfaces for Complete Control of\n  Electromagnetic Wavefronts: Complete control of electromagnetic fields requires particles that exhibit\nbianisotropic constituent parameters (i.e. permittivity, permeability, and\nchirality). Here, methods to analyze and synthesize two-dimensional,\nbianisotropic metamaterials (metasurfaces) are presented. First, closed-form\nexpressions are derived relating the reflection and transmission coefficients\nof a general bianisotropic metasurface to its constituent surface parameters.\nNext, a systematic method to design bianisotropic metasurfaces is presented. It\nis analytically shown that cascading anisotropic, patterned metallic sheets\n(electric sheet admittances) can provide electric, magnetic, and chiral\nresponses. To demonstrate the utility of the design procedure, four devices\nexhibiting exotic polarization transformations are presented: a polarization\nrotator, an asymmetric circular polarizer, an asymmetric linear polarizer, and\na symmetric circular polarizer. The optimal performance at centimeter,\nmillimeter, and micrometer wavelengths highlights the versatility of the design\nprocess.",
        "category": "physics_optics"
    },
    {
        "text": "Tunable circular dichroism through absorption in coupled optical modes\n  of twisted triskelia nanostructures: We present a system consisting of two stacked chiral plasmonic nanoelements,\nso-called triskelia, that exhibits a high degree of circular dichroism. The\noptical modes arising from the interactions between the two elements are the\nmain responsible for the dichroic signal. Their excitation in the absorption\ncross section is favored when the circular polarization of the light is\nopposite to the helicity of the system, so that an intense near-field\ndistribution with 3D character is excited between the two triskelia, which in\nturn causes the dichroic response. Therefore, the stacking, in itself, provides\na simple way to tune both the value of the circular dichroism, up to 60%, and\nits spectral distribution in the visible and near infrared range. We show how\nthese interaction-driven modes can be controlled by finely tuning the distance\nand the relative twist angle between the triskelia, yielding maximum values of\nthe dichroism at 20{\\deg} and 100{\\deg} for left- and right-handed circularly\npolarized light, respectively. Despite the three-fold symmetry of the elements,\nthese two situations are not completely equivalent since the interplay between\nthe handedness of the stack and the chirality of each single element breaks the\nsymmetry between clockwise and anticlockwise rotation angles around 0{\\deg}.\nThis reveals the occurrence of clear helicity-dependent resonances. The\nproposed structure can be thus finely tuned to tailor the dichroic signal for\napplications at will, such as highly efficient helicity-sensitive surface\nspectroscopies or single-photon polarization detectors, among others.",
        "category": "physics_optics"
    },
    {
        "text": "Optical Limiter Based on PT-Symmetry Breaking of Reflectionless Modes: The application of parity-time (PT) symmetry in optics, especially\nPT-symmetry breaking, has attracted considerable attention as a novel approach\nto controlling light propagation. Here, we report optical limiting by two\ncoupled optical cavities with a PT-symmetric spectrum of reflectionless modes.\nThe optical limiting is related to broken PT symmetry due to light-induced\nchanges in one of the cavities. Our experimental implementation is a\nthree-mirror resonator of alternating layers of ZnS and cryolite with a\nPT-symmetric spectral degeneracy of two reflectionless modes. The optical\nlimiting is demonstrated by measurements of single 532-nm 6-ns laser pulses. At\nfluences below 10 mJ/cm2, the multilayer exhibits a flat-top passband at 532\nnm. At higher fluences, laser heating combined with the thermo-optic effect in\nZnS leads to cavity detuning and PT-symmetry breaking of the reflectionless\nmodes. As a result, the entire multilayer structure quickly becomes highly\nreflective, protecting itself from laser-induced damage. The cavity detuning\nmechanism can differ at much higher limiting thresholds and include\nnonlinearity.",
        "category": "physics_optics"
    },
    {
        "text": "Synthetic non-Abelian gauge fields for non-Hermitian systems: Non-Abelian gauge fields are versatile tools for synthesizing topological\nphenomena but have so far been mostly studied in Hermitian systems, where gauge\nflux has to be defined from a closed loop in order for gauge fields, whether\nAbelian or non-Abelian, to become physically meaningful. We show that this\ncondition can be relaxed in non-Hermitian systems by proposing and studying a\ngeneralized Hatano--Nelson model with imbalanced non-Abelian hopping. Despite\nlacking gauge flux in one dimension, non-Abelian gauge fields create rich\nnon-Hermitian topological consequences. Under only nearest-neighbor coupling,\nnon-Abelian gauge fields enable Hopf-link bulk braiding topology, whose phase\ntransition accompanies the emergence of exceptional points (EPs). At both ends\nof an open chain, non-Abelian gauge fields lead to the simultaneous presence of\nnon-Hermitian skin modes, whose population can be effectively tuned. Asymptotic\nanalysis shows that this tuning mechanism stems from the interplay between the\nAbelian Hatano--Nelson coupling and effective high-order hopping, which becomes\nsubstantial near the EP phase transition condition. The predicted non-Hermitian\nphenomena, enabled by non-Abelian gauge fields, could be realized in synthetic\ndimensional optical platforms such as time-multiplexed photonic mesh lattices\nand driven ring resonators.",
        "category": "physics_optics"
    },
    {
        "text": "Producing an Efficient, Collimated and Thin Annular Beam with a Binary\n  Axicon: We propose and demonstrate a method to produce a thin and highly collimated\nannular beam that propagates similarly to an ideal thin Gaussian ring beam,\nmaintaining its excellent propagation properties. Our optical configuration is\ncomposed of a binary axicon - a circular binary phase grating, and a lens,\nmaking it robust and well suited for high-power lasers. It has a near-perfect\ncircular profile with a dark center, and its large radius to waist ratio is\nachieved with high conversion efficiency. The measured profile and propagation\nare in excellent agreement with a numerical Fourier simulation we perform.",
        "category": "physics_optics"
    },
    {
        "text": "Manipulating orbital angular momentum entanglement by using the\n  Heisenberg Uncertainty principle: Orbital angular momentum entanglement is one of the most intriguing topics in\nquantum physics. A broad range of research have been dedicated either to\nunravel its underlying physics or to expand the entanglement dimensions and\ndegrees. In this paper, we present a theoretical study on the orbital angular\nmomentum entanglement by employing the Heisenberg uncertainty principle to\nquantum position correlation within the azimuthal region. In this study, we\ndecompose the pump light into a set of pump cone states characterized by their\nradii. The OAM entanglement can be manipulated by controlling the radius of the\npump cone state, the length of the nonlinear crystal and the OAM carried by the\npump field,which is followed by a detailed discussion. We expect that our\nresearch will bring us a deeper understanding of the OAM entanglement, and will\ndo help to the high-dimensional quantum information tasks based on OAM\nentanglement.",
        "category": "physics_optics"
    },
    {
        "text": "Design of spontaneous parametric down-conversion in integrated hybrid\n  SixNy-PPLN waveguides: High-efficient and high-purity photon sources are highly desired for quantum\ninformation processing. We report the design of a chip-scale hybrid SixNy and\nthin film periodically-poled lithium niobate waveguide for generating\nhigh-purity type-II spontaneous parametric down conversion (SPDC) photons in\ntelecommunication band. The modeled second harmonic generation efficiency of\n225% W^(-1)*cm^(-2) is obtained at 1560nm. Joint spectral analysis is performed\nto estimate the frequency correlation of SPDC photons, yielding intrinsic\npurity with up to 95.17%. The generation rate of these high-purity photon pairs\nis estimated to be 2.87 * 10^7 pairs/s/mW within the bandwidth of SPDC. Our\nchip-scale hybrid waveguide design has the potential for large scale on-chip\nquantum information processing and integrated photon-efficient quantum key\ndistribution through high-dimensional time-energy encoding.",
        "category": "physics_optics"
    },
    {
        "text": "Optimised brightness from solid-state lasers: Laser brightness is a measure of the ability to de- liver intense light to a\ntarget, and encapsulates both the energy content and the beam quality. High\nbrightness lasers requires that both parameters be maximised, yet standard\nlaser cavities do not allow this. For example, in solid-state lasers multimode\nbeams have a high energy content but low beam quality, while Gaussian modes\nhave a small mode volume and hence low energy extraction, but in a good quality\nmode. Here we over- come this fundamental limitation and demonstrate an optimal\napproach to realising high brightness lasers. We employ intra- cavity beam\nshaping to produce a Gaussian mode that carries all the energy of the multimode\nbeam, thus energy extraction and beam quality are simultaneously maximised.\nThis work will have a significant influence on the design of future high\nbrightness laser cavities.",
        "category": "physics_optics"
    },
    {
        "text": "Stimulated Brillouin scattering from surface acoustic waves in\n  sub-wavelength photonic microwires: Brillouin scattering in optical fibres is a fundamental interaction between\nlight and sound with important implications ranging from optical sensors to\nslow and fast light. In usual optical fibres, light both excites and feels\nshear and longitudinal bulk elastic waves, giving rise to forward guided\nacoustic wave Brillouin scattering and backward stimulated Brillouin\nscattering. In a subwavelength-diameter optical fibre, the situation changes\ndramatically, as we here report with the first experimental observation of\nstimulated Brillouin scattering from surface acoustic waves. These\nRayleigh-type hypersound waves travel the wire surface at a specific velocity\nof 3400 m.s$^{\\mathrm{-1}}$ and backscatter the light with a Doppler shift of\nabout 6 GHz. As these acoustic resonances are highly sensitive to surface\ndefects or features, surface acoustic wave Brillouin scattering opens new\nopportunities for various sensing applications, but also in other domains such\nas microwave photonics and nonlinear plasmonics.",
        "category": "physics_optics"
    },
    {
        "text": "Topological Invariants in Point Group Symmetric Photonic Topological\n  Insulators: We proposed a group-theory method to calculate topological invariant in\nbi-isotropic photonic crystals invariant under crystallographic point group\nsymmetries. Spin Chern number has been evaluated by the eigenvalues of rotation\noperators at high symmetry k-points after the pseudo-spin polarized fields are\nretrieved. Topological characters of photonic edge states and photonic band\ngaps can be well predicted by total spin Chern number. Nontrivial phase\ntransition is found in large magnetoelectric coupling due to the jump of total\nspin Chern number. Light transport is also issued at the {\\epsilon}/{\\mu}\nmismatching boundary between air and the bi-isotropic photonic crystal. This\nfinding presents the relationship between group symmetry and photonic\ntopological systems, which enables the design of photonic nontrivial states in\na rational manner.",
        "category": "physics_optics"
    },
    {
        "text": "Highly Efficient Ultrathin Light Emitting Diodes based on Perovskite\n  Nanocrystals: Light-emitting diodes based on perovskite nanocrystals (PNCs-LEDs) have\ngained great interest for next-generation display and lighting technologies\nprized for their color purity, high brightness and luminous efficiency\napproaching the intrinsic limit imposed by extraction of electroluminescence\nfrom the device structure. Although the time is ripe for the development of\neffective light outcoupling strategies to further boost the device performance,\nthis technologically relevant aspect of PNC-LEDs is still without a definitive\nsolution. Here, following theoretical guidelines and without the integration of\ncomplex photonic structures, we realize stable PNC-LEDs with EQE as high as\n29.2% (average EQE=24.7%), which substantially break the outcoupling limit of\ncommon PNC-LEDs and systematically surpass any previous perovskite-based\ndevice. Key to such unprecedented performance is channeling the recombination\nzone in PNC emissive layers as thin as 10 nm, which we achieve by finely\nbalancing the electron and hole transport using CsPbBr3 PNCs resurfaced with a\nnickel oxide layer. The ultra-thin approach general and, in principle,\napplicable to other perovskite nanostructures for fabricating highly efficient,\ncolor tunable transparent LEDs ideal for unobtrusive screens and displays and\nis compatible with the integration of photonic components for further enhanced\nperformance.",
        "category": "physics_optics"
    },
    {
        "text": "Wave channeling of X-rays in narrow rough capillaries - non\n  Andronov-Leontovich theory: The effect of capture of X-ray beam into narrow submicron capillary was\ninvestigated with account for diffraction and decay of coherency by roughness\nscattering in transitional boundary layer. In contrast to well-known\nAndronov-Leontovich approach the losses do not vanish at zero gliding angle and\nscale proportional to the first power of roughness amplitude for small gliding\nangles. It was shown that for small correlation radius of roughness the\nscattering decay of coherency can be made of the same order as absorption decay\nof lower channeling modes to produce angular collimation of X-ray beams.\nEstimates were given for optimum capillary length at different roughness\namplitudes for angular sensitivity of X-ray transmission and chenneling effects\nthat can be usefull for designing of detector systems.",
        "category": "physics_optics"
    },
    {
        "text": "Suppression of narrow-band transparency in a metasurface induced by a\n  strongly enhanced electric field: We realize a suppression of an electromagnetically induced transparency (EIT)\nlike transmission in a metasurface induced by a local electric field that is\nstrongly enhanced based on two approaches: squeezing of electromagnetic energy\nin resonant metasurfaces and enhancement of electromagnetic energy density\nassociated with a low group velocity. The EIT-like metasurface consists of a\npair of radiatively coupled cut-wire resonators, and it can effect both field\nenhancement approaches simultaneously. The strongly enhanced local electric\nfield generates an air discharge plasma at either of the gaps of the cut-wire\nresonators, which causes the EIT-like metasurface to change into two kinds of\nLorentz type metasurfaces.",
        "category": "physics_optics"
    },
    {
        "text": "Probing Phase Transition of Band Topology via Radiation Topology: Topological photonics has received extensive attention from researchers\nbecause it provides brand new physical principles to manipulate light. Band\ntopology of optical materials is characterized using the Berry phase defined by\nBloch states. Until now, the criteria for experimentally probing the\ntopological phase transition of band topology has always been relatively\nlacking in topological physics. Moreover, radiation topology can be aroused by\nthe far-field polarizations of the radiating Bloch states, which is described\nby the Stokes phase. Although such two types of topologies are both related to\nBloch states on the band structure, it is rather surprising that their\ndevelopment is almost independent. Here, we reveal that the phase transition of\nband topology can be probed by the radiation topology. We theoretically design\nand experimentally demonstrate such an intriguing phenomenon by constructing\nphotonic crystals that support optical analogs of quantum spin Hall effects.\nThe results show that the topological charge of the far-field polarization\nvortex changes from +1 to -2 or from -2 to +1 when the band topology changes\nfrom trivial to non-trivial, which provides a new criterion to probe the phase\ntransition of band topology using radiation topology. Our findings not only\nprovide an insightful understanding of band topology and radiation topology,\nbut also can serve as a novel route to manipulate the near and far fields of\nlight.",
        "category": "physics_optics"
    },
    {
        "text": "Nonlinear optics in gallium phosphide cavities: simultaneous second and\n  third harmonic generation: We demonstrate the simultaneous generation of second and third harmonic\nsignals from a telecom wavelength pump in a gallium phosphide (GaP) microdisk.\nUsing analysis of the power scaling of both the second and third harmonic\noutputs and calculations of nonlinear cavity mode coupling factors, we study\ncontributions to the third harmonic signal from direct and cascaded sum\nfrequency generation processes. We find that despite the relatively high\nmaterial absorption in gallium phosphide at the third harmonic wavelength, both\nof these processes can be significant, with relative magnitudes that depend\nclosely on the detuning between the second harmonic wavelength of the cavity\nmodes.",
        "category": "physics_optics"
    },
    {
        "text": "Optimization of diamond optomechanical crystal cavities: Due to recent development of growing and processing techniques for\nhigh-quality single crystal diamond, the large scale production of diamond\noptomechanical crystal cavities becomes feasible, enabling optomechanical\ndevices that can operate in higher mechanical frequencies and be coupled to\ntwo-level systems based on diamond color centers. In this paper we describe a\ndesign optimization method to produce diamond optomechanical crystal (OMC)\ncavities operating at the high-cooperativity regime (close to unity) at room\ntemperature.",
        "category": "physics_optics"
    },
    {
        "text": "Topological states in partially-PT-symmetric azimuthal potentials: We introduce partially-parity-time-symmetric (pPT-symmetric) azimuthal\npotentials composed from individual PT-symmetric cells located on a ring, where\ntwo azimuthal directions are nonequivalent in a sense that in such potentials\nexcitations carrying topological dislo-cations exhibit different dynamics for\ndifferent directions of energy circulation in the initial field distribution.\nSuch non-conservative ratchet-like structures support rich families of stable\nvortex solitons in cubic nonlinear media, whose properties depend on the sign\nof the topological charge due to the nonequivalence of azimuthal directions. In\ncontrast, oppositely charged vortex solitons remain equivalent in similar fully\nPT-symmetric potentials. The vortex solitons in the pPT- and PT-symmetric\npotentials are shown to feature qualitatively different internal current\ndistributions, which are described by different discrete rotation symmetries of\nthe intensity profiles.",
        "category": "physics_optics"
    },
    {
        "text": "Fundamental constraints on particle tracking with optical tweezers: A general quantum limit to the sensitivity of particle position measurements\nis derived following the simple principle of the Heisenberg microscope. The\nvalue of this limit is calculated for particles in the Rayleigh and Mie\nscattering regimes, and with parameters which are relevant to optical tweezers\nexperiments. The minimum power required to observe the zero-point motion of a\nlevitating bead is also calculated, with the optimal particle diameter always\nsmaller than the wavelength. We show that recent optical tweezers experiments\nare within two orders of magnitude of quantum limited sensitivity, suggesting\nthat quantum optical resources may soon play an important role in high\nsensitivity tracking applications.",
        "category": "physics_optics"
    },
    {
        "text": "Plasmonic Heterodyne Spectrometry for Resolving the Spectral Signatures\n  of Ammonia over a 1-5 THz Frequency Range: We present a heterodyne terahertz spectrometry platform based on plasmonic\nphotomixing, which enables the resolution of narrow spectral signatures of\ngases over a broad terahertz frequency range. This plasmonic heterodyne\nspectrometer replaces the terahertz mixer and local oscillator of conventional\nheterodyne spectrometers with a plasmonic photomixer and a heterodyning optical\npump beam, respectively. The heterodyning optical pump beam is formed by two\ncontinuous-wave, wavelength-tunable lasers with a broadly tunable terahertz\nbeat frequency. This broadly tunable terahertz beat frequency enables\nspectrometry over a broad bandwidth, which is not restricted by the bandwidth\nlimitations of conventional terahertz mixers and local oscillators. We use this\nplasmonic heterodyne spectrometry platform to resolve the spectral signatures\nof ammonia over a 1-5 THz frequency range.",
        "category": "physics_optics"
    },
    {
        "text": "On the time evolution at a fluctuating exceptional point: We theoretically evaluate the impact of drift-free noise on the dynamics of\n$\\mathcal{PT}$-symmetric non-Hermitian systems with an exceptional point, which\nhave recently been proposed for sensors. Such systems are currently considered\nas promising templates for sensing applications, because of their intrinsically\nextremely sensitive response to external perturbations. However, this applies\nequally to the impact of fabrication imperfections and fluctuations in the\nsystem parameters. Here we focus on the influence of such fluctuations caused\nby inevitable (thermal) noise and show that the exceptional-point eigenstate is\nnot stable in its presence. To this end, we derive an effective differential\nequation for the mean time evolution operator averaged over all realizations of\nthe noise field and via numerical analysis we find that the presence of noise\nleads to exponential divergence of any initial state after some characteristic\nperiod of time. We therefore show that it is rather demanding to design sensor\nsystems based on continuous operation at an exceptional point.",
        "category": "physics_optics"
    },
    {
        "text": "Demonstration of spectral correlation control in a source of\n  polarization entangled photon pairs at telecom wavelength: Spectrally correlated photon pairs can be used to improve performance of long\nrange fiber based quantum communication protocols. We present a source based on\nspontaneous parametric down-conversion producing polarization entangled photons\nwithout spectral filtering. In addition, the spectral correlation within the\nphoton pair can be controlled by changing the pump pulse duration or coupled\nspatial modes characteristics. The spectral and polarization correlations were\ncharacterized. The generated photon pairs feature both positive spectral\ncorrelations, no correlations, or negative correlations and polarization\nentanglement with the fidelity as high as 0.97 (no background subtraction) with\nthe expected Bell state.",
        "category": "physics_optics"
    },
    {
        "text": "Computational snapshot angular-spectral lensless imaging: By placing a diffractive element in front of an image sensor, we are able to\nmultiplex the spectral and angular information of a scene onto the image\nsensor. Reconstruction of the angular-spectral distribution is attained by\nfirst calibrating the angular-spectral response of the system and then,\napplying optimization-based matrix inversion. In our proof-of-concept\ndemonstration, we imaged the 1D angle and the spectrum with resolutions of\n0.15o and 6nm, respectively. The information is reconstructed from a single\nframe, thereby enabling snapshot functionality for video-rate imaging.",
        "category": "physics_optics"
    },
    {
        "text": "Nonlinear polariton waves in dielectric medium: We theoretically investigate the properties of phonon-polariton inhomogeneous\nharmonic wave, cnoidal wave and spatial soliton propagating in boundless\ndielectric medium and compute the shape of nonlinear vector polariton wave. We\nobtain analytically the envelopes of linearly polarized nonlinear polariton\nwaves in the self-focusing and self-defocusing media.",
        "category": "physics_optics"
    },
    {
        "text": "Generation of high-quality terahertz OAM mode based on soft-aperture\n  difference frequency generation: We demonstrate the generation of high-quality tunable terahertz (THz)\nvortices in an eigenmode by employing soft-aperture difference frequency\ngeneration of a vortex output and a Gaussian mode. The generated THz vortex\noutput exhibits a high-quality orbital angular momentum (OAM) mode with a\ntopological charge of $\\ell_{THz}=\\pm1$ in a frequency range of 2-6 THz. The\nmaximum average power of the THz vortex output obtained was ~3.3 $\\mu$W at 4\nTHz.",
        "category": "physics_optics"
    },
    {
        "text": "Large bandwidth, highly efficient optical gratings through high index\n  materials: We analyze the diffraction characteristics of dielectric gratings that\nfeature a high index grating layer, and devise, through rigorous numerical\ncalculations, large bandwidth, highly efficient, high dispersion dielectric\ngratings in reflection, transmission, and immersed transmission geometry. A\ndielectric TIR grating is suggested, whose -1dB spectral bandwidth is doubled\nas compared to its all-glass equivalent. The short wavelength diffraction\nefficiency is additionally improved by allowing for slanted lamella. The\ngrating surpasses a blazed gold grating over the full octave. An immersed\ntransmission grating is devised, whose -1dB bandwidth is tripled as compared to\nits all-glass equivalent, and that surpasses an equivalent classical\ntransmission grating over nearly the full octave. A transmission grating in the\nclassical scattering geometry is suggested, that features a buried high index\nlayer. This grating provides effectively 100% diffraction efficiency at its\ndesign wavelegth, and surpasses an equivalent fused silica grating over the\nfull octave.",
        "category": "physics_optics"
    },
    {
        "text": "Optical guiding of absorbing nanoclusters in air: We introduce a novel approach for all-optical trapping and manipulation of\nabsorbing aerosol particles based on a photophoretic force. We demonstrate\nexperimentally, in open air, the robust three-dimensional guiding of\nagglomerates of carbon nanoparticles with the size spanned for two orders of\nmagnitude, from 100 nanometers to 10 microns, over the distances of a few\nmillimeters, as well as their acceleration up to velocities of 1 cm/s and\nsimultaneous trapping of a large number of particles.",
        "category": "physics_optics"
    },
    {
        "text": "Photonic integration of an optical atomic clock: Laboratory optical atomic clocks achieve remarkable accuracy (now counted to\n18 digits or more), opening possibilities to explore fundamental physics and\nenable new measurements. However, their size and use of bulk components prevent\nthem from being more widely adopted in applications that require precision\ntiming. By leveraging silicon-chip photonics for integration and to reduce\ncomponent size and complexity, we demonstrate a compact optical-clock\narchitecture. Here a semiconductor laser is stabilized to an optical transition\nin a microfabricated rubidium vapor cell, and a pair of interlocked\nKerr-microresonator frequency combs provide fully coherent optical division of\nthe clock laser to generate an electronic 22 GHz clock signal with a fractional\nfrequency instability of one part in 10^13. These results demonstrate key\nconcepts of how to use silicon-chip devices in future portable and ultraprecise\noptical clocks.",
        "category": "physics_optics"
    },
    {
        "text": "Spatio-temporal characterization of ultrashort vector pulses: Ultrafast vectorially polarized pulses have found many applications in\ninformation and energy transfer owing mainly to the presence of strong\nlongitudinal components and their space-polarization non-separability. Due to\ntheir broad spectrum, such pulses often exhibit space-time couplings, which\nsignificantly affect the pulse propagation dynamics leading to reduced energy\ndensity or utilized to create new effects like a rotating or sliding wavefront\nat focus. Here, we present a new method for the spatio-temporal\ncharacterization of ultrashort cylindrical vector pulses based on a combination\nof spatially resolved Fourier transform spectroscopy and Mach-Zehnder\ninterferometry. The method provides access to spatially resolved spectral\namplitudes and phases of all polarization components of the pulse. We\ndemonstrate the capabilities of the method by completely characterizing a\n$10$~fs radially polarized pulse from a Ti:sapphire laser at $800$~nm.",
        "category": "physics_optics"
    },
    {
        "text": "Nano-lens diffraction around a single heated nano particle: The action of a nanoscopic spherically symmetric refractive index profile on\na focused Gaussian beam may easily be envisaged as the action of a\nphase-modifying element, i.e. a lens: Rays traversing the inhomogeneous\nrefractive index field n(r) collect an additional phase along their trajectory\nwhich advances or retards their phase with respect to the unperturbed ray. This\nlens-like action has long been understood as being the mechanism behind the\nsignal of thin sample photothermal absorption measurements [1, 2], where a\ncylindrical symmetry and a different lengthscale is present. In photothermal\nsingle (nano-)particle microscopy, however, a complicated, though\nprediction-wise limited, electrodynamic (EM) scattering treatment was\nestablished [3] during the emergence of this new technique. Our recent study\nextended [4] this EM-approach into a full ab-initio model describing the\nreality of the situation encountered and showed for the first time that the\nmechanism behind the signal, despite its nanoscopic origin, is also the\nlens-like action of the induced refractive index profile only hidden in the\ncomplicated guise of the theoretical Mie-like framework. The diffraction model\nproposed here yields succinct analytical expressions for the axial PT signal\nshape and magnitude and its angular distribution, all showing the clear\nlens-signature. It is further demonstrated, that the Gouy-phase of a Gaussian\nbeam does not contribute to the relative photothermal signal in forward\ndirection, a fact which is not easily evident from the more rigorous EM\ntreatment. The model may thus be used to estimate the signal shape and\nmagnitude in photothermal single particle microscopy.",
        "category": "physics_optics"
    },
    {
        "text": "Computational Imaging Without a Computer: Seeing Through Random\n  Diffusers at the Speed of Light: Imaging through diffusers presents a challenging problem with various digital\nimage reconstruction solutions demonstrated to date using computers. We present\na computer-free, all-optical image reconstruction method to see through random\ndiffusers at the speed of light. Using deep learning, a set of diffractive\nsurfaces are designed/trained to all-optically reconstruct images of objects\nthat are covered by random phase diffusers. We experimentally demonstrated this\nconcept using coherent THz illumination and all-optically reconstructed objects\ndistorted by unknown, random diffusers, never used during training. Unlike\ndigital methods, all-optical diffractive reconstructions do not require power\nexcept for the illumination light. This diffractive solution to see through\ndiffusers can be extended to other wavelengths, and might fuel various\napplications in biomedical imaging, astronomy, atmospheric sciences,\noceanography, security, robotics, among others.",
        "category": "physics_optics"
    },
    {
        "text": "A new strategy to conceal an object from electromagnetic wave: A new recipe for concealing objects from detection is suggested. Different\nwith traditional cloak which deflects light around the core of the cloak to\nmake the object inside invisible, our cloak guides the light to penetrate the\ncore of the cloak but without striking some region of the cloak shell - the so\ncalled folded region. Full wave analytical calculation shows that this cloak\nwill lead to a scattering enhancement instead of scattering reduction in\ncontrast to the traditional cloak; the scattered field distribution can also be\nchanged as if the scatterer is moved from its original position. Such\ninteresting phenomenon indicates the proposed cloak can be used to disguise the\ntrue information of the object, e.g. the position, the size, etc, and further\nmislead the observer and avoid being detected.",
        "category": "physics_optics"
    },
    {
        "text": "Resonant-state expansion of three-dimensional open optical systems:\n  Light scattering: A rigorous method of calculating the electromagnetic field, the scattering\nmatrix, and scattering cross-sections of an arbitrary finite three-dimensional\noptical system described by its permittivity distribution is presented. The\nmethod is based on the expansion of the Green's function into the resonant\nstates of the system. These can be calculated by any means, including the\npopular finite element and finite-difference time-domain methods. However,\nusing the resonant-state expansion with a spherically-symmetric analytical\nbasis, such as that of a homogeneous sphere, allows to determine a complete set\nof the resonant states of the system within a given frequency range.\nFurthermore, it enables to take full advantage of the expansion of the field\noutside the system into vector spherical harmonics, resulting in simple\nanalytic expressions. We verify and illustrate the developed approach on an\nexample of a dielectric sphere in vacuum, which has an exact analytic solution\nknown as Mie scattering.",
        "category": "physics_optics"
    },
    {
        "text": "Singularities in Speckled Speckle: Speckle patterns produced by random optical fields with two (or more) widely\ndifferent correlation lengths exhibit speckle spots that are themselves highly\nspeckled. Using computer simulations and analytic theory we present results for\nthe point singularities of speckled speckle fields: optical vortices in scalar\n(one polarization component) fields; C points in vector (two polarization\ncomponent) fields. In single correlation length fields both types of\nsingularities tend to be more{}-or{}-less uniformly distributed. In contrast,\nthe singularity structure of speckled speckle is anomalous: for some sets of\nsource parameters vortices and C points tend to form widely separated giant\nclusters, for other parameter sets these singularities tend to form chains that\nsurround large empty regions. The critical point statistics of speckled speckle\nis also anomalous. In scalar (vector) single correlation length fields phase\n(azimuthal) extrema are always outnumbered by vortices (C points). In contrast,\nin speckled speckle fields, phase extrema can outnumber vortices, and azimuthal\nextrema can outnumber C points, by factors that can easily exceed $10^{4}$ for\nexperimentally realistic source parameters.",
        "category": "physics_optics"
    },
    {
        "text": "A wide band low profile linear cross polarizer for millimeter wave\n  applications: In this article, two wide band high-efficiency metasurface structures as a\nlinear-cross polarizer in transmission mode based on asymmetric split-ring\nresonators (ASRR) are proposed and tested to indicate that this is not a purely\ntheoretical work. A unit cell of the first structure consists of double\npolarization-sensitive S-shape resonators (SRs) on the bottom side of the\ndielectric layer. The superiority of this proposed structure is its filtering\nproperty and high conversion efficiency at its resonant frequency, 100 GHz,\nwhere its cross-polarized transmission coefficient reaches 0.87. The simulation\nresults show that the polarization conversion ratio (PCR) exceeds 85% in the\nfrequency range of 86 to 139 GHz. Additionally, the second structure consistsed\nof a wire-grid array as the bottom layer. This structure has extremely low\nco-polarized transmission amplitude which represents a good polarization\ntransformer in cross-direction. Furthermore, this structure has a polarization\nconversion ration more than 95% in frequency range of 50 to 200GHz, having\nwider relative bandwidth compared to the first proposed polarizer. The\nperformance of both polarizers has very weak angular dependence under the\noblique incident of x-polarized EM wave with from to .A prototype of these\nproposed polarizers has been fabricated and a good agreement between\nexperimental and simulation results has been observed. These proposed\nstructures can find application in mm-wave and THz sensing and imaging systems\nwhich benefit from polarimetric measurements.",
        "category": "physics_optics"
    },
    {
        "text": "Designing and using prior data in Ankylography: Recovering a 3D object\n  from a single diffraction intensity pattern: We present a novel method for Ankylography: three-dimensional structure\nreconstruction from a single shot diffraction intensity pattern. Our approach\nallows reconstruction of objects containing many more details than was ever\ndemonstrated, in a faster and more accurate fashion",
        "category": "physics_optics"
    },
    {
        "text": "Angle-resolved photoemission spectroscopy with 9-eV photon-energy pulses\n  generated in a gas-filled hollow-core photonic crystal fiber: A recently developed source of ultraviolet radiation, based on optical\nsoliton propagation in a gas-filled hollow-core photonic crystal fiber, is\napplied here to angle-resolved photoemission spectroscopy (ARPES).\nNear-infrared femtosecond pulses of only few {\\mu}J energy generate vacuum\nultraviolet (VUV) radiation between 5.5 and 9 eV inside the gas-filled fiber.\nThese pulses are used to measure the band structure of the topological\ninsulator Bi2Se3 with a signal to noise ratio comparable to that obtained with\nhigh order harmonics from a gas jet. The two-order-of-magnitude gain in\nefficiency promises time-resolved ARPES measurements at repetition rates of\nhundreds of kHz or even MHz, with photon energies that cover the first\nBrillouin zone of most materials.",
        "category": "physics_optics"
    },
    {
        "text": "Lasing on nonlinear localized waves in curved geometry: The use of geometrical constraints opens many new perspectives in photonics\nand in fundamental studies of nonlinear waves. By implementing surface\nstructures in vertical cavity surface emitting lasers as manifolds for curved\nspace, we experimentally study the impacts of geometrical constraints on\nnonlinear wave localization. We observe localized waves pinned to the maximal\ncurvature in an elliptical-ring, and confirm the reduction in the localization\nlength of waves by measuring near and far field patterns, as well as the\ncorresponding dispersion relation. Theoretically, analyses based on a\ndissipative model with a parabola curve give good agreement remarkably to\nexperimental measurement on the transition from delocalized to localized waves.\nThe introduction of curved geometry allows to control and design lasing modes\nin the nonlinear regime.",
        "category": "physics_optics"
    },
    {
        "text": "Theory of chiral edge state lasing in a two-dimensional topological\n  system: We theoretically study topological laser operation in a bosonic\nHarper-Hofstadter model featuring a saturable optical gain. Crucial\nconsequences of the chirality of the lasing edge modes are highlighted, such as\na sharp dependence of the lasing threshold on the geometrical shape of the\namplifying region and the possibility of ultraslow relaxation times and of\nconvectively unstable regimes. The different unstable regimes are characterized\nin terms of spatio-temporal structures sustained by noise and a strong\namplification of a propagating probe beam is anticipated to occur in between\nthe convective and the absolute (lasing) thresholds. The robustness of\ntopological laser operation against static disorder is assessed.",
        "category": "physics_optics"
    },
    {
        "text": "A heuristic resolution of the Abraham-Minkowski controversy: This paper reviews the history and origin of the Abraham-Minkowski\ncontroversy and points out that it is a continuation of the controversy over\nthe speed of light in medium. Upon considering an aircraft flying at a constant\nspeed along the great-circle route from the perspective of a geosynchronous\nspace station, we show that the A-M controversy arises from non-local\nobservation. The relative motion refractive index and the gravitational field\nrefractive index are defined by the space-time metric tensor, which reveals the\nA-M controversy hidden in special and general relativity. As another example,\nwe take light propagating over the surface of the sun, and show that Minkowski\nand Abraham forces are responsible for the gravitational deflection of light\nand the Shapiro time delay, respectively. Overall, we heuristically conclude\nthat non-local observation is the cause of the A-M controversy.",
        "category": "physics_optics"
    },
    {
        "text": "Super-resolution quantum sensing using NV centers based on rotating\n  linear polarized light and Monte-Carlo method: The nitrogen vacancy (NV) center in diamond has been widely applied for\nquantum information and sensing in last decade. Based on the laser polarization\ndependent excitation of fluorescence emission, we propose a super-resolution\nmicroscopy of NV center. A series of wide field images of NV centers are taken\nwith different polarizations of the linear polarized excitation laser. The\nfluorescence intensity of NV center is changed with the relative angle between\nexcitation laser polarization and the orientation of NV center dipole. The\nimages pumped by different excitation laser polarizations are analyzed with\nMonte Carlo method. Then the symmetry axis and position of NV center are\nobtained with sub-diffraction resolution.",
        "category": "physics_optics"
    },
    {
        "text": "Coherent perfect absorption in a weakly absorbing fiber: Short-length fiber lasers are key elements for device integration in fiber\nsystems. However, efficiently absorbing a pump beam in a short ion-doped fiber\nremains a challenge. We present an approach that renders a weakly absorbing\nshort-length Er-doped fiber completely absorbing. We exploit the concept of\ncoherent perfect absorption, whereby two appropriately designed fiber Bragg\ngratings define a short-length cavity that enforces complete absorption of an\nincident wave on resonance independently of the fiber intrinsic absorption.\nThis approach applies to any spectral window and may lead to efficient\nsingle-longitudinal-mode fiber lasers for applications in optical\ncommunication, sensing, and metrology.",
        "category": "physics_optics"
    },
    {
        "text": "High-performance lasers for fully integrated silicon nitride photonics: Silicon nitride (SiN) waveguides with ultra-low optical loss enable\nintegrated photonic applications including low noise, narrow linewidth lasers,\nchip-scale nonlinear photonics, and microwave photonics. Lasers are key\ncomponents to SiN photonic integrated circuits (PICs), but are difficult to\nfully integrate with low-index SiN waveguides due to their large mismatch with\nthe high-index III-V gain materials. The recent demonstration of multilayer\nheterogeneous integration provides a practical solution and enabled the\nfirst-generation of lasers fully integrated with SiN waveguides. However a\nlaser with high device yield and high output power at telecommunication\nwavelengths, where photonics applications are clustered, is still missing,\nhindered by large mode transition loss, nonoptimized cavity design, and a\ncomplicated fabrication process. Here, we report high-performance lasers on SiN\nwith tens of milliwatts output through the SiN waveguide and sub-kHz\nfundamental linewidth, addressing all of the aforementioned issues. We also\nshow Hertz-level linewidth lasers are achievable with the developed integration\ntechniques. These lasers, together with high-$Q$ SiN resonators, mark a\nmilestone towards a fully-integrated low-noise silicon nitride photonics\nplatform. This laser should find potential applications in LIDAR, microwave\nphotonics and coherent optical communications.",
        "category": "physics_optics"
    },
    {
        "text": "Amplification and ellipticity enhancement of sub-femtosecond XUV pulses\n  in IR-field-dressed neon-like active medium of a plasma-based X-ray laser: In [I.R. Khairulin et al., submitted to Phys. Rev. Lett.] we propose a method\nfor amplifying a train of sub-femtosecond pulses of circularly or elliptically\npolarized extreme ultraviolet (XUV) radiation, constituted by high-order\nharmonics of an infrared (IR) laser field, in a neon-like active medium of a\nplasma-based X-ray laser, additionally irradiated with a replica of a\nfundamental frequency laser field used to generate harmonics, and show the\npossibility of maintaining or enhancing the ellipticity of high-harmonic\nradiation during its amplification. In the present paper we describe this\nprocess in detail both for a single harmonic component and a sub-femtosecond\npulse train formed by a set of harmonics. We derive the analytical theory and\ndescribe both analytically and numerically the evolution of the high-harmonic\nfield during its propagation through the medium. We discuss also the\npossibility of an experimental implementation of the suggested technique in an\nactive medium of an X-ray laser based on neon-like Ti^{12+} ions irradiated by\nan IR laser field with a wavelength of 3.9 microns.",
        "category": "physics_optics"
    },
    {
        "text": "Atoms and Molecules in Intense Laser Fields: Gauge Invariance of Theory\n  and Models: Gauge invariance was discovered in the development of classical\nelectromagnetism and was required when the latter was formulated in terms of\nthe scalar and vector potentials. It is now considered to be a fundamental\nprinciple of nature, stating that different forms of these potentials yield the\nsame physical description: they describe the same electromagnetic field as long\nas they are related to each other by gauge transformations. Gauge invariance\ncan also be included into the quantum description of matter interacting with an\nelectromagnetic field by assuming that the wave function transforms under a\ngiven local unitary transformation. The result of this procedure is a quantum\ntheory describing the coupling of electrons, nuclei and photons. Therefore, it\nis a very important concept: it is used in almost every fields of physics and\nit has been generalized to describe electroweak and strong interactions in the\nstandard model of particles. A review of quantum mechanical gauge invariance\nand general unitary transformations is presented for atoms and molecules in\ninteraction with intense short laser pulses, spanning the perturbative to\nhighly nonlinear nonperturbative interaction regimes. Various unitary\ntransformations for single spinless particle Time Dependent Schr\\\"odinger\nEquations, TDSE, are shown to correspond to different time-dependent\nHamiltonians and wave functions. Accuracy of approximation methods involved in\nsolutions of TDSE's such as perturbation theory and popular numerical methods\ndepend on gauge or representation choices which can be more convenient due to\nfaster convergence criteria. We focus on three main representations: length and\nvelocity gauges, in addition to the acceleration form which is not a gauge, to\ndescribe perturbative and nonperturbative radiative interactions. Numerical\nschemes for solving TDSE's in different representations are also discussed.",
        "category": "physics_optics"
    },
    {
        "text": "Photon correlation spectroscopy with incoherent light: Photon correlation spectroscopy (PCS) is based on measuring the temporal\ncorrelation of the light intensity scattered by the investigated sample. A\ntypical setup requires a temporally coherent light source. Here, we show that a\nshort-coherence light source can be used as well, provided that its coherence\nproperties are suitably modified. This results in a \"skewed-coherence\" light\nbeam allowing that restores the coherence requirements. This approach overcomes\nthe usual need for beam filtering, which would reduce the total brightness of\nthe beam.",
        "category": "physics_optics"
    },
    {
        "text": "Silicon Carbide photonic platform based on suspended subwavelength\n  waveguides: Silicon carbide (SiC) displays a unique combination of optical and\nspin-related properties that make it interesting for photonics and quantum\ntechnologies. However, guiding light by total internal reflection can be\ndifficult to achieve, especially when SiC is grown as thin films on higher\nindex substrates, like Silicon. Fabricating suspended, subwavelength waveguides\nrequires a single lithography step and offers a solution to the confinement\nproblem, while preserving the design flexibility required for a scalable and\ncomplete photonic platform. Here we present a design for such platform, that\ncan be used for both classical and quantum optics operation. We simulate the\nkey optical components and analyze how to exploit the high nonlinearities of\nSiC and its defects.",
        "category": "physics_optics"
    },
    {
        "text": "Double-pass multiple-plate continuum for high temporal contrast\n  nonlinear pulse compression: We propose a new architecture, double-pass multiple-plate continuum (DPMPC),\nfor nonlinear pulse compression. In addition to smaller footprint, a\ndouble-pass configuration is designed to achieve substantial bandwidth\nbroadening without incurring noticeable higher-order dispersion, thus improving\nthe temporal contrast over those of traditional single-pass geometry when only\nquadratic spectral phase can be compensated. In our proof-of-concept\nexperiment, 187~$\\mu$J, 190-fs Yb-based laser pulse is compressed to 20~fs with\nhigh throughput (75%), high Strehl ratio (0.76) and excellent beam homogeneity\nby using DPMPC. Subsequently generated octave-spanning spectrum exhibits a\nsignificantly raised blue tail compared with that driven by pulses from a\nsingle-pass counterpart.",
        "category": "physics_optics"
    },
    {
        "text": "Beyond symmetry-protected BICs: transmission through asymmetric crossbar\n  junctions in one-dimensional waveguides: Over the last few decades, the study of Bound States in the Continuum, their\nformation, and properties has attracted lots of attention, especially in optics\nand photonics. It is particularly noticeable that most of these investigations\nbase their studies on symmetric systems. In this article, we study the\nformation of bound states in the continuum in electronic and photonic transport\nsystems consisting of crossbar junctions formed by one-dimensional waveguides,\nconsidering asymmetric junctions with commensurable lengths for the upper and\nlower arms. We also study how BICs form in linear junction arrays as a function\nof the distance between consecutive junctions and their commensurability with\nthe upper and lower arms. We solve the Helmholtz equation for the crossbar\njunctions and calculate the transmission probability, probability density in\nthe intersections, and quality factor. The presence of quasi-BICs is reflected\nin the transmission probability as a sharp resonance in the middle of a\nsymmetric Fano resonance along with Dirac's delta functions in the probability\ndensity and divergence in the quality factors.",
        "category": "physics_optics"
    },
    {
        "text": "Optical mirages from spinless beams: Spin-orbit interactions of light are ubiquitous in multiple branches of\nnanophotonics, including optical wave localization. In that framework, it is\nwidely accepted that circularly polarized beams lead to spin-dependent apparent\nshifts of dipolar targets commonly referred to as optical mirages. In contrast,\nthese optical mirages vanish when the illumination comes from a spinless beam\nsuch as a linearly polarized wave. Here we show that optical localization\nerrors emerge for particles sustaining electric and magnetic dipolar response\nunder the illumination of spinless beams. As an example, we calculate the\noptical mirage for the scattering by a high refractive index nanosphere under\nthe illumination of a linearly polarized plane wave carrying null spin,\norbital, and total angular momentum. Our results point to an overlooked\ninterference between the electric and magnetic dipoles rather than the\nspin-orbit interactions of light as the origin for the tilted position of the\nnanosphere.",
        "category": "physics_optics"
    },
    {
        "text": "Characterization of dipolar current-based metamaterials showing a single\n  resonance in the terahertz spectrum: We will report on the electromagnetic response due to induced dipolar\ncurrents in metamaterials of 2-dimensional array of metallic elements. Used as\nfrequency selectors, the metamaterial transmittance presents a single resonance\nin the region from 1 to 8 THz that can be easily selected and scaled\nmaintaining unaltered the quality factor by choosing the size and shape of the\nplanar metallic element and exploiting the scalability properties of the\nMaxwell equations. Basing on these studies, we have designed and tested a\nseries of simple and inexpensive frequency selective metamaterials fabricated\nby using lithographic processes.",
        "category": "physics_optics"
    },
    {
        "text": "Enabling self-induced back-action trapping of gold nanoparticles in\n  metamaterial plasmonic tweezers: The pursuit for efficient nanoparticle trapping with low powers has led to\noptical tweezers technology moving from the conventional free-space\nconfiguration to advanced plasmonic tweezers systems. However, trapping\nnanoparticles smaller than 10 nm still remains a challenge even for plasmonic\ntweezers. Proper nanocavity design and excitation has given rise to the\nself-induced back-action (SIBA) effect offering enhanced trapping stiffness\nwith decreased laser power. In this work, we investigate the SIBA effect in\nmetamaterial tweezers and its synergy with the exhibited Fano resonance. We\ndemonstrate stable trapping of 20 nm gold particles for on-resonant and\noff-resonant conditions with experimental trap stiffnesses as high as 4.18\nfN/(nm*mW/$\\mu$m$^2$ and very low excitation intensity of about 1\nmW/$\\mu$m$^2$. Simulations reveal the existence of two different groups of\nhotspots per unit cell of the metamaterial array. The two hotspots exhibit\ntunable trap stiffnesses and this is a unique feature of these systems. It can\nallow for sorting of particles and biological molecules based on their size,\nshape, and refractive index.",
        "category": "physics_optics"
    },
    {
        "text": "The Prevailing Role of Hotspots in Plasmon-Enhanced Sum-Frequency\n  Generation Spectroscopy: The plasmonic amplification of non-linear vibrational sum frequency\nspectroscopy (SFG) at the surfaces of gold nanoparticles is systematically\ninvestigated by tuning the incident visible wavelength. The SFG spectra of\ndodecanethiol-coated gold nanoparticles chemically deposited on silicon are\nrecorded for twenty visible wavelengths. The vibrational intensities of thiol\nmethyl stretches extracted from the experimental measurements vary with the\nvisible color of the SFG process and show amplification by coupling to\nplasmonics. Since the enhancement is maximal in the orange-red region rather\nthan in the green, as expected from the dipolar model for surface plasmon\nresonances, it is attributed mostly to hotspots created in particle multimers,\nin spite of their low surface densities.",
        "category": "physics_optics"
    },
    {
        "text": "Coexistence of Scattering Enhancement and Suppression by Plasmonic\n  Cavity Modes in Loaded Dimer Gap-Antennas: Plasmonic nanoantenna is of promising applications in optical sensing,\nsingle-molecular detection, and enhancement of optical nonlinear effect,\nsurface optical spectroscopy, photochemistry, photoemission, photovoltaics,\netc. Here we show that in a carefully-designed dimer gap-antenna made by two\nmetallic nanorods, the longitudinal plasmon antenna mode (AM) of bonding\ndipoles can compete with the transverse plasmonic cavity modes (CMs), yielding\ndramatically enhanced or suppressed scattering efficiency, depending on the CMs\nsymmetry characteristics (e.g., the radial order n and the azimuthal quantum\nnumber m ). More specifically, it is demonstrated that an appropriately loaded\ngap layer enables substantial excitation of toroidal moment and its strong\ninteraction with the AM dipole moment, resulting in Fano- or\nelectromagnetically induced transparency (EIT)-like profile in the scattering\nspectrum. However, for CMs with nonzero azimuthal number, the spectrum features\na cumulative signature of the respective AM and CM resonances. We supply both\ndetailed near-field and far-field analysis for these phenomena, showing that\nthe modal overlap and phase relationship between the fundamental moments of\ndifferent order play a crucial role. A classical coupled oscillator model is\nproposed which clearly reproduces the coexistence of scattering enhancement and\nsuppression. Finally, we show that the resonance bands of the AM and CMs can be\ntuned by adjusting the geometry parameters and the permittivity of the load.\nOur results may be useful in plasmonic cloaking, spin-polarized directional\nlight emission, ultra-sensitive optical sensing, and plasmon-mediated\nphotoluminescence.",
        "category": "physics_optics"
    },
    {
        "text": "Passive frequency conversion of ultraviolet images into the visible\n  using perovskite nanocrystals: We demonstrate a passive down-conversion imaging system that converts\nbroadband ultraviolet light to narrow-band green light while preserving the\ndirectionality of rays, and thus enabling direct down-conversion imaging. At\nthe same time our system has high transparency in the visible, enabling\nsuperimposed visible and ultraviolet imaging. The frequency conversion is\nperformed by a subwavelength-thickness transparent downconverter based on\nhighly efficient CsPbBr3 nanocrystals incorporated into the focal plane of a\nsimple telescope or relay-lens geometry. The resulting imaging performance of\nthis down-conversion system approaches the diffraction limit. This\ndemonstration sets the stage for the incorporation of other high-efficiency\nperovskite nanocrystal materials to enable passive multi-frequency conversion\nimaging systems.",
        "category": "physics_optics"
    },
    {
        "text": "Broadband cloaking with volumetric structures composed of\n  two-dimensional transmission-line networks: The cloaking performance of two microwave cloaks, both based on the recently\nproposed transmission-line approach, are studied using commercial full-wave\nsimulation software. The cloaks are shown to be able to reduce the total\nscattering cross sections of metallic objects of some restricted shapes and\nsizes. One of the studied cloaks is electrically small in diameter, and the\nother is electrically large, with the diameter equal to several wavelengths.",
        "category": "physics_optics"
    },
    {
        "text": "Bifurcating trajectory of non-diffractive electromagnetic Airy pulse: The explicit expression for spatial-temporal Airy pulse is derived from the\nMaxwell's equations in paraxial approximation. The trajectory of the pulse in\nthe time-space coordinates is analysed. The existence of a bifurcation point\nthat separates regions with qualitatively different features of the pulse\npropagation is demonstrated. At this point the velocity of the pulse becomes\ninfinite and the orientation of it changes to the opposite.",
        "category": "physics_optics"
    },
    {
        "text": "Trapping polarization of light in nonlinear optical fibers: An ideal\n  Raman polarizer: The main subject of this contribution is the all-optical control over the\nstate of polarization (SOP) of light, understood as the control over the SOP of\na signal beam by the SOP of a pump beam. We will show how the possibility of\nsuch control arises naturally from a vectorial study of pump-probe Raman\ninteractions in optical fibers. Most studies on the Raman effect in optical\nfibers assume a scalar model, which is only valid for high-PMD fibers (here,\nPMD stands for the polarization-mode dispersion). Modern technology enables\nmanufacturing of low-PMD fibers, the description of which requires a full\nvectorial model. Within this model we gain full control over the SOP of the\nsignal beam. In particular we show how the signal SOP is pulled towards and\ntrapped by the pump SOP. The isotropic symmetry of the fiber is broken by the\npresence of the polarized pump. This trapping effect is used in experiments for\nthe design of new nonlinear optical devices named Raman polarizers. Along with\nthe property of improved signal amplification, these devices transform an\narbitrary input SOP of the signal beam into one and the same SOP towards the\noutput end. This output SOP is fully controlled by the SOP of the pump beam. We\noverview the sate-of-the-art of the subject and introduce the notion of an\n\"ideal Raman polarizer\".",
        "category": "physics_optics"
    },
    {
        "text": "Electrical tuning and switching of an optical frequency comb generated\n  in aluminum nitride microring resonators: Aluminum nitride has been shown to possess both strong Kerr nonlinearity and\nelectro-optic Pockels effect. By combining these two effects, here we\ndemonstrate on-chip reversible on/off switching of the optical frequency comb\ngenerated by an aluminum nitride microring resonator. We optimize the design of\ngating electrodes and the underneath resonator structure to effectively apply\nelectric field without increasing the optical loss. The switching of the comb\nis monitored by measuring one of the frequency comb peaks while varying the\nelectric field. The controlled comb electro-optic response is investigated for\ndirect comparison with the transient thermal effect.",
        "category": "physics_optics"
    },
    {
        "text": "Heterogeneously Integrated Laser on Silicon with Non-Volatile Wavelength\n  Tuning: The von-Neumann bottleneck has constrained computing systems from efficiently\noperating on the increasingly large demand in data from networks and devices.\nSilicon (Si) photonics offers a powerful solution for this issue by providing a\nplatform for high-bandwidth, energy-efficient interconnects. Furthermore,\nmemristors have emerged as a fundamental building block for non-volatile data\nstorage and novel computing architectures with powerful in-memory processing\ncapabilities. In this paper, we integrate an Al2O3 memristor into a\nheterogeneous Si quantum dot microring laser to demonstrate the first laser\nwith non-volatile optical memory. The memristor alters the effective optical\nmodal index of the microring laser cavity by the plasma dispersion effect in\nthe high resistance state (HRS) or Joule heating in the low resistance state\n(LRS), subsequently controlling the output wavelength of the laser in a\nnon-volatile manner. This device enables a novel pathway for future\noptoelectronic neuromorphic computers and optical memory chips.",
        "category": "physics_optics"
    },
    {
        "text": "Iterative phase retrieval for digital holography: This paper provides a tutorial of iterative phase retrieval algorithms based\non the Gerchberg-Saxton (GS) algorithm applied in digital holography. In\naddition, a novel GS-based algorithm that allows reconstruction of 3D samples\nis demonstrated. The GS-based algorithms recover complex-valued wavefront by\nwavefront back-and forth propagation between two planes with constraints\nsuperimposed in these two planes. Iterative phase retrieval allows\nquantitatively correct and twin-image-free reconstructions of object amplitude\nand phase distributions from its in-line hologram. The present work derives the\nquantitative criteria on how many holograms are required to reconstruct a\ncomplex-valued object distribution, be it a 2D or 3D sample. It is shown that\nfor a sample that can be approximated as a 2D sample, a single-shot in-line\nhologram is sufficient to reconstruct the absorption and phase distributions of\nthe sample. Previously, the GS-based algorithms have been successfully employed\nto reconstruct samples that are limited to a 2D plane. However, realistic\nphysical objects always have some finite thickness and therefore are 3D rather\nthan 2D objects. This study demonstrates that 3D samples, including 3D phase\nobjects, can be reconstructed from two or more holograms. It is shown that in\nprinciple, two holograms are sufficient to recover the entire wavefront\ndiffracted by a 3D sample distribution. In this method, the reconstruction is\nperformed by applying iterative phase retrieval between the planes where\nintensity was measured. The recovered complex-valued wavefront is then\npropagated back to the sample planes, thus reconstructing the 3D distribution\nof the sample. This method can be applied for 3D samples such as 3D\ndistribution of particles, thick biological samples, and other 3D phase\nobjects. Examples of reconstructions of 3D objects, including phase objects,\nare provided.",
        "category": "physics_optics"
    },
    {
        "text": "Free-space optomechanical liquid probes using a twin-microbottle\n  resonator: Cavity optomechanics provides high-performance sensor technology, and the\nscheme is also applicable to liquid samples for biological and rheological\napplications. However, previously reported methods using fluidic capillary\nchannels and liquid droplets are based on fixed-by-design structures and\ntherefore do not allow an active free-space approach to the samples. Here, we\ndemonstrate an alternate technique using a probe-based architecture with a\ntwin-microbottle resonator. The probe consists of two microbottle\noptomechanical resonators, where one bottle (for detection) is immersed in\nliquid and the other bottle (for readout) is placed in air, which retains\nexcellent detection performance through the high optical-Q (~107) of the\nreadout bottle. The scheme allows the detection of thermomechanical motion of\nthe detection bottle as well as its optomechanical sideband drive. This\ntechnique could lead to in-situ metrology at the target location in arbitrary\nmedia, and could be extended to ultrasensitive biochips and rheometers.",
        "category": "physics_optics"
    },
    {
        "text": "Engineering photonic environments for two-dimensional materials: A fascinating photonic platform with a small device scale, fast operating\nspeed, as well as low energy consumption is two-dimensional (2D) materials,\nthanks to their in-plane crystalline structures and out-of-plane quantum\nconfinement. The key to further advancement in this research field is the\nability to modify the optical properties of the 2D materials. The modifications\ntypically come from the materials themselves, for example, altering their\nchemical compositions. This article reviews a comparably less explored but\npromising means, through engineering the photonic surroundings. Rather than\nmodifying materials themselves, this means manipulates the dielectric and\nmetallic environments, both uniform and nanostructured, that directly interact\nwith the materials. For 2D materials that are only one or a few atoms thick,\nthe interaction with the environment can be remarkably efficient. This review\nsummarizes the three degrees of freedom of this interaction: weak coupling,\nstrong coupling, and multi-functionality. Also, it reviews a relatively timing\nconcept of engineering that directly applied to the 2D materials by patterning.\nBenefiting from the burgeoning development of nanophotonics, the engineering of\nphotonic environments provides a versatile and creative methodology of\nreshaping light-matter interaction in 2D materials.",
        "category": "physics_optics"
    },
    {
        "text": "Generalized and multiplexed $q$-plates: experimental implementation: In this paper we generalize the concept of $q$-plate, allowing arbitrary\nfunctions of both the radial and the azimuthal variables, and study their\neffect on uniformly polarized beams in the near and far-field regime. This\ngives a tool for achieving beams with hybrid states of polarization (SoPs), and\nalternative phase and intensity distributions. We also implement an\nexperimental device based on a liquid crystal on silicon (LCoS) display for\nemulating these generalized $q$-plates. Moreover, we propose an application\nthat takes advantage of the pixelated nature of this kind of devices for\ncreating arbitrary superpositions of vector and vortex beams by representing\nonto the LCoS randomized combinations of two different $q$-plates, i.e.\nmultiplexed $q$-plates. Great agreement is found between theoretical and\nexperimental results.",
        "category": "physics_optics"
    },
    {
        "text": "Controllable spiking patterns in long-wavelength VCSELs for neuromorphic\n  photonics systems: Multiple controllable spiking patterns are obtained in a 1310 nm Vertical\nCavity Surface Emitting Laser (VCSEL) in response to induced perturbations and\nfor two different cases of polarized optical injection, namely parallel and\northogonal. Achievement of reproducible spiking responses in VCSELs operating\nat the telecom wavelengths offers great promise for future uses of these\ndevices in ultrafast neuromorphic photonic systems for non-traditional\ncomputing applications.",
        "category": "physics_optics"
    },
    {
        "text": "Non-Hermitian two-dimensional photonic crystal flat lens: In this paper, a non-Hermitian two-dimensional photonic crystal flat lens is\nproposed. The negative refraction of the second band of photonic crystal is\nutilized to realize super-resolution imaging of the point source. Based on the\nprinciples of non-Hermitian systems, a negative imaginary part is introduced\ninto the imaging frequency, in which case the imaging intensity and resolution\nare improved. The results indicate that the non-Hermitian system provides a new\nmethod to improve the imaging performance of the photonic crystal lens.",
        "category": "physics_optics"
    },
    {
        "text": "Integrated Fabry-Perot cavities as a mechanism for enhancing micro-ring\n  resonator performance: We propose and experimentally demonstrate the enhancement in the filtering\nquality (Q) factor of an integrated micro-ring resonator (MRR) by embedding it\nin an integrated Fabry-Perot (FP) cavity formed by cascaded Sagnac loop\nreflectors (SLRs). By utilizing coherent interference within the FP cavity to\nreshape the transmission spectrum of the MRR, both the Q factor and the\nextinction ratio (ER) can be significantly improved. The device is\ntheoretically analyzed, and practically fabricated on a silicon-on-insulator\n(SOI) wafer. Experimental results show that up to 11-times improvement in Q\nfactor, together with an 8-dB increase in ER, can be achieved via our proposed\nmethod. The impact of varying structural parameters on the device performance\nis also investigated and verified by the measured spectra of the fabricated\ndevices with different structural parameters.",
        "category": "physics_optics"
    },
    {
        "text": "High-Q slow light and its localization in a photonic crystal microring: We introduce a photonic crystal ring cavity that resembles an internal gear\nand unites photonic crystal (PhC) and whispering gallery mode (WGM) concepts.\nThis `microgear' photonic crystal ring (MPhCR) is created by applying a\nperiodic modulation to the inside boundary of a microring resonator to open a\nlarge bandgap, as in a PhC cavity, while maintaining the ring's circularly\nsymmetric outside boundary and high quality factor ($Q$), as in a WGM cavity.\nThe MPhCR targets a specific WGM to open a large PhC bandgap up to tens of free\nspectral ranges, compressing the mode spectrum while maintaining the high-$Q$,\nangular momenta, and waveguide coupling properties of the WGM modes. In\nparticular, near the dielectric band-edge, we observe modes whose group\nvelocity is slowed down by 10 times relative to conventional microring modes\nwhile supporting $Q~=~(1.1\\pm0.1)\\times10^6$. This $Q$ is $\\approx$50$\\times$\nthat of the previous record in slow light devices. Using the slow light design\nas a starting point, we further demonstrate the ability to localize WGMs into\nphotonic crystal defect (dPhC) modes for the first time, enabling a more than\n10$\\times$ reduction of mode volume compared to conventional WGMs while\nmaintaining high-$Q$ up to (5.6$\\pm$0.1)$\\times$10$^5$. Importantly, this\nadditional dPhC localization is achievable without requiring detailed\nelectromagnetic design. Moreover, controlling their frequencies and waveguide\ncoupling is straightforward in the MPhCR, thanks to its WGM heritage. By using\na PhC to strongly modify fundamental properties of WGMs, such as group velocity\nand localization, the MPhCR provides an exciting platform for a broad range of\nphotonics applications, including sensing/metrology, nonlinear optics, and\ncavity quantum electrodynamics.",
        "category": "physics_optics"
    },
    {
        "text": "Stationary plasmon-soliton waves in metal-dielectric nonlinear planar\n  structures: modeling and properties: We present three complementary methods to study stationary nonlinear\nsolutions in one-dimensional nonlinear metal-dielectric structures. Two of them\nuse an approximate treatment of the Kerr type nonlinear term taking into\naccount only the leading electric field component, while the third one allows\nfor an exact treatment of the nonlinearity. A direct comparison of the results\nobtained with all three models is presented and the excellent agreement between\nthem justifies the assumptions that have been used to construct the models. A\nsystematic study of the configurations made of two, three, or four layers, that\ncontain a semi-infinite Kerr type nonlinear dielectric, a metal film and linear\ndielectrics is presented. Detailed analysis of properties, type and number of\nsolutions in these three types of structures is performed. The parameter ranges\nwhere plasmon-soliton waves exist are found. The structures with realistic\nopto-geometric parameters where plasmon-solitons exist at power levels already\nused in spatial soliton studies are proposed and studied.",
        "category": "physics_optics"
    },
    {
        "text": "Design and analytically full-wave validation of the invisibility cloaks,\n  concentrators, and field rotators created with a general class of\n  transformations: We investigate a general class of electromagnetic devices created with any\ncontinuous transformation functions by rigorously calculating the analytical\nexpressions of the electromagnetic field in the whole space. Some interesting\nphenomena associated with these transformation devices, including the\ninvisibility cloaks, concentrators, and field rotators, are discussed. By\ncarefully choosing the transformation function, we can realize cloaks which are\ninsensitive to perturbations at both the inner and outer boundaries.\nFurthermore, we find that when the coating layer of the concentrator is\nrealized with left-handed materials, energy will circulate between the coating\nand the core, and the energy transmits through the core of the concentrator can\nbe much bigger than that transmits through the concentrator. Therefore, such\nconcentrator is also a power flux amplifier. Finally, we propose a spherical\nfield rotator, which functions as not only a wave vector rotator, but also a\npolarization rotator, depending on the orientations of the spherical rotator\nwith respect to the incident wave direction. The functionality of these novel\ntransformation devices are all successfully confirmed by our analytical full\nwave method, which also provides an alternate computational efficient\nvalidation method in contrast to numerical validation methods.",
        "category": "physics_optics"
    },
    {
        "text": "Dynamics of a class A nonlinear mirror mode-locked laser: Using a delay differential equation model we study theoretically the dynamics\nof a unidirectional class-A ring laser with a nonlinear amplifying loop mirror.\nWe perform linear stability analysis of the CW regimes in the large delay limit\nand demonstrate that these regimes can be destabilized via modulational and\nTuring-type instabilities, as well as by an instability leading to the\nappearance of square-waves. We investigate the formation of square-waves and\nmode-locked pulses in the system. We show that mode-locked pulses are\nasymmetric with exponential decay of the trailing edge in positive time and\nfaster-than-exponential (super-exponential) decay of the leading edge in\nnegative time. We discuss asymmetric interaction of these pulses leading to a\nformation of harmonic mode-locked regimes.",
        "category": "physics_optics"
    },
    {
        "text": "Tunneling mediated by conical waves in a 1D lattice: The nonlinear propagation of 3D wave-packets in a 1D Bragg-induced band-gap\nsystem, shows that tranverse effects (free space diffraction) affect the\ninterplay of periodicity and nonlinearity, leading to the spontaneous formation\nof fast and slow conical localized waves. Such excitation corresponds to\nenhanced nonlinear transmission (tunneling) in the gap, with peculiar features\nwhich differ on the two edges of the band-gap, as dictated by the full\ndispersion relationship of the localized waves.",
        "category": "physics_optics"
    },
    {
        "text": "Hidden Black: Coherent Enhancement of Absorption in Strongly-scattering\n  Media: We show that a weakly absorbing, strongly scattering (white) medium can be\nmade very strongly absorbing at any frequency within its strong-scattering\nbandwidth by optimizing the input electromagnetic field. For uniform\nabsorption, results from random matrix theory imply that the reflectivity of\nthe medium can be suppressed by a factor ~(l_a/lN^2), where N is the number of\nincident channels and l,l_a are the elastic and absorption mean free paths\nrespectively. It is thus possible to increase absorption from a few percent to\n> 99%. For a localized weak absorber buried in a non-absorbing scattering\nmedium, we find a large but bounded enhancement.",
        "category": "physics_optics"
    },
    {
        "text": "Maintaining Constant Pulse-duration in Highly Dispersive Media using\n  Nonlinear Potentials: A method is shown for preventing temporal broadening of ultrafast optical\npulses in highly dispersive and fluctuating media for arbitrary signal-pulse\nprofiles. Pulse pairs, consisting of a strong-field control-pulse and a\nweak-field signal-pulse co-propagate, whereby the specific profile of the\nstrong-field pulse precisely compensates for dispersive phase in the weak\npulse. A numerical example is presented in an optical system consisting of both\nresonant and gain dispersive effects. Here we show signal-pulses that do not\ntemporally broaden across a vast propagation distance, even in the presence of\ndispersion that fluctuates several orders of magnitude and in sign (for example\nwithin a material resonance) across the pulse's bandwidth. Our approach\nillustrates the potential for using cross-phase modulation to compensate for\ndispersive effects on a signal-pulse and fills the gap in the current\nunderstanding of this nonlinear phenomenon.",
        "category": "physics_optics"
    },
    {
        "text": "Kerr nonlinearity effect on light transmission in one dimensional\n  photonic crystal: We investigate numerically the effect of Kerr nonlinearity on the\ntransmission spectrum of a one dimensional $\\delta$-function photonic crystal.\nIt is found that the photonic band gap (PBG) width either increases or\ndecreases depending on both sign and strength of Kerr nonlinearity. We found\nthat any amount of self-focusing nonlinearity $(\\alpha >0)$ leads to an\nincrease of the PBG width leading to light localization. However, for\ndefocusing nonlinearity, we found a range of non-linearity strengths for which\nthe photonic band gap width decreases when the nonlinearity strength increases\nand a critical non-linearity strength $|\\alpha_{c}|$ above which the behaviour\nis reversed. At this critical value the photonic crystal become transparent and\nthe photonic band gap is suppressed. We have also studied the dependence on the\nangle of incidence and polarization in the transmission spectrum of our\none-dimensional photonic crystal. We found that the minimum of the transmission\nincreases with incident angle but seems to be polarization-independent. We also\nfound that position of the photonic band gap (PBG) shifts to lower wavelengths\nwhen the angle of incidence increases for TE mode while it shifts to longer\nwavelengths for TM mode.",
        "category": "physics_optics"
    },
    {
        "text": "Exciton-Plasmon-Photon Conversion in silver nanowire: polarization\n  dependence: Polarization dependence of the exciton-plasmon-photon conversion in silver\nnanowire-quantum dots structure was investigated using a scanning confocal\nmicroscope system. We found that the fluorescence enhancement of the CdSe\nnanocrystals was correlated with the angle between the excitation light\npolarization and the silver nanowire direction. The polarization of the\nemission was also related with the nanowire direction. It was in majority in\nthe direction parallel with nanowire.",
        "category": "physics_optics"
    },
    {
        "text": "Spectral Peak Recovery in Parametrically Amplified THz-Repetition-Rate\n  Bursts: Multi-photon resonant spectroscopies require tunable narrowband excitation to\ndeliver spectral selectivity and, simultaneously, high temporal intensity to\ndrive a nonlinear-optical process. These contradictory requirements are\nachievable with bursts of ultrashort pulses, which provides both high intensity\nand tunable narrowband peaks in the frequency domain arising from spectral\ninterference. However, femtosecond pulse bursts need special attention during\ntheir amplification [Optica 7, 1758 (2020)], which requires spectral peak\nsuppression to increase the energy safely extractable from a chirped-pulse\namplifier (CPA). Here, we present a method combining safe laser CPA, relying on\nspectral scrambling, with a parametric frequency converter that automatically\nrestores the desired spectral peak structure and delivers narrow linewidths for\nbursts of ultrashort pulses at microjoule energies. The shown results pave the\nway to new high-energy ultrafast laser sources with controllable spectral\nselectivity.",
        "category": "physics_optics"
    },
    {
        "text": "Generation of optical vortices imitating water vortices: In optics, we can generate vortex beams using specific methods such as spiral\nphase plates or computer generated holograms. While, in nature, it is worth\nnoting that water can produce vortices by a circularly symmetrical hole. So, if\na light beam can generate vortex when it is diffracted by an aperture? Here, we\nshow that the light field in the Fresnel region of the diffracted circularly\npolarized beam carries orbital angular momentum, which can transfer to the\ntrapped particles and make orbital rotation.",
        "category": "physics_optics"
    },
    {
        "text": "Tree-wave mixing of ordinary and backward electromagnetic waves:\n  extraordinary transients: Three-wave mixing of ordinary and backward electromagnetic waves in pulsed\nregime is investigated in the metamaterials, which enable co-existence and\nphase matching of such waves. It is shown that opposite direction of phase\nvelocity and energy flux in backward waves gives rise to extraordinary\ntransient processes in greatly enhanced optical parametric amplification and in\nfrequency up or down shifting nonlinear reflectivity. The discovered transients\nresemble slowed response of an oscillator on pulsed excitation in the vicinity\nof its resonance",
        "category": "physics_optics"
    },
    {
        "text": "Enhanced second harmonic generation from resonant GaAs gratings: We study second harmonic generation in nonlinear, GaAs gratings. We find\nlarge enhancement of conversion efficiency when the pump field excites the\nguided mode resonances of the grating. Under these circumstances the spectrum\nnear the pump wavelength displays sharp resonances characterized by dramatic\nenhancements of local fields and favorable conditions for second harmonic\ngeneration, even in regimes of strong linear absorption at the harmonic\nwavelength. In particular, in a GaAs grating pumped at 1064nm, we predict\nsecond harmonic conversion efficiencies approximately five orders of magnitude\nlarger than conversion rates achievable in either bulk or etalon structures of\nthe same material.",
        "category": "physics_optics"
    },
    {
        "text": "Tunable optical bistability in grapheme Tamm plasmon/Bragg reflector\n  hybrid structure at terahertz frequencies: We propose a composite multilayer structure consist of graphene Tamm plasmon\nand Bragg reflector with defect layer to realize the low threshold and tunable\noptical bistability (OB) at the terahertz frequencies. This low-threshold OB\noriginates from the couple of the Tamm plasmon (TP) and the defect mode (DM).\nWe discuss the influence of graphene and the DM on the hysteretic response of\nthe TM-polarized reflected light. It is found that the switch-up and\nswitch-down threshold required to observe the optical bistable behavior are\nlowered markedly due to the excitation of the TP and DM. Besides, the switching\nthreshold value can be further reduced by coupling the TP and DM. We believe\nthese results will provide a new avenue for realizing the low threshold and\ntunable optical bistable devices and other nonlinear optical devices.",
        "category": "physics_optics"
    },
    {
        "text": "Thermal-light heterodyne spectroscopy with frequency comb calibration: Precision laser spectroscopy is key to many developments in atomic and\nmolecular physics and the advancement of related technologies such as atomic\nclocks and sensors. However, in important spectroscopic scenarios, such as\nastronomy and remote sensing, the light is of thermal origin and\ninterferometric or diffractive spectrometers typically replace laser\nspectroscopy. In this work, we employ laser-based heterodyne radiometry to\nmeasure incoherent light sources in the near-infrared and introduce techniques\nfor absolute frequency calibration with a laser frequency comb. Measuring the\nsolar continuum, we obtain a signal-to-noise ratio that matches the fundamental\nquantum-limited prediction given by the thermal photon distribution and our\nsystem's efficiency, bandwidth, and averaging time. With resolving power\nR~1,000,000 we determine the center frequency of an iron line in the solar\nspectrum to sub-MHz absolute frequency uncertainty in under 10 minutes, a\nfractional precision 1/4000 the linewidth. Additionally, we propose concepts\nthat take advantage of refractive beam shaping to decrease the effects of\npointing instabilities by 100x, and of frequency comb multiplexing to increase\ndata acquisition rates and spectral bandwidths by comparable factors. Taken\ntogether, our work brings the power of telecommunications photonics and the\nprecision of frequency comb metrology to laser heterodyne radiometry, with\nimplications for solar and astronomical spectroscopy, remote sensing, and\nprecise Doppler velocimetry.",
        "category": "physics_optics"
    },
    {
        "text": "Coupled parametric processes in binary nonlinear photonic structures: We study parametric interactions in a new type of nonlinear photonic\nstructures, which is realized in the vicinity of a pair of nonlinear crystals.\nIn this kind of structure, which we call binary, multiple nonlinear optical\nprocesses can be implemented simultaneously, owing to multiple phase-matching\nconditions, fulfilled separately in the constituent crystals. The coupling\nbetween the nonlinear processes by means of modes sharing similar frequency is\nattained by the spatially-broadband nature of the parametric fields. We\ninvestigate the spatial properties of the fields generated in the binary\nstructure constructed from periodically poled crystals for the two examples: 1)\nsingle parametric down-conversion, and 2) coupled parametric down-conversion\nand up-conversion processes. The efficacy of the fields' generation in these\nexamples is analyzed through comparison with the cases of traditional single\nperiodically poled crystal and aperiodic photonic structure, respectively. It\nhas been shown that the relative shift between the periodic crystal lattices\nhas a crucial effect on the generated spatial field spectrum and the overall\nefficiency. In addition, the influence of the inter-crystal distance on these\ncharacteristics has been studied. Therefore, our study suggests that one can\nconstruct optical elements with sophisticated nonlinear properties from simpler\nelements without significant sacrifice of the efficacy.",
        "category": "physics_optics"
    },
    {
        "text": "Near-field spectroscopy of bimodal size distribution of InAs/AlGaAs\n  quantum dots: We report on high-resolution photoluminescence (PL) spectroscopy of spatial\nstructure of InAs/AlGaAs quantum dots (QDs) by using a near-field scanning\noptical microscope (NSOM). The double-peaked distribution of PL spectra is\nclearly observed, which is associated with the bimodal size distribution of\nsingle QDs. In particular, the size difference of single QDs, represented by\nthe doublet spectral distribution, can be directly observed by the NSOM images\nof PL.",
        "category": "physics_optics"
    },
    {
        "text": "A Hamiltonian treatment of stimulated Brillouin scattering in nanoscale\n  integrated waveguides: We present a multimode Hamiltonian formulation for the problem of\nopto-acoustic interactions in optical waveguides. We establish a Hamiltonian\nrepresentation of the acoustic field and then introduce a full system with a\nsimple opto-acoustic coupling that includes both photoelastic/electrostrictive\nand radiation pressure/moving boundary effects. The Heisenberg equations of\nmotion are used to obtain coupled mode equations for quantized envelope\noperators for the optical and acoustic fields. We show that the coupling\ncoefficients obtained coincide with those established earlier, but our\nformalism provides a much simpler demonstration of the connection between\nradiation pressure and moving boundary effects than in previous work [C. Wolff\net al, Physical Review A 92, 013836 (2015)].",
        "category": "physics_optics"
    },
    {
        "text": "Electrostatic theory for designing lossless negative permittivity\n  metamaterials: In this Letter, we develop an electrostatic theory for designing bulk\ncomposites with effective lossless negative permittivities. The theory and\nassociated design procedure are validated by comparing their predictions with\nthose of rigorous full-wave simulations. It is demonstrated that the excitation\nof the Frohlich mode (the first-order surface mode) of the constitutive\nnanoparticles plays a key role in achieving negative permittivities with\ncompensated losses.",
        "category": "physics_optics"
    },
    {
        "text": "Fourier ptychography multi-parameter neural network with composite\n  physical priori optimization: Fourier ptychography microscopy(FP) is a recently developed computational\nimaging approach for microscopic super-resolution imaging. By turning on each\nlight-emitting-diode (LED) located on different position on the LED array\nsequentially and acquiring the corresponding images that contain different\nspatial frequency components, high spatial resolution and quantitative phase\nimaging can be achieved in the case of large field-of-view. Nevertheless, FPM\nhas high requirements for the system construction and data acquisition\nprocesses, such as precise LEDs position, accurate focusing and appropriate\nexposure time, which brings many limitations to its practical applications. In\nthis paper, inspired by artificial neural network, we propose a Fourier\nptychography multi-parameter neural network (FPMN) with composite physical\nprior optimization. A hybrid parameter determination strategy combining\nphysical imaging model and data-driven network training is proposed to recover\nthe multi layers of the network corresponding to different physical parameters,\nincluding sample complex function, system pupil function, defocus distance, LED\narray position deviation and illumination intensity fluctuation, etc. Among\nthese parameters, LED array position deviation is recovered based on the\nfeatures of brightfield to darkfield transition low-resolution images while the\nothers are recovered in the process of training of the neural network. The\nfeasibility and effectiveness of FPMN are verified through simulations and\nactual experiments. Therefore FPMN can evidently reduce the requirement for\npractical applications of FPM.",
        "category": "physics_optics"
    },
    {
        "text": "Neighboring Interactions in a Periodic Plasmonic Material for\n  Solar-Thermal Energy Conversion: A periodic plasmonic meta-material was studied using finite-difference time\ndomain (FDTD) method to investigate the influence of neighboring particles on\nthe near unity optical absorptivity. The meta-material was constructed as a\nsilver nanoparticle (20-90nm) situated above an alumina (Al$_2$O$_3$)\ndielectric environment. A full parametric sweep of the particle width and the\ndielectric thickness was conducted. Computational results identified several\nresonances between the metal-dielectric and metal-air that have potential to\nbroadening the response through stacked geometry. A significant coupled\nresonance between the metal-dielectric resonance and a cavity resonance between\nparticles was capture as a function of dielectric thickness. This coupled\nresonance was not evident below dielectric thicknesses of 40nm and above cavity\nwidths of 20nm. Additionally, a noticeable propagating surface plasmon\npolariton resonance was predicted when the particle width was half the unit\ncell length.",
        "category": "physics_optics"
    },
    {
        "text": "Exact Analytical Solution of the One-Dimensional Time-Dependent\n  Radiative Transfer Equation with Linear Scattering: The radiative transfer equation (RTE) is a cornerstone for describing the\npropagation of electromagnetic radiation in a medium, with applications\nspanning atmospheric science, astrophysics, remote sensing, and biomedical\noptics. Despite its importance, an exact analytical solution to the RTE has\nremained elusive, necessitating the use of numerical approximations such as\nMonte Carlo, discrete ordinate, and spherical harmonics methods.\n  In this paper, we present an exact solution to the one-dimensional\ntime-dependent RTE. We delve into the moments of the photon distribution,\nproviding a clear view of the transition to the diffusion regime. This analysis\noffers a deeper understanding of light propagation in the medium.\n  Furthermore, we demonstrate that the one-dimensional RTE is equivalent to the\nKlein-Gordon equation with an imaginary mass term determined by the inverse\nreduced scattering length. Contrary to naive expectations of superluminal\nsolutions, we find that our solution is strictly causal under appropriate\nboundary conditions, determined by the light transport problem.\n  We validate the found solution using Monte Carlo simulations and benchmark\nthe performance of the latter. Our analysis reveals that even for highly\nforward scattering, dozens of random light scatterings are required for an\naccurate estimate, underscoring the complexity of the problem.\n  Moreover, we propose a method for faster convergence by adjusting the\nparameters of Monte Carlo sampling. We show that a Monte Carlo method sampling\nphoton scatterings with input parameters $(\\mu_s,g)$, where $\\mu_s$ is the\ninverse scattering length and $g$ is the scattering anisotropy parameter, is\nequivalent to that with $(\\mu_s(1-g)/2,-1)$. This equivalence leads to a\nsignificantly faster convergence to the exact solution, offering a substantial\nimprovement of the Monte Carlo method for the one-dimensional RTE.",
        "category": "physics_optics"
    },
    {
        "text": "Quasi-parallel X-ray microbeam obtained using a parabolic monocapillary\n  X-ray lens with an embedded square-shaped lead occluder: A parabolic monocapillary X-ray lens (PMXRL) is designed to effectively\nconstrain a laboratory point X-ray source into a parallel beam. A square-shaped\nlead occluder (SSLO) is used to block direct X-rays in the PMXRL. To design the\nPMXRL, we use Python to simulate the conic parameter (p = 0.001 mm) of the lens\nand then use a drawing machine to draw a corresponding lens (p = 0.000939 mm)\nwith a total length of 60.8 mm. We place the SSLO at the lens inlet for optical\ntesting. The results show that the controlled outgoing beam has a divergence of\nless than 0.4 mrad in the range of 15-45 mm of the lens outlet, which achieves\nexcellent optical performance in X-ray imaging methodology. The design details\nare reported in this paper.",
        "category": "physics_optics"
    },
    {
        "text": "Self-Frequency Shift of Cavity Soliton in Kerr Frequency Comb: We show that the ultrashort cavity soliton in octave-spanning Kerr frequency\ncomb generation exhibits striking self-adaptiveness and robustness to external\nperturbations, resulting in a novel frequency shifting/cancellation mechanism\nand gigantic dispersive wave generation in response to the strong frequency\ndependence of Kerr nonlinearity, Raman scattering, chromatic dispersion, and\ncavity Q. These observations open up a great avenue towards versatile\nmanipulation of nonlinear soliton dynamics, flexible spectrum engineering of\nmode-locked Kerr frequency combs, and highly efficient frequency translation of\noptical waves.",
        "category": "physics_optics"
    },
    {
        "text": "Temperature dependent moir\u00e9 trapping of interlayer excitons in\n  MoSe2-WSe2 heterostructures: MoSe2-WSe2 heterostructures host strongly bound interlayer excitons (IXs)\nwhich exhibit bright photoluminescence (PL) when the twist-angle is near\n0{\\deg} or 60{\\deg}. Over the past several years, there have been numerous\nreports on the optical response of these heterostructures but no unifying model\nto understand the dynamics of IXs and their temperature dependence. Here, we\nperform a comprehensive study of the temperature, excitation power, and\ntime-dependent PL of IXs. We observe a significant decrease in PL intensity\nabove a transition temperature that we attribute to a transition from localized\nto delocalized IXs. Astoundingly, we find a simple inverse relationship between\nthe IX PL energy and the transition temperature, which exhibits opposite power\ndependent behaviors for near 0{\\deg} and 60{\\deg} samples. We conclude that\nthis temperature dependence is a result of IX-IX exchange interactions, whose\neffect is suppressed by the moir\\'e potential trapping IXs at low temperature.",
        "category": "physics_optics"
    },
    {
        "text": "Optically Addressed Spatial Light Modulator based on Nonlinear\n  Metasurface: Spatial light modulators (SLMs) are devices for modulating amplitude, phase\nor polarization of a light beam on demand. Such devices have been playing an\nindispensable inuence in many areas from our daily entertainments to scientific\nresearches. In the past decades, the SLMs have been mainly operated in\nelectrical addressing (EASLM) manner, wherein the writing images are created\nand loaded via conventional electronic interfaces. However, adoption of\npixelated electrodes puts limits on both resolution and efficiency of the\nEASLMs. Here, we present an optically addressed SLM based on a nonlinear\nmetasurface (MS-OASLM), by which signal light is directly modulated by another\nwriting beam requiring no electrode. The MS-OASLM shows unprecedented\ncompactness and is 400 nm in total thickness benefitting from the outstanding\nnonlinearity of the metasurface. And their subwavelength feature size enables a\nhigh resolution up to 250 line pairs per millimeter, which is more than one\norder of magnitude better than any currently commercial SLMs. Such MS-OASLMs\ncould provide opportunities to develop the next generation of high resolution\ndisplays and all-optical information processing technologies.",
        "category": "physics_optics"
    },
    {
        "text": "A ray-optical Poincar\u00e9 sphere for structured Gaussian beams: A general family of structured Gaussian beams naturally emerges from a\nconsideration of families of rays. These ray families, with the property that\ntheir transverse profile is invariant upon propagation (except for cycling of\nthe rays and a global rescaling), have two parameters, the first giving a\nposition on an ellipse naturally represented by a point on the Poincar\\'e\nsphere (familiar from polarization optics), and the other determining the\nposition of a curve traced out on this Poincar\\'e sphere. This construction\nnaturally accounts for the familiar families of Gaussian beams, including\nHermite-Gauss, Laguerre-Gauss and Generalized Hermite-Laguerre-Gauss beams, but\nis far more general. The conformal mapping between a projection of the\nPoincar\\'e sphere and the physical space of the transverse plane of a Gaussian\nbeam naturally involves caustics. In addition to providing new insight into the\nphysics of propagating Gaussian beams, the ray-based approach allows effective\napproximation of the propagating amplitude without explicit diffraction\ncalculations.",
        "category": "physics_optics"
    },
    {
        "text": "High-Q silica zipper cavity for optical radiation pressure driven MOMS\n  switch: We design a silica zipper cavity that has high optical and mechanical Q\n(quality factor) values and demonstrate numerically the feasibility of a\nradiation pressure driven micro opto-mechanical system (MOMS) directional\nswitch. The silica zipper cavity has an optical Q of 6.0x10^4 and an effective\nmode volume Vmode of 0.66{\\lambda}^3 when the gap between two cavities is 34\nnm. We found that this Q/V_mode value is five times higher than can be obtained\nwith a single nanocavity design. The mechanical Q (Q_m) is determined by\nthermo-elastic damping and is 2.0x10^6 in a vacuum at room temperature. The\nopto-mechanical coupling rate g_OM is as high as 100 GHz/nm, which allows us to\nmove the directional cavity-waveguide system and switch 1550-nm light with\n770-nm light by controlling the radiation pressure.",
        "category": "physics_optics"
    },
    {
        "text": "Ptychography Intensity Interferometry Imaging for Dynamic Distant Object: As a promising lensless imaging method for distance objects, intensity\ninterferometry imaging (III) had been suffering from the unreliable phase\nretrieval process, hindering the development of III for decades. Recently, the\nintroduction of the ptychographic detection in III overcame this challenge, and\na method called ptychographic III (PIII) was proposed. We here experimentally\ndemonstrate that PIII can image a dynamic distance object. A reasonable image\nfor the moving object can be retrieved with only two speckle patterns for each\nprobe, and only 10 to 20 iterations are needed. Meanwhile, PIII exhibits robust\nto the inaccurate information of the probe. Furthermore, PIII successfully\nrecovers the image through a fog obfuscating the imaging light path, under\nwhich a conventional camera relying on lenses fails to provide a recognizable\nimage.",
        "category": "physics_optics"
    },
    {
        "text": "Ultraviolet Dual Comb Spectroscopy: A Roadmap: Dual Comb Spectroscopy proved its versatile capabilities in molecular\nfingerprinting in different spectral regions, but not yet in the ultraviolet\n(UV). Unlocking this spectral window would expand fingerprinting to the\nelectronic energy structure of matter.This will access the prime triggers of\nphoto-chemical reactions with unprecedented spectral resolution. In this\nresearch article, we discuss the milestones marking the way to the first UV\ndual comb spectrometer. We present experimental and simulated studies towards\nUV dual comb spectroscopy, directly applied to planned absorption measurements\nof formaldehyde (centered at 343 nm, 3.6 eV) and argon (80 nm, 16 eV). This\nwill enable an unparalleled relative resolution of up to $10^{-9}$ - with a\ntable-top UV source surpassing any synchrotron linked spectrometer by at least\ntwo and any grating-based UV spectrometer by up to six orders of magnitude.",
        "category": "physics_optics"
    },
    {
        "text": "On the Generation of Topological Vector Solitons from Bessel-Like Beams: Coupled solitary waves in optics literature, are coined vector solitons to\nreflect their particle-like nature that remains intact even after mutual\ncollisions. They are born from a nonlinear change in the refractive index of an\noptical material induced by the light intensity. We've discovered that the\nsecond harmonic intensity profile generated by Bessel-like beams, is composed\nof solitons of various geometries surrounded by concentric rings; one of which\nis two central solitons of similar radius knotted by ellipsoidal concentric\nrings. We observe that their geometry and intensity distribution is dependent\non the topological charge of the fundamental Bessel beams incident on the\nnonlinear medium. We show that their spatial profile is invariant against\npropagation. We observe that their behavior is similar to that of screw\ndislocations in wave trains: they collide and rebound at a 90$^\\circ$ angle\nfrom the beam propagation direction. In this way, we have generated linked\nfrequency doubled Bessel-type vector solitons with different topologies, that\nare knotted as they oscillate along the optical axis, when propagating in the\nlaboratory environment.",
        "category": "physics_optics"
    },
    {
        "text": "Topological analysis of paraxially scattered electron vortex beams: We investigate topological aspects of sub-nm electron vortex beams upon\nelastic propagation through atomic scattering potentials. Two main aspects can\nbe distinguished: (i) Significantly reduced delocalization compared to a\nsimilar non-vortex beam if the beam centers on an atomic column and (ii) site\nsymmetry dependent splitting of higher-order vortex beams. Furthermore, the\nresults provide insight into the complex vortex line fabric within the\nelastically scattered wave containing characteristic vortex loops predominantly\nattached to atomic columns and characteristic twists of vortex lines around\natomic columns.",
        "category": "physics_optics"
    },
    {
        "text": "Polarization-sensitive propagation in an anisotropic metamaterial with\n  double-sheeted hyperboloid dispersion relation: The polarization-sensitive propagation in the anisotropic metamaterial (AMM)\nwith double-sheeted hyperboloid dispersion relation is investigated from a\npurely wave propagation point of view. We show that TE and TM polarized waves\npresent significantly different characteristics which depend on the\npolarization. The omnidirectional total reflection and oblique total\ntransmission can occur in the interface associated with the AMM. If appropriate\nconditions are satisfied, one polarized wave exhibits the total refraction,\nwhile the other presents the total reflection. We find that the opposite\namphoteric refractions can be realized by rotating the principle axis of AMM,\nsuch that one polarized wave performs the negative refraction, while the other\nundergoes positive refraction. The polarization-sensitive characteristics allow\nus to construct two types of efficient polarizing beam splitters under certain\nachievable conditions.",
        "category": "physics_optics"
    },
    {
        "text": "A Step-by-step Guide to the Realisation of Advanced Optical Tweezers: Since the pioneering work of Arthur Ashkin, optical tweezers have become an\nindispensable tool for contactless manipulation of micro- and nanoparticles.\nNowadays optical tweezers are employed in a myriad of applications\ndemonstrating the importance of these tools. While the basic principle of\noptical tweezers is the use of a strongly focused laser beam to trap and\nmanipulate particles, ever more complex experimental set-ups are required in\norder to perform novel and challenging experiments. With this article, we\nprovide a detailed step- by-step guide for the construction of advanced optical\nmanipulation systems. First, we explain how to build a single-beam optical\ntweezers on a home-made microscope and how to calibrate it. Improving on this\ndesign, we realize a holographic optical tweezers, which can manipulate\nindependently multiple particles and generate more sophisticated wavefronts\nsuch as Laguerre-Gaussian beams. Finally, we explain how to implement a speckle\noptical tweezers, which permit one to employ random speckle light fields for\ndeterministic optical manipulation.",
        "category": "physics_optics"
    },
    {
        "text": "Optical Hardware Accelerators using Nonlinear Dispersion Modes for\n  Energy Efficient Computing: This paper proposes a new class of hardware accelerators to alleviate\nbottlenecks in the acquisition, analytics, storage and computation of\ninformation carried by wideband streaming signals.",
        "category": "physics_optics"
    },
    {
        "text": "Geometry-dependent skin effects in reciprocal photonic crystals: Skin effect that all eigenmodes within a frequency range become edge states\nis dictated by the topological properties of complex eigenvalues unique in\nnon-Hermitian systems. The prevailing attempts to realize such a fascinating\neffect are confined to either one-dimensional or nonreciprocal systems\nexhibiting asymmetric couplings. Here, inspired by a recent model Hamiltonian\ntheory, we propose a realistic reciprocal two-dimensional (2D) photonic crystal\n(PhC) system that shows the desired skin effect. Specifically, we establish a\nroutine for designing such non-Hermitian systems via revealing the inherent\nconnections between the non-trivial eigenvalue topology of order-2 exceptional\npoints (EPs) and the skin effects. Guided by the proposed strategy, we\nsuccessfully design a 2D PhC that possesses the EPs with nonzero eigenvalue\nwinding numbers. The spectral area along a specific wavevector direction is\nthen formed by leveraging the symmetry of the macroscopic geometry and the unit\ncell. The projected-band-structure calculations are performed to demonstrate\nthat the desired skin effect exists at the specific crystalline interfaces. We\nfinally employ time-domain simulations to vividly illustrate this phenomenon by\nexciting a pulse at the center of a finite-sized PhC. Our results form a solid\nbasis for further experimental confirmations and applications of the skin\neffect.",
        "category": "physics_optics"
    },
    {
        "text": "Analysis of Laser & Detector Placement in MIMO Multimode Optical Fiber\n  Systems: Multimode fibers (MMFs) offer a cost-effective connection solution for small\nand medium length networks. However, data rates through multimode fibers are\ntraditionally limited by modal dispersion. Signal processing and Multiple-Input\nMultiple-Output (MIMO) have been shown to be effective at combating these\nlimitations, but device design for the specific purpose of MIMO in MMFs is\nstill an open issue. This paper utilizes a statistical field propagation model\nfor MMFs to aid the analysis and designs of MMF laser and detector arrays, and\naims to improve data rates of the fiber. Simulations reveal that optimal device\ndesigns could possess 2-3 times the data carrying capacity of suboptimal ones.",
        "category": "physics_optics"
    },
    {
        "text": "Comment on \"Role of spatial coherence in Goos-H\u00e4nchen and\n  Imbert-Fedorov shifts\" [arXiv:0804.1895]: It is shown that the spatial Goos-H\\\"anchen shift is greatly affected by\nspatial coherence. A typical example is given.",
        "category": "physics_optics"
    },
    {
        "text": "Investigations of dynamic light scattering properties in fluorescent\n  solution: A comprehensive study using plasmonic enhanced Flow-Cytometry: In this article we present a new diagnostic approach utilizing flow-cytometry\nto study compounds of nanoparticle samples in solution by analysis of their\nscattering patterns retrieved from the cytometric measurements. As a specific\nenhancement of this technique we study as well the scattering pattern of\nnanoparticles in a fluorescent solution (529 nm). A significant enhancement of\nthe cytometry measurements is observed supporting an improved separation of\nparticle formations that are clearly resolved in the cytograms. The samples in\nthis experiment are prepared from 80 nm citrate-capped gold nanoparticles\n(AuNP). They are stabilized providing 2-(dimethylamino)ethanol (DMAE) in the\naqueous solution. A laser diode with a wavelength of 488 nm is used as\nfundamental illumination for the flow-cytometry measurements (FCM). Dynamic\nLight Scattering (DLS) measurements are performed separately and demonstrate a\nvery good agreement with the Flow-Cytometry measurements both of which allow to\ngive an effective size calibration. For further analysis light transport\nsimulations are presented. They provide information on the key-process to form\nthe correlation of the fluorescent solutions FCM to the studied particles of\ninterest. From this we extract the volume nature of the scattering process\nensuring the correlation.",
        "category": "physics_optics"
    },
    {
        "text": "On the attenuation coefficient of monomode periodic waveguides: It is widely accepted that, on ensemble average, the transmission T of guided\nmodes decays exponentially with the waveguide length L due to small\nimperfections, leading to the important figure of merit defined as the\nattenuation-rate coefficient alpha = -<ln(T)>/L. In this letter, we evidence\nthat the exponential-damping law is not valid in general for periodic monomode\nwaveguides, especially as the group velocity decreases. This result that\ncontradicts common beliefs and experimental practices aiming at measuring alpha\nis supported by a theoretical study of light transport in the limit of very\nsmall imperfections, and by numerical results obtained for two waveguide\ngeometries that offer contrasted damping behaviours.",
        "category": "physics_optics"
    },
    {
        "text": "Two-photon superbunching of pseudothermal light in a Hanbury Brown-Twiss\n  interferometer: Two-photon superbunching of pseudothermal light is observed with single-mode\ncontinuous-wave laser light in a linear optical system. By adding more\ntwo-photon paths via three rotating ground glasses,g(2)(0) = 7.10 is\nexperimentally observed. The second-order temporal coherence function of\nsuperbunching pseudothermal light is theoretically and experimentally studied\nin detail. It is predicted that the degree of coherence of light can be\nincreased dramatically by adding more multi-photon paths. For instance, the\ndegree of the second- and third-order coherence of the superbunching\npseudothermal light with five rotating ground glasses can reach 32 and 7776,\nrespectively. The results are helpful to understand the physics of\nsuperbunching and to improve the visibility of thermal light ghost imaging.",
        "category": "physics_optics"
    },
    {
        "text": "Tamm states and nonlinear surface modes in photonic crystals: We predict the existence of surface gap modes, known as Tamm states for\nelectronic systems, in truncated photonic crystals formed by two types of\ndielectric rods. We investigate the energy threshold, dispersion, and modal\nsymmetries of the surface modes, and also demonstrate the existence and\ntunability of nonlinear Tamm states in binary photonic crystals with nonlinear\nresponse.",
        "category": "physics_optics"
    },
    {
        "text": "Nanoscale X-ray imaging with high spectral sensitivity using\n  fluorescence intensity correlations: This paper introduces Spectral Incoherent Diffractive Imaging (SIDI) as a\nnovel method for achieving dark-field imaging of nanostructures with\nheterogeneous oxidation states. With SIDI, shifts in photoemission profiles can\nbe spatially resolved, enabling the independent imaging of the underlying\nemitter distributions contributing to each spectral line. In the X-ray domain,\nthis approach offers unique insights beyond the conventional combination of\ndiffraction and X-ray Emission Spectroscopy (XES). When applied at X-ray\nFree-Electron Lasers (XFELs), SIDI promises to be a versatile tool for\ninvestigating a broad range of systems, offering unprecedented opportunities\nfor detailed characterization of heterogeneous nanostructures for catalysis and\nenergy storage, including of their ultrafast dynamics.",
        "category": "physics_optics"
    },
    {
        "text": "Mathematical Synthesis and Analysis of Nonlinear Metasurfaces: We propose a discussion on the synthesis and scattering analysis of nonlinear\nmetasurfaces. For simplicity, we investigate the case of a second-order\nnonlinear isotropic metasurface possessing both electric and magnetic linear\nand nonlinear susceptibility components. We next find the synthesis expressions\nrelating the susceptibilities to the specified fields, which leads to the\ndefinition of the nonlinear metasurface conditions for no reflection,\nthemselves revealing the nonreciprocal nature of such structures. Finally, we\nprovide the approximate expressions of the scattered fields based on\nperturbation theory and compare the corresponding results to finite-difference\ntime-domain simulations.",
        "category": "physics_optics"
    },
    {
        "text": "Quantum spin Hall effect of light: Maxwell's equations, formulated 150 years ago, ultimately describe properties\nof light, from classical electromagnetism to quantum and relativistic aspects.\nThe latter ones result in remarkable geometric and topological phenomena\nrelated to the spin-1 massless nature of photons. By analyzing fundamental spin\nproperties of Maxwell waves, we show that free-space light exhibits an\nintrinsic quantum spin Hall effect, i.e., surface modes with strong\nspin-momentum locking. These modes are evanescent waves that form, e.g.,\nsurface plasmon-polaritons at vacuum-metal interfaces. Our findings illuminate\nthe unusual transverse spin in evanescent waves and explain recent experiments\ndemonstrating the transverse spin-direction locking in the excitation of\nsurface optical modes. This deepens our understanding of Maxwell's theory,\nreveals analogies with topological insulators for electrons, and offers\napplications for robust spin-directional optical interfaces.",
        "category": "physics_optics"
    },
    {
        "text": "Time-dependent theory of optical electro- and magnetostriction: Electrostriction, the deformation of dielectric materials under the influence\nof an electric field, is of continuous interest in optics. The classic\nexperiment by Hakim and Higham [Proc. Phys. Soc. 80, 190 (1962)] for a\nstationary field supports a different formula of the electrostrictive force\ndensity than the recent experiment by Astrath et al. [Light Sci. Appl. 11, 103\n(2022)] for an optical field. In this work, we study the origin of this\ndifference by developing a time-dependent covariant theory of optical force\ndensities in photonic materials. When a light pulse propagates in a bulk\ndielectric, the field-induced force density consists of two parts: (i) The\noptical wave momentum force density carries the wave momentum of light and\ndrives forward a mass density wave of the covariant coupled field-material\nstate of light. (ii) The optostrictive force density arises from the atomic\ndensity dependence of the electric and magnetic field energy densities. It\nrepresents an optical Lorentz-force-law-based generalization of the electro-\nand magnetostrictive force densities well known for static electromagnetic\nfields and derived from the principle of virtual work. Since the work done by\nthe optostrictive force density is not equal to the change of the field energy\ndensity during the contraction of the material, we have to describe this\ndifference by optostriction-related dissipation terms to fulfill the energy\nconservation. The detailed physical model of the dissipation is left for\nfurther works. The optostrictive force density can be understood in terms of\nfield-induced pair interactions inside the material. Because of the related\naction and reaction effects, this force density cannot contribute to the net\nmomentum transfer of the optical field. We also use the theory to simulate the\npropagation of a Gaussian light pulse through a dielectric material.",
        "category": "physics_optics"
    },
    {
        "text": "Retrieving the Complex Intracavity Pump Field of a Kerr Comb from the\n  Through Port Data: A method of retrieving the complex intracavity pump field from the through\nport is proposed, and verified through characterizing the time-domain waveform\nof a mode-locked comb related to dark soliton formation in a normal-dispersion\nmicroresonator.",
        "category": "physics_optics"
    },
    {
        "text": "Excitable-like chaotic pulses in the bounded-phase regime of an\n  opto-radiofrequency oscillator: We report theoretical and experimental evidence of chaotic pulses with\nexcitable-like properties in an opto-radiofrequency oscillator based on a\nself-injected dual-frequency laser. The chaotic attractor involved in the\ndynamics produces pulses that, albeit chaotic, are quite regular: They all have\nsimilar amplitudes, and are almost periodic in time. Thanks to these features,\nthe system displays properties that are similar to those of excitable systems.\nIn particular, the pulses exhibit a threshold-like response, of well-defined\namplitude, to perturbations, and it appears possible to define a refractory\ntime. At variance with excitability in injected lasers, here the excitable-like\npulses are not accompanied by phase slips.",
        "category": "physics_optics"
    },
    {
        "text": "Globally-Linked Vortex Clusters in Trapped Wave Fields: We put forward the existence of a rich variety of fully stationary vortex\nstructures, termed H-clusters, made of an increasing number of vortices nested\nin paraxial wave fields confined by trapping potentials. However, we show that\nthe constituent vortices are globally linked, rather than products of\nindependent vortices. Also, they always feature a monopolar global wave front\nand exist in nonlinear systems, such as Bose-Einstein condensates. Clusters\nwith multipolar global wave fronts are non-stationary or at best flipping.",
        "category": "physics_optics"
    },
    {
        "text": "Quantum non-Gaussianity certification of photon-number-resolving\n  detectors: We report on direct experimental certification of the quantum non-Gaussian\ncharacter of a photon-number resolving detector. The certification protocol is\nbased on an adaptation of the existing quantum non-Gaussianity criteria for\nquantum states to quantum measurements. In our approach, it suffices to probe\nthe detector with a vacuum state and two different thermal states to test its\nquantum non-Gaussianity. The certification is experimentally demonstrated for\nthe detector formed by a spatially multiplexed array of ten single-photon\navalanche photodiodes. We confirm the quantum non-Gaussianity of POVM elements\n$\\hat{\\Pi}_m$ associated with the $m$-fold coincidence counts, up to $m=7$. The\nexperimental ability to certify from the first principles the quantum\nnon-Gaussian character of $\\hat{\\Pi}_m$ is for large $m$ limited by low\nprobability of the measurement outcomes, especially for vacuum input state. We\nfind that the injection of independent Gaussian background noise into the\ndetector can be helpful and may reduce the measurement time required for\nreliable confirmation of quantum non-Gaussianity. In addition, we modified and\nexperimentally verified the quantum non-Gaussianity certification protocol\nemploying a third thermal state instead of a vacuum to speed up the whole\nmeasurement. Our findings demonstrate the existence of efficient tools for the\npractical characterization of fundamental non-classical properties and\nbenchmarking of complex optical quantum detectors.",
        "category": "physics_optics"
    },
    {
        "text": "Edge Modes of Scattering Chains with Aperiodic Order: We study the scattering resonances of one-dimensional deterministic aperiodic\nchains of electric dipoles using the vectorial Green's matrix method, which\naccounts for both short- and long-range electromagnetic interactions in open\nscattering systems. We discover the existence of edge-localized scattering\nstates within fractal energy gaps with characteristic topological band\nstructures. Notably, we report and characterize edge-localized modes in the\nclassical wave analogues of the Su-Schrieffer-Heeger (SHH) dimer model,\nquasiperiodic Harper and Fibonacci crystals, as well as in more complex\nThue-Morse aperiodic systems. Our study demonstrates that topological\nedge-modes with characteristic power-law envelope appear in open aperiodic\nsystems and coexist with traditional exponentially localized ones. Our results\nextend the concept of topological states to the scattering resonances of\ncomplex open systems with aperiodic order, thus providing an important step\ntowards the predictive design of topological optical metamaterials and devices\nbeyond tightbinding models.",
        "category": "physics_optics"
    },
    {
        "text": "Optical Properties of Bismuth Nanostructures Towards the Ultrathin Film\n  Regime: Bulk bismuth presents outstanding optical properties, such as a giant\ninfrared refractive index (n near 10) and a negative ultraviolet visible\npermittivity induced by giant interband electronic transitions. Although such\nproperties are very appealing for applications in nanophotonics, the dielectric\nfunction of bismuth nanostructures has been scarcely studied. Here, we\ndetermine by spectroscopic ellipsometry the far infrared to ultraviolet\ndielectric function of pulsed laser deposited bismuth thin films with nominal\nthickness tBi varied from near 10 nm to several tens of nm. For tBi above 15\nnm, the films display a continuous structure and their dielectric function is\ncomparable with that of bulk bismuth. For tBi below 15 nm, the film structure\nis discontinuous, and the dielectric function differs markedly from that of\nbulk bismuth. It is proposed from FDTD simulations that this marked difference\narises mainly from effective medium effects induced by the discontinuous film\nstructure, where quantum electronic confinement does not play a dominant role.\nThis suggests that ultrathin and continuous bismuth films should present the\nsame outstanding optical properties as bulk bismuth for high performance\nnanophotonic devices.",
        "category": "physics_optics"
    },
    {
        "text": "Calibrating an interferometric laser frequency stabilization to MHz\n  precision: We report on a calibration procedure that enhances the precision of an\ninterferometer based frequency stabilization by several orders of magnitude.\nFor this purpose the frequency deviations of the stabilization are measured\nprecisely by means of a frequency comb. This allows to implement several\ncalibration steps that compensate different systematic errors. The resulting\nfrequency deviation is shown to be less than $5.7 $MHz (rms $1.6 $MHz) in the\nwhole wavelength interval $750 - 795 $nm. Wide tuning of a stabilized laser at\nthis exceptional precision is demonstrated.",
        "category": "physics_optics"
    },
    {
        "text": "Approximate equivalence between guided modes in a low-contrast photonic\n  bandgap fiber and Maxwell TM modes of a high-contrast two-dimensional\n  photonic structure: We present a formal analogy between the eigenvalue problem for guided scalar\nmodes in a low-contrast photonic bandgap fiber and quasi-stationary TM modes of\na two-dimensional (2D) photonic structure. Using this analogy, we numerically\nstudy the confinement losses of disordered microstructured fibers through the\nleakage rate of an open 2D system with high refractive index inclusions. Our\nresults show that for large values of the disorder, the confinement losses\nincrease. However, they also suggest that losses might be improved in strongly\ndisordered fibers by exploring ranges of physical parameters where Anderson\nlocalization sets in.",
        "category": "physics_optics"
    },
    {
        "text": "RF Injection Locking of THz Metasurface Quantum-Cascade VECSEL: RF injection locking and spectral broadening of a terahertz (THz)\nquantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) is\ndemonstrated. An intra-cryostat VECSEL focusing cavity design is used to enable\ncontinuous-wave lasing with a cavity length over 30 mm which corresponds to a\nround-trip frequency near 5 GHz. Strong RF current modulation is injected to\nthe QC-metasurface electrical bias to pull and lock the round-trip frequency.\nThe injection locking range at various RF injection powers is recorded and\ncompared with the injection locking theory. Moreover, the lasing spectrum\nbroadens from 14 GHz in free-running mode to a maximum spectral width around\n100 GHz with 20 dBm of injected RF power. This experimental setup is suitable\nfor further exploration of active mode-locking and picosecond pulse generation\nin THz QC-VECSELs.",
        "category": "physics_optics"
    },
    {
        "text": "Nonlinear optics in Xe-filled hollow-core PCF in high pressure and\n  supercritical regimes: Supercritical Xe at 293 K offers a Kerr nonlinearity that can exceed that of\nfused silica while being free of Raman scattering. It also has a much higher\noptical damage threshold and a transparency window that extends from the UV to\nthe infrared. We report the observation of nonlinear phenomena, such as\nself-phase modulation, in hollow-core photonic crystal fiber filled with\nsupercritical Xe. In the subcritical regime, intermodal four-wave-mixing\nresulted in the generation of UV light in the HE12 mode. The normal dispersion\nof the fiber at high pressures means that spectral broadening can clearly\nobtained without influence from soliton effects or material damage.",
        "category": "physics_optics"
    },
    {
        "text": "High efficiency spin-decoupled modulation using chiral C2-symmetric\n  meta-atoms: Orthogonal circularly polarized light is essential for multiplexing tunable\nmetasurfaces. Mainstream spin-decoupled metasurfaces, consisting of numerous\nmeta-atoms with mirror symmetry, rely on the cooperative modulation of the\nPancharatnam-Berry (PB) phase and the propagation phase. This paper\ndemonstrates spin-decoupled functionality through the synergistic utilization\nof planar chiral meta-atom phase response and PB phase. Based on the Jones\ncalculus, it has been found that meta-atoms with chiral C2-symmetry owns a\nlarger geometric parameter range with high cross-polarization ratio compared to\nthose with mirror symmetry or higher symmetries at the same aspect ratio. This\ncharacteristic is advantageous in terms of enabling high-efficiency\nmanipulation and enhancing the signal-to-noise ratio. As an example, 10 kinds\nof C2-symmetry chiral meta-atoms with a H-like shape are selected by the\nself-adaptive genetic algorithm to attain a full 2$\\pi$ phase span with an\ninterval of $\\pi$/5. To mitigate the additional propagation phase change of the\nguided modes originated from the arrangement alternation upon the rotation of\nthe meta-atoms, the enantiomer of chiral meta-atoms and its PB phase delay are\nadopted to minimize the difference between the actual and desired target\nphases. A polarization-insensitive metalens and a chiral virtual-moving\nmetalens array are designed to demonstrate the spin-decoupled function with\nboth high efficiency and signal-to-noise ratio. The work in this paper may\ntrigger more exciting and interesting spin-decoupled multiplexing metasurfaces\nand broaden the prospect of chiroptical applications.",
        "category": "physics_optics"
    },
    {
        "text": "Measurement of infrared optical constants with visible photons: We demonstrate a new scheme of infrared spectroscopy with visible light\nsources and detectors. The technique relies on the nonlinear interference of\ncorrelated photons, produced via spontaneous parametric down conversion in a\nnonlinear crystal. Visible and infrared photons are split into two paths and\nthe infrared photons interact with the sample under study. The photons are\nreflected back to the crystal, resembling a conventional Michelson\ninterferometer. Interference of the visible photons is observed and it is\ndependent on the phases of all three interacting photons: pump, visible and\ninfrared. The transmission coefficient and the refractive index of the sample\nin the infrared range can be inferred from the interference pattern of visible\nphotons. The method does not require the use of potentially expensive and\ninefficient infrared detectors and sources, it can be applied to a broad\nvariety of samples, and it does not require a priori knowledge of sample\nproperties in the visible range.",
        "category": "physics_optics"
    },
    {
        "text": "Four-terminal perovskite/silicon tandem solar cell with integrated\n  Mie-resonant spectral splitter metagrating: A spectral splitting, light trapping dielectric metasurface is designed,\nfabricated and integrated into a four-terminal perovskite/silicon hybrid tandem\nsolar cell to increase the absorption of light close to the bandgap of the\nperovskite top cell, and enhance transmission of the near-infrared spectral\nband towards the bottom cell. The metagrating is composed of a hexagonal array\nof unit cells of 150-nm-tall hydrogenated amorphous silicon trimer\nnanostructures with dielectric Mie resonances in the 600-800 nm perovskite\nnear-gap region, made using substrate-conformal imprint lithography. By\ntailoring the metasurface resonant scattering modes and their interference with\nthe direct reflection paths we minimize specular reflection and obtain high\ndiffraction efficiency that leads to improved light trapping in the perovskite\ntop cell. The measured short-circuit current increase in the perovskite top\ncell is 0.5 mA/cm2 corresponding to an estimated efficiency gain of 0.26%\n(absolute) for the metasurface-integrated 4T perovskite/silicon tandem cell.\nSimulations for a further optimized metasurface spectrum splitter geometry\npredict a short-circuit current gain in the perovskite top cell of 1.4 mA/cm2\nand an efficiency gain for the 4T tandem cell of 0.4% (absolute). The\nmetagrating approach for simultaneous spectral splitting, light trapping and\nreflectance reduction provides a flexible platform that can be applied to many\ntandem cell geometries.",
        "category": "physics_optics"
    },
    {
        "text": "Novel approaches to tailor and tune light-matter interactions at the\n  nanoscale: In this thesis we propose new, versatile schemes to control light-matter\ninteractions at the nanoscale.\n  In the first part of the thesis, we envisage a new class of plasmonic cloaks\nmade of magneto-optical (MO) materials. We demonstrate that the application of\na uniform magnetic field B in these cloaks may not only switch on and off the\ncloaking mechanism, but also mitigate the electromagnetic (EM) absorption. We\nalso prove that the scattered field profile can be effectively controlled by\nchanging B.\n  The second part of the thesis is devoted to the study of light-matter\ninteractions mediated by fluctuations of the vacuum EM field. Firstly, we\ndemonstrate that the Purcell effect can be effectively suppressed for an\nexcited atom near a cloaking device. Furthermore, the decay rate of a quantum\nemitter near a graphene-coated wall under the influence of an external magnetic\nfield is studied. We show that the MO properties of graphene strongly affect\nthe atomic lifetime and that B allows for an unprecedented control of the decay\nchannels of the system. In addition, we discuss the dispersive interaction\nbetween an atom and suspended graphene in a magnetic field. For large\natom-graphene separations and low temperatures we show that the interaction\nenergy is a quantized function of B. Besides, we show that at room temperature,\nthermal effects must be taken into account even in the extreme near-field\nregime.\n  Finally, the third part of the thesis deals with the study of near-field heat\ntransfer. We analyze the energy transfered from a semi-infinite medium to a\ncomposite sphere made of metallic inclusions embedded in a dielectric host\nmedium. We show that the heat transfer can be strongly enhanced at the\npercolation phase transition. We show that our results apply for different\neffective medium models and are robust against changes in the inclusions' shape\nand materials.",
        "category": "physics_optics"
    },
    {
        "text": "Comment on \"Far-field microscopy with a nanometer-scale resolution based\n  on the in-plane image magnification by surface plasmon polaritons\": This is a small comment concerning the work by Smolyaninov et al. in Phys.\nRev. Lett.94, 057401 (2005).",
        "category": "physics_optics"
    },
    {
        "text": "Four-wave mixing based orbital angular momentum translation: We theoretically study the generation of orbital angular momentum(OAM) based\non four-wave mixing (FWM) process in a diamond-type inhomogeneously broadened\n$^{85}$Rb atomic system. We use density matrix formalism at weak probe limit to\nexplain the origin of vortex translation between different optical fields and\ngenerated signal. We show how the singularities which are omnipresent in phase\nof the input optical vortex beams can be profoundly mapped to atomic coherence\nin the transverse plane that hold the origin of OAM translation. This\ntranslation process works well even for moderately intense control field which\nenhances medium nonlinearity. Further we have manoeuvred an additional rotation\nof the phase wavefront in both clockwise and anti-clockwise direction\ncontrolled by the single photon detuning. The generation and manipulation of\nOAM of light beam in nonlinear medium may have important applications in\noptical tweezers and quantum information processing systems.",
        "category": "physics_optics"
    },
    {
        "text": "Performance evaluation of on-chip wavelength conversion based on\n  InP/In$_{1-x}$Ga$_x$As$_y$P$_{1-y}$ semiconductor waveguide platforms: We propose and design the high confinement InP/In1-xGaxAsyP1-y semiconductor\nwaveguides and report the results of effective wavelength conversion based on\nthis platform. Efficient confinement and mode field area fluctuation at\ndifferent wavelength is analyzed to achieve the high nonlinear coefficient. The\nnumerical results show that nearly zero phase-mismatch condition can be\nsatisfied through dispersion tailoring of InP/In1-xGaxAsyP1-y waveguides, and\nthe wavelength conversion ranging over 40 nm with the maximum conversion\nefficiency -26.3 dB is achieved for fixing pump power 100 mW. Meanwhile, the\ninfluences of the doping parameter y and pumping wavelength on the bandwidth\nand conversion efficiency are also discussed and optimized. It is indicated the\nexcellent optical properties of the InP/In1-xGaxAsyP1-y waveguides and pave the\nway towards direct integration telecom band devices on stand semiconductor\nplatforms.",
        "category": "physics_optics"
    },
    {
        "text": "Analysis of Dispersive Fourier Transform dataset using Dynamic Mode\n  Decomposition: evidence of multiple vibrational modes, and their interplay in\n  a three-soliton molecule: We demonstrate that the Dynamic Mode Decomposition technique can effectively\nreduce the amount of noise in Dispersive Fourier Transform dataset; and allow\nfor finer quantitative analysis of the experimental data. We therefore were\nable to demonstrate that the oscillation pattern of a soliton molecule actually\nresults from the interplay of several elementary vibration modes.",
        "category": "physics_optics"
    },
    {
        "text": "Focusing membrane metamirrors for integrated cavity optomechanics: We have realized a suspended, high-reflectivity focusing metamirror\n($f\\approx 10$ cm, $\\mathcal{R} \\approx 99\\%$) by non-periodic photonic crystal\npatterning of a Si$_3$N$_4$ membrane. The design enables construction of a\nstable, short ($L$ = 30 $\\mu$m), high-finesse ($\\mathcal{F}>600$) membrane\ncavity optomechanical system using a single plano dielectric end-mirror. We\npresent the metamirror design, fabrication process, and characterization of its\nreflectivity using both free space and cavity-based transmission measurements.\nThe mirror's effective radius of curvature is inferred from the transverse mode\nspectrum of the cavity. In combination with phononic engineering and\nmetallization, focusing membrane mirrors offer a route towards\nhigh-cooperativity, vertically-integrated cavity optomechanical systems with\napplications ranging from precision force sensing to hybrid quantum\ntransduction.",
        "category": "physics_optics"
    },
    {
        "text": "Tuning Multipolar Mie Scattering of Particles on a Dielectric-Covered\n  Mirror: Optically resonant particles are key building blocks of many nanophotonic\ndevices such as optical antennas and metasurfaces. Because the functionalities\nof such devices are largely determined by the optical properties of individual\nresonators, extending the attainable responses from a given particle is highly\ndesirable. Practically, this is usually achieved by introducing an asymmetric\ndielectric environment. However, commonly used simple substrates have limited\ninfluences on the optical properties of the particles atop. Here, we show that\nthe multipolar scattering of silicon microspheres can be effectively modified\nby placing the particles on a dielectric-covered mirror, which tunes the\ncoupling between the Mie resonances of microspheres and the standing waves and\nwaveguide modes in the dielectric spacer. This tunability allows selective\nexcitation, enhancement, and suppression of the multipolar resonances and\nenables scattering at extended wavelengths, providing new opportunities in\ncontrolling light-matter interactions for various applications. We further\ndemonstrate with experiments the detection of molecular fingerprints by\nsingle-particle mid-infrared spectroscopy, and, with simulations strong optical\nrepulsive forces that could elevate the particles from a substrate.",
        "category": "physics_optics"
    },
    {
        "text": "Role of electron scattering on the high-order harmonic generation from\n  solids: We extend the semi-classical trajectory description for the high-order\nharmonic generation~(HHG) from solids by integrating the effect of\nelectron-scattering. Comparing the extended semi-classical trajectory model\nwith a one-dimensional quantum mechanical simulation, we find that the\nmulti-plateau feature of the HHG spectrum is formed by Umklapp scattering under\nthe electron-hole acceleration dynamics by laser fields. Furthermore, by\ntracing the scattered trajectories in real-space, the model fairly describes\nthe emitted photon energy and the emission timing of the HHG even in the higher\nplateau regions. We further consider the loss of trajectories by scattering\nprocesses with a mean-free-path approximation and evaluate the HHG cutoff\nenergy as a function of laser wavelength. As a result, we find that the\ntrajectory loss by scattering causes the wavelength independence of the HHG\nfrom solids.",
        "category": "physics_optics"
    },
    {
        "text": "Localized Surface Plasmons in Vibrating Graphene Nanodisks: Localized surface plasmons are confined collective oscillations of electrons\nin metallic nanoparticles. When driven by light, the optical response is\ndictated by geometrical parameters and the dielectric environment and plasmons\nare therefore extremely important for sensing applications. Plasmons in\ngraphene disks have the additional benefit to be highly tunable via electrical\nstimulation. Mechanical vibrations create structural deformations in ways where\nthe excitation of localized surface plasmons can be strongly modulated. We show\nthat the spectral shift in such a scenario is determined by a complex interplay\nbetween the symmetry and shape of the modal vibrations and the plasmonic mode\npattern. Tuning confined modes of light in graphene via acoustic excitations,\npaves new avenues in shaping the sensitivity of plasmonic detectors, and in the\nenhancement of the interaction with optical emitters, such as molecules, for\nfuture nanophotonic devices.",
        "category": "physics_optics"
    },
    {
        "text": "Application of the anisotropic bond model to second-harmonic generation\n  from amorphous media: As a step toward analyzing second-harmonic generation (SHG) from crystalline\nSi nanospheres in glass, we develop an anisotropic bond model (ABM) that\nexpresses SHG in terms of physically meaningful parameters and provides a\ndetailed understanding of the basic physics of SHG on the atomic scale.\nNonlinear-optical (NLO) responses are calculated classically via the four\nfundamental steps of optics: evaluate the local field at a given bond site,\nsolve the force equation for the acceleration of the charge, calculate the\nresulting radiation, then superpose the radiation from all charges. The ABM\ngoes beyond previous bond models by including the complete set of underlying\ncontributions: retardation (RD), spatial-dispersion (SD), and magnetic (MG)\neffects, in addition to the anharmonic restoring force acting on the bond\ncharge. We apply the ABM to obtain analytic expressions for SHG from amorphous\nmaterials under Gaussian-beam excitation. These materials represent an\ninteresting test case not only because they are ubiquitous but also because the\nanharmonic-force contribution that dominates the SHG response of crystalline\nmaterials and ordered interfaces vanishes by symmetry. Using the paraxial-ray\napproximation, we reduce the results to the isotropic case in two limits, that\nwhere the linear restoring force dominates (glasses), and that where it is\nabsent (metals). Both forward- and backscattering geometries are discussed.\nEstimated signal strengths and conversion efficiencies for fused silica appear\nto be in general agreement with data, where available. Predictions are made\nthat allow additional critical tests of these results.",
        "category": "physics_optics"
    },
    {
        "text": "Layer-Resolved Absorption of Light in Arbitrarily Anisotropic\n  Heterostructures: We present a generalized formalism to describe the optical energy flow and\nspatially resolved absorption in arbitrarily anisotropic layered structures.\nThe algorithm is capable of treating any number of layers of arbitrarily\nanisotropic, birefringent, and absorbing media and is implemented in an open\naccess computer program. We derive explicit expressions for the transmitted and\nabsorbed power at any point in the multilayer structure, using the electric\nfield distribution from a 4 $\\times$ 4 transfer matrix formalism. As a test\nground, we study three nanophotonic device structures featuring unique\nlayer-resolved absorption characteristics, making use of (i) in-plane\nhyperbolic phonon polaritons, (ii) layer-selective, cavity-enhanced exciton\nabsorption in transition metal dichalcogenide monolayers, and (iii)\nintersubband-cavity polaritons in quantum wells. Covering such a broad spectral\nrange from the far-infrared to the visible, the case studies demonstrate the\ngenerality and wide applicability of our approach.",
        "category": "physics_optics"
    },
    {
        "text": "Two-color pulse compounds in waveguides with a zero-nonlinearity point: We study incoherently coupled two-frequency pulse compounds in waveguides\nwith single zero-dispersion and zero-nonlinearity points. In such waveguides,\nsupported by a negative nonlinearity, soliton dynamics can be obtained even in\ndomains of normal dispersion. We demonstrate trapping of weak pulses by\nsolitary-wave wells, forming nonlinear-photonics meta-atoms, and molecule-like\nbound-states of pulses. We study the impact of Raman effect on these pulse\ncompounds, finding that, depending on the precise subpulse configuration, they\ndecelerate, accelerate, or are completely unaffected. Our results extend the\nrange of systems in which two-frequency pulse compounds can be expected to\nexist and demonstrate further unique and unexpected behavior.",
        "category": "physics_optics"
    },
    {
        "text": "Investigation by the Method the Raman of Spectroscopy of Allocation of\n  Molecules in Ternary Mix-Crystals: The Method a Raman of spectroscopy studies allocation of molecules in ternary\nmix-crystals of a p-dibromobenzene of p-dichlorobenzene and\np-bromochlorobenzene. It is shown, that the mutual concentration of builders\ndepends on requirements of growing. Was possibly as a uniform modification of\nconcentration of all builders along a specimen, and a wavy modification of\nconcentration of two substances.",
        "category": "physics_optics"
    },
    {
        "text": "Laser-guided lightning: Electric currents circulating between charged clouds and the earth surface\nduring lightning discharges are responsible for considerable damages and\ncasualties. It is therefore important to develop better protection methods in\naddition to the traditional Franklin rod. Here we present the first\ndemonstration that filaments formed by short and intense laser pulses can guide\nlightning discharges over considerable distances. We believe that this\nexperimental breakthrough will lead to progress in lightning protection and\nlightning physics. An experimental campaign was carried out on the S\\\"antis\nMountain in Northeastern Switzerland during the Summer of 2021 with a high\nrepetition rate terawatt laser. The guiding of an upward negative lightning\nleader over a distance of 50 m was recorded by two separate high-speed cameras.\nThe guiding of negative lightning leaders by laser filaments was corroborated\nin three other instances by VHF interferometric measurements, and the number of\nX-ray bursts detected during guided lightning events was significantly\nincreased. While this research field has been very active for more than 20\nyears with many research groups around the world working to achieve this goal,\nthis result demonstrates lightning guiding by lasers, which may lead to the\ndevelopment of a laser lightning rod. This work paves the way for new\natmospheric applications of ultrashort lasers and represents a significant step\nforward in the development of a laser based lightning protection for airports,\nlaunchpads or large infrastructures.",
        "category": "physics_optics"
    },
    {
        "text": "Noise-induced dynamics and photon statistics in bimodal quantum-dot\n  micropillar lasers: Emission characteristics of quantum-dot micropillar lasers (QDMLs) are\nlocated at the intersection of nanophotonics and nonlinear dynamics, which\nprovides an ideal platform for studying the optical interface between classical\nand quantum systems. In this work, a noise-induced bimodal QDML with orthogonal\ndual-mode outputs is modeled, and nonlinear dynamics, stochastic mode jumping\nand quantum statistics with the variation of stochastic noise intensity are\ninvestigated. Noise-induced effects lead to the emergence of two intensity\nbifurcation points for the strong and the weak mode, and the maximum output\npower of the strong mode becomes larger as the noise intensity increases. The\nanti-correlation of the two modes reaches the maximum at the second intensity\nbifurcation point. The dual-mode stochastic jumping frequency and effective\nbandwidth can exceed 100 GHz and 30 GHz under the noise-induced effect.\nMoreover, the noise-induced photon correlations of both modes simultaneously\nexhibit super-thermal bunching effects ($g^{(2)}(0)>2$) in the low injection\ncurrent region. The $g^{(2)}(0)$-value of the strong mode can reach over 6 in\nthe high injection current region. Photon bunching ($g^{(2)}(0)>1$) of both\nmodes is observed over a wide range of noise intensities and injection\ncurrents. In the presence of the noise-induced effect, the photon number\ndistribution of the strong or the weak mode is a mixture of Bose-Einstein and\nPoisson distributions. As the noise intensity increases, the photon number\ndistribution of the strong mode is dominated by the Bose-Einstein distribution,\nand the proportion of the Poisson distribution is increased in the high\ninjection current region, while that of the weak mode is reduced. Our results\ncontribute to the development preparation of super-bunching quantum integrated\nlight sources for improving the spatiotemporal resolution of quantum sensing\nmeasurements.",
        "category": "physics_optics"
    },
    {
        "text": "Semiconductor-Dielectric-Metal Solar Absorbers with High Spectral\n  Selectivity: An ideal solar thermal absorber has a sharp transition between high and low\nabsorptance at the wavelength where the blackbody emissive power begins to\nexceed the solar irradiance. However, most real selective absorbers have a\nfairly broad transition, leading to both solar absorption and thermal emission\nlosses. Here, we model, fabricate, and characterize a highly selective\nsemiconductor-dielectric-metal (Ga0.46In0.54As - MgF2 - Ag) solar absorber with\nan extremely sharp transition from high to low absorptance. The thin\nsemiconductor serves as a selective filter, absorbing photons with wavelengths\nshorter than the bandgap and transmitting those with longer wavelengths. The\nhighly reflective dielectric-metal rear mirror allows the structure to have\nvery low emittance for longer wavelengths. These characteristics provide the\nabsorber with a measured solar absorptance >91% below the bandgap wavelength\nand infrared emittance <5% at 100 C above the bandgap wavelength. This\ntransition wavelength can be tuned by modifying the semiconductor composition,\nand modeling indicates that the absorber's optical properties should be stable\nat high temperatures, making the structure a good candidate for unconcentrated\nto highly concentrated solar thermal energy conversion.",
        "category": "physics_optics"
    },
    {
        "text": "Exact computation of image disruption under reflection on a smooth\n  surface and Ronchigrams: We use geometrical optics and the caustic-touching theorem to study, in an\nexact way, the change in the topology of the image of an object obtained by\nreflections on an arbitrary smooth surface. Since the procedure that we use to\ncompute the image is exactly the same as that used to simulate the ideal\npatterns, referred to as Ronchigrams, in the Ronchi test used to test mirrors,\nwe remark that the closed loop fringes commonly observed in the Ronchigrams\nwhen the grating, referred to as a Ronchi ruling, is located at the caustic\nplace are due to a disruption of fringes, or, more correctly, as disruption of\nshadows corresponding to the ruling bands. To illustrate our results, we assume\nthat the reflecting surface is a spherical mirror and we consider two kinds of\nobjects: circles and line segments.",
        "category": "physics_optics"
    },
    {
        "text": "Optical generation of non-diffracting beams via photorefractive\n  holography: This work presents, for the first time the optical generation of\nnon-diffracting beams via photorefractive holography. Optical generation of\nnon-diffracting beams using conventional optics components is difficult and, in\nsome instances, unfeasible, as it is wave fields given by superposition of\nnon-diffracting beams. It is known that computer generated holograms and\nspatial light modulators (SLMs) successfully generate such beams. With\nphotorefractive holography technique, the hologram of a non-diffracting beam is\nconstructed (recorded) and reconstructed (reading) optically in a nonlinear\nphotorefractive medium. The experimental realization of a non-diffracting beam\nwas made in a photorefractive holography setup using a photorefractive\nBi12SiO20 (BSO) crystal as the holographic recording medium, where the\nnon-diffracting beams, the Bessel beam arrays and superposition of\nco-propagating Bessel beams (Frozen Waves) were obtained experimentally. The\nexperimental results are in agreement with the theoretically predicted results,\npresenting excellent prospects for implementation of this technique for\ndynamical systems at applications in optics and photonics.",
        "category": "physics_optics"
    },
    {
        "text": "Self-consistent theory for a plane wave in a moving medium and\n  light-momentum criterion: A self-consistent theory is developed based on the principle of relativity\nfor a plane wave in a moving non-dispersive, lossless, non-conducting,\nisotropic, uniform medium. A light-momentum criterion is set up for the first\ntime, which states that the momentum of light in a medium is parallel to the\nwave vector in all inertial frames of reference. By rigorous analysis, novel\nbasic properties of the plane wave are exposed: (a) Poynting vector does not\nnecessarily represent the electromagnetic (EM) power flow when a medium moves;\n(b) Minkowski light momentum and energy constitute a Lorentz four-vector in a\nform of single EM-field cell or single photon, and Planck constant is a Lorentz\ninvariant; (c) there is no momentum transfer taking place between the plane\nwave and the uniform medium, and the EM momentum conservation equation cannot\nbe uniquely determined without resorting to the principle of relativity; and\n(d) when the medium moves opposite to the wave vector at a\nfaster-than-dielectric light speed, negative frequency and negative EM energy\ndensity occur, with the plane wave becoming left-handed. Finally, a new physics\nof so-called \"intrinsic Lorentz violation\" is presented as well.",
        "category": "physics_optics"
    },
    {
        "text": "Ultrafast single-channel machine vision based on neuro-inspired photonic\n  computing: High-speed machine vision is increasing its importance in both scientific and\ntechnological applications. Neuro-inspired photonic computing is a promising\napproach to speed-up machine vision processing with ultralow latency. However,\nthe processing rate is fundamentally limited by the low frame rate of image\nsensors, typically operating at tens of hertz. Here, we propose an\nimage-sensor-free machine vision framework, which optically processes\nreal-world visual information with only a single input channel, based on a\nrandom temporal encoding technique. This approach allows for compressive\nacquisitions of visual information with a single channel at gigahertz rates,\noutperforming conventional approaches, and enables its direct photonic\nprocessing using a photonic reservoir computer in a time domain. We\nexperimentally demonstrate that the proposed approach is capable of high-speed\nimage recognition and anomaly detection, and furthermore, it can be used for\nhigh-speed imaging. The proposed approach is multipurpose and can be extended\nfor a wide range of applications, including tracking, controlling, and\ncapturing sub-nanosecond phenomena.",
        "category": "physics_optics"
    },
    {
        "text": "Darker than black: radiation-absorbing metamaterial: We show that corrugated surfaces of hyperbolic metamaterials scatter light\npreferentially inside the media, resulting in a very low reflectance and\nultimate dark appearance in the spectral range of hyperbolic dispersion. This\nphenomenon of fundamental importance, demonstrated experimentally in arrays of\nsilver nanowires grown in alumina membranes, originates from a broad-band\nsingularity in the density of photonic states. It paves the road to a variety\nof applications ranging from the stealth technology to high-efficiency solar\ncells and photodetectors.",
        "category": "physics_optics"
    },
    {
        "text": "Strong self-induced nonreciprocal transmission by using nonlinear\n  PT-symmetric epsilon-near-zero metamaterials: Nonreciprocal transmission is the fundamental process behind unidirectional\nwave propagation phenomena. In our work, a compact and practical parity-time\n(PT) symmetric metamaterial is designed based on two Silicon Carbide (SiC)\nmedia separated by an air gap and photonically doped with gain and loss\ndefects. We demonstrate that an exceptional point (EP) is formed in this\nPT-symmetric system when SiC operates as a practical epsilon-near-zero (ENZ)\nmaterial and by taking into account its moderate optical loss. Furthermore and\neven more importantly, strong self-induced nonreciprocal transmission is\nexcited due to the nonlinear Kerr effect at a frequency slightly shifted off\nthe EP but without breaking the PT-symmetric phase. The transmittance from one\ndirection is exactly unity while the transmittance from the other direction is\ndecreased to very low values, achieving very high optical isolation. The\nproposed active nonlinear metamaterial overcomes the fundamental physical\nbounds on nonreciprocity compared with a passive nonlinear nonreciprocal\nresonator. The strong self-induced nonreciprocal transmission arises from the\nextreme asymmetric field distribution achieved upon excitation from opposite\nincident directions. The significant enhancement of the electric field in the\ndefects effectively decreases the required optical power to trigger the\npresented nonlinear response. This work can have a plethora of applications,\nsuch as nonreciprocal ultrathin coatings for the protection of sources or other\nsensitive equipment from external pulsed signals, circulators, and isolators.",
        "category": "physics_optics"
    },
    {
        "text": "Floquet Mode Resonance: Trapping light in the bulk mode of a Floquet\n  topological insulator by quantum self-interference: Floquet topological photonic insulators characterized by periodically-varying\nHamiltonians are known to exhibit much richer topological behaviors than static\nsystems. In a Floquet insulator, the phase evolution of the Floquet-Bloch modes\nplays a crucial role in determining its topological behaviors. Here we show\nthat by perturbing the driving sequence, it is possible to manipulate the\ncyclic phase change of the system over each evolution period to induce quantum\nself-interference of a bulk mode, leading to a new topological resonance\nphenomenon called Floquet Mode Resonance (FMR). The FMR is fundamentally\ndifferent from other types of optical resonances in that it is cavity-less\nsince it does not require physical boundaries. Its spatial localization pattern\nis instead dictated by the driving sequence and can thus be used to probe the\ntopological characteristics of the system. We demonstrated excitation of FMRs\nby edge modes in a Floquet octagon lattice on silicon-on-insulator, achieving\nextrinsic quality factors greater than 10^4. Imaging of the scattered light\npattern directly revealed the hopping sequence of the Floquet system and\nconfirmed the spatial localization of FMR in a bulk-mode loop. The new\ntopological resonance effect could enable new applications in lasers, optical\nfilters and switches, nonlinear cavity optics and quantum optics.",
        "category": "physics_optics"
    },
    {
        "text": "Realization of all-band-flat photonic lattices: Flatbands play an important role in correlated quantum matter and have novel\napplications in photonic lattices. Synthetic magnetic fields and destructive\ninterference in lattices are traditionally used to obtain flatbands. However,\nsuch methods can only obtain a few flatbands with most bands remaining\ndispersive. Here we realize all-band-flat photonic lattices of an arbitrary\nsize by precisely controlling the coupling strengths between lattice sites to\nmimic those in Fock-state lattices. This allows us to go beyond the\nperturbative regime of strain engineering and group all eigenmodes in\nflatbands, which simultaneously achieves high band flatness and large usable\nbandwidth. We map out the distribution of each flatband in the lattices and\nselectively excite the eigenmodes with different chiralities. Our method paves\na new way in controlling band structure and topology of photonic lattices.",
        "category": "physics_optics"
    },
    {
        "text": "Two-photon scattering by a driven three-level emitter in a\n  one-dimensional waveguide and electromagnetically induced transparency: We study correlated two-photon transport in a (quasi) one-dimensional\nphotonic waveguide coupled to a three-level $\\Lambda$-type emitter driven by a\nclassical light field. Two-photon correlation is much stronger in the waveguide\nfor a driven three-level emitter (3LE) than a two-level emitter. The driven 3LE\nwaveguide shows electromagnetically induced transparency (EIT), and we\ninvestigate the scaling of EIT for one and two photons. We show that the two\ntransmitted photons are bunched together at any distance separation when energy\nof the incident photons meets \"two-photon resonance\" criterion for EIT.",
        "category": "physics_optics"
    },
    {
        "text": "Beam shaping by a layered structure with left-handed materials: We analyze transmission of a layered photonic structure (a one-dimensional\nphotonic crystal) consisting of alternating slabs of two materials with\npositive and negative refractive index. For the periodic structure with zero\naveraged refractive index, we demonstrate a number of unique properties of the\nbeam transmission observed in strong beam modification and reshaping.",
        "category": "physics_optics"
    },
    {
        "text": "Coherently amplified ultrafast imaging in a free-electron interferometer: Accessing the low-energy non-equilibrium dynamics of materials with\nsimultaneous spatial and temporal resolutions has been a bold frontier of\nelectron microscopy in recent years. One of the main challenges is the ability\nto retrieve extremely weak signals while simultaneously disentangling amplitude\nand phase information. Here, we present an algorithm-based microscopy approach\nthat uses light-induced electron modulation to demonstrate the coherent\namplification effect in electron imaging of optical near-fields. We provide a\nsimultaneous time-, space-, and phase-resolved measurement in a micro-drum made\nfrom a hexagonal boron nitride membrane, visualizing the sub-cycle\nspatio-temporal dynamics of 2D polariton wavepackets therein. The\nphase-resolved measurement reveals vortex-anti-vortex singularities on the\npolariton wavefronts, together with an intriguing phenomenon of a traveling\nwave mimicking the amplitude profile of a standing wave. Our experiments show a\n20-fold coherent amplification of the near-field signal compared to\nconventional electron near-field imaging, resolving peak field intensities of\n~W/cm2 (field amplitude of few kV/m). As a result, our work opens a path toward\nspatio-temporal electron microscopy of biological specimens and quantum\nmaterials - exciting yet sensitive samples, which are currently difficult to\ninvestigate.",
        "category": "physics_optics"
    },
    {
        "text": "A quantum cascade laser-pumped molecular laser tunable over 1 THz: By introducing methyl fluoride (CH$_3$F) as a new gain medium for a quantum\ncascade laser-pumped molecular laser (QPML), we demonstrate continuous-wave\nlasing from more than 120 discrete transitions spanning the frequency range\n0.25 to 1.3 THz. The unprecedented degree of spectral tuning achieved with\nCH$_3$F also confirms the universality of the QPML concept: for all polar gas\nmolecules, lasing can be induced on any dipole-allowed rotational transition by\nsufficient pumping of a related roto-vibrational transition using a\ncontinuously tunable quantum cascade laser.",
        "category": "physics_optics"
    },
    {
        "text": "Interferometric measurement of the deflection of light by light in air: The aim of the DeLLight (Deflection of Light by Light) experiment is to\nobserve for the first time the optical nonlinearity in vacuum, as predicted by\nQuantum Electrodynamics, by measuring the refraction of a low-intensity focused\nlaser pulse (probe) when crossing the effective vacuum index gradient induced\nby a high-intensity focused laser pulse (pump). The deflection signal is\namplified by using a Sagnac interferometer. Here, we report the first\nmeasurement performed with the DeLLight pilot interferometer, of the deflection\nof light by light in air, with a low-intensity pump. We show that the\ndeflection signal measured by the interferometer is amplified, and is in\nagreement with the expected signal induced by the optical Kerr effect in air.\nMoreover, we verify that the signal varies as expected as a function of the\npump intensity, the temporal delay between the pump and the probe, and their\nrelative polarisation. These results represent a proof of concept of the\nDeLLight experimental method based on interferometric amplification.",
        "category": "physics_optics"
    },
    {
        "text": "Bound-state-in-continuum guided modes in a multilayer electro-optically\n  active photonic integrated circuit platform: Bound states in the continuum (BICs) are localized states existing within a\ncontinuous spectrum of delocalized waves. Emerging multilayer photonic\nintegrated circuit (PIC) platforms allow implementation of low index 1D guided\nmodes within a high-index 2D slab mode continuum; however, conventional wisdom\nsuggests that this always leads to large radiation losses. Here we demonstrate\nlow-loss BIC guided modes for multiple mode polarizations and spatial orders in\nsingle- and multi-ridge low-index waveguides within a two-layer heterogeneously\nintegrated electro-optically active photonic platform. The transverse electric\n(TE) polarized quasi-BIC guided mode with low, <1.4 dB/cm loss enables a\nMach-Zehnder electro-optic amplitude modulator comprising a single straight\nSi3N4 ridge waveguide integrated with a continuous LiNbO3 slab layer. The\nabrupt optical transitions at the edges of the slab function as compact and\nefficient directional couplers eliminating the need for additional components.\nThe modulator exhibits a low insertion loss of 2.3 dB and a high extinction\nratio of 25 dB. The developed general theoretical model may enable innovative\nBIC-based approaches for important PIC functions, such as agile spectral\nfiltering and switching, and may suggest new photonic architectures for quantum\nand neural network applications based on controlled interactions between\nmultiple guided and delocalized modes.",
        "category": "physics_optics"
    },
    {
        "text": "Ultrawide Edge State Supercontinuum in a Floquet-Lieb Topological\n  Photonic Insulator: Conventional topological photonic insulators typically have narrow nontrivial\nband gaps truncated by broad dispersive bulk bands, resulting in limited edge\nmode transmission bandwidths that can be exploited for potential applications.\nHere we propose and demonstrate the first Floquet-Lieb topological photonic\ninsulator with all flat bands which can support continuous edge mode\ntransmission across multiple Floquet-Brillouin zones. This supercontinuum of\nedge states results from the orthogonality between the flat-band modes and the\nedge states, allowing for continuous excitation of the latter without\nscattering into the bulk modes. Moreover, we show that these flat bands are\nperfectly immune to random variations in the on-site potential, regardless of\nhow large the perturbations are, thus ensuring complete robustness of the edge\nmodes to this type of disorder. We realized Floquet-Lieb insulators using 2D\nmicroring resonator lattices with perfect nearest-neighbor couplings.\nTransmission measurements and direct imaging of the scattered light\ndistributions showed an edge mode supercontinuum spanning more than three\nmicroring free spectral ranges. These broad edge mode transmission bands\npersisted even in the presence of lattice disorder caused by fabrication\nimperfection. The proposed Floquet-Lieb insulator can potentially be used to\nrealize topological photonic devices with ultrawide bandwidths and super\nrobustness for applications in integrated quantum photonics and programmable\nphotonic circuits.",
        "category": "physics_optics"
    },
    {
        "text": "Orthogonal thermal noise and transmission signals: A new coherent\n  perfect absorption's feature: Coherent perfect absorption (CPA) is an interference process associated with\nthe zeros of the scattering matrix that enables light-with-light interactions\nin linear systems, of interest for optical computing, data processing and\nsensing. However, the noise properties of CPA remain relatively unexplored.\nHere, we demonstrate that CPA thermal noise signals exhibit a unique property:\nthey are orthogonal to the signals transmitted through the network. In turn,\nsuch property enables a variety of thermal noise management effects, such as\nthe physical separability of thermal noise and transmitted signals, and\n\"externally lossless\" networks that internally host radiative heat transfer\nprocesses. We believe that our results provide a new perspective on the many\nCPA technologies currently under development.",
        "category": "physics_optics"
    },
    {
        "text": "Single mode waveguide platform for spontaneous and surface-enhanced\n  on-chip Raman spectroscopy: We review an on-chip approach for spontaneous Raman spectroscopy and Surface\nEnhanced Raman Spectroscopy (SERS) based on evanescent excitation of the\nanalyte as well as evanescent collection of the Raman signal using\nComplementary Metal Oxide Semiconductor (CMOS) compatible single mode\nwaveguides. The signal is either directly collected from the analyte molecules\nor via plasmonic nanoantennas integrated on top of the waveguides. Flexibility\nin the design of the geometry of the waveguide, and/or the geometry of the\nantennas, enables optimization of the collection efficiency. Furthermore the\nsensor can be integrated with additional functionality (sources, detectors,\nspectrometers) on the same chip. In this paper, the basic theoretical concepts\nare introduced to identify the key design parameters and some proof-of-concept\nexperimental results are reviewed.",
        "category": "physics_optics"
    },
    {
        "text": "Excitation of plasmonic nanoantennas with nonresonant and resonant\n  electron tunnelling: A rigorous theory of photon emission accompanied inelastic tunnelling inside\nthe gap of plasmonic nanoantennas has been developed. The disappointingly low\nefficiency of the electrical excitation of surface plasmon polaritons in these\nstructures can be increased by orders of magnitude when a resonant tunnelling\nstructure is incorporated inside the gap. Resonant tunnelling assisted surface\nplasmon emitter may become a key element in future electrically-driven\nnanoplasmonic circuits.",
        "category": "physics_optics"
    },
    {
        "text": "Optimized detection modality for double resonance alignment based\n  optical magnetometer: In this work, we present a comprehensive and comparative analysis of two\ndetection modalities, i.e., polarization rotation and absorption measurement of\nlight, for a double resonance alignment based optical magnetometer (DRAM). We\nderive algebraic expressions for magnetometry signals based on multipole\nmoments description. Experiments are carried out using a room-temperature\nparaffin-coated Caesium vapour cell and measuring either the polarization\nrotation or absorption of the transmitted laser light. A detailed experimental\nanalysis of the resonance spectra is performed to validate the theoretical\nfindings for various input parameters. The results signify the use of a single\nisotropic relaxation rate thus simplifying the data analysis for optimization\nof the DRAM. The sensitivity measurements are performed and reveal that the\npolarization rotation detection mode yields larger signals and better\nsensitivity than absorption measurement of light.",
        "category": "physics_optics"
    },
    {
        "text": "Special scattering regimes for conical all-dielectric nanoparticles: All-dielectric nanophotonics opens a venue for a variety of novel phenomena\nand scattering regimes driven by unique optical effects in semiconductor and\ndielectric nanoresonators. Their peculiar optical signatures enabled by\nsimultaneous electric and magnetic responses in the visible range pave a way\nfor a plenty of new applications in nano-optics, biology, sensing, etc. In this\nwork, we investigate fabrication-friendly truncated cone resonators and achieve\nseveral important scattering regimes due to the inherent property of cones -\nbroken symmetry along the main axis without involving complex geometries or\nstructured beams. We show this symmetry breaking to deliver various kinds of\nKerker effects (Generalized and Transverse Kerker effects), non-scattering\nhybrid anapole regime (simultaneous anapole conditions for all the multipoles\nin a particle leading to the nearly full scattering suppression) and, vice\nversa, superscattering regime. Being governed by the same straightforward\ngeometrical paradigm, discussed effects could greatly simplify the\nmanufacturing process of photonic devices with different functionalities.\nMoreover, the additional degrees of freedom driven by the conicity open new\nhorizons to tailor light-matter interactions at the nanoscale.",
        "category": "physics_optics"
    },
    {
        "text": "Structured light analogy of squeezed state: Control of structured light is of great importance to explore fundamental\nphysical effects and extend practical scientific applications, which has been\nadvanced by accepting methods of quantum optics - many classical analogies of\nexotic quantum states were designed using structured modes. However, the\nprevailing quantum-like structured modes are limited by discrete states where\nthe mode index is analog to the photon number state. Yet, beyond discrete\nstates, there is a broad range of quantum states to be explored in the field of\nstructured light -- continuous-variable (CV) states. As a typical example of CV\nstates, squeezed state plays a prominent role in high-sensitivity\ninterferometry and gravitational wave detection. In this work, we bring\ntogether two seemingly disparate branches of physics, namely, classical\nstructured light and quantum squeezed state. We propose the structured light\nanalogy of squeezed state (SLASS), which can break the spatial limit following\nthe process of surpassing the standard quantum limit (SQL) with quantum\nsqueezed states. This work paves the way for adopting methods from CV quantum\nstates into structured light, opening new research directions of CV\nentanglement, teleportation, classical and quantum informatics of structured\nlight in the future.",
        "category": "physics_optics"
    },
    {
        "text": "Backward air lasing actions induced by femtosecond laser filamentation:\n  influence of population inversion lifetime: We experimentally investigate generation of backward 357 nm N2 laser in a gas\nmixture of N2/Ar using 800-nm femtosecond laser pulses, and examine the\ninvolved gain dynamics based on pump-probe measurements. Our findings show that\na minimum lifetime of population inversion in the excited N2 molecules is\nrequired for generating intense backward nitrogen lasers, which is ~0.8 ns\nunder our experimental conditions. The results shed new light on the mechanism\nfor generating intense backward lasers from ambient air, which are highly in\ndemand for high sensitivity remote atmospheric sensing application.",
        "category": "physics_optics"
    },
    {
        "text": "Speckled cross-spectral densities and their associated correlation\n  singularities for a modern source of partially coherent x rays: We consider a realistic model for calculating the cross-spectral density of\npartially coherent beams from an x-ray undulator in a modern storage ring. This\ntwo-point coherence function is seen to have a speckled structure associated\nwith the presence of x-ray coherence vortices and domain walls. Such\ncross-spectral density speckle is associated with a network of spatial pairs of\npoints for which there is zero correlation. X-ray coherence vortices and domain\nwalls are seen to emerge naturally as the number of coherent modes required\nincreases. An understanding of the existence and nature of such correlation\nsingularities enhances our ability to exploit partially coherent x-ray\nradiation from new or upgraded synchrotron sources, for both imaging and\ndiffraction applications.",
        "category": "physics_optics"
    },
    {
        "text": "Direct S-matrix calculation for diffractive structures and metasurfaces: The paper presents a derivation of analytical components of S-matrices for\narbitrary planar diffractive structures and metasurfaces in the Fourier domain.\nAttained general formulas for S-matrix components can be applied within both\nformulations in the Cartesian and curvilinear metric. A numerical method based\non these results can benefit from all previous improvements of the Fourier\ndomain methods. In addition, we provide expressions for S-matrix calculation in\ncase of periodically corrugated layers of 2D materials, which are valid for\narbitrary corrugation depth-to-period ratios. As an example the derived\nequations are used to simulate resonant grating excitation of graphene plasmons\nand an impact of silica interlayer on corresponding reflection curves.",
        "category": "physics_optics"
    },
    {
        "text": "Optical Characterization of Disordered Yb-doped Silica Glass Anderson\n  Localizing Optical Fiber: We investigate and report the optical and laser characteristics of a\nytterbium-doped transverse Anderson localizing optical fiber to develop a\nfundamental understanding of the light propagation, generation, and\namplification processes in this novel fiber. Ultimately, the goal based on the\nmeasurements and calculations conducted herein is to design and build a random\nfiber laser with a highly directional beam. The measurements are based on\ncertain observations of the laser pump propagation and amplified spontaneous\nemission generation in this fiber. Judicious approximations are used in the\npropagation equations to obtain the relevant desired parameters in simple\ntheoretical fits to the experimental observations, without resorting to\nspeculations based on the intended construction from the fiber preform.",
        "category": "physics_optics"
    }
]