Disordered optical media are an emerging class of materials capable ofstrongly scattering light. Their study is relevant to investigate transportphenomena and for applications in imaging, sensing and energy storage. Whilesuch materials can be used to generate coherent light, their directionalemission is typically hampered by their very multiple scattering nature. Here,we tune the out-of-plane directionality of coherent Raman light scattered by afractal network of silicon nanowires. By visualizing Rayleigh scattering,photoluminescence and weakly localized Raman light from the random network ofnanowires via real-space microscopy and Fourier imaging, we gain insight on thelight transport mechanisms responsible for the material's inelastic coherentsignal and for its directionality. The possibility of visualizing andmanipulating directional coherent light in such networks of nanowires opensvenues for fundamental studies of light propagation in disordered media as wellas for the development of next generation optical devices based on disorderedstructures, inclusive of sensors, light sources and optical switches.
On the modeling of thermal and free carrier nonlinearities in Silicon On Insulator microring resonatorsThe temporal dynamics of integrated silicon resonators has been modeled usinga set of equations coupling the internal energy, the temperature and the freecarrier population. Owing to its simplicity, Newton's law of cooling is thetraditional choice for describing the thermal evolution of such systems. Inthis work, we theoretically and experimentally prove that this can beinadequate in monolithic planar devices, leading to inaccurate predictions. Anew equation, that we train to reproduce the correct temperature behaviour, isintroduced to fix the discrepancies with the experimental results. We discussthe limitations and the range of validity of our refined model, identifyingthose cases where Netwon's law provides, nevertheless, accurate solutions. Ourmodeling describes the phenomena underlying thermal and free carrierinstabilities, and is a valuable tool for the engineering of photonic systemswhich relay on resonator dynamical states, such as all optical spiking neuralnetworks or reservoirs for neuromorphic computing.
Single-pixel Tracking and Imaging under Weak IlluminationUnder weak illumination, tracking and imaging moving object turns out to behard. By spatially collecting the signal, single pixel imaging schemes promisethe capability of image reconstruction from low photon flux. However, due tothe requirement on large number of samplings, how to clearly image movingobjects is an essential problem for such schemes. Here we present a principleof single pixel tracking and imaging method. Velocity vector of the object isobtained from temporal correlation of the bucket signals in a typicalcomputational ghost imaging system. Then the illumination beam is steeredaccordingly. Taking the velocity into account, both trajectory and clear imageof the object are achieved during its evolution. Since tracking is achievedwith bucket signals independently, this scheme is valid for capturing movingobject as fast as its displacement within the interval of every sampling keepslarger than the resolution of the optical system. Experimentally, our methodworks well with the average number of detected photons down to 1.88photons/speckle.
Simultaneous gain profile design and noise figure prediction for Raman amplifiers using machine learningA machine learning framework predicting pump powers and noise figure profilefor a target distributed Raman amplifier gain profile is experimentallydemonstrated. We employ a single-layer neural network to learn the mapping fromthe gain profiles to the pump powers and noise figures. The obtained resultsshow highly-accurate gain profile designs and noise figure predictions, with amaximum error on average of ~0.3dB. This framework provides the comprehensivecharacterization of the Raman amplifier and thus is a valuable tool forpredicting the performance of the next-generation optical communicationsystems, expected to employ Raman amplification.
Free electron nonlinearities in heavily doped semiconductors plasmonicsHeavily doped semiconductors have emerged as tunable low-loss plasmonicmaterials at mid-infrared frequencies. In this article we investigate nonlinearoptical phenomena associated with high concentration of free electrons. We usea hydrodynamic description to study free electron dynamics in heavily dopedsemiconductors up to third-order terms, which are usually negligible for noblemetals. We find that cascaded third-harmonic generation due to second-harmonicsignals can be as strong as direct third-harmonic generation contributions evenwhen the second-harmonic generation efficiency is zero. Moreover, we show thatwhen coupled with plasmonic enhancement free electron nonlinearities could beup to two orders of magnitude larger than conventional semiconductornonlinearities. Our study might open a new route for nonlinear opticalintegrated devices at mid-infrared frequencies.
Non-invasive imaging of object behind turbid media via cross-spectrumWe develop a method based on the cross-spectrum of an intensity-modulated CWlaser, which can extract a signal from an extremely noisy environment and imageobjects hidden in turbid media. We theoretically analyzed our scheme andperformed the experiment by scanning the object placed in between two groundglass diffusers. The image of the object is retrieved by collecting theamplitudes at the modulation frequency of all the cross-spectra. Our method isnon-invasive, easy-to-implement, and can work for both static and dynamicmedia.
Metasurfaces constituted of a continuously-tuned lattice of coupled nanopillars: inter-coupled metasurfacesMetasurfaces are subwavelength-thick constructs, consisting of discretemeta-cells providing discretized levels of phase accumulation that collectivelyapproximate a designed optical functionality. The meta-cells utilizingPancharatnam-Berry phase with polarization-converting structures producedencouraging implementations of optical components including metasurface lenses(metalenses). However, a pending and fundamental problem of this approach isthe low device efficiency that the resulting metasurface components suffer, anunwanted side effect of large lattice constants that are used for preventinginter-coupling of their meta-cells. Here, we propose and show that, instead ofsuch uncoupled unit cells with fixed periodicity, tightly coupled fabric ofidentical dielectric nanopillars with continuously-tuned edge-to-edge distancesmake excellent and complete metasurface elements. This paradigm shift enablesthe scatterers to interact with the incoming wave extremely efficiently. As aproof-of-concept demonstration, we showed an achromatic cylindrical metalens,constructed from strongly coupled dielectric nanopillars of a single geometryas continuously-set phase elements, working in the entirety 400-700 nm band.This metalens achieves over 85 percent focusing efficiency in transmission modeacross this spectral range. To combat polarization-sensitivity, we found thatstacking the nanopillars in a honeycomb lattice may be used for building up apolarization-independent scatterer library. Finally, a circular metalens withpolarization-independent operation and achromatic focusing was obtained. To thebest of our knowledge, this is the first account of a metasurface architecturewoven of identical nanopillars coupled into a lattice laterally constructed byseamlessly bringing them in close proximity with carefully-tuned inter-celldistances.
Polarization and phase textures in lattice plasmon condensatesPolarization textures of light may reflect fundamental phenomena such astopological defects, and can be utilized in engineering light beams. Three mainroutes are applied during their creation: spontaneous appearance in phasetransitions, steering by an excitation beam, or structural engineering of themedium. We present an approach that uses all three in a platform offeringadvantages that are not simultaneously provided in any previous system:advanced structural engineering, strong-coupling condensate with effectivephotonic interactions, as well as room temperature and sub-picosecondoperation. We demonstrate domain wall polarization textures in a plasmoniclattice Bose-Einstein condensate, by combining the dipole structure of thelattice with a non-trivial condensate phase revealed by phase retrieval. Theseresults open new prospects for fundamental studies of non-equilibriumcondensation and sources of polarization-structured beams.
Hybrid microwave-optical scanning probe for addressing solid-state spins in nanophotonic cavitiesSpin-photon interfaces based on solid-state atomic defects have enabled avariety of key applications in quantum information processing. To maximize thelight-matter coupling strength, defects are often placed inside nanoscaledevices. Efficiently coupling light and microwave radiation into thesestructures is an experimental challenge, especially in cryogenic or high vacuumenvironments with limited sample access. In this work, we demonstrate afiber-based scanning probe that simultaneously couples light into a planarphotonic circuit and delivers high power microwaves for driving electron spintransitions. The optical portion achieves 46% one-way coupling efficiency,while the microwave portion supplies an AC magnetic field with strength up to 9Gauss. The entire probe can be scanned across a large number of devices insidea $^3$He cryostat without free-space optical access. We demonstrate thistechnique with silicon nanophotonic circuits coupled to single Er$^{3+}$ ions.
Direct Measurement of a Non-Hermitian Topological Invariant in a Hybrid Light-Matter SystemTopology is central to understanding and engineering materials that displayrobust physical phenomena immune to imperfections. The topological character ofa material is quantified by topological invariants that simplify theclassification of topological phases. In energy-conserving systems, thetopological invariants, e.g., the Chern number, are determined by the windingof the eigenstates in momentum (wavevector) space, which have beenexperimentally measured in ultracold atoms, microwaves, and photonic systems.Recently, new topological phenomena have been theoretically uncovered indissipative, non-Hermitian systems. A novel, non-Hermitian topologicalinvariant, yet to be observed in experiments, is predicted to emerge from thewinding of the complex eigenvalues in momentum space. Here, we directly measurethe non-Hermitian topological invariant arising from spectral degeneracies(exceptional points) in the momentum space of exciton polaritons. These hybridlight-matter quasiparticles are formed by photons strongly coupled toelectron-hole pairs (excitons) in a halide perovskite semiconductor microcavityat room temperature. By performing momentum-resolved photoluminescencespectroscopy of exciton polaritons, we map out both the real (energy) andimaginary (linewidth) parts of the exciton-polariton eigenvalues near theexceptional point, and extract a new topological invariant - fractionalspectral winding. Our work represents an essential step towards realisation ofnon-Hermitian topological phases in a solid-state system.
Exactly unitary discrete representations of the metaplectic transform for linear-time algorithmsThe metaplectic transform (MT), sometimes called the linear canonicaltransform, is a tool used ubiquitously in modern optics, for example, whencalculating the transformations of light beams in paraxial optical systems. TheMT is also an essential ingredient of the geometrical-optics modeling ofcaustics that was recently proposed by the authors. In particular, thisapplication relies on the near-identity MT (NIMT); however, the NIMTapproximation used so far is not exactly unitary and leads to numericalinstability. Here, we develop a discrete MT that is exactly unitary, andapproximate it to obtain a discrete NIMT that is also unitary and can becomputed in linear time. We prove that the discrete NIMT converges to thediscrete MT when iterated, thereby allowing the NIMT to compute MTs that arenot necessarily near-identity. We then demonstrate the new algorithms with aseries of examples.
Spatial distributions of the fields in guided normal modes of two coupled parallel nanofibersWe study the cross-sectional profiles and spatial distributions of the fieldsin guided normal modes of two coupled parallel nanofibers. We show that thedistributions of the components of the field in a guided normal mode of twoidentical nanofibers are either symmetric or antisymmetric with respect to theradial principal axis $x$ and the tangential principal axis $y$ in thecross-sectional plane of the fibers. The symmetry of the magnetic fieldcomponents with respect to the principal axes is opposite to that of theelectric field components. We show that, in the case of even$\mathcal{E}_z$-cosine modes, the electric intensity distribution is dominantin the area between the fibers, with a saddle point at the two-fiber center.Meanwhile, in the case of odd $\mathcal{E}_z$-sine modes, the electricintensity distribution at the two-fiber center attains a local minimum ofexactly zero. We find that the differences between the results of the coupledmode theory and the exact mode theory are large when the fiber separationdistance is small and either the fiber radius is small or the light wavelengthis large. We show that, in the case where the two nanofibers are not identical,the intensity distribution is symmetric about the radial principal axis $x$ andasymmetric about the tangential principal axis $y$.
Ultra-bright multiplexed energy-time entangled photon generation from lithium niobate on insulator chipHigh-flux entangled photon source is the key resource for quantum opticalstudy and application. Here it is realized in a lithium niobate on isolator(LNOI) chip, with 2.79*10^11 Hz/mW photon pair rate and 1.53*10^9 Hz/nm/mWspectral brightness. These data are boosted by over two orders of magnitudecompared to existing technologies. A 130-nm broad bandwidth is engineered for8-channel multiplexed energy-time entanglement. Harnessed by high-extinctionfrequency correlation and Franson interferences up to 99.17% visibility, suchenergy-time entanglement multiplexing further enhances high-flux data rate, andwarrants broad applications in quantum information processing on a chip.
Propagation dynamics of the circular Airy Gaussian vortex beams in the fractional nonlinear Schrödinger equationWe have investigated the propagation dynamics of the circular Airy Gaussianvortex beams (CAGVBs) in a (2+1)-dimesional optical system discribed byfractional nonlinear Schrödinger equation (FNSE). By combining fractionaldiffraction with nonlinear effects, the abruptly autofocusing effect becomesweaker, the radius of the focusing beams becomes bigger and the autofocusinglength will be shorter with increase of fractional diffraction Lévy index. Ithas been found that the abruptly autofocusing effect becomes weaker and theabruptly autofocusing length becomes longer if distribution factor of CAGVBsincreases for fixing the Lévy index. The roles of the input power and thetopological charge in determining the autofocusing properties are alsodiscussed. Then, we have found the CAGVBs with outward acceleration and shownthe autodefocusing properties. Finally, the off-axis CAGVBs with positivevortex pairs in the FNSE optical system have shown interesting features duringpropagation.
Photonic realization of the kappa-deformed Dirac equationWe show an implementation of a kappa-deformed Dirac equation in tight-bindingarrays of photonic waveguides. This is done with a special configuration ofcouplings extending to second nearest neighbors. Geometric manipulations cancontrol these evanescent couplings. A careful study of wave packet propagationis presented, including the effects of deformation parameters on Zitterbewegungor trembling motion. In this way, we demonstrate how to recreate the effects ofa flat noncommutative spacetime -i.e., kappa-Minkowski spacetime -in simpleexperimental setups. We touch upon elastic realizations in the section ofConclusions.
PyLlama: a stable and versatile Python toolkit for the electromagnetic modeling of multilayered anisotropic mediaPyLlama is a handy Python toolkit to compute the electromagnetic reflectionand transmission properties of arbitrary multilayered linear media, includingthe case of anisotropy. Relying on a $4 \times 4$-matrix formalism, PyLlamaimplements not only the transfer matrix method, that is the most popular choicein existing codes, but also the scattering matrix method, which is numericallystable in all situations (e.g., thick, highly birefringent cholestericstructures at grazing incident angles). PyLlama is also designed to suit thepractical needs by allowing the user to create, edit and assemble layers ormultilayered domains with great ease. In this article, we present theelectromagnetic theory underlying the transfer matrix and scattering matrixmethods and outline the architecture and main features of PyLlama. Finally, wevalidate the code by comparison with available analytical solutions anddemonstrate its versatility and numerical stability by modelling cholestericmedia of varying complexity. A detailed documentation and tutorial are providedin a separate user manual. Applications of PyLlama range from the design ofoptical components to the study of structurally coloured materials in theliving world.