Robust Classical and Quantum Polarimetry with a Single Nanostructured Metagrating

We formulate a new conceptual approach for one-shot complete polarization state measurement with nanostructured metasurfaces applicable to classical light and multiphoton quantum states by drawing on the principles of generalized quantum measurements based on positive operator-valued measures. Accurate polarization reconstruction from a combination of photon counts or correlations from several diffraction orders is robust with respect to even strong fabrication inaccuracies, requiring only a single classical calibration of the metasurface transmission. Furthermore, this approach operates with a single metagrating without interleaving, allowing for a reduction in metasurface size while preserving high transmission efficiency and output beam quality. We theoretically obtained original metasurface designs, fabricated the metasurface from amorphous silicon nanostructures deposited on glass, and experimentally confirmed accurate polarization reconstruction of laser beams. We also anticipate robust operation under changes in environmental conditions, opening new possibilities for space-based imaging and satellite optics.

Engineering Quantum Light Sources with Flat Optics

Quantum light sources are essential building blocks for many quantum technologies, enabling secure communication, powerful computing, and precise sensing and imaging. Recent advancements have witnessed a significant shift toward the utilization of "flat" optics with thickness at subwavelength scales for the development of quantum light sources. This approach offers notable advantages over conventional bulky counterparts, including compactness, scalability, and improved efficiency, along with added functionalities. This review focuses on the recent advances in leveraging flat optics to generate quantum light sources. Specifically, the generation of entangled photon pairs through spontaneous parametric down-conversion in nonlinear metasurfaces, and single photon emission from quantum emitters including quantum dots and color centers in 3D and 2D materials are explored. The review covers theoretical principles, fabrication techniques, and properties of these sources, with particular emphasis on the enhanced generation and engineering of quantum light sources using optical resonances supported by nanostructures. The diverse application range of these sources is discussed and the current challenges and perspectives in the field are highlighted.

Polarization Engineering of Entangled Photons from a Lithium Niobate Nonlinear Metasurface

Complex polarization states of photon pairs are indispensable in various quantum technologies. Conventional methods for preparing desired two-photon polarization states are realized through bulky nonlinear crystals, which can restrict the versatility and tunability of the generated quantum states due to the fixed crystal nonlinear susceptibility. Here we present a solution using a nonlinear metasurface incorporating multiplexed silica metagratings on a lithium niobate film of 300 nm thickness. We fabricate two orthogonal metagratings on a single substrate with an identical resonant wavelength, thereby enabling the spectral indistinguishability of the emitted photons, and we demonstrate in experiments that the two-photon polarization states can be shaped by the metagrating orientation. Leveraging this essential property, we formulate a theoretical approach for generating arbitrary polarization-entangled qutrit states by combining three metagratings on a single metasurface, allowing the encoding of the desired quantum states or information. Our findings enable miniaturized optically controlled quantum devices by using ultrathin metasurfaces as polarization-entangled photon sources.

Spatially entangled photon pairs from lithium niobate nonlocal metasurfaces

Metasurfaces consisting of nanoscale structures are underpinning new physical principles for the creation and shaping of quantum states of light. Multiphoton states that are entangled in spatial or angular domains are an essential resource for many quantum applications; however, their production traditionally relies on bulky nonlinear crystals. We predict and demonstrate experimentally the generation of spatially entangled photon pairs through spontaneous parametric down-conversion from a metasurface incorporating a nonlinear thin film of lithium niobate covered by a silica meta-grating. We measure the correlations of photon pairs and identify their spatial antibunching through violation of the classical Cauchy-Schwarz inequality, witnessing the presence of multimode entanglement. Simultaneously, the photon-pair rate is strongly enhanced by 450 times as compared to unpatterned films because of high-quality-factor resonances. These results pave the way to miniaturization of various quantum devices by incorporating ultrathin metasurfaces functioning as room temperature sources of quantum-entangled photons.

Nonlinear transition between PT-symmetric and PT-broken modes in coupled fiber lasers

We present a systematic analysis of the stationary regimes of nonlinear parity-time (PT) symmetric laser composed of two coupled fiber cavities. We find that power-dependent nonlinear phase shifters broaden regions of existence of both PT-symmetric and PT-broken modes, and can facilitate transitions between modes of different types. We show the existence of non-stationary regimes and demonstrate an ambiguity of the transition process for some of the unstable states. We also identify the presence of higher-order stationary modes, which return to the initial state periodically after a certain number of round-trips.

Multidimensional synthetic chiral-tube lattices via nonlinear frequency conversion

Geometrical dimensionality plays a fundamentally important role in the topological effects arising in discrete lattices. Although direct experiments are limited by three spatial dimensions, the research topic of synthetic dimensions implemented by the frequency degree of freedom in photonics is rapidly advancing. The manipulation of light in these artificial lattices is typically realized through electro-optic modulation; yet, their operating bandwidth imposes practical constraints on the range of interactions between different frequency components. Here we propose and experimentally realize all-optical synthetic dimensions involving specially tailored simultaneous short- and long-range interactions between discrete spectral lines mediated by frequency conversion in a nonlinear waveguide. We realize triangular chiral-tube lattices in three-dimensional space and explore their four-dimensional generalization. We implement a synthetic gauge field with nonzero magnetic flux and observe the associated multidimensional dynamics of frequency combs, all within one physical spatial port. We anticipate that our method will provide a new means for the fundamental study of high-dimensional physics and act as an important step towards using topological effects in optical devices operating in the time and frequency domains.

Nonlinear optical lattices with a void impurity

We examine a one-dimensional nonlinear (Kerr) waveguide array which contains a single "void" waveguide where the nonlinearity is identically zero. We uncover a family of nonlinear localized modes centered at or near the void, and their stability properties. Unlike a usual impurity problem, here the void acts like a repulsive impurity causing the center of the simplest mode to lie to the side of the void's position. We also compute the stability of extended nonlinear modes showing significant differences from the usual homogeneous nonlinear array. The transmission of a nonlinear pulse across the void shows three main regimes-transmission, reflection, and trapping at the void's position-and we identify a critical momentum for the pulse below (above) where the pulse gets reflected (transmitted), or trapped indefinitely at the void's position. For relatively wide pulses, we observe a steep increase from a regime of no transmission to a regime of high transmission, as the amplitude of the soliton increases beyond a critical wave-vector value. Finally, we consider the transmission of an extended nonlinear wave across the void impurity numerically, finding a rather complex structure of the transmission as a function of wave vector, with the creation of more and more transmission gaps as nonlinearity increases. The overall transmittance decreases and disappears eventually, where the boundaries separating passing from nonpassing regions are complex and fractal-like.

Observation of localized modes at effective gauge field interface in synthetic mesh lattice

We predict a generic mechanism of wave localization at an interface between uniform artificial gauge fields, arising due to propagation-dependent phase accumulation similar to Aharonov-Bohm phenomenon. We realize experimentally a synthetic mesh lattice with real-time control over the vector gauge field, and observe robust localization under a broad variation of gauge strength and direction, as well as structural lattice parameters. This suggests new possibilities for confining and guiding waves in diverse physical systems through the synthetic gauge fields.

Broadband on-chip polarization mode splitters in lithium niobate integrated adiabatic couplers

We report, to the best of our knowledge, the first broadband polarization mode splitter (PMS) based on the adiabatic light passage mechanism in the lithium niobate (LiNbO₃) waveguide platform. A broad bandwidth of ~140 nm spanning telecom S, C, and L bands at polarization-extinction ratios (PER) of >20 dB and >18 dB for the TE and TM polarization modes, respectively, is found in a five-waveguide adiabatic coupler scheme whose structure is optimized by an adiabaticity engineering process in titanium-diffused LiNbO₃ waveguides. When the five-waveguide PMS is integrated with a three-waveguide "shortcut to adiabaticity" structure, we realize a broadband, high splitting-ratio (ηc) mode splitter for spatial separation of TE- (H-) polarized pump (700-850 nm for ηc>99%), TM- (V-) polarized signal (1510-1630 nm for ηc>97%), and TE- (H-) polarized idler (1480-1650 nm for ηc>97%) modes. Such a unique integrated-optical device is of potential for facilitating the on-chip implementation of a pump-filtered, broadband tunable entangled quantum-state generator.

Quantum metasurface for multiphoton interference and state reconstruction

Metasurfaces based on resonant nanophotonic structures have enabled innovative types of flat-optics devices that often outperform the capabilities of bulk components, yet these advances remain largely unexplored for quantum applications. We show that nonclassical multiphoton interferences can be achieved at the subwavelength scale in all-dielectric metasurfaces. We simultaneously image multiple projections of quantum states with a single metasurface, enabling a robust reconstruction of amplitude, phase, coherence, and entanglement of multiphoton polarization-encoded states. One- and two-photon states are reconstructed through nonlocal photon correlation measurements with polarization-insensitive click detectors positioned after the metasurface, and the scalability to higher photon numbers is established theoretically. Our work illustrates the feasibility of ultrathin quantum metadevices for the manipulation and measurement of multiphoton quantum states, with applications in free-space quantum imaging and communications.

Direct characterization of a nonlinear photonic circuit's wave function with laser light

Integrated photonics is a leading platform for quantum technologies including nonclassical state generation^{1, 2, 3, 4}, demonstration of quantum computational complexity⁵ and secure quantum communications⁶. As photonic circuits grow in complexity, full quantum tomography becomes impractical, and therefore an efficient method for their characterization^{7, 8} is essential. Here we propose and demonstrate a fast, reliable method for reconstructing the two-photon state produced by an arbitrary quadratically nonlinear optical circuit. By establishing a rigorous correspondence between the generated quantum state and classical sum-frequency generation measurements from laser light, we overcome the limitations of previous approaches for lossy multi-mode devices^{9, 10}. We applied this protocol to a multi-channel nonlinear waveguide network and measured a 99.28±0.31% fidelity between classical and quantum characterization. This technique enables fast and precise evaluation of nonlinear quantum photonic networks, a crucial step towards complex, large-scale, device production.

Asymmetric adiabatic couplers for fully-integrated broadband quantum-polarization state preparation

Spontaneous parametric down-conversion (SPDC) is a widely used method to generate entangled photons, enabling a range of applications from secure communication to tests of quantum physics. Integrating SPDC on a chip provides interferometric stability, allows to reduce a physical footprint, and opens a pathway to true scalability. However, dealing with different photon polarizations and wavelengths on a chip presents a number of challenging problems. In this work, we demonstrate an on-chip polarization beam-splitter based on z-cut titanium-diffused lithium niobate asymmetric adiabatic couplers (AAC) designed for integration with a type-II SPDC source. Our experimental measurements reveal unique polarization beam-splitting regime with the ability to tune the splitting ratios based on wavelength. In particular, we measured a splitting ratio of 17 dB over broadband regions (>60 nm) for both H- and V-polarized lights and a specific 50%/50% splitting ratio for a cross-polarized photon pair from the AAC. The results show that such a system can be used for preparing different quantum polarization-path states that are controllable by changing the phase-matching conditions in the SPDC over a broad band. Furthermore, we propose a fully integrated electro-optically tunable type-II SPDC polarization-path-entangled state preparation circuit on a single lithium niobate photonic chip.

Atom-mediated spontaneous parametric down-conversion in periodic waveguides

We propose the concept of atom-mediated spontaneous parametric down-conversion, in which photon-pair generation can take place only in the presence of a single two-level emitter, relying on the bandgap evanescent modes of a nonlinear periodic waveguide. Using a guided signal mode, an evanescent idler mode, and an atom-like emitter with the idler's transition frequency embedded in the structure, we find a heralded excitation mechanism, in which the detection of a signal photon outside the structure heralds the excitation of the embedded emitter. We use a rigorous Green's function quantization method to model this heralding mechanism in a 1D periodic waveguide and determine its robustness against losses.

Anderson localization in synthetic photonic lattices

Synthetic photonic lattices provide unique capabilities to realize theoretical concepts emerging in different fields of wave physics via the utilization of powerful photonic technologies. Here we observe experimentally Anderson localization for optical pulses in time domain, using a photonic mesh lattice composed of coupled fiber loops. We introduce a random potential through programmed electro-optic pulse phase modulation, and identify the localization features associated with varying degree of disorder. Furthermore, we present a practical approach to control the band-gap width in photonic lattices by varying the coupling between the fiber loops, and reveal that the strongest degree of localization is limited and increases in lattices with wider band-gaps. Importantly, this opens a possibility to enhance or reduce the effect of disorder and associated localization of optical pulses.

Frequency comb generation in SNAP bottle resonators

We develop a theory of optical frequency comb generation in ultra-compact surface nanoscale axial photonic (SNAP) bottle microresonators, employing the nonlinear interaction of whispering gallery modes which are confined along an optical fiber with nanoscale radius variation. We predict that a SNAP microresonator with a radius of a few micrometers can generate a frequency comb with an ultra-fine sub-gigahertz spectral spacing, which would require traditional ring resonators of centimeter radius. We identify regimes of stable or quasi-periodic comb dynamics due to soliton excitation, and show that special engineering of the SNAP radius profile can be used to compensate for nonlinearity-induced dispersion.

Non-reciprocal geometric phase in nonlinear frequency conversion

We describe analytically and numerically the geometric phase arising from nonlinear frequency conversion and show that such a phase can be made non-reciprocal by momentum-dependent photonic transition. Such non-reciprocity is immune to the shortcomings imposed by dynamic reciprocity in Kerr and Kerr-like devices. We propose a simple and practical implementation, requiring only a single waveguide and one pump, while the geometric phase is controllable by the pump and promises robustness against fabrication errors.

Nonlocal splitting of photons on a nonlinear chip

In spontaneous parametric downconversion (SPDC), a pump photon spontaneously splits into signal and idler photons in media with quadratic nonlinearity. This phenomenon is the most widely utilized source of entangled photons with multiple applications in quantum information technology. SPDC on a chip is usually treated as a local process, meaning that signal and idler photons are created in the same position at which the pump photon is destroyed. We reveal that this locality condition can be violated in an array of coupled waveguides. By utilizing higher-order modes of individual waveguides, it is possible to destroy a pump photon in one waveguide and to generate signal and idler photons in other waveguides. This phenomenon of nonlocal photon-pair generation opens new opportunities for the engineering of spatial photon entanglement.

Experimental demonstration of bidirectional light transfer in adiabatic waveguide structures

We propose and demonstrate a novel type of optical integrated structure consisting of three adiabatically coupled waveguides arranged in an N-shaped geometry. Unlike conventional adiabatic three-waveguide couplers mimicking the stimulated Raman adiabatic passage process which utilize solely the counter-intuitive coupling and, thus, operate only in one direction, our structure achieves complete bidirectional light transfer between two waveguides through the counter-intuitive and intuitive coupling in either direction over a wide wavelength range. Moreover, the light transfer through the intuitive coupling is more efficient and robust than through the counter-intuitive coupling.

Generation of Photon-Plasmon Quantum States in Nonlinear Hyperbolic Metamaterials

We develop a general theoretical framework of integrated paired photon-plasmon generation through spontaneous wave mixing in nonlinear plasmonic and metamaterial nanostructures, rigorously accounting for material dispersion and losses in the quantum regime through the electromagnetic Green function. We identify photon-plasmon correlations in layered metal-dielectric structures with 70% internal heralding quantum efficiency and reveal a novel mechanism of broadband generation enhancement due to topological transition in hyperbolic metamaterials.

Two-photon tomography using on-chip quantum walks

We present an approach to quantum tomography based on first expanding a quantum state across extra degrees of freedom and then exploiting the introduced sparsity to perform reconstruction. We formulate its application to photonic circuits and show that measured spatial photon correlations at the output of a specially tailored discrete-continuous quantum walk can enable full reconstruction of any two-photon spatially entangled and mixed state at the input. This approach does not require any tunable elements, so it is well suited for integration with on-chip superconducting photon detectors.

Fabrication of free-standing lithium niobate nanowaveguides down to 50 nm in width

Nonlinear optical nanoscale waveguides are a compact and powerful platform for efficient wavelength conversion. The free-standing waveguide geometry opens a range of applications in microscopy for local delivery of light, where in situ wavelength conversion helps to overcome various wavelength-dependent issues, such as biological tissue damage. In this paper, we present an original patterning method for high-precision fabrication of free-standing nanoscale waveguides based on lithium niobate, a material with a strong second-order nonlinearity and a broad transparency window covering the visible and mid-infrared wavelength ranges. The fabrication process combines electron-beam lithography with ion-beam enhanced etching and produces nanowaveguides with lengths from 5 to 50 μm, widths from 50 to 1000 nm and heights from 50 to 500 nm, each with a precision of few nanometers. The fabricated nanowaveguides are tested in an optical characterization experiment showing efficient second-harmonic generation.

Parity-time anti-symmetric parametric amplifier

We describe the process of parametric amplification in a directional coupler of quadratically nonlinear and lossy waveguides, which belongs to a class of optical systems with spatial parity-time (PT) symmetry in the linear regime. We identify a distinct spectral PT anti-symmetry associated with optical parametric interaction, and show that pump-controlled symmetry breaking can facilitate spectrally selective mode amplification in analogy with PT lasers. We also establish a connection between the breaking of spectral and spatial mode symmetries, revealing the potential to implement unconventional regimes of spatial light switching through ultrafast control of PT breaking by pump pulses.

Modulated coupled nanowires for ultrashort pulses

We predict analytically and confirm with numerical simulations that intermode dispersion in nanowire waveguide arrays can be tailored through periodic waveguide bending, facilitating flexible spatiotemporal reshaping without breakup of femtosecond pulses. This approach allows simultaneous and independent control of temporal dispersion and spatial diffraction that are often strongly connected in nanophotonic structures.

Reflectionless potentials for slow whispering gallery modes in surface nanoscale axial photonic fiber resonators

We consider an optical fiber with a nanoscale variation of the effective fiber radius that supports whispering gallery modes slowly propagating along the fiber, and reveal that the radius variation can be designed to support the reflectionless propagation of these modes. We show that reflectionless modulations can realize control of the transmission amplitude and temporal delay, while enabling close packing due to the absence of cross talk, in contrast to the conventional potentials.

Unidirectional flow of discrete solitons in waveguide arrays

We present a proposal for realizing a unidirectional flow of discrete solitons in optical waveguide arrays or coupled resonator optical waveguides. Our proposal is based on modulating the coupling coefficients between the waveguides by two reflectionless potentials with different depths and finite separation. For a single reflectionless potential, we demonstrate numerically the possibility of nonlinear switching with a critical nonlinearity strength at which sharp transition in the transport coefficients takes place.

Chaotic oscillations of coupled nanobeam cavities with tailored optomechanical potentials

We reveal novel features of nonlinear optomechanical interactions in coupled suspended nanocavities that are driven by two detuned laser frequencies. Such driving enables simultaneous excitation of odd and even optical supermodes, which induce gradient forces of opposite signs, and the competition between these forces enables the realization of optomechanical potentials with large barriers and narrow wells. These types of potentials were suggested for precise displacement control, or "spectral bonding," in the static regime. However we find that self-induced oscillations appear even at the deep global potential minima when the mechanical damping rate is below a certain threshold, including a new regime of chaotic switching between mechanical deformations of opposite signs.

Photon pair generation and pump filtering in nonlinear adiabatic waveguiding structures

We propose a novel integrated scheme for generation of Bell states, which allows simultaneous spatial filtering of pump photons. It is achieved through spontaneous parametric down-conversion in the system of nonlinear adiabatically coupled waveguides. We perform detailed analytic study of photon-pair generation in coupled waveguides and reveal the optimal conditions for the generation of each particular Bell state. Furthermore, we simulate the performance of the device under realistic assumptions and show that adiabatic coupling allows us to spatially filter the pump from modal-entangled photon pairs. Finally, we demonstrate that adiabatic couplers open the possibility of maintaining the purity of generated Bell states in a relatively fabrication-fault-tolerant way.

Nonlinear coupled-mode theory for periodic plasmonic waveguides and metamaterials with loss and gain

We derive general coupled-mode equations describing the nonlinear interaction of electromagnetic modes in periodic media with loss and gain. Our approach is rigorously based on the Lorentz reciprocity theorem, and it can be applied to a broad range of metal-dielectric photonic structures, including plasmonic waveguides and metamaterials. We verify that our general results agree with the previous analysis of particular cases, and predict novel effects on self- and cross-phase modulation in multilayer nonlinear fishnet metamaterials.

Compact surface Fano states embedded in the continuum of waveguide arrays

We describe theoretically and observe experimentally the formation of a surface state in a semi-infinite waveguide array with a side-coupled waveguide, designed to simultaneously achieve Fano and Fabry-Perot resonances. We demonstrate that the surface mode is compact, with all energy concentrated in a few waveguides at the edge and no field penetration beyond the side-coupled waveguide position. Furthermore, we show that by broadening the spectral band in the rest of the waveguide array it is possible to suppress exponentially localized modes, while the Fano state having the eigenvalue embedded in the continuum is preserved.

Coherent perfect absorption and reflection in slow-light waveguides

We identify a family of unusual slow-light modes occurring in lossy multimode grating waveguides, for which either the forward or backward mode components, or both, are degenerate. In the fully degenerate case, the response can be modulated between coherent perfect absorption (zero reflection) and perfect reflection by varying the wave amplitudes in a uniform input waveguide. The perfectly absorbed wave has anomalously short absorption length, scaling as the inverse one-third power of the absorptivity.

Nonlocality in PT-symmetric waveguide arrays with gain and loss

We demonstrate that light propagation in waveguide arrays that include PT-symmetric structures can exhibit strongly nonlocal sensitivity to topology of the array at fixed other parameters. We consider an array composed of lossless waveguides, that includes a pair of PT-symmetric waveguides with balanced gain and loss, and reveal that PT-symmetry breaking thresholds are different for planar and circular array configurations. These results demonstrate that PT-symmetric structures can offer new regimes for optical beam shaping compared to conservative structures.

Slow-light enhanced optical forces between longitudinally shifted photonic-crystal nanowire waveguides

We reveal that slow-light enhanced optical forces between side-coupled photonic-crystal nanowire waveguides can be flexibly controlled by introducing a relative longitudinal shift. We predict that close to the photonic band edge, where the group velocity is reduced, the transverse force can be tuned from repulsive to attractive, and the force is suppressed for a particular shift value. Additionally the shift leads to symmetry breaking that can facilitate longitudinal forces acting on the waveguides, in contrast to unshifted structures where such forces vanish.

Combined frequency conversion and pulse compression in nonlinear tapered waveguides

We suggest an application of pump-degenerate four-wave mixing process in tapered waveguides for generation of ultrashort pulses with central frequency tunable over the material transparency range. Our method can produce strongly compressed frequency-converted pulses in presence of group-velocity mismatch and group-velocity dispersion. Additionally, the proposed technique does not require pulse phase synchronization and effectively operates for strongly chirped pump pulses, thus enabling the use of longer nonlinear media for high conversion efficiency.

Frozen and broadband slow light in coupled periodic nanowire waveguides

We develop novel designs enabling slow-light propagation with vanishing group-velocity dispersion ("frozen light") and slow-light with large delay-bandwidth product, in periodic nanowires. Our design is based on symmetry-breaking of periodic nanowire waveguides and we demonstrate its vailidy through two- and three-dimensional simulations. The slow-light is associated with a stationary inflection point which appears through coupling between forward and backward waveguide modes. The mode coupling also leads to evanescent modes, which enable efficient light coupling to the slow mode.

Spontaneous parametric down-conversion and quantum walks in arrays of quadratic nonlinear waveguides

We analyze the process of photon-pair generation with simultaneous quantum walks in a quadratic nonlinear waveguide array. We demonstrate that the spontaneous parametric down-conversion in the array allows for creating quantum states with strongly pronounced spatial correlations, which are qualitatively different from those possible in bulk crystals or through quantum walks in linear waveguide arrays. Most importantly, the photon correlations can be controlled entirely classically by varying the spatial profile of the pump beam or the phase-matching conditions.

Spectral pulse transformations and phase transitions in quadratic nonlinear waveguide arrays

We study experimentally and numerically the dynamics of a recently found topological phase transition for discrete quadratic solitons with linearly coupled SH waves. We find that, although no stationary states are excited in the experimental situation, the generic feature of the phase transition of the SH is preserved. By utilizing simulations of the coupled mode equations we identify the complex processes leading to the phase transition involving spatial focusing and the generation of new frequency components. These distinct signatures of the dynamic phase transition are also demonstrated experimentally.

Degenerate band edges in optical fiber with multiple grating: efficient coupling to slow light

Degenerate band edges (DBEs) of a photonic bandgap have the form (ω-ω(D)) ∝k(2m) for integers m>1, with ω(D) the frequency at the band edge. We show theoretically that DBEs lead to efficient coupling into slow-light modes without a transition region, and that the field strength in the slow mode can far exceed that in the incoming medium. A method is proposed to create a DBE of arbitrary order m by coupling m optical modes with multiple superimposed gratings. The enhanced coupling near a DBE occurs because of the presence of one or more evanescent modes, which are absent at conventional quadratic band edges. We furthermore show that the coupling can be increased or suppressed by varying the number of excited evanescent waves.

Realization of reflectionless potentials in photonic lattices

We realize experimentally a true reflectionless potential, which facilitates nonresonant unity transmission for all incident waves and at the same time supports localized modes. We utilize arrays of evanescently coupled optical waveguides, where a particular modulation of the transverse waveguide separations provides a physical realization of reflectionless Ablowitz-Ladik soliton potentials.

Control of light transmission in laser-written phase-shifted Bragg grating couplers

We demonstrate the fabrication by direct laser writing and the operation of a directional coupler containing Bragg gratings in each waveguide. We achieve high-precision control over the longitudinal shift between the gratings, which feature first-order Bragg resonance at telecommunication wavelengths. We observe fundamental differences between light transmission characteristics in couplers with unshifted and shifted gratings in agreement with theoretical predictions.

Experimental observation of evanescent modes at the interface to slow-light photonic crystal waveguides

We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is v(g) ~ c/10 to a waveguide in which it is v(g) ~ c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results.

Paired modes of heterostructure cavities in photonic crystal waveguides with split band edges

We investigate the modes of double heterostructure cavities where the underlying photonic crystal waveguide has been dispersion engineered to have two band-edges inside the Brillouin zone. By deriving and using a perturbative method, we show that these structures possess two modes. For unapodized cavities, the relative detuning of the two modes can be controlled by changing the cavity length, and for particular lengths, a resonant-like effect makes the modes degenerate. For apodized cavities no such resonances exist and the modes are always non-degenerate.

Phase transitions of nonlinear waves in quadratic waveguide arrays

We study two-color parametric nonlinear modes in waveguide arrays with a quadratic nonlinear response. We predict theoretically and observe experimentally a new type of phase transition manifested in an abrupt power-controlled change of the mode structure from unstaggered to staggered, due to the interplay of localization and synchronization in parametrically driven discrete systems.

All-optical switching of dark states in nonlinear coupled microring resonators

We propose and analyze an on-chip all-optical dynamic tuning scheme for coupled nonlinear resonators employing a single control beam injected in parallel with a signal beam. We show that the nonlinear Kerr response can be used to dynamically switch the spectral properties between a "dark state" and electromagnetically induced transparency configurations. Such a scheme can be realized in integrated optical applications for pulse trapping and delaying.

Lattice-controlled modulation instability in photorefractive feedback systems

We study the modulation instability in a two-dimensional nonlinear single feedback system with a photonic lattice and reveal a sharp transition in the instability regimes as the lattice strength is increased. For a shallow lattice, the instability modes are enhanced parallel to the lattice wave vector, while in stronger lattices, these modes are suppressed.

Focusing-to-defocusing crossover in nonlinear periodic structures

We demonstrate experimentally a transition from nonlinear beam trapping to defocusing in a two-dimensional periodic photonic structures by varying the modulation depth of the lattice. The observed effect illustrates the fundamental crossover from discrete to cw transport mechanisms. At the threshold modulation, the output beam is highly sensitive to refractive index and power variations, which can be potentially applied for high-sensitivity refractive index or temperature sensing.

Binary parity-time-symmetric nonlinear lattices with balanced gain and loss

We study nonlinear binary arrays composed of parity-time-symmetric optical waveguides with gain and loss. We demonstrate that such nonlinear binary lattices support stable discrete solitons, which can be adiabatically tuned and switched through nonlinear symmetry breaking by varying gain and loss parameters.

Analytical study of pulse amplification in silicon Raman amplifiers

The nonlinear process of stimulated Raman scattering is important for silicon photonics as it enables optical amplification and lasing. To understand the dynamics of silicon Raman amplifiers (SRAs), a numerical approach is generally employed, even though it provides little insight into the contribution of different SRA parameters to the signal amplification process. In this paper, we solve the coupled pump-signal equations analytically under realistic conditions, and derive an exact formula for the envelope of a signal pulse when picosecond optical pulses are amplified inside a SRA pumped by a continuous-wave laser beam. Our solution is valid for an arbitrary pulse shape and fully accounts for the Raman gain-dispersion effects, including temporal broadening and group-velocity reduction (a slow-light effect). It can be applied to any pumping scenario and leads to a simple analytic expression for the maximum optical delay produced by the Raman dispersion in a unidirectionally pumped SRA. We employ our analytical formulation to study the evolution of optical pulses with Gaussian, exponential, and Lorentzian shapes. The ability of a Gaussian pulse to maintain its shape through the amplifier makes it possible to realize soliton-like propagation of chirped Gaussian pulses in SRAs. We obtain analytical expressions for the required linear chirp and temporal width of a soliton-like pulse in terms of the net signal gain and the Raman-dispersion parameter. Our results are useful for optimizing the performance of SRAs and for engineering controllable signal delays.

Observation of two-dimensional dynamic localization of light

We report on the first experimental observation of dynamic localization of light in two-dimensional photonic lattices. We demonstrate suppression of beam diffraction in hexagonal lattices created by weakly coupled waveguides with axis bending. We also reveal that this effect is strongly related to dynamic localization in zigzag waveguide arrays with next-nearest neighboring interactions.

Polychromatic solitons and symmetry breaking in curved waveguide arrays

We theoretically and experimentally study the nonlinear propagation of polychromatic light in curved waveguide arrays. We show that at moderate light powers the nonlinear self-action breaks the left-right symmetry of the polychromatic beam, resulting in the separation of different spectral components owing to the wavelength-dependent spatial shift. At high light powers a diffraction-managed polychromatic soliton is formed. These results demonstrate new possibilities for tunable demultiplexing and spatial filtering of supercontinuum light.

Reflectionless potentials and cavities in waveguide arrays and coupled-resonator structures

We present an approach for the realization of reflectionless potentials in linear coupled-resonator optical waveguides or optical waveguide arrays with modulated coupling. We show that soliton solutions of nonlinear Ablowitz-Ladik equations can be used to define analytically reflectionless modulations for linear photonic structures. Such structures fully transmit incident waves and simultaneously support a pair of localized modes. These results suggest new possibilities for pulse and beam shaping, including a realization of cavities that do not reflect incident light.