Observation of soliton compression in silicon photonic crystals

Shortcuts to adiabatic soliton compression in active nonlinear Kerr media

We implement variational shortcuts to adiabaticity for optical pulse compression in an active nonlinear Kerr medium with distributed amplification and spatially varying dispersion and nonlinearity. Starting with the hyperbolic secant ansatz, we employ a variational approximation to systematically derive dynamical equations, establishing analytical relationships linking the amplitude, width, and chirp of the pulse. Through the inverse engineering approach, we manipulate the distributed gain/loss, nonlinearity and dispersion profiles to efficiently compress the optical pulse over a reduced distance with high fidelity. In addition, we explore the dynamical stability of the system to illustrate the advantage of our protocol over conventional adiabatic approaches. Finally, we analyze the impact of tailored higher-order dispersion on soliton self-compression and derive physical constraints on the final soliton width for the complementary case of soliton expansion. The broader implications of our findings extend beyond optical systems, encompassing areas such as cold-atom and magnetic systems highlighting the versatility and relevance of our approach in various physical contexts.

Experimental observation of linear pulses affected by high-order dispersion

We experimentally study the linear propagation of optical pulses affected by high-order dispersion. We use a programmable spectral pulse-shaper that applies a phase that equals that which would result from dispersive propagation. The temporal intensity profiles of the pulses are characterized using phase-resolved measurements. Our results are in very good agreement with previous numerical and theoretical results, confirming that for high dispersion orders m the central part of the pulses follow the same evolution, with m only determining the rate of evolution.

Supercontinuum in integrated photonics: generation, applications, challenges, and perspectives

Frequency conversion in nonlinear materials is an extremely useful solution to the generation of new optical frequencies. Often, it is the only viable solution to realize light sources highly relevant for applications in science and industry. In particular, supercontinuum generation in waveguides, defined as the extreme spectral broadening of an input pulsed laser light, is a powerful technique to bridge distant spectral regions based on single-pass geometry, without requiring additional seed lasers or temporal synchronization. Owing to the influence of dispersion on the nonlinear broadening physics, supercontinuum generation had its breakthrough with the advent of photonic crystal fibers, which permitted an advanced control of light confinement, thereby greatly improving our understanding of the underlying phenomena responsible for supercontinuum generation. More recently, maturing in fabrication of photonic integrated waveguides has resulted in access to supercontinuum generation platforms benefiting from precise lithographic control of dispersion, high yield, compact footprint, and improved power consumption. This Review aims to present a comprehensive overview of supercontinuum generation in chip-based platforms, from underlying physics mechanisms up to the most recent and significant demonstrations. The diversity of integrated material platforms, as well as specific features of waveguides, is opening new opportunities, as will be discussed here.

Design of multifunctional all-optical logic gates based on photonic crystal waveguides

The explosive development of the big data era has driven the rapid growth of silicon photonics, and logic operators based on photonic circuits have also been intensively investigated. Photonic integrated logic operators possess a high degree of design freedom and novel prospects, and they are regarded as promising platforms for future signaling and data processing. In this work, considering all-optical logic operation with lower power consumption and a smaller device footprint, multifunctional all-optical logic gates based on silicon photonic crystal (PhC) waveguides and phase-encoded light beams are proposed and applied to realize several logic operators, including XNOR, XOR, NOR, AND gates as well as a half adder and half subtractor. The initial phases (π and 0) of incident light represent the input digital states (1 and 0), and the logic operation results are determined by the output light intensity. Also, simulations are carried out to verify the proposed concept and to investigate the rise time, fall time, and cross talk of the devices. Theoretically, the bit rate for the proposed device can reach 1.25 Tb/s, and the proposed structures have the potential to be extremely compact due to PhC waveguides.

Thickness-dependent slow light gap solitons in three-dimensional coupled photonic crystal waveguides

The thickness-dependent multimodal nature of three-dimensional (3D) coupled photonic crystal waveguides is investigated with the aim of realizing a medium for controlled optical gap soliton formation in the slow light regime. In the linear case, spectral properties of the modes (dispersion diagrams), location of the gap regions versus the thickness of the 3D photonic crystal, and the near-field distributions at frequencies in the slow light region are analyzed using a full-wave electromagnetic solver. In the nonlinear regime (Kerr-type nonlinearity), we infer an existence of crystal-thickness-dependent temporal solitons with stable pulse envelope and use the solitonic pulses for driving quantum transitions in localized quantum systems within the photonic crystal waveguide. The results may be useful for applications in optical communications, multiplexing systems, nonlinear physics, and ultrafast spectroscopy.

Quantum fluids of light in all-optical scatterer lattices

One of the recently established paradigms in condensed matter physics is examining a system's behaviour in artificial potentials, giving insight into phenomena of quantum fluids in hard-to-reach settings. A prominent example is the matter-wave scatterer lattice, where high energy matter waves undergo transmission and reflection through narrow width barriers leading to stringent phase matching conditions with lattice band formation. In contrast to evanescently coupled lattice sites, the realisation of a scatterer lattice for macroscopic matter-wave fluids has remained elusive. Here, we implement a system of exciton-polariton condensates in a non-Hermitian Lieb lattice of scatterer potentials. By fine tuning the lattice parameters, we reveal a nonequilibrium phase transition between distinct regimes of polariton condensation: a scatterer lattice of gain guided polaritons condensing on the lattice potential maxima, and trapped polaritons condensing in the potential minima. Our results pave the way towards unexplored physics of non-Hermitian fluids in non-stationary mixtures of confined and freely expanding waves.

Soliton-effect compression of picosecond pulses on a photonic chip

We report soliton-effect pulse compression of low energy (∼25pJ), picosecond pulses on a photonic chip. An ultra-low-loss, dispersion-engineered 40-cm-long waveguide is used to compress 1.2-ps pulses by a factor of 18, which represents, to our knowledge, the largest compression factor yet experimentally demonstrated on-chip. Our scheme allows for interfacing with an on-chip picosecond source and offers a path towards a fully integrated stabilized frequency comb source.

Observation of Ultrashort Laser Pulse Evolution in a Silicon Photonic Crystal Waveguide

Using the sum frequency generation cross-correlation frequency-resolved optical gating (SFG-XFROG) measurement setup, we observed the soliton evolution of low energy pulse in an Si photonic crystal waveguide, and it exhibited the pulse broadening, blue shift, and evident pulse acceleration. The soliton evolution was also investigated by nonlinear Schrödinger equation (NLSE) modelling simulation, and the simulated results agreed well with the experimental measurements. The effects of waveguide length on the pulse evolution were analyzed; the results showed that the pulse width changed periodically with increasing waveguide length. The results further the understanding of the ultra-fast nonlinear dynamics of solitons in silicon waveguides, and are helpful to soliton-based functional elements on CMOS-compatible platforms.

High spectro-temporal compression on a nonlinear CMOS-chip

Optical pulses are fundamentally defined by their temporal and spectral properties. The ability to control pulse properties allows practitioners to efficiently leverage them for advanced metrology, high speed optical communications and attosecond science. Here, we report 11× temporal compression of 5.8 ps pulses to 0.55 ps using a low power of 13.3 W. The result is accompanied by a significant increase in the pulse peak power by 9.4×. These results represent the strongest temporal compression demonstrated to date on a complementary metal–oxide–semiconductor (CMOS) chip. In addition, we report the first demonstration of on-chip spectral compression, 3.0× spectral compression of 480 fs pulses, importantly while preserving the pulse energy. The strong compression achieved at low powers harnesses advanced on-chip device design, and the strong nonlinear properties of backend-CMOS compatible ultra-silicon-rich nitride, which possesses absence of two-photon absorption and 500× larger nonlinear parameter than in stoichiometric silicon nitride waveguides. The demonstrated work introduces an important new paradigm for spectro-temporal compression of optical pulses toward turn-key, on-chip integrated systems for all-optical pulse control.

Numerical simulation of all-optical logic gates based on hybrid-cavity semiconductor lasers

All-optical switch and multiple logic gates have been demonstrated using a hybrid-cavity semiconductor laser composed of a square microcavity and a Fabry-Perot cavity experimentally. In this paper, two-section tri-mode rate equations with optical injection terms are proposed and applied to study all-optical logic gates of NOT, NOR, and NAND operations utilizing the hybrid-cavity laser. Steady-state and dynamical characteristics of all-optical multiple logic gates are simulated, taking into account the influence of mode frequency detuning, gain suppression coefficients, mode Q factor, injection energy, and biasing current. All-optical logic NOT, NOR, and NAND gates up to 20, 15, and 20 Gbit/s are obtained numerically with dynamic extinction ratios of over 20, 20, and 10 dB, respectively, which are potential response speeds of the all-optical logic gates based on the hybrid-cavity semiconductor lasers.

Quantum Noise in Fibers with Arbitrary Nonlinear Profiles

In this Letter we introduce a novel equation addressing the effect of quantum noise in optical fibers with arbitrary frequency-dependent nonlinear profiles. To the best of our knowledge, such an endeavor has not been undertaken before despite the growing relevance of fiber optics in the design of new quantum devices. We show that the stochastic generalized nonlinear Schrödinger equation, derived from a quantum theory of optical fibers, leads to unphysical results such as a negative photon number and the appearance of a dominant anti-Stokes sideband when applied to this kind of waveguides. Starting from a recently introduced master-equation approach to propagation in fibers, we derive a novel stochastic photon-conserving nonlinear Schrödinger equation suitable for modeling arbitrary nonlinear profiles, thus greatly enhancing the study of fiber-based quantum devices.

Particle swarm optimization of silicon photonic crystal waveguide transition

Slow light generated through silicon (Si) photonic crystal waveguides (PCWs) is useful for improving the performance of Si photonic devices. However, the accumulation of coupling loss between a PCW and Si optical wiring waveguides is a problem when slow-light devices are connected in a series in a photonic integrated circuit. Previously, we reported a tapered transition structure between these waveguides and observed a coupling loss of 0.46 dB per transition. This Letter employed particle swarm optimization to engineer the arrangement of photonic crystal holes to reduce loss and succeeded in demonstrating theoretical loss value of 0.12 dB on average in the wavelength range of 1540-1560 nm and an experimental one of 0.21 dB. Crucially, this structure enhances the versatility of slow light.

Theory of soliton propagation in nonlinear photonic crystal waveguides

Propagation of the temporal soliton in Kerr-type photonic crystal waveguide is investigated theoretically and numerically. An expression describing the evolution of the envelope of the soliton based on the full-wave modal analysis, taking into account all space-harmonics, is rigorously obtained. The nonlinear coefficient is derived, for the first time, based on a modification of the refractive indices for each space-harmonic due to the Kerr-type nonlinearity. For illustrating the general formulation and results, we performed extensive computational electromagnetics simulations for the propagation of gap solitons in an experimentally feasible photonic crystal waveguides, endorsing the correctness and usefulness of the proposed formalism.

Linear Schrödinger equation with temporal evolution for front induced transitions

The nonlinear Schrödinger equation based on slowly varying approximation is usually applied to describe the pulse propagation in nonlinear waveguides. However, for the case of the front induced transitions (FITs), the pump effect is well described by the dielectric constant perturbation in space and time. Thus, a linear Schrödinger equation (LSE) can be used. Also, in waveguides with weak dispersion the spatial evolution of the pulse temporal profile is usually tracked. Such a formulation becomes impossible for optical systems for which the group index or higher dispersion terms diverge as is the case near the band edge of photonic crystals. For the description of FITs in such systems a linear Schrödinger equation can be used where temporal evolution of the pulse spatial profile is tracked instead of tracking the spatial evolution. This representation provides the same descriptive power and can easily deal with zero group velocities. Furthermore, the Schrödinger equation with temporal evolution can describe signal pulse reflection from both static and counter-propagating fronts, in contrast to the Schrödinger equation with spatial evolution which is bound to forward propagation. Here, we discuss the two approaches and apply the LSE with temporal evolution for systems close to the band edge where the group velocity vanishes by simulating intraband indirect photonic transitions. We also compare the numerical results with the theoretical predictions from the phase continuity criterion for complete transitions.

Photon acceleration and tunable broadband harmonics generation in nonlinear time-dependent metasurfaces

Time-dependent nonlinear media, such as rapidly generated plasmas produced via laser ionization of gases, can increase the energy of individual laser photons and generate tunable high-order harmonic pulses. This phenomenon, known as photon acceleration, has traditionally required extreme-intensity laser pulses and macroscopic propagation lengths. Here, we report on a novel nonlinear material—an ultrathin semiconductor metasurface—that exhibits efficient photon acceleration at low intensities. We observe a signature nonlinear manifestation of photon acceleration: third-harmonic generation of near-infrared photons with tunable frequencies reaching up to ≈3.1ω. A simple time-dependent coupled-mode theory, found to be in good agreement with experimental results, is utilized to predict a new path towards nonlinear radiation sources that combine resonant upconversion with broadband operation.

Photon acceleration, which can be used to generate tunable high harmonic radiation, typically requires high-intensity lasers and long propagation distances. Here, Shcherbakov et al. show efficient photon acceleration at low power input power from a semiconductor metasurface, less than a micron thin.

Femtosecond filamentation and supercontinuum generation in bulk silicon

We experimentally study filamentation and supercontinuum generation in bulk silicon crystal using femtosecond mid-infrared pulses with carrier wavelengths in the range of 3.25-4.7 μm, in the presence of three-, four-, and five-photon absorption. Spectral measurements show a fairly stable blueshifted cutoff in the 2.5-2.7 μm range and gradual increase of the long-wave extent with increasing wavelength of the incident pulses, eventually yielding an octave-spanning supercontinuum, covering the wavelength range from 2.5 to 5.8 μm with the input pulses at 4.7 μm. The recorded spatiotemporal intensity distributions of a single filament revealed pulse splitting after the nonlinear focus, in line with the pulse-splitting-based filamentation scenario inherent to normally dispersive dielectric nonlinear media.

Topological protection of biphoton states

The robust generation and propagation of multiphoton quantum states are crucial for applications in quantum information, computing, and communications. Although photons are intrinsically well isolated from the thermal environment, scaling to large quantum optical devices is still limited by scattering loss and other errors arising from random fabrication imperfections. The recent discoveries regarding topological phases have introduced avenues to construct quantum systems that are protected against scattering and imperfections. We experimentally demonstrate topological protection of biphoton states, the building block for quantum information systems. We provide clear evidence of the robustness of the spatial features and the propagation constant of biphoton states generated within a nanophotonics lattice with nontrivial topology and propose a concrete path to build robust entangled states for quantum gates.

Physical origin of higher-order soliton fission in nanophotonic semiconductor waveguides

Supercontinuum generation in Kerr media has become a staple of nonlinear optics. It has been celebrated for advancing the understanding of soliton propagation as well as its many applications in a broad range of fields. Coherent spectral broadening of laser light is now commonly performed in laboratories and used in commercial “white light” sources. The prospect of miniaturizing the technology is currently driving experiments in different integrated platforms such as semiconductor on insulator waveguides. Central to the spectral broadening is the concept of higher-order soliton fission. While widely accepted in silica fibers, the dynamics of soliton decay in semiconductor waveguides is yet poorly understood. In particular, the role of nonlinear loss and free carriers, absent in silica, remains an open question. Here, through experiments and simulations, we show that nonlinear loss is the dominant perturbation in wire waveguides, while free-carrier dispersion is dominant in photonic crystal waveguides.

Realization of true all-optical AND logic gate based on nonlinear coupled air-hole type photonic crystal waveguides

In this manuscript we propose an easily scalable true all-optical AND logic gate for pulsed signal operation based on band-gap transmission within nonlinear realistic air-hole type coupled photonic crystal waveguides (C-PCW). We call it "true" all-optical AND logic gate, because all AND gate topologies operate with temporal solitons that maintain a stable pulse envelope during the optical signal processing along the different C-PCW modules yielding ultrafast full-optical digital signal processing. We directly use the registered (output) signal pulse as new input signal between multiple concatenated nonlinear C-PCW modules (i.e. AND gates) to setup a multiple-input true all-optical AND logic gate. Extensive full-wave computational electromagnetic analysis proves the correctness of our theoretical studies and the proposed operation principle of the multiple-input AND logic gate is vividly demonstrated for realistic C-PCWs.

Reflection from a free carrier front via an intraband indirect photonic transition

The reflection of light from moving boundaries is of interest both fundamentally and for applications in frequency conversion, but typically requires high pump power. By using a dispersion-engineered silicon photonic crystal waveguide, we are able to achieve a propagating free carrier front with only a moderate on-chip peak power of 6 W in a 6 ps-long pump pulse. We employ an intraband indirect photonic transition of a co-propagating probe, whereby the probe practically escapes from the front in the forward direction. This forward reflection has up to 35% efficiency and it is accompanied by a strong frequency upshift, which significantly exceeds that expected from the refractive index change and which is a function of group velocity, waveguide dispersion and pump power. Pump, probe and shifted probe all are around 1.5 µm wavelength which opens new possibilities for “on-chip” frequency manipulation and all-optical switching in optical telecommunications.

Here the authors present an experimental demonstration of a free carrier front induced intraband indirect photonic transition and show how the waveguide dispersion can be exploited to decrease the required free carrier concentration for substantial reflection via an indirect intraband transition.

On-chip temporal focusing of elastic waves in a phononic crystal waveguide

The ability to manipulate acoustic and elastic waveforms in continuous media has attracted significant research interest and is crucial for practical applications ranging from biological imaging to material characterization. Although several spatial focusing techniques have been developed, these systems require sophisticated resonant structures with narrow bandwidth, which limit their practical applications. Here we demonstrate temporal pulse manipulation in a dispersive one-dimensional phononic crystal waveguide, which enables the temporal control of ultrasonic wave propagation. On-chip pulse focusing is realized at a desired time and position with chirped input pulses that agree perfectly with the theoretical prediction. Moreover, traveling four-wave mixing experiments are implemented, providing a platform on which to realize novel nonlinear phenomena in the system. Incorporating this dispersive pulse engineering scheme into nonlinear phononic crystal architecture opens up the possibility of investigating novel phenomena such as phononic solitons.

Here the authors demonstrate the temporal control of ultrasonic wave propagation in a one-dimensional phononic crystal waveguide. Four-wave mixing experiments are implemented, providing a platform on which to realize novel nonlinear phenomena in the system.

Aberrated surface soliton formation in a nonlinear 1D and 2D photonic crystal

We discuss a novel type of surface soliton-aberrated surface soliton-appearance in a nonlinear one dimensional photonic crystal and a possibility of this surface soliton formation in two dimensional photonic crystal. An aberrated surface soliton possesses a nonlinear distribution of the wavefront. We show that, in one dimensional photonic crystal, the surface soliton is formed at the photonic crystal boundary with the ambient medium. Essentially, that it occupies several layers at the photonic crystal boundary and penetrates into the ambient medium at a distance also equal to several layers, so that one can infer about light energy localization at the lateral surface of the photonic crystal. In the one dimensional case, the surface soliton is formed from an earlier formed soliton that falls along the photonic crystal layers at an angle which differs slightly from the normal to the photonic crystal face. In the two dimensional case, the soliton can appear if an incident Gaussian beam falls on the photonic crystal face. The influence of laser radiation parameters, optical properties of photonic crystal layers and ambient medium on the one dimensional surface soliton formation is investigated. We also discuss the influence of two dimensional photonic crystal configuration on light energy localization near the photonic crystal surface. It is important that aberrated surface solitons can be created at relatively low laser pulse intensity and for close values of alternating layers dielectric permittivity which allows their experimental observation.

Analysis and design of fibers for pure-quartic solitons

The recently discovered pure-quartic solitons, arising from the interaction of quartic dispersion and Kerr nonlinearity, open the door to unexplored soliton regimes and ultrafast laser science. Here, we report a general analysis of the dispersion and nonlinear properties necessary to observe pure-quartic solitons in optical platforms. We apply this analysis, in combination with numerical calculations, to the design of pure-quartic soliton supporting microstructured optical fibers. The designs presented here, which have realistic fabrication tolerances, support unperturbed pure-quartic soliton propagation providing access to an unmatched platform to study novel soliton physics.

Integrated Optical Modulator Based on Transition between Photonic Bands

An area efficient novel optical modulator with low operation voltage is designed based on integrated Mach-Zehnder Interferometer with a photonic crystal slab structure as the phase shifter. Plasma dispersion effect is utilized so that photonic band-to-band transition occurs at the operating frequency leading to a high index change (Δn = ~4) for π-phase shift on the modulator. This approach reduces the phase shifter length to a few micrometers (~5 µm) in a silicon on insulator platform and operating voltage required is around 1 V. Low voltage together with short optical interaction length decrease optical losses and power consumption during modulation process providing a great opportunity for size and system cost optimization.

Optimized optical coupling to silica-clad photonic crystal waveguides

Silica-clad silicon photonic crystal waveguides (PCWs) are promising components for various applications because of their simple fabrication and generation of slow light. However, an optical loss higher than 4 dB occurs when they are simply coupled to input/output silicon wire waveguides. To reduce the optical loss, we proposed a junction structure in which light in the waveguide is first coupled to a high-group-velocity radiation mode at an expanded core and subsequently converted to the slow-light mode in a tapered core of the PCW. The coupling loss at a junction is calculated to be 0.28 dB at its minimum and less than 0.5 dB for the wavelength range of 12 nm. We measured a coupling loss of 0.46 dB for the device fabricated by the silicon photonics process. This low-loss junction well supports the practical use of PCWs.

Real-time dynamics and cross-correlation gating spectroscopy of free-carrier Drude slow-light solitons

Optical solitons-stable waves balancing delicately between nonlinearities and dispersive effects-have advanced the field of ultrafast optics and dynamics, with contributions spanning from supercontinuum generation to soliton fission, optical event horizons, Hawking radiation and optical rogue waves, among others. Here, we investigate picojoule soliton dynamics in silicon slow-light, photonic-bandgap waveguides under the influence of Drude-modeled, free-carrier-induced nonlinear effects. Using real-time and single-shot amplified dispersive Fourier transform spectroscopy simultaneously with high-fidelity cross-correlation frequency resolved optical gating at femtojoule sensitivity and femtosecond resolution, we examine the soliton stability limits, the soliton dynamics including free-carrier quartic slow-light scaling and acceleration, and the Drude electron-hole plasma-induced perturbations in the Cherenkov radiation and modulation instability. Our real-time single-shot and time-averaged cross-correlation measurements are matched with our detailed theoretical modeling, examining the reduced group velocity free-carrier kinetics on solitons at the picojoule scale.

Parasitic Photon-Pair Suppression via Photonic Stop-Band Engineering

We calculate that an appropriate modification of the field associated with only one of the photons of a photon pair can suppress generation of the pair entirely. From this general result, we develop a method for suppressing the generation of undesired photon pairs utilizing photonic stop bands. For a third-order nonlinear optical source of frequency-degenerate photons, we calculate the modified frequency spectrum (joint spectral intensity) and show a significant increase in a standard metric, the coincidence to accidental ratio. These results open a new avenue for photon-pair frequency correlation engineering.

Pushing the limits of CMOS optical parametric amplifiers with USRN:Si7N3 above the two-photon absorption edge

CMOS platforms operating at the telecommunications wavelength either reside within the highly dissipative two-photon regime in silicon-based optical devices, or possess small nonlinearities. Bandgap engineering of non-stoichiometric silicon nitride using state-of-the-art fabrication techniques has led to our development of USRN (ultra-silicon-rich nitride) in the form of Si₇N₃, that possesses a high Kerr nonlinearity (2.8 × 10⁻¹³ cm² W⁻¹), an order of magnitude larger than that in stoichiometric silicon nitride. Here we experimentally demonstrate high-gain optical parametric amplification using USRN, which is compositionally tailored such that the 1,550 nm wavelength resides above the two-photon absorption edge, while still possessing large nonlinearities. Optical parametric gain of 42.5 dB, as well as cascaded four-wave mixing with gain down to the third idler is observed and attributed to the high photon efficiency achieved through operating above the two-photon absorption edge, representing one of the largest optical parametric gains to date on a CMOS platform.

Typical CMOS materials in the telecommunications band suffer from two-photon absorption or possess weak Kerr nonlinearities. Here, Ooi et al. demonstrate 42.5 dB optical parametric amplification in ultra-silicon-rich nitride waveguides, designed to have strong nonlinearities with negligible losses.

Finely engineered slow light photonic crystal waveguides for efficient wideband wavelength-independent higher-order temporal solitons

By orthogonally dual-shifting the air-hole rows in the triangular photonic crystal waveguide, a novel finely engineered slow light silicon photonic crystal waveguide is designed for higher-order temporal solitons and ultrashort temporal pulse compression with a large fabrication tolerance. The engineering of dispersion provides the waveguide with a wide wavelength range with only low anomalous dispersion covering, which makes the compression ratio wavelength-independent and stable even under ultralow input pulse energy. The simulation results are based on nonlinear Schrödinger equation modeling, which demonstrates that the input picosecond pulses in the broad wavelength range with ultralow pJ pulse energy can be stably compressed by a factor of 6 to higher-order temporal solitons in a 250 μm short waveguide.

Free-carrier-induced soliton fission unveiled by in situ measurements in nanophotonic waveguides

Solitons are localized waves formed by a balance of focusing and defocusing effects. These nonlinear waves exist in diverse forms of matter yet exhibit similar properties including stability, periodic recurrence and particle-like trajectories. One important property is soliton fission, a process by which an energetic higher-order soliton breaks apart due to dispersive or nonlinear perturbations. Here we demonstrate through both experiment and theory that nonlinear photocarrier generation can induce soliton fission. Using near-field measurements, we directly observe the nonlinear spatial and temporal evolution of optical pulses in situ in a nanophotonic semiconductor waveguide. We develop an analytic formalism describing the free-carrier dispersion (FCD) perturbation and show the experiment exceeds the minimum threshold by an order of magnitude. We confirm these observations with a numerical nonlinear Schrödinger equation model. These results provide a fundamental explanation and physical scaling of optical pulse evolution in free-carrier media and could enable improved supercontinuum sources in gas based and integrated semiconductor waveguides.

Solitons are nonlinear waves that exist in diverse forms of matter. Here, Husko et al. use near-field measurements to observe the spatio-temporal evolution of optical pulses in a nanophotonic semiconductor waveguide, demonstrating that nonlinear photo-carrier generation can induce fission of solitons.

Pure-quartic solitons

Temporal optical solitons have been the subject of intense research due to their intriguing physics and applications in ultrafast optics and supercontinuum generation. Conventional bright optical solitons result from the interaction of anomalous group-velocity dispersion and self-phase modulation. Here we experimentally demonstrate a class of bright soliton arising purely from the interaction of negative fourth-order dispersion and self-phase modulation, which can occur even for normal group-velocity dispersion. We provide experimental and numerical evidence of shape-preserving propagation and flat temporal phase for the fundamental pure-quartic soliton and periodically modulated propagation for the higher-order pure-quartic solitons. We derive the approximate shape of the fundamental pure-quartic soliton and discover that is surprisingly Gaussian, exhibiting excellent agreement with our experimental observations. Our discovery, enabled by precise dispersion engineering, could find applications in communications, frequency combs and ultrafast lasers.

Optical solitons are pulses that propagate undistorted. Here, the authors demonstrate a class of soliton arising from the interaction of self-phase modulation with quartic dispersion, rather than with quadratic dispersion as occurs in conventional solitons.

Ultra-fast pulse propagation in nonlinear graphene/silicon ridge waveguide

We report the femtosecond laser propagation in a hybrid graphene/silicon ridge waveguide with demonstration of the ultra-large Kerr coefficient of graphene. We also fabricated a slot-like graphene/silicon ridge waveguide which can enhance its effective Kerr coefficient 1.5 times compared with the graphene/silicon ridge waveguide. Both transverse-electric-like (TE-like) mode and transverse-magnetic-like (TM-like) mode are experimentally measured and numerically analyzed. The results show nonlinearity dependence on mode polarization not in graphene/silicon ridge waveguide but in slot-like graphene/silicon ridge waveguide. Great spectral broadening was observed due to self-phase modulation (SPM) after propagation in the hybrid waveguide with length of 2 mm. Power dependence property of the slot-like hybrid waveguide is also measured and numerically analyzed. The results also confirm the effective Kerr coefficient estimation of the hybrid structures. Spectral blue shift of the output pulse was observed in the slot-like graphene/silicon ridge waveguide. One possible explanation is that the blue shift was caused by the ultra-fast free carrier effect with the optical absorption of the doped graphene. This interesting effect can be used for soliton compression in femtosecond region. We also discussed the broadband anomalous dispersion of the Kerr coefficient of graphene.

Ultra-low-power hybrid light-matter solitons

New functionalities in nonlinear optics will require systems with giant optical nonlinearity as well as compatibility with photonic circuit fabrication techniques. Here we introduce a platform based on strong light-matter coupling between waveguide photons and quantum-well excitons. On a sub-millimetre length scale we generate picosecond bright temporal solitons at a pulse energy of only 0.5 pJ. From this we deduce a nonlinear refractive index three orders of magnitude larger than in any other ultrafast system. We study both temporal and spatio-temporal nonlinear effects and observe dark-bright spatio-temporal polariton solitons. Theoretical modelling of soliton formation in the strongly coupled system confirms the experimental observations. These results show the promise of our system as a high speed, low power, integrated platform for physics and devices based on strong interactions between photons.

Unambiguous demonstration of soliton evolution in slow-light silicon photonic crystal waveguides with SFG-XFROG

We demonstrate the temporal and spectral evolution of picosecond soliton in the slow light silicon photonic crystal waveguides (PhCWs) by sum frequency generation cross-correlation frequency resolved optical grating (SFG-XFROG) and nonlinear Schrödinger equation (NLSE) modeling. The reference pulses for the SFG-XFROG measurements are unambiguously pre-characterized by the second harmonic generation frequency resolved optical gating (SHG-FROG) assisted with the combination of NLSE simulations and optical spectrum analyzer (OSA) measurements. Regardless of the inevitable nonlinear two photon absorption, high order soliton compressions have been observed remarkably owing to the slow light enhanced nonlinear effects in the silicon PhCWs. Both the measurements and the further numerical analyses of the pulse dynamics indicate that, the free carrier dispersion (FCD) enhanced by the slow light effects is mainly responsible for the compression, the acceleration, and the spectral blue shift of the soliton.

Ultra-large nonlinear parameter in graphene-silicon waveguide structures

Mono-layer graphene integrated with optical waveguides is studied for the purpose of maximizing E-field interaction with the graphene layer, for the generation of ultra-large nonlinear parameters. It is shown that the common approach used to minimize the waveguide effective modal area does not accurately predict the configuration with the maximum nonlinear parameter. Both photonic and plasmonic waveguide configurations and graphene integration techniques realizable with today's fabrication tools are studied. Importantly, nonlinear parameters exceeding 10(4) W(-1)/m, two orders of magnitude larger than that in silicon on insulator waveguides without graphene, are obtained for the quasi-TE mode in silicon waveguides incorporating mono-layer graphene in the evanescent part of the optical field. Dielectric loaded surface plasmon polariton waveguides incorporating mono-layer graphene are observed to generate nonlinear parameters as large as 10(5) W(-1)/m, three orders of magnitude larger than that in silicon on insulator waveguides without graphene. The ultra-large nonlinear parameters make such waveguides promising platforms for nonlinear integrated optics at ultra-low powers, and for previously unobserved nonlinear optical effects to be studied in a waveguide platform.

Dispersive wave emission and supercontinuum generation in a silicon wire waveguide pumped around the 1550 nm telecommunication wavelength

We experimentally and numerically study dispersive wave emission, soliton fission, and supercontinuum generation in a silicon wire at telecommunication wavelengths. Through dispersion engineering, we experimentally confirm a previously reported numerical study and show that the emission of resonant radiation from the solitons can lead to the generation of a supercontinuum spanning over 500 nm. An excellent agreement with numerical simulations is observed.

Pulse compression in adiabatically tapered silicon photonic wires

We present a comprehensive analysis of pulse compression in adiabatically tapered silicon photonic wire waveguides (Si-PhWWGs), both at telecom (λ ∼ 1.55 μm) and mid-IR (λ ≳ 2.1 μm) wavelengths. Our theoretical and computational study is based on a rigorous model that describes the coupled dynamics of the optical field and photogenerated free carriers, as well as the influence of the physical and geometrical parameters of the Si-PhWWGs on these dynamics. We consider both the soliton and non-soliton pulse propagation regimes, rendering the conclusions of this study relevant to a broad range of experimental settings and practical applications. In particular, we show that by engineering the linear and nonlinear optical properties of Si-PhWWGs through adiabatically varying their width, one can achieve more than 10× pulse compression in millimeter-long waveguides. The inter-dependence between the pulse characteristics and compression efficiency is also discussed.