Integrated optical auto-correlator based on third-harmonic generation in a silicon photonic crystal waveguide

Phase-matched third-harmonic generation in silicon nitride waveguides

Third-harmonic generation (THG) in silicon nitride waveguides is an ideal source of coherent visible light, suited for ultrafast pulse characterization, telecom signal monitoring and self-referenced comb generation due to its relatively large nonlinear susceptibility and CMOS compatibility. We demonstrate third-harmonic generation in silicon nitride waveguides where a fundamental transverse mode at 1,596 nm is phase-matched to a TM02 mode at 532 nm, confirmed by the far-field image. We experimentally measure the waveguide width-dependent phase-matched wavelength with a peak-power-normalized conversion efficiency of 5.78 × 10-7 %/W2 over a 660-μm-long interaction length.

Active control of mid-wavelength infrared non-linearity in silicon photonic crystal slab

Natural materials' inherently weak nonlinear response demands the design of artificial substitutes to avoid optically large samples and complex phase-matching techniques. Silicon photonic crystals are promising artificial materials for this quest. Their nonlinear properties can be modulated optically, paving the way for applications ranging from ultrafast information processing to quantum technologies. A two-dimensional 15-μm-thick silicon photonic structure, comprising a hexagonal array of air holes traversing the slab's thickness, has been designed to support a guided resonance for the light with a wavelength of 4-μm. At the resonance conditions, a transverse mode of the light is strongly confined between the holes in the "veins" of the silicon component. Owing to the confinement, the structure exhibits a ratio of nonlinear to linear absorption coefficients threefold higher than the uniform silicon slab of the same thickness. A customised time-resolved Z-scan method with provisions to accommodate ultrafast pump-probe measurements was used to investigate and quantify the non-linear response. We show that optically pumping free charge carriers into the structure decouples the incoming light from the resonance and reduces the non-linear response. The time-resolved measurements suggest that the decoupling is a relatively long-lived effect on the scale comparable to the non-radiative recombination in the bulk material. Moreover, we demonstrate that the excited free carriers are not the source of the nonlinearity, as this property is determined by the structure design.

Third-Order Optical Nonlinearities of 2D Materials at Telecommunications Wavelengths

All-optical signal processing based on nonlinear optical devices is promising for ultrafast information processing in optical communication systems. Recent advances in two-dimensional (2D) layered materials with unique structures and distinctive properties have opened up new avenues for nonlinear optics and the fabrication of related devices with high performance. This paper reviews the recent advances in research on third-order optical nonlinearities of 2D materials, focusing on all-optical processing applications in the optical telecommunications band near 1550 nm. First, we provide an overview of the material properties of different 2D materials. Next, we review different methods for characterizing the third-order optical nonlinearities of 2D materials, including the Z-scan technique, third-harmonic generation (THG) measurement, and hybrid device characterization, together with a summary of the measured n₂ values in the telecommunications band. Finally, the current challenges and future perspectives are discussed.

Transmission characteristics of femtosecond laser pulses in a polymer waveguide

Femtosecond lasers have been widely employed in scientific and industrial applications, including the study of material properties, fabrication of structures on the sub-micrometer scale, surgical and medical treatment, etc. In these applications, the ultrafast laser is implemented either in free space or via an optical fiber-based channel. To investigate the light-matter interaction on a chip-based dimension, laser pulses with extremely high peak power need to be injected into an integrated optical waveguide. This requires the waveguide to be transparent and linear at this power, but also capable of providing a highly efficient and reliable interface for fiber-chip coupling. Contrary to the common belief that polymer materials may suffer from stability issues, we show that a polymer waveguide fabricated under simple and low-cost technology using only commercial materials can indeed transmit femtosecond laser pulses with similar characteristics as low-power continuous-wave laser. The coupling efficiency with a lensed fiber is ∼76% per facet. The pulse broadening effect in the polymer waveguide is also well fitted by the material and waveguide dispersion without nonlinear behavior. This study paves the way for developing a low-cost, highly efficient, polymer-based waveguide platform for the investigation of ultrafast phenomena on a chip.

Coherent modulation of chiral nonlinear optics with crystal symmetry

Light modulation is of paramount importance for photonics and optoelectronics. Here we report all-optical coherent modulation of third-harmonic generation (THG) with chiral light via the symmetry enabled polarization selectivity. The concept is experimentally validated in monolayer materials (MoS₂) with modulation depth approaching ~100%, ultra-fast modulation speed (<~130 fs), and wavelength-independence features. Moreover, the power and polarization of the incident optical beams can be used to tune the output chirality and modulation performance. Major performance of our demonstration reaches the fundamental limits of optical modulation: near-unity modulation depth, instantaneous speed (ultra-fast coherent interaction), compact footprint (atomic thickness), and unlimited operation bandwidth, which hold an ideal optical modulation solution for emerging and future nonlinear optical applications (e.g., interconnection, imaging, computing, and quantum technologies).

Polarization selective ultra-broadband wavelength conversion in silicon nitride waveguides

We experimentally demonstrate broadband degenerate continuous-wave four-wave mixing in long silicon nitride (Si₃N₄) waveguides for operation both in the telecommunication L-band and the thulium band near 2 µm by leveraging polarization dependence of the waveguide dispersion. Broadband conversion is typically demonstrated in short milimeter long waveguides as the bandwidth is linked to the interaction length. This makes it challenging to simultaneously push bandwidth and efficiency, imposing stringent constraints on dispersion engineering. In this work, we show conversion bandwidths larger than 150 nm in the L-band when pumping in the transverse magnetic (TM) mode and larger than 120 nm at 2 µm when using transverse electric excitation, despite the use of 0.5 m long waveguides. In addition, we also show how extreme polarization selectivity can be leveraged in a single waveguide to enable switchable distant phase-matching based on higher-order dispersion. Relying on this approach, we demonstrate the selective conversion of light from the telecom band to the O-band for TM polarization or to the mid-infrared light up to 2.5 µm in TE. Our experiments are in excellent agreement with simulations, showing the high potential of the platform for broadband and distant conversion beyond the telecom band.

Graphene Oxide for Integrated Photonics and Flat Optics

With superior optical properties, high flexibility in engineering its material properties, and strong capability for large-scale on-chip integration, graphene oxide (GO) is an attractive solution for on-chip integration of 2D materials to implement functional integrated photonic devices capable of new features. Over the past decade, integrated GO photonics, representing an innovative merging of integrated photonic devices and thin GO films, has experienced significant development, leading to a surge in many applications covering almost every field of optical sciences such as photovoltaics, optical imaging, sensing, nonlinear optics, and light emitting. This paper reviews the recent advances in this emerging field, providing an overview of the optical properties of GO as well as methods for the on-chip integration of GO. The main achievements made in GO hybrid integrated photonic devices for diverse applications are summarized. The open challenges as well as the potential for future improvement are also discussed.

2D Layered Graphene Oxide Films Integrated with Micro-Ring Resonators for Enhanced Nonlinear Optics

Layered 2D graphene oxide (GO) films are integrated with micro-ring resonators (MRRs) to experimentally demonstrate enhanced nonlinear optics. Both uniformly coated (1-5 layers) and patterned (10-50 layers) GO films are integrated on complementary-metal-oxide-semiconductor (CMOS)-compatible doped silica MRRs using a large-area, transfer-free, layer-by-layer GO coating method with precise control of the film thickness. The patterned devices further employ photolithography and lift-off processes to enable precise control of the film placement and coating length. Four-wave-mixing (FWM) measurements for different pump powers and resonant wavelengths show a significant improvement in efficiency of ≈7.6 dB for a uniformly coated device with 1 GO layer and ≈10.3 dB for a patterned device with 50 GO layers. The measurements agree well with theory, with the enhancement in FWM efficiency resulting from the high Kerr nonlinearity and low loss of the GO films combined with the strong light-matter interaction within the MRRs. The dependence of GO's third-order nonlinearity on layer number and pump power is also extracted from the FWM measurements, revealing interesting physical insights about the evolution of the GO films from 2D monolayers to quasi bulk-like behavior. These results confirm the high nonlinear optical performance of integrated photonic resonators incorporated with 2D layered GO films.

A pulsewidth measurement technology based on carbon-nanotube saturable absorber

We demonstrate a proof-of-concept saturable absorption based pulsewidth measurement (SAPM) by exploring the intensity dependent nonlinear transmission (i.e., saturable absorption) of low-dimensional material (LDM) carbon nanotubes. A minimum pulse energy of 75 fJ is experimentally detected with an average-power-peak-power product (P_av⋅ P_pk) of 5.44×10^-7 W² near 1550 nm. A minimum detectable pulse energy of 10 fJ with a P_av⋅ P_pk of 1.3×10^-9 W² is estimated with further optimization. The nanometer-level thickness and femtosecond-level decay time of LDMs allow ultrafast light interaction on a very small footprint, which potentially supports chip-scale characterization of ultrafast pulses with minimum distortion.

Toroidal Dipole-Enhanced Third Harmonic Generation of Deep Ultraviolet Light Using Plasmonic Meta-atoms

The harmonic generation of light with plasmonic and all-dielectric nanostructures has gained much recent interest. This approach is especially promising for short wavelength (i.e., ultraviolet (UV)) generation, where conventional nonlinear crystals reach their limits both in transparency and in their ability to achieve phase-matching between the input and output fields. Here, we demonstrate that the third harmonic generation of deep UV light in an indium tin oxide (ITO) film can be substantially enhanced by a metasurface consisting of metallic toroidal meta-atoms covered with an alumina layer for protection against laser-induced damage. This approach combines the benefits of the large nonlinear susceptibility of ITO with the unique field enhancement properties of a toroidal metasurface. This ITO-meta-atom combination produces a third harmonic signal at a wavelength of 262 nm that is nominally five times larger than that of an ITO film patterned with a conventional hotspot-enhanced plasmonic dimer array. This result demonstrates the potential for toroidal meta-atoms as the active engineered element in a new generation of enhanced nonlinear optical materials and devices.

Fano-resonant silicon photonic crystal slab for efficient third-harmonic generation

Strong light-matter interactions in resonant photonic nanostructures open up opportunities for enhancing nonlinear responses. In this work, by applying Fano resonances, we experimentally demonstrate efficient third-harmonic generation (THG) obtained with 2D silicon photonic crystal slabs (PCSs) thanks to the field enhancement in the dielectric layer. A 160-fold enhancement of THG is observed in the silicon PCS compared to the unpatterned silicon film. Through slightly changing the radius of the PCS, tunable THG on a single chip is obtained, paving a way for the optical manipulation of harmonic generation.

High-Efficiency Plasmonic Third-Harmonic Generation with Graphene on a Silicon Diffractive Grating in Mid-infrared Region

Benefiting from the large third-order nonlinear susceptibility of graphene and significantly enhanced field intensity of graphene plasmons (GPs), graphene has shown great potentials to enhance plasmonic third-harmonic generation conversion efficiency. However, it still lacks an effective configuration that can excite the fundamental frequency (FF) GPs and guide the generated third-harmonic frequency (THF) GPs simultaneously. Here, we have proposed a diffractive silicon grating underneath a graphene sheet to generate and transmit THF GPs. The FF GPs are efficiently excited by illuminating a normal-incidence plane wave due to guided-mode resonance and then are converted to the THF GPs with a large conversion efficiency, originating from the giant field intensity of the FF GPs. We numerically demonstrate that, a large third-harmonic generation conversion efficiency of 3.68 × 10^-7 can be realized with a small incident power density of 0.19 MW/cm² at 28.62 μm. Furthermore, the generated THF GPs can be efficiently guided along low-loss GP waveguides that are connected to both sides of grating section. Our results can stimulate making graphene-based light sources for mid- and far-infrared silicon photonics.

Interferometric autocorrelation of ultrafast optical pulses in silicon sub-micrometer p-i-n waveguides

We investigated the high-sensitivity interferometric autocorrelation of ultrafast optical pulses utilizing two-photon absorption in sub-micrometer silicon p-i-n waveguides. The autocorrelation sensitivities were evaluated to be about 0.5 and 4.5 × 10^-8 W² for 1- and 0.5-mm devices, respectively. Such sensitivities are about 100 times higher than the traditional two-photon conductivity photodetectors in commercial autocorrelators; thus favor weak pulse characterization. We comprehensively studied the interferometric autocorrelation performances by the experiment and FDTD (finite-difference time-domain) simulation. The pulse energy dependences of measured autocorrelation photocurrents and pulse widths were well explained by the simulation with the free carrier absorption and free carrier plasma effect considered. The autocorrelation error tends to occur if the pulse energy is high enough to cause strong free carrier effects and the threshold pulse energy for error occurrence is increased for shorter devices, but accurate autocorrelation measurement was achieved for sub-Watts pulses at which the influences of free carrier effects on interferometric autocorrelation was negligible. The minimum applicable range of pulse widths was estimated from waveguide dispersion analysis to be ~0.09 and 0.13 ps with a 10% target error for 0.5-mm and 1-mm devices, respectively. The interferometric autocorrelation in sub-micrometer silicon p-i-n waveguides is promising as a monolithic photonic device for on-chip monitor and diagnostics of weak ultrafast pulses.

Optical Third Harmonic Generation Using Nickel Nanostructure-Covered Microcube Structures

We investigated the optical third harmonic generation (THG) signal from nanostructure-covered microcubes on Ni. We found that the hierarchical structures greatly change the third-order optical nonlinearity of the metallic surface. While the symmetry and lightning rod (LR) effects on microstructures did not significantly influence the THG, the localized surface plasmon (LSP) effect on the nanostructures enhanced it. By removing the nanostructures on the hierarchical structures, THG intensity could be strongly suppressed. In the present paper, we also discuss the mechanism that enhances THG in nano/micro structures.

High-performance on-chip autocorrelator using a rib waveguide loaded with two-photon absorption diodes

We report an on-chip autocorrelator comprising a Si rib waveguide and two-photon-absorption photodiodes in which two pulses simultaneously propagate in opposite directions, and their correlation is acquired. The rib waveguide's broadband guiding mode and moderately low group index allow the device to operate over a wide wavelength range with high resolution. We confirm that the waveguide has a transmission band of at least 1300-1630 nm and observed correlation waveforms for pulses of the order of several hundred femtoseconds to several picoseconds, with a range of at least the entire C band region.

Photonic crystal waveguides on silicon rich nitride platform

We demonstrate design, fabrication, and characterization of two-dimensional photonic crystal (PhC) waveguides on a suspended silicon rich nitride (SRN) platform for applications at telecom wavelengths. Simulation results suggest that a 210 nm photonic band gap can be achieved in such PhC structures. We also developed a fabrication process to realize suspended PhC waveguides with a transmission bandwidth of 20 nm for a W1 PhC waveguide and over 70 nm for a W0.7 PhC waveguide. Using the Fabry-Pérot oscillations of the transmission spectrum we estimated a group index of over 110 for W1 PhC waveguides. For a W1 waveguide we estimated a propagation loss of 53 dB/cm for a group index of 37 and for a W0.7 waveguide the lowest propagation was 4.6 dB/cm.

Optical autocorrelation performance of silicon wire p-i-n waveguides utilizing the enhanced two-photon absorption

Optical autocorrelation accuracy was for the first time analyzed for the silicon waveguide based autocorrelators utilizing two-photon absorption (TPA) under various short pulse conditions by numerical simulation. As for autocorrelation operation in the sub-μm silicon p-i-n rib waveguides on the 220 nm SOI (silicon on insulator) wafers, the autocorrelation error of pulse width measurement gradually increases with the increase of the peak power for both Gaussian and hyperbolic secant pulses due to the influence of free-carrier absorption (FCA). For the same pulse type, the relative error is independent of the input pulse width; however different pulse type has different peak power dependency of the accuracy. It was verified that this thin rib waveguide has a TPA responsivity >60 times higher than the thick rib waveguides and the correct pulse width can be measured with a <1% relative error for characterizing ps/sub-ps short pulses of sub-watt peak powers by utilizing the silicon wire p-i-n waveguides as the autocorrelator detector.

Single-shot laser pulse reconstruction based on self-phase modulated spectra measurements

We report a method for ultrashort pulse reconstruction based only on the pulse spectrum and two self-phase modulated (SPM) spectra measured after pulse propagation through thin media with a Kerr nonlinearity. The advantage of this method is that it is a simple and very effective tool for characterization of complex signals. We have developed a new retrieval algorithm that was verified by reconstructing numerically generated fields, such as a complex electric field of double pulses and few-cycle pulses with noises, pedestals and dips down to zero spectral intensity, which is challenging for commonly used techniques. We have also demonstrated a single-shot implementation of the technique for the reconstruction of experimentally obtained pulses. This method can be used for high power laser systems operating in a single-shot mode in the optical, near- and mid-IR spectral ranges. The method is robust, low cost, stable to noise, does not require a priori information, and has no ambiguity related to time direction.

Enhanced third harmonic generation in a silicon metasurface using trapped mode

We experimentally demonstrate enhanced third harmonic generation (THG) using a silicon metasurface, which is consist of symmetric spindle-shape nanoparticle array. Relying on the trapped mode supported by the high quality factor all-dielectric metasurface, the conversion efficiency of THG is about 300 times larger than that of bulk silicon slab. The maximum extinction ratio of the intensity of THG reaches about 25 dB by tuning the polarization of incident light. The simulation results agree with the experimental performances.

Single CdTe Nanowire Optical Correlator for Femtojoule Pulses

On the basis of the transverse second harmonic generation (TSHG) in a highly nonlinear subwavelength-diameter CdTe nanowire, we demonstrate a single-nanowire optical correlator for femto-second pulse measurement with pulse energy down to femtojoule (fJ) level. Pulses to be measured were equally split and coupled into two ends of a suspending nanowire via tapered optical fibers. The couterpropagating pulses meet each other around the central area of the nanowire, and emit TSHG signal perpendicular to the axis of the nanowire. By transferring the spatial intensity profile of the transverse second harmonic (TSH) image into the time-domain temporal profile of the input pulses, we operate the nanowire as a miniaturized optical correlator. Benefitted from the high nonlinearity and the very small effective mode area of the waveguiding CdTe nanowire, the input energy of the single-nanowire correlator can go down to fJ-level (e.g., 2 fJ/pulse for 1064 nm 200 fs pulses). The miniature fJ-pulse correlator may find applications from low power on-chip optical communication, biophotonics to ultracompact laser spectroscopy.

Channel add-drop filter based on dual photonic crystal cavities in push-pull mode

We demonstrate an add-drop filter based on a dual photonic crystal nanobeam cavity system that emulates the operation of a traveling wave resonator, and, thus, provides separation of the through and drop port transmission from the input port. The device is on a 3×3  mm chip fabricated in an advanced microelectronics silicon-on-insulator complementary metal-oxide semiconductor (SOI CMOS) process (IBM 45 nm SOI) without any foundry process modifications. The filter shows 1 dB of insertion loss in the drop port with a 3 dB bandwidth of 64 GHz, and 16 dB extinction in the through port. To the best of our knowledge, this is the first implementation of a port-separating, add-drop filter based on standing wave cavities coupled to conventional waveguides, and demonstrates a performance that suggests potential for photonic crystal devices within optical immersion lithography-based advanced CMOS electronics-photonics integration.

One-chip integration of optical correlator based on slow-light devices

We propose and demonstrate an on-chip optical correlator, in which two types of photonic crystal slow-light waveguides are integrated and operated as an optical delay scanner and a two-photon-absorption photodetector. The footprint of the device, which was fabricated using a CMOS-compatible process, was 1.0 × 0.3 mm(2), which is substantially smaller than that of conventional optical correlators with free-space optics. We observed optical pulses using this device and confirmed the correspondence of pulse waveforms with those observed using a commercial correlator when the pulse width was 5-7 ps. This device will achieve one-chipping of an optical correlator and related measurement instruments.

Photonic crystal cavity-assisted upconversion infrared photodetector

We describe an upconversion infrared photodetector assisted by a gallium phosphide photonic crystal nanocavity directly coupled to a silicon photodiode. The strongly cavity-enhanced second harmonic signal radiating from the gallium phosphide membrane can thus be efficiently collected by the silicon photodiode, which promises a high photoresponsivity of the upconversion detector as 0.81 A/W with the coupled power of 1W. The integrated upconversion photodetector also functions as a compact autocorrelator with sub-ps resolution for measuring pulse width and chirp.

Nonlinear optical response of low loss silicon germanium waveguides in the mid-infrared

We have investigated the nonlinear optical response of low loss Si(0.6)Ge(0.4) / Si waveguides in the mid-infrared wavelength range from 3.25- 4.75μm using picosecond optical pulses. We observed and measured the three and four-photon absorption coefficients as well as the Kerr nonlinear refractive index. The dynamics of the spectral broadening suggests that, in addition to multiphoton absorption, the corresponding higher order nonlinear refractive phenomena also needs to be included when high optical pulse intensities are used at mid-infrared wavelengths in this material.

Extraction of group index of lossy photonic crystal waveguides

We present a numerical approach to extract group index in photonic crystal (PhC) waveguides using two- and three-dimensional finite-difference time-domain methods and make a quantitative study of the effects of loss on slow light propagation in PhC waveguides. PhC waveguides are simulated with varying material loss and varying PhC waveguide length. Finally, we validate our method by comparing three-dimensional simulation results with experimental results.