Ultrafast All-optical Signal Processing Research Articles

Localized surface plasmon resonance induced nonlinear absorption and optical limiting activity of gold decorated graphene/MoS2 hybrid

The synergistic supremacy of a hybrid nanocomposite made of reduced graphene oxide (rGO), molybdenum disulfide (MoS2) and gold nanoparticles (Au) for Q-switching and optical limiting applications is investigated. Hydrothermally synthesized Au-rGO-MoS2 hybrid with varying concentrations of Au (2.5, 5, 7.5 and 10 wt%) was subjected to interact with Q-switched Nd:YAG nanosecond green laser pulses. The intensity dependent nonlinear absorption studies revealed a switch over in the nonlinear behaviour from saturable absorption (SA) to reverse saturable absorption (RSA) behaviour. SA occurs due to ground state bleaching at comparatively lower laser irradiances serving the need for Q-switching and optical communications. The RSA trait at higher laser irradiance is attributed to sequential two-photon absorption which serves as the platform for optical limiting behaviour prompting laser safety and effective photothermal therapy applications. The strong mid-visible and UV absorption promotes the availability of multiple energy bands for energy transitions of excited state absorption (ESA). Enhanced local fields due to localized surface plasmon resonance effect, structural defects, hierarchical morphology, increased absorption cross-section and plasmon induced energy transfer promotes robust ESA process. These tunable nonlinear optical properties make Au-rGO-MoS2 hybrid promising futuristic candidates for applications in nonlinear photonic devices, ultrafast optical switches and all-optical signal processing systems.

Optical Bullets and Domain Walls with Cross Spatio-Dispersion and Having Kudryashov’s form of Self-Phase Modulation

In this paper, we investigate the retrieval of optical bullets and domain walls in a novel context, considering the interplay of cross spatio-dispersive effects and employing Kudryashov' s proposed form of self-phase modulation. The integration of these two key elements represents a significant advancement in the field of nonlinear optics, enabling the recovery of optical structures with enhanced precision and control. We propose an enhanced Kudryashov’s approach, which overcomes the limitations of conventional methods and facilitates the retrieval of optical bullets and domain walls even in the presence of cross spatio-dispersive effects. Kudryashov' s self-phase modulation mechanism has proven to be a powerful tool in controlling the nonlinear dynamics of optical systems, offering unique opportunities to manipulate light waves through phase variations induced by the medium' s response to intense optical fields. The results presented herein offer new insights into the intricate interplay between self-phase modulation and spatio-dispersive effects in nonlinear optical systems. Moreover, our findings provide a promising avenue for developing novel applications in ultrafast all-optical signal processing, such as the manipulation of light bullets for high-speed data transmission and storage. The enhanced retrieval and control of optical bullets and domain walls are expected to have a profound impact on the advancement of nonlinear optics and its practical implications.

High-contrast optical bistability using a subwavelength epsilon-near-zero material.

Optical bistability opens up a promising avenue toward various optical nonlinear functions analogous to their electrical counterparts, such as switches, logic gates, and memory. Free-space bistable devices have unique advantages in large-scale integration. However, most proposed free-space schemes for optical bistability have limitations in one or more aspects of low contrast ratio, compromised compatibility, slow switching speed, and bulk size. Epsilon-near-zero (ENZ) materials have recently shown an ultrafast and giant optical nonlinearity within a subwavelength scale, potentially overcoming these obstacles. Using large-mobility indium-doped cadmium oxide (CdO) as the ENZ material, we numerically demonstrate two efficient schemes for high-contrast optical bistability within a deep subwavelength size based on the ENZ mode and the Berreman mode. The ENZ wavelength can be optically tuned with a typical time scale of sub-picoseconds, giving rise to a switchable bistability between the near-zero state and the high-reflection state. Our work contributes to the advances on compact and ultrafast all-optical signal processing.

On-chip all-optical quantizer based on the cross polarization modulation effect in a double-ridge a-Si:H-Si3N4 waveguide.

The on-chip all-optical quantizer is a key functional module to realize all-optical analog-to-digital conversion. It has important applications in future optical communication and ultrafast all-optical signal processing systems. To the best of our knowledge, a novel all-optical quantization scheme based on the cross-polarization modulation effect in a double-ridge a-Si:H-Si3N4 waveguide is proposed. For the optimized waveguide, the maximum nonlinear coefficient and transmission loss at the probe wavelength are 2680.5m-1W-1 and 2.246 dB/cm, respectively. The simulation results of the proposed all-optical quantizer show that a nonlinear phase shift of 8 can be generated in the 8.79 mm long waveguide when the peak pump light power is 0.8 W. A 16-level quantization and a 4-bit quantization resolution are obtained. The effective number of bits is 3.79-bit, which is only 0.21-bit away from the ideal resolution.

Amplitude modulator of Multi-walled Carbon nanotubes/Chitin in the C-band region

In ultrafast all-optical signal processing, the all-optical method is crucial, and all-fiber technique offers a wide range of applications in optical communications. This study investigated the amplitude modulation using multi-walled carbon nanotubes (MWCNTs) embedded into chitin as saturable absorber (SA). The MWCNTs-chitin SA is fabricate using a liquid phase exfoliation method to reduce complexity and produce an excellent material quality. In this paper, an optical amplitude modulator produced a linear region with a regression line of the peak intensity at pump power range from 17.92 mW to 67.92 mW with modulation efficiency of 0.50 dB/mW.

Manipulation of the dielectric properties of narrow-band gap material by an ultrashort laser pulse

The control of dielectric responses of narrow-band gap materials by an ultrashort laser pulse is investigated theoretically using time-dependent density functional theory, taking gallium phosphide as a concrete example. We demonstrate that, by using properly selected laser parameters an ultrashort laser pulse can induce reversible change of the electric and optical properties. The anisotropy of the transient permittivity of the material throughout the pulse duration can be enhanced so strongly that the dispersion of electromagnetic waves in the material becomes hyperbolic in some frequency interval with increasing laser energy. We describe the evolution of the transient permittivity throughout the duration of the laser pulse, pointing out to a fundamental possibility for applications of ultrafast all-optical signal processing and optical-field effect devices.

Controlling dielectric properties of cBN by an ultrashort double-pulse light

The optical responses of wide band gap materials can be manipulated by a purposely designed ultrashort double-pulse light, based on the real-time time-dependent density functional theory. Taking the cubic boron nitride (cBN) as a typical example, we calculate the energy transfer from an ultrashort double-pulse light field to dielectric materials. We show that the purposely designed ultrashort double-pulse light and pulse train can reversibly manipulate the dielectric properties of cBN without damage. By tuning the parameters of light pulse, the cBN will change between isotropic, elliptic and hyperbolic for a particular light frequency at certain time points during the pulse light. We propose that the cBN with reversible but deep nonlinear optical responses excited by the external light field can be used in the ultrafast all-optical signal processing and optical-field-effect devices.

All-optical AND, OR, and XOR logic gates based on coherent perfect absorption in graphene-based metasurface at terahertz region

In this paper, we propose a novel method for designing all-optical logic gates based on the coherent perfect absorption principle in graphene-based metasurface in terahertz (THz) region. The proposed structure allows us to access AND, OR, and XOR logic operations by adjusting the relative phase difference of two input signals and creating destructive and/or constructive interference. We have calculated the contrast ratio to analyze the performance of these logic gates. The working frequency of the proposed logic gates can be manipulated by varying the gate-controlled Fermi energy of graphene. These structures can be used in ultrafast all-optical signal processing and ultra-compact integrated circuits.

Sub-20 fs All-Optical Switching in a Single Au-Clad Si Nanodisk.

Dielectric nanoantennas have recently emerged as promising elements for nonlinear and ultrafast nanophotonics due to their ability to concentrate light on the nanometer scale with low losses, while exhibiting large nonlinear susceptibilities. In this work, we demonstrate that single Si nanodisks covered with a thin 30 nm thick layer of Au can generate positive and negative sub-20 fs reflectivity modulations of ∼0.3% in the vicinity of the first-order anapole mode, when excited around the second-order anapole mode. The experimental results, characterized in the visible to near-infrared spectral range, suggest that the nonlinear optical Kerr effect is the responsible mechanism for the observed all-optical switching phenomena. These findings represent an important step toward nanoscale ultrafast all-optical signal processing.

Controlling anisotropy of dielectrics by an ultrashort double-pulse laser

We theoretically investigate the control of optical responses of dielectrics by an ultrashort double-pulse laser, based on the real-time time-dependent density functional theory. Taking the diamond as a concrete example, we demonstrate the unique capability of a purposely designed double-pulse laser in manipulating the strong anisotropy of dielectrics, achieving significant benefits compared with a single-pulse laser. With easily tunable laser parameters, the dielectric material can even be switched by reversal of isotropic, elliptic and hyperbolic for a particular light frequency at certain time points during the pulse laser. We describe the evolution of the transient permittivity throughout the duration of the pulse laser, which can exhibit reversible but deeply nonlinear modification in optical responses driven by a purposely designed pulse laser without imposing physical damage, demonstrating a fundamental possibility for applications of ultrafast all-optical signal processing and optical-field effect devices.

Spatial integration and differentiation of optical beams in a slab waveguide by a dielectric ridge supporting high-Q resonances.

We show that a very simple structure consisting of a single subwavelength dielectric ridge on the surface of a slab waveguide enables spatial integration and differentiation of the profile of optical beams propagating in the waveguide. The integration and differentiation operations are performed in reflection and in transmission, respectively, at oblique incidence of the beam impinging on the ridge. The implementation of these operations is associated with the resonant excitation of a cross-polarized eigenmode of the ridge. We demonstrate that the quality factor of the resonances strongly varies along the dispersion curves and allows one to achieve the required tradeoff between the integration (or differentiation) quality and the amplitude of the resulting beam. The presented rigorous numerical simulation results confirm high-quality integration and differentiation. The proposed integrated structure may find application in ultrafast all-optical analog computing and signal processing systems.

Wideband and continuously-tunable fractional photonic Hilbert transformer based on a single high-birefringence planar Bragg grating.

We propose and experimentally demonstrate wideband and continuously tunable fractional-order photonic Hilbert transformers (FrHT). These are realized by a single apodized planar Bragg grating within a high-birefringence planar substrate. The fractional order of the FrHT is continuously tuned and precisely controlled by changing the polarization state of the input light. The experimental characterization demonstrates an operating bandwidth up to 120 GHz with amplitude ripples below 3 dB. The optical phase shift response is directly measured to verify the proposed tuning property, demonstrating transform orders of around 1, 0.7, and 0.5. This approach is simple, stable, and compact compared to other existing methods and has great potential in the fields of ultrafast all-optical signal processing.

All-optical modulator based on MoS2-PVA thin film

The all-optical approach plays an important role in ultrafast all-optical signal processing, and the all-fiber scheme has a wide application in optical communications. In this letter, we investigate an all-optical modulator using few-layer molybdenum disulfide (MoS2)-polyvinyl alcohol (PVA) thin films based on the thermo-optic effect and obtain a long-time stable modulated output by applying polarization interference. By absorbing the injected 980 nm pump (control light), MoS2 generates heat, changes the refractive index of MoS2, and modulates the polarization of light. The obtained thermal all-optical modulator has a rise time of 526 μs.

Integrated reconfigurable photonic filters based on interferometric fractional Hilbert transforms.

In this paper, we present integrated reconfigurable photonic filters using fractional Hilbert transformers (FrHTs) and optical phase tuning structure within the silica-on-silicon platform. The proposed structure, including grating-based FrHTs, an X-coupler, and a pair of thermal tuning filaments, is fabricated through the direct UV grating writing technique. The thermal tuning effect is realized by the controllable microheaters located on the two arms of the X-coupler. We investigate the 200GHz maximum bandwidth photonic FrHTs based on apodized planar Bragg gratings, and analyze the reflection spectrum responses. Through device integration and thermal modulation, the device could operate as photonic notch filters with 5GHz linewidth and controllable single sideband suppression filters with measured 12dB suppression ratio. A 50GHz instantaneous frequency measuring system using this device is also schematically proposed and analyzed with potential 3dB measurement improvement. The device could be configured with these multiple functions according to need. The reconfigurable structure has great potential in ultrafast all-optical signal processing fields.

Nonlinear Transient Dynamics of Photoexcited Resonant Silicon Nanostructures

Optically generated electron–hole plasma in high-index dielectric nanostructures was demonstrated as a means of tuning their optical properties. However, until now an ultrafast operation regime of such plasma-driven nanostructures has not been attained. Here, we perform pump–probe experiments with resonant silicon nanoparticles and report on dense optical plasma generation near the magnetic dipole resonance with an ultrafast (about 2.5 ps) relaxation rate. On the basis of experimental results, we develop an analytical model describing the transient response of a nanocrystalline silicon nanoparticle to an intense laser pulse and show theoretically that plasma-induced optical nonlinearity leads to ultrafast reconfiguration of the scattering power pattern. We demonstrate 100 fs switching to a unidirectional scattering regime upon irradiation of the nanoparticle by an intense femtosecond pulse. Our work lays the foundation for developing ultracompact and ultrafast all-optical signal processing devices.

Compact picosecond nondegenerate four-wave mixing mirrorless optical parametric oscillator in silicon waveguide

The compact picosecond nondegenerate four-wave mixing mirrorless optical parametric oscillator based on multimode silicon waveguide is proposed and investigated numerically. Two counterpropagating picosecond pulses of fundamental mode can generate new pulses of second-order mode at different wavelengths due to the large modal dispersion between the fundamental mode and the second-order mode. The frequency of the newly generated waves can be tuned to 0.6 THz by adjusting the pump frequency difference of 5 THz. The output signal wave exhibits pulse width of 50 ps when the pump pulse is 100 ps. The proposed mirrorless optical parametric oscillator exhibits compact configuration and low threshold, which can find important applications in integrated optical source and ultrafast all-optical signal processing.

Integrated nonlinear Mach Zehnder for 40 Gbit/s all-optical switching

We report on the experimental demonstration of a novel silicon based fully integrated nonlinear Mach Zehnder device. A standard silicon waveguide is used as a nonlinear arm, conversely a large mode SU-8 waveguide acts as a purely linear arm. Given this asymmetry, an intensity dependent phase shift can be introduced between the two interferometric arms. Thanks to a fine tuning of the silicon arm optical properties, a low power, ultrafast, picosecond operation is demonstrated, allowing the use of this device for ultrafast all-optical signal processing in high density communication networks.

Optical and structure properties of amorphous Ge–Sb–Se films for ultrafast all-optical signal processing

In this paper, we deposited amorphous chalcogenide Ge–Sb–Se films using the RF sputtering method, then measured their optical and structural properties using various diagnosis tools. The linear refractive index and optical band-gap for as-deposited films were analyzed as a function of the chemical composition and the mean coordination number (MCN). The third-order optical nonlinearities were predicted by applying the Z-scan method combined with a model developed by Sheik-Bahae. The relationship between film compositions and the optical properties was analyzed by Raman spectra in terms of structural evolution. The data suggests that the “defect” gap states in the network play an important role in the third-order optical nonlinearity of these chalcogenide films. The Ge20Sb15Se65 films most suitable for all-optical signal processing applications had large nonlinear refractive indices (−8.735 × 10−15 m2/W), moderate nonlinear absorption (8.592 × 10−9m/W) and showed an ultrafast nonlinear response time (66 fs).

Design of Ultrafast All-Optical Signal Processing Devices Based on Fiber Bragg Gratings in Transmission

A general and practical approach for designing ultra-fast all-optical (all-fiber) signal processing devices based on chirped fiber Bragg gratings (C-FBGs) working in transmission is presented. The approach can be used to design any minimum-phase linear optical device, e.g., first and high-order all-optical time differentiators and integrators and a large variety of optical pulse shapers, significantly overcoming the bandwidth limitations of previous FBG-based designs. Processing speeds in the THz range, corresponding to sub-picosecond time features, can be achieved using readily feasible grating apodization profiles. The approach is successfully proved through the design of two relevant functionalities, namely a THz-bandwidth optical differentiator and a 1-ps flat-top optical pulse shaper.

High conversion efficiency femtosecond optical parametric amplifier based on silicon waveguide in mid-infrared

The mid-infrared femtosecond optical parametric amplifier (OPA) in silicon waveguide is proposed and investigated numerically. Through appropriately engineering the waveguide dimensions, the zero-dispersion wavelength of the silicon waveguide can be tailored to 2.1μm and the broadband phase-matching can be realized. We achieve a conversion efficiency of −40dB and a conversion bandwidth of over 300nm for a low pump peak power in 8-mm-long silicon waveguide. We also analyze the evolution of the pump, signal and idler pulses in the process of optical parametric amplification. The proposed OPA exhibits compact configuration and can find important applications in integrated broadband optical source and ultrafast all-optical signal processing in mid-infrared.