Nonlinear Signal Processing Applications Research Articles

High-Q quasi-bound states in the continuum in C2-symmetric metasurface with enhanced second harmonic generation in two-dimensional materials

Quasi-bound states in the continuum (Quasi-BIC) possess a high quality-factor (Q-factor) that can amplify local electromagnetic fields and nonlinear effects such as second harmonic generation (SHG). In this paper, we propose a silicon metasurface with C2 symmetry. By tuning the structural parameters without breaking the C2 symmetry, three symmetric protected BICs (SP-BICs) are transformed into sharp quasi-BICs dominated by different modes in the nearinfrared spectrum. The quasi-BICs exhibit Q-factors as high as 10370 in C2 symmetric metasurface. Numerical simulations reveal that the toroidal dipole (TD) and magnetic dipole (MD) enhance intercellular coupling and local electromagnetic fields, laying the foundation for generating metasurface with high Q-factor. Furthermore, the longitudinal radiative component of electric quadrupole (EQ) outside the facet promotes enhanced light interaction with the 2D material. Leveraging the robust local electromagnetic fields and radiation characteristics of these quasi-BICs, we have achieved substantial SHG enhancement across multiple wavelengths under continuous wave (CW) operation. The SHG from 2D GaSe flake reaches 14189-fold enhancement. Our results not only offer insights into designing metasurface structures with high Q-factors but also explore the methods of enhancing the interaction between metasurface and 2D materials. The proposed nanostructure has numerous potential applications in nonlinear optics, optoelectronics and signal processing.

Large second-order susceptibility from a quantized indium tin oxide monolayer.

Due to their high optical transparency and electrical conductivity, indium tin oxide thin films are a promising material for photonic circuit design and applications. However, their weak optical nonlinearity has been a substantial barrier to nonlinear signal processing applications. In this study, we show that an atomically thin (~1.5 nm) indium tin oxide film in the form of an air/indium tin oxide/SiO2 quantum well exhibits a second-order susceptibility χ2 of ~1,800 pm V-1. First-principles calculations and quantum electrostatic modelling point to an electronic interband transition resonance in the asymmetric potential energy of the quantum well as the reason for this large χ2 value. As the χ2 value is more than 20 times higher than that of the traditional nonlinear LiNbO3 crystal, our indium tin oxide quantum well design can be an important step towards nonlinear photonic circuit applications.

Polarization-sensitive optical responses from natural layered hydrated sodium sulfosalt gerstleyite

Multi-element layered materials have gained substantial attention in the context of achieving the customized light-matter interactions at subwavelength scale via stoichiometric engineering, which is crucial for the realization of miniaturized polarization-sensitive optoelectronic and nanophotonic devices. Herein, naturally occurring hydrated sodium sulfosalt gerstleyite is introduced as one new multi-element van der Waals (vdW) layered material. The mechanically exfoliated thin gerstleyite flakes are demonstrated to exhibit polarization-sensitive anisotropic linear and nonlinear optical responses including angle-resolved Raman scattering, anomalous wavelength-dependent linear dichroism transition, birefringence effect, and polarization-dependent third-harmonic generation (THG). Furthermore, the third-order nonlinear susceptibility of gerstleyite crystal is estimated by the probed flake thickness-dependent THG response. We envisage that our findings in the context of polarization-sensitive light-matter interactions in the exfoliated hydrated sulfosalt layers will be a valuable addition to the vdW layered material family and will have many implications in compact waveplates, on-chip photodetectors, optical sensors and switches, integrated photonic circuits, and nonlinear signal processing applications.

Polarization-dependent optical responses in natural 2D layered mineral teallite

Multi-element layered materials enable the use of stoichiometric variation to engineer their optical responses at subwavelength scale. In this regard, naturally occurring van der Waals minerals allow us to harness a wide range of chemical compositions, crystal structures and lattice symmetries for layered materials under atomically thin limit. Recently, one type of naturally occurring sulfide mineral, ternary teallite has attained significant interest in the context of thermoelectric, optoelectronic, and photovoltaic applications, but understanding of light-matter interactions in such ternary teallite crystals is scarcely available. Herein, polarization-dependent linear and nonlinear optical responses in mechanically exfoliated teallite crystals are investigated including anisotropic Raman modes, wavelength-dependent linear dichroism, optical band gap evolution, and anisotropic third-harmonic generation (THG). Furthermore, the third-order nonlinear susceptibility of teallite crystal is estimated using the thickness-dependent THG emission process. We anticipate that our findings will open the avenue to a better understanding of the tailored light-matter interactions in complex multi-element layered materials and their implications in optical sensors, frequency modulators, integrated photonic circuits, and other nonlinear signal processing applications.

A shower of mid-infrared Raman solitons at designed wavelength of ∼3 μm from a tapered fluorotellurite fiber

Optical temporal soliton generation in optical fibers and resonators has achieved great breakthroughs and found significant applications in many fields. Despite recent advances on optical soliton generation in optical fibers, it remains challenging to realize a shower of Raman solitons at a certain wavelength in optical fibers, i.e. put all the generated Raman solitons at a same wavelength on demand. Here we propose and experimentally demonstrate a shower of mid-infrared Raman solitons at designed wavelength of 3 μm from a tapered fluorotellurite fiber pumped by a 1960 nm picosecond fiber laser. Tapered fluorotellurite fibers have an untapered region length of ∼49 cm and a tapered transition region length of ∼1 cm. The fiber segment at the end of the tapered region has a 2nd zero-dispersion wavelength of ∼3 μm. Firstly, multiple Raman solitons with different wavelengths are generated in the untapered region pumped by the 1960 nm picosecond laser. Then, as Raman solitons propagate into the tapered region, the redshift of all the generated Raman solitons with different wavelengths are accelerated due to the dramatic increase of the nonlinear coefficient. Finally, as Raman solitons move out from the tapered region, all the accelerated Raman solitons are stopped around 3 μm owing to the occurrence of soliton self-frequency shift cancellation, causing the generation of a shower of mid-infrared Raman solitons around 3 μm. This phenomenon may find applications in super-efficient nonlinear light generation and signal processing.

Total gain of InTlAsSb quantum dot structures

Abstract This work studies the total gain of the InTlAsSb quantum dot structure, which is not studied earlier. Adding thallium to structures makes it emit at larger wavelengths. The nonlinear effect of the injected signal power is examined for three quaternary thallium structures: In0.85Tl0.15AsSb, In0.93Tl0.07AsSb and In0.97Tl0.03AsSb. The gain peak was increased by four times and the wavelength was shifted to longer one for the In0.97Tl0.03AsSb quantum dot (QD) structure. This quaternary QD structure extends the emission wavelength to more than 12 μm which is important in long-wavelength infrared applications. The nonlinear behavior of these QD structures is also addressed. It is shown that the structure In0.97Tl0.03AsSb has a deeper spectral hole burning which is adequate for nonlinear signal processing applications.

Quasi-BIC Resonant Enhancement of Second-Harmonic Generation in WS2 Monolayers.

Atomically thin monolayers of transition metal dichalcogenides (TMDs) have emerged as a promising class of novel materials for optoelectronics and nonlinear optics. However, the intrinsic nonlinearity of TMD monolayers is weak, limiting their functionalities for nonlinear optical processes such as frequency conversion. Here we boost the effective nonlinear susceptibility of a TMD monolayer by integrating it with a resonant dielectric metasurface that supports pronounced optical resonances with high quality factors: bound states in the continuum (BICs). We demonstrate that a WS2 monolayer combined with a silicon metasurface hosting BICs exhibits enhanced second-harmonic intensity by more than 3 orders of magnitude relative to a WS2 monolayer on top of a flat silicon film of the same thickness. Our work suggests a pathway to employ high-index dielectric metasurfaces as hybrid structures for enhancement of TMD nonlinearities with applications in nonlinear microscopy, optoelectronics, and signal processing.

Effect of alpha and Gaussian refractive index profiles on the design of highly nonlinear optical fibre for efficient nonlinear optical signal processing

We propose an index profiled, highly nonlinear ultra-flattened dispersion fibre (HN-UFF) with appreciable values of fibre parameters such as dispersion, dispersion slope, effective area, nonlinearity, bending loss and splice loss. The designed fibre has normal zero flattened dispersion over S, C, L, U bands and extends up to . The maximum dispersion variation observed for this fibre is as low as over the optical fibre transmission spectrum. This fibre also has two zero dispersion wavelengths at 1.487 and and the respective dispersion slopes are 0.02476 and . The fibre has a very low ITU-T cutoff wavelength of and a virtuous nonlinear coefficient of . The wide spectrum of zero flattened dispersion and a good nonlinear coefficient make the designed fibre very promising for different nonlinear optical signal processing applications.

CO2 Laser‐Induced Directional Recrystallization to Produce Single Crystal Silicon‐Core Optical Fibers with Low Loss

Reduced losses in silicon-core fibers are obtained using CO2 laser directional recrystallization of the core. Single crystals with aspect ratios up to 1500:1 are reported, limited by the scan range of the equipment. This processing technique holds promise for bringing crystalline silicon-core fibers to a central role in nonlinear optics and signal processing applications.

Ultrahigh Birefringent Nonlinear Slot Silicon Microfiber With Low Dispersion

A new simple structure of ultrahigh birefringent and ultrahigh nonlinear slot silicon microfiber with highly efficient dispersion reduction is proposed and numerically simulated using the full-vector finite-element method with anisotropic perfectly matched layers. Benefiting from the slot effect-induced sub-wavelength mode confinement, ultrahigh birefringence up to the order of $10^{\mathrm {{-1}}}$ can be realized within a wide wavelength range from 1.4 to 1.7 $\mu \text{m}$ , which is comparable with the results in these ultrahigh birefringent photonic crystal fibers, but cannot be achieved in traditional microfibers. Meanwhile, the nonlinear coefficients of quasi-TE mode and quasi-TM mode at the wavelength of 1.55 $\mu \text{m}$ are as high as 969.58 $\text{W}^{\mathrm {{-1}}}\text{m}^{\mathrm {{-1}}}$ and 156.74 $\text{W}^{\mathrm {{-1}}}\text{m}^{\mathrm {{-1}}}$ , respectively. Furthermore, the dispersion value of quasi-TE mode at 1.55 $\mu \text{m}$ can be decreased from $1.2358\times 10^{\mathrm {{3}}}$ ps/(nm $\cdot $ km) to 0 ps/(nm $\cdot $ km) simply by modifying the slot size. Owing to its excellent performance, the proposed slot silicon microfiber will have great potential for polarization maintaining nonlinear signal processing applications.

A review of recent progress in plasmon-assisted nanophotonic devices

Plasmonics squeezes light into dimensions far beyond the diffraction limit by coupling the light with the surface collective oscillation of free electrons at the interface of a metal and a dielectric. Plasmonics, referred to as a promising candidate for high-speed and high-density integrated circuits, bridges microscale photonics and nanoscale electronics and offers similar speed of photonic devices and similar dimension of electronic devices. Various types of passive and active surface plasmon polariton (SPP) enabled devices with enhanced deep-subwavelength mode confinement have attracted increasing interest including waveguides, lasers and biosensors. Despite the trade-off between the unavoidable metal absorption loss and extreme light concentration, the ever-increasing research efforts have been devoted to seeking low-loss plasmon-assisted nanophotonic devices with deep-subwavelength mode confinement, which might find potential applications in high-density nanophotonic integration and efficient nonlinear signal processing. In addition, other plasmon-assisted nanophotonic devices might also promote grooming functionalities and applications benefiting from plasmonics.

Designing slow-light photonic crystal waveguides for four-wave mixing applications

We discuss the optimization of photonic crystal waveguides for four-wave mixing (FWM) applications, taking into account linear loss and free-carrier effects. Suitable figures of merit are introduced in order to guide us through the choice of practical, high-efficiency designs requiring relatively low pump power and small waveguide length. In order to realistically perform the waveguide optimization process, we propose and validate an approximate expression for the FWM efficiency, which significantly alleviates our numerical calculations. Promising waveguide designs are identified by means of an exhaustive search, altering some structural parameters. Our approach aims to optimize the waveguides for nonlinear signal-processing applications based on the FWM.

Approximate expressions for estimation of four-wave mixing efficiency in slow-light photonic crystal waveguides

We present approximate analytical expressions for the estimation of the degenerate four-wave mixing conversion efficiency in slow-light photonic crystal waveguides (PCWs). The derived formulas incorporate the different effective modal areas and the frequency-dependent linear and nonlinear parameters of the pump, signal, and idler waves. The influence of linear loss, two-photon absorption, and free-carrier generation is also accounted for. Numerical solution of the coupled propagation equations is used to verify the validity of the proposed expressions under different values of the linear and nonlinear parameters of the waveguide. It is shown that the derived expressions provide an accurate estimation of the conversion efficiency and are thus expected to be useful in the design of PCWs for nonlinear signal-processing applications.

Long-Range Hybrid Plasmonic Slot Waveguide

We propose and design a silicon-based long-range hybrid plasmonic slot (LRHPS) waveguide. The waveguide structure consists of a thin metal film (silver) inserted into a vertical low-index slot [filled with silicon nanocrystal (Si-nc)] between two high-index dielectrics (silicon), forming two nanoscale low-index Si-nc slot regions. The modes are confined within the two vertical regions. The designed LRHPS waveguide takes advantages of both traditional long-range surface plasmon polariton waveguide and hybrid plasmonic waveguide. The combined effects of long-range surface plasmon polaritons and discontinuity of electric field at the interface between two dielectrics with high-contrast refractive index enable a millimeter-scale propagation range together with a subwavelength mode confinement for potential high-density nanophotonic integration. The resultant quasi-TE mode properties of LRHPS waveguide, including long-range hybrid (LRH) mode and short-range hybrid (SRH) mode, are analyzed, showing a long propagation length up to 14.55 mm (corresponding to a low loss of $3\times 10^{-4} \hbox{dB}/\mu\hbox{m}$ ) with a slot size of 150 nm $\times$ 200 nm. Normalized power and intensity are also calculated, indicating tight mode confinement within subwavelength low-index slot regions. Moreover, nonlinearity and chromatic dispersion are also studied. Due to the design freedom of double-slot structure, a high nonlinearity of $7.82 \times 10^{6} \hbox{W}^{-1}\hbox{km}^{-1}$ and a low chromatic dispersion of $-$ 28.29 ps/nm/km at 1550 nm are achieved, implying possible applications in efficient nonlinear optical signal processing.

Design of optical pumping scheme for quantum‐dot semiconductor optical amplifiers

In this study, design of optical pumping (OP) scheme for quantum-dot semiconductor optical amplifiers (QD-SOAs) is investigated. To evaluate the static and dynamic responses of the QD-SOA under OP, a non-linear state-space model is derived in which the effects of the homogeneous and inhomogeneous broadening of the gain medium and the spectral dependence of the input signals are taken into account. Using this model, the effects of the wavelength and power of the pump signal and the homogeneous and inhomogeneous linewidths are studied in detail. Simulation results demonstrate that optimal operation of the QD-SOA under the OP scheme is realised if the detuning between the pump wavelength and the input signal wavelength is constant (around 100 nm for our investigated device). The authors found that OP scheme can be employed for non-linear signal processing applications, where the superiorities of the OP against the electrical pumping appear for small values of the homogeneous and inhomogeneous linewidths.

Numerical analysis of soliton propagation in photonic crystal slab waveguides for signal processing applications

The delay performance of slow light optical pulses inside photonic crystal slab waveguides is considered in the linear and nonlinear propagation regime from both a theoretical and an application point of view. The numerical model used relies on a nonlinear envelope propagation equation that includes the effects of second- and third-order dispersion, optical losses, and self-phase modulation. It is numerically shown that for rates of 40  Gb/s and 100  Gb/s, nonlinear solitary pulses experience less broadening than the linear case and can therefore be used to obtain larger delays. The influence of propagation losses on the soliton broadening factor is also incorporated and discussed. The results demonstrate the potential of implementing a variety of linear and nonlinear signal processing applications in photonic crystal waveguides including optical buffering.

SOA-MZI-Based Nonlinear Optical Signal Processing: A Frequency Domain Transfer Function for Wavelength Conversion, Clock Recovery, and Packet Envelope Detection

We present analytic expressions for the frequency-domain transfer function of semiconductor optical amplifier Mach-Zehnder interferometric (SOA-MZI) switches that employ a single optical control signal and a continuous wave input optical beam. Our analysis relies on first-order perturbation theory approximations applied both to the SOA response as well as to the SOA-MZI characteristics, yielding a frequency response that enables a qualitative insight into the different SOA-MZI operational regimes. The final transfer function expression is utilized for the analysis and evaluation of the multifunctional potential of SOA-MZI switches, concluding with the necessary conditions for supporting a number of completely different SOA-MZI-based nonlinear signal processing applications that have been demonstrated experimentally: wavelength conversion, packet envelope detection (PED), and clock recovery (CR). The theoretically obtained operational conditions are in close agreement with experimental observations, showing that SOA-MZIs can serve as functional circuit elements in applications with different requirements depending on its operational parameters: as low-pass filtering devices with cut-off frequencies in the megahertz regime or in the multi-gigahertz regime, and as resonant modules resembling band-pass filtering structures. The validity of our theoretical SOA-MZI frequency-domain system model is further confirmed by its successful incorporation in a Fabry-Perot assisted SOA-MZI subsystem, demonstrating PED and CR operations through the exploitation of typical systems theory tools.

Microresonators in CMOS Compatible Substrate

We report our recent progress in designing and developing traveling-wave microresonators on CMOS compatible substrate, with a focus on ultra-compact Si-based microring and microdisk resonators in the visible and near infrared wavelengths with ultra-high Q and small mode volume, suitable for strong light-matter interaction. The performance of these resonators is discussed, and the design and fabrication challenges and solutions to achieve efficient coupling from the bus-waveguide to the resonator is mentioned. Coupled-resonator architectures for the design of filters are analyzed and theoretical and experimental results for the flat-band filters with wide bandwidth and large free spectral range are presented. Furthermore, by using a detailed thermal model we demonstrate optimized microresonator structures on semiconductor substrates with improved thermal conductivity that can sustain a high circulating optical intensity at the resonance wavelength, which is particularly useful for nonlinear optics and optical signal processing applications. Finally, we address some of the applications and future prospects of such microresonators in CMOS-compatible substrate technology, including portable multi-purpose bio/chemical sensing systems, reconfigurable optical signal processing modules and chip-scale optical amplifiers and lasers.

Two-photon Autocorrelation in a MQW GaAs Laser at 1.55μm

A quadratic two-photon photocurrent is measured in a novel multiple-quantum well GaAs laser waveguide at 1550nm. We show that this device is ideal for nonlinear detection and signal processing applications due to its large efficiency and high sensitivity. In particular, we measure the autocorrelation trace of a 100 fs pulse using sub-watt peak power levels.

Efficient self-phase modulation in low loss, high index doped silica glass integrated waveguides

We demonstrate efficient self phase modulation, as well as negligible nonlinear absorption, in low loss (<0.06 dB/cm), high index silica glass-based waveguides. Using approximately 1 ps pulses near 1560 nm we achieve a 1.5pi nonlinear phase shift in an integrated 45 cm long spiral waveguide with <60 W of peak input power, corresponding to a large nonlinearity (gamma) of 220 W(-1)km(-1). Further, we observe negligible nonlinear absorption for input intensities > 25 GW/cm(2). The high nonlinearity and low linear and nonlinear losses of these waveguides make them promising for nonlinear all-optical signal processing applications.