Asymmetric Gratings Research Articles

Silicon photonic modulators with a 2 × 1 Fabry–Perot cavity

Abstract Silicon photonics modulators based on a 2 × 1 Fabry–Perot (FP) cavity, which is circulator-free, are proposed and demonstrated by introducing two asymmetric multimode-waveguide grating (AMWG) reflectors and a short straight modulation section with interleaved PN junctions. In particular, the straight modulation section in the FP cavity is broadened to be far beyond the single-mode regime, alleviating the inherent sensitivity to the variations of waveguide dimensions and thus reducing stochastic resonance-wavelength variations. The Q factor of the FP cavity is manipulated by optimally manipulating the reflection of the AMWGs, and the modulation bandwidth is enhanced to be over 40 GHz by utilizing the optical peaking enhancement effect, which happens when operating at the wavelength slightly detuning to its resonance wavelength. Eye diagrams for high-speed modulation with 50 Gbps are also demonstrated in experiments. Finally, wafer-level measurement is conducted by characterizing the silicon photonic modulators based on the 2 × 1 FP cavity and a conventional microring fabricated on the same chip, experimentally revealing an average improvement of 43 % in minimizing the random resonance-wavelength variation, which is attributed to the implementation of broadening the straight modulation section in the FP cavity.

Using asymmetric surface relief gratings to unify the efficiency in certain field of view for see-through display

Using asymmetric surface relief gratings to unify the efficiency in certain field of view for see-through display

Extensive Q-factor tuning for leaky modes with minimal frequency variation in asymmetric slab grating structures

We investigated an asymmetric slab grating structure to achieve significant tuning of the quality (Q) factor for a leaky mode while minimizing frequency variation. This structure comprises two identical gratings placed on the top and bottom of a slab waveguide, with one grating laterally shifted to introduce asymmetry. Simulations demonstrate that lateral shifting of one grating induces extensive changes in the Q-factor with minimal frequency variation, particularly near the band-flip filling fraction because the band-flip filling fraction remains unaffected by the shifting. The independence of the band-flip filling fraction from lateral shifting is attributed to the superposition property of Bragg scattering processes in the asymmetric grating structure. Experimental verification in the terahertz range confirms significant control over the Q-factor of the leaky mode of the structure. The proposed asymmetric slab grating structure offers possibilities for mechanically controllable optical devices, which are applicable to tunable filters and sensors. This study advances our understanding and application of leaky modes in asymmetric grating structures, revealing a previously unexplored aspect of asymmetric optical lattice.

Engineering quasi-bound states in the continuum in asymmetric waveguide gratings

Abstract The occurrence of quasi-bound states in the continuum (qBIC) in all-dielectric asymmetric grating waveguide couplers with different degrees of asymmetry under normal light incidence is analysed from the viewpoint of identifying the most promising configuration for realizing the highest quality (Q) factor under the condition of utmost efficiency (i.e. total extinction). Considering asymmetric gratings produced by altering every Nth ridge of a conventional (symmetric) grating coupler, we analyse different regimes corresponding to the interplay between diffractive coupling to waveguide modes and band gap effects caused by the Bragg reflection of waveguide modes. The symmetric and double- and triple-period asymmetric grating couplers are considered in detail for the same unperturbed two-mode waveguide and the grating coupler parameters that ensure the occurrence of total transmission extinction at the same wavelengths. It is found that the highest Q is expected for the double-period asymmetric grating, a feature that we explain by the circumstance that the first-order distributed Bragg resonator (DBR) is realized for this configuration while, for other configurations, the second-order DBR comes into play. Experiments conducted at telecom wavelengths for all three cases using thin-film Al2O3-on-MgF2 waveguides and Ge diffraction gratings exhibit the transmission spectra in qualitative agreement with numerical simulations. Since the occurrence of considered qBIC can be analytically predicted, the results obtained may serve as reliable guidelines for intelligent engineering of asymmetric grating waveguide couplers enabling highly resonant, linear and nonlinear, electromagnetic interactions.

Compact device for the generation of toroidal spatiotemporal optical vortices

Due to the unique spatiotemporal coupling characteristics in phase, spatiotemporal optical vortices have attracted extensive attention. Toroidal vortices, as high-dimensional spatiotemporal optical vortices, have become a research hotspot in recent years due to their unique topological structures. In this paper, we propose an asymmetric grating structure for the generation of optical toroidal vortices in a compact way. A cylindrical vector wave packet is transformed by the structure into a transmitted toroidal vortex pulse. Such a compact toroidal vortex generator may find applications in optical topology and high-dimensional optical communications.

Design of high power evanescent quantum dot distributed feedback lasers on Si

Great advancements in III–V/Si epitaxy have pushed quantum dot lasers to the forefront of silicon photonics. In this work, we designed the structures of evanescent coupled quantum dot distributed feedback lasers with asymmetric gratings, which made significant improvement in on-chip output power while maintaining single-longitudinal-mode stability. The optimal λ/4 phase-shift position (the ratio of the grating length from the rear-end of λ/4 phase-shift to the total grating length) from conventional position of 0.50 to 0.64 allows the ratio of the output power at both sides of silicon waveguide to be increased from 1.0 to 5.9. Moreover, the optimal duty cycle at one side of the phase-shift from 0.50 to 0.8 allows the ratio to be increased from 1.0 to 3.7. Meanwhile, the ratio could be dramatically improved from 1.0 to 9.2 by changed the duty cycle at one side of phase-shift to 0.7 while maintaining the phase-shift position of 0.64. With those designed structures, evanescent coupled quantum dot lasers could challenge the state-of-the-art bonded quantum well lasers and may eventually become ubiquitous and affordable for future commercial production.

Low-threshold lasing behavior assisted by the asymmetric grating profile in the guided-mode resonance structure

We present a nanoscale laser based on an asymmetric guided-mode resonance (AGMR) structure. The AGMR creates hybrid resonant modes, which consist of the newly generated quasi-bound states in the continuum (quasi-BICs) and the intrinsic GMR mode. By modifying the structural parameters, the GMR and quasi-BICs modes can be matched with the pump and emission wavelengths of the gain medium, respectively. Therefore, high-intensity near fields of the GMR as the resonant optical pumping mode and high quality (Q) factor of the quasi-BICs as the resonant-emitting mode jointly contribute to the low-threshold lasing behavior. The finite-difference time-domain method is used to analyze the lasing characteristics, and a threshold of 14.2 μJ/cm2 is obtained, which is approximately ten times lower than that of a single-coupled mode laser. By adjusting the asymmetry parameters and fill factor of the grating, the threshold can be further reduced to 8.9 μJ/cm2. Additionally, we have also studied the feasibility of the AGMR structure by adjusting the simulation boundary conditions from periodic to perfectly matched layer to simulate a finite-sized structure. The findings provide a new way to excite hybrid modes, opening up possibilities for applications that demand highly confined fields and high Q-factors.

Mitigating the Impact of Asymmetric Deformation on Advanced Metrology for Photolithography

Controlling overlay in lithography is crucial for improving the yield of integrated circuit manufacturing. The process disturbances can cause undesirable morphology changes of overlay targets (such as asymmetric grating), which can significantly impact the accuracy of overlay metrology. It is essential to decouple the overlay target asymmetry from the wafer deformation, ensuring that the overlay metrology is free from the influence of process-induced asymmetry (e.g., grating asymmetry and grating imbalance). Herein, we use an asymmetric grating as a model and show that using high-diffraction-order light can mitigate the impact of asymmetric grating through the rigorous coupled-wave analysis (RCWA) method. In addition, we demonstrate the diffraction efficiency as a function of the diffraction order, wavelength, and pitch, which has guiding significance for improving the measurement accuracy of diffraction-based overlay (DBO) metrology.

MoS2-based polarization-sensitive photodetectors with asymmetric plasmonic structures and decreased detection time

This study analyzes the response time and sensitivity of photosensors that use 2D MoS2 semiconductor films with a high concentration of defects resulting from technological process sequences. In the developed photosensors, we introduce asymmetric plasmonic gratings providing their sensitivity enhancement as well as light polarization sensitivity. The photodetectors, as initially fabricated, exhibit an exceptionally long response time, surpassing a duration of a thousand seconds. This prolonged response time significantly impedes the practical application of these detectors. Nevertheless, the implementation of a specifically designed algorithm enables a dramatic reduction in detection time, by several orders of magnitude in comparison to the response time. It is based on a mathematical model that accounts for the dynamic features of these devices. Our study demonstrates that photodetectors, when enhanced with asymmetric plasmonic gratings, exhibit significantly improved performance metrics. These include a photosensitivity of approximately 60 mA/W and a degree of linear polarization at 0.78. Furthermore, the implementation of our proposed algorithm, which optimizes the calculation of detection time, dramatically reduces the characteristic detection time from over a thousand seconds to just around 10 s.

Independent Dual‐Band Bound States in the Continuum Supported by Double Asymmetric Periodic Gratings in Germanium‐Based Structure (Laser Photonics Rev. 18(4)/2024)

Independent Dual‐Band Bound States in the Continuum Supported by Double Asymmetric Periodic Gratings in Germanium‐Based Structure (Laser Photonics Rev. 18(4)/2024)

2D asymmetric diffraction grating controlled by vortex light in double-Λ-type atomic system

A two-dimensional (2D) asymmetric diffraction grating controlled by vortex light in a double-Λ-type atomic system is studied. Such an atomic system is driven by a weak traveling-wave probe field and a signal field, a position-dependent strong standing-wave (SW) control field, and a Laguerre–Gaussian (LG) vortex field. Due to the asymmetric properties of the LG vortex field, the probe photons can be asymmetrically diffracted into four different domains after passing through the atomic media. The Diffraction patterns and intensities of the 2D asymmetric diffraction grating can be manipulated by the detuning of the probe field, the interaction length, and the intensity of SW control field. In addition, the relative phase and the azimuth parameter which is closely related to the vortex light also can be used to regulate the asymmetric diffraction grating effectively. This work may provide useful reference for optical information processing, especially for the design of optical beam dividers with desired intensities and novel quantum devices requiring asymmetric optical transmission.

Design of enlarged-asymmetric-power single-mode silicon evanescent lasers with asymmetric distributed feedback gratings

Great achievements in the bonded III–V lasers have pushed the productization process of silicon photonic chips. In this work, we report the design of silicon evanescent lasers with asymmetric gratings, which can provide high output power and stable single-longitudinal-mode operation. The high coupling efficiency and low threshold current density are achieved by linearly expanding the width of rib waveguide from 0.4 µm to 2 µm through 150 µm taper coupler. By optimizing the λ/4 phase-shift position ratio from conventional 0.50 to 0.64, the ratio of output power at both sides of the bonded lasers (P2/P1) is improved from1.0 to 5.0, and the normalized grating coupling coefficient (κL) remains at 2.78. By optimizing the duty cycle at one side of λ/4 phase-shift from 0.5 to 0.8 and another side remains at 0.5, P2/P1 is 3.3, and κL lowers from 2.78 to 2.21. Besides, keeping the phase-shift position ratio at 0.64 while changing the duty cycle to 0.7, P2/P1 increases significantly to 7.5 with κL of 2.59. This work provides a step forward towards the development of high-power silicon-based light sources.

The study on application of high-order tilted asymmetric Bragg gratings in quantum cascade lasers

High-order gratings eliminate the need for high-resolution lithography in the definition process. However, they are not widely adopted due to inability to provide sufficient coupling coefficients in long-wave mid-infrared quantum cascade lasers (QCLs). Fifth order tilted asymmetric gratings (TAS-gratings) distributed feedback (DFB) QCLs with stable single longitudinal mode are firstly reported in this paper. TAS-gratings exhibit longitudinal mode selection capability across a broad range of asymmetric distances around 1.18 μm and duty cycles around 0.5, which is attributed to their larger effective duty cycles and phase differences caused by an 18° tilt angle of the grating in the middle and two asymmetrical distributions of gratings on both sides. Further experimental results indicate that DFB QCLs with TAS-gratings exhibit stable excitation of single longitudinal modes within the duty cycle range of 0.48–0.54 due to the large coupling coefficients, the temperature drift coefficient of 0.29 nm/K of TAS-grating device is reduced by 64% compared with symmetric gratings (S-gratings) devices, the maximum side-mode suppression ratio (SMSR) of the devices exceeds 25 dB. TAS-gratings offer greater process tolerance, design flexibility and lower preparation difficulty. This study provides a solution to achieve sufficiently large coupling coefficients for high-order gratings applied to long-wave mid-infrared QCLs.

Rigorous coupled-wave analysis of unilateral Smith-Purcell radiation from asymmetric resonators.

The Smith-Purcell radiation produced by electrons moving closely to a grating can be enhanced by resonances. Here, we show a method to manipulate the directionality of the resonance-enhanced radiation. Using the rigorous coupled-wave analysis method, we compare the radiation from symmetric and asymmetric gratings, showing that the enhanced Smith-Purcell radiation can become unilateral with a perturbation that breaks the structural symmetry. Our work provides an effective method for frequency-domain calculation of Smith-Purcell radiation and also an approach to realize more efficient use of the radiation.

Ultra-high-Q and ultra-sensitive refractive index sensing in low refractive index dielectric gratings based on bound states in the continuum

The realization of a high quality factor (Q-factor) and strong local optical fields has long been of great interest in the field of nanophotonics. Unfortunately, it is still challenging to achieve high-Q and strong localized fields in nanostructures made of low refractive index materials. In this study, drawing upon the concept of bound states in the continuum (BICs), an asymmetric dielectric grating composed of low refractive index materials is demonstrated to generate an ultra-high-Q symmetry-protected quasi-BIC in the visible wavelength. Importantly, the design of BIC (quasi-BIC) mode enables the strong localized confinement of light in air. By leveraging this property, we realize ultra-sensitive refractive index sensing with a remarkable sensitivity of 669 nm/RIU and a high figure of merit (FOM) of 45,314RIU−1. This study offers an approach to achieve highly sensitive and high precision refractive index sensing with potential applications in the practical realization of strong light–matter interactions using low index materials.

Perfect Absorption and Reflection Modulation Based on Asymmetric Slot-Assisted Gratings without Mirrors.

As a perfect graphene absorber without any external mirrors, we proposed asymmetric slot-assisted grating structures supporting two degenerate resonant modes of the guided-mode resonances (GMR) and the quasi-bound states in the continuum (quasi-BIC). The GMR mode functions as an internal mirror in conjunction with the background scattering, while the quasi-BIC, which is responsible for perfect graphene absorption, stems from the horizontal symmetry breaking by an asymmetric slot. By properly shifting the slot center from the grating center, the leakage rate of quasi-BIC can be controlled in such a way as to satisfy the critical coupling condition. We provide a comprehensive study on the coupling mechanism of two degenerate resonant modes for a one-port system mimicking the resonance. We also numerically demonstrated that our proposed grating structures show an excellent reflection-type modulation performance at optical wavelength ranges when doped double-layer graphene is applied. Due to the perfect absorption at the OFF state, a high modulation depth of ~50 dB can be achieved via a small Fermi level variation of ~0.05 eV. To obtain the lower insertion loss at the ON state, the higher Fermi level is required to decrease the graphene absorption coefficient.

All-dielectric metasurface refractive index sensor with high figure of merit based on quasi-bound states in the continuum

All-Dielectric metasurfaces are widely used in the field of sense due to their small size, freedom of design, and low radiation loss. However, the optical sensor still has the problem of wide full width at half maximum (FWHM), which leads to a low figure of merit (FOM). In order to solve this problem, an all-dielectric metasurface based on quasi-bound states in the continuum (q-BIC) was proposed for refractive index sensing. The sensor structure consisted of a prism/metasurface/analyte, where the metasurface was composed of periodically arranged subwavelength asymmetric silicon gratings. The asymmetry of the structure led to the coupling of the tangential and radiative field components in the superficial layer at the resonance position, which formed a q-BIC resonance peak with a narrow FWHM in the reflection spectrum. We explained the causes of the q-BIC resonance peak generation by finite element analysis, and analyzed the factors affecting the FWHM, and also simulated the machining errors. The results showed that by reducing the asymmetry of the sensor, it could have a high FOM, with a FOM of 1.35 × 107 RIU−1. In addition, simulated machining errors within ±5 nm showed that small machining errors did not lead to the disappearance of q-BIC, which proved that the sensor had good robustness. These findings had provided ideas for the accurate detection of trace substances.

Enhancing the Goos–Hänchen shift based on quasi-bound states in the continuum through material asymmetric dielectric compound gratings

Quasi-bound state in the continuum (QBIC) resonance is gradually attracting attention and being applied in Goos–Hänchen (GH) shift enhancement due to its high quality (Q) factor and superior optical confinement. Currently, symmetry-protected QBIC resonance is often achieved by breaking the geometric symmetry, but few cases are achieved by breaking the material symmetry. This paper proposes a dielectric compound grating to achieve a high Q factor and high-reflection symmetry-protectede QBIC resonance based on material asymmetry. Theoretical calculations show that the symmetry-protected QBIC resonance achieved by material asymmetry can significantly increase the GH shift up to −980 times the resonance wavelength, and the maximum GH shift is located at the reflection peak with unity reflectance. This paper provides a theoretical basis for designing and fabricating high-performance GH shift tunable metasurfaces/dielectric gratings in the future.

Enhancement of longitudinal mode characteristic of quantum cascade laser by high-order tilted asymmetric surface grating

The high-order tilted asymmetric surface grating (TASG) is firstly investigated and applied to quantum cascade lasers (QCLs) here. Due to the tilted grating of 16°in the middle of ridge, the longitudinal mode loss difference of TASG is about 4% higher than that of the uniform grating (UNG), which results in better single mode selectivity in reflectance spectrum. Further experimental results indicate that the TASG distributed feedback (DFB) QCL maintains more stable single longitudinal mode performance with increasing the current and temperature because that TASG have a larger longitudinal mode loss difference. While the UNG QCL with the equivalent coupling coefficient exhibits mode jumping. Hence, the drift coefficient of 0.311 nm/K of TASG QCL is 60% lower than that of the UNG device and the maximum side-mode suppression ratio (SMSR) achieves 30 dB. The characteristic temperature of TASG QCL device is 36K, which is higher than 32K of UNG device. This study showcases the feasibility and superiority of the TASG in mid-infrared active devices.

Highly efficient second harmonic generation assisted by the quasi-bound states in the continuum from AlGaAs meta-gratings

Due to the design freedom of metasurfaces in tailoring wavefronts, they have become a general platform for miniaturized functional nonlinear optics. Recently, with the help of the quasi-bound states in the continuum (Q-BIC), the all-dielectric resonance metasurfaces with high quality factor (Q) boost the interaction between pump light and dielectric structure, which have become an effective way to achieve high nonlinear efficiency. Here, an asymmetric grating structure composed of AlGaAs is utilized to enhance the second harmonic generation (SHG) near Q-BIC. By breaking the symmetry of the grating structure, a simple normal incidence linearly polarized plane wave can be treated to excite stronger localized fields within the metasurfaces, allowing strong light–matter​ interactions in nonlinear metasurfaces. Numerical investigations depict that the Q-factor and resonant field enhancement noticed on the metasurface are intimately related to the asymmetric structure of the system. In addition, the high Q-factor resonance facilitates the SHG conversion process in AlGaAs metasurfaces. Especially, for the AlGaAs metasurface with Q-factor of 4.0314 × 104, the conversion efficiency of SHG can reach 10.5% when the pumping intensity is 5 MW/cm 2. This study is of great importance for analyzing the efficient nonlinear metasurfaces with high Q-factors.