Subwavelength Grating Research Articles

All-silicon and ultra-compact transverse electric-pass polarizer with large bandwidth and high polarization extinction ratio based on subwavelength metamaterials

In this paper, we have proposed an all-silicon and compact transverse electric (TE)-pass polarizer with large bandwidth and high polarization extinction ratio (PER), where the polarizer uses a semi-arc waveguide to cause polarization-dependent bending loss (PDBL). The combination of the subwavelength grating (SWG) and double adiabatic tapers serve as an equivalent cladding to get strengthened PDBL. The parameters of SWG and double adiabatic tapers are analyzed in detail to improve the performance of the device. In the proposed TE-pass polarizer, the injected fundamental TE mode (i.e., TE0) gets protection and the injected fundamental transverse magnetic mode (i.e., TM0) lets out. The footprint is only about 6 μm × 10 μm, which is substantially reduced compared with the previous counterparts. From the results, its bandwidth is as high as 228 nm (from 1.482 to 1.710 μm) with PER > 39 dB and low excess loss (EL < 1 dB), and it has an ultra-high PER of 42.5 dB and a low EL of 0.1 dB at 1.55 μm. In addition, large fabrication tolerances of 20 nm ( ± 10 nm) to the key structural parameters are analyzed.

Compact and broadband on-chip silicon polarization beam splitter based on tilted subwavelength grating

Efficient polarization management is a key issue for on-chip silicon photonic integrated circuits. Here, we propose a compact and broadband on-chip silicon polarization beam splitter (PBS) using tilted subwavelength grating (SWG) structure. The key polarization separation region consists of an input tapered SWG, a titled SWG, and two output tapered SWGs at both output ports. The tilted SWG has a strong polarization dependence for the mode coupling, and we have introduced different grating duty cycles, positional offset, and a new, to the best of our knowledge, output mode transition structure to enhance the device performance. Based on these techniques, the key coupling length of the proposed PBS is only 5 µm (total length, 12 µm), together with polarization extinction ratio (ER) 21.2 dB (19.7 dB) and insertion loss 0.21 dB (0.39 dB) for the TE (TM) mode. Moreover, the device working bandwidth can be extended to 311 nm (154 nm) by keeping E R &gt; 15 d B ( E R &gt; 18 d B ), satisfying the on-chip broadband applications. We believe the proposed device could become a good candidate to boost the on-chip polarization management toward compact and broadband directions.

Dynamically switchable triple-band absorption enhancement of graphene by a subwavelength grating coupled hybrid structure

The dynamically switchable triple-band absorption enhancement over 96% of monolayer graphene in the visible to the near-infrared range is achieved by a simple configuration consisting of subwavelength grating and periodically distributed Bragg reflectors. The isolated optical field distribution enabled by the optical Tamm state (OTS), guided mode resonance (GMR) and Fabry–Perot (FP) mode allows each graphene layer to selectively absorb the incident light in different wavelengths at different parts of the structure, creating favorable conditions for the independent extraction of multi-channel signals. Moreover, the continuous regulation of the absorption wavelength is futher realized by varying the geometrical parameters and dielectric permittivity. Also, the electrical tunability of graphene allows the independent step modulation of the absorption efficiency of three eigenmodes, resulting in the real-time dynamic switching of the system's triple-band/dual-band/single-band operation modes. The proposed structure can be integrated as a dynamic component into other graphene-based multifunctional and switchable optoelectronic devices.

Compact and broadband design of an 850nm 2 × 2 3-dB directional coupler with a shallowly etched SWG gap.

A compact and broadband 2×2 3dB directional coupler (DC) is designed at the 850nm wavelength region based on the silicon nitride platform. The proposed DC is equipped with a shallowly etched subwavelength gratings (SWG) gap so that the length of the coupling region is effectively reduced to 7.8µm for equally splitting the fundamental TE polarization state. Such a DC coupling region is much more compact than that using empty and pure SWG gaps. Meanwhile, the DC working bandwidth is determined to be broader than 106nm, and the insertion loss is always kept under an acceptable level of 0.6dB over the entire wavelength region. Considering that the shallowly etched device requires two photoetching processes in manufacturing, we did a numerical analysis to characterize the fabrication tolerance under a minimum overlay accuracy of 20nm in both x and y directions. Such overlay mismatch only increases the maximal imbalance from 0.55 to 0.6dB at the center wavelength. In addition, we discussed the impact of the proposed DC in its potential application, i.e.,optical coherence tomography. The results show that the proposed DC is able to support a high axial resolution of 3.77µm, and the 20nm overlay mismatch leads to neglected axial resolution degradation.

Integrated Subwavelength Grating Waveguide Bragg Gratings for Microwave Photonics

Bragg gratings (BGs) have numerous applications in microwave photonic (MWP) signal processing and MWP systems due to the ability to obtain custom-designed amplitude and phase responses. Moreover, waveguide BGs (WBGs) are compact and can be readily integrated with other passive and active components to realize MWP building blocks and processing systems. Subwavelength grating (SWG) waveguides and passive devices in silicon-on-insulator (SOI) have emerged as an enabler for developing break-through devices based on anisotropic metamaterials. In this paper, we review recent advances on developing SWG WBGs in SOI, including chirped, phase-shifted, sampled, and polarization (in)dependent structures. Applications of these SWG WBGs in MWP are described.

A Design Method of Two-Dimensional Subwavelength Grating Filter Based on Deep Learning Series Feedback Neural Network.

Subwavelength grating structure has excellent filtering characteristics, and its traditional design method needs a lot of computational costs. This work proposed a design method of two-dimensional subwavelength grating filter based on a series feedback neural network, which can realize forward simulation and backward design. It was programed in Python to study the filtering characteristics of two-dimensional subwavelength grating in the range of 0.4–0.7 µm. The shape, height, period, duty cycle, and waveguide layer height of two-dimensional subwavelength grating were taken into consideration. The dataset, containing 46,080 groups of data, was generated through numerical simulation of rigorous coupled-wave analysis (RCWA). The optimal network was five layers, 128 × 512 × 512 × 128 × 61 nodes, and 64 batch size. The loss function of the series feedback neural network is as low as 0.024. Meanwhile, it solves the problem of non-convergence of the network reverse design due to the non-uniqueness of data. The series feedback neural network can give the geometrical structure parameters of two-dimensional subwavelength grating within 1.12 s, and the correlation between the design results and the theoretical spectrum is greater than 0.65, which belongs to a strong correlation. This study provides a new method for the design of two-dimensional subwavelength grating, which is quicker and more accurate compared with the traditional method.

Ultracompact, polarization-independent, and highly scalable optical power splitting model employing fan-out bending metamaterials

Optical power splitters (OPSs) are essential components in the photonic integrated circuits. Considerable power splitting schemes have been reported on the silicon-on-insulator platform. However, the corresponding device lengths are enlarged, and polarization-sensitive operations are usually encountered when the splitting channels are increased from two to five. In this paper, a novel power splitting model is proposed to overcome these limitations. Here, fan-out bending subwavelength grating (FBSWG) metamaterials instead of classical straight SWGs are leveraged to expand the input TE/TM mode in an ultracompact region and further bend its wavefronts. By using N -angled tapers to match bending wavefronts, the light expanded by FBSWGs can be collected and evenly distributed into N output channels. Based on such a model, three OPSs are designed and experimentally demonstrated, which are the shortest polarization-independent 1 × 3 , 1 × 4 , and 1 × 5 OPSs reported until now to our knowledge. The characterizations show low insertion losses ( &lt; 1.2 dB , &lt; 1.35 dB , and &lt; 1.65 dB ) and uniformities ( &lt; 0.9 dB , &lt; 1 dB , and &lt; 1 dB ) over bandwidths of 54 nm, 49 nm, and 38 nm for the 1 × 3 , 1 × 4 , and 1 × 5 OPSs, respectively. For the first time, an ultracompact device length of &lt; 4.3 μm and a polarization-independent operation can be maintained simultaneously as the output splitting channels are increased.

An Optimization Framework for Silicon Photonic Evanescent-Field Biosensors Using Sub-Wavelength Gratings.

Silicon photonic (SiP) evanescent-field biosensors aim to combine the information-rich readouts offered by lab-scale diagnostics, at a significantly lower cost, and with the portability and rapid time to result offered by paper-based assays. While SiP biosensors fabricated with conventional strip waveguides can offer good sensitivity for label-free detection in some applications, there is still opportunity for improvement. Efforts have been made to design higher-sensitivity SiP sensors with alternative waveguide geometries, including sub-wavelength gratings (SWGs). However, SWG-based devices are fragile and prone to damage, limiting their suitability for scalable and portable sensing. Here, we investigate SiP microring resonator sensors designed with SWG waveguides that contain a “fishbone” and highlight the improved robustness offered by this design. We present a framework for optimizing fishbone-style SWG waveguide geometries based on numerical simulations, then experimentally measure the performance of ring resonator sensors fabricated with the optimized waveguides, targeting operation in the O-band and C-band. For the O-band and C-band devices, we report bulk sensitivities up to 349 nm/RIU and 438 nm/RIU, respectively, and intrinsic limits of detection as low as 5.1 × 10−4 RIU and 7.1 × 10−4 RIU, respectively. This performance is comparable to the state of the art in SWG-based sensors, positioning fishbone SWG resonators as an attractive, more robust, alternative to conventional SWG designs.

Simple measurement of a wedge angle based on subwavelength grating

In this study, a high-precision and low-cost instrument was developed for the measurement of the wedge angle of wedge plates. The module was designed using two prism-patterned subwavelength gratings, with the −1 order rays extracted using a semiconductor laser beam-splitting device. Subsequently, in reference to Snell’s Law and the diffractive characteristic, the wedge angles of wedge plates were tested and analyzed when the two points on the movable receiving screen were overlapped through adjustment of the distance. This result revealed that the proposed system achieved sub-second ultimate resolution for the wedge angles of the wedge plate under a 1-µm precision requirement for measuring instruments.

Ultracompact Polarization Splitter–Rotator Based on Shallowly Etched Subwavelength Gratings and Anisotropic Metasurfaces

Polarization splitter–rotators (PSRs) are an essential component in on-chip polarization-sensitive and polarization–division multiplexing systems. In this work, we propose an ultracompact and high-performance silicon-based polarization splitter–rotator utilizing anisotropic metasurfaces, which is the first to combine the two, to our knowledge. The tilted periodic metasurface structure has different modulation effects on different polarized light fields, such as the transverse–electric (TE) mode and the transverse–magnetic (TM) mode, which are beneficial for designing polarization management devices. According to the results, the entire length of the silicon PSR was ~13.5 μm. The TE-to-TM conversion loss and polarization conversion ratio ere −0.154 dB and 96.5% at 1.55 μm, respectively. In the meanwhile, the cross talk and reflection loss were −27.0 dB and −37.3 dB, when the fundamental TE mode was input. The insertion loss and cross talk were −0.19 dB and −25.01 dB at the central wavelength when the fundamental TM mode was input. In addition, the bandwidth reached up to ~112 nm with polarization conversion loss and insertion loss higher than −0.46 dB and −0.36 dB. The simulations also show that the designed devices had good fabrication tolerance.

On-Chip Refractive Index Sensor With Ultra-High Sensitivity Based on Sub-Wavelength Grating Racetrack Microring Resonators and Vernier Effect

An approach to optimize the sensitivity and the limit of detection of on-chip refractive index sensor is proposed and demonstrated based on sub-wavelength grating racetrack microring resonator and Vernier effect. The sub-wavelength grating waveguide can reduce the structure limitation of the light field, which is beneficial to enhancing the interaction between the photon and analyte. By optimizing the parameters of the sub-wavelength grating racetrack microring resonator, the sensitivity of the sensor could be significantly improved to 664 nm/RIU. Subsequently, capitalizing the Vernier effect, a two cascaded microring-based refractive index sensor is designed. Owing to the Vernier effect, the wavelength spacings among the overlapped peaks could be effectively amplified more than ten times, leading to a high performance. The results demonstrate that an ultra-high sensitivity of 7061 nm/RIU and a low limit of detection of 1.74 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> RIU. With the advantages of ultra-high sensitivity and low limit of detection, the integrated device has important value in the fields of environmental monitoring and biosensors.

Dual-Mode Silicon Ring Resonator Based on Subwavelength Grating Directional Couplers

We propose and experimentally demonstrate a dual-mode silicon ring resonator which could process TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> modes simultaneously. By introducing subwavelength grating into the coupling region, compact footprint of ∼90 μm × 20 μm is achieved. The proposed device is fabricated on the standard silicon-on-insulator platform. The measured results indicate the mode crosstalk within C band are less than −12.2 and −11.5 dB for TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> modes, respectively. Furthermore, heaters for resonant wavelength tuning are fabricated and measured with good uniformity for both modes.

On‐Chip Chiral Mode Switching by Encircling an Exceptional Point in an Anti‐Parity‐Time Symmetric System

AbstractDynamically encircling the exceptional point (EP) of an anti‐parity‐time (anti‐PT) symmetric system can achieve chiral mode switching between symmetry‐broken eigenmodes, which is especially important for developing nanophotonic devices. However, the realization of anti‐PT‐symmetric systems on a chip still remains significantly challenging as precisely controlling the parameters of nanophotonic structures is difficult. Herein, a novel anti‐PT‐symmetric system is designed and constructed on a silicon‐on‐insulator chip by taking advantage of subwavelength gratings to help engineer nanophotonic structures. Consequently, on‐chip chiral switching between symmetry‐broken eigenmodes is experimentally realized by dynamically encircling the EP in parametric space of the proposed anti‐PT‐symmetric system. Moreover, the achieved chiral mode switching exhibits high output power ratios (≥ 6.4 dB) at 1550 nm, wide‐band response that covers C‐band completely, and great thermal stability up to 90 °C, which indicate desirable performance features for practical applications in optical communication. This study paves an avenue for realizing on‐chip chiral switching between symmetry‐broken modes, which shows great promise for developing nanophotonic devices, such as optical switching, logic gates, and isolators, in optical telecommunication bands.

Polarization control of 795 nm vertical-cavity surface-emitting lasers by in-phase surface gratings.

795 nm vertical-cavity surface-emitting lasers (VCSELs) with in-phase surface gratings are fabricated and investigated theoretically and experimentally. The polarization characteristics of 795 nm VCSELs with various grating periods and depths are analyzed using the rigorous coupled-wave analysis (RCWA) method; the dependence of polarization stability on the profile of gratings demonstrates that the trapezoid grating ridge slightly enhances the orthogonal polarization suppression ratio (OPSR), but increases the threshold current. The fabricated VCSELs with a sub-wavelength in-phase surface grating of a duty cycle of 0.5 show stabilized output polarization at the cost of increasing the threshold current which is in agreement with the calculations. The grating VCSELs with a period of 200 nm and an oxide aperture of 3.43µm×4.39µm produce a single-mode output with an OPSR of 16.6 dB and a slope efficiency of 0.42 W/A at 85°C.

Highly efficient and tunable terahertz polarization converter based on double subwavelength metallic gratings infiltrated with liquid crystal.

In this work, a tunable cross-polarized transmission structure at the terahertz frequency was demonstrated, and the polarization state during modulation was investigated. The proposed structure can significantly enhance the polarization conversion performance of nematic liquid crystals by leveraging the formation of a Fabry-Perot-like resonant cavity that consists of two metal gratings. As a result, the incident waves are continuously reflected in the liquid crystal layer to complete the accumulation of polarization angle changes. From the experimental results acquired, it was concluded that the insertion loss of the cross-polarization transmission was less than 3 dB and the extinction ratio was larger than 28 dB in the frequency range of 388-426 GHz. Our work provides useful insights for improving the efficiency of cross-polarization conversion by enhancing the resonance process in a Fabry-Perot-like resonant cavity and, thus, significantly extending the equivalent optical path.

Efficient mode converters and filters using asymmetrical directional couplers with subwavelength gratings.

Mode-division multiplexing (MDM) is a promising solution to improve data transmission capacity for future optical interconnect networks. Mode converters and mode filters play a key role in on-chip MDM systems. Here, we propose and experimentally demonstrate a device, enabling mode conversion and filtering simultaneously, which is composed of asymmetrical directional couplers with subwavelength gratings, in a small footprint of 14.7 µm × 1.42 µm. The device can realize optical mode conversion between the first-order transverse electric (TE) mode and the fundamental TE mode, and can also filter the fundamental TE mode efficiently. The conversion efficiency is over 95%, with a broad 1 dB bandwidth over 80 nm and a high mode extinction ratio of >29 dB. As a mode filter, strong mode elimination of >30 dB is achieved.

A silicon TM mode multiplexer based on three waveguide directional coupler with subwavelength grating

In this work, a silicon TM mode multiplexer based on three waveguide directional coupler constructed by subwavelength gratings (SWG) is proposed. The working bandwidth is greatly increased by using the SWG waveguides and a mode block based on SWG is added to reduce the crosstalk. FDTD simulation results show that the insertion loss and crosstalk at 1550 nm are 0.7 dB and −35.9 dB respectively. Large working bandwidth of 270 nm (1400 nm∼1670 nm) can be achieved under the conditions of less than 3 dB insertion loss and −15 dB crosstalk. In addition, the fabrication tolerance of the proposed device is also investigated.

Numerical Simulation of GaAs/AlOx High Index Contrast Subwavelength Gratings for GaAs-Based Vertical Cavity Surface Emitting Lasers

Numerical Simulation of GaAs/AlOx High Index Contrast Subwavelength Gratings for GaAs-Based Vertical Cavity Surface Emitting Lasers

Grating-based microcavity with independent control of resonance energy and linewidth for non-Hermitian polariton system

Exciton–polaritons have become an emerging platform for implementing non-Hermitian physics. The implementation commonly requires control of both the real and imaginary parts of the eigenmodes of the system. We present an experimental method to achieve this purpose using microcavities with sub-wavelength gratings as reflectors. The reflectivity and reflection phase of the grating can be changed by its geometric parameters, and they determine the energy and linewidth of the polariton modes. We demonstrate that this method allows a wide range of possible polariton energy and linewidth, suitable for implementing non-Hermitian polariton systems with coupled modes.

Subwavelength dielectric grating structures with tunable higher order resonance for achromatic augmented reality display.

Color non-uniformities caused by a dispersion effect can seriously affect the image quality for a diffractive waveguide display system. In this work, we propose a subwavelength multilayered dielectric grating structure by a rigorous coupled wave analysis as a novel coupling grating, to the best of our knowledge, for waveguide-based near-eye displays to overcome the "rainbow" effect. Such a grating structure exhibits a tunable high-efficiency resonance in first-order diffraction due to resonant coupling of incident light with the grating structure. A further analysis of the resonant behaviors helps us get a clear understanding of the underlying physics for the mode excitation and resonant coupling process. The first-order resonance with a diffraction efficiency of more than 60% can be achieved with the resonant angle continuously shifted to get a large field of view. The resonant angle, diffraction efficiency, and spectral linewidth can be easily tuned by the geometrical parameters of the grating structure.