Top Grating Research Articles

Numerical Modeling of Hybrid Solar/Thermal Conversion Efficiency Enhanced by Metamaterial Light Scattering for Ultrathin PbS QDs-STPV Cell

Ultrathin cells are gaining popularity due to their lower weight, reduced cost, and enhanced flexibility. However, compared to bulk cells, light absorption in ultrathin cells is generally much lower. This study presents a numerical simulation of a metamaterial light management structure made of ultrathin lead sulfide colloidal quantum dots (PbS CQDs) sandwiched between a top ITO grating and a tungsten backing to develop an efficient hybrid solar/thermophotovoltaic cell (HSTPVC). The optical properties were computed using both the finite integration technique (FIT) and the finite element method (FEM). The absorptance enhancement was attributed to the excitations of magnetic polaritons (MP), surface plasmon polaritons (SPP), and lossy mode resonance (LMR). The HSTPVC with the metamaterial optical light management structure was assessed for short-circuit current density, open-circuit voltage, and conversion efficiency. The results show a conversion efficiency of 18.02% under AM 1.5 solar illumination and a maximum thermophotovoltaic conversion efficiency of 12.96% at TB = 1600 K. The HSTPVC can operate in a hybrid solar/thermal conversion state when the ITO grating is included by combining the advantages of QDs and metamaterials. This work highlights the potential for developing a new generation of hybrid STPV cells through theoretical modeling and numerical simulations.

Quasi-bound states in the continuum in asymmetric hetero-bilayer metasurfaces

Bound states in the continuum (BICs) are completely confined states of open optical structures existing above the light line. However, BICs must be turned into quasi-BICs having a finite Q-factor to be accessible. To achieve quasi-BICs with a high Q-factor at normal incidence, diverse optical structures have been explored by inducing asymmetry by etching a tiny part of the structure. However, it is difficult to introduce a small asymmetry by the etching process for a high Q-factor. Here, we propose novel asymmetric hetero-bilayer metasurface using a low-refractive-index top grating and high-refractive-index bottom grating. The thin upper grating is intended to introduce perturbation accurately, and the bottom grating functions as a broadband reflector. We identified the effect of the geometric perturbation and the top grating material on the Q-factor, resonance wavelength, and background reflectance. As the geometric asymmetry increases, both the Q-factor and the background reflectance decreased, and the resonance wavelength changed accordingly. The Q-factor followed the inverse quadratic of the geometric perturbation and the inverse cube of the relative permittivity. The effect of geometric perturbation decreased up to 10 times with a lower refractive index. Taking advantage of a high-Q hetero-bilayer metasurface, a gas sensor was numerically investigated, and it distinguished a change in the refractive index of 5 × 10−7. Furthermore, the asymmetric hetero-bilayer metasurface showed a quasi-BIC with a high Q-factor on a rigid substrate.

Two-dimensional simple structured ultranarrow-band metamaterial perfect absorber with dielectric nanocylindrical array

A two-dimensional (2D) simple structured ultranarrow-band metamaterial perfect absorber (MPA) with nanocylindrical array is studied both theoretically and experimentally. Using a dielectric 2D cylindrical array as the top grating layer of the MPA, an ultranarrow nonpolarizing absorption peak at normal incidence is obtained. Furthermore, we use the same dielectric material to simplify the topology of the ultranarrow-band MPA, and the number of film layers in the MPA is reduced from 3 to 1. By optimizing its structural parameters and metal substrate, the highest ultranarrow nonpolarizing absorption peak that approaches 1 at normal incidence is obtained. To analyze the absorption physical mechanism, the heat power density distributions of the optimized MPA are simulated to see where the incident light is absorbed. Moreover, numerical simulation results show that the main transverse-electric absorption peak of the optimized MPA is insensitive to the incident angle, and the main transverse magnetic absorption peak is sensitive to the incident angle. Finally, we fabricate an MPA sample with dielectric nanocylindrical array and measure its absorption spectra to make a further comparison. The experimental data are consistent with the theoretical ones. This method might be helpful to reduce the manufacture difficulty and cost of the ultranarrow-band MPAs and also to promote the development of applications of the ultranarrow-band MPAs.

Enhanced Optical Confinement Enriching the Power Conversion Efficiency of Integrated 3D Grating Organic Solar Cell.

In this paper, we examine the impact of three-dimensional grating layers embedded at selected locations in an organic solar cell structure to obtain enhanced efficiency. The design, simulations, and optimizations were carried out using an in-house tool based on the rigorous coupled-wave analysis (RCWA) method developed on the MATLAB R2019a platform. An optimal organic solar cell structure design with a top grating layer exhibited an increase of in the short-circuit current density compared to an organic solar cell structure with a smooth top layer. The power conversion efficiency (PCE) increase was mainly due to increased light confinement in the thin absorbing layer. Adding an embedded grating layer in the absorption layer resulted in a significant increase in the absorptance spectral bandwidth, where the short-circuit current density increased by . In addition, the grating cells yielded a substantial improvement in the cell’s conical absorptance since the existence of a surface plasmon polariton (SPP) in the back metal gratings increases the confinement properties. Further, the effect of a pyramid-shaped embedded grating array was a slight improvement in the PCE compared to the rectangular-shaped grating arrays. We showed that a pyramid-grating can act as a nano black-body layer, increasing the absorption for a wide range of azimuthal and polar incident angles.

Directly modulated azimuthally polarized vector beam laser design.

Directly modulated vector beam lasers are increasingly desirable for wide applications ranging from optical manipulation to optical communications. We report the first, to our knowledge, high-speed directly modulated vector beam laser with azimuthally polarized emission. It is a microcylinder cavity interacted with a proper second-order grating on top, which enables single mode lasing and efficient surface emission. Through theoretical and numerical analysis, the laser is designed in detail. With an optimized top grating, the emission of the laser is an azimuthally polarized vector beam. With high-differential-gain material and a small active region, the laser can be directly modulated with a high 3 dB bandwidth reach of 40 GHz in simulation. The proposed high-speed directly modulated semiconductor laser with an azimuthally polarized vector beam is promising for applications in fiber space communications or quantum information.

Near-infrared spectroscopy using period-chirped Si/SiO/SiO2-based guided mode resonance filter.

We demonstrate a Si/SiO/SiO2-based period-chirped guided mode resonance (GMR) filter to discriminate telecom o-band wavelengths by spatially resolved horizontal movement. Continuously period-chirped silicon gratings were fabricated by using a Lloyd's laser interferometer with a convex mirror. Due to the large waveguide effective index, the GMR filter can be realized with a short grating period, thus enabling a slow grating period transition along the sample position and high optical resolution in wavelength discrimination. Depositing a SiO/SiO2 stack on top of silicon gratings enables a narrowband GMR filter with a linewidth of 1-1.5 nm over a wavelength range of 1260-1360 nm. By using the chirped GMR filter as a dispersive device, the optical spectra of a near-infrared broadband light source are reconstructed. An optimized aspheric mirror is proposed to further improve the linearity of chirped gratings. Such a period-chirped GMR filter is promising for compact on-chip spectroscopy and sensing applications.

Tunable nonreciprocal thermal emitter based on metal grating and graphene

Various nonreciprocal thermal emitters, which provide opportunities for higher energy conversion efficiency, have been proposed to completely violate the traditional Kirchhoff's law. However, the tunable nonreciprocal thermal emitters remain barely investigated. In this paper, by sandwiching a graphene monolayer between a top metallic grating and a bottom magneto-optical film backed with a metallic mirror, we achieve tunable nonreciprocal radiation effect. It is shown that strong nonreciprocal radiation for graphene under initial state is realized at wavelength around 14.845 μm when the incident angle is 30° and the external magnetic field is 3 T. The physical origin behind such phenomenon is disclosed by investigating both the distribution of magnetic field at the resonant peak and the coupled-mode theory. Besides, the nonreciprocal radiation performance remains perfectly within certain range of structure parameters, which is benefit for practical fabrication and applications. More importantly, the strong nonreciprocal radiation of the proposed scheme can be flexibly tuned by the gate voltage of graphene without redesigning and refabricating the structure. We believe that this work will provide new approaches for the design of novel energy harvesting systems and nonreciprocal thermal emitters.

Ultra-wideband Transmissive Linear Polarization Device Based on Graphene

In order to achieve both adjustable wideband and high Polarization Conversion Rate (PCR) of the transmitted waves, a novelty tri-layered structure is proposed for terahertz applications. The Rhombus Hollow Square (RHS) is built up by top and bottom gold gratings on Silicon Dioxide and Polyamide substrate with graphene strips. The proposed polarizer broadens the bandwidth and has well performance. As chemical potential increases, the bandwidth is also broadened by adjusting the graphene. From 0.5 THz to 3 THz, the PCR is greater than 90%, and the relative bandwidth up to 142.9%. The transmission and absorption of polarizer are analyzed at the oblique incidence with chemical potential 0.1eV. By simulating and analyzing the performance, a new result of maintaining broadband and high transmittance in oblique incidence is obtained.

High-absorption grating-insulator-metal structures.

A thin grating-insulator-metal (GIM) structure consisting of a top metal grating layer on a dielectric layer and a bottom metal layer is proposed, which shows a broadband high absorption at a small thickness. This phenomenon is attributed to the appropriate effective surface permittivity of the top grating layer and the cavity resonance of the middle insulator layer. By optimizing the structural and material parameters, the materials of the GIM structure from top to bottom are Mn, Al2O3, and Mn with thicknesses of 10, 70, and 70 nm, respectively. The structure with these optimum parameters is fabricated and characterized, and an improved performance with absorption exceeding 90% in the visible region is obtained using Mn as the metal layers. The experimental results are in good agreement with the numerical values, depicting an ultrabroad absorption bandwidth. The conclusions presented here could have potential applications in optical devices used for optical displacement detection and visible light absorption.

Analysis of Leaky-Wave Radiation From Corrugated Parallel-Plate Waveguides With Steerable Beams by Rotatable Corrugations

This article presents a modal analysis entailing classical vector potentials and asymptotic corrugations boundary conditions for treating a leaky-wave antenna (LWA) composed of a corrugated parallel-plate waveguide with mutually rotatable grated surfaces and whose upper grille is penetrable to the exterior space. Beam steering at fixed frequencies is achieved by rotating both or either of the base and top gratings, thus finding roles that conventional frequency-steered LWAs are unable to serve. Within millimeter-wave (mm-wave) and THz regimes where components become strongly miniaturized, the advantages of electronic beam-scanning at lower microwave frequencies begin to get offset by the benefits of mechanical steering. Not only does the rotational form of steering enjoy the benefits of simplicity and low cost, it also circumvents the need for complex networks at mm-wave/THz regimes required by electronic-steering that annuls the strengths of LWA in the first place. The rotation in the horizontal plane also offers high weight-handling capability, making it all the more able to manage miniaturized structures at mm-wave/THz regimes. Numerical results computed by the modal approach are validated with those simulated by commercial software. Measurements on a manufactured prototype demonstrate the efficacy of the steerable LWA just as the numerical designs have predicted.

Nanoimprinting metal-containing nanoparticle-doped gratings to enhance the polarization of light-emitting chips by induced scattering

Polarized radiative luminous semiconductor chips have huge application potential in many highly value-added fields. The integration of a subwavelength grating is recognized to be the most promising method for the development of polarized chips, but still faces the challenge of low polarized radiative performance. This paper describes a proposal for, and the development of, a scattering-induced enhanced-polarization light-emitting diode chip by directly nanoimprinting a metal-containing nanoparticle-doped grating onto the top surface of a common flip chip. The rate at which quantum-well light emission is used by the developed polarized chip is improved by more than 30%. More attractively, the doped scattering nanoparticles function as a scattering-induced polarization state converter that is sandwiched in between the top aluminum grating and the bottom silver reflector of the chips. The originally non-radiated light, with an electric-field vector parallel to the grating lines, is reflected back and forth inside the sandwich until it changes to the perpendicular vibration mode and is radiated outside the chip. Therefore, the polarization extinction ratio is greatly improved, compared to undoped samples.

Design of a single-mode directly modulated orbital angular momentum laser

We propose a design of single-mode orbital angular momentum (OAM) beam laser with high direct-modulation bandwidth. It is a microcylinder/microring cavity interacted with two types of second-order gratings: the complex top grating containing the real part and the imaginary part modulations and the side grating. The side grating etched on the periphery of the microcylinder/microring cavity can select a whispering gallery mode with a specific azimuthal mode number, while the complex top grating can scatter the lasing mode with travelling-wave pattern vertically. With the cooperation of the gratings, the laser works with a single mode and emits radially polarized OAM beams. With an asymmetrical pad metal on the top of the cavity, the OAM on-chip laser can firstly be directly modulated with electrical pumping. Due to the small active volume, the laser with low threshold current is predicted to have a high direct modulation bandwidth about 29 GHz with the bias current of ten times the threshold from the simulation. The semiconductor OAM laser can be rather easily realized at different wavelengths such as the O band, C band, and L band.

Demonstration of Low-Threshold and Directly Modulated Grating-Assisted Microcylinder Surface-Emitting Lasers

In this article, we report the demonstration of directly modulated grating-assisted microcylinder surface-emitting lasers (GAMSELs) with low threshold. Two second-order gratings are added onto the side and top of the microcylinder cavity to select a single mode with a radially polarized emission pattern and efficiently emit the mode vertically, respectively. Laser with cavity radius of 6.05 μm is demonstrated with continuous-wave lasing ranging from 1 °C to 50 °C. This radius is approaching the theoretically smallest radius for the microcylinder laser to work at O band. A low threshold of 3.6 mA at 15 °C is obtained for the GAMSEL laser. Single-mode lasing with side-mode suppression-ratio (SMSR) larger than 40 dB is also shown here. GAMSELs with different cavity radii are demonstrated as laser arrays. 3 dB direct modulation bandwidth of 12 and 14 GHz of GAMSELs with radius of about 8 and 10 μm are obtained, respectively. Besides, effects of the duty cycle of the top and side gratings, scattering losses of the cavity, gain peak of the epitaxial wafer and ion implantation are experimentally investigated.

Concentric microcavities for cylindrical vector beam lasers.

Cylindrical vector (CV) beams with polarization singularities have attracted intense research interest because of their important applications in optical trapping and manipulation, imaging, and high-speed optical communication. In this Letter, we propose a high-speed integrated device designed to emit fundamental CV beams, including both radially and azimuthally polarized beams. The device is composed of two grating-assisted concentric microcavities based on an InP platform. The microcavity with only a second-order grating shallowly etched on the top is optimized and used for improved azimuthally polarized CV beam emission. Another microcavity with both a triangular-shaped side grating and a rectangular-shaped top grating is employed for radially polarized CV beam lasing. The proposed devices can be further developed to be compatible with wavelength-division multiplexing and mode-division multiplexing techniques. They hold great potential in CV beam-based classical and quantum communication systems.

Aluminum nitride two-dimensional-resonant-rods

In the last few decades, bulk-acoustic-wave filters have been essential components of 3G-to-4G radios. These devices rely on the high electromechanical coupling coefficient (kt2 ∼ 7%), attained by aluminum nitride (AlN) film-bulk-acoustic-resonators (FBARs), to achieve a wideband and low-loss frequency response. As the resonance frequency of FBARs is set by their thickness, the integration of multiple FBARs, to form filters, can only be attained through the adoption of frequency tuning fabrication steps, such as mass loading or trimming. However, as the ability to reliably control these steps significantly decays for thinner (or higher frequency) FBARs, manufacturing FBAR-based filters, addressing the needs of emerging IoT and 5G applications, is becoming more and more challenging. Consequently, there is a quest for new acoustic resonant components, simultaneously exhibiting high-kt2 and a lithographic frequency tunability. In this work, a novel class of AlN resonators is presented. These radio frequency devices, labeled as two-dimensional-resonant-rods (2DRRs), exploit, for the first time, the unconventional acoustic behavior exhibited by a forest of locally resonant rods, built in the body of a profiled AlN layer that is sandwiched between a bottom un-patterned metal plate and a top metallic grating. 2DRRs exhibit unexplored modal features that make them able to achieve high-kt2, a significant lithographic frequency tunability, and a relaxed lithographic resolution, while relying on an optimal AlN crystalline orientation. The operation of 2DRRs is discussed, in this work, by means of analytical and finite-element-methods. The measured performance of the first fabricated 2DRR, operating around 2.4 GHz and showing a kt2 in excess of 7.4%, is also reported.

Enhanced terahertz absorption of quantum wells sandwiched between heavily doped contacts based on micro-cavity resonance

A novel high-efficiency microcavity structure of quantum wells sandwiched between periodic heavily Si-doped GaAs top contact gratings and bottom contact film has been proposed as the optical coupler of a terahertz quantum well photodetector (THz QWP). Similar to metal at visible light, highly doped semiconductors exhibit plasma frequencies at mid- and far-infrared wavelengths. The intersubband absorption spectra and electric field distribution of the microcavity THz QWP are calculated with the finite difference time-domain method. Our results indicate that the frequency of the surface plasmon polariton can be tuned to the microcavity resonant mode under an optimized structure and the intersubband absorption is efficiently enhanced by the microcavity structure. When the doping concentration of the contact exceeds 1018 cm−3, the intersubband absorption of the microcavity THz QWP at the response wavelength is over one order of magnitude higher than that of the standard 45° device. In addition, the angle of the incident light only influences the intensity of the absorptivity, indicating that the designed device was independent of the periodic surface structure.

Light-trapping schemes for silicon thin-film solar cells via super-quadratic subwavelength gratings.

We systematically investigate the light-trapping schemes of crystalline silicon thin-film solar cells (TFSCs) for three common grating layouts via one-dimensional super-quadratic subwavelength gratings. The effects of antireflective coating, absorber layer thickness, and grating geometry on the light-trapping performance of TFSCs are numerically studied using the finite-difference time-domain method. The results suggest that the conformal aluminum-doped zinc oxide (AZO) coatings have better optical properties than the plane AZO coatings. For the case of only top Si gratings, the grating geometry of degree $n={4}$n=4 can achieve a good trade-off between the shape-dependent light-trapping and antireflection properties, showing the best light-trapping effect; for the case of only bottom Ag gratings, the optical performance of TFSCs is significantly degraded as the degree $n$n increases from $n={1}$n=1 to $n\to\infty$n→∞. The above findings are analyzed and demonstrated in detail from the optical and electrical perspectives, and they can be utilized to guide the design of light-trapping structures for TFSCs.

Metal-insulator-metal plasmonic grating filter with suppressed Rayleigh anomaly

Plasmonic grating filters can be fabricated in single lithography process and reduce the cost of colour filters used in hyperspectral cameras. Due to the presence of Rayleigh Anomaly (RA) peak, however, it has not been possible to design filter array spanning wide-spectral-range without sacrificing spectral purity. In this paper, a plasmonic grating filter design using Metal-Insulator-Metal (MIM) with suppressed RA peak is presented. Proposed filter allows extending spectral range without sacrificing spectral purity. Using proposed MIM structure, surface plasmon polariton (SPP) mode supported on air side of bottom grating structure is cancelled by second set of SPP mode on top grating structure. This allows designing filter array with improved spectral range and achieves better than 2× improvement in suppression of the Rayleigh Anomaly peak.

Single-mode surface-emitting microcylinder/microring laser assisted by a shallowly-etched top grating.

Semiconductor lasers based on microcylinder/microring cavities supporting high-quality factor modes are promising candidates of optical sources for optical interconnect. However, their multi-mode lasing performance and non-directional emission characteristic restrict their applications. In this paper, a single mode surface-emitting laser at O-band based on a second-order grating shallowly etched on the top of the microcylinder/microring cavity is proposed and demonstrated. The second-order top grating cannot only scatter the whispering gallery modes vertically to form surface emission but also select a single mode to lase. The laser is electrically pumped and continuous wave operated in a wide temperature range with side-mode suppression-ratio larger than 40 dB. The upward surface-emitting optical power exceeds 1 mW. Except for O-band, the laser can be easily realized at other long-wavelength communication bands, such as C-band and L-band.

Understanding the nano-photonics absorption limit in both front-side and front/rear-side textured slabs

Surface texturing is one of the main techniques to enhance light absorption in solar cells. In thin film devices, periodic texturing can be used to excite the guided resonances supported by the structure. Therefore, total absorption is enhanced largely due to the excitation of these resonances. Although the maximum absorption enhancement limit in both bulk and photonic structures is known already, the weight of each resonance type in this limit is not yet clear. In this contribution, we extend the temporal couple-mode theory, deriving a closed formula to distinguish the contribution of Fabry-Perot and wave-guided modes within the absorption limit for 1-D grating structures. Secondly, using this analytical approach, we can clearly address cases of bulk and thin absorber thicknesses. Our results, supported by rigorous electromagnetic calculation, show that absorption enhancement in a 1-D grating structure can be much higher than the nano-photonic limit (2πn) reported by Yu et al. Thirdly, beyond the framework put forward by Yu et al., we extended our theory to describe the absorption enhancement in double side textured slabs. We have found that when the periods of top and bottom gratings are aliquant, absorption is enhanced in a wider frequency range. We provide rigorous numerical calculations to support our theoretical approach.