Simple Grating Structure Research Articles

Metagrating-based acoustic wavelength division multiplexing enabled by deterministic and probabilistic deep learning models

Wavelength division multiplexing (WDM), the technology that utilizes different wavelengths of carrier signal as independent information channels and helps to realize enhanced information density and functional diversity, is desired in various acoustic applications for the growing demand of higher transmission efficiency and lower cost. In this paper, we marry the rising concept of acoustic metagrating with the technology of WDM and demonstrate that interesting and distinct wavelength-dependent functionalities with unitary efficiency in prespecified frequency ranges can be realized by a simple grating structure. To this end, we integrate the particle swarm optimization with the deep learning technique to resolve the uncertainty issue and multi-objective problem and develop both deterministic and probabilistic neural networks for the inverse design of a metagrating-based WDM device. As a result, the computational cost is greatly reduced, and the probabilistic model reveals the sensitivity and flexibility of the parameters of the metagrating with respect to the desired functionality. In this paper, we not only provide an intelligent inverse design paradigm of high-performance WDM devices for multiple manipulation purposes but also present a feasible solution for the development of integrated acoustic devices for wavelength-dependent applications.

Enhancing ultrasound transmission and focusing through a stiff plate with inversely optimized auxiliary meta-lens

Effective sound energy transmission and beam manipulation through stiff and dense materials such as metal remain daunting tasks. It is in part attributable to the vast impedance mismatch between those materials and ambient media. Adding openings may facilitate to better bridge energy over, yet ineffective in many applications and may also damage the structural integrity. Here, we present an auxiliary ultrasound focusing meta-lens for stiff and dense materials. It offers significantly enhanced ultrasound transmission and focusing through a stiff metal plate yet without enforcing any through holes or openings. The simple, one-sided only meta-structures are designed and optimized by an inverse strategy based on the genetic algorithm. We numerically and experimentally demonstrate the much enhanced ultrasound transmission when the meta-lens is added to a flat brass plate, along with the capability to offer simultaneous ultrasound focusing. This design methodology can be easily extended to deal with more complex shaped target in a straightforward manner, offering a practical solution to the efficient tunneling of ultrasound energy through stiff and dense materials. With simple grating structures, the meta-lens can be easily fabricated, showing great application prospects in medical imaging and disease treatment.

Radiative cooling with multilayered periodic grating under sunlight

Radiative cooling is an energy-saving cooling technology and is achieved when the grating structure emits infrared radiant heat energy into outer space through an atmospheric window (8∼13μm). The radiation cooling effect is low for the simple grating structure due to its low absorptivity, thus was a novel multilayered composite grating structure proposed in the paper, an aluminum substrate composite whose top layer is polycarbonate coating and intermediate layers are composed of SrTiO2, SiO and Al2O3. The influencing factors of the absorptivity of the simple grating structure were analyzed by rigorous coupled wave analysis (RCWA), which consider grating period (1∼5μm), grating height (1∼20μm), duty cycle (0.75∼1) and angle of incidence (0∼75°), it was found that grating height is the crucial factor and increasing grating height can improve the absorptivity of the grating structure Hence, the proposed composite grating structure was studied with 20μm grating height and 0.5 to 2μm thickness of the polycarbonate (PC) coating, 1μm thickness polycarbonate structure was found to have good absorptivity, most is over 0.9, and the net cooling power of this grating structure could reach 71.39 W/m2 at ambient temperature.

Near-field thermophotovoltaic system design and calculation based on coupled-mode analysis

The coupling of resonant modes between two surfaces is important in near-field heat transfer and near-field thermophotovoltaic (TPV) systems. Recently, coupled-mode theory (CMT) has been developed for the analysis and optimal design of TPV systems. We use CMT to analyze the “emitter-vacuum-PV cell” configuration and quantitatively show how the emitter of a nanostructure can drastically improve the near-field TPV device performance. The key feature of the nanostructure is the additional geometry-induced resonant mode, whose energy is lower than the original surface plasmon polariton resonant frequency and much closer to the bandgap of the PV cell. Specifically, we show that, with a simple grating structure, the generated power density of a TPV cell is increased from 13 to 34 W/cm2 when the PV cell is fixed at 300 K and the emitter is at 1000 K. The increase is over 20 times higher when both planar and grating emitters are at a lower temperature of 500 K.

Rigorous Coupled-Wave Approach for Sandwich Gratings

Rigorous Coupled-Wave Approach (RCWA) has been used successfully and accurately to study simple grating structures, such as one-layer gratings, one-whole gratings. In this paper, RCWA is expanded to solve Sandwich gratings (SG), which is composed of two identical planar dielectric gratings adjoined by thin metallic or dielectric film. The electromagnetic analytic expressions for each layer of SG structure are given and rigorous coupled-wave equations are deduced. The numerical investigations for the diffraction spectra of SG by our theoretical and computer programs are in good agreement with the results of classical RCWA in the condition when a Sandwish grating is simplified to a classical one-layer grating. The calculations by our programs of another condition when a Sandwish grating is degenerated to a classical single planar structure also conform to the results of classical electromagnetic theory. The research results above indicate that the extended theoretical formula has backwards compatibility and is self-consistent with the classical theory.

Enhancing light absorption within the carrier transport length in quantum junction solar cells.

Colloidal quantum dot (CQD) solar cells have attracted tremendous attention because of their tunable absorption spectrum window and potentially low processing cost. Recently reported quantum junction solar cells represent a promising approach to building a rectifying photovoltaic device that employs CQD layers on each side of the p-n junction. However, the ultimate efficiency of CQD solar cells is still highly limited by their high trap state density in both p- and n-type CQDs. By modeling photonic structures to enhance the light absorption within the carrier transport length and by ensuring that the carrier generation and collection efficiencies were both augmented, our work shows that overall device current density could be improved. We utilized a two-dimensional numerical model to calculate the characteristics of patterned CQD solar cells based on a simple grating structure. Our calculation predicts a short circuit current density as high as 31 mA/cm2, a value nearly 1.5 times larger than that of the conventional flat design, showing the great potential value of patterned quantum junction solar cells.

Modeling photovoltaic performance in periodic patterned colloidal quantum dot solar cells.

Colloidal quantum dot (CQD) solar cells have attracted tremendous attention mostly due to their wide absorption spectrum window and potentially low processability cost. The ultimate efficiency of CQD solar cells is highly limited by their high trap state density. Here we show that the overall device power conversion efficiency could be improved by employing photonic structures that enhance both charge generation and collection efficiencies. By employing a two-dimensional numerical model, we have calculated the characteristics of patterned CQD solar cells based of a simple grating structure. Our calculation predicts a power conversion efficiency as high as 11.2%, with a short circuit current density of 35.2 mA/cm2, a value nearly 1.5 times larger than the conventional flat design, showing the great potential value of patterned quantum dot solar cells.

Control of thermal radiative properties using two-dimensional complex gratings

The present work theoretically investigates the electromagnetic resonance modes on two-dimensional (2-D) complex gratings consisting of a metallic grating, a dielectric film, and a metal substrate. We treat a complex grating as geometrical combination of two square grating elements with different sizes in a single unit cell. In order to identify various resonance modes, such as surface plasmon and magnetic resonance (or magnetic polariton) existing in gratings, the finite-difference time-domain method is employed in solving Maxwell’s equation. Differently from 2-D simple grating structures, we observe the diverse magnetic polariton modes associated with each grating elements in a single unit cell. Furthermore, if two different grating elements are merged together, magnetic polariton will be branched out into multiple modes. These characteristics of 2-D complex grating will be desirable for achieving broad-band absorption, which is critical for energy harvesting applications.

Metasurfaces for manipulating surface plasmons

Metasurfaces have recently emerged as an innovative approach to control light propagation with unprecedented capabilities. Different from previous work concentrating on steering far-field propagating waves, here we demonstrate that metallic metasurfaces can efficiently and effectively manipulate surface plasmons in the near-field regime. By engineering the dispersion of surface plasmons on a simple grating structure, we are able to realize normal, non-divergent as well as anomalous diffraction of surface plasmons. In particular, all-angle and broadband negative refraction of surface plasmons is achieved, largely attributed to the uniquely designed hyperbolic constant frequency contour of surface plasmons propagating along the metasurface.

Vertical-coupling optical interface for on-chip optical interconnection

We present a vertical-coupling optical interface with a grating coupler for transmitting and receiving optical signals between single-mode optical fibers and microphotonic waveguides with a view to realize on-chip optical interconnection. The optical interface consisting of a simple grating structure with a reflective mirror and an optical power combiner exhibits high optical coupling efficiency and wide tolerance range for the misalignment of optical fibers. The optical interface exhibits high coupling efficiency even if the optical input is almost vertical to the chip surface.

Analysis of Plasmonic Waveguides and Gratings Using Implicit Finite-Difference Methods

Various metal-insulator-metal- (MIM-) type plasmonic waveguides and gratings are investigated numerically. Three gratings are treated: one is formed by alternately stacking two kinds of MIM waveguides, another by periodic changes in the dielectric insulator materials of an MIM waveguide, and the other by a periodic variation of the air core width in an MIM waveguide. The dispersion property of each MIM waveguide of which the grating consists is analyzed using the implicit Yee-mesh-based beam-propagation method. It is shown that the third one has a relatively large effective index modulation of the guided mode with a simple grating structure, while maintaining a low propagation loss. Further examination is given to modifications of this grating structure. The transmission characteristics are examined using the frequency-dependent implicit locally one-dimensional FDTD method. We discuss how the modified grating structure affects the bandgap of the transmission characteristics.

Design of tungsten complex gratings for thermophotovoltaic radiators

The concept of a kind of complex grating based on superposition of two simple binary gratings is described for potential application as thermophotovoltaic (TPV) radiators. The grating with one-dimensional microstructured surface is relatively easy to fabricate and the predicted directional-spectral emittance exhibits apparent enhancement over simple grating structures. Specifically, the emittance of the complex grating has a wider peak in the spectral region where the quantum efficiency of TPV cells is high. This enhancement can be explained by the excitation of surface plasmon polaritons coupled with the grating microstructures. At longer wavelengths, the emittance remains low to reduce the radiative heat transfer from the radiator to the TPV cells by low-energy photons that do not produce any photocurrent. Calculations using the rigorous coupled-wave analysis demonstrate that the emittance peak is insensitive to the direction, suggesting that the proposed structure may be well suitable for TPV applications.

Optical computing without using optical elements

We present an optical algorithm for parallel multiplication of vectors and matrices using simple grating structures. The proposed scheme has the advantage that it does not require optical elements. Light while passing through the spatial light modulators, consisting of grating structures, gets multiplied and summed at the output plane. The scheme has the flexibility to perform matrix operations without changing the hardware.

Diffractive optics in nonlinear media with periodic structure

The diffraction of time-harmonic waves in a nonlinear medium with periodic structure is studied in this paper. In particular second harmonic generation – an important phenomenon in nonlinear optics-is modelled. The model, derived from a general nonlinear system of Maxwell's equations, is shown to have a unique solution for all but a discrete number of frequencies. The problem is solved numerically by combining a method of finite elements and a fixed-point iteration scheme. Numerical experiments for some simple grating structures are presented and discussed.