Rigorous Coupled-wave Analysis Research Articles

Polarization-driven thermal emission regulator based on self-aligned GST nanocolumns

The increasing advances in thermal radiation regulators have attracted growing interest, particularly in infrared sources, thermal management, and camouflage. Despite many advances in dynamic thermal emitters with great controllability, sustained external energy is required to maintain the desired emission. In this study, we present a polarization-driven thermal emission regulator based on a two-way control: i) phase change and ii) polarization tuning. Based on a conventional, non-volatile phase change material, i.e., Ge2Sb2Te5 (GST), we newly introduce an anisotropic medium for facile emissivity regulation without heat energy consumption. A rigorous coupled-wave analysis method provides design guidelines for finding optimal structural parameters. We utilized a simple glancing angle deposition process which induces tilted self-aligned nanocolumns with anisotropic properties. The fabricated sample shows polarization-sensitive thermal regulation through thermal imaging spectroscopic measurement. Additionally, we manufactured a multispectral visibly/thermally camouflaged patch that identifies encrypted information at a specific polarization state for a proof-of-concept demonstration.

Design and Fabrication of Tapered Dielectric for Broadband and Wide Incident Angle Transmission

We propose a novel design procedure for a 2-D array of tapered dielectric structures for broadband and wide-incident-angle transmission. We exploit a stereolithography apparatus (SLA)-type 3-D printer to fabricate tapered dielectric samples. This technology eliminates the costly and complicated processes (e.g., perforation and etching) that are required in the fabrication of tapered dielectric structures. Specifically, we adopt the rigorous coupled wave analysis (RCWA) technique to extract the effective permittivity (EP) of a tapered dielectric unit structure. An optimization algorithm is employed to determine the ultimate optimal design parameters for the maximum transmittance. The free-space measurement is performed to evaluate the electromagnetic (EM) performance of designed samples. We demonstrate that a 2-D array of optimized tapered dielectric unit structures can exhibit enhanced transmission property in comparison to a half-wavelength dielectric layer and tapered dielectric structure without optimization, viz. above 60% transmittance over the entire Ka-band up to 60° of the incidence angle for both transverse electric (TE) and magnetic polarizations.

Grating-Based Surface Plasmon Resonance Sensor for Visible Light Employing a Metal/Semiconductor Junction for Electrical Readout

In this study, we proposed an electrical readout surface plasmon resonance (SPR) chemical sensor coupled with visible light to avoid significant light absorption of water. A gold (Au) diffraction grating with a submicrometer pitch, designed using the rigorous coupled-wave analysis (RCWA) method to have a submicrometer pitch and 30-nm-tall Au grating, was fabricated on an n-type silicon (n-Si) substrate by nanofabrication/microfabrication. The hot electrons generated by SPR on the grating are converted to photocurrent at the Schottky barrier at the Au/n-Si interface. The Schottky barrier height was estimated based on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I-V$ </tex-math></inline-formula> characteristics of the device to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$q{\Phi}_{\mathrm{B}} = 0.72$ </tex-math></inline-formula> eV. The peak shift of photocurrent was observed to be consistentwith the SPR theory when the concentration of glucose aqueous solution changed the refractive index on the Au grating. Assuming that the incident wavelength and angle were fixed, a noise analysis revealed a refractive index resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.4-4.0 \times 10^{-5}$ </tex-math></inline-formula> refractive index unit (RIU). Since this structure eliminated bulky optics in SPR experiments, it would contribute to the realization of a thin single-chip label-free chemical sensor.

Design and fabrication of diffraction grating with optimized efficiency for transient grating spectroscopy.

Transient grating spectroscopy (TGS) based on diffraction gratings is a powerful optical method for studying the transport of energy carriers such as phonons and electrons. The diffraction grating in a TGS system is a key component to form a large-area interference pattern, i.e., transient grating, and to study the mean free path distribution of energy carriers. In this work, a design method for polarization-insensitive diffraction gratings with periods in the range 2-50 µm for TGS by a combination of rigorous coupled wave analysis and genetic algorithm was discussed. The method was tested for pump/probe wavelength of 515/532 or 1030/808nm. Each ±1st diffraction order carries 35%-40% of the incident energy and the diffraction efficiencies of the other orders are lower than 10%. The optimized diffraction gratings were fabricated by a combination of photolithography and inductively coupled plasma etching, with the processing parameters introduced in detail, and their optical characteristics were evaluated. Finally, as a demonstration, the diffraction gratings for 1030/808nm were applied to TGS to study the thermal transport properties of Ge. This work provides a useful guide for future applications and the development of TGS.

Gas sensing of organophosphorous compounds with III–V semiconductor plasmonics

Chemical warfare agents including nerve agents, e.g., sarin, are part of the organophosphorous compounds group known for their extreme lethal potency towards humans. With increasing risks of exposure to these molecules, there is a need for sensitive and selective detection. Infrared absorption spectroscopy is a powerful technique for the identification of molecules by probing their vibro-rotational molecular resonances. However, infrared light wavelength and molecule absorption cross-section dimensions mismatch, significantly hampering light-matter interaction especially at low concentrations, therefore lowering the sensitivity. To overcome this challenge, we propose a rapid chemistry-free III–V semiconductors InAsSb plasmonic sensor working in the mid-infrared with ribbon-shape nano-antennas associated to a confined enhanced electric field at the nanoscale. Exploiting well-known Salisbury perfect absorber structure coupled to an epsilon-near-zero layer benefit both spectral and spatial overlap, two required conditions for surface-enhanced infrared spectroscopy. Finite difference time domain and rigorous coupled-wave analysis coupled to Drude formalism were performed to design the sensor. The plasmonic sensor was exposed to hundreds of ppm-level concentration vapors of dimethyl methylphosphonate, known as DMMP, a commonly used sarin simulant. We determined that DMMP bound to the sensor native oxide to form a 5 Å estimated monolayer. The sensor response results from the coupling between surface plasmons and DMMP molecular vibrations upon adsorption at resonances frequency. Further simulations confirmed experimental results. This work demonstrates the application of III–V semiconductor plasmonics for gas sensing of complex molecules exploiting surface-enhanced infrared absorption.

Modified rigorous coupled-wave analysis for multi-layer deformable gratings with arbitrary profiles and materials.

In this paper, the rigorous coupled-wave analysis (RCWA) is extended for general multi-layer deformable gratings with arbitrary numbers of layers, surface profiles, layer offsets, and materials. The contribution from the offset between grating layers and/or due to the movement of the deformable grating layer is included in the expansion of the relative permittivity by the Fourier series, enabling the calculations of deformable gratings commonly used in many optical-based displacement sensing devices. The accuracy and efficiency of the extended RCWA are verified by a number of grating models. It is found that the numerical results are in excellent agreement with those from the finite element method, while the RCWA method costs only ∼1/10 in computation time when compared to its counterpart. Our approach can be used for fast calculation and optimization of multi-layer deformable gratings for optical displacement sensing applications.

Efficient Rigorous Coupled-Wave Analysis Simulation of Mueller Matrix Ellipsometry of Three-Dimensional Multilayer Nanostructures.

Mueller matrix ellipsometry (MME) is a powerful metrology tool for nanomanufacturing. The application of MME necessitates electromagnetic computations for inverse problems of metrology determination in both the conventional optimization process and the recent neutral network approach. In this study, we present an efficient, rigorous coupled-wave analysis (RCWA) simulation of multilayer nanostructures to quantify reflected waves, enabling the fast simulation of the corresponding Mueller matrix. Wave propagations in the component layers are characterized by local scattering matrices (s-matrices), which are efficiently computed and integrated into the global s-matrix of the structures to describe the optical responses. The performance of our work is demonstrated through three-dimensional (3D) multilayer nanohole structures in the practical case of industrial Muller matrix measurements of optical diffusers. Another case of plasmonic biosensing is also used to validate our work in simulating full optical responses. The results show significant numerical improvements for the examples, demonstrating the gain in using the RCWA method to address the metrological studies of multilayer nanodevices.

Bioinspired antireflective subwavelength nanostructures induced by femtosecond laser for high transparency glass

In nature, the Cicada wing exhibiting a high transparency at wide viewing angles can be bioinspired for the formation of broadband antireflective subwavelength nanostructures (ASS), which can significantly improve the transmittance of optical devices. In this study, a femtosecond laser micro-machining is developed to fabricate a biomimic nanostructure array on optical glass for antireflection. The underlying mechanism for the transmittance enhancement and geometry design of the artificial structure is thoroughly explored by applying a rigorous coupled-wave analysis method both in the visible and the near-infrared band. In addition, the height, diameter, and period of the ASS experimentally induced by the femtosecond laser are adjusted by controlling the laser-processing parameters of pulse energy and scanning velocity. Accordingly, the proposed bioinspired nanostructures featuring a wide-angle omnidirectional antireflection in the visible band, as well as hydrophobicity properties, are anticipated to be widely used in glass displays applications.

A proposal for eliminating the zeroth order with polarization independence by mixing SiO2/Ta2O5 metamaterials

A novel splitter with zeroth order eliminated is proposed based on mixing SiO2/Ta2O5 metamaterials. By combing rigorous coupled-wave analysis and the key equation of simulated annealing, the efficiencies of the ± 1st orders, the 0th order are 48.52 %, 0.17 % for TE polarization and 47.96 %, 1.14 % for TM polarization. In addition, for TE and TM polarizations, the grating keeps the high efficiencies which can be more than 47 % in the ± 1st orders, with the wide incident bandwidth of 92 nm, the period d of 2.140–2.725 µm or the duty cycle f of 0.312–0.372. Most importantly, the novel metamaterials have the new polarization-independent property and zeroth-order eliminating. It indicates that the manuscript on the advance in the novel mixing SiO2/Ta2O5 metamaterials has been validated by the finite-difference time-domain method, the coupled-mode theory, and the physical mechanism.

Stack of cellular lamellae forms a silvered cortex to conceal the opaque organ in a transparent gastropod in epipelagic habitat.

Gelatinous zooplankton in epipelagic environments often have highly transparent bodies to avoid detection by their visual predators and prey; however, the digestive systems are often exceptionally opaque even in these organisms. In a holoplanktonic gastropod, Pterotrachea coronata, the visceral nucleus is an opaque organ located at the posterior end of its alimentary system, but this organ has a mirrored surface to conceal its internal opaque tissue. Our ultrastructural observation proved that the cortex of the visceral nucleus comprised a stack of thin cellular lamellae forming a Bragg reflector, and the thickness of lamellae (0.16 µm in average) and the spaces between the lamellae (0.1 µm in average) tended to become thinner toward inner lamellae. Based on the measured values, we built virtual models of the multilamellar layer comprising 50 lamellae and spaces, and the light reflection on the models was calculated using rigorous coupled wave analysis to evaluate their properties as reflectors. Our simulation supported the idea that the layer is a reflective tissue, and the thickness of the lamella/space must be chirped to reflect sunlight as white/silver light, mostly independent of the angle of incidence. In P. coronata, the cortex of the visceral nucleus comprised multicellular lamellae that form a chirped Bragg reflector. It is distinct in structure from the intracellular Bragg structures of common iridophores. This novel Bragg reflector demonstrates the diversity and convergent evolution of reflective tissue using reflectin-like proteins in Mollusca.

Design and numerical analysis of metallic Ronchi diffraction gratings acting as reflective beam-splitter

ObjectiveIn this work, a reflective beam-splitter based on a metallic Ronchi diffraction grating normally illuminated is designed and analysed. This kind of beam-splitter could have potential applications in photonics and optical technologies in which robustness is necessary since it may be manufactured over malleable metallic substrates. MethodsThe main idea under the design is as simple as obligating the zero-th diffraction order to be null. Firstly, scalar approach is performed, showing an approximation to the parameters of the grating necessary to cancel zero-th diffraction order. After that, a more rigorous approach such as Rigorous Coupled Waves Analysis (TE and TM polarization) is used to evaluate the proposed diffraction gratings as reflective beam-splitters. ResultsBeam-splitting is demonstrated for TE and TM polarization with slightly different dimensional parameters of the diffraction grating. Besides, we show how physical height of the grating grooves that allows cancelling zero-th diffraction order for a certain illumination wavelength depends on the metals used to manufacture the grating and its period. The dependence of the grooves height on the period is exponentially decreasing. To complete the analysis, we demonstrate how for a given grating period, the grooves height also depends on the illumination wavelength.

Fast algorithm for the simulation of 3D-printed microoptics based on the vector wave propagation method.

In this work, we propose the Fast Polarized Wave Propagation Method (FPWPM), which is an efficient method for vector wave optical simulations of microoptics. The FPWPM is capable of handling comparably large simulation volumes while maintaining quick runtime. This allows for real-world application of this method for the rapid development process of 3D-printed microoptics. By comparison to established routines like the rigorous coupled wave analysis (RCWA) or the Richards-Wolf-Integral, accuracy and superior runtime efficiency of the FPWPM are demonstrated by simulation of interfaces, gratings, and lenses. By considering polarization in simulations, the FPWPM facilitates the analysis of optical elements which employ this property of electromagnetic waves as a feature in their optical design, e.g., diffractive elements, gratings, or optics with high angle of incidence like high numerical aperture lenses.

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.

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.

Subwavelength Quasi-Periodic Array for Infrared Antireflection.

Infrared antireflection of a zinc sulfide (ZnS) surface is important to improve performance of infrared detector systems. In this paper, double-pulse femtosecond laser micro-machining is proposed to fabricate a subwavelength quasi-periodic array (SQA) on ZnS substrate for infrared antireflection. The SQA consisting of approximately 30 million holes within a 2 × 2 cm2 area is uniformly formed in a short time. The double-pulse beam can effectively suppress the surface plasma shielding effect, resulting in obtaining a larger array depth. Further, the SQA depth is tunable by changing pulse energy and pulse delay, and can be used to readily regulate the infrared transmittance spectra as well as hydrophobicity. Additionally, the optical field intensity distributions of the SQA simulated by the rigorous coupled-wave analysis method indicate the modulation effect by the array depth. Finally, the infrared imaging quality captured through an infrared window embedded SQA is evaluated by a self-built infrared detection system.

Microstructural characteristics of sandwiched T-shaped metamaterials for three-port splitting

This paper presents a polarization-insensitive sandwiched three-port beam splitting grating at normal incidence. The grating adopts a sandwiched structure. Based on the rigorous coupled-wave analysis (RCWA) and the simulated annealing algorithm, the grating parameters are optimized to obtain the diffraction efficiencies of 32.83% at 0th order and 32.87% at ±1st orders for transverse electric polarization, and 32.63% at 0th order and 32.39% at ±1st orders for transverse magnetic polarization. Meanwhile, the calculation results of the finite element method are in general agreement with those of RCWA. In addition, the analysis results of grating period, duty cycle, and wavelength bandwidth further demonstrate the good wavelength bandwidth of the grating and its feasibility for practical production. Therefore, the grating can be widely used in actual production.

Experimental and numerical study of the emissivity of rolled aluminum

Directional spectral emissivity measurements on a rolled aluminum sheet are reported between 423 and 823 K in vacuum. The results are compared to available literature data and to theoretical predictions, revealing the crucial role of the surface state in explaining the observed scatter of values. In particular, it is argued that the cold-rolling process induces a multi-scale roughness profile that significantly enhances emission at all wavelengths, a phenomenon that can be described using rigorous coupled-wave analysis (RCWA). A small peak in the p-polarized component at oblique angles is formed by the native oxide layer. Aside from the intrinsic value of the emissivity data for the application of thermographic techniques to rolled aluminum materials, the results contained in this work also serve to validate the usefulness of RCWA to simulate the emissivities of randomly rough metal surfaces, highlighting directions of further research.

In situ spectroscopic ellipsometry and rigorous coupled wave analysis for real time profile evolution of atomic layer deposited films inside SiO2 nanotrenches

Rigorous coupled wave analysis (RCWA) is conducted on in situ spectroscopic ellipsometry data to understand profile evolution during film deposition inside nanotrenches. Lithographically patterned SiO2 nanotrenches are used as test structures. The nanotrenches are 170 nm wide at the top with a taper angle of 4.5° and are 300 nm in depth. Atomic layer deposition of ZnO is used as a model process where the thickness (cycles) of the film is varied from 0 (0 cycles) to 46 nm (300 cycles). The analysis predicts transient behavior in deposition affecting film conformality and changes to the trench taper angle. In the process, the aspect ratio varies from 2.05 at the start of the process to 6.67 at the end. The model predicts changes in the refractive index of the ZnO film as a function of thickness. The real and imaginary parts of the refractive index at a wavelength of 350 nm change from 1.81 to 2.37 and 0.25 to 0.87, respectively. Scanning electron microscopy cross sections confirm thickness at the top and bottom of the trench to within 13% of those predicted by RCWA. The experimentally measured conformality degrades as film deposition proceeds from 97.3% at 100 cycles to 91.1% at 300 cycles. These results demonstrate the potential of using RCWA for continuous and in situ monitoring of growth inside 3D nanostructures.

Al/Mo/SiC multilayer diffraction gratings with broadband efficiency in the extreme ultraviolet.

Al/Mo/SiC periodic and aperiodic multilayers were optimized and deposited on high groove density gratings to achieve broadband efficiency in the extreme ultraviolet (EUV). Grating efficiencies were measured by monochromatic synchrotron radiation under 5° and 45° incident angles in the wavelength ranges 17-25 nm and 22-31 nm, respectively. We study the influence of the number of deposited periods on the initial trapezoidal profile and the EUV diffraction efficiency. We propose models of periodic and aperiodic coatings based on a combination of characterizations and compare rigorous coupled-wave analysis (RCWA) simulations with experimental data. We demonstrate the possibility to select the optimal balance between peak efficiency and bandwidth by adjusting the number of periods in the case of periodic multilayer grating. We also report unprecedented broadband diffraction efficiency with an Al/Mo/SiC aperiodic multilayer grating.

Eyebox uniformity optimization over the full field of view for optical waveguide displays based on linked list processing.

Eyebox uniformity is an important indicator to evaluate the performance of optical waveguide displays. However, there is currently no standard design approach that achieves ideal uniformity over the full field of view (FOV) within the eyebox. Here, a novel method for eyebox uniformity optimization based on linked list processing is proposed. The linked list processing method can fast record the light trajectory and calculate the optimal numerical diffraction efficiency distribution of the coupler. We use the linked list method for an L-shaped diffractive optical waveguide and solve the matched coupler structure by combining rigorous coupled-wave analysis (RCWA) and the simplex method. By building the model on LightTools and demonstrating the illuminance uniformity, the feasibility of the method is verified. In the FOV range of 15°× 15°, the eyebox uniformity reaches 0.9 at the central viewing angle and the overall eyebox uniformity is 0.617.