Rigorous Coupled-wave Analysis Calculations Research Articles

Global rigorous coupled wave analysis for design of multilayer metasurface absorbers.

In this work, a novel Global NV-ETM RCWA method is proposed to accelerate the optimization of the periodic stepped radar absorbing structure. This method is based on the rigorous coupled-wave analysis (RCWA) utilizing the normal vector field (NV) and enhanced transmittance matrix (ETM) approach. The NV field dramatically improves the convergence rate for both dielectric and magnetic metasurfaces. The Global NV-ETM RCWA algorithm is developed to further accelerate the complete search calculations. Using the proposed method, the periodic stepped radar absorbing structures are efficiently optimized to realize the entire band absorption in 2-18 GHz. The optimization results demonstrate the Global NV-ETM RCWA method significantly increase the computational efficiency, with a 38-fold improvement over direct NV-ETM RCWA calculations when the truncation order N=3. This method provides a powerful tool for designing metasurface absorbers with various desired functionalities.

Conditions for Minimizing the Computational Complexity of the RCWA Calculation of the Diffraction Efficiency of Sawtooth Two-Layer Double-Relief Microstructures

In this study, novel recommendations are presented and substantiated for selecting the number of modes and optical thicknesses of flat lattice slabs that make up microreliefs, which minimize the computational complexity of the rigorous coupled-wave analysis calculation of the diffraction efficiency (DE) of a sawtooth two-layer two-relief microstructure, while maintaining the specified reliability of the calculation results. The computational complexity can be controlled by allowing one or another level of oscillation of the DE curves, depending on the angle of incidence of the radiation incident on the microstructure. In particular, the complexity of the thousands of DE calculations in the optimization process can be reduced by using the proposed methodology as well as increased computational complexity to verify the accuracy of the solution obtained as a result of the implemented optimization.

A combination of library search and Levenberg-Marquardt algorithm in optical scatterometry

Optical scatterometry has been widely used for nanostructure metrology in integrated circuit (IC) given the fast, low-cost, non-contact, non-destructive, and in-line technique. Library search is a common solution for optical scatterometry, but the deterministic error in this method is hard to be eliminated due to the limitation of the grid interval in the spectral library. Another solution is the nonlinear regression algorithm, which faces difficulty in practical applications in meeting the in-line real-time requirements as it involves the time-consuming rigorous coupled-wave analysis (RCWA) calculations in the forward modeling. In this paper, we propose a combination of library search and Levenberg-Marquardt (LSLM) algorithm to solve the problem. We apply a presorted K-dimensional tree (KD-tree) algorithm to speed up searching the initial parameters in the spectral library and replace the time-consuming RCWA calculations with the Taylor expansion in the spectral library to compute the Jacobian matrix in the Levenberg-Marquardt (LM) procedure. Simulations and experiments demonstrate that the results in the proposed method are accurate, and in particular, the speed is much faster than that in previous methods.

Wide-band enhancement of the transverse magneto-optical Kerr effect in magnetite-based plasmonic crystals

The transverse magneto-optical Kerr effect (TMOKE) in magnetite-based magneto-plasmonic crystals is studied experimentally and theoretically. We analyze angle-resolved TMOKE spectra from two types of structures where noble metallic stripes are incorporated inside a thin magnetite film or located on top of a homogeneous film. A multiple wide band enhancement of the TMOKE signal in transmission is demonstrated. The complex dielectric permittivity and gyration are experimentally determined using the ellipsometry technique as well as Faraday rotation and ellipticity measurements. The obtained parameters are used in rigorous coupled wave analysis (RCWA) calculations for studying the optical resonances. Our RCWA calculations of transmittance and TMOKE are in good agreement with the experimental data. The role of guiding and plasmonic modes in the TMOKE enhancement is revealed. We demonstrate that the TMOKE provides rich information about the studied optical resonances.

Light extraction enhancement of BGO scintillator by monolayers of SiO2 periodic array

Utilizing the self-assembly method, we have successfully prepared a two-dimensional photonic crystal (PhC) of monolayer SiO2 spheres onto the Bismuth Germinate (Bi4Ge3O12, BGO) crystal and therefore supply another photonic crystal structure with the advantages of low-cost and large-area preparation to improve the light extraction efficiency of scintillator. A light extraction enhancement ratio up to 145.3% as compared the BGO coated by the PhC with the plain reference sample is observed and the underlying mechanism of scintillator light extraction is discussed in detail via a rigorous coupled-wave analysis calculation. The calculated transmission spectra and electric-field distribution exhibit the electric-field eigenmodes confined in SiO2 spheres resonating by the way of whispering gallery modes (WGMs) and becoming leaky modes due to the diffraction effect. Since almost no SiO2 nanospheres over the polystyrene spheres are immersed in preparation, we have demonstrated that the introduction of PhC structures using the SiO2 nanospheres onto the scintillator surface is an effective way to solve the light-trapping problem and therefore the design is very useful to be applied to the particle detection of radiation field.

Tailoring Fano Resonance for Flat-Top Broadband Reflectors Based on Single Guided-Mode Resonance

We investigated on the interaction between a single guided-mode resonance and Fabry-Perot resonance (FPR) in a high contrast grating (HCG) from a viewpoint of Fano resonance. It has been found that the background reflection due to the FPR in the HCG is useful to implement flat-top broadband reflectors via tailoring the phase polarity of Fano resonance. The novel conceptual approach was elucidated theoretically with the temporal coupled-mode theory (TCMT), which was confirmed by the Si HCG broadband reflector design of 380-nm bandwidth for R > 99.9 from 1400 to 1780 nm (a fractional bandwidth of 24.5%). The TCMT model was in good agreement with the rigorous coupled wave analysis calculation. We believe that our approach provides expanded understanding on Fano resonance in a grating, enabling more intuitive and simpler design of a flat-top broadband reflector with grating structures.

Tunable Fano Resonances in Mid-Infrared Waveguide-Coupled Otto Configuration

A planar silicon carbide/dielectric multilayer structure is investigated in Otto geometry, where surface phonon polaritons and planar waveguide mode can be coupled to realize Fano resonances under transverse magnetic polarization. The resonance coupling is analytically demonstrated using the coupled harmonic oscillator model and numerically presented through rigorous coupled-wave analysis calculations, which shows that the coupling strength between different resonances and the resonant wavelength matching condition plays an important role in the bandwidth and position of the Fano resonance (FR); the magnetic field distribution was also shown to explain the origin of FRs qualitatively.

Broadband and omnidirectional highly-transparent coverglasses coated with biomimetic moth-eye nanopatterned polymer films for solar photovoltaic system applications

Highly-transparent coverglasses coated with broadband and omnidirectional antireflective (AR) moth-eye nanopatterned ultra-violet (UV) curable polymer, i.e., Norland Optical Adhesive (NOA) 63, films are reported. The moth-eye nanopatterns on the NOA63 polymer films/glasses are transferred from the silicon molds with conical nanogratings (NGs) using poly-dimethylsiloxane stamps by a soft imprint lithography technique. For the NOA63 NGs/glasses, surface wetting behaviors, UV radiation/thermal effects, and optical properties, together with theoretical analyses using rigorous coupled-wave analysis calculations, are investigated. For a period of 380nm, the NOA63 NGs/glass exhibits a hydrophobic surface with a water contact angle of ~112°. Furthermore, it increases the transmittance of the bare glass over a wide range of wavelengths and incident light angles, showing the higher solar weighted transmittance (TSW) of ~93.2% at an incident angle (θi) of 0° and the larger average TSW value of ~83.6% at θi=10–70° than those (i.e., TSW~90.1% at θi=0° and average TSW value of ~81.5% at θi=10–70°) of the bare glass in the wavelength range of 350–1800nm. In addition, it shows a relatively good stability under intense UV radiation and high temperature. By employing the 380nm-period NG patterned NOA63 films into an AR coating of coverglasses in encapsulated III-V GaInP/GaInAs/Ge triple-junction solar photovoltaic (PV) sub-receiver modules, an enhanced power conversion efficiency (PCE) of 31.33% is obtained mainly due to an increased short-circuit current density (JSC) of 14.68mA/cm2 compared to reference PV modules with the bare coverglass (i.e., PCE=30.12% and JSC=14.07mA/cm2) at θi=0°. For incident angle-dependent PV module characteristics, it also has more outstanding solar energy conversion properties in a wide θi range of 10–70°.

Localized input fields in rigorous coupled-wave analysis.

We introduce a new treatment for localized input fields in coupled-wave theory. It consists of a combination of angular spectrum decomposition and a cyclic formulation of rigorous coupled-wave analysis (RCWA), which does not require the computation of several standard RCWA calculations. The cyclic approach especially concerns the definition of the convolution matrix of the permittivity and enables the processing of multiple input plane waves in a single RCWA calculation. At the same time it ensures physically meaningful simulation results for finite truncation limits and thus inherently conserves energy. By adapting the three-dimensional (3D)-RCWA algorithm to efficiently simulate the effects of localized and arbitrarily polarized input fields to 3D volume gratings, various advanced diffraction problems such as fiber coupling, the influence of distorted plane waves, or the effects of focused beams on diffraction gratings can be investigated. Therefore, the impact of this work should concern many fields of application.

Fabrication of Gallium Phosphide Tapered Nanostructures on Selective Surfaces

We present tapered nanostructures fabricated on a selective area of gallium phosphide substrates for advanced optoelectronic device applications. A lithography-free fabrication process was accomplished by dry etching of metal nanoparticles. Thermal dewetting of micro-patterned metal thin films provides etch masks for tapered nanostructures. This simple process also allows the formation of plasmonic surfaces with corrugated shapes. Rigorous coupled-wave analysis calculations provide design guidelines for tapered nanostructures on gallium phosphide substrates.

Strong guided mode resonant local field enhanced visible harmonic generation in an azo-polymer resonant waveguide grating

Guided mode resonance (GMR) enhanced second- and third-harmonic generation (SHG and THG) is demonstrated in an azo-polymer resonant waveguide grating (RWG), comprised of a poled azo-polymer layer on top of a textured SU8 substrate with a thin intervening layer of TiO2. Strong SHG and THG outputs are observed by matching either in-coming fundamental- or out-going harmonic-wavelength to the GMR wavelengths of the azo-polymer RWG. Without the azo-polymer coating, pure TiO2 RWGs, do not generate any detectable SHG using a fundamental beam peak intensity of 2 MW/cm(2). Without the textured TiO2 layer, a planar poled azo-polymer layer results in 3650 times less SHG than the full nonlinear RWG structure under identical excitation conditions. Rigorous coupled-wave analysis calculations confirm that this enhancement of the nonlinear conversion is due to strong local electric fields that are generated at the interfaces of the TiO2 and azo-polymer layers when the RWG is excited at resonant wavelengths associated with both SHG and THG conversion processes.

Nanostructured Surfaces: Bioinspiration for Transparency, Coloration and Wettability

Natural nanostructures rarely come with a single biological function to fulfil. Moreover, from a bio-inspiration perspective, it sounds attractive to develop multifunctional coatings, devices or sensors. Suppression of light reflection from body parts, such as the wings of insects, is useful for hiding from predators. The transparent parts of the wings of Cacostatia ossa (moth) inherit their antireflective property from non-close-packed nano-scale nipple arrays on both sides of the wings. Through modelling and optical simulations, we show that effective medium approaches, commonly used to characterize antireflection, slightly overestimate the reflectance with respect to detailed Rigorous Coupled Wave Analysis calculation. Coloration due to light interference in nanostructure, on the other hand, sometimes comes with an additional, unexpected and maybe non-biologically significant function: hygrochromism, i.e. change of color with humidity. The male Hoplia coerulea (beetle), for instance, exhibits iridescent blue-violet color which turns to emerald green when the elytron is put in contact with water. Impregnation experiments with various liquids revealed intriguing color change dynamics which could be related to the wetting properties of porous chitinous cuticle. Super-hydrophobicity is another function of biological significance, which helps, for instance, insects to keep dry in humid environments. Through morphological, optical and contact angle measurements, we show the existence of entangled levels of functionality on the wings of Cicada orni, namely a superhydrophobic stage at the upper part of the nipple array corrugated surface and an antireflective stage and the lower part.

Antireflective grassy surface on glass substrates with self-masked dry etching

Although recently developed bio-inspired nanostructures exhibit superior optic performance, their practical applications are limited due to cost issues. We present highly transparent glasses with grassy surface fabricated with self-masked dry etch process. Simultaneously generated nanoclusters during reactive ion etch process with simple gas mixture (i.e., CF4/O2) enables lithography-free, one-step nanostructure fabrication. The resulting grassy surfaces, composed of tapered subwavelength structures, exhibit antireflective (AR) properties in 300 to 1,800-nm wavelength ranges as well as improved hydrophilicity for antifogging. Rigorous coupled-wave analysis calculation provides design guidelines for AR surface on glass substrates.

Guided-mode resonance enhanced excitation and extraction of two-photon photoluminescence in a resonant waveguide grating

Guided-mode resonances enhanced excitation and extraction of two-photon photoluminescence (TPP) is demonstrated with a one-dimensional resonant waveguide grating (RWG) with a layer of fluorescent polymer (polyfluorene, PFO) on top. In this work, we design and fabricate a PFO RWG, in which two dispersive resonant modes in TE-polarization were measured. By aligning the red-shifting resonant mode with excitation wavelength in the infrared range, and the blue-shifting resonant mode with TPP spectrum in the visible range, the intensity of TPP can be enhanced up to 300-fold compared with that from a flat film with the same thickness coated on a glass slide. Such high enhancement results from firstly the strong evanescent local field in the waveguide layer due to the resonance between the incident light and the waveguide structure according to the results of rigorous coupled-wave analysis calculation, and secondly the enhanced extraction of the emission light which also resonates with the waveguide structure.

Influence of groove depth and surface profile on fluorescence enhancement by grating-coupled surface plasmon resonance

The fluorescence intensity of a sample placed on a metal grating pattern is enhanced due to excitation by the electric field of the grating-coupled surface plasmon resonance (GC-SPR). The dependence of the enhancement on groove depth and surface profile was studied with the aim of improving the sensitivity of fluorescence detection. The enhancement was found to depend on the groove depth, with intensity most enhanced on grating substrate of about 20 nm depth, which produced an intensity about 30 times greater than that on a flat borosilicate glass substrate. Rigorous coupled wave analysis calculation showed that the shape of the groove influenced GC-SPR, suggesting that controlling not only the depth but also the shape of the grating surface profile can be an important factor in improving the sensitivity of detection by fluorescence microscopy.