Reflection analysis of absorbing film with diffractive structures for incoherent light by rigorous coupled-wave analysis.

Reflectivity is useful for evaluating the extinction coefficient; however, it is highly sensitive to the refractive index structure. In this study, we propose a novel, to the best of our knowledge, method for evaluating the influence of the structure on reflectivity using rigorous coupled-wave analysis (RCWA), and apply it to analyze the reflectivity of the dye rhodamine B. The reflection-absorption spectrum of the film was significantly affected by its surface and internal structure. We found that simulating the reflectivity of a film with an unknown internal structure, using the coherency parameter is convenient. The RCWA facilitated simultaneous treatment of the coherent diffraction by the surface structure and incoherent reflection in the film.

Diffraction pattern of triangular grating in the resonance domain.

We propose a combination of ray optics and Fraunhofer multiple-slit diffraction theory for calculating the two-dimensional triangular periodic grating in the resonance domain. The peak of the envelope pattern of angular distribution of diffraction efficiency is calculated by ray optics while the peak width is calculated using Fraunhofer theory. It was clarified, using rigorous coupled-wave analysis and a nonstandard-finite-difference time-domain method, that the envelope pattern of the diffraction of the grating could be calculated easily and understood intuitively for the design of displays and lighting.

Simulation of subwavelength metallic gratings using a new implementation of the recursive convolution finite-difference time-domain algorithm.

We introduce a new implementation of the finite-difference time-domain (FDTD) algorithm with recursive convolution (RC) for first-order Drude metals. We implemented RC for both Maxwell's equations for light polarized in the plane of incidence (TM mode) and the wave equation for light polarized normal to the plane of incidence (TE mode). We computed the Drude parameters at each wavelength using the measured value of the dielectric constant as a function of the spatial and temporal discretization to ensure both the accuracy of the material model and algorithm stability. For the TE mode, where Maxwell's equations reduce to the wave equation (even in a region of nonuniform permittivity) we introduced a wave equation formulation of RC-FDTD. This greatly reduces the computational cost. We used our methods to compute the diffraction characteristics of metallic gratings in the visible wavelength band and compared our results with frequency-domain calculations.

Design of a wavelength independent grating in the resonance domain.

We propose using blazed gratings in the resonance domain with period larger than the wavelength for anti-reflection and polarization selection. The inherent problem in this region is wavelength dispersion, which is solved by analyzing the total reflectivity and electric field distribution. The positional relationship between the area of strong electric field, and the side and tip of the grating is crucial to the wavelength dispersion of total reflectivity.

Colour characterization of a Morpho butterfly wing-scale using a high accuracy nonstandard finite-difference time-domain method.

In certain species of moths and butterflies iridescent colours arise from subwavelength diffractive structures. The optical properties of such a structure depend strongly on wavelength, incidence angle and state of polarization of illuminating radiation and on the viewing angle. Such structures can be analyzed only by solving Maxwell's equations, but since analytical solutions exist for only a few simple, highly symmetric structures numerical methods must be employed. We investigated the optical properties of butterfly wings in two dimensions by simulating a scale structure using a high accuracy version of nonstandard finite-difference time-domain algorithm. The simulated structure is a computer-generated model of a certain quasi-periodic arrangement of tree-like structures observed in the transmission electron micrograph (TEM) image of a transverse cross-section of a single scale from Morpho butterfly wings. We assumed that the structure is made of a slightly lossy dielectric material. We checked the accuracy and validity of our approach, by computing scattered field intensities due to an infinite cylinder and compared the results with analytical calculations using Mie theory. Next we deduced the wavelength dependence of a real refractive index and an absorption coefficient for the ground scales on the wings of Morpho sulkowskyi butterfly by computing the reflectivity and transmissivity spectrum of a scale at normal incidence, and comparing with experimental measurements. Finally, we calculated the tristimulus values and corresponding colour coordinates for various viewing directions from the scale's far-field reflectivity and transmissivity spectra to characterize its colour rendering abilities.

High accuracy simulations of optical propagation in nanophotonic devices.

We introduce a new high accuracy second-order finite-difference time-domain (FDTD) algorithm based on non-standard finite differences, and use it to investigate light propagation in subwavelength structures such as photonic crystals.