Ilya Valuev

Minimizing light reflection from dielectric textured surfaces

In this paper, we consider antireflective properties of textured surfaces for all texture size-to-wavelength ratios. Existence and location of the global reflection minimum with respect to geometrical parameters of the texture is a subject of our study. We also investigate asymptotic behavior of the reflection with the change of the texture geometry for the long and short wavelength limits. As a particular example, we consider silicon-textured surfaces used in solar cells technology. Most of our results are obtained with the help of the finite-difference time-domain (FDTD) method. We also use effective medium theory and geometric optics approximation for the limiting cases. The FDTD results for these limits are in agreement with the corresponding approximations.

Standards for molecular dynamics modelling and simulation of relaxation

An attempt is made to formulate a set of requirements for simulation and modelling of relaxation in dense media. Each requirement is illustrated by examples of numerical simulation of particles with different types of interaction given by soft-sphere, Lennard–Jones, embedded atom method or Coulomb potential. The approaches developed are expected to be universal for some classes of relaxation processes in liquids, fluids, crystals and plasmas.

Subpixel smoothing for conductive and dispersive media in the finite-difference time-domain method.

Staircasing of media properties is one of the intrinsic problems of the finite-difference time-domain method, which reduces its accuracy. There are different approaches for solving this problem, and the most successful of them are based on correct approximation of inverse permittivity tensor epsilon(-1) at the material interface. We report an application of this tensor method for conductive and dispersive media. For validation, comparisons with analytical solutions and various other subpixel smoothing methods are performed for the Mie scattering from a small sphere.

Iterative technique for analysis of periodic structures at oblique incidence in the finite-difference time-domain method

Normal incidence of a plane electromagnetic wave on a periodical structure can be simulated by the finite-difference time-domain method using a single unit cell with periodical boundary conditions imposed on its borders. For the oblique wave incidence, the boundary conditions would contain time delays and thus are difficult to implement in the time-domain method. We propose a method of oblique incidence simulation, based on an iterative algorithm. The accuracy of this method is demonstrated by comparing it with the layer Korringa-Kohn-Rostoker frequency-domain method for calculation of transmission spectra of a monolayered photonic crystal.

Long-time behavior of PML absorbing boundaries for layered periodic structures

In this work we consider a special case of the Perfectly Matched Layer (PML) divergence which is observed by the simulation of the planar periodic structures such as photonic crystal slabs or antenna arrays. This divergence is caused by an excitation of long-living artefact evanescent waves in these structures by an incident external pulse. We study the application of the known remedies to this problem: increasing the distance between the structure and PML, employing the κ parameter, employing non-PML absorbers. We also suggest a new simple and effective solution, where the usual PML is backed by an additional absorbing layer.

Antireflective properties of pyramidally textured surfaces

Antireflective properties of pyramidally textured surfaces at normal light incidence are studied by the finite-difference time-domain (FDTD) method. Optimal parameters for the period of the texture and the pyramid height are found. The asymptotic behavior of the reflection coefficient with an increasing height-to-base size ratio for the pyramids is also estimated for two limiting approximations: the effective medium theory (EMT) and geometric optics. For calculations in the geometric optics limit the ray tracing method was applied. The FDTD results for these limits are in agreement with the EMT and with the ray tracing calculations. It was found that the key factor influencing the optimal scatterer size is the character of the substrate tiling by the pyramid bases.

Hybrid transfer-matrix FDTD method for layered periodic structures

A hybrid transfer-matrix finite-difference time-domain (FDTD) method is proposed for modeling the optical properties of finite-width planar periodic structures. This method can also be applied for calculation of the photonic bands in infinite photonic crystals. We describe the procedure of evaluating the transfer-matrix elements by a special numerical FDTD simulation. The accuracy of the new method is tested by comparing computed transmission spectra of a 32-layered photonic crystal composed of spherical or ellipsoidal scatterers with the results of direct FDTD and layer-multiple-scattering calculations.

Using metallic photonic crystals as visible light sources

In this paper we study numerically and experimentally the possibility of using metallic photonic crystals (PCs) of different geometries (log-piles, direct and inverse opals) as visible light sources. It is found that by tuning geometrical parameters of a direct opal PC one can achieve substantial reduction of the emissivity in the infrared along with its increase in the visible. We take into account disorder of the PC elements in their sizes and positions, and get quantitative agreement between the numerical and experimental results. We analyze the influence of known temperature-resistant refractory host materials necessary for fixing the PC elements, and find that PC effects become completely destroyed at high temperatures due to the host absorption. Therefore, creating PC-based visible light sources requires that low-absorbing refractory materials for embedding medium be found.

Creating numerically efficient FDTD simulations using generic C++ programming

In the present work we propose a strategy for developing reusable multi-model simulation library for solving Finite-Difference Time-Domain (FDTD) problem for Maxwells equations. The described EMTL (Electromagnetic Template Library) architecture is based on the selection of a small number of primitive low-level physical and numerical concepts which are used as parameters and building blocks for higher-level algorithms and structures. In the present work we demonstrate that a large set of FDTD techniques may be formulated using the same primitives. The basic concept for this representation is a discretized field contour entering the integral form of Maxwells equations. We also describe the proposed architecture in terms of FDTD C++ template class library and discuss the performance and the usage of this library for various FDTD-based simulations.

Multiscale computer design of photonic crystal based materials for optical chemosensors

A multiscale method is proposed for modeling elements of optical chemosensors based on photonic crystals. The method is based on the first-principles quantum-chemical and electrodynamic calculations. A technique is proposed that takes into account electromagnetic emission sources in the framework of the finite-difference time-domain method. An end-to-end simulation of fluorescence in a photonic crystal is performed: the absorption and emission spectra of a dye on a substrate are calculated by quantum chemistry, and it is shown how the dye emission spectra are modified in a three-dimensional photonic crystal.