Layer Homogenization Method for Modeling Additive Color Effect in Natural Photonic Polycrystals --- The Case of Entimus imperialis Weevil

An original method for computing the re∞ectance of photonic crystal whose unit cell is composed of form-birefringence anisotropic elements (e.g., cylindres) is proposed. This method, based on the layer homogenization of the photonic crystal, is particularly useful when the re∞ectance is calculated for difierent crystal orientations. The far-fleld color due to an additive color efiect in a natural photonic polycrystal found on Entimus imperialis weevil was investigated. 1. INTRODUCTION Modeling the optical properties of photonic polycrystals is crucial for the investigation of natural photonic structures as well as for the design and fabrication of bioinspired devices (1,2). The computation of re∞ectance (or transmittance) of photonic crystals (PCs) by the Rigorous Coupled Wave Analysis (RCWA) method (1,3) is sometimes cumbersome to perform due to the presence of form-birefringence anisotropic elements in the crystal unit cell (e.g., cylinders or parallelepipeds), particularly when it has to be calculated for various crystal orientations. The Layer Homogenization (LH) method proposed here solves this issue speciflcally for crystals with periodicities such that there is only specular re∞ection and no other difiraction orders (4). For a single crystal orientation, the dielectric function is homogenized within the layers in which the crystal is sliced. A standard thin fllm solver is then used for the calculation of the re∞ectance. The disorder in the crystal orientations of the photonic polycrystal is taken into account by averaging re∞ectance spectra computed for various crystal orientations and incidence angles.