Saswatee BANERJEE^* and Zhu DONG¹

Sumitomo Chemical Co. Ltd., Tsukuba, Ibaraki 300-3294, Japan
¹Graduate School of Systems and Information Engineering, University of Tsukuba, Ibaraki 305-8577, Japan

(Received February 5, 2007; Accepted July 30, 2007)

In certain species of moths and butterflies iridescent colors arise from subwavelength diffractive surface corrugation of the wing-scales. The optical properties of such structures depend strongly on wavelength, incidence angle, and state of polarization of illuminating radiation, and the viewing angle. In this paper, we study the reflection spectra of the wings of the Morpho didius butterfly by simulating a double-layered model of a transverse cross-section comprised of the ground scale and the cover scale. Each layer contains a certain quasi-periodic arrangement of tree-like subwavelength microstructures. The simulation is done using a high accuracy nonstandard finite-difference time-domain (FDTD) method in two dimensions. We assume that the structure is made of a slightly lossy dielectric material. The wavelength dependence of the complex refractive index for the ground scale of Morpho didius is assumed to be similar to that of Morpho sulkowskyi. The complex refractive index in the latter case was obtained by comparing the computed reflection/transmission spectra with corresponding experimental measurements at normal incidence.

Key words: Morpho butterflies, butterfly iridescence, structural color, subwavelength diffractive optical elements, gratings and diffraction, interference, electromagnetic theory, finite-difference time-domain method

^*E-mail address: banerjee@tuc.sumitomo-chem.co.jp