Anisotropicoptical response of the cuticle of arthropods

Abstract

The structure of the cuticle of arthropods consists of layers of micro-filamentary chitin particles. The layers are stacked one on top of the other realizing a Bouligand helical structure which generates optical phenomena such as structural colors and birefringence that produce a metallic or iridescent appearance, polarization upon reflection and optical activity. We model the anisotropic optical response of a constitutive layer in the Bouligand helix of the cuticle using the Photonic computational package, obtaining the macroscopic dielectric tensor. Its anisotropy depends on the shape of the chitin particles, their reticular arrangement and their filling fractions. We build a model for the cuticle with tunable reflectance band-gaps that are very sensitive to geometrical parameters such as the angle θ that controls the helix pitch. We can explain the structural colors in terms of the reflectance band-gap induced by the rotation of the anisotropy mediated by θ, instead of following the usual approach that considers arrangements of pairs of layers with contrasting optical properties.