Anisotropic optical response of arthopods's cuticle

The structure of the cuticle of arthropods consists of layers of microfilamentary chitin particles. The layers are stacked one on top of the other realizing a helical (Bouligand helix) pattern. This structure generates optical phenomena such as structural colors and birefringence that produce a metalic or iridescent appearance, polarization upon reflection and optical activity. We model the anisotropic optical responses of the cuticle of arthropods using the Photonic package to obtain the macroscopic dielectric tensor of a constitutive layer in the Bouligand helix. We find that its anisotropy depends on the shape of the chitin particles, their reticular arrangement and their filling fractions. As a main result, we obtained a cuticle model with tunable reflectance band gaps that are very sensitive to geometrical parameters such as the angle $\theta$ that controls the helix pitch. We can explain the structural colors in terms of the reflectance band gap induced by the modulation of the anisotropy mediated by $\theta$, instead of following the usual approach that consider multiple arrangement of layers pairs with different thickness and optical properties.