Elucidating the stop bands of structurally colored systems through recursion

TL;DR

This paper models the optical stop bands in multilayer structures using recursion relations, demonstrating how interference and absorption lead to observable iridescent colors in beetles, linking physical theory with biological phenomena.

Contribution

It introduces a minimal recursive model for optical band structures in multilayer systems and connects theoretical predictions with experimental beetle data.

Findings

Stop bands emerge with infinite layers via fixed points of recursion.

Absorption is necessary for convergence of the recursive relations.

The model explains high reflectance and iridescence in beetles.

Abstract

Interference phenomena are the source of some of the spectacular colors of animals and plants in nature. In some of these systems, the physical structure consists of an ordered array of layers with alternating high and low refractive indices. This periodicity leads to an optical band structure that is analogous to the electronic band structure encountered in semiconductor physics; namely, specific bands of wavelengths (the stop bands) are perfectly reflected. Here, we present a minimal model for optical band structure in a periodic multilayer and solve it using recursion relations. We present experimental data for various beetles, whose optical structure resembles the proposed model. The stop bands emerge in the limit of an infinite number of layers by finding the fixed point of the recursive relations. In order for these to converge, an infinitesimal amount of absorption needs to be present, reminiscent of the regularization procedures commonly used in physics calculations. Thus, using only the phenomenon of interference and the idea of recursion, we are able to elucidate the concepts of band structure and regularization in the context of experimentally observed phenomena, such as the high reflectance and the iridescent color appearance of structurally colored beetles.