Beyond butterflies - the diversity of biological photonic crystals

When biological photonic crystals are discussed, butterfly photonic crystals are often cited as representative; in fact, numerous diverse biological photonic crystals exist and butterfly photonic crystals have several quirks when compared with others, with the consequence that considering them typical is in many ways unhelpful. In this paper, we give an overview of biological photonic crystals and discuss their typical features, specifically with regard to their periodicities, geometries, chemical compositions, the wavelengths they reflect and their band gaps. The low refractive index contrast and low mean refractive index: a universal feature of biological photonic crystals compared with artificial ones is highlighted and attention is drawn to their comparatively complex band diagrams.