Circularly polarised reflection from the scarab beetle Chalcothea smaragdina: light scattering by a dual photonic structure.

Abstract

Helicoidal architectures comprising various polysaccharides such as chitin and cellulose have been reported in biological systems. In some cases, these architectures exhibit stunning optical properties analogous to ordered cholesteric liquid crystal phases. In this work, we charac- terise the circularly polarised re ectance and optical scattering from the cuticle of the beetle Chalcothea smaragdina (Coleoptera: Scarabaeidae: Cetoniinae) using optical experiments, sim- ulations and structural analysis. The selective re ection of left-handed circularly polarised light is attributed to a Bouligand-type helicoidal morphology within the beetle's exocuticle. Using electron microscopy to inform electromagnetic simulations of this anisotropic strati ed medium, the inextricable connection between the colour appearance of C. smaragdina and the periodic- ity of its helicoidal rotation is shown. A close agreement between the model and the measured re ectance spectra is obtained. In addition, the elytral surface of C. smaragdina possesses a blazed di raction grating-like surface structure, which a ects the di use appearance of the beetle's reected colour, and therefore potentially enhances crypsis amongst the dense foliage of its rainforest habitat.