The evolution of structural colour mechanisms in biological systems has given rise to many interesting optical effects in animals and plants. The instance of the scarab beetle is particularly distinctive. Its exoskeleton has a bright, golden appearance and reflects both right-handed and left-handed circularly polarized light concurrently. The chiral nanostructure responsible for these properties is a helicoid, in which birefringent dielectric planes are assembled with an incremental rotation. This study correlates details of the beetle's circularly polarized reflectance spectra directly with physical aspects of its structural morphology. Electron micrography is used to identify and measure the physical dimensions of the key constituent components. These include a chiral multilayer configuration comprising two chirped, left-handed helicoids that are separated by a birefringent retarder. A scattering matrix technique is used to simulate the system's optical behaviour in which the roles of each component of the morphological substructure are elucidated by calculation of the fields throughout its depth.
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 characterize the circularly polarized reflectance and optical scattering from the cuticle of the beetle (Coleoptera: Scarabaeidae: Cetoniinae)using optical experiments, simulations and structural analysis. The selective reflection of left-handed circularly polarized light is attributed to a Bouligand-type helicoidal morphology within the beetle's exocuticle. Using electron microscopy to inform electromagnetic simulations of this anisotropic stratified medium, the inextricable connection between the colour appearance of and the periodicity of its helicoidal rotation is shown. A close agreement between the model and the measured reflectance spectra is obtained. In addition, the elytral surface of possesses a blazed diffraction grating-like surface structure, which affects the diffuse appearance of the beetle's reflected colour, and therefore potentially enhances crypsis among the dense foliage of its rainforest habitat.
Photonic structures in ordered, quasi-ordered or disordered forms have evolved across many different animals and plant systems. They can produce complex and often functional optical responses through coherent and incoherent...
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