Structure-property-relations of cuticular photonic crystals evolved by different beetle groups (Insecta, Coleoptera)
Photonic crystals are optically active materials that can inhibit the propagation of light within certain frequency ranges. This type of material has shown many new and exciting optical effects which have led to considerable advances in technology. Investigating the design of the periodic structures of photonic crystals is one of the essential research fields aiming at optimized optical properties for technical applications. In nature, many living organisms have evolved biological photonic crystals which are able to generate vivid iridescent colors. These biological model systems, especially the photonic structures formed by the cuticle of insects, have already been shown to be a rich source of inspiration for the design and development of synthetic optical materials.
The author investigated the relations of structure and optical properties of the cuticular photonic crystals of five beetle species from three different systematic groups with the goal to evaluate their design principles. A large variety of methods for microscopic investigations and chemical analysis were applied to characterize the microstructure and composition of these photonic crystals experimentally. Their optical properties were both experimentally measured and theoretically characterized based on models derived from the structural analysis.
The results show that the investigated species of Ground beetles and Scarab beetles have evolved multilayer structures and helicoidal structures in different regions of their cuticles: epicuticle and exocuticle, respectively. The weevil species has formed three-dimensional photonic crystals based on a D-surface structure (diamond) in scales which are outgrowths of the cuticle. Through modifications of structure and chemical composition of these basic structures, the investigated beetles obtain optical properties that are optimized to meet the imposed ecophysiological strains. In some cases, unusual and unexpected optical effects were observed. The principles of both the basic structuring of these photonic crystals and the additional modifications provide us inspirations for the design of synthetic composite materials with not only novel optical properties, but also combinations with other physical properties in a balanced equilibrium.