The secret to the shimmering brilliance of two beetles that shine naturally with silver and gold colors has been unlocked by researchers at Universidad de Costa Rica (San José, Costa Rica). In a straightforward application of biomimetics, the researchers used optical analysis to identify the natural physical structures that create the metallic “structural coloration” of the beetles—structures that can be mimicked in the laboratory to create novel optical materials.
By illuminating the beetles at normal incidence using unpolarized light with wavelengths between 300 and 750 nm and modeling the broadband reflectance spectra with a multilayer-transfer-matrix formalism (and taking into account the curvature of the beetle’s body), the data showed a sharp drop in reflectance for wavelengths shorter than 525 nm for the gold beetle Chrysina aurigans, and low reflectance values for ultraviolet wavelengths (300 to 400 nm) and high reflectance in the whole visible spectrum for the silver beetle Chrysina limbata. Careful modeling of the reflectance spectra indicates that the beetle coloration is due to a chirped multilayer structure with about 68 layers (of different thickness and different refractive index for C. aurigans and C. limbata) of chitin in the exoskeleton of the beetles.
Contact William E. Vargas at [email protected].