Uric acid profiles through helical structures and the unidirectional layer in the cuticle of the <i>Chrysina cupreomarginata</i> scarab and photonic crystal characterization

Reflectance spectra of Chrysina cupreomarginata ’s elytra are measured for visible wavelengths. The spectra consist of both left- and right-handed circularly polarized light, which suggests the presence of two chitin-based helicoidal structures separated by a unidirectional layer, as reported for the Chrysina resplendens scarabs. This fact has been corroborated by the analysis of scanning electron microscopy imaging. The structural and effective pitches of each helicoid have been obtained from these images. The height and width of the reflectance bands are linked with the presence of uric acid crystallites through the arrangement of oriented chitin fibrils embedded in the proteinaceous matrix. Calculated left- and right-handed circularly polarized reflectance spectra are evaluated from a radiative transfer matrix formalism. The photonic characterization of each helicoid is carried out in terms of the variation of the optical gap with depth through the twisted arrangements. A novel, to our knowledge, approach is developed to obtain the depth-dependence of both the uric acid volume fractions through each helical structure and the spectral positions and widths of the photonic band gaps. When modeling reflectance spectra to resemble measured ones, it is necessary to significantly increase the volume fraction of uric acid in the unidirectional layer for it to function as a half-wave plate.