Photonic nano-structure of R. Gigantea

Rhomborhina Gigantea (R. Gigantea) was examined for its iridescent properties in its exocuticle. The iridescence was explained by the theories of the optical reflectors causing the structural colors and the concepts of cholesteric liquid crystals (CLCs). The exocuticle was examined for its optical properties through experiments of reflection and diffraction. Reflection spectra showed contrast results in transverse electric (TE) and transverse magnetic (TM) modes. The cross-section of the beetle cuticle structure was investigated using scanning electron microscope (SEM). The experimental values of average refraction index (n_av) and the half-pitch (P_h) were derived from the optimal Bragg condition. The observed P_h from SEM images was similar to the experimental values in linear polarization. The average refractive indices determined from the optical data in both TE and TM modes were in god agreement to the predicted value. The consistency of the results implied that the structure responsible for the iridescent beetle was a biological analogue to CLCs.