Light Reflection by Cuticles of Chrysina Jewel Scarabs: Optical Measurements, Morphology Characterization, and Theoretical Modeling

Abstract

Cuticles of some Chrysina scarabs are characterized by flat, graded, and twisted structures of nanosized chitin fibrils. As inferred from SEM images, each species has its own spatial period or pitch P which is dependent on the depth z through the cuticle. From Berreman's formalism, taking into account the corresponding P(z) dependence, we evaluate reflection spectra of C. aurigans and C. chrysargyrea scarabs. The spectra display the main spectral features observed in the measured ones when small sections of the cuticles are illuminated with non-polarized light, for wavelengths between 300 and 1100 nm. By considering these twisted structures as 1D photonic crystals, an approach is developed to show how the broad band characterizing the reflection spectra arises from a narrow intrinsic photonic band width, whose spectral position moves through visible and near infrared wavelengths. The role of the epicuticle that covers the twisted structures is analyzed in terms of a waxy layer acting as an anti-reflecting coating that also shows low levels of light scattering.

Graded pitch profile for the helicoidal broadband reflector and left-handed circularly polarizing cuticle of the scarab beetle Chrysina chrysargyrea

“…Other authors reported a graded pitch ranging between 240 and 360 nm from the analysis of SEM images taken on the cuticle of silver-like C . chrysargyrea 18 . This variation of pitch is not consistent with the reflection of left-handed polarized light in the near infrared.…”

“…Both silvery and golden beetles can be found in different specimens of the same species as in C . chrysargyrea 18 . Figure 1(b) shows a photo of a silvery C .…”

“…aurigans , C . chrysargyrea ) 15–18 . TEM, co-polarized and cross-polarized spectra of right- and left-handed circularly polarized light studies have been reported for C .…”

“…aurigans as well as silvery and golden C . chrysargyrea beetles were determined from SEM images 18 . Using this graded pitch profile, the authors calculated the total reflectance as a sum of two irradiance contributions: one coherent reflection from a perfect left-handed twisted structure and one non-coherent contribution due to variations of the azimuth angle that specifies the orientation of chitin-protein nanofibrils.…”

Optical and Structural Characterization of Natural Nanostructures

“…Recently, there has been a growing interest on the polarization properties of these helicoidal structures [5,6]. This has lead to the creation of models of the structure that reproduce its optical response [7] and to biomimetic fabrication of similar structures [8].…”