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.
Highlights
As seen in reflection or transmission modes, nanosized non-light absorbing materials can display specific colors when they form part of organized micrometric structures
Regarding the golden-like C. aurigans, the presence of a depth dependent spatial period leads to a broadening of the photonic band gap, which goes from 580 to about 900 nm, but within our zero order approach this is not sufficient to explain the broader width of the reflection band only in terms of ∆λ
The presence of defects through the twisted structures corresponding to C. aurigans and golden-like C. chrysargyrea contributes to the spectra by adding a small amount of non-coherent left-handed reflected light, with a broad band centered at 800 nm in the near infrared
Summary
As seen in reflection or transmission modes, nanosized non-light absorbing materials can display specific colors when they form part of organized micrometric structures. This is the case of chitin nano-fibrils which are the main constituent found in the cuticle of scarab beetles, where the micrometric structure consists of a twisted arrangement that shows a left- or righthanded chirality. Our group has recently reported for Chrysina aurigans scarab beetles the presence of a pitch P that shows a smooth dependence with depth z through the cuticle [7]-[13], i.e. P = P(z) Such dependence has been obtained from the SEM images that show a cut perpendicular or nearly perpendicular to the layered structure, by assuming a change of π in φ when following the depth z from a black layer to the one. A novel analysis of the optical role played by the epicuticle that covers each chiral structure is developed in the subsection 4.5
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