Photonic Crystal Characterization of the Cuticles of Chrysina chrysargyrea and Chrysina optima Jewel Scarab Beetles.

William Vargas,Ángel Solís,Daniel Azofeifa,Marcela Hernández-Jiménez,Cynthia Barboza-Aguilar,Eduardo Libby,Esteban Avendano

doi:10.3390/biomimetics3040030

William Vargas, Ángel Solís + Show 5 more

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https://doi.org/10.3390/biomimetics3040030

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Abstract

A unified description involving structural morphology and composition, dispersion of optical constants, modeled and measured reflection spectra and photonic crystal characterization is devised. Light reflection spectra by the cuticles of scarab beetles (Chrysina chrysargyrea and Chrysina optima), measured in the wavelength range 300–1000 nm, show spectrally structured broad bands. Scanning electron microscopy analysis shows that the pitches of the twisted structures responsible for the left-handed circularly polarized reflected light change monotonically with depth through the cuticles, making it possible to obtain the explicit depth-dependence for each cuticle arrangement considered. This variation is a key aspect, and it will be introduced in the context of Berreman’s formalism, which allows us to evaluate reflection spectra whose main features coincide in those displayed in measurements. Through the dispersion relation obtained from the Helmholtz’s equation satisfied by the circular components of the propagating fields, the presence of a photonic band gap is established for each case considered. These band gaps depend on depth through the cuticle, and their spectral positions change with depth. This explains the presence of broad bands in the reflection spectra, and their spectral features correlate with details in the variation of the pitch with depth. The twisted structures consist of chitin nanofibrils whose optical anisotropy is not large enough so as to be approached from modeling the measured reflection spectra. The presence of a high birefringence substance embedded in the chitin matrix is required. In this sense, the presence of uric acid crystallites through the cuticle is strongly suggested by frustrated attenuated total reflection and Raman spectroscopy analysis. The complete optical modeling is performed incorporating the wavelength-dependent optical constants of chitin and uric acid.

Highlights

Reflection of light by cuticles of scarab beetles has attracted the attention of researchers since the beginning of the 20th Century, even more so after the circularly polarized nature of the reflectedBiomimetics 2018, 3, 30; doi:10.3390/biomimetics3040030 www.mdpi.com/journal/biomimeticsBiomimetics 2018, 3, 30 light was established [1]
The optical properties of cuticles of Chrysina scarab beetles have been considered from spectrophotometric measurements and modeled with Berreman’s formalism, which incorporates the anisotropy in the optical constants of the materials: the natural polymer chitin nanofibrils and the uric acid high birefringent embedded crystals
In addition to the optical constants with their wavelength dependences, variations with depth of the pitches characterizing the chiral structures found through the cuticles have been provided to the model

Summary

Introduction

Reflection of light by cuticles of scarab beetles has attracted the attention of researchers since the beginning of the 20th Century, even more so after the circularly polarized nature of the reflectedBiomimetics 2018, 3, 30; doi:10.3390/biomimetics3040030 www.mdpi.com/journal/biomimeticsBiomimetics 2018, 3, 30 light was established [1]. The iridescent green and purple colors of the elytra of the Japanese beetle Chrysochroa fulgidissima have been correlated with the presence of corresponding multilayered structures through its cuticle [7] In this case, successive layers differ slightly in refractive index, and each one is assumed as optically homogeneous in the context of the radiative transfer model used to calculate reflection spectra consistent with the observed colors and linear polarization of the reflected light. Regarding the twisted arrangements found through the cuticle of scarab beetles, structures characterized by a single pitch value with a small variation of the pitch from a certain depth through the cuticle and helicoidal arrangements with various pitches have been considered in connection with the spectral properties of the reflected light [18,19] Both spectrophotometricand ellipsometry-based analyses have been reported [20,21]. Photometric and ellipsometric measurements were reported by Goldstein for Chrysina resplendens, Chrysina clypealis and Chrysina gloriosa scarabs [22]

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