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

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Centro de Investigación en Ciencia e Ingeniería de Materiales, Escuela de Física, Universidad de Costa Rica, San José 2060-11501, Costa Rica
2
Academia Nacional de Ciencias de Costa Rica, San José 1367-2050, Costa Rica
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Centro de Investigación en Ciencia e Ingeniería de Materiales, Escuela de Química, Universidad de Costa Rica, San José 2060-11501, Costa Rica
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Departamento de Historia Natural, Museo Nacional de Costa Rica, San José 749-1000, Costa Rica
5
Centro de Investigación en Estructuras Microscópicas, Universidad de Costa Rica, San José 2060-11501, Costa Rica
*
Author to whom correspondence should be addressed.
Biomimetics 2018, 3(4), 30; https://doi.org/10.3390/biomimetics3040030
Received: 31 July 2018 / Revised: 8 September 2018 / Accepted: 27 September 2018 / Published: 11 October 2018
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. View Full-Text
Keywords: circular polarization; structural color; natural photonic material; photonic band gap; photonic crystal circular polarization; structural color; natural photonic material; photonic band gap; photonic crystal
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Vargas, W.E.; Avendano, E.; Hernández-Jiménez, M.; Azofeifa, D.E.; Libby, E.; Solís, Á.; Barboza-Aguilar, C. Photonic Crystal Characterization of the Cuticles of Chrysina chrysargyrea and Chrysina optima Jewel Scarab Beetles. Biomimetics 2018, 3, 30.

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