TITLE:
Light Reflection by Cuticles of Chrysina Jewel Scarabs: Optical Measurements, Morphology Characterization, and Theoretical Modeling
AUTHORS:
W. E. Vargas, M. Hernández-Jiménez, E. Libby, D. E. Azofeifa, C. Barboza, Á. Solis
KEYWORDS:
Structural Color, Structural Chirality, Natural Broad Band Reflector, Circular Polarization, Jewel Scarab, Chitin Refractive Index, Uric Acid Refractive Index, Left-Handed Twisted Structure, Bouligand-Type Structure
JOURNAL NAME:
Optics and Photonics Journal,
Vol.6 No.7,
July
13,
2016
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.