Abstract
Several beetle species in the Scarabaeoidea superfamily reflect left-handed polarized light due to a circular Bragg structure in their cuticle. The right-handed polarized light is transmitted. The objective here is to evaluate cuticle chiral properties in an effective medium approach using transmission Mueller matrices assuming the cuticle to be a bianisotropic continuum. Both differential decomposition and nonlinear regression were used in the spectral range of 500–1690 nm. The former method provides the sample cumulated birefringence and dichroic optical properties and is model-free but requires a homogeneous sample. The materials chirality is deduced from the circular birefringence and circular dichroic spectra obtained. The regression method requires dispersion models for the optical functions but can also be used in more complex structures including multilayered and graded media. It delivers the material properties in terms of model functions of materials’ permittivity and chirality. The two methods show excellent agreement for the complex-valued chirality spectrum of the cuticle.
Highlights
Several beetle species in the Scarabaeoidea superfamily reflect left-handed polarized light due to a circular Bragg structure in their exoskeleton, called cuticle.[1]
The objective here is to evaluate cuticle chiral properties in an effective medium approach using transmission Mueller matrices assuming the cuticle to be a bianisotropic continuum. Both differential decomposition and nonlinear regression were used in the spectral range of 500–1690 nm
The effect is illustrated for the scarab beetle Cetonia aurata (Linnaeus, 1758) in Fig. 1, where it is seen that the beetle appears nonreflecting when viewed through a right-handed polarizer and preserves its color when viewed through a left-handed polarizer
Summary
Several beetle species in the Scarabaeoidea superfamily reflect left-handed polarized light due to a circular Bragg structure in their exoskeleton, called cuticle.[1]. The circular Bragg structure is chiral and is called a Bouligand structure and has been visualized in several beetles by electron microscopy.[3,4] The optical features of such biological reflectors include structural colors, polarization, and depolarization and have been explored in numerous investigations based on spectral reflectance.[5–8] A more advanced methodology, compared to reflectance measurements, is Mueller matrix ellipsometry, which has the advantage that it allows polarization and depolarization features to be quantified. A sum decomposition of a Mueller matrix provides a phenomenological description of beetle cuticle reflection in terms of optical elements like retarders and polarizers.[15]. Transmission Mueller matrices and to compare with results from nonlinear regression of Mueller matrices using dispersion models
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