Circularly polarized reflection from the scarab beetle Chalcothea smaragdina: light scattering by a dual photonic structure

Abstract

Helicoidal architectures comprising various polysaccharides, such as chitin and cellulose, have been reported in biological systems. In some cases, these architectures exhibit stunning optical properties analogous to ordered cholesteric liquid crystal phases. In this work, we characterize the circularly polarized reflectance and optical scattering from the cuticle of the beetle Chalcothea smaragdina (Coleoptera: Scarabaeidae: Cetoniinae) using optical experiments, simulations and structural analysis. The selective reflection of left-handed circularly polarized light is attributed to a Bouligand-type helicoidal morphology within the beetle's exocuticle. Using electron microscopy to inform electromagnetic simulations of this anisotropic stratified medium, the inextricable connection between the colour appearance of C. smaragdina and the periodicity of its helicoidal rotation is shown. A close agreement between the model and the measured reflectance spectra is obtained. In addition, the elytral surface of C. smaragdina possesses a blazed diffraction grating-like surface structure, which affects the diffuse appearance of the beetle's reflected colour, and therefore potentially enhances crypsis among the dense foliage of its rainforest habitat.

Keywords: Bouligand structure; Coleoptera; blazed grating; circular polarization.

Figure 1.

(a–c) Photographs of C. smaragdina capturing the beetle's appearance for the following configurations: (a) without an analysing filter, (b) through an LCP analyser and (c) through an RCP analyser. The photographs show that the beetle selectively reflects LCP light. (d–g) Optical micrographs of the elytral surface corresponding to: (d) unpolarized bright-field illumination; (e) unpolarized dark-field illumination; (f) with the reflected light analysed using an LCP filter; (g) with the reflected light analysed using an RCP filter. A reticulated surface patterning is revealed which scatters light to oblique angles under dark-field illumination. Scale bars, (a) 1 cm and (d–g) 50 μm.

Figure 2.

(a) Spectrophotometry measurements showing both the co-polarized (LCP and RCP) and cross-polarized (LCP/RCP) CP reflectance spectra for C. smaragdina. (b) Optical modelling of the co-polarized and cross-polarized reflectance from the specimen.

Figure 3.

(a,b) SEM images detailing the reticulated elytral surface structure for C. smaragdina. (c,d) AFM measurements reveal the form of the blazed grating-like surface topography, with the dashed black line in (c) corresponding to the line profile shown in (d). Both techniques provided physical dimensions describing the blazed surface topography. Scale bars, (a) 30 μm and (b) 5 μm.

Figure 4.

(a) TEM cross-section showing the full extent of the photonic ultrastructure within the specimen's dorsal pygidial cuticle. (b) High-magnification TEM cross-section showing the Bouligand structure within the beetle's elytral exocuticle. (c) Schematic of the helicoidal morphology within the C. smaragdina exocuticle. The helicoid is formed by a lamellar stack comprising successively rotated chitin microfibril layers. (d) Lamellar pitch profile describing the distribution of helicoidal periods in C. smaragdina. Each lamellar pitch measurement (black crosses) represents the distance for which the chitin microfibrils comprising the helicoid rotate through 180°. The mean lamellar pitch measurements (green circles) fitted using a spline interpolation (green line) are also shown. Scale bars, (a) 3 μm and (b) 200 nm.

Figure 5.

ISM results obtained for C. smaragdina, showing the far-field scattering arising following (a) narrow-aperture illumination and (b–d) wide-aperture illumination using the ISM. (c,d) Angle-dependent scattering pattern upon insertion of a left- and right-handed circular polarizer in the detection pathway. The images are cropped around the central axis due to polarization accuracy. Moving radially outwards from the scatterogram centres, the red circles denote scattering angles of 5°, 30°, 60° and 90°.