Bright-White Beetle Scales Optimise Multiple Scattering of Light

Abstract

Whiteness arises from diffuse and broadband reflection of light typically achieved through optical scattering in randomly structured media. In contrast to structural colour due to coherent scattering, white appearance generally requires a relatively thick system comprising randomly positioned high refractive-index scattering centres. Here, we show that the exceptionally bright white appearance of Cyphochilus and Lepidiota stigma beetles arises from a remarkably optimised anisotropy of intra-scale chitin networks, which act as a dense scattering media. Using time-resolved measurements, we show that light propagating in the scales of the beetles undergoes pronounced multiple scattering that is associated with the lowest transport mean free path reported to date for low-refractive-index systems. Our light transport investigation unveil high level of optimisation that achieves high-brightness white in a thin low-mass-per-unit-area anisotropic disordered nanostructure.

Evolutionary‐Optimized Photonic Network Structure in White Beetle Wing Scales

“…[11] Distortion-free 3D imaging of biological tissue with submicrometer resolution in a large volume is cumbersome and often relies on destructive techniques, i.e., focused ion beam (FIB)-scanning electron microscopy (SEM) serial tomography or electron tomography. [7,12] Multi-keV ptychographic X-ray computed tomography (PXCT) [13] overcomes this limitation, achieving below 15 nm resolution in radiation-hard materials at room temperature. [14] In radiation-sensitive materials, however, the PXCT resolution used to be much worse (>100 nm), limited by either radiation damage at room temperature or by drifts of the setup at cryogenic temperatures.…”

“…[7] Closer inspection of the white scales using SEM reveals that the scale interior comprises a network of interconnected, chitinous fibers surrounded by air. [7][8][9]18,19] Earlier interpretation of the optical response of the scales [7,9] was hampered by the lack of artifact-free, 3D visualization of the complex chitinous cuticle structure, and claims concerning the evolutionary optimized white color of the beetle were therefore anecdotal.To gain insight into the optical function of the scales, we investigated the wing scales using PXCT. A first attempt was aimed at reconstructing the network morphology inside the beetle scale using conventional room-temperature PXCT, [20] Adv.…”

“…[6] A striking example of such whiteness is found in the chitinous networks of white beetles, e.g., Lepidiota stigma and Cyphochilus sp. [7][8][9] Previous research investigating these beetle structures has shown that the chitinous network is one of the most strongly scattering materials in nature, and therefore the question arises whether this structure is evolutionary optimized for strong scattering while minimizing the Most studies of structural color in nature concern periodic arrays, which through the interference of light create color. The "color" white however relies on the multiple scattering of light within a randomly structured medium, which randomizes the direction and phase of incident light.…”

“…[2,3,9,10] For these complex cases, the validity of the diffusion approximation is limited, since single scattering elements are difficult to be identified. [7] To fully understand the correlation between the structure and (optical) properties of complex materials, the detailed real-space structure in combination with a suitable model unravels this relationship. In optics, these include finite-element modeling or finite-difference time-domain (FDTD) techniques, which can be employed for materials with arbitrary structures.…”

“…These scales strongly scatter incident light, and the reflectance is more or less constant across the visible wavelength range ( Figure S3 of the Supporting Information). [7] Closer inspection of the white scales using SEM reveals that the scale interior comprises a network of interconnected, chitinous fibers surrounded by air. [7][8][9]18,19] Earlier interpretation of the optical response of the scales [7,9] was hampered by the lack of artifact-free, 3D visualization of the complex chitinous cuticle structure, and claims concerning the evolutionary optimized white color of the beetle were therefore anecdotal.…”