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.
Highlights
Whiteness arises from diffuse and broadband reflection of light typically achieved through optical scattering in randomly structured 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
These structural colours arise from complex nanostructures such as ordered and quasi-ordered photonic crystals and random assemblies[18,19,20]
Summary
Matteo Burresi[1,2], Lorenzo Cortese[1,3], Lorenzo Pattelli[1,3], Mathias Kolle[4], Peter Vukusic[5], Diederik S. 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. Functional structures in insects mostly consist of chitin and melanin[12] The assembly of these materials in various parts of the body often creates intriguing optical effects ranging from matte to iridescent colours[12,13], and from black[14] to substitutes extremely with bright white[15,16,17]. These structural colours arise from complex nanostructures such as ordered and quasi-ordered photonic crystals and random assemblies[18,19,20]. Nanostructure combined with its remarkably low mass per unit area – crucial for flying insects – are evidence for optimisation by evolution
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