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Bright-white Beetle Scales Optimise Multiple Scattering of Light


Bright-white Beetle Scales Optimise Multiple Scattering of Light

Matteo Burresi et al. Sci Rep.

Erratum in

  • Sci Rep. 2014;4:7271


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.


Figure 1
Figure 1. White reflection from beetle scales.
(a,c), Images of Cyphochilus and Lepidiota stigma beetles, respectively. (b,d), Scanning electron micrographs (SEM) of the cross-section of the scales of the respective species.
Figure 2
Figure 2. Time-resolved measurements of the scales.
(a,b) Time-of-flight of light transmitted through the scales of Cyphochilus and L. stigma, respectively (open triangles). The reference measurement (cross-correlation) of the probe pulse is shown as black squares. The fit of the probe pulse (green line) yields a pulse duration, in semilog scale of 130 fs. In contrast, the pulse transmitted through the scales exhibits an exponential tail over three orders of magnitude in intensity. The exponential fit (red line) yields lifetimes of τ ≈ 140 fs and τ ≈ 210 fs for (a,b), respectively.
Figure 3
Figure 3. Transport mean free path in the scales.
(a,b), Transport mean free path t as a function of filling fraction f of the chitin random network and scale thickness t calculated by equation 1 using measured τ-values, for Cyphochilus and L. stigma, respectively. The black areas correspond to unphysical solutions. t is smaller than 5 μm in the relevant f and t range. (c,d), Corresponding variation of t with f for fixed t (t = 8.1 μm and t = 13.7 μm for (c,d), respectively). The grey and black symbols correspond to the predictions of equation 1 and 2, respectively. The crossing points provide estimates of t and f, delimited by the confidence range indicated by dashed lines.
Figure 4
Figure 4. Reflection spectra of the white scales.
(a–b) The measured spectra for Cyphochilus and L. stigma scales, respectively, reveal the whiteness of a single scale which extends to the near-infrared. The dashed lines are predictions from equation 2 (R = 1 − T). The mean free path used in equation 2 is calculated by modelling the chitin continuous random network with a distribution of particle havening different shape and orientations: only spheres, spheres and rods randomly oriented in space, and spheres and rods with a random in-plane orientation (no vertically oriented rods are present).

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