Abstract
Light in biological media is known as freely diffusing because interference is negligible. Here, we show Anderson light localization in quasi-two-dimensional protein nanostructures produced by silkworms (Bombyx mori). For transmission channels in native silk, the light flux is governed by a few localized modes. Relative spatial fluctuations in transmission quantities are proximal to the Anderson regime. The sizes of passive cavities (smaller than a single fibre) and the statistics of modes (decomposed from excitation at the gain–loss equilibrium) differentiate silk from other diffusive structures sharing microscopic morphological similarity. Because the strong reflectivity from Anderson localization is combined with the high emissivity of the biomolecules in infra-red radiation, silk radiates heat more than it absorbs for passive cooling. This collective evidence explains how a silkworm designs a nanoarchitectured optical window of resonant tunnelling in the physically closed structures, while suppressing most of transmission in the visible spectrum and emitting thermal radiation.
Highlights
When light waves undergo multiple scattering through inhomogeneous dielectric biomacromolecules, interference is conventionally ignored[1,2], because the phases of most scattered waves are uncorrelated and their interference is cancelled out
A single silk fibroin filament with a size of L ≈ 20 μm can contain ~3800 nanofibrils (Fig. 1d)
If each individual nanofibril serves as a scattering centre and the arrangement of scattering centres is favourable for constructive interference, Anderson localization could potentially be realized (Fig. 1a)
Summary
When light waves undergo multiple scattering through inhomogeneous dielectric biomacromolecules, interference is conventionally ignored[1,2], because the phases of most scattered waves are uncorrelated and their interference is cancelled out. Anderson localization has been used to describe the metal–insulator transition, resulting from scattering of the electronic wavefunction in random defects of the electronic potential[8]. Since this concept was proven for a system where the total energy is conserved, it has recently been extended to nonconservative bosonic fields (e.g. microwaves and light waves)[6,7,9,10,11,12,13]. The statistics of decomposed modes differentiate silk from other diffusive structures, such as cellulose fibre microstructures of white paper This approach is relatively insensitive to structural nonuniformity and light absorption of materials and limited numerical apertures (NA) in optics. Because Anderson light localization is a physical phenomenon not typically found in nature, our results from native silk may suggest an idea of ‘natural’ metamaterials such that nature overcomes constituent material limitations for exceptional properties, which are considered to be only possible by human engineering
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