Physics of structural colors

Abstract

In recent years, structural colors have attracted great attention in a wide variety of research fields. This is because they are originated from complex interaction between light and sophisticated nanostructures generated in the natural world. In addition, their inherent regular structures are one of the most conspicuous examples of non-equilibrium order formation. Structural colors are deeply connected with recent rapidly growing fields of photonics and have been extensively studied to clarify their peculiar optical phenomena. Their mechanisms are, in principle, of a purely physical origin, which differs considerably from the ordinary coloration mechanisms such as in pigments, dyes and metals, where the colors are produced by virtue of the energy consumption of light. It is generally recognized that structural colors are mainly based on several elementary optical processes including thin-layer interference, diffraction grating, light scattering, photonic crystals and so on. However, in nature, these processes are somehow mixed together to produce complex optical phenomena. In many cases, they are combined with the irregularity of the structure to produce the diffusive nature of the reflected light, while in some cases they are accompanied by large-scale structures to generate the macroscopic effect on the coloration. Further, it is well known that structural colors cooperate with pigmentary colors to enhance or to reduce the brilliancy and to produce special effects. Thus, structure-based optical phenomena in nature appear to be quite multi-functional, the variety of which is far beyond our understanding. In this article, we overview these phenomena appearing particularly in the diversity of the animal world, to shed light on this rapidly developing research field.

Structure and optical function of amorphous photonic nanostructures from avian feather barbs: a comparative small angle X-ray scattering (SAXS) analysis of 230 bird species

“…Like photonic crystals [6], biophotonic nanostructures vary in nanostructure in either one, two or three dimensions (figure 1a-c). However, they may also vary in whether they have long-range, crystalline periodicity, or only short-range (nearest neighbour), structural correlations [1][2][3][4][5] (figure 1d -f ). The latter referred to as quasi-ordered or amorphous biophotonic nanostructures are characterized by unimodal distributions of scatterer size and inter-scatterer spacing, and a notable lack of any underlying periodicity beyond the span of a few nearest neighbours (figure 2e,f ) [3,7 -9].…”

Evolutionary‐Optimized Photonic Network Structure in White Beetle Wing Scales

“…[1] Opaque white materials have to contain a sufficiently large number of scatterers and therefore usually require thicker, material-rich nanostructures than structural color arising from the coherent interference of light. [2,3] In nature, bright white appearance arises from the dense arrays of pterin pigments in pierid butterflies, [4] guanine crystals in spiders, [5] or leucophore cells in the flexible skin of cuttlefish. [6] A striking example of such whiteness is found in the chitinous networks of white beetles, e.g., Lepidiota stigma and Cyphochilus sp.…”

“…[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.…”

Graded index photonic hole: Analytical and rigorous full wave solution

“…For a long time, the natureinspired designs of optical systems are predominantly based on mimicking structures appearing in the diversity of the biological world, leading to a thriving area of biomimetic or bioinspired photonics. [1][2][3][4] Typical examples in this area include, among others, the study of structural color [5][6][7] and artificial compound eyes. 8 In addition to imitating terrestially occurring structures, the design of novel optical components is recently inspired by mimicking celestial phenomena.…”