Structural coloration and photonic pseudogap in natural random close-packing photonic structures.

Scales on the elytra of longhorn beetle Anoplophora graafi display diverse non-iridescent colors ranging from blue, green, yellow, and red to purple. By structural characterizations, optical measurements, and theoretical calculations, we found that the scale colors stem from an amorphous photonic structure possessing only short-range order: random close-packing of chitin nanoparticles. Our results showed that direction-independent photonic pseudogaps found in the photon density of states of the random close-packing photonic structure are the ultimate physical origin for non-iridescent coloration of scales. The color steering strategy of scales is ingenious, simply by varying the size of chitin nanoparticles. Revealed natural random close-packing photonic structures and the color steering strategy of scales could render valuable inspiration for the artificial fabrication and design of photonic structures and devices as well.