Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), in collaboration with King Abdullah University of Science and Technology, have developed a system to build large-scale metamaterials with structural color. The research is described in the journal Nature Light: Science and Applications.

A peacock's feather or a butterfly's wing rely on photonic crystals or highly ordered arrays of nanofibers to produce colors. Reproducing those structures in a lab requires precision and expensive fabrication. SEAS researchers were inspired by a different kind of feather.

Light-matter interaction with the sample with different thickness of the alumina coating. Credit: Henning GalinskiUnlike a peacock's ordered array of nanostructures, contingas get their hues from a disordered and porous nanonetwork of keratin that looks like a sponge or piece of coral. When light strikes the feather, the porous keratin pattern causes red and yellow wavelengths to cancel each other out, while blue wavelengths of light amplify one another.

Inspired by the cotinga feather, the researchers used an etching process to create a complex but random porous nanonetwork in a metallic alloy. The structure was then coated with an ultra-thin transparent alumina layer.

The porous nanostructure creates localized hotspots of different colors in the alloy. The color that is reflected by the localized states depends on the thickness of the transparent coating.

With no alumina overlayer, the material looks dark. With a coating 33-nanometers thick, the material reflects blue light. At 45 nanometers, the material turns red and with a coating 53 nanometers thick, the material is yellow. By changing the thickness of the coating, the researchers could create a gradient of colors.

The metasurface is lightweight and scratch-proof and could be used in large-scale commercial applications such as lightweight coatings for the automotive sector, biomimetic tissues, and camouflage.