Heat-induced perfect light absorption in thin-film metasurfaces for structural coloring

Heating reflective metals is known to produce a wide range of colors due to oxidation of the metal surface. In fact, the most vibrant colors used in the pre-industrial era came from oxides, acetates and carbonates of metal ores and minerals. In this work, we show that heating low reflectivity metals, e.g., Ni and Ti, creates structural colors through perfect light absorption. We tune the absorption across the visible and NIR spectrum by changing the heating duration and, consequently, the oxide thickness. We demonstrate experimentally angle-insensitive perfect and near-perfect absorption in the visible and NIR regimes up to ±60°. The absorption is insensitive to the incidence angle due to the relatively high refractive index of the formed oxides, which create iridescent free coloration. We demonstrate that the oxide layer thickness, with refractive index n, is < λ/4n due to non-trivial phase change at the oxide/metal interfaces, which makes these systems the simplest example of meta-surfaces based on thin films. The results show that oxidized metals can have applications beyond producing vibrant colors.