Color Tunable, Lithography-Free Refractory Metal–Oxide Metacoatings with a Graded Refractive Index Profile

Abstract

The refractory metal–oxide semiconductors are an overlooked platform for nanophononics that offer alloys with high melting points and tunable optical constants through stoichiometry changes and ion intercalation. We show that these semiconductors can form metamaterial coatings (metacoatings) made from a set of highly subwavelength, periodic metal–oxide layers (≤20 nm) with a varying and graded refractive index profile that includes a combination of high and low refractive indices and plasmonic layers. These metacoatings exhibit vibrant, structural color arising from the periodic index profile that can be tuned across the visible spectrum, over ultralarge lateral areas through bottom-up thermal annealing techniques.

Recent progress on structural coloration

“…In general, structural colors can be realized by nanophotonics systems ranging from multilayered thin films with engineered thicknesses to dielectric and/or metallic nanoparticles and the array they form. One-dimensional (1D) photonic crystals, formed by stacking multilayers of thin-film materials, realize the structural colors by prompting constructive and destructive interferences for photons with different wavelengths via the thin-film interference effect [22][23][24][25][26][27] , where the limited degrees of freedom of the multilayer film system constrains the variety of colors that can be realized [28] . Additionally, changes in the angle of incidence of light can cause iridescence in multilayer pairs, which are often considered as undesirable display artifacts for common applications.…”