Quarter-wave design criteria for omnidirectional structural colors

Structural colors are caused by light interference from periodic multi-layer dielectric stacks and exhibit rich, highly-chromatic colors. However, avoiding a color shift due to angular variation remains a challenge. The design criteria for angle-independent structural colors have been studied by treating a quarter-wave stack of alternating dielectric material layers as a one-dimensional photonic crystal. A refractive index zone has been identified that exhibits narrowband omnidirectionality for structural colors in the visible wavelength range. It was found that a quarter-wave stack of high refractive index (RI) paired-layers (n H = 2.85 and n L = 2.5), which have low RI contrast (n H /n L = 1.14), is key in fabricating omnidirectional structural color materials. The theoretical predictions discussed here are verified experimentally using a titania/halfnia (TiO2/HfO2) multilayer stack.