Generation of Reflection Colors from Metal-Insulator-Metal Cavity Structure Enabled by Thickness-Dependent Refractive Indices of Metal Thin Film

Tunable structural colors have diverse applications ranging from displays and photovoltaics to surface decoration and art. A metal-insulator-metal (MIM) cavity structure formed by thin continuous layers has drawn great interest as a lithography-free and scalable optical structure to control light transmission and reflection at the surface of a material. However, the production of distinct reflection colors from the structure is challenging because the typical MIM cavity absorbs a narrow wavelength range and reflects the rest of the spectrum. This study shows that the MIM structure can exhibit a reflection peak instead of a reflection dip if the metal layer has proper optical constants. Vivid reflection colors are generated by using thermally evaporated Au and Ag thin films whose refractive indices are much different from the standard handbook data. The strong thickness dependence of the refractive indices also enables color tuning by varying the thickness of the metal layer only. Consequently, color images can be printed by locally controlling the thickness of either the insulating spacer or the metal layer. The results of the study are attractive and useful for both practical and artistic purposes.