Full-color, multi-level transmittance modulators: From reflectivity/gradient absorption coupling mechanism to materials map

Abstract Full-color multi-level transmittance-modulated films have important applications in many emerging fields, such as decorative smart windows and camouflage laser-protection windows. However, the existing transmission-modulated films cannot obtain rich colors due to the limitation of selective absorption. Although the traditional selective reflection-enhanced Fabry-Perot (FP) cavity has rich colors, it cannot obtain the transmission-modulated properties due to the use of a metal reflective layer. Herein, we design an asymmetric FP cavity with a pair of high- and low-activation energy chalcogenide phase change materials as the reflective layer (named as “PCM pair” FP cavity), achieving a combination of full color and multi-level transmittance modulation in a thin film. This is attributed to the “PCM pair” having the coupling effect of selective reflection enhancement and gradient absorption that traditional FP cavities do not have. According to simulations and experiments of ITO/“PCM pair” (“PCM pair” = GeTe/Sb2Te3), we prove that the gradient absorption is attributed to the multi-stage change of “PCM pair” from amorphous to crystalline state. This increases carrier absorption and induces additional interface absorption, thus resulting in four-level transmittance modulation. The selective reflectivity enhancement is due to light interference effect between “PCM pair” and ITO with different thicknesses, which realizes full-color modulations. Based on new insights into the activation energy, we propose the design principles of “PCM pair” and present a materials map. Therefore, this study provides the new FP cavity and candidate materials for the problem that the color and transmittance modulation cannot be integrated in a thin film.