High-purity reflective color filters based on thin film cavities embedded with an ultrathin Ge2Sb2Te5 absorption layer

Abstract

A thin film cavity formed by stacking metal–insulator–metal (MIM) continuous layers is of significant interest as a lithography-free and scalable color-filtering structure. Such a cavity can selectively transmit a certain frequency range of incident light, thus producing vivid transmission colors. However, the generation of reflection colors with high purity and reflectivity is a challenge because a cavity in reflection mode resonantly absorbs a narrow range of wavelengths and reflects the remaining spectrum. This study shows that highly pure and reflective colors can be obtained by embedding an ultrathin Ge₂Sb₂Te₅ layer within the cavity. Because the MIM structure exhibits a nonuniform intensity distribution across the insulator layer, the approach is to place the Ge₂Sb₂Te₅ layer in a high-intensity region within the insulator and thereby create another absorption band in addition to the cavity resonance mode. When combined with the refractive-index engineering of the metal layer, this approach leads to red, green, and blue colors having a bandwidth of ∼100 nm and a reflection efficiency of 90%. The results of the study may be effectively utilized in numerous applications, including reflective color filters, colorimetric sensors, and surface decorations.