Wide‐Angle Tunable Critical Coupling in Nanophotonic Optical Coatings with Low‐Loss Phase Change Material

Abstract Realizing perfect light absorption in stacked thin films of dielectrics and metals through critical light coupling has recently received intensive research attention. In addition, realizing ultra‐thin perfect absorber and tunable perfect absorber in the visible spectrum is essential for novel optoelectronics applications. However, the existing thin film stacks cannot show tunable perfect absorption in a wide‐angle range. Here, a tunable perfect absorption from normal incidence to a wide‐angle range (0° to 70°) by utilizing a two‐layer stack consisting of a high refractive index low‐loss dielectric on a high reflecting metal is proposed. This is experimentally demonstrated by depositing a thin layer of low‐loss phase change material such as stibnite (Sb₂S₃) on a thin layer of silver. This structure shows tunable perfect absorption with large spectral tunability in the visible wavelength. Furthermore, the absorption enhancement in 2D materials by transferring monolayer molybdenum disulfide on the stack, which shows 96% light absorption with enhanced photoluminescence, is demonstrated. In addition, the thin film stack can work as a scalable phase modulator offering a maximum phase tunability of ≈140° by changing the structural state of Sb₂S₃ from amorphous to crystalline.