Actively tunable thin films for visible light by thermo‐optic modulation of ZnO

Abstract

Applications of active control of light matter interactions within integrated photonics, hyper‐spectral imaging, reconfigurable lasers, and selective bio‐surfaces have enormously increased the demand for realization of optical modulation covering the spectrum from ultraviolet (UV) up to infrared (IR) wavelength range. In this study, we demonstrate ZnO‐based actively tunable perfect absorber operating within UV and visible spectrum with more than 5 nm shift in the resonant absorption wavelength. Using spectroscopic ellipsometry technique, we extract temperature‐dependent optical constants of atomic layer‐deposited ZnO within 0.3–1.6 and 4–40 μm spectra. We also observe bandgap narrowing of ZnO at elevated temperatures due to lattice relaxation verified by the red‐shift of phonon‐modes. At around its bandgap, refractive index variations up to 0.2 is obtained and ZnO is shown to exhibit thermo‐optic coefficient as high as 9.17 × 10−4 K−1 around the bandgap which is the largest among well‐known large bandgap materials.