Design and energy analysis of tunable nanophotonic infrared filter based on thermochromic vanadium dioxide

In this work, a mid-infrared tunable nanophotonic filter is designed by placing a silicon spacer layer between two nanometric vanadium dioxide films on a thick infrared-transparent substrate. Transfer matrix formulation incorporated with the ray-tracing method is used for the optical modeling, and a transmittance peak near 4 µm wavelength is achieved based on Fabry–Perot resonance effect. Thermochromic vanadium dioxide allows the filter to be switched between a semi-transparent state when cooled below its phase transition temperature and an opaque state when heated above its phase transition temperature. The underlying mechanism is elucidated by calculating the total phase shift of electromagnetic waves through the spacer layer, and the effect of layer thicknesses on the spectral transmittance peak is discussed. The filter can find potential applications in energy dissipation where it can actively promote or suppress radiative cooling, and in thermal camouflage where the apparent temperature of a hot surface through the filter can be greatly reduced. Detailed energy analysis is conducted for both applications with the proposed tunable filter, whose structure is further optimized through the thicknesses of the thin-film layers to achieve maximum performance.