Tunable filters for visible light based on resonant VO2planar thin films

Vanadium dioxide is a phase change material that undergoes an abrupt insulator-to-metal phase transition when its temperature is raised above 68°C. When the temperature is reduced, the structure reversibly switches back to the insulating phase. Since the transition is reversible over many cycles and it occurs by using different stimuli, i.e. thermal, electrical, optical and via strain, VO2 is currently receiving attention for the development of novel, tunable photonic devices. A number of designs have been proposed, including planar thin-film resonators, nanoantennas and metasurfaces. In the solutions proposed so far, tunability is usually achieved by exploiting the large change of absorption coefficient in the infrared and at lower frequencies. Here we propose a Salisbury-screen configuration based on a mirror-backed VO2 thin-film geometry, which works in the visible range and hence it relies on the change of VO2 refractive-index rather than its absorption coefficient. The structure is designed to support a critical-coupling resonant condition with nearly-perfect absorption in a narrow band and it acts as a tunable filter for visible light under the application of a stimulus. Possible applications include nano-pixeled ultrafast solid-state displays, 'smart' glasses, optical limiters and switches.