Frequency tunable perfect absorber in visible and near-infrared regimes based on VO_2 phase transition using planar layered thin films

We show a frequency tunable perfect absorber in visible and near-infrared regimes based on the vanadium dioxide (${\mathrm{VO}}_2$) phase transition with a wide tuning capability typical of asymmetric Fabry–Perot (FP) cavities, comprising only an unpatterned, ultrathin ($\sim \lambda /15$) film of ${\mathrm{VO}}_2$ deposited on a metal-reflecting substrate. ${\mathrm{VO}}_2$ undergoes a structural transition from an insulating phase ($\mathrm{i}-{\mathrm{VO}}_2$) to a metallic phase ($\mathrm{m}-{\mathrm{VO}}_2$) at 68°C. A perfect absorption wavelength can be dynamically tuned over a wide wavelength range over 175 nm (from 905 to 730 nm in the measured spectrum) in visible and near-infrared regimes by controlling the temperature. More importantly, its absorption peaks become even stronger during the process of increasing temperature (from 96.8% at 905 nm to 98.4% at 730 nm). The frequency tunable absorption peaks were explained because both indices of $\mathrm{i}-{\mathrm{VO}}_2$ and $\mathrm{m}-{\mathrm{VO}}_2$ are very close to the maximum absorption point in $n-k$ parameter space.