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.

In this study, an ordered SiO₂ nanosphere array coated with vanadium dioxide (VO₂) has been fabricated to enhance transmittance with the potential application as an energy-efficient coating in the field of smart windows. SiO₂ arrays were formed using the methods of self-assembly, and VO₂ thin films were prepared by rapid thermal annealing (RTA) of sputtered vanadium films. VO₂@SiO₂ arrays were characterized by scanning electron microscopy, X-ray diffraction, a four-point probe, and UV-vis-NIR spectrophotometry. Compared with the planar films, the films deposited on 300 nm diameter SiO₂ nanospheres can offer approximately 18% enhancement of luminous transmission (T_lum) because the diameter is smaller than the given wavelength and the protuberance of the surface array behaves as a gradation of refractive index producing antireflection. The solar regulation efficiency was not much deteriorated.