Vanadium dioxide thermochromic opals grown by chemical vapour deposition

The semiconductor–metal transition that occurs in vanadium dioxide at 68 °C is the object of increasing interest in modern optics due to its applications in ultrafast optical devices. In a three-dimensional photonic crystal, the fine control of the structure is important for optimizing the switching magnitude and the spatial homogeneity. We report a controlled process to fabricate large-area high quality VO2–SiO2 opals with fine control over the filling volume. The method comprises two stages. The first stage is a chemical vapour deposition synthesis whereby the vanadium pentoxide is grown. The second one is a thermal annealing that allows reducing the vanadium pentoxide to vanadium dioxide. We have carried out a steady-state study of the semiconductor–metal transition, for opals with the Bragg peak around the 1.55 µm spectral region, by means of reflectance spectroscopy. As the temperature increases approaching the phase transition the intensity of the reflectance peak decreases and a small blueshift can be observed at 65 °C. When the phase transition is achieved at 68 °C the intensity of the reflectance peak decreases drastically and the Fabry–Perot oscillations disappear.