Vanadium dioxide thermochromic opals grown by chemical vapour deposition

The semiconductor-metal transition that occurs in vanadium dioxide at 68 degrees 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 mu m 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 degrees C. When the phase transition is achieved at 68 degrees C the intensity of the reflectance peak decreases drastically and the Fabry-Perot oscillations disappear.