We demonstrate ultrafast shifting of a photonic stop band driven by a photoinduced phase transition in vanadium dioxide (VO2) forming a three-dimensional photonic crystal. An ultrashort 120-fs laser pulse induces a phase transition in VO2 filling the pores of an artificial silica opal, thus changing the effective dielectric constant of the opal. Consequently, the spectral position of the photonic stop band blue-shifts producing large changes in the reflectivity. The observed switching of the photonic crystal is faster that 350 fs. The demonstrated properties of opal-VO2 composite are relevant for potential applications in all-optical switches, optical memories, low-threshold lasers, and optical computers.
Three-dimensional opal-VO2 photonic crystals were synthesized by the chemical bath deposition technique. The Bragg reflection spectra from the (111) planes of the crystals were measured as a function of the temperature in the range between 15 and 100°C. The thermal hysteresis loop of the reflection peak position due to the phase transition in VO2 filling the opal voids was observed. A theoretical model of the periodic layered medium was proposed to describe quantitatively the reflection spectra of opal-like structures. The values of the dielectric constants of the VO2 below and above the phase transition temperature have been estimated which give the best fit within the model considered.
Three-dimensional (3D) photonic crystals entirely consisting of GaN have been fabricated for the first time. Detailed investigations of optical Bragg diffraction spectra have shown a high quality of the prepared photonic crystals.