Excited state dynamics and semiconductor-to-metallic phase transition of VO2 thin film

VO2 thin films deposited on fused-quartz substrates were prepared by a pulsed laser deposition technique. Vanadium dioxide shows an ultrafast, passive phase transition (PT) from a monoclinic semiconductor phase to a metallic tetragonal rutile structure when the sample temperature is above 68°C. The fast PT can also be optically induced by laser excitation. In this paper, we report the optical properties of prepared thin films by absorption and photoluminescence (PL) measurements. In addition, a degenerate-four-wave-mixing measurement was also conducted using a 30 ps YAG pulsed laser operated at 532 nm. It is the first time, to our knowledge, that a large polarizability and relatively slow nanosecond excited state dynamical processes have been identified in VO2. The polarizability is found to be ∼300 times greater than the χ(3) value of a CS2 reference. The mechanism for slow process is believed to be associated with the structural change.