Orientation Dependent Phase Transition in VO_2 Thin Film by Ultrafast Laser Excitation

摘要

Using femtosecond laser excitation the light-induced ultrafast phase transition (PT) was observed in VO_2 thin films deposited on different optically transparent substrates. The PT was characterized by transient absorption, reflection and nonlinear optical (NLO) response measurements. Degenerate-four-wave-mixing (DFWM) technique was also applied for time-resolved measurement of light-induced PT. While strong NLO response can always be observed in films on fused quartz substrates the response in films on C-cut sapphire substrates was diminished. However, it is much enhanced in films on R-cut sapphire. Further orientation dependent measurements show that the light-induced PT from insulator to metallic state is connected to the following lattice relaxation. In the interface layers a lattice distortion along c axis of the rutile structure or off-axis displacement may be engaged in the resultant PT. While the substrate lattice symmetry is in favour of this type of distortion the light-induced PT becomes enhanced.