Abstract
Using femtosecond laser excitation the light-induced ultrafast phase transition (PT) was observed in VO2 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.
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