Photoinduced insulator-to-metal phase transition in VO_2 crystalline films and model of dielectric susceptibility

摘要

A light-induced insulator-to-metal ultrafast phase transition (PT) was observed in VO_2 thin films deposited on single-crystal sapphire and amorphous glass substrates. The PT and optical properties of VO_2 were characterized by transient reflection, absorption, and transient grating nonlinear optical measurements. VO_2 films deposited on crystalline substrates are spatially ordered and show noticeable anisotropy in optical response upon PT. Structural and optical properties are dependent on the shear plane of the sapphire substrate. Significant twinning was found for VO_2 deposited on a (001)Al_2O_3 substrate while a VO_2 film on (012)Al_2O_3 is highly textured, without detectable twinning. VO_2 dielectric susceptibility is described by a proposed model. The V-O-V charge transfer was considered as a major process contributing to the linear and nonlinear dielectric susceptibility. The PT time and relaxation dynamics were found to be dependent on the film morphology and concentration of structural defects.