Femtosecond laser excitation dynamics of the semiconductor-metal phase transition in VO2

We have measured the subpicosecond optical response of a solid-state, semiconductor-to-metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of polycrystalline VO2 thin-films by making pump-probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties very close to the high-temperature metallic state being attained within about 5 ps. The equilibration of the metallic state after femtosecond excitation was modeled by non-exponentially decaying perturbations in the metallic state electron density and collision frequency. The decay of both these plasma parameters was well fit by a 1/√t time dependence. This indicated that a diffusion process governed the equilibration of the metallic phase of VO2.