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 p ulses. We have determined the dynamic response of the complex refracti ve index of polycrystalline VO2 thin-films by making pump-probe optica l transmission and reflection measurements at 780 nm. The phase transi tion was found to be largely prompt with the optical properties very c lose to the high-temperature metallic state being attained within abou t 5 ps. The equilibration of the metallic state after femtosecond exci tation 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/root t time dependenc e. This indicated that a diffusion process governed the equilibration of the metallic phase of VO2. (C) 1996 American Institute of Physics.