Temperature-Dependent Femtosecond Pump-Probe Spectroscopy of Thin-Film Vanadium Dioxide

Vanadium dioxide (VO2) is characterized as a transitional-metal oxide that undergoes an abrupt insulator-to-metal phase transition upon a threshold-crossing external stimulus such as heating above a critical temperature of ~344 K, applying a sufficient electric field, or optical excitation above a threshold fluence. In this thesis, temperature-dependent, degenerate femtosecond pump-probe spectroscopy measurements of ~300-nm-thick VO2 grown on MgO via pulsed laser deposition are described and reveal a significant qualitative difference between the optical absorption and relaxation dynamics of the material’s insulating and metallic states. Upon perturbation by a ~100fs-wide, 800-nm-wavelength pumping laser pulse, insulating VO2 displays an initial, system-limited 300 fs-long decrease in reflectivity followed by a bi-exponential relaxation to its equilibrium state with time constants = 0.50 ps and = 2.63 ps attributed to electron thermalization and optical phonon scattering, respectively. In VO2’s metallic state, the perturbing pump pulse induces an initial ~320 fs FWHM Gaussian-shaped decrease in reflectivity tentatively attributed to a decrease in probe beam reflection due to enhanced two-photon absorption at the pump and probe pulse correlation time. The remainder of the metallic state reflection change transient is characterized by a ~10 ps rise-time followed by an acoustic-phonon-scattering-attributed exponential relaxation with a time constant of = 402.5 ps. Degenerate pump-probe spectroscopy measurements of current-carrying VO2 are also presented and show that the measured transients at transition-threshold-crossing currents display qualitatively similar