Temperature-Dependent Femtosecond Pump-Probe Spectroscopy of Thin-Film Vanadium Dioxide
Abstract
Vanadium dioxide (VO2) is characterized as a transitional-metal oxide that undergoes an abrupt insulator-to-metalphase 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 materials insulating and metallic states. Upon perturbation by a ~100-fs-wide, 800-nm-wavelength pumping laser pulse, insulating VO2 displays an initial, system-limited 300 fs-long decrease inreflectivity followed by a bi-exponential relaxation to its equilibrium state with time constants tau1 = 0.50 ps and tau2= 2.63 ps attributed to electron thermalization and optical phonon scattering, respectively. In VO2s metallic state,the perturbing pump pulse induces an initial ~320 fs FWHM Gaussian-shaped decrease in reflectivity tentativelyattributed 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 tau3 = 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 responses to thermally induced metallic state VO2 transients, suggesting that the two stimuli perturb the system into a common state.
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 24, 2014
- Accession Number
- AD1051690
Entities
People
- Graham H. Jensen
Organizations
- University of Rochester