Dynamics of the blue pump-induced ultrafast insulator-to-metal transition and relaxation in VO2/TiO2 and VO2/TiO2:Nb thin films

We study the ultrafast time resolved response of 30 nm films of VO2 on a TiO2 substrate when 3.1 eV (400 nm wavelength) pump pulses were used to excite the insulator to metal transition (IMT). We found that the IMT threshold for these samples (<= 30 mu J/cm(2)) is more than 3 orders of magnitude lower than that generally reported for a more traditional 1.55 eV (800 nm wavelength) excitation. The samples also exhibited unusual reflectivity dynamics at near-threshold values of pump fluence where their fractional relative reflectivity Delta R/R initially increased before becoming negative after several hundreds of picoseconds, in stark contrast with uniformly negative Delta R/R observed for both higher 400 nm pump fluences and for 800 nm pump pulses. We explain the observed behavior by the interference of the reflected probe beam from the inhomogeneous layers formed inside the film by different phases of VO2 and use a simple diffusion model of the VO2 phase transition to support qualitatively this hypothesis. We also compare the characteristics of the VO2 films grown on undoped TiO2 and on doped TiO2:Nb substrates and observe more pronounced reflectivity variation during IMT and faster relaxation to the insulating state for the VO2/TiO2:Nb sample. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.