Search for: All records

We study the ultrafast time resolved response of 30 nm films of VO₂on a TiO₂substrate 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 (≤30µJ/cm²) 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 ΔR/R initially increased before becoming negative after several hundreds of picoseconds, in stark contrast with uniformly negative ΔR/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 VO₂and use a simple diffusion model of the VO₂phase transition to support qualitatively this hypothesis. We also compare the characteristics of the VO₂films grown on undoped TiO₂and 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.