Ultrafast Charge Carrier Dynamics in Vanadium Dioxide, VO2: Nonequilibrium Contributions to the Photoinduced Phase Transitions

Vanadium oxide (VO2) is an exotic phase-change material with diverse applications ranging from thermochromic smart windows to thermal sensors, neuromorphic computing, and tunable metasurfaces. Nonetheless, the mechanism responsible for its metal-insulator phase transition remains a subject of vigorous debate. Here, we investigate the ultrafast dynamics of the photoinduced phase transition in VO2 under low perturbation conditions. By experimentally examining carrier relaxation dynamics at energy levels near the VO2 band gap (0.6-0.92 eV), we note that numerous optical features do not correspond to the first-order phase transition. Previous studies indeed induced such a phase transition, but they relied on fluences at least an order of magnitude higher, leading to temperature increases well above the transition threshold (340 K). Instead, for excitation fluences that correspond to lattice temperatures only in slight excess of the phase transition (absolute temperatures < 500 K), we find that the marked changes in optical properties are dominated by a shift in the electronic density of states/Fermi level. We find that this effect is a lattice-driven process and does not occur until sufficient energy has been transferred from the excited electrons into the phonon subsystem.