Electron and hole dynamics in the electronic and structural phase transitions of VO$_{2}$*

The ultrafast, optically induced insulator-to-metal transition (IMT) and the associated structural phase transition (SPT) in vanadium dioxide (VO$_{2})$ have been studied for over a decade. However, only recently have effects due to the combined presence of electron-hole pairs and injected electrons been observed. Here we compare and contrast IMT dynamics when both hot electrons and optically excited electron-hole pairs are involved, in (1) thin films of VO$_{2}$ overlaid by a thin gold foil, in which hot electrons are generated by 1.5~eV photons absorbed in the foil and accelerated through the VO$_{2}$ by an applied electric field; (2) VO$_{2}$ nanoparticles covered with a sparse mesh of gold nanoparticles averaging 20-30 nm in diameter in which hot electrons are generated by resonant excitation and decay of the localized surface plasmon; and (3) bare VO$_{2}$ thin films excited by intense near-single-cycle THz pulses. In the first case, the IMT is driven by excitation of the bulk gold plasmon, and the SPT appears on a few-picosecond time scale. In the second case, density-functional calculations indicate that above a critical carrier density, the addition of a single electron to a 27-unit supercell drives the catastrophic collapse of the coherent phonon associated with, and leading to, the SPT. In the third case, sub-bandgap-energy photons (approximately 0.1~eV) initiate the IMT, but exhibit the same sub-100 femtosecond switching time and coherent phonon dynamics as observed when the IMT is initiated by 1.5~eV photons. This suggests that the underlying mechanism must be quite different, possibly THz-field induced interband tunneling of spatially separated electron-hole pairs. The implications of these findings for ultrafast switching in opto-electronic devices -- such as hybrid VO$_{2}$ silicon ring resonators -- are briefly considered.