Optical control of structural evolution of ultrathin VO$_2$ film following a photoinduced metal-insulator phase transition

Using ultrafast x-ray diffraction technique, we have demonstrated that the structural symmetry in VO$_{2}$ thin film across the metal-insulator transition (MIT) can be controlled by the optical pump fluence. After an 800 nm, 50 fs laser excitation, the structural evolution of a 13nm-thick, MBE-grown VO$_{2}$ film on TiO$_{2}$ substrate was monitored by a synchrotron based x-ray diffraction with 100 picosecond (ps) time resolution. By adjusting the pump fluence, the VO$_{2}$ film can be prepared in selected structural state 100 ps after excitation, and transits through the known structural phases (tetragonal, monoclinic M2, monoclinic M1) in tens of nanoseconds time scale. When pumping at high fluence (~28 mJ/cm$^{2}$), a new transient state that exhibits lower symmetry than the thermal equilibrium tetragonal phase has been identified and its evolution pathway has been measured.