Optical Characterization of Structural Evolution of UltrathinVO2 Films across Metal-Insulator Transition

We use ultrafast x-ray diffraction, time resolved pulsed THz spectroscopy and infrared transient absorption techniques to study the structural and electronic evolution of photo-induced ultrathin VO$_{2}$ film epitaxied on Titanium Dioxide (TiO$_{2})$ substrate. We have demonstrated the lattice structural state of VO$_{2}$ film can be tunable by the amount of excitation fluence. The structural evolution in nanosecond scale can be understood by thermal transport model simulation done by comsol. At high excitation fluence (as high as 29mj/cm$^{2})$, a new transient state is observed based on the x-ray diffraction result. The transient structure is unambiguously identified to benon-thermally induced and decoupled from metal-insulator electronic phase transition. It recovers to the tetragonal phase within one nanosecond, and further to the monoclinic phase in tens of nanoseconds. Moreover, the lattice vary properties are highly dependent on the orientation of substrate. The lattice expands along surface normal axis in VO$_{2}$/TiO$_{2}$ (100) films while contracts in VO$_{2}$/TiO$_{2}$ (001) films when going through the phase transition region.