  The structural and electronic recovery pathways of a photoexcited ultrathin
VO2 film at nanosecond time scales have been studied using time-resolved x-ray
diffraction and transient optical absorption techniques. The recovery pathways
from the tetragonal metallic phase to the monoclinic insulating phase are
highly dependent on the optical pump fluence. At pump fluences higher than the
saturation fluence of 14.7 mJ/cm2, we observed a transient structural state
with lattice parameter larger than that of the tetragonal phase, which is
decoupled from the metal-to-insulator phase transition. Subsequently, the
photoexcited VO2 film recovered to the ground state at characteristic times
dependent upon the pump fluence, as a result of heat transport from the film to
the substrate. We present a procedure to measure the time-resolved film
temperature by correlating photoexcited and temperature-dependent x-ray
diffraction measurements. A thermal transport model that incorporates changes
of the thermal parameters across the phase transition reproduces the observed
recovery dynamics. The optical excitation and fast recovery of ultrathin VO2
films provides a practical method to reversibly switch between the monoclinic
insulating and tetragonal metallic state at nanosecond time scales.
