  Dynamical phase separation during a solid-solid phase transition poses a
challenge for understanding the fundamental processes in correlated materials.
Critical information underlying a phase transition, such as localized phase
competition, is difficult to reveal by measurements that are spatially averaged
over many phase separated regions. The ability to simultaneously track the
spatial and temporal evolution of such systems is essential to understanding
mesoscopic processes during a phase transition. Using state-of-the-art
time-resolved hard x-ray diffraction microscopy, we directly visualize the
structural phase progression in a VO2 film upon photoexcitation. Following a
homogenous in-plane optical excitation, the phase transformation is initiated
at discrete sites and completed by the growth of one lattice structure into the
other, instead of a simultaneous isotropic lattice symmetry change. The
time-dependent x-ray diffraction spatial maps show that the in-plane phase
progression in laser-superheated VO2 is via a displacive lattice transformation
as a result of relaxation from an excited monoclinic phase into a rutile phase.
The speed of the phase front progression is quantitatively measured, and is
faster than the process driven by in-plane thermal diffusion but slower than
the sound speed in VO2. The direct visualization of localized structural
changes in the time domain opens a new avenue to study mesoscopic processes in
driven systems.
