  Photo-doped Mott insulators can exhibit novel photocarrier transport and
relaxation dynamics and non-equilibrium phases. However, time-resolved
real-space imaging of these processes are still lacking. Here, we use scanning
ultrafast electron microscopy (SUEM) to directly visualize the spatial-temporal
evolution of photoexcited species in a spin-orbit assisted Mott insulator
{\alpha}-RuCl3. At low optical fluences, we observe extremely long hot
photocarrier transport time over one nanosecond, almost an order of magnitude
longer than any known values in conventional semiconductors. At higher optical
fluences, we observe nonlinear features suggesting a photo-induced
insulator-to-metal transition, which is unusual in a large-gap Mott insulator.
Our results demonstrate the rich physics in a photo-doped Mott insulator that
can be extracted from spatial-temporal imaging and showcase the capability of
SUEM to sensitively probe photoexcitations in strongly correlated electron
systems.
