Imaging a Photo-Doped Mott Insulator with Ultrafast Electron Microscopy

Scanning ultrafast electron microscopy (SUEM) is an emerging optical-pump-electron-probe technique that can image spatial-temporal dynamics of photo-induced surface charges with high resolutions and surface sensitivity. In this talk, I will describe our preliminary study of photo-doped Mott insulator ��-RuCl3 using SUEM, where we observed qualitatively different dynamics of photo-induced charge with varying optical excitation fluence. At low fluence, we observe the diffusion of a Gaussian population with a long hot carrier cooling time beyond 1 nanosecond, which is almost one order of magnitude longer than that in conventional semiconductors. At higher fluence above a threshold, we observe a highly nonlinear response in the SUEM images showing a ring-shaped feature and an overall suppressed contrast intensity. By comparing to time-dependent DFT simulations, we attribute the observation to a photo-induced insulator-to-metal transition in ��-RuCl3. 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. This work is supported by US Air Force Office of Scientific Research under the award number FA9550-22-1-0468 and US Army Research Office under the award number W911NF2310188.