Scanning ultrafast electron microscopy: A novel technique to probe photocarrier dynamics with high spatial and temporal resolutions

The dynamics of photo-excited charge carriers, particularly their transport and interactions with defects and interfaces, play an essential role in determining the performance of a wide range of solar and optoelectronic devices. A thorough understanding of these processes requires tracking the motion of photocarriers in both space and time simultaneously with extremely high resolutions, which poses a significant challenge for previously developed techniques, mostly based on ultrafast optical spectroscopy. Scanning ultrafast electron microscopy (SUEM) is a recently developed photon-pump-electron-probe technique that combines the spatial resolution of scanning electron microscopes (SEM) and the temporal resolution of femtosecond ultrafast lasers. Despite many recent excellent reviews for the ultrafast electron microscopy, we dedicate this article specifically to SUEM, where we review the working principle and contrast mechanisms of SUEM in the secondary-electron-detection mode from a users' perspective and discuss the applications of SUEM to directly image photocarrier dynamics in various materials. Furthermore, we propose future theoretical and experimental directions for better understanding and fully utilizing the SUEM measurements to obtain detailed information about the dynamics of photocarriers. To conclude, we envision the potential of expanding SUEM into a versatile platform for probing photophysical processes beyond photocarrier dynamics.