Ultrafast electron imaging of surface charge carrier dynamics at low voltage

The performance of optoelectronic devices strongly depends on charge carrier dynamics on top of surfaces of the absorber layers. Unfortunately, this information cannot be selectively probed using conventional ultrafast laser spectroscopic methods, due to the large penetration depth (tens of nm to μm) of the photon pulses in the pump-probe configurations. Therefore, ultrafast time-resolved approaches that can directly and selectively visualize the behavior of the surface carrier dynamics are urgently needed. Here, we introduce a novel methodology of low-voltage scanning ultrafast electron microscopy that can take ultrafast time-resolved images (snapshots) of the surface of materials at the sub-nanometer level. By this approach, the surface of the photoactive materials is optically excited and imaged, using a pulsed low-voltage electron beam (1 keV) that interacts with the surface to generate secondary electrons with an energy of a few eV, and that are emitted only from the top surface of materials, providing direct information about the carrier dynamics and the localization of electron/holes in real space and time. An outlook on the potential applications of this low voltage approach in different disciplines will also be discussed.