Pulling apart photoexcited electrons by photoinducing an in-plane surface electric field

The study and control of spatiotemporal dynamics of photocarriers at the interfaces of materials have led to transformative modern technologies, such as light-harvesting devices and photodetectors. At the heart of these technologies is the ability to separate oppositely charged electrons and holes. Going further, the ability to separate like charges and manipulate their distribution could provide a powerful new paradigm in opto-electronic control, more so when done on ultrafast time scales. However, this requires one to selectively address subpopulations of the photoexcited electrons within the distribution-a challenging task, particularly on ultrafast time scales. By exploiting the spatial intensity variations in an ultrafast light pulse, we generate local surface fields within the optical spot of a doped semiconductor and thereby pull apart the electrons into two separate distributions. Using time-resolved photoemission microscopy, we directly record a movie of this redistribution process lasting a few hundred picoseconds, which we control via the spatial profile and intensity of the photoexciting pulse. Our quantitative model explains the underlying charge transport phenomena, thus providing a roadmap to the more generalized ability to manipulate photocarrier distributions with high spatiotemporal resolution.