Imaging electrons in motion across semiconductor heterojunctions!

Imaging the dynamics of photoexcited electrons in complex, heterostructure materials with simultaneous high spatial and temporal resolution would provide great insight intothe science and technology of photoactive condensed matter systems anddevices. Today, we will watch movies of electrons as they traverse space, time and energy in a GaAs/InSe semiconductor heterostructure [Nat. Nano. In press].Using a two-color pump-probe technique, we photoexcite carriers in the conductions bands of GaAs and InSe in our heterostructure sample with a near-infrared pump pulse. Then, wephotoemit carriers with a time-delayed near-ultraviolet probe pulse. Thephotoemitted electrons are imaged in a photoemission electron microscope with and without energy resolution.At the instant of photoexcitation, our spectrally resolved measurements reveal the highly non-equilibrium distribution of carriers in space and energy. At later time delays, we image the spatialredistribution of electrons from high energy to low energy states, thus making a movie of the fundamental operating principle of photoactive semiconductor devices.The observed spatio-temporal electron dynamics provide insight into the operation of solar cells, the physics of 2D van der Waals flakes and optoelectronic devices in general.I will end by briefly describing the unique institutional context in which these experiments were done at the Okinawa Institute of Science and Technology (OIST)!