Physics

Dr. Keshav M Dani
Okinawa institute of science and technology, Japan

Imaging the dynamics of photoexcited electrons in complex, heterostructure materials with simultaneous high spatial and temporal resolution would provide great insight into the science and technology of photoactive condensed matter systems and devices. 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 femtosecond, near-infrared pump pulse. Then, we photoemit carriers with a time-delayed near-ultraviolet probe pulse. The photoemitted 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 spatial redistribution of electrons from high energy to low energy states, thus making a movie – lasting just a few trillionths of a second, of the fundamental operating principle of photoactive semiconductor devices, like solar cells. 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), and offering a peak into its future!