Picosecond real-space imaging of hot electron spin diffusion in GaAs

T. Henn, L.W. Molenkamp, and W. Ossau

IBM Research, Zurich Research Laboratory, Switzerland
Department of Physics, University of Würzburg, Germany

Pump-probe magneto-optical Kerr effect (MOKE) microscopy is widely used for the investigation of electron spin transport in semiconductors, with the ultimate goal to approach the understanding of solid state spin phenomena needed to realize spintronic devices. However, while the significance of pump-induced electron heating for the diffusion of photoexcited carriers has been understood for a long time in the related context of exciton transport in quantum wells, such hot carrier effects have been widely neglected in the interpretation of data obtained by MOKE microscopy.
We here present the first direct measurements of the strong influence of photocarrier heating on electron spin diffusion. We use time-resolved two-color MOKE microscopy to study low-temperature electron spin diffusion in bulk GaAs with picosecond time and micrometer spatial resolution. This method enables real-space imaging of the advancing spread of optically excited electron spin packets by spin diffusion. We observe a high initial expansion rate of the spin packets which is strongly reduced with increasing time. By comparison with continuous-wave MOKE microscopy we demonstrate that this decrease is caused by the influence of the transient cooling of hot photocarriers on the electron spin diffusion coefficient.