Date of Degree
PhD (Doctor of Philosophy)
Arthur L. Smirl
Charge transport and spin transport (spintronics) over nanometer spatial scales are topics of fundamental scientific and technological interest. If the potential of nano-devices and spintronics is to be realized, ways must be developed to inject and control ballistic charge and spin currents, as well as to measure their motion. Here, using novel polarization and phase sensitive optical pump probe techniques, we not only inject ballistic charge and spin currents in GaAs, Ge, and Si but also follow the subsequent carrier motion with < 1 nm spatial and 200 fs temporal resolution. Unlike most free space measurements, the spatial resolution of these techniques is not limited by diffraction, and therefore these techniques provide a unique platform for studying ballistic transport in semiconductors and semiconductor structures.
The injection process relies on quantum interference between absorption pathways associated with two-photon absorption of a fundamental optical field and one-photon absorption of the corresponding second harmonic. By utilizing the phase, polarization, photon energy, and intensity of the optical fields we can control the type of current injection (spin current or charge current) and the direction and magnitude. In GaAs we present the first time resolved measurements of charge and spin currents injected by this process and also show the ballistic direct and inverse Spin Hall Effect. These techniques are extended to the more technologically relevant group IV semiconductors Si and Ge. The charge currents injected in these materials show similar qualitative behavior. The electrons and holes are injected with oppositely directed average ballistic velocities that move apart and return to a common position on sub-picosecond time scales. The spin currents however, are very different. The spin up and spin down carrier profiles move apart and remain apart until their spin profiles decay. In GaAs the profile decay on picosecond time scales however, in Ge they decay on femtosecond time scales since the electrons quickly scatter to the side valleys. Unlike GaAs and Ge, the spin orbit coupling in Si is much too small to produce measurable spin currents.
Copyright 2011 Eric Justin Loren