Spins in semiconductors provide a pathway towards the development of spin-based electronics. The appeal of spin logic devices lies in the fact that the spin current is even under time reversal symmetry, yielding non-dissipative coupling to the electric field. To exploit the energy-saving potential of spin current it is essential to be able to control it. While recent demonstrations of electrical-gate control in spin-transistor configurations show great promise, operation at room temperature remains elusive. Further progress requires a deeper understanding of the propagation of spin polarization, particularly in the high mobility semiconductors used for devices.;This thesis presents the demonstration and application of a powerful new optical technique, Doppler spin velocimetry, for probing the motion of spin polarization at the level of 1 nm on a picosecond time scale. We discuss experiments in which this technique is used to measure the motion of spin helices in high mobility n-GaAs quantum wells as a function of temperature, in-plane electric field, and photoinduced spin polarization amplitude. We find that the spin helix velocity changes sign as a function of wave vector and is zero at the wave vector that yields the largest spin lifetime. This observation is quite striking, but can be explained by the random walk model that we have developed. We discover that coherent spin precession within a propagating spin density wave is lost at temperatures near 150 K. This finding is critical to understanding why room temperature operation of devices based on electrical gate control of spin current has so far remained elusive. We report that, at all temperatures, electron spin polarization co-propagates with the high-mobility electron sea, even when this requires an unusual form of separation of spin density from photoinjected electron density. Furthermore, although the spin packet co-propagates with the two-dimensional electron gas, spin diffusion is strongly suppressed by electron-electron interactions, leading to remarkable resistance to diffusive spreading of the drifting pulse of spin polarization. Finally, we show that spin helices continue propagate at the same speed as the Fermi sea even when the electron drift velocity exceeds the Fermi velocity of 107 cm s-1.;We also use this phase-resolved Doppler velocimetry technique to perform the first simultaneous measurements of drift and diffusion of electron-hole packets in the same two-dimensional electron gas. The results that we obtain strongly violate the picture of electron-hole transport that is presented in the classic textbook treatments of ambipolar dynamics. We find that the rates of transport are controlled almost entirely by the intrinsic frictional force exerted between electrons and holes, rather than the interaction of carriers with phonons or impurities. From the experimental data we obtain the first measurement of the "Coulomb drag" friction between electrons and holes coexisting in the same two-dimensional layer. Moreover, we show that the frictional force thus obtained is in quantitative agreement with theoretically predicted values, which follow entirely from electron density, temperature and fundamental constants, i.e. no adjustable parameters. The understanding of ambipolar transport that we have achieved is an essential prerequisite to the design of those spintronic devices in which spin current is carried by the drift of polarized electrons and holes.
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