Coherent propagation of spin helices in a quantum-well confined electron gas.

We use phase-resolved transient grating spectroscopy to measure the propagation of spin helices in a high mobility n-GaAs/AlGaAs quantum well with an applied in-plane electric field. At relatively low fields helical modes crossover from overdamped excitations where the spin-precession period exceeds the spin lifetime, to a regime of coherent propagation where several spin-precession periods can be observed. We demonstrate that the envelope of a spin polarization packet reaches a current-driven velocity of 10(7) cm s(-1) in an applied field of 70 V cm(-1).

Measurement of electron-hole friction in an n-doped GaAs/AlGaAs quantum well using optical transient grating spectroscopy.

We use phase-resolved transient grating spectroscopy to measure the drift and diffusion of electron-hole density waves in a semiconductor quantum well. The unique aspects of this optical probe allow us to determine the frictional force between a two-dimensional Fermi liquid of electrons and a dilute gas of holes. Knowledge of electron-hole friction enables prediction of ambipolar dynamics in high-mobility electron systems.