Optical excitation and control of electron spins in semiconductor quantum wells

We present our recent work on optical excitation of electron spins and the resulting dynamics in semiconductor quantum wells. We primarily use the transient Faraday/Kerr rotation technique in several different experiments to give a more complete picture of spin polarization, manipulation, transport, decoherence, and the influence of disorder. Using two color time-resolved techniques is particularly valuable for resolving spin dynamics of different species by spectral selection. Experiments in lightly n-doped quantum wells show that spin polarization of the electron gas is generated through trion formation, with spin coherence partially lost through exciton spin relaxation. We also demonstrate that existing electron spin polarization can be rotated without excitation of excitons or trions through a below-resonance adiabatic Raman process. Diffusion of electron spins is also measured using transient spin gratings for different optical excitation conditions. Spin diffusion is shown to accelerate with increasing density or energy of the optically excited carriers. Finally, we examine the spin coherence of the electron gas with different densities, in CdTe quantum wells with different doping densities, and through a mixed type GaAs quantum well, where the electron density can be varied through optical excitation. We characterize the disorder potential by measuring the electron g-factor dependence on density, and show that spin coherence is lost from the interplay between localization by disorder and dynamical scattering.