Abstract:

The talk presents the results of recent experimental and theoretical studies of electron spin dynamics.
1. Introduction. Optical Orientation of Free Carriers and Excitons. Generation of carrier spin coherence by circularly polarized laser pulses: (a) empty QW, only photogenerated carriers which form excitons; (b) low density two-dimensional electron gas (2DEG), trions with a singlet ground state formed by a photogenerated exciton and a resident electron; (c) dense 2DEG with a Fermi energy exceeding the exciton binding energy.
2. Spin Faraday and Kerr Rotation Technique. The pump-probe technique serves as a very convenient tool to study the coherent spin dynamics. A short circularly polarized pump pulse generates the electron spin orientation, which is monitored by the weaker linearly polarized probe pulse delayed in respect to the pump one. In external magnetic elds a coherent spin precession of the electrons can be detected opening an access to the electron spin dephasing times.
3. Decoherence and Dispersion of Electron g Factors in Quantum-Dot Arrays. The formalism presented provides a complete theoretical description of single- and twocolor pump-probe Faraday or Kerr rotation experiments in an ensemble of singly charged quantum dots (QDs). The modeling of time-dependent traces of the Faraday rotation signal shows their high sensitivity to the inhomogeneous properties of the QD ensemble, such as the transition-frequency dependence of electron g-factor and the nuclear-induced dispersion, as well as to the excitation conditions, such as pump and probe pulse detuning, single pulse versus train of pulses excitation, and the pumping intensity.
4. Decoherence of Neutral Zero-Dimensional Excitons. Entangled Photons. A quite general property of actual QDs is the existence of a splitting of the exciton radiative doublet into linearly polarized components due to an anisotropic contribution in the electronhole exchange interaction. In some circumstances the Zeeman effect and the orbital effect of an in-plane magnetic field can lead to a cancellation of the native anisotropic splitting. This allows one to produce polarization-entangled photon pairs in the quantum-dot biexcitonexciton radiative cascade.
5. Spin Relaxation Controlled by Electron-Electron Interaction. In high-mobility quantum wells, the electron-electron interaction controls the Dyakonov-Perel spin relaxation. In a weakly spin polarized electron gas, the spin relaxation rate is limited by electronelectron collisions. In a highly polarized gas, the spin is stabilized by the exchange interaction: the Hartree-Fock exchange eld acts as an external magnetic eld and suppresses the Dyakonov-Perel' spin relaxation.
 
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