An indirect exciton is a bound pair of an electron and a hole confined in spatially separated semiconductor layers. Long lifetimes of indirect excitons allow them to cool down to low temperatures below the temperature of quantum degeneracy. In coherent exciton gases the spin relaxation mechanisms linked with the scattering of particles are strongly suppressed. Recent experiments evidenced spontaneous coherence and condensation, phase singularities, spatial ordering, and polarization textures in a cold excitons gas . We develop a theoretical model accounting for the spin-orbit interaction of electrons and holes, exchange splitting of exciton states and Zeeman effect. This model describes the polarization patterns observed experimentally, including helical patterns, four-leaf patterns, spiral patterns, and bell patterns, and confirms that the polarization textures at low temperature manifest ballistic propagation of cold excitons over the distances exceeding the exciton Bohr radius by 3 orders of magnitude and the exciton thermal De Broglie wavelength by more than an order of magnitude. The comparison of theory and experiment allows extracting the spin currents carried by electrons and holes bound to excitons. The spins tend to align along the effective spin-orbit field induced due to the Dresselhaus effect.

In various manganese-based II-VI dilutedmagnetic semiconductors and their quantum structures, the measured variations of electron spin dephasing time in an external magnetic field are conflicting with the most advanced spin relaxation theory based on quantum kinetic equations. We demonstrate, by time-resolved optical spin beat measurements performed on high-mobility n-doped CdMnTe quantum wells, that these contradictions are resolved if one takes into account the electron spin dephasing induced by laser heating of the manganese spins.We then provide a test of the spin relaxation theory inMn-based quantum wells by a careful comparison with existing data, including our measurements. It turns out that the theory systematically underestimates the relaxation rates by a factor of 5.

We report on the nondestructive measurement of nuclear magnetization in n-GaAs via cavity enhanced Faraday rotation. In contrast with the existing optical methods, this detection scheme does not require the presence of detrimental out-of-equilibrium electrons. Specific mechanisms of the Faraday rotation are identified for (i) nuclear spins situated within the localized electron orbits, these spins are characterized by fast dynamics, (ii) all other nuclear spins in the sample characterized by much slower dynamics. Our results suggest that even in degenerate semiconductors nuclear spin relaxation is limited by the presence of localized electron states and spin diffusion, rather than by Korringa mechanism.

Nuclear spin dynamics in semiconductors has been studied extensively by optical means for more than forty years. Yet generally the existing techniques require the injection of out of equilibrium carriers in the semiconductor in order to probe the nuclear polarization, which in turn disturbs the nuclei to be measured. This is the case for photoluminescence experiments, but also for time resolved Faraday/Kerr rotation, in which a shift in the electron Larmor frequency is induced by the nuclear field.1 In this work, we demonstrate a non-destructive detection of nuclear magnetization in n-GaAs via cavity enhanced Faraday rotation of an off-resonant light beam. We use a two-step protocol, as proposed by Artemova and Merkulov2, where preparation and measurement are separated in time. The nuclear spins are first cooled down by optical pumping, then the pump beam is switched off and the subsequent evolution of nuclear polarization is detected by the Faraday rotation of a below band gap probe beam. This experimental protocol applied to metallic GaAs just above the metal-insulator transition reveals a non- monotonous evolution of the nuclear induced Faraday rotation. We interpret this behavior as a consequence of the existence of two contributions in the Faraday rotation, which are related to optical transitions involving either localized or delocalized states. Therefore, this detection scheme turns out to be sensitive to nuclear spins situated within the localization radius of bound electron states, as well as to all other nuclear spins in the sample. The contributions to the Faraday rotation of different nuclear species with different localizations in the sample are identified in nuclear magnetic resonance experiments. Our results suggest that the nuclear relaxation in metallic n-GaAs is dominated by diffusion towards fast relaxing centers, rather than by the Korringa mechanism.

We report on the nondestructive measurement of nuclear magnetization in n-GaAs via cavity enhanced Faraday rotation. In contrast with the existing optical methods, this detection scheme does not require the presence of detrimental out-of-equilibrium electrons. Specific mechanisms of the Faraday rotation are identified for (i) nuclear spins situated within the localized electron orbits, they are characterized by fast dynamics, (ii) all other nuclear spins in the sample characterized by much slower dynamics. Our results suggest, that even in degenerate semiconductors nuclear spin relaxation is limited by the presence of localized electron states and spin diffusion, rather than by Korringa mechanism.

Mn spin dynamics is measured in very diluted (Cd,Mn)Te crystals by time-resolved Kerr rotation. Spin beats due to the hyperfine interaction between the 3d electrons of the Mn ions and their own nuclei are detected. It is shown that the effect of the crystal field can be strongly suppressed for "magic" orientations of the magnetic field. This particular orientation of the field permits the optical read-out of the Mn nuclear spin state. Manganese ions trapped on a semiconductor lattice have uniform properties and relatively long spin lifetimes, which make them promising for optical manipulation. In particular, Mn2+ ions embedded in a II-VI semiconductor are S-state ions, weakly coupled to the lattice. For this reason quite long Mn electronic spin relaxation times are expected and have been observed [1]. Here we show that the spin coherence time is mainly limited by dipole-dipole interactions at a Mn concentration x=0.001, and reaches up to 15 ns. At this low concentration, fine and hyperfine structures of Mn2+ are resolved in electron spin resonance experiments. In the time-domain it corresponds to low-frequency beats as shown in Fig. 1.

We propose a new experimental technique for the optical study of dark and bright spatially indirect excitons in coupled quantum wells. We demonstrate that indirect excitons strongly contribute to nonlinear optical effects: photoinduced reflectivity and Kerr rotation, detected at the direct exciton transition frequency. In contrast to conventional photoluminescence measurements which are sensitive only to optically active |±1> states inside the light cone, the proposed technique allows to access yet hidden dark components of excitonic systems.