Low-temperature electron-spin relaxation is studied by the optical orientation method in bulk n-GaAs with donor concentrations from ${10}^{14}{\mathrm{cm}}^{-3}$ to $5\times {10}^{17}{\mathrm{cm}}^{-3}.$ A peculiarity related to the metal-to-insulator transition is observed in the dependence of the spin lifetime on doping near $n_D=2\mathrm{}\times {10}^{16}{\mathrm{cm}}^{-3}.$ In the metallic phase, spin relaxation is governed by the Dyakonov-Perel mechanism, while in the insulator phase it is due to anisotropic exchange interaction and hyperfine interaction

Resonant transmission of light through a microcavity in the strong coupling regime is used to estimate the strength of the interaction between polaritons with parallel $\left({\alpha }_1\right)$ or antiparallel $\left({\alpha }_2\right)$ spins. The ratio ${\alpha }_2/{\alpha }_1$ is found to be strongly dependent on the detuning between exciton and photon modes. From negative to zero detuning it changes from about 0 to less than $-1$. Our observations indicate that at certain conditions the polaritons might rather condense in the real space than form a Bose-Einstein condensate. We analyze theoretically different mechanisms of polariton-polariton interaction including the mean-field electrostatic interaction, the direct exchange interaction, the Van-der-Waals interaction and the indirect exchange interaction via dark exciton and biexciton states.

We have studied the electron spin relaxation in semiconductor $\mathrm{InAs}/\mathrm{GaAs}$ quantum dots by time-resolved optical spectroscopy. The average spin polarization of the electrons in an ensemble of $p$-doped quantum dots decays down to $1/3$ of its initial value with a characteristic time $T_{\Delta }\approx 500\mathrm{ps}$, which is attributed to the hyperfine interaction with randomly oriented nuclear spins. We show that this efficient electron spin relaxation mechanism can be suppressed by an external magnetic field as small as 100 mT.

The spin-orbit interaction in semiconductors is shown to result in an anisotropic contribution into the exchange Hamiltonian of a pair of localized conduction-band electrons. The anisotropic exchange interaction exists in semiconductor structures that are not symmetric with respect to spatial inversion, for instance in bulk zinc-blende semiconductors. The interaction has both symmetric and antisymmetric parts with respect to permutation of spin components. The antisymmetric (Dzyaloshinskii-Moriya) interaction is the strongest one. It contributes significantly into spin relaxation of localized electrons; in particular, it governs low-temperature spin relaxation in n-GaAs with the donor concentration near ${10}^{16}\hspace{0.5em}{\mathrm{cm}}^{-3}.$ The interaction must be allowed for in designing spintronic devices, especially spin-based quantum computers, where it may be a major source of decoherence and errors.

Microcavity polariton spin dynamics is investigated in high-finesse microcavities under resonant excitation using polarization- and time-resolved secondary emission spectroscopy. Using an appropriate calibration procedure, the instantaneous polariton occupation numbers of the modes with $\mathbf{k}\approx \mathbf{0}$ wave vectors can be determined. When the phase-matching conditions are achieved, we show that Coulomb scattering dominates the other processes, and that stimulation occurs when the occupation of the $\mathbf{k}\approx \mathbf{0}$ final state with a given mode polarization approaches unity. Under elliptically polarized light, we show that the stimulated polariton scattering leads to an increase with time of the circular polarization of the $\mathbf{k}\approx \mathbf{0}$ states, while the linear polarization dynamics is characterized first by a fast drop to negative or zero values immediately after the excitation pulse, followed by weak-amplitude nonperiodic oscillations. The latter are interpreted as the manifestation of the pseudospin precession of the pumped polariton modes around the self-induced exchange effective field.

We present the quantum theory of momentum and spin relaxation of exciton-polaritons in microcavities. We show that giant longitudinal-transverse splitting of the polaritons mixes their spin states, which results in beats between right- and left-circularly polarized photoluminescence of microcavities, as was recently experimentally observed [M. D. Martin et al., Phys. Rev. Lett. 89, 077402 (2002)]. This effect is strongly sensitive to the bosonic stimulation of polariton scattering.