The dynamics of electron and nuclear spin polarization in an ensemble of singly negatively charged quantum dots subject to optical excitation is theoretically studied using a graded box model, in which the electron density is approximated by a sequence of steps. The model is numerically implemented for a limited number of nuclei (up to $N=192$) with spins $I=1/2$ or $I=3/2$. The polarization dynamics is found to depend strongly on the polarized excitation protocol. For excitation by periodic laser pulses, the electron-nuclear spin dynamics evolves coherently and gives rise to a recurrence effect in the electron spin dynamics as well as to a decelerated nuclear polarization. The validity of the model is justified by comparison with experimental data on the electron spin polarization in (In,Ga)As/GaAs quantum dots.

We study the formation of exciton-polariton condensates in potlike traps created by optical pumping in a planar microcavity with embedded quantum wells. The trap is formed by a repulsive reservoir of incoherent excitons excited by a ring-shaped nonresonant laser beam. Polariton condensates confined in a trapping potential are subject to spatial confinement leading to energy quantization. We reveal experimentally the discrete spectrum of polariton eigenstates in an optical trap that can be characterized by a pair of quantum numbers, azimuthal and radial quantum numbers, that correspond to the number of nodes of a condensate wave function in the corresponding directions. The occupation numbers of the eigenstates of a polariton condensate are determined by the overlap integral of the condensate wave function and the exciton reservoir spatial density distribution. The nonresonant pumping scheme enables engineering the shape and size of the trap, that allows to selectively excite specific superpositions of the eigenstates of a polariton condensate in each experiment. We demonstrate both single- and multiple-mode polariton lasing in an optical trap.

The efficiency of the adiabatic demagnetization of the nuclear spin system (NSS) of a solid is limited, if quadrupole effects are present. Nevertheless, despite a considerable quadrupole interaction, recent experiments validated the thermodynamic description of the NSS in GaAs. This suggests that nuclear spin temperature can be used as a probe of nuclear magnetic resonances. We implement this idea by analyzing the modification of the NSS temperature in response to an oscillating magnetic field at various frequencies, an approach termed as the warm-up spectroscopy. It is tested in a $n$-GaAs sample where both mechanical strain and built-in electric field may contribute to the quadrupole splitting, yielding the parameters of electric field gradient tensors for ${}^{75}\mathrm{As}$ and both Ga isotopes, ${}^{69}\mathrm{Ga}$ and ${}^{71}\mathrm{Ga}$.

We report on the observation of resonant optical pumping of nuclear spin polarization in an ensemble of singly charged (In,Ga)As/GaAs quantum dots subject to a transverse magnetic field. Electron spin orientation by circularly polarized light with the polarization modulated at the nuclear spin transition frequency is found to create a significant nuclear spin polarization, precessing about the magnetic field. Synchronous rf field application along the optical excitation axis considerably enhances the effect. Nuclear spin resonances for all isotopes in the quantum dots are found in that way. In particular, transitions between states split off from the $|\pm 1/2\rangle$ doublets by the nuclear quadrupole interaction are identified.

The avian magnetic compass allows orientation during migration and is shown to function properly under short-wavelength but not long-wavelength visible light. Therefore, the magnetoreceptive system is assumed to be light- and wavelength-dependent and localized in the retina of the eye. Putative candidates for the role of primary magnetosensory molecules are the cryptochromes that are known to be expressed in the avian retina and must be able to interact with phototransduction proteins. Previously, we reported that in migratory birds change in magnetic field direction induces significant effects on electroretinogram amplitude in response to blue flashes, and such an effect was observed only in the nasal quadrant of the retina. Here, we report new electroretinographic, microscopic and microspectrophotometric data on European robins, confirming the magnetosensitivity of the retinal nasal quadrant after applying the background illumination. We hypothesized that magnetoreceptive distinction of this region may be related to its morphology and analyzed the retinal distribution and optical properties of oil droplets, the filtering structures within cones. We found that the nasal quadrant contains double cones with the most intensely colorized oil droplets compared to the rest of the retina, which may be related to its magnetosensory function.

At low lattice temperatures the nuclear spins in a solid form a closed thermodynamic system that is well isolated from the lattice. Thermodynamic properties of the nuclear spin system are characterized by the local field of spin-spin interactions, which determines its heat capacity and the minimal achievable nuclear spin temperature in demagnetization experiments. We report the results of measurement of the local field for the nuclear spin system in GaAs, which is a model material for semiconductor spintronics. The choice of the structure, a weakly doped GaAs epitaxial layer with weak residual deformations, and of the measurement method, the adiabatic demagnetization of optically cooled nuclear spins, allowed us to refine the value of the nuclear spin-spin local field, which turned out to be two times less than one previously obtained. Our experimental results are supported by calculations, which take into account dipole-dipole and indirect (pseudodipolar and “exchange”) nuclear spin interactions as well as quadrupole splitting of nuclear spins in the vicinity of charged impurity centers.