The photoluminescence polarizations of (In,Ga)As/GaAs quantum dots annealed at different temperatures are studied as a function of external magnetic field (Hanle curves). In these dependencies, remarkable resonant features appear due to all-optical nuclear magnetic resonances (NMR) for optical excitation with modulated circular polarization. Application of an additional radio-frequency field synchronously with the polarization modulation strongly modifies the NMR features. The resonances can be related to transitions between different nuclear spin states split by the strain-induced gradient of the crystal field and by the externally applied magnetic field. A theoretical model is developed to simulate quadrupole and Zeeman splittings of the nuclear spins in a strained quantum dot. Comparison with the experiment allows us to uniquely identify the observed resonances. The large broadening of the NMR resonances is attributed to variations of the quadrupole splitting within the quantum dot volume, which is well described by the model.
Resonance dielectric response of excitons is studied for the high-quality
GaAs/InGaAs heterostructures with wide asymmetric quantum wells (QWs). To
highlight effects of the QW asymmetry, we have grown and studied several
heterostructures with nominally square QWs as well as with triangle-like QWs.
Several quantum confined exciton states are experimentally observed as narrow
exciton resonances with various profiles. A standard approach for the
phenomenological analysis of the profiles is generalized by introducing of
different phase shifts for the light waves reflected from the QWs at different
exciton resonances. Perfect agreement of the phenomenological fit to the
experimentally observed exciton spectra for high-quality structures allowed us
to obtain reliable parameters of the exciton resonances including the exciton
transition energies, the radiative broadenings, and the phase shifts. A direct
numerical solution of Schr\"{o}dinger equation for the heavy-hole excitons in
asymmetric QWs is used for microscopic modeling of the exciton resonances.
Remarkable agreement with the experiment is achieved when the effect of indium
segregation during the heterostructure growth is taken into account. The
segregation results in a modification of the potential profile, in particular,
in an asymmetry of the nominally square QWs
The role of nuclear spin fluctuations in the dynamic polarization of nuclear spins by electrons is investigated in (In,Ga)As quantum dots. The photoluminescence polarization under circularly polarized optical pumping in transverse magnetic fields (Hanle effect) is studied. A weak additional magnetic field parallel to the optical axis is used to control the efficiency of nuclear spin cooling and the sign of nuclear spin temperature. The shape of the Hanle curve is drastically modified with changing this control field, as observed earlier in bulk semiconductors and quantum wells. However, the standard nuclear spin cooling theory, operating with the mean nuclear magnetic field (Overhauser field), fails to describe the experimental Hanle curves in a certain range of control fields. This controversy is resolved by taking into account the nuclear spin fluctuations owed to the finite number of nuclei in the quantum dot. We propose a model describing cooling of the nuclear spin system by electron spins experiencing fast vector precession in the random Overhauser fields of nuclear spin fluctuations. The model allows us to accurately describe the measured Hanle curves and to determine the parameters of the electron-nuclear spin system of the studied quantum dots.
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