  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.
