  The nuclear spin fluctuations (NSF) as well as the dynamic nuclear
polarization (DNP) and their effects on the electron spins in negatively
charged (In,Ga)As/GaAs quantum dots have been studied by polarized pump-probe
and photoluminescence spectroscopy techniques. The effective magnetic field of
the NSF is about 30 mT at low excitation power. The NSF distribution becomes
highly anisotropic at strong optical excitation by circularly polarized light
with periodically alternating helicity. This phenomenon is attributed to a
decrease of the nuclear spin entropy due to the hyperfine interaction with
polarized electron spins. The DNP is limited to small values for intense, but
short photoexcitation.
