  We investigate the dynamic nuclear polarization from the hyperfine
interaction between nonequilibrium electronic spins and nuclear spins coupled
to them in semiconductor nanostructures. We derive the time and position
dependence of the induced nuclear spin polarization and dipolar magnetic
fields. In GaAs/AlGaAs parabolic quantum wells the nuclear spin polarization
can be as high as 80% and the induced nuclear magnetic fields can approach a
few gauss with an associated nuclear resonance shift of the order of kHz when
the electronic system is 100% spin polarized. These fields and shifts can be
tuned using small electric fields. We discuss the implications of such control
for optical nuclear magnetic resonance experiments in low-dimensional
semiconductor nanostructures.
