  Optical and electrical control of the nuclear spin system allows enhancing
the sensitivity of NMR applications and spin-based information storage and
processing. Dynamic nuclear polarization in semiconductors is commonly achieved
in the presence of a stabilizing external magnetic field. Here we report
efficient optical pumping of nuclear spins at zero magnetic field in strain
free GaAs quantum dots. The strong interaction of a single, optically injected
electron spin with the nuclear spins acts as a stabilizing, effective magnetic
field (Knight field) on the nuclei. We optically tune the Knight field
amplitude and direction. In combination with a small transverse magnetic field,
we are able to control the longitudinal and transverse component of the nuclear
spin polarization in the absence of lattice strain i.e. nuclear quadrupole
effects, as reproduced by our model calculations.
