  We theoretically investigate dynamic nuclear spin polarization in a
self-assembled quantum dot pumped optically by two laser beams. With the
assumption that a noncollinear interaction between the hole spin and nuclear
spins leads to nuclear spin polarization, we find that both weak and strong
nuclear spin polarizations can arise, depending on the intensities and central
frequencies of the lasers. For weak nuclear spin polarization, we use a
perturbation method to show that the distribution of the nuclear spin
Overhauser field may become significantly narrower. Using Monte Carlo
simulations to study a single quantum dot, we find that strong nuclear spin
polarization can also be generated via appropriate optical pumping.
