  We show that detuned optical pulse trains with a modest spectral width can
polarize nuclear spins in InAs quantum dots. The pulse bandwidth is large
enough to excite a coherent superposition of both electron spin eigenstates in
these negatively charged dots but narrow enough to give partial spectral
selectivity between the eigenstates. The coherent precession of electron spin
states and periodic excitation focuses the nuclear spin distribution, producing
a discrete set of precession modes. The spectral selectivity generates a net
nuclear polarization, through a mechanism that relies on optical spin rotations
rather than electron spin relaxation.
