Light-polarization-independent nuclear spin alignment in a quantum dot

We study experimentally nuclear spin-pumping mechanisms in neutral InP/GaInP quantum dots under nonresonant optical excitation. We find two distinct regimes of dynamic nuclear polarization. At low optical powers when the dot is populated with dark excitons we observe nuclear spin polarization up to similar to 10% with direction insensitive to polarization and wavelength of light. Measurements of photoluminescence of both dark and bright excitons in single dots reveal that at low optical power nuclear spin pumping occurs via a virtual spin-flip transition between these states accompanied by photon emission. Under these conditions the sign of the nuclear spin polarization is determined by asymmetry in the exciton energy spectrum rather than by the sign of the exciton spin polarization. By contrast at high optical powers resulting in saturation of the quantum dot and suppression of exciton photoluminescence we detect nuclear spin polarization with direction and degree (up to similar to 50%) determined by the polarization of light.