Bistability of the nuclear polarization created through optical pumping in In1-xGaxAs quantum dots

We show that optical pumping of electron spins in individual InGaAs quantum dots leads to strong nuclear polarization that we measure via the Overhauser shift delta(n) in magnetophotoluminescence experiments between 0 and 4 T. We find a strongly nonmonotonous dependence of delta(n) on the applied magnetic field, with a maximum nuclear polarization of about 40% for intermediate magnetic fields. We observe that delta(n) is larger for nuclear fields antiparallel to the external field than in the parallel configuration. A bistability in the dependence of delta(n) on the spin polarization of the optically injected electrons is found. All our findings are qualitatively understood with a model based on a simple perturbative approach.