Dynamic nuclear polarization of a single charge-tunable InAs/GaAs quantum dot

We report on the dynamic nuclear polarization of a single charge-tunable self-assembled InAs/GaAs quantum dot in a longitudinal magnetic field of similar to 0.2 T. The hyperfine interaction between the optically oriented electron and nuclear spins leads to the polarization of the quantum dot nuclei measured by the Overhauser shift of the singly charged excitons (X+ and X-). When going from X+ to X-, we observe a reversal of this shift, which reflects the average electron spin optically written in the quantum dot either in the X+ state or in the final state of X- recombination. We discuss a theoretical model which indicates an efficient depolarization mechanism for the nuclei limiting their polarization to similar to 10%.