Electrically tunable dynamic nuclear spin polarization in GaAs quantum dots at zero magnetic field

In III-V semiconductor nano-structures, the electron and nuclear spin dynamics are strongly coupled. Both spin systems can be controlled optically. The nuclear spin dynamics are widely studied, but little is known about the initialization mechanisms. Here, we investigate optical pumping of carrier and nuclear spins in charge tunable GaAs dots grown on 111A substrates. We demonstrate dynamic nuclear polarization (DNP) at zero magnetic field in a single quantum dot for the positively charged exciton X⁺ state transition. We tune the DNP in both amplitude and sign by variation of an applied bias voltage V_g. Variation of ΔV_g on the order of 100 mV changes the Overhauser splitting (nuclear spin polarization) from -30 μeV (-22%) to +10 μeV (+7%) although the X⁺ photoluminescence polarization does not change sign over this voltage range. This indicates that absorption in the structure and energy relaxation towards the X⁺ ground state might provide favourable scenarios for efficient electron-nuclear spin flip-flops, generating DNP during the first tens of ps of the X⁺ lifetime which is on the order of hundreds of ps. Voltage control of DNP is further confirmed in Hanle experiments.