Long nuclear spin polarization decay times controlled by optical pumping in individual quantum dots

Nuclear polarization dynamics are measured in the nuclear spin bistability regime in a single optically pumped InGaAs/GaAs quantum dot. The controlling role of nuclear spin diffusion from the dot into the surrounding material is revealed in pump-probe measurements of the nonlinear nuclear spin dynamics. We measure nuclear spin polarization decay times in the range of 0.2-5 s, strongly dependent on the optical pumping time. The long nuclear spin decay arises from polarization of the material surrounding the dot by spin diffusion for long (>5s) pumping times. The time-resolved methods allow the detection of the unstable nuclear polarization state in the bistability regime otherwise undetectable in cw experiments.