  Nuclear polarization dynamics are measured in the nuclear spin bi-stability
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 non-linear nuclear spin dynamics. We
measure nuclear spin decay times in the range 0.2-5 sec, 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 (>5sec) pumping
times. The time-resolved methods allow the detection of the unstable nuclear
polarization state in the bi-stability regime otherwise undetectable in cw
experiments.

Mesoscale and Nanoscale Physics;Materials Science