Dynamics of optically induced nuclear spin polarization in individual InPOGaxIn1-xP quantum dots

We report on dynamics of optically induced nuclear spin polarization in individual InP/GaInP quantum dots at T=4.2 K. Dots with different charge states arising from residual doping in a nominally undoped sample have been studied. In the same sample, we find strong dot-to-dot variation in the nuclear spin decay times in the dark from similar to 85 to similar to 6000 s. The longest decay times measured are comparable to those previously measured in bulk InP and correspond to almost complete suppression of nuclear spin diffusion out of the dot. In the negatively charged dots, the spin decay times exceed 300 s (with the slowest decay of similar to 6000 s), about 105 times longer than those reported previously in electron charged dots in gated structures. We discuss possible mechanisms responsible for suppression of nuclear spin diffusion, including inhomogeneous quadrupolar shifts and stabilizing effect of the hyperfine interaction with the electron confined in the dot.