  We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with
circularly polarized light, the nuclei of atoms constituting the dot can be
driven into a bistable regime, in which either a threshold-like enhancement or
reduction of the local nuclear field by up to 3 Tesla can be generated by
varying the intensity of light. The excitation power threshold for such a
nuclear spin "switch" is found to depend on both external magnetic and electric
fields. The switch is shown to arise from the strong feedback of the nuclear
spin polarization on the dynamics of spin transfer from electrons to the nuclei
of the dot.
