  We report optically detected nuclear magnetic resonance (ODNMR) measurements
on small ensembles of nuclear spins in single GaAs quantum dots. Using ODNMR we
make direct measurements of the inhomogeneous Knight field from a photo-excited
electron which acts on the nuclei in the dot. The resulting shifts of the NMR
peak can be optically controlled by varying the electron occupancy and its spin
orientation, and lead to strongly asymmetric lineshapes at high optical
excitation. The all-optical control of the NMR lineshape will enable
position-selective control of small groups of nuclear spins in a dot. Our
calculations also show that the asymmetric NMR peak lineshapes can provide
information on the volume of the electron wave-function, and may be used for
measurements of non-uniform distributions of atoms in nano-structures.
