Optical Stark effect in a quantum dot: ultrafast control of single exciton polarizations.

We report the first experimental study of the optical Stark effect in single semiconductor quantum dots (QD). For below band gap excitation, two-color pump-probe spectra show dispersive line shapes caused by a light-induced blueshift of the excitonic resonance. The line shape depends strongly on the excitation field strength and is determined by the pump-induced phase shift of the coherent QD polarization. Transient spectral oscillations can be understood as rotations of the QD polarization phase with negligible population change. Ultrafast control of the QD polarization is demonstrated.