Optical Stark Effect on Excitons in GaAs Quantum Wells.

We describe the first experimental observation of an extremely fast shift of the n=1 exciton transition energy in GaAs quantum well heterostructures. The shift is produced by optical pumping below the bandgap and is not associated with a carrier or exciton population. We interpret the shift in terms of an optical Stark effect. We present a model for the Stark effect on the ground state exciton in quantum wells and find good agreement between the predictions of the model and our experimental results.