Femtosecond ac Stark effect in semiconductor quantum wells: Extreme low- and high-intensity limits

We show that in quasi-2D GaAs quantum wells, below-resonance low-intensity excitation produces a pure shift of the excitons, contrary to ac Stark shifts in atomic systems. At high pump intensities two-photon 3D-real-carrier generation competes with the virtual 2D-exciton effects. We also show that dc fields applied perpendicular to the layers reduce the excitonic ac Stark effect.