HIGH DENSITY OF VIRTUALLY CREATED EXCITONS IN GAAS MULTIPLE-QUANTUM-WELL STRUCTURES.

Summary form only given, as follows. A systematic study has been performed of the changes in the excitonic properties of GaAs multiple-quantum-well structures induced by a femtosecond laser pulse falling in the transparency region of the material, below the lowest heavy-hole exciton. Experiments have been performed as a function of laser detuning from the lowest exciton resonance (at fixed laser intensity) or alternatively as a function of input laser intensity at a fixed detuning. Furthermore, results obtained in several samples with different well and barrier sizes have been compared using similar excitation conditions. Two major effects are observed. The first is a dynamic Stark effect, leading to a high-energy shift of the lowest exciton, which can become comparable in magnitude to the exciton binding energy itself. The second effect is a strong reduction of the exciton oscillator strength, leading in the extreme cases to the total disappearance of the HH exciton. Both effects recover almost completely within 1 ps of the end of the excitation pulse (pulse duration, 200 fs). The results are compared to a recent theoretical treatment.