Femtosecond recovery of exciton bleaching in the optical Stark effect

The observation of a very fast recovery of the exciton bleaching for an off-resonance excitation condition in a quantum-well sample at room temperature is reported. The experiments were performed on a GaAs/AlGaAs multiple quantum well (MQW) with 76-angstrom well sizes. A hybridly mode-locked dye laser that yielded 70-fs (FWHM) pulses at an 82-MHz repetition rate centered at ≅865 nm was used. The time delay was varied by changing the optical path of the pump pulse. The absorption spectra of the probe and the numerically evaluated excitonic absorption spectra, both at various time delays, are shown and discussed. The good agreement between the experimental results and the numerical calculations of the time-resolved absorption spectra suggests a coherent transient population buildup and decay occurring on the time scale of the pump pulse.