Dynamics and collective properties of non-equilibrium carriers in highly photoexcited quantum wells

The carrier dynamics and the collective properties of a photogenerated quasi-two-dimensional electron gas in an asymmetric undoped GaAs/AlGaAs quantum well (QW) structure are investigated using an all-optical interband pump?intersubband probe absorption experiment. At a carrier density of 1 ? 1012 cm?2 per QW, an e1?e2 absorption band at 126 meV is found. Transient resonance frequency blueshifts with respect to the single-particle resonance as well as a transient spectral band narrowing are observed. At the carrier density 1 ? 1013 cm?2 per QW, the e2?e3 absorption band at 99 meV with a spectral width of 5 meV is measured. This band is blueshifted by 30 meV through collective effects and broadened at an excitation density of 3 ? 1013 cm?2 per QW.