Enhancement of the Radiative Lifetime of 2D Excitons in a GaAs Quantum Well by Dephasing Collisions

The nonlinear optical properties and the ultrafast dynamics of excitons in semiconductors are a major field of present semiconductor research. This interest is explained by the potential applications of excitonic nonlinearities as ultrafast optical switching devices in future optical communication systems. In a recent theoretical paper Hanamura /1/ discussed the advantages of excitons in a quantum well (QW) as a nonliner optical medium combining large 3rd order nonlinear susceptibility χ(3) with a fast response. The strong enhancement of is χ(3) explained as a consequence of both the macroscopic transition dipole moment of the exciton in a QW and the rapid radiative decay of the confined excitons. Hanamura calculated that an exciton in a QW should decay superradiantly through its macroscopic dipole transition moment within a few picoseconds. This superradiant decay requires, however, a coherent polarization of the material and will be strongly reduced if the spatial and temporal coherence of the excited excitons is destroyed by interaction of the excitons with their environment. Such a tight connection between the radiative lifetime τ r and the dephasing T2 has been recently predicted by Feldmann et al. /2/.