Resonantly excited photoluminescence with femtosecond time resolution reveals the essential role of momentum scattering in emission from excitons in quasi-two-dimensional systems. The gradual rise of the luminescence provides a direct measure of exciton momentum scattering rates. An analysis of quantum beats in the luminescence determines exciton dephasing rates. The results show that the momentum scattering rate is considerably larger than the dephasing rate, indicating a breakdown of the impact approximation commonly assumed in coherent optical studies.

• Received 13 December 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.3065