Coulomb Contributions to Exciton Saturation in Room Temperature GaAs-AlxGa1-xAs Multiple Quantum Wells

A number of optoelectronic device applications of quantum well semiconductors depend on the saturation of exciton absorption features. Studies of exciton saturation at room temperature have resolved exciton-exciton interactions on timescales less than 300fs, and two distinct mechanisms based on phase space filling (PSF) and Coulomb effects caused by free carriers on longer timescales. Nonequilibrium carrier distributions were originally employed to separate Pauli exclusion and long range Coulomb effects [1]. More recently, optically induced circular dichroism was used to identify PSF and Coulomb exchange contributions [2]. However, Coulomb contributions can arise from both screening and collisional broadening. In this work, we have extended the use of circularly polarised ultrashort pulses to distinguish the two related Coulomb effects of screening and broadening and in addition, compared the relative contributions of excitons and free carriers to Coulomb contributions.