Exciton saturation in GaAs multiple quantum wells at room temperature

Pump-probe experiments in GaAs/AlGaAs multiple quantum well samples are des cribed using both picosecond and femtosecond laser pulses in different pola rization configurations. The excitonic and free carrier components of excit on absorption saturation were analyzed as a function of well width. The rel ative importance of phase space filling, Coulomb screening and broadening c ontributions was determined from polarization and laser bandwidth dependenc ies via spin-dependent and spin-independent components. A five-level model was used to fit the data and nonlinear coefficients for the individual cont ributions were determined for each well width. The Coulomb contributions ar ising from screening and broadening of the excitons was found to dominate t he absorption bleaching using picosecond pulses, whereas phase space fillin g was largest in the femtosecond regime. Exciton phase space filling was fo und to be four times larger than free carrier phase space filling at narrow er well widths. A sublinear dependence of exciton broadening with carrier d ensity was observed. (C) 1999 American Institute of Physics. [S0021- 8979(9 9)06919-4].