Pump-probe experiments in GaAs/AlGaAs multiple quantum well samples are described using both picosecond and femtosecond laser pulses in different polarization configurations. The excitonic and free carrier components of exciton absorption saturation were analyzed as a function of well width. The relative importance of phase space filling, Coulomb screening and broadening contributions was determined from polarization and laser bandwidth dependencies via spin-dependent and spin-independent components. A five-level model was used to fit the data and nonlinear coefficients for the individual contributions were determined for each well width. The Coulomb contributions arising from screening and broadening of the excitons was found to dominate the absorption bleaching using picosecond pulses, whereas phase space filling was largest in the femtosecond regime. Exciton phase space filling was found to be four times larger than free carrier phase space filling at narrower well widths. A sublinear dependence of exciton broadening with carrier density was observed. (C) 1999 American Institute of Physics. [S0021- 8979(99)06919-4].
- OPTICAL NONLINEARITIES
- SPIN RELAXATION