Extraction of many-body excitation configurations from nonlinear absorption in semiconductor quantum wells

We extract detailed electronic many-body configurations by analyzing quantitatively measured, time-resolved nonlinear absorption spectra of resonantly excited GaAs quantum wells with our fully consistent microscopic theory. Nonlinear spectral changes observed in the probe absorption are attributed by our theory to a unique mixture of the effects of electron-hole plasma, exciton populations, and polarization. Studies of these effects include quantitative comparison of co-linear- and co-circular-polarization pump-probe excitation schemes that reveal the consequences of spin-selection rules on scattering. For co-circular excitation conditions, we observe strong transient gain. We attribute this gain to the transfer of pump-induced coherences to the probe. Unexpectedly, we also find that true exciton populations do not significantly contribute to spectral broadening or shifting; rather, the nonlinear modifications are dominated by the excited carrier densities.