Virtual excitation of the Fermi-edge singularity in modulation-doped quantum wells

The Fermi-edge singularity (FES) in modulation-doped GaAs quantum wells is a unique many-body feature that arises from strong Coulomb correlations within a dense sea of electrons. Using intense ultrashort excitation, we show experimentally and theoretically that the nonresonant nonlinearity (ac Stark shift) of the FES is dramatically different than that of the excitonic ac Stark shift of undoped-quantum-well excitons.