Nonlinear optical study of the Fermi edge singularity: differences from atomic excitons in the virtual excitation regime

We discuss how the different nature of excitonic effects in modulation-doped quantum wells or metals (Fermi edge singularity) and undoped semiconductors (atomic excitons) manifests itself strongly in the nonlinear absorption spectrum for off-resonant pump excitation. We study this problem by extending our previous theoretical study of the Fermi edge singularity in linear absorption and obtain a new intuitive picture that extracts the physics conveyed by our experimental results. We point out that, in addition to phase-space filling, the pump-induced polarization increases the carrier effective masses and, as a result, can enhance excitonic resonances.