Ultrafast optical response of confined Fermi seas

Summary form only given. Ultrafast spectroscopy offers a unique perspective into the role of many-body effects by providing time resolution shorter than the scattering times among the elementary excitations of the system. In modulation-doped quantum wells (QWs), the optical properties are dominated by the Fermi edge singularity (FES). Unlike in the undoped case, the FES is a continuum resonance whose line shape is governed by the Coulomb interactions of the photoexcited carriers with the low-lying Fermi sea (FS) excitations. In doped systems, the interactions are screened and the many-body correlations enter into the nonlinear response not via long-ranged exciton-exciton interactions, but through the unadiabatic dynamical response of the FS to the sudden appearance of a valence hole. For resonant pump excitation, the electron-electron scattering leads to dephasing and relaxation times that vary from a few ps close to the FES to a few fs away from the Fermi level. However, such incoherent effects are suppressed for off-resonant excitation. Furthermore, for negative time delays, the pump/probe signal is due to the coherent interaction of the pump and the probe polarization and, again, the effects of incoherent processes are strongly attenuated. Therefore, the difference in the coherent dynamics between the FES and the exciton, due to e-h Coulomb correlations with the FS, can be best observed under off-resonant excitation conditions and when the probe precedes the pump. We investigate the ultrafast pump/probe dynamics of the FES-exciton hybrid.