MANY-BODY EFFECTS IN NONLINEAR SPECTROSCOPY - A TIME-DEPENDENT TRANSFORMATION APPROACH

We discuss a recent theory for studying many-body effects in nonlinear spectroscopy. With a canonical transformation, we eliminate the optic ally-induced interband charge fluctuations and obtain a ''dressed'' Ha miltonian describing the Coulomb correlations leading to excitonic res onances. We then focus on a Fermi sea and use the Coupled Cluster expa nsion to study the dephasing and many-body processes. We provide an in tuitive picture of how the dynamics of the Coulomb correlations manife sts itself in nonlinear spectroscopy within the conventional picture o f interacting carriers moving inside effective bands. We then apply th is general method to extract the physics conveyed by recent experiment s, indicating that, for off-resonant pump excitation, the different na ture of the excitonic effects in doped quantum wells or metals (Fermi Edge Singularity) and undoped semiconductors (atomic exciton) leads to different nonlinear absorption. To interpret this, we demonstrate how a pump-induced increase in the carrier masses strongly enhances the F ermi Edge Singularity and why this depends on the pulse duration.