Coulomb correlation signatures in the excitonic optical nonlinearities of semiconductors This chapter focuses on the carrier interaction effects—that is, the many-body Coulomb correlations and their influence on band-gap semiconductor optical nonlinearities. The near band-gap semiconductor response is determined by the optical interaction with the resonant or near resonant material polarization that may result in the excitation of carriers (electron–hole pairs) and/or transient coherent nonlinearities. The microscopic analysis of these effects requires the investigation of the relevant quasi-particles and their interactions. Whereas the basic occurrence of a blue shift can be motivated already based on a simple two-level model, a more detailed understanding of the optical Stark effect in semiconductors requires a microscopic modeling of the relevant bands as well as the inclusion of the many-body Coulomb interaction. Compensations among the first- and higher-order Coulomb terms at the spectral positions of the exciton are obtained in the differential absorption induced by incoherent occupations. 67 231-313 231-313 Elsevier