Basic mechanisms of the optical nonlinearities of semiconductors near the band edge

Semiconductors are known to show in the band-gap region relative large changes of their optical properties with increasing light intensity. These changes are due to the creation of electron–hole pairs, which modify by band filling the intraband and interband contributions to the complex optical dielectric function. Most important, with increasing concentration of electron–hole pairs, the band gap shrinks, and the Coulomb forces are strongly reduced so that excitonic effects disappear. The theoretical description of these phenomena in three- and quasi-two-dimensional semiconductors is reviewed.