Introduction to Nonlinear Optics and High Excitation

In this chapter we describe two approaches to treat the optical properties of semiconductors under intense irradiation with light. The first one is the expansion of the field-dependence of the dielectric susceptibility into a power series of the electric-field amplitudes. This expansion allows treating various frequency-mixing phenomena like the generation of higher harmonics or four-wave mixing when discussing the real part of the nonlinear susceptibility. Its imaginary part on the other hand describes multi-photon absorption processes. In this context we illustrate the role of virtual intermediate states. The second approach deals with optical nonlinearites which are evoked by incoherent real particles. Interactions of these particles renormalize their energies resulting in high-excitation effects like band-filling and band-gap shrinkage. Also new composite particle like biexcitons or new phases like plasmas or condensates can form.