Semiconductors contain free carriers. That is the characteristic feature which makes them different from the other systems which we have considered hitherto. The optical nonlinearities discussed in previous chapters arose from bound charges. Similar effects arise from bound charges in semiconductors but we do not consider them here. The optical nonlinearities which arise from free carriers in semiconductors are particularly important for applications because of the high degree of control which we have over the free-carrier densities and therefore on the performance of devices which make use of them.

When the free-carrier densities are changed by optical excitation we are concerned with real transitions (see §6.6). The resulting nonlinear processes proceed via a real exchange of energy from the optical field to the medium, and are often referred to as ‘dynamic nonlinearities’ (Miller et al, 1981a; Oudar, 1985); this is the nomenclature used here.

Another feature of semiconductors which has become of particular significance for applications in recent years is the ability to fabricate multiple ‘quantum well’ structures, in which the carriers are confined in one direction in repeated layers of the order of 5 nm wide. Within the layers the carrier motion is two-dimensional, which drastically affects their behaviour. This topic belongs more naturally to the next chapter. In this chapter, we confine our attention to the nonlinear-optical properties of bulk semiconductors.

In §§8.1 and 8.2 we outline the one-electron band structure and the behaviour of phonons in Group IV and III – V semiconductors.