2.2 Nonlinear optical spectroscopy

The purpose of this chapter is to review experimental technique, particularly nonlinear optical spectroscopy, which have been or can be used for optical spectroscopy of semiconductor structures of reduced dimensionality. The chapter first mentions some necessary or typical equipment for the observation of optical nonlinearities, and then experimental methods and some possible results. It also gives some information on possible general problems. Further, it addresses inhomogeneities of the density of excited species, frequently electron-hole pairs or excitons. Finally, it explains experimental methods, which are subdivided into one-beam methods, pump-and-probe beam spectroscopy and four-wave mixing (FWM) and laser-induced gratings (LIG). The pump-and-probe beam spectroscopy and FWM and LIG use two (or more) beams. The difference is essentially that the two beams need not (or even must not) be coherent for the pump-and-probe beam spectroscopy, while they must be coherent for FWM and LIG.