The general principles of amplification and oscillation in semiconductor lasers are the same as those in solid state and gas lasers, as discussed in Chapter 6. A negative χ″ is obtained in an active region via induced transitions of the electrons. When the gain per unit distance is larger than the propagation loss, laser amplification is obtained. In order to achieve laser oscillation, the active material is enclosed in a cavity. Laser oscillation begins when the gain exceeds the losses, including the output. However, the details are quite different. In this chapter, the discussion on semiconductor lasers will use much of the analyses already developed in Chapters 5 and 6; however, the differences will be emphasized.

In semiconductor lasers, free electrons and holes are the particles that undertake stimulated emission and absorption. How such free carriers are generated, transported and recombined has been discussed extensively in the literature. We note here, in particular, that free electrons and holes are in a periodic crystalline material. The energy levels of electrons and holes in such a material are distributed within conduction and valence bands. The distribution of energy states within each band depends on the specific semiconductor material and its confinement within a given structure. For example, it is different for a bulk material (a three-dimensional periodic structure) and for a quantum well (a two-dimensional periodic structure).