TABLE OF CONTENTS The operation of semiconductor lasers is strongly affected by the materials from which they are made. Spontaneous emission which initiates lasing, optical gain which is essential to achieve lasing, and other processes involved in lasing all use quantum processes at the level of single atoms and electrons within the lasing material. Indeed, without simplifications, the physics and mathematics necessary to describe such atomic systems fully is too complicated, certainly for the level of this book. The simplification and approximations must, however, be done in a way which is adapted to the requirements of semiconductor lasers, and the limitations must be understood. The results and implications of quantum physics are discussed here but there are only illustrative outlines of any derivations. Greater depth may be obtained from specialist material [1, 2] (see Bibliography).
In the context of lasers, the atomic behaviour of semiconductors can be summarised under three headings:
the distribution of energy states;
the occupation probability of those states; and
transitions between states.
The first few sections of this chapter will cover these headings at the atomic level, while the later sections will apply them at a macroscopic level to optical gain, spontaneous emission and noise.
In a semiconductor, the electrons which are of interest in the conduction processes are not normally bound to any single atom, but may move around in the crystal. Simple Newtonian mechanics forms a starting point where the kinetic energy of a particle ?
