Fundamentals of spectroscopy of collective excitation in solids

Collective excitations are the intrinsic excitations of solids. A study of their physical properties is necessary for an adequate understanding of solid state systems. The purpose of this article is to examine the basic aspects of collective excitations and of their spectroscopic properties; it will introduce the fundamental concepts and will lay down the background material useful for the treatment of the subject of this book. In the first section we shall deal with a simple two-level system and we shall examine the effect of a time-dependent perturbation on such a system. In the second section we shall introduce the concept of collective excitation, keeping the treatment as general as possible. In the third section we shall treat the problem of the interaction of electromagnetic radiation with collective excitations. In the fourth section we shall examine the conditions for the propagation of electromagnetic radiation in a dispersive linear medium. Finally in the fifth section we shall apply the knowledge acquired in the previous sections to the treatment of different types of collective excitations.