Chapter One

Roland H. Tarkhanyan

Introduction

Propagation of electromagnetic waves in crystalline solids is the basis for understanding their optical properties in different ranges of frequencies and wavelengths. A large number of books are now available, concerning specific properties of the waves in solids, and in most of them crystals of cubic structure with isotropic optical behavior are considered. However, most semiconductors and ionic insulators possess anisotropic crystal structures. It is well known that 19 from 32 possible point groups for crystals are characterized by the presence of the axis of rotation of the third, fourth or sixth order. All these crystals relate to the group of optically uniaxial crystals, the optical properties of which are described by the permittivity tensor with two different principal eigenvalues along and across the crystal axis of symmetry. If a uniaxial crystal has a center of symmetry, that is, a point through which the crystal may be inverted and left invariant, the opposite directions along the axis of symmetry are equivalent.

A great number of general properties of electromagnetic waves in anisotropic nonconducting media have been discussed in Refs., which also hold for uniaxially stressed silicon and germanium, Te, Se, Cd[As.sub.2], InSe, [alpha]-SiC(6H), CdS and other uniaxial semiconductors. However, the presence of the free carriers and phonons and their interaction with the electromagnetic waves lead to new specific properties of the waves, which are absent in anisotropic insulators as well as