Optical Propagation in Solids

This chapter emphasizes the linear optical properties of solids as a function of frequency and temperature. Such information is basic to understanding the performance of optical fibers, lenses, dielectric and metallic mirrors, window materials, thin films, and solid-state photonic devices in general. Optical properties are comprehensively covered in terms of mathematical models of the complex index of refraction based on those discussed in Chapters 4 and 5. Parameters for these models are listed in Appendix 4. A general review of solid-state properties precedes this development because the choice of an optical material requires consideration of thermal, mechanical, chemical, and physical properties as well. This section introduces the classification of optical materials and surveys other material properties that must be considered as part of total optical system design involving solidstate optics. Solid-state materials can be classified in several ways. The following are relevant to optical materials. Three general classes of solids are insulators, semiconductors, and metals. Insulators and semiconductors are used in a variety of ways, such as lenses, windows materials, fibers, and thin films. Semiconductors are used in electrooptic devices and optical detectors. Metals are used as reflectors and high-pass filters in the ultraviolet. This type of classification is a function of the material’s electronic bandgap. Materials with a large room-temperature bandgap (Eg > 3eV) are insulators. Materials with bandgaps between 0 and 3 eV are semiconductors. Metals have no observable bandgap because the conduction and valence bands overlap. Optical properties change drastically from below the bandgap, where the medium is transparent, to above the bandgap, where the medium is highly reflective and opaque. Thus, knowledge of its location is important. Appendix 4 lists the bandgaps of a wide variety of optical materials. To characterize a medium within the region of transparency requires an understanding of the mechanisms of low-level absorption and scattering. These mechanisms are classified as intrinsic or extrinsic. Intrinsic properties are the fundamental properties of a perfect material, caused by lattice vibrations, electronic transitions, and so on, of the atoms composing the material.