ABSTRACT
The optical properties of solids are basically described using the dielectric functions of the materials. This chapter presents the fundamental concept of the geometry of the crystal structure and reviews the optical absorptions by electrons and lattice vibrations in solids. It summarizes Light scattering effects like Raman and Brillouin scatterings, which offer the complementary information to the absorption spectra in solids. The energy absorbed by the medium works to excite the atoms, ions, or electrons depending on the photon energy and also to excite elementary excitations or collective excitations like exciton, polariton, plasmon, pho-non, magnon, and so forth in solids. In the optical measurements, typically, using a spectrometer, the intensity of the electromagnetic wave is usually observed after some temporal average depending on the measuring system. In inhomogeneous crystals, both the dielectric constant and conductivity are tensors and depend on the direction of the crystal lattice.
