Normal Electromagnetic Waves and the Propagation of Light through Crystals

Abstract

Abstract The conditions under which monochromatic light propagates through a crystal in the region of exciton absorption frequencies are investigated. It is shown that the plane electromagnetic wave passing through a crystal with a broad exciton band is, in the general case, a superposition of two normal waves with different refractive indices and absorption coefficients. The relative contribution of these waves is defined by the crystal temperature and the frequency of light passing through a crystal. At temperatures above a critical one only one of the normal waves gives an essential contribution. At temperatures below a critical one both the waves play a comparable role. At frequencies less the resonance one the dominant contribution is made by one of the waves, and at frequencies greater than the resonance one by the other wave. In the resonance region both the waves with different refractive indices make comparable contributions. Therefore, the attenuation of the intensity of a plane wave is different from the exponential one and the concept of a refractive index in this frequency range gives no sense.