Dielectric Susceptibility of an Anharmonic Crystal

The dielectric susceptibility of a noncentrosymmetric anharmonic crystal is computed by finding and solving the equations of motion for the appropriate Green's functions. The effect of a static field is also investigated. The main absorption line has an additional shift and broadening when the crystal is in a static electric field. In the absence of a static electric field, the higher order electric dipole moments contribute only to the sideband structure of the absorption spectrum, while in a static electric field both the higher order electric dipole moments and the anharmonic terms contribute to the main absorption peak as well as to the sidebands.