Nonlinear refractive index for crystals at terahertz frequencies (Invited Paper)

The lecture is devoted to the analysis of the non-linearity of the refractive index for crystals at terahertz frequencies. We develop a simple analytical model for calculating the vibrational contribution to the nonlinear refractive index n₂ of a crystal in terms of known crystalline parameters such as the linear refractive index, the coefficient of thermal expansion, atomic density, and the reduced mass and the natural oscillation frequency of the vibrational modes of the crystal lattice. Significant part of the lecture is devoted to the theoretical methods of analysis and features of self-action of few-cycle terahertz waves in various nonlinear media.