Nonlinear refractive index for crystals in terahertz spectral range

We develop a simple analytical model for calculating the vibrational contribution to the nonlinear refractive index n
₂
of a crystal at terahertz frequencies in terms of known crystalline parameters such as the coefficient of thermal expansion, atomic density, and the natural oscillation frequency of the vibrational modes of the crystal lattice. Theoretical methods of analysis and features of self-action of few-cycle terahertz waves in nonlinear media are discussed.