Nonlinearity of the refractive index of doped insulators

Relationships are obtained for the nonlinear resonant polarization of a multilevel system in the field of a monochromatic light wave under noncoherent interaction conditions allowing for anharmonicity of the electron oscillator and the dynamic Stark shift. Estimates are made of the resonant nonlinear refractive index of glasses and crystals activated with Cr³⁺ and Nd³⁺ ions and it is found that this may be of the order of (or greater than) the same index of the insulating base of the active medium.