Saturation of nonlinear susceptibility of noble gas atom in intense laser field

We investigate the nonlinear susceptibility of noble gas atom interacting with intense laser field. The solution is obtained by solving the time dependent Schrodinger equation. It is shown that the behavior of nonlinear susceptibility strongly depends on the intensity; it increases almost linearly in low intensity regime, then becomes saturated at higher intensity. Furthermore, at intensity beyond the saturation value, the nonlinear susceptibility is substantially induced by the ionization of the atom, leading to rapid fall of the nonlinear refractive index.