Ionization-induced high-order nonlinear susceptibility

The nonlinear susceptibility of atomic hydrogen interacting with an intense laser field has been investigated using a numerical solution of the time-dependent Schr\"odinger equation. It was shown that the behavior of the nonlinear susceptibility strongly depends on the intensity; it increases linearly in the low-intensity regime (Kerr regime), then saturates at a higher intensity. Furthermore, it was also shown that the nonlinear susceptibility is substantially induced by the ionization, whose magnitude is simply related to the density of the ionized electrons. By analyzing the calculated refractive index, it was found that the refractive index induced by ionization is 7--20 % smaller than that derived from the plasma frequency.