Modeling the nonlinear refractive index in atomic gases

Accurate modeling of optical nonlinearities is crucial to describe macroscopic laser propagation in a medium, including sum frequency generation, spectral broadening due to self-phase modulation, various ionization processes and soliton formation. For incident laser light the response of the medium is given by the induced polarization of the microscopic system. The polarization is usually expanded in a Taylor series for the electric field amplitude, which is truncated after the first non-linear term being of third order for isotropic media. A third-order nonlinearity leads to the well-known optical Kerr effect, where the refractive index of the medium becomes intensity dependent via n = n₀ + n₂I. This leads to an inherent problem when modeling laser propagation in two or more spatial dimensions, linked to the formal divergence (n₂ > 0) of the refractive index for increasing intensity.