Two-color resonant filamentation in gases

In this paper, it is shown that two-photon resonance involving a fundamental field and one of its odd harmonic strongly influences the filamentation process, i.e., the nonlinear propagation of an ultrashort and ultraintense laser field. This particular situation happens, for instance, when a 400 nm fundamental field propagates together with its third harmonic in krypton. Using three-dimensional ab initio calculations, the optical response of krypton is evaluated and the underlying nonlinear refractive indices are extracted. It is found that the resonance also exacerbates higher-order nonlinear processes. Injecting the retrieved higher-order Kerr indices in a nonlinear propagation solver, it is found that the resonance leads to an enhanced defocusing cross-phase modulation that strongly participates to the filament stabilization. This work sheds a light on the mechanism of filamentation, in particular, in the ultraviolet range, where two-color two-photon resonances are expected to occur in many atomic gases.