Coulomb effects in directional current excitation in the ionization of gas by a two-color laser field

Macroscopic quasi-dc currents produced in gases ionized by intense two-color laser fields are known as an efficient source of highly intense broadband terahertz pulses. Recent experimental and theoretical studies give conflicting results regarding the optimal phase shift between the components of a two-color field to maximize the terahertz energy. To address these contradictions, we are studying the role of the effect of the Coulomb potential on the escaping electron in the formation of the ionization-induced directional photocurrents in a two-color scheme. We demonstrate that due to the Coulomb effects, the optimal phase shift between the fundamental field and its second harmonic strongly depends on the laser intensity. In a wide range of laser intensities, the Coulomb effects are shown to significantly influence the directional current generation. (paper)