Generation of terahertz radiation at optical breakdown of air: The dependence of the optimal phase shift between the components of a two-color laser pulse on their intensity

The generation of terahertz radiation at optical breakdown of air by two-color laser pulses composed of the first and second harmonics has been experimentally studied. A strong dependence of the optimal phase shift between the components of the two-color field on the intensity of a laser pulse has been demonstrated. The experimental data in a wide range of the parameters of laser radiation are in agreement with the theoretical results obtained beyond the strong field approximation. This agreement confirms an important role of Coulomb interactions in the formation of directional photocurrents generating terahertz radiation.