Frolov, M. V.; Manakov, N. L.; Sarantseva, T. S.; Emelin, M. Yu.; Ryabikin, M. Yu.; and

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Asymmetric photocurrent produced by ionizing laser pulses in a gas is known to be a highly efficient source of terahertz radiation. We examine the possibility of exploiting the asymmetry of the medium itself, rather than the properties of the laser field acting on it, to facilitate the generation of directional photocurrents. We show that the magnitude of directional current and the efficiency of its excitation in tunneling ionization of asymmetric molecules can be significantly enhanced compared to the case of symmetric systems. The results obtained both in a simple classical model and in quantum-mechanical numerical simulations favor the subcycle asymmetry of the ionization process in combination with the effect of the Coulomb potential on the escaping electron as a mechanism responsible for a high-efficiency generation of residual current in tunneling ionization of oriented asymmetric molecules.

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.