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.

We present a study of the possibility to significantly enhance the efficiency of high-order harmonic generation (HHG) using few-cycle optical waveforms obtained by superposing two laser pulses of different color delayed optimally relative to each other. Special attention is paid to the dynamics of the depopulation of atomic states, which, one the one hand, promotes electrons to the continuum to take part in the high-energy photon emission, but, on the other hand, depletes the nonlinear medium. The use of the waveforms proposed here gives extra flexibility to control both the bound-state depopulation and the electron acceleration in the continuum. We demonstrate that the approach proposed here allows to increase by up to order of magnitude the efficiency of optical frequency conversion into sub-keV or few-keV photon energy ranges. High efficiency of HHG in optimal conditions is explained by the peculiarities of the photoelectron dynamics, which are in this case characterized by a combination of high-probability ejection of the electron responsible for the highest-order harmonic production and its subsequent strong acceleration accompanied by a relatively low probability of the bound-state depletion during the time interval between ionization and recollision.

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