High-order above-threshold ionization by a few-cycle laser pulse in the presence of a terahertz pulse

Abstract

In our recent letter [D. B. Milo\ifmmode \check{s}\else \v{s}\fi{}evi\ifmmode \acute{c}\else \'{c}\fi{}, Opt. Lett. 47, 1669 (2022)] we proposed to apply a terahertz pulse to modify the ionization process induced by a strong midinfrared laser pulse. If the vector potentials of the laser and THz fields are comparable, the yield and the maximum energy of the photoelectrons emitted in the strong-field ionization process can be significantly increased. The time delay $\mathrm{\ensuremath{\Delta}}t$ between the THz pulse and the laser pulse is used as a control parameter. In the present work we further analyze this process, with particular emphasis on high-order above-threshold ionization (HATI). Using our improved strong-field approximation we calculate the ionization amplitude by numerical integration over the electron's ionization and travel times. We apply the quantum-orbit theory and the modified saddle-point method to provide a physical explanation. In addition to the previously considered ionization of Cs atoms by a 3100-nm midinfrared laser, we now show that qualitatively similar results can be obtained for HATI of Ar atoms by a much stronger 800-nm laser pulse. Furthermore, for HATI of Ar by a 3100-nm laser, assisted by a THz pulse of a few-MV/cm strength, we show that the photoelectron energy can be in the keV region.