Enhancing high harmonic generation in a short-pulse two-color laser field by controlling the atomic-electron subcycle detachment and acceleration dynamics

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.