We propose to use a low-frequency radiation in addition to the basic pump to enhance the efficiency of high-order harmonic generation following atom tunnel above-threshold ionization. This enhancement can be achieved without an increase of basic pump intensity and, consequently, without additional saturation of ionization. A static field added to the basic pump modifies photoelectron trajectories in the continuum that leads to a high-frequency extension of harmonic spectrum. This is caused by an increase in the part of recombining photoelectrons, kinetic energy of such electrons, and recombination efficiency (owing to reduced electron wave-packet spreading). An experimental realization of this effect is proposed with use of the radiation of a powerful CO<sub>2</sub> laser and ultrashort basic pump pulses. It is shown that the maximum-generated frequency can be tripled with this technique. Also, a multiplateau structure of the high-frequency region of the harmonic spectrum has been revealed and explained.
© 2000 Optical Society of America
(190.4160) Nonlinear optics : Multiharmonic generation
(190.4180) Nonlinear optics : Multiphoton processes
(270.4180) Quantum optics : Multiphoton processes
(270.6620) Quantum optics : Strong-field processes
Vladimir D. Taranukhin and Nickolay Yu. Shubin, "High-order harmonic generation by atoms with strong high-frequency and low-frequency pumping," J. Opt. Soc. Am. B 17, 1509-1516 (2000)