High-order harmonic generation by the atoms in strong bichromatic fields

It is proposed to use bichromatic laser fields to increase the efficiency of generation of coherent short-wavelength radiation by atoms in the process of above-threshold tunnel ionisation. It is shown in the framework of a semiclassical model that the use of a relatively weak static field, along with the basic pump radiation, results in the increase of the fraction of recombining photoelectrons and in the rise of their kinetic energy at the moment of recombination. It is shown that the experimental realisation of this effect is possible with the use of high-power CO₂ laser radiation (instead of the static field) and ultrashort pulses of the basic pump component with wavelength λ ~ 1 μm. Numerical calculations in the framework of a quantum mechanical model confirm the conclusions of the semiclassical model and show that the addition of long-wavelength radiation enables tripling the maximum generation frequency. Under specific intensity of CO₂ laser radiation, the so-called multi-plateau structure was discovered near the high-frequency edge of the generated radiation spectrum. An interpretation of this effect is given in the framework of a semiclassical model.