High-Order Harmonic Generation by Atoms in a Two-Color Laser Field: Phase Control of Recombination Radiation Spectrum and Duration

摘要

The feasibility of phase control over above-threshold tunnel ionization and subsequent recombination emission in two-frequency laser fields is studied. It is shown that, in such fields, we can control the instants of ionization t_0 (within optical cycle T) and recombination t_k. The conditions that minimize the characteristic times δt_0 T and δt_k T, within which effective ionization and recombination occur, were found. Phase control allows recombination radiation to be generated with the selection of a narrow spectral range, while additional high-frequency "background illumination" sets up high harmonic "amplification" conditions. It was shown that special two-frequency pumping with elliptically polarized radiation can generate coherent electromagnetic pulses of attosecond width. The width of the pulses decreases as the intensity of pumping increases and can reach subattosecond values. Experimental generation of such pulses may lead to a breakthrough in the development of new methods for femto- and attosecond diagnostics of fast processes.