Abstract
ABSTRACT Based on the three-step model of high harmonic generation, we propose a scheme to improve the harmonic spectra and to produce the intense water window attosecond pulses. It is found that, firstly, by introducing the single chirp and two chirps modulations, the instantaneous frequency on the falling part of the laser field is decreased. As a result, the electron can obtain larger energy during the acceleration-recombination process, showing a broad harmonic plateau in the water window region. Secondly, by properly adding a third controlling IR or UV pulse, the ionization process can be controlled and a larger ionization probability can be found, which leads to the enhancement of the harmonic intensity by 1 or 2 orders of magnitudes, respectively. Most important, using this scheme, the harmonic plateau is only contributed by the intense single harmonic emission peak, which can support the generation of an intense 36 as pulse.
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