Waveform control in generations of intense water window attosecond pulses via multi-color combined field

The waveform control in the improvements of high-order harmonic generation (HHG) spectra and attosecond pulse signals driven by the two-color and three-color combined fields has been theoretically investigated. (a) The results show that by properly controlling the omega-2 omega two-color laser beam (including the modulations of chirps, carrier envelope phases and delay time), either the harmonic cutoff can be extended, showing a water window spectral continuum, or the selective enhancement of the single-order and two-order harmonics can be found. Further, with the introduction of a third controlling field, the efficiency of spectral continuum can be enhanced by two orders of magnitude compared with that from the two-color field. Moreover, the enhancement of HHG is not very sensitive to the frequency of the third field (i.e., the frequency of the third field is chosen to be 3 omega, 4 omega and 6 omega). Thus, some water window attosecond pulses with the durations of 60 as can be obtained. (b) Furthermore, the harmonic cutoff can be further extended when using a half-cycle controlling pulse or introducing the inhomogeneous effect of the laser field. Moreover, the efficiency of HHG can be further improved when the initial state is prepared as the superposition state of the ground state and some excited state of He atom. Consequently, a much broader spectral continuum with an intensity enhancement of another two orders of magnitude can be found. Finally, through the Fourier transformation of some spectral continuum, the intense water window attosecond pulses with the durations of 60 as can be produced.