Generation of water window single attosecond pulse from multi-cycle mid-infrared laser field with chirp gating modulation

A potential scheme to produce the water window single attosecond pulses (SAPs) from the multi-cycle mid-infrared (MIR) laser field has been investigated with the help of the chirp gating modulation. It is found that with the introduction of the laser chirp (i.e. up-chirp or down-chirp modulation), the extension of the harmonic cutoff can be achieved and the single harmonic radiation peak (HRP) can be selected during the harmonic emission process. Moreover, the chirp gating modulation on the high-order harmonic generation (HHG) is sensitive to the few-cycle pulse duration, but it is not very sensitive to the multi-cycle pulse duration. Moreover, a larger extension of the harmonic cutoff covering the whole water window region can be found with the help of the multi-cycle down-chirp modulation. Further, by properly choosing the coherent superposition of the ground state and the high Rydberg state as the initial state, the efficiency of HHG can be enhanced by 5 orders of magnitude. Finally, a series of high-intensity sub-40as pulses covering the water window region can be obtained.