Sub-waveform optimization for producing water window single-order harmonic

Abstract

Through the sub-waveform optimization of the laser field, a potential method to produce the water window single-order harmonic (SOH) has been proposed. First, by properly introducing the chirps of two-color field, the SOHs from 303th order to 616th order can be obtained. Theoretical analyses show that the folding region on the harmonic emission process, caused by the multiple accelerations, is responsible for the enhanced SOH. Moreover, the folding region is dependent on the neighbor two half-cycle profiles. Thus, through further controlling the sub-waveform of the folding region by using the unipolar pulse, the folding region on the harmonic emission process will be extended to the higher photon energy region, including the water window region. Finally, by properly choosing the combinations of chirps and unipolar pulses, the water window SOH from 446th order to 833th order (from 345 eV to 645 eV) can be obtained.