Shaping extreme-ultraviolet attosecond pulses by tuning the minimum in high-order harmonic generation with two gas jets

We propose to shape attosecond pulses in the extreme ultraviolet (xuv) by creating and tuning a spectral minimum in the high-order harmonic generation (HHG) spectrum by using two gas jets. We show that the HHG spectrum is greatly modulated by moving the first gas jet upstream from the laser focus while the second gas jet is located at the laser focus and its pressure is low, thus the xuv attosecond pulse can be effectively shaped. This does not depend much on the profile of the gas density. Phase matching of HHG is determined by the interplay of the geometric phase and the single-atom induced dipole phase when the pressure of the first gas jet is low, while phase matching is mostly caused by reshaping of the driving laser in the first gas jet when it has the high pressure. We also show that the HHG spectrum cannot be modified much when pressures of both gas jets are high. This work provides an alternative approach to shape xuv attosecond pulses based on the available technique for generation of HHG.