Improvement of High-Order Harmonic Generation Via Controlling Multiple Acceleration-Recombination Process

The multiple acceleration-recombination process in high-order harmonic generation (HHG) has been investigated and discussed. Generally, the HHG can be explained through the ionization-acceleration- recombination process in each half-cycle waveform of the laser field. In this article, through the waveform control via the two-colour frequency-chirping laser field, the multiple acceleration-recombination process of the free electron in a specific "W" waveform structure can be found, which will lead to the larger emitted photon energies. Moreover, with the optimization of this "W" waveform by changing the frequency chirps, the carrier envelope phases, and the delay time, not only the efficiency of HHG can be enhanced compared with that from the original chirp-free pulse, but also the larger harmonic cutoff can be obtained. Further, with the assistance of the unipolar pulses, the cutoff and the efficiency of HHG can be further improved, showing a water window spectral continuum with the intensity enhancement of 66 times. As a result, by properly superposing some selected harmonics on the spectral continuum, three intense 38-as pulses in the water window region can be obtained.