Coherent control of high-order harmonic generation by phase jump pulses.

We theoretically investigate the high-order harmonic generation driven by laser pulses with a π-phase jump. The cutoff of high-order harmonic spectrum extends dramatically due to the phase jump which enlarges the asymmetry of the laser field. We find that the intensity and the coherence of the continuum can be controlled by the jump time. By selecting proper frequency of the continuum spectrum, a sub-100-attosecond pulse can be generated without any phase compensation.