Isolated sub-50 attosecond pulse generation driven by tricolor multi-cycle pulses

We theoretically investigate the high-order harmonic generation (HHG) driven by laser pulses with tri-color carrier wave in the multi-cycle regime. Through the modulation of the carrier wave, the peak of the return kinetic energy of the electron near the pulse center extends dramatically and the other peaks are suppressed by the envelope. Thus, a very broad continuum spectrum appears in the HHG. Moreover, due to the propagation effect, the long path of the electron for the continuum spectrum is eliminated effectively. Hence, the continuum spectrum is well-phase locked, from which an isolated sub-50 attosecond pulse could be obtained even for the driver pulse with duration of 30 fs.