Fig. 1. The Keldysh parameter (a) and the ionization rate (b) with different laser intensities

Fig. 2. Two few-cycle laser pulses with durations of 9 fs (a) and 5 fs (b), respectively, with several different phases

Fig. 3. The ionization rate induced by the few-cycle laser pulses with different phases

Fig. 4. The plasma density produced by the few-cycle laser pulses with different phases

Fig. 5. Plasma currents produced by the few-cycle laser pulses with different phases

Fig. 6. The ultra-broadband radiation in the terahertz to mid-infrared range generated by the plasma current, (a) the time waveform, (b) its frequency spectrum

Fig. 7. The phase evolution of electromagnetic pulse depending on the phase of few-cycle laser pulses, (a) the pulse waveforms, (b) the frequency spectra

Fig. 8. The amplitude of electromagnetic pulse as a function of the phase of the few-cycle laser pulse