A transient photocurrent model is used to explain terahertz ermission from gas plasma irritated by two-color laser pulses,with one the second harmonic of the other.Taking multiple degrees of ionization into account,the gas ionization process at different laser intensities from 1014 W/cm2 to 1015 W/cm2 is discussed.The results show that when Iω ≥ 6 × 1014 W/cm2,double ionization plays an important role in producing electrons.The corresponding terahertz spectra and waveforms are calculated,showing that increasing laser intensity can broaden the spectra to high frequencies and enhance the terahertz field.A promising method for generating terahertz (THz) waves involves emission from laser induced gas plasmas,which was first introduced hy Hamster et al.[1,2] By focusing laser femtosecond pulses with intensity greater than the thresholl for ionization of the gas molecules,one can obtain significant plasma formation.The ionized electrons will then accelerate by the lapser ponderomotive force,thus an electromagnetic pulse at THz frcqucncies can be produced.Since then,other plasma-based THz generation scheines have been proposed.L(o)ffler et al.,[3,4] applied an exteal dc bias to the plasma region,leading to an approximately one order of magnitude increase in the THz field strength.