Terahertz radiation by two-color lasers due to the field ionization of gases

Terahertz (THz) radiation via the two-color laser scheme is investigated theoretically and numerically when the second laser is at different harmonic orders of the main laser. It is found that THz radiation can be generated only when the second laser is an even harmonic and the THz field strength does not show a simple scaling with the two laser amplitudes, suggesting that this is different from normal optical rectification or frequency mixing processes. The THz strength generally tends to decrease with increase of the even harmonic order. The strength is also sensitive to the carrier envelope (CE) phases of both the laser pulses even if their durations are quite long. With different CE phases, the strength can be proportional to either the sine or cosine of the relative phase displacement of the two lasers. For a given gas target except hydrogen, it increases with the laser amplitude between several saturation plateaus, each corresponding to a different ionization level. For a given main laser intensity, there is an optimized intensity of the second laser for the strongest THz radiation. DOI: 10.1103/PhysRevE.87.033108