Abstract
According to the theory of perturbative nonlinear optics, the third harmonic signal generated in argon gas by two-color laser field (ω and 2ω) is contributed by two processes, namely, direct third harmonic generation (DTHG) by the ω beams, four wave mixing (FWM) by ω and 2ω beams. The interference between above two processes also affect the third harmonic signal. In intense laser field, plasma can be generated through the ionization process. In the multiphoton ionization region, the plasma density is estimated by the Perelomov-Popov-Terent’ev (PPT) model where the instantaneous electric field and frequency of laser are taken into account. Under the assumption that susceptibility and wave-vector mismatch depend on the plasma density, we show that plasma plays a significant role in the third harmonic signal. The simulation results are in good agreement with the experimental data in argon by employing the fundamental (1064 nm) and second harmonic (532 nm) fields of an injection-seeded Q-switched Nd:YAG laser. We also show that, phase modulation of the third harmonic signal is not only due to the interference term of DTHG and FWM but also affected by the variation of plasma density in two-color laser field. In another experiment, the two-color enhancement of high harmonic generation and vacuum ultraviolet (VUV) emission in argon can also be explained by the enhancement of ionization rate by the two-color laser field.