以微擾非線性光學為基礎，雙色雷射(基頻ω,二倍頻2ω)在各向同性介質的氬氣中產生之三階諧波來自於兩種非線性過程:直接三倍頻，四波混頻。此兩種非線性過程的干涉也會同時影響三階諧波訊號的產生。如果在高強度的雷射電場中，氬氣將被游離而形成電漿，電漿的密度在多光子電離的範圍裡能以Perelomov-Popov-Terent’ev (PPT)模型來估算，其密度與雷射的瞬時電場強度及震盪頻率有關。在假設三階非線性極化率和相位失配為電漿密度函數的條件下，我們成功模擬出電漿增強之三階諧波產生。其結果與利用波長為1064 nm 的Nd:YAG雷射以及其二倍頻532 nm在氬氣室中產生的三階諧波之實驗相互吻合。我們也預測了在雙色雷射激發時，三階諧波訊號的相位調控不僅來自於三倍頻與四波混頻的干涉的貢獻，還包括了被合成波型影響的電漿密度。我們也觀察到以雙色雷射激發氬氣產生的高階諧波訊號較用單一波長激發時為強，此結果同樣能利用電漿密度因雙色激發的提升解釋。

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.

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