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Measurement of harmonics in a underdense CO₂ laser produced plasma Zhu, Yueqiang


Second harmonic(2ω₀) and three halves harmonic(3/2ω₀) are studied in the interaction of CO₂ laser of frequency ω₀, with an underdense plasma. It is shown theoretically that filamentation can lead to sizable second harmonic generation. The 2ω₀-radiation is emitted into a forward directed cone, its angle determined by the wavelength of the fundamental radiation in the plasma filament. Experimental observations confirm the theoretical predications. Aside from the forward directed emission cone, non negligable 2ω₀ power is detected over a broad angular range in the backward direction showing a broadened red shifted spectrum. The dependence of the backward emitted 2ω₀-radiation on the plasma density, and the red shifted spectrum strongly support the argument that the backward emitted 2ω₀-radiation is related to the SBS(Stimulated Brillouin scattering) instability. Based on the wave vector matching conditions, the angular distribution of 3/2ω₀-radiation is predicted. The experimental results confirm the prediction. It is shown experimentally that the fusion of three plasma waves produced in the TPD(Two Plasma Decay) instability dominates the generation of 3/2ω₀-radiation when the incident beam energy is high.

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