Abstract：The authors observed the spectrum of third-order harmonic (TH) emission of the plasma channel in atmosphere by focused ultra-short laser pulses under various conditions. The authors used pulsed Ti: sapphire chirped pulse amplification (CPA) femtosecond laser system, with the central wavelength at 795 nm, repetition rate of 10 Hz, pulse duration of 30 fs and the pulse energy of 12 mJ, focused by a concave mirror with the focal length of 0.5 m, which can generate about 1013 W·cm－2 of power intensity. Under this condition, the dynamic balance between nonlinear Kerr self-focusing and plasma defocusing can support a long plasma channel in the interaction of the high intense laser pulses and gaseous media, and the interaction length between the laser pulse and air is greatly elongated, which is helpful to generating third-order harmonic emission. The full width at half maximum (FWHM) of the generated third-order harmonic spectrum is 15 nm with the central wavelength at 265 nm in the forward direction. The spectra of third-order harmonic emission red shift when the laser pulse is positive chirped. On the contrary, the spectra of third-order harmonic emission blue shift when the laser pulse is negative chirped. Proper dispersion can increase the intensity of third-order harmonic spectrum peak and sharped the spectrum peak. With the group velocity dispersion (GVD) of the pulses equal to +1.3×105 fs2, the peak of third-order harmonic spectrum red shifts and the group velocity dispersion of laser pulses equals to zero fs2. The experiment shows that the rising wing of pulses can obtain higher third-order harmonic conversion efficiency than the falling edge of the pulses. In addition, acoustic-optic programmable dispersive filter (AOPDF) in the laser system can control the spectrum shape of the laser. Changing the hole position of acoustic-optic programmable dispersive filter can also shift the spectrum shape of third-order harmonic emission in a certain range. Studies on spectral characteristics of the third-order harmonic emission in plasma channels at atmosphere are very useful to developing the technology of tuning the radiation in generation of ultraviolet or even soft X-ray domain.
李海宁,张丽平,吴洪,李贤,丁良恩*. 空气中激光等离子体通道的三次谐波光谱特性研究[J]. 光谱学与光谱分析, 2008, 28(06): 1201-1204.
LI Hai-ning,ZHANG Li-ping,WU Hong,LI Xian,DING Liang-en*. Study on Spectral Characteristics of Third-Order Harmonic Emission of Plasma Channels in Atmosphere. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2008, 28(06): 1201-1204.
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