空气中激光等离子体通道的三次谐波光谱特性研究

Study on Spectral Characteristics of Third-Order Harmonic Emission of Plasma Channels in Atmosphere

对不同条件下强激光在空气中形成等离子体通道的三次谐波光谱特性进行了研究。单脉冲能量12mJ,脉宽30 fs,重复频率10 Hz,中心波长795 nm的飞秒激光脉冲经0.5 m焦距的凹面镜聚焦,在空气中形成了等离子体通道,并在前向观测到谱线半峰全宽(FWHM)为15 nm的三次谐波。随着脉冲啁啾的变化,三次谐波的光谱出现红移或兰移,当激光脉冲附带+1.3×105fs2的二阶色散时,三次谐波谱线红移且谱峰强度增长了两倍。同时,通过改变可编程声光色散滤波器(AOPDF)光谱调制的位置(Hole position),三次谐波的光谱也发生频移。

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 10t3 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 × 10^5 fs^2 , the peak of third-order harmonic spectrum red shifts and the group velocity dispersion of laser pulses equals to zero fs^2. 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.