基于“泵浦-探测”效应的等离子体通道中三次谐波增强与光谱分析

利用“泵浦-探测”非共线双等离子体通道方法实现三次谐波增强。泵浦光与探测光间的耦合作用能有效改善探测光光丝中的饱和效应，克服光强钳制对谐波强度增加的限制。实验中将两束能量分别为4.4和10.2 mJ、中心波长均为810 nm、脉宽均为60 fs的超短脉冲在空气中非共线聚焦，能各自形成光丝且产生微弱三次谐波。当强光在时域上超前于弱光时，强光会预先成丝形成等离子体通道，对后续弱光产生调制，使探测光产生的三次谐波强度明显增强。实验发现270 nm谐波能量增加的显著区域内频谱宽度出现振荡变化，当两光以27.3 mrad小角度相交于几何焦点前约15 mm，且探测光滞后约55 fs时，获得的能量增长倍率达到近70倍，对应谱宽约为5 nm。

Third Harmonic Enhancement and Spectral Analysis within Plasma Channel Based on Pump-Probe Effect

The experiment realized third harmonic enhancement by using non-collinear dual plasma channels based on pump-probe effect. The coupling action between pump and probe beams can improve saturation effect within filament and overcome the limitation of harmonic enhancement due to intensity clamping. By non-collinearly focusing two ultrafast pulses in air which have respectively a single pulse energy of 4.4 and 10.2 mJ, both with duration of 60fs and central wavelength of 810 nm, dual filaments as well as weak third harmonics can be generated. When strong beam is ahead of weak one, the former induces a plasma channel in advance which can modulate later weak beam and make obvious harmonic increase produced by probe beam. It was found that oscillation variation of spectral bandwidth in the region of harmonic energy increased significantly. When two beams intersect in a small angle of 27.3 mrad about 15 mm before geometric focus and the probe beam lags behind about 55 fs, the energy increase rate nearly reaches 70, and the corresponding spectral bandwidth is approximately 5 nm.