大气中飞秒激光等离子体丝上荧光特性的研究

Fluorescence of the Plasma Filaments Induced by Femtosecond Laser Pulses in Air

本文通过研究超短强激光脉冲在空气中形成的等离子体丝上荧光的特性，提出了一种间接获得等离子体丝上光强分布的方法．实验研究表明，等离子体丝上荧光主要分布在290—430nm的紫外区域，该区域内的线状谱来自于N2的第二正带分子谱线和N2^＋的第一负带离子谱线．同时等离子体丝上荧光强度与入射激光能量呈非线性增长关系，当入射激光能量低于11mJ，荧光强度快速增强表明丝上光强增大，当激光能量进一步增大时，丝上的光强变化缓慢，此时成丝结构逐步由单丝向多丝演化．同时通过研究等离子体丝上光强随光脉冲传输距离的变化规律，进一步验证了等离子体成丝机制是基于光束自聚焦效应和等离子体散焦过程之间的动态平衡的正确性．

An indirect method to detect the gross intensity distribution of the plasma filament is developed by measuring the fluorescence on the filament created by femtosecond laser pulses in air. The fluorescence wavelength is in the range of 290 nm- 430 nm, which is mainly emitted by the second positive band system of N2 and the first negative band system of N2^＋ . The fluorescence intensity is nonlinearly dependent on the laser energy. When the laser energy is lower than 11 mJ, the intensity on the plasma filament enhances quickly with the energy. When the energy further increases, the spatial distribution of the plasma filament becomes from single filament into multiple, which leads to the slow increase in intensity. Besides, the Variation in intensity verse the propagation distance further verify the fact that the underlying mechanism of plasma filaments is based on the dynamic balance between the selffocusing and the plasma defocusing.