1.Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China 2.Jilin Provincial Key Laboratory of Applied Atomic and Molecular Spectroscopy, Jilin University, Changchun 130012, China 3.Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China
Fund Project:Project supported by the National Basic Research Program (Grant No. 2019YFA0307701), the National Natural Science Foundation of China (Grant Nos. 11704145, 11974138, 11674128, 11504129, 11674124), and the Scientific Research Project of 13th Five-year Plan of the Education Department of Jilin Province, China (Grant No. JJKH20190181KJ)
Received Date:14 October 2020
Accepted Date:02 February 2021
Available Online:22 June 2021
Published Online:05 July 2021
Abstract:As a major component in the air, nitrogen emits fluorescence when it interacts with intensive laser field. The fluorescence comes from the first negative band system (${{\rm{B}}^{{2}}}\Sigma _{\rm{u}}^{{ + }} \to {{\rm{X}}^{{2}}}\Sigma _{\rm{g}}^{{ + }}$ transition) of ${\rm{N}}_{{2}}^{{ + }}$ and the second positive band system (${{\rm{C}}^{{3}}}\Pi _{\rm{u}}^{{ + }} \to {{\rm{B}}^{{3}}}\Pi _{\rm{g}}^{{ + }}$ transition) of ${{\rm{N}}_{{2}}}$. Under the action of high-intensity femtosecond laser, ${{\rm{N}}_{{2}}}$ can be directly photo-ionized into ${\rm{N}}_{{2}}^{{ + }}{{(}}{{\rm{B}}^{{2}}}\Sigma _{\rm{u}}^{{ + }})$, which results in fluorescence emission of ${\rm{N}}_{{2}}^{{ + }}$. In the process of femtosecond laser filament formation, the dynamic processes such as ionization and excitation of nitrogen molecules are affected by the laser intensity distribution and laser polarization direction. The products show different distributions in the propagation direction and radial space, which, in turn, affects its light emission. Therefore, it is necessary to further ascertain its generation mechanism through the spatial distribution of nitrogen fluorescence. In this experiment, the spatial distribution of the nitrogen fluorescence emission generated by linearly polarized femtosecond laser pulse filaments in air is measured. By changing the polarization direction of the laser to study the distribution of nitrogen fluorescence in the radial plane, it is found that the fluorescence emission of ${\rm{N}}_2^ + $ is more intense in the direction perpendicular to the laser polarization, while it is weaker in the direction parallel to the laser polarization. The nitrogen fluorescence emission has the same intensity in all directions. The ionization probability of a linear molecule depends on the angle between the laser polarization direction and the molecular axis, which is maximum (minimum) when the angle is ${{{0}}^{\rm{o}}}$(${{9}}{{{0}}^{\rm{o}}}$). The ${{\rm{N}}_{{2}}}$ gas is more likely to be ionized in the laser polarization direction, the nitrogen molecular ions ${\rm{N}}_{{2}}^{{ + }}$ and electrons are separated in the direction parallel to the laser polarization. Therefore, more ions (${\rm{N}}_{{2}}^{{ + }}$) are generated in the direction parallel to the laser polarization, and the fluorescence emission of ${\rm{N}}_{{2}}^{{ + }}$ is more intense. Along the propagation direction of the laser, it is found that the fluorescence of ${{\rm{N}}_{{2}}}$ appears before the fluorescence of ${\rm{N}}_2^ + $ and disappears after the fluorescence of ${\rm{N}}_{{2}}^{{ + }}$ has vanished. This is due to the fact that ${{\rm{N}}_{{2}}}$ can be ionized into ${\rm{N}}_{{2}}^{{ + }}{{(}}{{\rm{B}}^{{2}}}\Sigma_{\rm{u}}^{{ + }})$ at the position of high enough laser intensity, thus emitting fluorescence of ${\rm{N}}_2^ + $. However, the laser energy is not enough to ionize nitrogen at the beginning and end of laser transmission, but it can generate ${\rm{N}}_2^ * $, which emits nitrogen fluorescence through the process of intersystem crossing ${\rm{N}}_2^*\xrightarrow{{{\rm{ISC}}}}{{\rm{N}}_2}({{\rm{C}}^3}\Pi _{\rm{u}}^ + )$. The spatial distribution of nitrogen fluorescence emission during femtosecond laser filament formation shows that in the case of short focal length, the intersystem crossing scheme can explain the formation of ${{\rm{N}}_{{2}}}{{(}}{{\rm{C}}^{{3}}}\Pi _{\rm{u}}^{{ + }})$. This research is helpful in understanding the mechanism of nitrogen fluorescence emission. Keywords:femtosecond laser pulse/ femtosecond laser filament/ nitrogen molecule/ fluorescence
3.结果与讨论为了清楚地研究氮荧光的径向发射行为, 沿着激光传播方向移动平移台, 选取${{\rm{N}}_{\rm{2}}}$荧光信号最强的位置对氮荧光的径向角分布进行了测量, 如图2所示. 图 2 当脉冲能量为 (a) 2.00, (b) 2.63和(c) 3.00 mJ时氮荧光的径向角分布 Figure2. Radial angular distribution of nitrogen fluorescence when pulse energy is (a) 2.00, (b) 2.63 and (c) 3.00 mJ.
图2给出了激光能量为2.00, 2.63和3.00 mJ时, 对应的氮荧光径向角分布图. 其中337和357 nm谱线分别对应于${{\rm{N}}_{\rm{2}}}$分子的${{\rm{C}}^{\rm{3}}}\Pi _{\rm{u}}^{{ + }}(v = 0) \to $$ {{\rm{B}}^{\rm{3}}}\Pi _{\rm{g}}^{{ + }}(v' = 0, 1)$跃迁, 从图中可以看出${{\rm{N}}_{\rm{2}}}$荧光在各个方向的发射强度几乎一样, 来自于${{\rm{N}}_{\rm{2}}}$的荧光的径向角分布呈圆形, 并且其径向角分布不受激光能量改变的影响. 391和428 nm谱线分别对应于${\rm{N}}_{\rm{2}}^{{ + }}$的${{\rm{B}}^{\rm{2}}}\Sigma _{\rm{u}}^{{ + }}(v = 0) \to {{\rm{X}}^{\rm{2}}}\Sigma _{\rm{g}}^{{ + }}(v' = 0, 1)$跃迁, 可以看出${\rm{N}}_{\rm{2}}^{{ + }}$的荧光发射在θ = 0o时(即激光偏振方向与z轴平行)比θ = 90o时(激光偏振方向与z轴垂直)强度更强, 来自于${\rm{N}}_{\rm{2}}^{{ + }}$的荧光的径向角分布呈椭圆形, 而且激光能量的大小并不影响其径向角分布特点. 这表明在激光偏振方向与z轴平行和垂直时${\rm{N}}_{\rm{2}}^{{ + }}$荧光发射表现出了较为明显的差异, 这与我们先前的发现一致[21], Su等[22]也发现了这一现象. 为了更清晰地反映这种差异, 我们给出了激光能量为3.00 mJ, 激光偏振方向平行和垂直于z轴时的氮荧光光谱, 如图3(a)所示. 图 3 (a)激光偏振方向平行(黑色实线)和垂直(红色虚线)于z轴时的氮荧光光谱; 激光偏振方向平行和垂直于z轴时(b) 337, (c) 357, (d) 391和(e) 428 nm荧光信号随传播距离的变化 Figure3. (a) Nitrogen fluorescence spectrum when the laser polarization direction is parallel (solid black line) and perpendicular (dashed red line) to the z-axis. Variations of the (b) 337, (c) 357, (d) 391 and (e) 428 nm fluorescence signals with the propagation distance when the laser polarization direction is parallel and perpendicular to z-axis.
从图3(a)中可以看出, 来自${{\rm{N}}_{\rm{2}}}$的荧光(337和357 nm谱线等)在激光偏振方向平行和垂直于z轴时发射强度近乎一致. 而对于${\rm{N}}_{\rm{2}}^{{ + }}$的荧光(391和428 nm谱线), 激光偏振方向垂直于z轴时的信号与平行于z轴时的信号之间的强度比约为0.85. 图3(b)和图3(c)表明在传播路径上, 激光偏振方向与z轴平行和垂直时${{\rm{N}}_{\rm{2}}}$荧光信号强度几乎一致, 不受激光偏振方向的影响. 而对${\rm{N}}_{\rm{2}}^{{ + }}$的荧光信号, 其强度始终在激光偏振方向平行于z轴时更强, 如图3(d)和图3(e)所示. ${\rm{N}}_{\rm{2}}^{{ + }}$荧光发射的各向异性可能是由其产生通道造成的. 当线偏振飞秒激光脉冲在空气中成丝时, 强激光与空气分子会发生隧穿/多光子电离和准直. 由旋转波包的拉曼激发产生的分子准直, 其发生的概率高于电离的概率, 这是因为从能量或多光子吸收的角度来看, 拉曼效应是低阶(三阶)非线性效应, 而电离是高阶非线性效应. 因此在激光强度低于电离阈值时, 分子就已经开始旋转, 这就意味着分子将以某种强度“同时”准直并电离, 并具有不同的概率, 而线性分子的电离几率取决于激光偏振方向和分子轴之间的夹角, 夹角为${{\rm{0}}^{\rm{o}}}$时电离概率最大, 夹角为${\rm{9}}{{\rm{0}}^{\rm{o}}}$时最小[22]. 若激光偏振方向固定, 当${\rm{N}}_{\rm{2}}^{{ + }}$转动到分子轴的方向与激光偏振方向一致时, 其荧光发射较强. 在实验中, ICCD的门宽为20 ns, 远大于${\rm{N}}_{\rm{2}}^{{ + }}$的转动周期(8.35 ps), 在这个时间尺度内, ${\rm{N}}_{\rm{2}}^{{ + }}$分子将转动很多圈, 因此观测到的是一个时间平均后的效应: ${\rm{N}}_{\rm{2}}^{{ + }}$荧光在垂直于激光偏振方向上更强. 在实验中使用的是单脉冲, 其目的是产生氮荧光, 准直效应弱. 如果利用另一束激光预先准直${{\rm{N}}_{\rm{2}}}$分子, 则${\rm{N}}_{\rm{2}}^{{ + }}$径向角分布的椭圆率更高[23,24]. 氮分子的电离和激发等动力学过程受激光强度分布和激光偏振方向的影响, 其产物呈现出不同的分布情况, 进而影响其光发射. 为了排除激光能量对等离子体荧光发射在传播方向上分布情况的影响, 给出了不同激光能量下, 激光偏振方向平行和垂直于z轴时${{\rm{N}}_{\rm{2}}}$和${\rm{N}}_{\rm{2}}^{{ + }}$荧光信号沿着传播方向的分布情况, 如图4所示. 图 4 当脉冲能量为2.00, 2.63和3.00 mJ时, 激光偏振方向平行 (a), (b), (c)和垂直(a'), (b'), (c')于z轴时${{\rm{N}}_{\rm{2}}}$和${\rm{N}}_{\rm{2}}^{{ + }}$荧光信号随传播距离的变化 Figure4. Variations of the ${{\rm{N}}_{\rm{2}}}$ and ${\rm{N}}_{\rm{2}}^{{ + }}$ fluorescence signal with the propagation distance when the laser polarization direction is parallel (a), (b), (c) and perpendicular (a'), (b'), (c') to the z-axis and the pulse energy is 2.00, 2.63 and 3.00 mJ.
图 1 测定飞秒激光成丝过程中氮荧光空间分布的实验装置示意图. A: 光阑; H: 半波片; L: 聚焦透镜; F: 光纤
图 2 当脉冲能量为 (a) 2.00, (b) 2.63和(c) 3.00 mJ时氮荧光的径向角分布
图 3 (a)激光偏振方向平行(黑色实线)和垂直(红色虚线)于z轴时的氮荧光光谱; 激光偏振方向平行和垂直于z轴时(b) 337, (c) 357, (d) 391和(e) 428 nm荧光信号随传播距离的变化
图 4 当脉冲能量为2.00, 2.63和3.00 mJ时, 激光偏振方向平行 (a), (b), (c)和垂直(a'), (b'), (c')于z轴时
