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测量了线偏振飞秒激光脉冲在空气中成丝产生的氮荧光发射的空间分布. 通过改变激光的偏振方向研究成丝过程中氮荧光发射的径向角分布, 发现
${\rm{N}}_{{2}}^{{ + }}$ 荧光发射在垂直于激光偏振方向上更强, 而在平行于激光偏振方向上较弱;${{\rm{N}}_{{2}}}$ 荧光发射在所有方向上具有近乎相同的强度. 原子和分子的激发、电离等动力学过程受激光强度的影响. 在飞秒激光成丝过程中沿着激光传播方向, 强度呈现先增强后减弱的分布, 从而影响这些过程的产物的空间分布及其荧光发射的空间分布. 沿着激光传播方向, 发现${{\rm{N}}_{{2}}}$ 荧光先于${\rm{N}}_{{2}}^{{ + }}$ 荧光出现且在${\rm{N}}_{{2}}^{{ + }}$ 荧光消失之后消失. 激光强度分布和激光偏振方向均会影响氮荧光的空间分布. 基于实验分析, 在短焦距情况下, 系间窜越过程能很好的解释${{\rm{N}}_{{2}}}{{(}}{{\rm{C}}^{{3}}}\Pi _{\rm{u}}^{{ + }})$ 的形成, 这项研究有助于理解飞秒激光成丝过程中氮荧光发射的产生机制.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.[1] Xu H, Cheng Y, Chin S L, Sun H B 2015 Laser Photonics Rev. 9 275
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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
Fig. 1. Schematic diagram of experimental setup for measuring the spatial distribution of nitrogen fluorescence during femtosecond laser filament formation. A: Aperture. H: half-wave plate. L: focusing lens. F: optical fiber.
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
Fig. 2. Radial angular distribution of nitrogen fluorescence when pulse energy is (a) 2.00, (b) 2.63 and (c) 3.00 mJ.
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
Fig. 3. (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.
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Fig. 4. 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. -
[1] Xu H, Cheng Y, Chin S L, Sun H B 2015 Laser Photonics Rev. 9 275
Google Scholar
[2] Dicaire I, Jukna V, Praz C, Milián C, Summerer L, Couairon A 2016 Laser Photonics Rev. 10 481
Google Scholar
[3] Lange H R, Chiron A, Ripoche J F, Mysyrowicz A, Breger P, Agostini P 1998 Phys. Rev. Lett. 81 1611
Google Scholar
[4] Kasparian J, Sauerbrey R, Chin S L 2000 Appl. Phys. B 71 877
Google Scholar
[5] Li S, Chen A, Jiang Y, Jin M 2018 Opt. Commun. 426 105
Google Scholar
[6] Chin S L, Xu H L, Luo Q, Théberge F, Liu W, Daigle J F, Kamali Y, Simard P T, Bernhardt J, Hosseini S A, Sharifi M, Méjean G, Azarm A, Marceau C, Kosareva O, Kandidov V P, Aközbek N, Becker A, Roy G, Mathieu P, Simard J R, Châteauneuf M, Dubois J 2009 Appl. Phys. B 95 1
Google Scholar
[7] Xu S, Sun X, Zeng B, Chu W, Zhao J, Liu W, Cheng Y, Xu Z, Chin S L 2012 Opt. Express 20 299
Google Scholar
[8] Sun X, Xu S, Zhao J, Liu W, Cheng Y, Xu Z, Chin S L, Mu G 2012 Opt. Express 20 4790
Google Scholar
[9] Hosseini S A, Luo Q, Ferland B, Liu W, Aközbek N, Roy G, Chin S L 2003 Appl. Phys. B 77 697
Google Scholar
[10] Daigle J F, Jaroń-Becker A, Hosseini S, Wang T J, Kamali Y, Roy G, Becker A, Chin S L 2010 Phys. Rev. A 82 023405
Google Scholar
[11] Lei M, Wu C, Liang Q, Zhang A, Li Y, Cheng Q, Wang S, Yang H, Gong Q, Jiang H 2017 J. Phys. B: At. Mol. Opt. Phys. 50 145101
Google Scholar
[12] Ran P, Li G, Liu T, Hou H, Luo S 2019 Opt. Express 27 19177
Google Scholar
[13] Wang P, Xu S, Li D, Yang H, Jiang H, Gong Q, Wu C 2014 Phys. Rev. A 90 033407
Google Scholar
[14] 朱竹青, 王晓雷 2011 物理学报 60 085205
Google Scholar
Zhu Z Q, Wang X L 2011 Acta Phys. Sin. 60 085205
Google Scholar
[15] Wu J, Wu Z, Chen T, Zhang H, Zhang Y, Zhang Y, Lin S, Cai X, Chen A, Jiang Y, Li S, Jin M 2020 Opt. Laser Technol. 131 106417
Google Scholar
[16] Mitryukovskiy S, Liu Y, Ding P, Houard A, Couairon A, Mysyrowicz A 2015 Phys. Rev. Lett. 114 063003
Google Scholar
[17] Xu H L, Azarm A, Bernhardt J, Kamali Y, Chin S L 2009 Chem. Phys. 360 171
Google Scholar
[18] Arnold B R, Roberson S D, Pellegrino P M 2012 Chem. Phys. 405 9
Google Scholar
[19] Li S, Jiang Y, Chen A, He L, Liu D, Jin M 2017 Phys. Plasmas 24 033111
Google Scholar
[20] Becker A, Bandrauk A D, Chin S L 2001 Chem. Phys. Lett. 343 345
Google Scholar
[21] Li S, Sui L, Chen A, Jiang Y, Liu D, Shi Z, Jin M 2016 Phys. Plasmas 23 023102
Google Scholar
[22] Su Q, Sun L, Chu C, Zhang Z, Zhang N, Lin L, Zeng Z, Kosareva O, Liu W, Chin S L 2020 J. Phys. Chem. Lett. 11 730
Google Scholar
[23] Pavičić D, Lee K F, Rayner D M, Corkum P B, Villeneuve D M 2007 Phys. Rev. Lett. 98 243001
Google Scholar
[24] 姚云华, 卢晨晖, 徐淑武, 丁晶新, 贾天卿, 张诗按, 孙真荣 2014 物理学报 63 184201
Google Scholar
Yao Y H, Lu C H, Xu S W, Ding J X, Jia T Q, Zhang S A, Sun Z R 2014 Acta Phys. Sin. 63 184201
Google Scholar
[25] Itikawa Y 2006 J. Phys. Chem. Ref. Data 35 31
Google Scholar
[26] Liu Y, Ding P, Lambert G, Houard A, Tikhonchuk V, Mysyrowicz A 2015 Phys. Rev. Lett. 115 133203
Google Scholar
[27] Chin S L, Xu H L, Cheng Y, Xu Z, Yamanouchi K 2013 Chin. Opt. Lett. 11 013201
Google Scholar
[28] Zhang Y, Lötstedt E, Yamanouchi K 2017 J. Phys. B: At. Mol. Opt. Phys. 50 185603
Google Scholar
[29] Xu H L, Lötstedt E, Iwasaki A, Yamanouchi K 2015 Nat. Commun. 6 8347
Google Scholar
[30] Ando T, Lötstedt E, Iwasaki A, Li H, Fu Y, Wang S, Xu H L, Yamanouchi K 2019 Phys. Rev. Lett. 123 203201
Google Scholar
[31] Yao J, Jiang S, Chu W, Zeng B, Wu C, Lu R, Li Z, Xie H, Li G, Yu C, Wang Z H, Jiang H, Gong Q, Cheng Y 2016 Phys. Rev. Lett. 116 143007
Google Scholar
[32] Bernhardt J, Liu W, Théberge F, Xu H L, Daigle J F, Châteauneuf M, Dubois J, Chin S L 2008 Opt. Commun. 281 1268
Google Scholar
[33] Plenge J, Wirsing A, Raschpichler C, Meyer M, Rühl E 2009 J. Chem. Phys. 130 244313
Google Scholar
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