通过圆偏振光提高飞秒激光诱导击穿光谱的发射强度 
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Objective Laser-induced breakdown spectroscopy (LIBS) is a novel atomic-emission spectroscopic technique that has been widely used for the elemental analysis of materials. Investigators in the field of LIBS research are actively working to improve the sensitivity of this technique, and many methods have been proposed to enhance its spectral intensity. With the development of chirped-pulse-amplification technology, femtosecond pulse lasers have been introduced into the study of LIBS. Compared with nanosecond lasers, femtosecond lasers have many advantages for LIBS, but it is important to improve their spectral intensity. In general, the output laser beam of a femtosecond laser system is linearly polarized. In a linearly polarized laser field, electrons undergo alternating acceleration and deceleration in each optical period of the laser pulse. However, femtosecond lasers with circular polarization can accelerate electrons continuously, so they attain higher energies. The energies of electrons after irradiation with a circularly polarized femtosecond laser beam are different from those of electrons after irradiation with a linearly polarized femtosecond laser beam. This makes the optical signal from a plasma induced by a circularly polarized femtosecond laser beam different from that induced by a linearly polarized femtosecond laser beam. It is therefore necessary to compare the plasma emission induced by a femtosecond laser under both linear and circular polarizations.
Methods We focused a femtosecond laser beam onto the surface of a brass sample to produce plasmas, and we analyzed the resulting plasma spectra. To compare the effects of linear and circular polarizations on the spectral-emission intensity, we adjusted the polarization of the femtosecond laser beam from linear polarization to circular polarization by using a quarter-wave plate. The sample was placed on a three-dimensional translation table to avoid over-ablation. We collected the light emission from the laser-induced plasma and focused the collected light into an optical fiber, which transmitted it to a spectrometer. The light dispersed by the spectrometer was detected with an intensified charge-coupled device (ICCD). The ICCD was synchronized using an electrical signal from the synchronization-and-delay generator of the femtosecond laser system. Each spectral datum was an average of 50 laser shots. The whole experiment was carried out in air.
Results and Discussions We first compared the time-integrated spectra from femtosecond-laser-induced brass plasmas obtained under circular and linear polarizations. The spectral intensities of Zn (I) and Cu (I) obtained with a circularly polarized laser beam were higher than those obtained with a linearly polarized laser beam, and the spectral intensity increased by about 15%. An important difference between linear polarization and circular polarization in the femtosecond laser field is that the kinetic energies of electrons subject to differently polarized laser fields are different. Under linear polarization, electrons undergo alternating acceleration and deceleration in each optical period of the pulse, so they attain low kinetic energy. In contrast, electrons are always accelerated under circular polarization, so they attain high kinetic energy. Second, we measured the time-resolved spectra of femtosecond LIBS. The time-resolved peak intensities of Cu (I) at 510.55nm and Zn (I) at 472.21nm under circularly polarized laser are higher than that under linearly polarized laser, and the atomic lines in circularly polarized laser-induced plasmas persist longer duration. We thus find that circularly polarized femtosecond pulsed laser irradiation can produce stronger plasmas, which emitting stronger time-resolved spectra during the process of plasma decay. Laser polarization thus plays an important role in femtosecond LIBS. Finally, we calculated the time-resolved electron temperature and density under irradiation with circular and linear polarizations based on the Boltzmann plot and Stark broadening. The changes in the electron temperature and density are similar to the changes in the spectral intensity. Electrons under circularly polarized laser collide with atoms or ions, leading to higher electron temperature and density than those under linearly polarized laser. The higher-energy electrons transfer more energy to the lattice to produce stronger plasmas.
Conclusions We produced plasmas on the surfaces of brass samples by using focused femtosecond pulse laser, and we measured the spectral lines of Zn (I) and Cu (I) emitted from the plasmas under linear and circular polarizations. The results show that the spectral intensities under circularly polarized femtosecond laser were higher than those under linearly polarized femtosecond laser, and the spectral intensities increased by about 15%. We also measured the time-resolved spectra of linearly and circularly polarized femtosecond-laser-induced brass plasmas. For the same laser energy, atomic lines in the plasma produced by the circularly polarized laser persisted longer duration than those produced by the linearly polarized laser. Compared with linear polarization, the electron temperature and density obtained with circular polarization were also higher. This is because a circularly polarized laser can accelerate the electrons continuously, so the kinetic energy of electron produced by a circularly polarized femtosecond laser are higher than that produced by a linearly polarized femtosecond laser. Electrons with higher kinetic energy collide in the plasma to produce higher electron temperature and density, which thus emit a higher spectral intensity. Therefore, we can improve the signal intensity of femtosecond LIBS by adjusting a femtosecond laser from linear polarization to circular polarization. We expect this work to be useful for the study of femtosecond LIBS.
1 引言
激光诱导击穿光谱(LIBS)是一种新颖的原子光谱技术,在材料的元素分析中得到了广泛应用[1-5]。LIBS采用高功率、短脉冲激光烧蚀样品表面,产生激光诱导的等离子体[6],等离子体中的原子和离子能辐射出与样品元素组成相关的特征谱线。LIBS可用于分析任何物质,无论其物理状态是固态、液态还是气态。目前,研究人员正在积极努力地提高LIBS的灵敏度,并已提出了许多增强LIBS光谱强度的方法,例如:空间约束LIBS[7]、火花放电辅助LIBS[8]、纳米粒子增强LIBS[9]、表面增强LIBS[10]、偏振分辨LIBS[11]以及双脉冲LIBS[12]等。LIBS有许多优点,如非常低的背景辐射,更快的连续背景衰减,允许大量脉冲信号累加[13]。
随着啁啾脉冲放大飞秒激光技术的发展[14],飞秒激光系统输出的能量越来越高,飞秒脉冲激光也被引入到LIBS的研究中。与纳秒激光相比,飞秒激光的脉冲具有更短的持续时间,能够实现更好的烧蚀深度控制和更高质量的烧蚀坑[15]。此外,飞秒激光可以提高LIBS元素分析的性能,例如:可以获得更稳定的光谱信号强度,减弱对待测样品的热损伤,实现更精准的三维元素分析[16]。飞秒激光脉冲的持续时间短于电子与晶格之间的弛豫时间[17],从而减弱了激光脉冲与等离子体羽之间的相互作用,导致光谱强度相对较弱[18]。因此,提高飞秒LIBS的强度是非常必要的。
通常情况下,飞秒激光系统输出的是线偏振的飞秒脉冲激光。在线偏振飞秒激光场中,电子在激光脉冲的每一个光周期内都经历着交替的加减速过程。而圆偏振飞秒激光可以连续加速自由电子,电子将获得更高的能量。由Kaganov等[19]在1957年和Anisimov等[20]在1974年提出的双温模型可知,在飞秒激光烧蚀金属过程中,光的吸收主要是由金属内部的类自由电子引起的。因此,圆偏振飞秒激光作用于金属后的自由电子能量将不同于线偏振飞秒激光作用后的自由电子能量,从而使得圆偏振飞秒激光诱导产生的等离子体的光信号可能不同于线偏振飞秒激光诱导产生的等离子体的光信号。
本文将飞秒激光聚焦到黄铜样品表面进行烧蚀产生等离子体,然后对等离子体光谱进行分析。为了对比线偏振和圆偏振飞秒激光对光谱的影响,利用四分之一波片将飞秒激光的偏振由线偏振调整为圆偏振,测量线偏振和圆偏振飞秒激光诱导黄铜等离子体的时间分辨光谱。结果表明,圆偏振飞秒激光产生的光谱强度高于线偏振下的光谱强度。此外,本文还利用获得的谱线计算了描述等离子体过程的两个重要参数:等离子体的电子温度和电子密度,并对计算结果进行了比较。
2 实验装置
飞秒LIBS实验装置示意图如
3 结果与讨论
首先,本文对比了圆偏振和线偏振下飞秒激光诱导黄铜等离子体的时间积分光谱,如
图 2. 圆偏振和线偏振下Zn (I)和Cu (I)的时间积分光谱(激光能量为1.5mJ)。(a)圆偏振下Zn (I)的时间积分光谱;(b)线偏振下Zn (I)的时间积分光谱;(c)圆偏振下Cu (I)的时间积分光谱;(d)线偏振下Cu (I)的时间积分光谱
Fig. 2. Time-integrated spectra of Zn (I) and Cu (I) lines with circular and linear polarizations (laser energy is 1.5mJ). (a) Time-integrated spectrum of Zn (I) line with circular polarization; (b) time-integrated spectrum of Zn (I) line with linear polarization; (c) time-integrated spectrum of Cu (I) line with circular polarization; (d) time-integrated spectrum of Cu (I) line with linear polarization
LIBS是一个动态衰减的过程,在激光产生等离子体之后,LIBS的光谱辐射强度将会发生变化[27-28]。本文将ICCD的门宽设置为25ns,测量了飞秒LIBS的时间分辨光谱。
表 1. Cu (I)光谱线对应的相关参数值
Table 1. Spectral parameters of Cu (I)
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图 3. 圆偏振和线偏振下Cu (I)的时间分辨光谱(激光能量为1.5mJ)。(a)圆偏振;(b)线偏振
Fig. 3. Time-resolved spectra of Cu (I) lines with circular and linear polarizations (laser energy is 1.5mJ). (a) Circular polarization; (b) linear polarization
为了更清晰地比较偏振对时间分辨光谱的影响,本文选择Cu (I) 510.55nm和Zn (I) 472.21nm讨论不同偏振下的时间分辨光谱。
图 4. 圆偏振和线偏振下Cu (I) 510.55nm和Zn (I) 472.21nm的峰值强度随着延迟时间的变化(激光能量为1.5mJ)
Fig. 4. Evolution of peak intensities of Cu (I) 510.55nm and Zn (I) 472.21nm lines with delay time under circular and linear polarizations (laser energy is 1.5mJ)
飞秒激光脉冲的偏振影响等离子体光谱的机理一直是人们研究的热点。先前的研究发现,不同偏振光作用后的等离子体之间存在一些不同。例如,Lemos等[29]通过实验证明了圆偏振飞秒激光产生的等离子体羽的膨胀速度比线偏振光的快。众所周知,飞秒激光脉冲可以通过电离激发等离子体,这就是所谓的阈上电离。电离电子的速度是由电离时激光电场的相位决定的,当电子被电离成自由电子时,电子获得的能量与电场的相位有关。Corkum等[30]指出,通过改变电离激光脉冲的偏振和波长可以控制等离子体的电子温度。他们还计算了圆偏振和线偏振下激光诱导等离子体的电子能谱,结果发现,圆偏振激光作用后的能谱更宽,而且圆偏振激光会产生更高的等离子体温度,从而加速了等离子体的膨胀。因此,圆偏振光能使电子获得更高的能量[31-32],在圆偏振条件下等离子体内的电子与原子或离子之间的碰撞更为频繁,从而导致等离子体的密度更大。为了验证上述讨论,本文测量了不同偏振激光脉冲在不同激光能量下激发的铜等离子体的光谱强度分布,结果如
假设激光诱导等离子体处于局部热力学平衡和光学薄的状态,采用Boltzmann图法可以很容易计算出等离子体的电子温度,计算公式为
式中:I为强度;λ为波长;k和i分别表示上下能级;Aki为跃迁概率;gk为上能级简并;Ek为上能级能量;kB为Boltzmann常数;Te为等离子体的电子温度;C为常数。光谱参数gk、Aki和Ek都来自NIST数据库,见
选择Cu(I) 510.55nm、Cu(I) 515.32nm和Cu(I) 521.82nm谱线建立用于等离子体电子温度计算的Boltzmann图,因为这三条谱线具有较宽的能级范围,可以大大提高计算等离子体电子温度的准确性[35]。典型的线性Boltzmann图如
图 5. 典型的Boltzmann图。(a)圆偏振,延迟时间为0.4μs;(b)圆偏振,延迟时间为0.5μs
Fig. 5. Typical Boltzmann plots. (a) Circular polarization, delay time of 0.4μs; (b) circular polarization, delay time of 0.5μs
在LIBS中,可以忽略除Stark展宽之外的其他展宽机制,例如Doppler展宽和自然展宽。因此,Stark展宽成为谱线加宽的主要展宽过程。谱线宽度和等离子体中电子的数密度之间的关系为
式中:Δλ1/2为谱线宽度;ω为电子的碰撞参数;Ne为电子的数密度。谱线展宽包括Doppler展宽(Δ
图 6. Cu(I) 510.55 nm 谱线的Stark展宽轮廓。(a)圆偏振,延迟时间为0.4μs;(b)圆偏振,延迟时间为0.5μs
Fig. 6. Stark broadening frofile of Cu(I) 510.55 nm spectral line. (a) Circular polarization, delay time of 0.4μs; (b) circular polarization, delay time of 0.5μs
图 7. 圆偏振和线偏振下电子温度和电子密度随着延迟时间的变化(激光能量为1.5mJ)。(a)电子温度;(b)电子密度
Fig. 7. Evolution of electron temperature and density with delay time under circular and linear polarizations(laser energy is 1.5mJ). (a) Electron temperature; (b) electron density
4 结论
本文通过聚焦飞秒脉冲激光到黄铜样品表面产生等离子体,测量了线偏振和圆偏振下等离子体辐射的Zn (I)和Cu (I)原子谱线。结果表明,圆偏振光下的谱线强度高于线偏振光下的谱线强度,谱线强度大约提高了15%。本文还测量了线偏振和圆偏振飞秒激光诱导黄铜等离子体的时间分辨光谱,在激光能量相同的情况下,圆偏振激光诱导等离子体中原子谱线的持续时间较长;同时,本文利用测量的Cu (I)线获得了时间分辨的等离子体电子温度和电子密度,结果表明,与线偏振相比,圆偏振下等离子体的电子温度和电子密度也较高。这是由于圆偏振激光能不断地加速电子,圆偏振飞秒激光脉冲作用后的电子动能高于线偏振光作用后的电子动能,动能较高的电子在等离子体中碰撞,产生更高温度和密度的等离子体,从而产生更高的光谱强度。因此,可以采用将线偏振激光调整成圆偏振激光的方法提高飞秒LIBS光谱信号的强度。希望本工作能为飞秒LIBS的研究提供有益的参考。
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于丹, 孙艳, 冯志书, 代玉银, 王秋云, 陈安民, 金明星. 通过圆偏振光提高飞秒激光诱导击穿光谱的发射强度[J]. 中国激光, 2021, 48(1): 0111001. Dan Yu, Yan Sun, Zhishu Feng, Yuyin Dai, Qiuyun Wang, Anmin Chen, Mingxing Jin. Improving Emission Intensity of Femtosecond Laser-Induced Breakdown Spectroscopy by Using Circular Polarization[J]. Chinese Journal of Lasers, 2021, 48(1): 0111001.
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