Femtosecond laser-induced Cu plasma spectra at different laser polarizations and sample temperatures

Laser-induced breakdown spectroscopy (LIBS) is a good technique for detecting and analyzing material elements due to the plasma emission produced by the high-power laser pulse. Currently, a significant topic of LIBS research is improving the emission intensity of LIBS. This study investigated the effect of laser-polarization on femtosecond laser-ablated Cu plasma spectra at different sample temperatures. The measured lines under circularly polarized lasers were higher than those under linearly and elliptically polarized lasers. The enhancement effect was evident at higher Cu temperatures when comparing the plasma spectra that have circular and linear polarizations for different target temperatures. To understand the influence of laser-polarization and sample temperature on signal intensity, we calculated the plasma temperature (PT) and electron density (ED) . The change in PT and ED was consistent with the change in the atomic lines as the laser polarization was being adjusted. When raising the Cu temperature, the PT increased while the ED decreased. Raising the Cu temperature whilst adjusting the laser-polarization is effective for improving the signal of femtosecond LIBS compared to raising the initial sample temperature alone or only changing the laser polarization.     

飞秒激光成丝诱导Cu等离子体的温度和电子密度

研究了飞秒激光成丝诱导铜击穿光谱,利用光发射光谱对产生的铜等离子体光谱强度沿着丝长度进行了测量,获得了在不同样品与聚焦透镜间距离的Cu(I)的强度分布.结果显示,由于强度钳箍效应成丝诱导的光谱在较大的透镜样品间距离范围内有较强的辐射强度.另外,利用玻尔兹曼图和斯塔克展宽计算了整个成丝繁衍距离中Cu等离子体温度和电子密度.     

The multielectron dissociative ionization dynamics of N2 molecule in intense femtosecond laser fields with arbitrary polarization

The field-ionization Coulomb explosion model is extended to investigate the multielectron dissociative ionization process of N₂ molecule irradiated by an intense femtosecond laser field with an arbitrary polarization. The ionization process of N₂ molecule is found to be optimal at the critical internuclear distance R_c=7a.u., which is independent of the laser polarization state, the molecular explosion channel and the angle between the molecular axis and the direction of laser electric field. The kinetic energies of the ion fragments are identical in the cases of linear and circular polarizations at the same incident laser intensity. However,the probability of electron ionization is very sensitive to the above three parameters. At the critical distance R_c=7a.u. the angular dependence of the threshold intensity for the over-the-barrier ionization leads to the geometric alignment of molecules in the case of linear polarization. The threshold intensity in the case of circular polarization is apparently higher than that in the case of linear polarization, which can well explain the significant decrease of ionization in the case of circular polarization. The numerical calculations are compared with the experimental measurements.     

Polarization-dependent supercontinuum generation from light filaments in air

We investigate polarization-dependent properties of the supercontinuum emission generated from filaments produced by intense femtosecond laser pulses propagating through air over a long distance. The conversion efficiency from the 800-nm fundamental to white light is observed to be higher for circular polarization than for linear polarization when the laser intensity exceeds the threshold of the breakdown of air.     

PMMA材料在线偏振和圆偏振光下的超连续光谱特性分析

飞秒激光在透明材料加工过程中会出现超连续光谱现象。在阐述超连续光谱产生的原理的基础上,为了分析PMMA材料在不同偏振光下产生的超连续光谱的阈值,设计了线偏光和圆偏光及不同能量加工PMMA材料的实验方案。利用光谱仪对产生的超连续光谱信号进行采集及处理,分析出不同能量下的线偏振(TE和TM)和圆偏振两种偏振态的超连续光谱的变化规律,并对比了相同能量下线偏振和圆偏振的超连续光谱的区别。实验中采用脉宽160 fs、中心波长为775 nm的飞秒激光,实验结果表明,同一偏振下能量越大,光谱谱宽越宽;通过对比不同能量下的光谱特性,观测出产生超连续光谱的阈值, 线偏振光的阈值为0.46 μJ,圆偏振光的阈值为0.586 μJ;对比相同能量下的线偏振和圆偏振光,线偏振的谱宽比圆偏振的宽。     

激光诱导击穿光谱机理及爆炸物研究

激光诱导击穿光谱(LIBS)技术是基于激光诱导等离子体发光探测物质成分的方法。激光诱导等离子体的发射谱线由两部分组成:一部分是电子-离子复合及自由电子相互作用辐射的宽带连续谱;另一部分是原子和离子发射的特征谱线。对这些谱线进行识别并测量其强度,给出物质成分的定性和定量信息。分别讨论了样品所附着的基质为金属和电介质的两种情况,并对ns和fs级激光脉宽的影响作了介绍。     

样品温度和空间约束两种方法相结合对激光诱导击穿光谱的影响

升高样品温度和采用空间约束能提高激光诱导击穿光谱的信号强度,两种技术的结合可以进一步提高激光诱导击穿光谱的光谱强度.本文在空气环境中研究了升高样品温度和空间约束效应两种方法相结合对激光诱导击穿光谱的影响,测量了激光诱导铝等离子体的时间分辨光谱.实验结果表明:升高样品温度能增加激光诱导击穿光谱的信号强度,高温样品能耦合更...     

Spectroscopic analysis using a hybrid LIBS-Raman system

A novel setup, combining two spectroscopic techniques, laser induced breakdown spectroscopy (LIBS) and Raman spectroscopy in a hybrid unit, is described. The work presented herein is part of a broader project that aims to demonstrate the applicability of the hybrid LIBS-Raman unit as an analytical tool for the investigation of samples and objects of cultural heritage. The system utilizes a nanosecond pulsed Nd:YAG laser (532 nm) for both LIBS and Raman analysis. In the Raman mode, a low intensity beam from the laser probes the sample surface and the scattering signal is collected into a grating spectrograph coupled to an intensified charge-coupled device (ICCD) detector, which records the Raman spectrum. In the LIBS mode a single high intensity pulse from the laser irradiates the sample surface and the time- and spectrally-resolved emission from the resulting laser ablation plume yields the LIBS spectrum. The use of a non-gated CCD detector was found to produce similar quality data (in terms of S/N ratio and fluorescence background) in the Raman mode, while in the LIBS mode spectral features were clearly broader but did not prevent identification of prominent atomic emission lines. Several model pigment samples were examined and the data obtained show the ability of the hybrid unit to record both Raman and LIBS spectra from the same point on the sample, a clear advantage over the use of different analytical setups. PACS 39.30.+w; 82.80.Dx; 82.80.Gk; 52.38.Mf     

High stability breakdown of noble gases with femtosecond laser pulses

In the past, laser-induced breakdown spectroscopy (LIBS) signals have been reported to have a stability independent of the pulse length in solids. In this Letter, we perform the first stability study of femtosecond LIBS in gases (to our best knowledge) and show a significant improvement in signal stability over those achieved with longer pulses. Our study shows that ultrashort-pulse LIBS has an intrinsically higher stability in gas compared to nanosecond-pulse LIBS because of a deterministic ionization process at work in the femtosecond pulse. Relative standard deviations below 1% are demonstrated and are likely only limited by our laser output fluctuations. This enhanced emission stability may open up possibilities for a range of applications, from monitoring rapid gas dynamics to high-quality broadband light sources.     

金纳米与磁场作用下LIBS检测灵敏度改善研究

针对激光诱导击穿光谱技术（LIBS）中等离子体的发射光谱增强问题,提出一种磁场增强LIBS与纳米颗粒增强LIBS（NELIBS）相结合的方法。采用热蒸发法在样品表面沉积一层直径20 nm的金纳米颗粒。利用波长为1 064 nm,最大能量为200 mJ的Nd∶YAG脉冲激光器在室温,一个标准大气压下对纯铜和黄铜进行诱导击穿。调整激光能量为30~110 mJ,分别使用传统LIBS、磁场增强LIBS、NELIBS以及两种方法结合对纯铜进行激光诱导击穿,得到特征谱线(Cu Ⅰ 521.8 nm)的强度增强因子和信噪比,并对其增强机理进行分析。在相同环境下使用四种方式对黄铜和纯铜进行诱导击穿以探测样品中的微量元素。当在样品表面沉淀金纳米颗粒或者将沉淀有金纳米颗粒的样品放在磁场中进行诱导击穿时,发现纯铜样品的光谱中存在Mg元素的特征谱线Mg Ⅱ 279.569 nm,黄铜样品的光谱中存在Si元素的特征谱线 Si Ⅰ 251.611 nm。实验结果表明：单独施加磁场约束或增加纳米金颗粒均可以有效增强等离子体光谱强度,但增强效果弱于两种方法结合,磁场约束对光谱的增强效果弱于NELIBS的增强效果。当结合NELIBS与磁场约束LIBS时,谱线增强因子最高可达14.3(Cu Ⅰ 521.8 nm),相比于磁场增强LIBS和NELIBS,最大增强因子分别提高了28%和59%。四种情况中当激光脉冲能量逐渐增大时,等离子体向外膨胀的强度增大,磁场产生的洛伦兹力束缚等离子的能力相对减弱,同时纳米金颗粒对等离子体发射光谱的增强作用被削弱,谱线强度降低,等离子体的增强因子逐渐减小后趋于稳定。通过NELIBS与磁场约束LIBS结合方式,不仅可以有效提高等离子体的发射谱线强度,改善光谱信号信噪比,而且传统LIBS方法中由于谱线强度低、背景噪声大而无法探测的微量元素可以被探测到,LIBS技术对微量元素的探测能力得到显著提高,微量元素的探测下限变得更低。NELIBS与磁场约束LIBS结合的方法具有更高的灵敏度和准确度,为激光诱导击穿光谱技术的谱线增强方法提供了新的思路,在该领域具有广阔的应用前景。     

Ultraviolet laser filaments for remote laser-induced breakdown spectroscopy (LIBS) analysis: applications in cultural heritage monitoring

We report experiments with subpicosecond UV laser filaments for the remote analysis of samples related to objects of cultural heritage. The classic laser-induced breakdown spectroscopy (LIBS) technique finds new avenues through femtosecond filamentation, and dynamic remote LIBS becomes possible. Advantages such as self-regulated laser intensity deposition on the target suggest that there is significant potential for using UV femtosecond filaments for the remote analysis of sculpture and large monuments.     

普铝中Fe,Si元素的激光诱导击穿光谱测试条件及定量分析研究

为推广LIBS技术在电解铝行业中的应用，充分发挥其快速、免制样、多元素同时检测的优势。利用激光诱导击穿光谱技术首次对铝电解生产得到的普铝中Fe和Si元素进行测试研究，探索了合理的实验参数条件，在合理的实验条件基础上建立定标曲线并对普铝中Fe和Si元素进行定量分析，结合国标GB/T 7999-2015《铝及铝合金光电直读发射光谱分析方法》考察LIBS测试结果的准确性。以Nd∶YAG脉冲激光器基频1 064 nm激光作为光源激发等离子体，采用多通道光栅光谱仪和ICCD检测器检测、记录光谱信息。首先探测了LIBS光谱谱线，并对谱线进行了归属；综合分析，选取AlⅠ 266.04 nm，SiⅠ 288.15 nm与FeⅠ 259.92 nm作为分析谱线用于定量分析研究。分别研究了触发延迟时间、1Q延迟时间、激光器设定电压对光谱信号强度及信噪比SNR的影响。实验结果表明，触发延迟时间4 μs、1Q延迟时间170～190 μs、激光器设定电压560 V对于Si与Fe元素定量测试分析而言是较为合理的实验参数。根据谱线强度与元素浓度的关系，采用内标法建立了定标曲线，Si与Fe元素定标曲线中相关系数分别为0.919 72与0.952 11，其相对标准偏差（RSD）分别为7.25%与6.34%，说明谱线强度与元素浓度具有良好的线性关系，并基于此模型对12个样品进行了定量测试分析。将测试结果与光电直读发射光谱测得的结果进行比对，结果表明，Fe含量的相对误差绝对值在0~17.3%之间，Si含量相对误差绝对值在0~14.3%之间。依照国标GB/T 7999-2015《铝及铝合金光电直读发射光谱分析方法》中规定的实验室之间分析结果相对误差≤17%的规定，12个测试样品中，试样Si含量测试100%符合允许差要求，试样Fe含量测试91.7%符合允许差要求。该实验结果表明，LIBS技术在电解铝普铝Fe和Si元素检测中具有一定的推广利用价值。     

High resolution scanning microanalysis on material surfaces using UV femtosecond laser induced breakdown spectroscopy

Femtosecond lasers together with high resolution optics have given us the ability to achieve submicron ablation spots which can play an important role in specific micromachining applications. Light emitted from the plasma at the sample surface created by a focused femtosecond laser pulse can also be used in laser induced breakdown spectroscopy (LIBS) and allows us to characterize the chemical composition of the target surface with micron-level lateral resolution. The spatial resolution using LIBS has often been defined by measuring the FWHM of the crater size. In this report, we study the application of femtosecond 266 nm laser pulses with very low energies of 10׳s of nanojoules. We have investigated spatial resolution using the detection of thin strips of chromium on silicon substrates and compared the actual width of the chromium versus the experimentally obtained width using LIBS detection. The variation of signal levels for low pulse energies is investigated on chromium surfaces. A spatial resolution of ~1 μm was obtained for detection of chromium from the emission.     

Early stage expansion and time-resolved spectral emission of laser-induced plasma from polymer

In the nanosecond laser ablation regime, absorption of laser energy by the plasma during its early stage expansion critically influences the properties of the plasma and thus its interaction with ambient air. These influences can significantly alter spectral emission of the plasma. For organic samples especially, recombination of the plasma with the ambient air leads to interfering emissions with respect to emissions due to native species evaporated from the sample. Distinguishing interfering emissions due to ambient air represents a critical issue for the application of laser-induced breakdown spectroscopy (LIBS) to the analysis of organic materials. In this paper, we report observations of early stage expansion and interaction with ambient air of the plasma induced on a typical organic sample (nylon) using time-resolved shadowgraph. We compare, in the nanosecond ablation regime, plasmas induced by infrared (IR) laser pulses (1064 nm) and ultraviolet (UV) laser pulses (266 nm). Nanosecond ablation is compared with femtosecond ablation where the post-ablation interaction is absent. Subsequent to the early stage expansion, we observe for each studied ablation regime, spectral emission from CN, a typical radical for organic and biological samples. Time-resolved LIBS allows identifying emissions from native molecular species and those due to recombination with ambient air through their different time evolution behaviors.     

超强激光作用下相对论电子振荡所导致的谐波辐射

研究发现，在超强激光作用下电子运动的相对论效应可导致高次谐波辐射，采用单电子模型计算分析了不同偏振微光作用下的高次谐波发射，表明圆偏振激光较线偏振激光更有利于高次谐波产生。     

C4分子在激光场中的密度泛函理论研究

采用含时局域密度近似与分子动力学相结合的方法计算了C4分子的光学吸收谱及它在激光场中的响应。研究表明,在激光场中C4分子的诱导偶极矩与电离之间有着直接的联系,最外层电子对电离的贡献最大。此外,通过对激光的极化方向与C4分子的激发的关系的研究表明,x方向的激光场唯一的激发起C4分子x方向的偶极矩,电离增强而且离子的振动增强。     

Weibel instability in plasma produced by a superintense femtosecond laser pulse

The Weibel instability increment is analytically derived for plasma produced at the barrier-suppression ionization of atoms and atomic ions by a superintense femtosecond laser pulse. The cases of linear and circular polarization are considered. Relativistic effects are discussed. It is found that the instability increment is larger for the circular polarization than for the linear polarization. This increment can attain the plasma frequency. Barrier-suppression ionization decreases the increment compared with the case of tunneling ionization. Relativistic effects also decrease the value of the increment. Estimates of the produced maximum quasistatic magnetic field are given.     

基于Abel逆变换的激光诱导等离子体辐射特性研究

激光诱导击穿光谱(LIBS)作为一种新型的物质成分测量方法已经在越来越多的领域得到广泛应用，但是与传统的分析方法相比，LIBS技术的分析性能还需进一步提高。LIBS技术的理论基础是激光诱导等离子体，从物理机理上研究等离子体特性，对LIBS系统实验参数的优化具有指导作用，也为提高LIBS技术的检测能力奠定理论基础。激光诱导等离子体是一个与空间相关的非稳态辐射源，空间分辨光谱测量是探究等离子体物理特性的重要手段之一。为研究激光诱导等离子体的辐射特性，采用1 064 nm的Nd∶YAG调Q固体激光器烧蚀合金钢样品产生等离子体，利用空间分辨装置测量二维空间的等离子体辐射光谱信号，通过分析可知实验采集的光谱信号是信号探测器测量路径上的积分光谱强度，由此计算得到的等离子体参数也是观测路径上的平均值。为了深入研究等离子体由内层到外层的辐射规律，首先测量得到等离子体路径积分光谱强度的横向空间分布，然后以等离子体为光学薄和圆柱对称的前提条件，采用三次样条函数算法对路径积分光谱强度进行Abel逆变换，反演得到等离子体由内层到外层谱线辐射率的径向空间分布。选取等离子体辐射光谱中的原子谱线Fe Ⅰ: 374.55 nm和Mn Ⅰ: 403.08 nm为研究对象，分析等离子体辐射光谱的空间分布特征，研究结果表明，等离子体辐射路径积分光谱强度的横向分布呈现出中心位置强度大边缘位置强度小的特征，这是由于等离子体膨胀扩张的结果引起的；通过Abel逆变换得到等离子体光谱辐射率的径向分布，结果表明等离子体从内层到外层谱线的辐射率经过了先增加后减小的变化规律，等离子体中心处出现辐射率的极小值，造成这种现象的主要原因是由于等离子体辐射源中心区域具有较低的电子密度；选取等离子体辐射光谱中Fe元素的11条原子谱线，采用Boltzman法分别由谱线相应的积分光谱强度和辐射率计算等离子体温度，得到等离子体温度的横向空间和径向空间的二维分布，两者具有类似的变化规律；由等离子体温度的横向空间分布可以看出，随着离样品表面距离的增加，等离子体温度呈现单调减小的趋势，等离子体中心到边缘区域等离子体温度逐渐降低，这是由等离子体膨胀扩张以及与环境气体相互作用共同的结果；由等离子体温度的径向空间分布可以看出等离子体由内层到外层等离子体温度逐渐降低，这是由于等离子体膨胀扩张冷却引起的。由此可见，采用Abel逆变换能够实现等离子体由内层到外层的辐射特性分析，为深入理解等离子体产生和演变的物理机理提供实验依据，从而为提高激光诱导击穿光谱技术的分析性能奠定理论基础。     

Polarization-dependent femtosecond laser filamentation in air

We report on a systematic study of the laser polarization effect on a femtosecond laser filamentation in air.By changing the laser's ellipticity from linear polarization to circular polarization, the onset position of laser filament formation becomes farther from the focusing optics, the filament length is shorter, and less laser energy is deposited. The laser polarization effect on air filaments is supported by a simulation and analysis of the polarization-dependent critical power and ionization rates in air.     

BEPCII横向束流反馈系统的梳状滤波器

BEPCII是一个多束团、大流强的装置, 由于高频腔的高次模和电阻壁阻抗等因素，不可避免地会出现束流不稳定性。BEPCII中采用束流反馈系统来抑制束流不稳定性。横向束流反馈系统主要包括前端电子学、信号处理电子学、反馈器件和放大器等几个部分。梳状滤波器是信号处理电子学的重要部件，利用两根长短不同的电缆以及一个功分器和合成器构成了一种简单有效的梳状滤波器，其梳状深度达到-41 dB，使用这种梳状滤波器的横向反馈系统成功地抑制了束流中出现的不稳定性。