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Abstract
Aiming to enhance the ns-LIBS signal, in this work, we introduced orbital angular momentum to modulate the laser phase of the Gaussian beam into the vortex beam. Under similar incident laser energy, the vortex beam promoted more uniform ablation and more ablation mass compared to the Gaussian beam, leading to elevated temperature and electron density in the laser-induced plasma. Consequently, the intensity of the ns-LIBS signal was improved. The enhancement effects based on the laser phase modulation were investigated on both metallic and non-metallic samples. The results showed that laser phase modulation resulted in a maximum 1.26-times increase in the peak intensities and a maximum 1.25-times increase in the signal-to-background ratio (SBR) of the Cu spectral lines of pure copper for a laser energy of 10 mJ. The peak intensities of Si atomic spectral lines were enhanced by 1.58-1.94 times using the vortex beam. Throughout the plasma evolution process, the plasma induced by the vortex beam exhibited prolonged duration and a longer continuous background, accompanied by a noticeable reduction in the relative standard deviation (RSD). The experimental results demonstrated that modulation the laser phase based on orbital angular momentum is a promising approach to enhancing the ns-LIBS signal.
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1. Introduction
Laser-induced breakdown spectroscopy (LIBS), a viable emission spectroscopy analytical technology, shows rapid detection, simple pretreatment, and high-efficiency analysis capabilities combined with intelligent algorithms. By focusing a laser pulse on the surface of the sample, the chemical bonds of surface substances are broken and chemical elements are ionized, forming a high-temperature plasma composed of atoms, free electrons, and ions. The spectral information of laser-induced plasma can be utilized for the analysis of the compositional constituents of the sample. The application of LIBS technology is not restricted by the physical form of the sample material, including but not limited to solid [1], liquid [2], and gaseous [3] samples. Benefiting from this advantage, it has been regarded as the next-generation trend in various fields such as environmental monitoring [4,5], metallurgical process analysis [6,7], cultural relics restoration [8], deep sea exploration [9,10], biomedical detection [11–14], and planetary exploration [15–17], etc.
As mentioned earlier, the reason it is referred to as the next-generation trend in technological development is due to the existing gap in the stability and repeatability of measurements between LIBS and traditional detection methods. Achieving optimal signal intensity and signal-to-background ratio is the primary avenue for bridging this gap. In order to improve the LIBS signal, some scientists use extra energy sources to interact with plasma and prolong its life. Yuan Lu et al. used an ultraviolet (UV) femtosecond (fs)- nanosecond (ns) double pulse method to detect silicon, which improved the signal intensity by roughly 360 times as compared to a single-fs laser pulse LIBS of Si [18]. Weidong Zhou et al. used laser ablation combined with rapid discharge technology (LA-FPDPS) to detect Pb and As in soil, and the signal intensity of Pb (I) 283.31 nm obtained by this technology was 38.5 times that of traditional LIBS [19]. Zhe Ye et al. used spark-discharge-assisted LIBS (SD-LIBS) to enhance the spectral intensity of Cu and significantly reduced the limit of detection of LIBS [20]. Shahab Ahmed Abbasi et al. used a magnetic field-assisted LIBS system and proved that optical signals of Pd-I and Pd-II lines could be enhanced by 3–4 times [21].
In addition to the energy introduced by the excitation source, some researchers enhanced the intensity of the spectrum signals by spatial constraints on the plasma. Lianbo Guo et al. proved that the emission intensities of V, Cr, and Mn lines are significantly increased by 4.2, 3.1, and 2.87 times respectively due to the limitation of a hemispherical cavity [22].
Such enhancement methods can improve the LIBS signal in a sense, but the more complex systems with extra costs cannot be completely suitable to industry and commercial detection. Furthermore, the bulky system also restricts its applications for outdoor explorations [23].
In recent years, researchers have found that the physical properties of plasma can be changed by optical field modulation, thus improving the signal quality of LIBS. Vasily Lednev et al. compared the signal repeatability from a single-mode laser beam with that from a multi-mode laser beam. Results of single-mode laser beam have better reproducibility of analytical signals compared to the multimode laser beam while laser energy was the same for both cases [24]. If it is possible to balance the repeatability of single-mode lasers and the uniformity of multi-mode lasers, there may be better results. Zongyu Hou et al. modulated the evolution of the laser-induced plasma by shaping the laser beam from a commonly used Gaussian profile to a flat-top profile and found that it can improve signal intensity and reduce the RSD of the LIBS signal through the more uniform laser energy density [25]. As a novel method for enhancing LIBS signals, beam shaping may not exhibit effects as pronounced as those observed with previously mentioned methods. Nonetheless, it offers substantial advantages, notably in significantly reducing the complexity and cost associated with the LIBS system. Importantly, this approach is universally applicable to both atomic spectral lines and molecular spectral bands. This versatility suggests considerable potential for enhancing LIBS analysis performance and facilitating commercialization.
In beam shaping, the primary consideration revolves around the influence of energy distribution, but another crucial factor affecting the beam is its phase. A new type of laser beam called vortex beam has attracted great attention due to its unique spiral phase and has been widely explored in various fields [26–28]. It has already been proved that the vortex beam can enhance the intensity of spectra by 1.55 times in femtosecond LIBS (fs-LIBS) with the explanation that the ring-shaped plasma generated by the vortex beam pulse led to plasma collision, forming a strong stagnation layer which could result in the kinetic energy of the plasma converted into heat energy, and the plasma was heated. Thus, the plasma emission can be further enhanced by the colliding plasma [29].
However, the conclusion from this study is limited to explaining the enhancement effect of vortex beams on fs-LIBS, and it’s still not sure that the impact on ns-LIBS. During the plasma ignition process, the mechanisms and plasma properties strongly depend on laser irradiance and pulse duration. For irradiances higher than 1013 W/cm2 with the femtosecond laser pulse, Coulomb explosion is the main bond-breaking mechanism. For a nanosecond laser pulse with irradiances less than 108 W/cm2, the dominant mechanism is thermal vaporization: the temperature of the solid surface increases, and a well-defined phase transition occurs from solid to liquid, liquid to vapor, and vapor to plasma [30]. While fs-LIBS has a higher spatial resolution and a lower ablation threshold, the stronger self-absorption effect and higher limits of detection (LOD) pose limitations compared to ns-LIBS [31,32]. Furthermore, femtosecond laser systems are substantially bulkier and costlier than nanosecond laser systems [33]. Therefore, it has more commercial and industrial value to explore the effect of the vortex beam on the plasma spectrum generated by a nanosecond laser pulse. In addition, the comparison between the Gaussian beam and the vortex beam in previous research was limited to metallic samples and lacks investigation of signal evolution.
In this work, the influence of nanosecond vortex beams on LIBS signal quality was investigated. Firstly, the original Gaussian beam was modulated into a vortex beam by introducing orbital angular momentum (using a vortex retarder). Then, the LIBS signal characteristics of the Gaussian beam and vortex beam were compared with different laser energies. Furthermore, the signal evolution and the universality on both metallic and non-metallic samples were also investigated.
2. Experimental setup
2.1 Vortex-beam-generating setup
The experimental setup of generating the vortex beam was designed as shown in Fig. 1. A home-made Q-switched Nd: YAG laser (wavelength: 1064 nm; repetition frequency: 1 Hz; pulse duration: 5 ns; beam diameter: Ø 6 mm) was employed in the experiment. A half-wave plate and a Glan prism were used to adjust the energy of the laser and the output of Glan prism was a linearly polarized laser. Neutral Density Filters were used to attenuate laser energy and avoid the NIR CMOS saturation. A vortex retarder (Lubon, topological charge m = 1, transmission rate >98%, conversion efficiency ≥99.5%) was used to modulate the Gaussian beam. The energies of 100 pulses were measured with and without the vortex retarder in order to calculated the energy attenuation of the vortex retarder. The results showed that the average energy attenuation after passing through the vortex retarder was 1%. In order to produce a vortex beam with a spiral phase, the input laser of the vortex retarder should be a circularly polarized laser. The angle between the vibration direction of linearly polarized laser and the fast axis of quarter-wave plate was set to 45° in order to generate a circularly polarized laser. So, the laser beam turned into a vortex beam after passing through the vortex retarder.
The center of the Gaussian beam must pass through the center of the vortex retarder, otherwise, the light field distribution of the generated vortex beam will have obvious inhomogeneity. In order to verify the vortex beam, we used a NIR CMOS (Daheng Imaging, MER-530-20GM-P NIR) and a tilted convex lens [34] as shown in Fig. 2. When the vortex beam passed through the tilted convex lens, its spot would turn into the Hermite-Gaussian-like spot. Due to the different topological charges, vortex beams and Gaussian beams would form two and one spot (as shown in Fig. 3(c) and (d)), respectively, after passing through the tilted convex lens.
Fig. 2. Facula for nanosecond vortex (a) and Gaussian (b) beams. The Hermite-Gaussian-like spots of the vortex beam (c) and the Gaussian beam(d), generated by the tilted convex lens were used to verify the generated beam.
2.2 LIBS experimental setup
The nanosecond LIBS experimental setup of the vortex beam was designed as shown in Fig. 3. After the Gaussian beam turned into to the vortex beam, the laser beam was reflected by three plane mirrors (M1, M2, and M3) and focused onto the sample surface by a microscope objective lens (Mitutoyo objective,10×, numerical aperture = 0.26, working distance = 30.5 mm, and focal length = 20 mm). A He-Ne laser (wavelength: 632.8 nm) was used to point out the position of the focus point. The samples were fixed on a three-dimensional motorized stage. The plasma emission was collected by a collector and entered into a two-channel spectrometer (AvaSpec 2048-2-USB2, Avantes; spectral range: 190 - 1100 nm; resolution: 0.20–0.30 nm). The spectrometer and the laser were controlled by a digital delay generator (SRS-DG535, Stanford Research Systems). The gate width of CCD was 1.05 ms, and the delay time was set among 0.28-5.78 µs. The experiment was performed under ambient air conditions. The nanosecond LIBS experimental setup of the Gaussian beam was without the vortex retarder.
In this work, pure copper (99.9% Cu), brass (62% Cu and 38% Zn), Silicon (99.9% Si) and aluminum (99.9% Al) samples were used as shown in Fig. 4. Before the experiment, the sample surface was cleaned with absolute alcohol and then dried in the air. After each laser shot, the sample was moved to a fresh spot to avoid repetitive striking on the same spot. The focusing point of the laser was below the sample surface, where the spectral intensity was the strongest. The spectral signals were collected from an angle of 45° between the sample normal and the axis of the collector. Morphologies of the ablated craters were measured by Zeiss scanning confocal microscope.
3. Results and discussion
3.1 Comparison of morphologies and plasma characteristics
Spectral signals of pure copper excited by the Gaussian beam and the vortex beam were collected at 10mJ and 20mJ respectively, and 200 spectra were collected in each case. The morphology of three ablated craters which were randomly selected from 200 craters of the Gaussian and vortex beams with the laser energy of 10 mJ were measured, respectively. The images of the typical craters were shown in Fig. 5. It could be seen that the crater ablated by the vortex beam was more regular, but it was not annular. This might be because the topological charge of the vortex retarder would affect the inner diameter of the annular spot, and the inner diameter with topological charge equals to one was quite small. After focusing, the center area without energy distribution was almost nonexistent. Also, during the pulse duration, there was enough time for the laser energy to propagate into the metal target and to create a relatively large molten layer, which might change the original morphology of the craters [35,36].
Fig. 5. True color view of representative ablation craters caused by the Gaussian beam (a) and the vortex beam (b).
The measurement values of depths, diameters, and volume of the ablation craters were listed in Table 1. The average diameter of craters produced by the vortex beam was slightly smaller than that of the Gaussian beam. Though the average maximum depth of the Gaussian beam ablated was deeper than that of the vortex beam, the average depth of the vortex beam was higher. This indicated that the laser ablation was more dispersed rather than over-concentrated in the middle because of the ring-like laser energy distribution, which could be seen more clearly in Fig. 6. In Fig. 6, the profile of ablation crater produced by the Gaussian beam was sharper, similar to an inverted triangle, while the profile of ablation crater produced by the vortex beam was gentler, similar to a bowl. Meanwhile, the volumes of the craters caused by the vortex beam were larger than those of the Gaussian beam, which meant that the vortex beam could cause more ablation.
Fig. 6. The typical cross section of the ablation crater generated by (a) the Gaussian beam and (b) the vortex beam.
Table 1. Depths, diameters, and volume of the ablation craters ablated by the Gaussian and vortex beams
The average temperature and electron density of the two types of plasma have been calculated to elucidate the impact of laser phase modulation. the temperature was calculated through the Boltzmann plot using four Cu atomic lines (510.55 nm, 515.32 nm, 521.82 nm and 578.21 nm) as shown in Fig. 7 According to the Eq. (1), the plasma temperature T could be calculated from the slope $- \frac{1}{{{K_\textrm{b}}T}}$.
where I is the peak intensity, ${A_{ki}}$ is the transition probability, ${g_k}$ is the statistical weight of the k level, ${E_k}$ is the energy of k level, ${K_b}$ is the Boltzmann constant, and $C$is a constant. The spectral parameters of ${g_k}$, ${A_{ki}}$, and ${E_k}$ are from the NIST database. Among all the broadening mechanisms of plasma spectral lines, Stark broadening is the most important mechanism, and other broadening can be ignored. The line shape of the spectral line under the Stark broadening mechanism is the Lorentzian profile, and its linewidth $\varDelta \lambda $ can be expressed as where $\omega $ is the electron collision coefficient, ${n_e}$ is the electron density. The electron density was estimated using the full width at half maximum (FWHM) of the Cu (I) 521.82 nm line.The plasma temperature and electron density for the plasma induced by Gaussian beam and vortex beam were listed in Table 2, where the results were the average value over 200 laser shots. The results showed that the plasma produced by the vortex beam had higher temperatures and higher electron density. This proved that the plasma shielding effect for the Gaussian beam was rather intense.
Table 2. Temperature and electron density of Cu plasma at 10 mJ
As the signals were collected from the angle of 45° between the sample normal and the axis of the collector, the spectra were obtained from the inner plasma. Because of the higher central energy density compared to the vortex beam, the Gaussian beam led to intenser plasma shielding effect, which would result in less energy available for heating the inner plasma, resulting in lower temperature and electron density of the plasma. The vortex beam could reduce the plasma shielding effect so that more energy could be utilized to heat the plasma and thus obtain higher plasma temperature and electron density [25].
3.2 Comparison of spectral signals
Figure 8 shows the spectra of pure copper induced by the Gaussian and vortex beams, averaged over 200 shots, with the laser energies of 10 mJ and 20 mJ, respectively. It was clear that plasma induced by the vortex beam had stronger intensities of all the Cu lines. The average enhancement ratios and SBRs of Cu (324.75 nm, 327.40 nm, 510.55 nm, 515.32 nm, 521.82 nm and 578.21 nm) were listed in Table 3. The enhancement ratio represented the ratio of the spectral peak intensity obtained after background subtraction for the vortex beam and Gaussian beam excitations. This provided a more intuitive display of the variation in peak intensity, unaffected by background interference. The best enhancement ratio of Cu lines could reach 1.26 and 1.36 at the laser energy of 10 mJ and 20 mJ, respectively. And the enhancement ratio increased with the laser pulse energy. The Cu lines at 324.75 nm and 327.40 nm were resonance lines, and the enhancement effect of vortex beam at these two lines did not increase with the energy because of self-absorption effect. Similar to the case of line intensity enhancement, the SBRs for the vortex beam were much higher than those for the Gaussian beam. The average SBRs of the vortex beam had been improved 1.21 times and 1.25 times compared to the average SBRs of the Gaussian beam with the laser pulse energy of 10 mJ and 20 mJ, respectively. A higher SBR was expected to help improve the detection limit of LIBS. Compared with the femtosecond results [29], the enhancement effect was slightly lower, which may be due to the melting effect, that is, the interaction between laser pulse and plasma has a certain interference with the result.
Fig. 8. Comparison of spectral intensities for the Gaussian and vortex beams at 10 mJ (a) and 20 mJ (b).
Table 3. The enhancement ratios and SBR of Cu (I) at 10 mJ and 20 mJ
The average evolution of emission peak intensities and SBR at Cu (I) 521.82 nm line of the vortex beam and Gaussian beam from 0.28 µs to 5.78 µs were compared as shown in Fig. 9(a) and (b). The spectral intensities of the vortex beam were always stronger than those of the Gaussian beam while both of the intensities decreased with the delay time, which meant that the plasma induced by the vortex beam lasted longer than that induced by the Gaussian beam. The SBRs of Cu (I) 521.82 nm line increased first and then decreased as shown in Fig. 9(b). The highest SBR of the spectra generated by the vortex beam was at the delay time of 1.28 µs while the highest SBR of the spectra generated by the Gaussian beam was at the delay time of 0.78 µs. The highest SBR for the vortex beam appeared later, which indicated that the continuous radiation duration of plasma induced by the vortex beam was longer. During the evolution of the plasma, two primary factors influenced the SBR of the vortex beam. Firstly, the vortex beam resulted in a higher plasma electron density due to increased ablation mass, leading to more intense collisions between electrons and ions, thereby prolonging the duration of continuous background radiation. Secondly, the elevated temperature of the vortex beams induced plasma also contributed to a prolonged period of continuous background radiation.
Fig. 9. The average evolution of emission peak intensities (a), and SBRs (b). The RSD of the spectral intensities (c) and the RSD of the SBR (d) at Cu (I) 521.82 nm line of the vortex beam and Gaussian beam from 0.28 µs to 5.78 µs.
Figure 9(c) and (d) show the RSDs of the spectral intensities and the SBRs of the Cu (I) 521.82 nm line. The RSDs were calculated using the peak intensities from 200 laser pulses at each delay time. According to Fig. 9(c), the RSDs of the vortex beam-induced plasma increased with the delay time similarly to the Gaussian beam induced plasma. The increasing RSDs during the plasma evolution was attributed to the Rayleigh-Taylor Instability (RTI), which means the drastic material interpenetration at the plasma-ambient gas interface at an early stage led to considerable plasma fluctuation at a later stage [37]. In addition, the vortex beam-induced plasma exhibited much lower RSDs during the plasma evolution compared to the Gaussian beam, indicating improved repeatability and reduced uncertainty. The RSD of the Cu (I) 521.82 nm line was reduced from 26% to 17% at 1.28 µs. The reason is that the plasma induced by the vortex beam had a flatter distribution, weakening the back-pressed downward process of the frontier material, and resulting in less morphological fluctuation. Also, the vortex beam would increase the ablated mass, which could enlarge the relatively stable and hot core region of plasma, which would help to decrease the RSD [25].
3.3 Comparison of other samples
In order to prove the universal improving effect of laser phase modulating on both atomic lines and molecular bands in spectra of both metallic and non-metallic samples, we repeated the experiment on other three samples, including silicon, aluminum, and brass, with 10 mJ laser pulse energy. The average spectra of these samples induced by the vortex laser beam and the Gaussian laser beam were shown in Fig. 10. It could be seen that laser phase reshaping had an evident enhancement effect on metallic and non-metallic samples. According to the experimental results, the spectral intensities of brass were an order of magnitude higher than those of the pure copper samples under the same experimental condition. This is because the existence of Zn in the brass samples increased the sample hardness, which facilitated the laser energy to more easily couple into the sample for ablation [38]. At the same time, lower Cu concentration also led to the increase in signal intensity [39]. The low spectral intensities of non-metallic samples, Si, were due to the polished surface on silicon as surfaces with lower roughness values yield lower signal intensity [40].
Fig. 10. Comparison of spectral intensities for the Gaussian and vortex beams based on silicon, aluminum, and brass at 10 mJ.
The enhancement ratios of several feature spectral lines of the three samples were listed in Table 4. The increase in line intensity was in the range of 1.31–2.83 times. Bold figures indicated the highest enhancement ratios of each element. As mentioned above, Zn element and the lower element concentration led to more ablation mass and thus lead to stronger spectral intensity. Therefore, the enhancement ratios of Cu lines of brass were clearly higher than those of pure copper. This indicates that the enhancement effect of the vortex beam of Cu lines can be amplified due to the lower element concentration and the addition of Zn. The highest enhancement ratio of all the elements was the Zn line at 334.50 nm in brass, reaching a maximum value of 2.83. Even though the intensities of the Si atomic spectral lines were low, using the vortex beam can obtain obvious enhancement ratios on Si samples, ranging from 1.58-1.94. The enhancement ratios of AlO were at range of 1.34-1.46. The above results verify the universality of the enhancement effect based on the vortex beam on the ns-LIBS signal.
Table 4. The enhancement ratio of Al, Si and brass spectral lines at 10 mJ
4. Conclusions
In this work, we modulated the Gaussian beam into a vortex beam by leading in orbital angular momentum to enhance ns-LIBS signals. According to the morphology of the ablation craters and the plasma characteristics, the vortex beam was capable of ablating more mass and improving the plasma temperature and electron density under identical conditions. The ring-shaped energy distribution of the vortex beam can effectively avoid excessive concentration of ablation, thereby reducing plasma shielding effects. The results showed that the signal peak intensities of Cu lines were enhanced by 1.26 and 1.36 times, and the SBR of the Cu (I) 521.82 nm line was enhanced by 1.21 and 1.25 times compared to the results of the Gaussian beam for the energy of 10 mJ and 20 mJ, respectively. During the evolution of emission peak intensities at Cu (I) 521.82 nm line, the SBR reached its maximum value later by using the vortex beam compared to the Gaussian beam. The mechanism was attributed to longer continuous background period because of more ablation, higher plasma temperature and electron density. The lower RSD exhibited by the vortex beam was attributed to expanding the relatively stable and hotter core region of the plasma and the annular plasma distribution, which weakens the downward backpressure process of the front material, resulting in smaller morphology fluctuations. In the comparison of other samples, the experiment showed the universality of the enhancement effects on metallic atomic spectral lines, non-metallic atomic spectral lines, and molecular bands. Furthermore, the enhancement effects of identical Cu lines on pure copper and brass samples, potentially attributed to variations in elemental concentrations and the addition of Zn. The results demonstrated that the phase modulation based on orbital angular momentum can enhance the LIBS signal effectively, and provides a potential method for improving the analytical ability of LIBS.
Funding
National Natural Science Foundation of China (62075011); Graduate Interdisciplinary Innovation Project of Yangtze Delta Region Academy of Beijing Institute of Technology (Jiaxing) (GIIP2022-011); BIT Research and Innovation Promoting Project (2023YCXY026).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the corresponding author upon reasonable request.
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