Femtosecond Laser-induced Breakdown Spectroscopy Research Articles

Quantitative spectral analysis by femtosecond pulse laser-induced breakdown spectroscopy

Quantitative spectral analysis by femtosecond pulse laser-induced breakdown spectroscopy

Effect of Laser Pulse Repetition Rate on the Detection Limits of the Elemental Composition of Pollutants in Aqueous Solutions by Femtosecond Laser Induced Breakdown Spectroscopy

Effect of Laser Pulse Repetition Rate on the Detection Limits of the Elemental Composition of Pollutants in Aqueous Solutions by Femtosecond Laser Induced Breakdown Spectroscopy

Femtosecond Single-Pulse and Orthogonal Double-Pulse Laser-Induced Breakdown Spectroscopy (LIBS): Femtogram Mass Detection and Chemical Imaging with Micrometer Spatial Resolution.

Femtosecond laser-induced breakdown spectroscopy (fs-LIBS) is employed to detect tiny amounts of mass ablated from macroscopic specimens and to measure chemical images of microstructured samples with high spatial resolution. Frequency-doubled fs-pulses (length 400 fs, wavelength 520 nm) are tightly focused with a Schwarzschild microscope objective to ablate the sample surface. The optical emission of laser-induced plasma (LIP) is collected by the objective and measured with an echelle spectrometer equipped with an intensified charge-coupled device camera. A second fs-laser pulse (1040 nm) in orthogonal beam arrangement is reheating the LIP. The optimization of the experimental setup and measurement parameters enables us to record single-pulse fs-LIBS spectra of 5 nm thin metal layers with an ablated mass per pulse of 100 femtogram (fg) for Cu and 370 fg for Ag films. The orthogonal double-pulse fs-LIBS enhances the recorded emission line intensities (two to three times) and improves the contrast of chemical images in comparison to single-pulse measurements. The size of ablation craters (diameters as small as 1.5 µm) is not increased by the second laser pulse. The combination of minimally invasive sampling by a tightly focused low-energy fs-pulse and of strong enhancement of plasma emission by an orthogonal high-energy fs-pulse appears promising for future LIBS chemical imaging with high spatial resolution and with high spectrochemical sensitivity.

Improving the signal-to-noise ratio of atomic transitions in LIBS using two-dimensional correlation analysis

In this study, two-dimensional (2D) correlation analysis was utilized for achieving a significant improvement in the signal-to-noise (S/N) ratio of laser-induced breakdown spectroscopy (LIBS) data. Time-resolved LIBS spectra of metallic, bimetallic targets and the normal LIBS spectra of bimetallic targets with varying compositions were used for the detailed analysis. The diagonal of the matrix in the synchronous spectra was used to demonstrate the improvement in the signal-to-noise ratio (SNR). An improvement in the peak intensities by few orders of magnitude accompanied by suppression in the noise was observed. The correlations between LIBS peaks were also visualized using the 2-D plots. The correlation strengths of atomic transitions were visualized in aluminium (Al), copper (Cu), and brass whereas correlation strengths of atomic and ionic transitions were visualized in Au-Ag bimetallic targets with different compositions (Au30Ag70, Au50Ag50, Au80Ag20). The improved spectra were subsequently used in the principal component analysis for classification studies of four compositions of bimetallic targets (Au20Ag80, Au30Ag70, Au50Ag50, and Au80Ag20). The variance of the first three principal components was found to be improved from the analysis. The accumulated percentage of explained variance of ∼95 was achieved with the first three components from improved spectra whereas only ∼80 was achieved with the regular LIBS spectra from PCA studies. Furthermore, using this correlation analysis we demonstrate a significant improvement in the SNR of CN and C2 peaks in the femtosecond LIBS spectra of two polymer samples obtained from a standoff distance of 6 m, which may prove substantial for improved classifications studies.

Evaluation of femtosecond laser-induced breakdown spectroscopy system as an offline coal analyzer

Developments in femtosecond laser induced breakdown spectroscopy (fs-LIBS) applications during the last two decades have further centered on innovative métier tie-in to the advantageous properties of femtosecond laser ablation (fs-LA) introduced into LIBS. Yet, for industrially-oriented application like coal analysis, no research has exposed to view the analytical capabilities of fs-LA in enhancing the physical processes of coal ablation and the impact into quantitative correlation of spectra and data modeling. In a huge coal market, fast and accurate analysis of coal property is eminently important for coal pricing, combustion optimization, and pollution reduction. Moreover, there is a thirst need of precision standardization for coal analyzers in use. In this letter, the analytical performance of a one-box femtosecond laser system is evaluated relative to an industrially applied coal analyzer based on five objectives/measures: spectral correlation, relative sensitivity factors, craters topology, plasma parameters, and repeatability. Despite high-threshold operation parameters of the fs system, competitive results are achieved compared to the optimized analytical conditions of the ns-coal analyzer. Studies targeting the in-field optimization of fs-LIBS systems for coal analysis can potentially provide insights into fs-plasma hydrodynamics under harsh conditions, instrumental customization, and pave the way for a competitive next-generation of coal analyzers.

Reduction of self-absorption in femtosecond laser-induced breakdown spectroscopy using spark discharge

The self-absorption effect of femtosecond laser-induced breakdown spectroscopy (LIBS) of aluminum with and without spark discharge is investigated in air. The measured time-resolved spectra show strong self-absorption/self-reversal in the absence of spark discharge. Still, when spark discharge is used to enhance the spectral intensity of LIBS, almost no self-reversal effect can be observed from time-resolved spectra. The results show that spark discharge can effectively reduce the self-absorption effect and improve spectral intensity in femtosecond-LIBS.

Comparison of sample temperature effect on femtosecond and nanosecond laser-induced breakdown spectroscopy

In this paper, we investigated the emission spectra of plasmas produced from femtosecond and nanosecond laser ablations at different target temperatures in air. A brass was selected as ablated target of the experiment. The results indicated that spectral emission intensity and plasma temperature showed similar trend for femtosecond and nanosecond lasers, and the two parameters were improved by increasing the sample temperature in both cases. Moreover, the temperature of nanosecond laser-excited plasma was higher compared with that of femtosecond laser-excited plasma, and the increase of the plasma temperature in the case of nanosecond laser was more evident. In addition, there was a significant difference in electron density between femtosecond and nanosecond laser-induced plasmas. The electron density for femtosecond laser decreased with increasing the target temperature, while for nanosecond laser, the electron density was almost unchanged at different sample temperatures.

Comparison of emission signals for different polarizations in femtosecond laser-induced breakdown spectroscopy

In this study, a femtosecond laser was focused to ablate brass target and generate plasma emission in air. The influence of lens to sample distance (LTSD) on spectral emission of brass plasma under linearly and circularly polarized pulses with different pulse energies was investigated. The results indicated that the position with the strongest spectral emission moved toward focusing lens with increasing the energy. At the same laser energy, the line emission under circularly polarized pulse was stronger compared with linearly polarized pulse for different LTSDs. Next, electron temperature and density of the plasma were obtained with Cu (I) lines, indicating that the electron temperature and density under circularly polarized pulse were higher compared to that under linearly polarized pulse. Therefore, changing the laser polarization is a simple and effective way to improve the spectral emission intensity of femtosecond laser-induced plasma.

Influence of target temperature on AlO emission of femtosecond laser-induced Al plasmas

The influence of the target temperature on the molecular emission of femtosecond laser-induced breakdown spectroscopy (LIBS) was investigated experimentally. An Al target was ablated to produce laser-induced plasma. The Al target was uniformly heated to a maximum of 250 °C. The measured molecular emission was AlO (Δν = 0) from the femtosecond LIBS of the Al target. The measurements indicated that the molecular emission of AlO increased as the temperature of the Al target increased. In addition, a two-temperature model was used to simulate the evolution of the electron and lattice temperature of the Al target with different initial temperatures. The simulated results showed that the electron and lattice temperatures of Al irradiated by the femtosecond laser increased as the initial temperature of the Al target increased; also, the simulated ablated depth increased. Therefore, an increase in the initial Al target temperature resulted in an enhancement in the spectral signal of AlO from the femtosecond LIBS of Al, which was directly related to the increase in the size of the ablated crater. The study suggested that increasing the temperature of the target improves the intensity of molecular emission in femtosecond LIBS.

Effect of the repetition rate of laser pulses on the limits of detection of the elemental composition of pollutants in aqueous solutions by femtosecond laser induced breakdown spectroscopy

Effect of the repetition rate of laser pulses on the limits of detection of the elemental composition of pollutants in aqueous solutions by femtosecond laser induced breakdown spectroscopy

通过圆偏振光提高飞秒激光诱导击穿光谱的发射强度

通过圆偏振光提高飞秒激光诱导击穿光谱的发射强度

Time-resolved electron temperature and density of spark discharge assisted femtosecond laser-induced breakdown spectroscopy

In laser-induced breakdown spectroscopy (LIBS), spark-discharge (SP) is an effective auxiliary method to improve spectral emission intensity, and can effectively affect plasma temperature and electron density. In traditional LIBS technology, nanosecond laser is usually used as excitation source. Compared with nanosecond laser, femtosecond (fs) pulse laser has many advantages in the interaction between laser and matter. The SP can also enhance the emission intensity of fs-LIBS. However, the discussion on plasma temperature and electron density of SP assisted fs-LIBS is rare. In this study, we measured time-resolved spectra of fs-LIBS with the SP. By using the Boltzmann plot and the Stark broadening, we calculated time-resolved plasma temperatures and electron densities for different laser energies at different discharge voltages. We hope that this study can provide a better understanding for SP assisted fs-LIBS.

Influence of polarization of laser beam on emission intensity of femtosecond laser-induced breakdown spectroscopy**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674128, 11674124, and 11974138), the Jilin Provincial Scientific and Technological Development Program, China (Grant No. 20170101063JC), and the Scientific and Technological Research Project of the Education Department of Jilin Province, China (Grant No. JJKH20200937KJ).

Laser-induced breakdown spectroscopy (LIBS) is an important technique which is widely used to analyze element composition. In order to improve the sensitivity of LIBS, much effort has been made to enhance the spectral intensity of LIBS by proposing a number of methods. In addition, we find that laser polarization has great influence on the emission intensity of femtosecond LIBS. By comparing the emission intensity of femtosecond LIBS in the circular polarization with that in the linear polarization, the spectral intensity in the case of circular polarization is stronger than that in the case of linear polarization. Moreover, this phenomenon is more obvious as laser energy increases. The polarization plays an important role in LIBS signal intensity. Based on the observation, the enhanced mechanism of the laser polarization for the spectral intensity is discussed in this paper, which will be helpful in spectral analysis and component analysis.

Influence of target temperature on femtosecond laser-ablated brass plasma spectroscopy

Spectral intensity, electron temperature and density of laser-induced plasma (LIP) are important parameters for affecting sensitivity of laser-induced breakdown spectroscopy (LIBS). Increasing target temperature is an easy and feasible method to improve the sensitivity. In this paper, a brass target in a temperature range from 25 °C to 200 °C was ablated to generate the LIP using femtosecond pulse. Time-resolved spectral emission of the femtosecond LIBS was measured under different target temperatures. The results showed that, compared with the experimental condition of 25 °C, the spectral intensity of the femtosecond LIP was enhanced with more temperature target. In addition, the electron temperature and density were calculated by Boltzmann equation and Stark broadening, indicating that the changes in the electron temperature and density of femtosecond LIP with the increase of the target temperature were different from each other. By increasing the target temperature, the electron temperature increased while the electron density decreased. Therefore, in femtosecond LIBS, a high-temperature and low-density plasma with high emission can be generated by increasing the target temperature. The increase in the target temperature can improve the resolution and sensitivity of femtosecond LIBS.

Enhanced optical emission in laser-induced breakdown spectroscopy by combining femtosecond and nanosecond laser pulses

The role of inter-pulse delay time in laser-induced Cu plasma with femtosecond and nanosecond double-pulse irradiation had been investigated by optical emission spectroscopy. The experimental results showed that a significant enhancement effect in the atomic line emission was observed by changing the inter-pulse delay time. The spectral intensity of Cu plasma produced by a femtosecond + nanosecond double-pulse was higher than that produced by a nanosecond + femtosecond dual-pulse. That is to say, the shorter laser pulse (femtosecond) was efficient in coupling to the sample and the longer laser pulse (nanosecond) was suitable for re-exciting the produced plasma. In addition, the corresponding plasma temperature and electron density were also calculated. The plasma temperature in the case of femtosecond + nanosecond was lower than that in the case of nanosecond + femtosecond, while the electron density in the case of femtosecond + nanosecond was higher than that in the case of nanosecond + femtosecond. This study is helpful for the research on sample detection using femtosecond and nanosecond double-pulse laser-induced breakdown spectroscopy.

Effect of lens focusing distance on AlO molecular emission from femtosecond laser-induced aluminum plasma in air

This paper investigated the effect of lens focusing distance (LFD) on optical emission from femtosecond laser-induced breakdown spectroscopy (LIBS) of Al in air. The measurements showed that the emission intensities of Al (I) line and AlO molecular band strongly depended on the LFD. Along with the change in the LFD, two extreme emission intensities appeared in Al (I) line and AlO (0-0). It was due to self-focusing, defocusing, refocusing effects during femtosecond laser propagation in air. That is to say, there were two convergent points. The spectral intensity of Al (I) line at first focus point was higher than that at second focus point. However, the spectral intensity of AlO (0-0) at second focus point was higher compared with that at first focus point. For molecular emission of femtosecond LIBS, the second focus point is more suitable for generation of AlO molecule. In addition, the plasma temperature was also calculated. The temperature at first focus point was higher than that at second focus point. Therefore, the mechanism of weaker molecular emission at first focus point is due to higher plasma temperature. This research will be beneficial to optimize the spectral intensity of molecular emission in the future femtosecond LIBS.

Femtosecond laser-induced breakdown spectroscopy of a preheated Cu target

We present a study detailing femtosecond laser-induced breakdown spectroscopy (LIBS) of Cu with the sample preheated to different temperatures (22–120 °C). The detected spectral intensity of the plasma emission increased with the target temperature. Additionally, laser-induced plasma temperature was calculated by Boltzmann plot method. The plasma temperature rose along with the target temperature, indicating an enhanced interaction between the laser and the target in the case of a higher target temperature. We also simulated the thermal behavior of Cu irradiated by the femtosecond laser via a two-temperature model. As the sample temperature increased from 22 °C to 120 °C, the increase in the lattice temperature under femtosecond laser irradiation was approximately 200–300 K, and the ablated depth increased. The simulated results suggested that increasing the sample temperature led to an increase in the ablated mass. Based on this work, it can be seen that the method of preheating the ablation target can help enhance the spectral intensity and the detection sensitivity of femtosecond LIBS.

Multiscale Visualization of Colloidal Particle Lens Array Mediated Plasma Dynamics for Dielectric Nanoparticle Enhanced Femtosecond Laser-Induced Breakdown Spectroscopy

A multiscale visualization of silica colloidal particle lens array (CPLA) assisted laser ablation of copper is investigated. The distributed holes on a crater of CPLA-deposited Cu (CPLA-Cu) show a near-field effect by the silica nanoparticles (NPs), and the plasma emission signal of CPLA-Cu is 3-5 times as strong as that of Cu. Time-resolved plasma expansion, shockwave propagation, plasma plume emission, and nanoparticle distribution are observed and analyzed for ablations on both Cu and CPLA-Cu substrates. The initial expansion of plasma generated on CPLA-Cu is faster than that of pristine Cu. The shockwave of CPLA-Cu is rounder and its plasma plume is wider than those of Cu. The nanoparticle distribution shows a strong lateral collision during plume ejection for CPLA-Cu. Plasma characterization shows the increased plasma temperature is the key reason for femtosecond laser-induced breakdown spectroscopy (fs-LIBS) signal enhancement. This work demonstrates the signal enhancement effect of dielectric NPs on fs-LIBS and provides insights into hydrodynamics of the fs laser-induced plasma generated on CPLA-deposited substrate.

Time-resolved spectroscopy of femtosecond laser-induced Cu plasma with spark discharge

The combination of spark discharge and laser-induced breakdown spectroscopy (LIBS) is called spark discharge assisted LIBS. It works under laser-plasma triggered spark discharge mode, and shows its ability to enhance spectral emission intensity. This work uses a femtosecond laser as the light source, since femtosecond laser has many advantages in laser-induced plasma compared with nanosecond laser, meanwhile, the study on femtosecond LIBS with spark discharge is rare. Time-resolved spectroscopy of spark discharge assisted femtosecond LIBS was investigated under different discharge voltages and laser energies. The results showed that the spectral intensity was significantly enhanced by using spark discharge compared with LIBS alone. And, the spectral emission intensity using spark discharge assisted LIBS increased with the increase in the laser energy. In addition, at low laser energy, there was an obvious delay on the discharge time compared with high laser energy, and the discharge time with positive voltage was different from that with negative voltage.

The role of laser fluence and ambient environments on femtosecond laser induced breakdown spectroscopy and on surface morphology of Mg and Zr

The effects of laser fluence and ambient environments on plasma parameters and on surface modifications of femtosecond laser irradiated Mg and Zr have been investigated. A Ti:Sapphire laser (800 nm, 35 fs) was employed to irradiate the metallic targets under vacuum and Ar environments at various fluences. Laser induced breakdown spectroscopy analyses reveal that the optical emission spectra, excitation temperature (Te), and electron number density (ne) of metallic plasmas exhibit increasing trend with increasing fluence irrespective of the target under both the environments. This increasing tendency is because of the enhanced ablation rate with the increase in the fluence. However, the values of these parameters are significantly higher in the presence of Ar as compared to that of vacuum, which is attributable to confinement effects offered by the gas. The plasma parameters, Te and ne, have higher values in the case of Zr under both environments (vacuum and Ar) due to its higher melting point and lower thermal conductivity as compared to Mg. Field emission SEM analyses for both of the metals irradiated under vacuum exhibit a non-uniform distribution of nanoglobules, nanocones, and micrometer-sized cavities in the case of irradiated Mg, whereas for Zr, there is growth of laser induced periodic surface structures along with the formation of a deep crater. When both the metals were irradiated in Ar, a significant difference in surface morphologies of both Mg and Zr has been observed. In the case of Mg, SEM discloses the formation of micro-inhomogeneities and micrometer-sized cones covered with nanoglobules, whereas for Zr, high-spatial-frequency laser induced periodic surface structures covered with nanoroughness and micro-columns have been detected. In the present work, by exploring the optimum conditions in terms of laser fluence, environmental conditions, and material response, a correlation has been established between the calculated plasma parameters and observed micro- and nanostructures for both of the metals. This established correlation will enable us to better understand the plasma to be utilized for ion-implantation, thin film deposition, and surface structuring in a more effective manner.