Effect of crater volume on laser-induced plasma lasers and Laser-Induced Breakdown Spectroscopy intensity

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Highlights

  • LIBS/LIPL intensities vary with ablation laser pulses in about the same manner.

  • The second peak is found in LIBS/LIPL intensities dependencies on laser pulses.

  • The existence of the second peak is determined mostly by the specific heat and thermal conductivity of the sample.

Abstract

Laser-Induced Breakdown Spectroscopy (LIBS) and Laser-Induced Plasma Lasers (LIPL) in elongated Laser-Induced Plasma (LIP) are investigated. It is shown that LIBS and LIPL behavior versus ablation laser pulses are about the same, though LIBS emission is proportional to the atoms in excited states, but LIPL generation is proportional to the atoms in the ground state. Furthermore, two peaks are found in LIBS/LIPL intensities dependencies on laser pulses. The first peak is well-known in LIBS. The second, observed at large ablation pulses number (≥104 pulses in our experimental conditions), may be attributed to the sample's thermal properties, especially thermal capacity and thermal diffusivity.

Graphical abstract

Al (red circles) and Fe (black squares) LIPL intensities versus ablation laser pulses.

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Introduction

The lasing effect was demonstrated in laser-induced plasmas (LIP) of elements from the 13th and 14th groups and Ca, Ti, Zr, Fe, Cu, and V [[1], [2], [3], [4], [5], [6], [7], [8]]. Very briefly, LIP, created on the sample surface, is pumped by an OPO laser tuned in resonance with a strong optical transition of the specific element. As a result, intense (up to six orders of magnitude stronger than LIBS [2]), sometimes polarized, and low-divergent radiation (stimulated emission (SE)) is emitted. Placing plasma inside the optical resonator strongly increases its intensity and diminishes the beam divergence. We call this effect laser-induced plasma laser (LIPL). The unique generation mechanism of LIPLs allows one to study various LIP properties and fundamental laser-plasma physics problems. A well-collimated SE, emitted in both forward and backward directions regarding the pumping beam, can be used in sensing systems for the remote detection and identification of objects under investigation. LIPLs may be used as coherent light sources also.

The starting step of the LIBS/LIPL processes is the ablation of the material by intense laser light. Laser ablation is utilized in processes, such as chemical analysis by LIBS [[9], [10], [11]] nanoparticles [12], depth profiling [13], thin-film investigations [14,15]. Repetitive ablation during successive laser pulses creates a crater on a sample. The crater's influence on LIBS parameters has been investigated theoretically and experimentally by many researchers (see for example [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]]. In many cases, increasing crater depth at the beginning of the laser shot series increases LIBS signal, but subsequent increasing crater depth diminishes LIBS intensity. The common acceptable explanation is that there are some competing processes: laser beam interaction with crater walls and plasma confining that leads to signal enhancement at the shallow craters, and plasma cooling by deep crater walls diminish LIBS signal [[23], [24], [25]]. In artificial craters (a preliminary drilled hole, external cylinder placed on sample surface e. d.), LIBS enhancement also depends on cavity height/diameter ratio [23,[26], [27], [28]]. The ratio optimum varies from 1/5 [27] to 3/0.5 [28], which, obviously, depends on experimental conditions.

Many LIBS investigations, reported in the literature, use Gaussian beam lasers, focused by the spherical lens and producing irradiance 109–1010 W/cm2. Such irradiance creates semispherical LIP plumes and produces conical craters.

An elongated plasma shape is preferable for LIPL because it produces a much stronger generation in the long plasma plume axis direction. This shape of the plasma plume is rarely used in LIBS practice; exceptions may be the investigations from Moscow University [[29], [30], [31]]. Effects of the repetitive laser shots on elongated plasma and LIPL properties are not investigated to the best of our knowledge.

The repetitive ablation laser shots and elongated crater shape on LIBS/LIPL are investigated in the present research. Furthermore, effects are investigated on samples with different thermo-mechanical properties, and significant differences were revealed. Here, we were focused on improving LIPL reproducibility to develop a stable laser source based on LIPL principles.

Section snippets

Experimental

The experimental setup is similar to that described elsewhere [2,7,8] and is schematically shown in Fig. 1. A transverse pumping scheme is used for LIPL generation. The sample plate is placed inside a plane parallel resonator on an X-Y-Z micrometer translation stage so that the sample location could be changed after each laser pulses series. Z translation allows one to change the sample's position relative to lens 1. The output resonator mirror reflection is optimized for maximal lasing output.

Crater

Fig. 3 presents examples of 3D craters imaging measured after 200 (a), 2000 (c), and 8000 pulses (e). Fig. 3b, d, f show examples of crater cross-sections.

Craters have a truncated rectangular pyramidal shape under weak ablation laser focusing. A crater formation occurs due to the melt-flush mechanism; peaks and valleys on the crater's bottom and rim around the crater are due to the evaporated material's subsequent partly resolidification [[15], [16], [17], [18], [19],37]. Crater volume Vc was

Conclusions

It is shown that elongated crater shapes depend mostly on ablation laser fluence and do not depend much on the sample.

LIBS and LIPL intensities versus ablation laser pulses are about the same despite that LIBS emission is proportional to the number of atoms in excited states, but LIPL intensity is proportional to the number of atoms in the ground state.

LIBS/LIPL intensities vary on ablation laser pulses from a single peak (Al) to plateau (Cu) or double peaks (iron, bronze, brass). These

Author statement

We respond on all reviewers questions.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (49)

  • A. Zakuskin et al.

    Emission spectroscopy of long cylindrical laser spark with additional coaxial excitation

    Spectrochim. Acta Part B Atom. Spectrosc.

    (2019)
  • L. Cabalin et al.

    Experimental determination of laser induced breakdown thresholds of metals under nanosecond Q-switched laser operation

    Spectrochim. Acta B

    (1998)
  • A. De Giacomo

    A novel approach to elemental analysis by Laser Induced Breakdown Spectroscopy based on direct correlation between the electron impact excitation cross section and the optical emission intensity

    Spectrochim. Acta B

    (2011)
  • I. Gornushkin et al.

    Radiative models of laser-induced plasma and pump-probe diagnostics relevant to laser-induced breakdown spectroscopy

    Spectrochim. Acta B

    (2010)
  • R. Noll et al.

    Laser-induced breakdown spectroscopy – from research to industry, new frontiers for process control

    Spectrochim. Acta B

    (2008)
  • A. El Sherbini et al.

    Evaluation of self-absorption coefficients of aluminum emission lines in laser-induced breakdown spectroscopy measurements

    Spectrochim. Acta Part B

    (2005)
  • R. Glaus et al.

    Stimulated emission in aluminum laser-induced plasma: experimental study

    Appl. Opt.

    (2017)
  • I. Gornushkin et al.

    Stimulated emission in aluminum laser-induced plasma: the kinetic model of population inversion

    Appl. Opt.

    (2017)
  • I. Gornushkin et al.

    Kinetic model of stimulated emission created by resonance pumping of aluminum laser-induced plasma

    J. Appl. Phys.

    (2017)
  • L. Nagli et al.

    Polarization effects in laser-induced plasma lasers based on elements from the 13th group

    J. Appl. Phys.

    (2021)
  • D. Cramers et al.

    Handbook of Laser-Induced Breakdown Spectroscopy

    (2006)
  • A. Miziolek et al.

    Laser-Induced Breakdown Spectroscopy (LIBS) Fundamental and Applications

    (2006)
  • R. Noll

    Laser-Induced Breakdown Spectroscopy, Fundamentals and Applications

    (2012)
  • G. Asimellis et al.

    Rapid, automated measurement of layer thicknesses on steel coin blanks using laser-induced-breakdown spectroscopy depth profiling

    Appl. Opt.

    (2007)
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