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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 20 — Oct. 1, 2007
  • pp: 12905–12915
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Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse

Dennis K. Killinger, Susan D. Allen, Robert D. Waterbury, Chris Stefano, and Edwin L. Dottery  »View Author Affiliations


Optics Express, Vol. 15, Issue 20, pp. 12905-12915 (2007)
http://dx.doi.org/10.1364/OE.15.012905


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Abstract

For the first time to the best of our knowledge, a simultaneous 10.6 μm CO2 laser pulse has been used to enhance the Laser Induced Breakdown Spectroscopy (LIBS) emission from a 1.064 μm Nd:YAG laser induced plasma on a hard target. The enhancement factor was on the order of 25 to 300 times, depending upon the emission lines observed. For an alumina ceramic substrate the Al emission lines at 308 nm and Fe impurity line at 278 nm showed an increase of 60x and 119x, respectively. The output energy of the Nd:YAG laser was 50 mJ/pulse focused to a 1 mm diameter spot to produce breakdown. The CO2 laser pulse had a similar energy density of 40 mJ/mm2. Timing overlap of the two laser pulses within 1 microsecond was important for enhancement to be observed. An observed feature was the differential enhancement between different elemental species and also between different ionization states, which may be helpful in the application of LIBS for multi-element analysis.

© 2007 Optical Society of America

1. Introduction and background

Laser-Induced-Breakdown Spectroscopy (LIBS) is a recognized laser detection technique for sensing the chemical composition of a wide range of materials including minerals, chemical substances, and trace species [1–3

1. K. Song, Y. I. Lee, and J. Sneddon, “Applications of laser-induced breakdown spectrometry,” Appl. Spectrosc. Rev. 32, 183–235 (1997). [CrossRef]

]. Recently, LIBS is being studied for the remote detection of a wide variety of substances such as surface contaminants and other trace materials [4

4. C. Lopez-Moreno, S. Palanco, J. J. Laserna, F. DeLucia Jr., A. W. Miziolek, J. Rose, R. A. Walters, and A. I. Whitehouse, “Test of a stand-off laser-induced-breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces,” J. Anal. At. Spectrom. 21, 55 (2006). [CrossRef]

]. There is a need to increase the detection range of such standoff LIBS systems, and, as a result, the need to increase the strength of the LIBS signal.

Toward this end, the work presented in the paper is the first demonstration of significant enhancement of a LIBS emission by several orders of magnitude using a simultaneous CO2 laser pulse to enhance the initial Nd:YAG induced LIBS plasma emission. We found that enhancements on the order of 25 to 300 could be obtained for the LIBS emission from ceramic (alumina) targets. Laser pulse energy density on target was about 50 mJ/mm2 for both lasers, with LIBS emission depending upon both the Nd:YAG and the CO2 laser pulse intensities. We have also studied the overlap dependence of the two laser pulses and found that the CO2 laser pulse needs to be simultaneous with the plasma formed (on the order of the pulse width of the CO2 laser). While additional studies need to be conducted to better quantify the effects of laser beam mode, temporal pulse shaping, individual emission line ionization states, and optimization for different LIBS target composition, our new measurements qualitatively indicate that significant enhancements may be feasible and may enhance the use of LIBS for detection and identification of chemical compounds, biological species, and trace chemical films within a background matrix. It is important to note that the use of a CO2 laser for enhancement is a technique that may have eye-safety advantages for stand-off applications; laser wavelengths greater than about 1.4 μm have several orders of magnitude greater maximum permissible exposure limits for direct ocular view.

2. Experimental apparatus

A schematic of our simultaneous dual-laser enhanced LIBS system is shown in Fig. 1.

Fig. 1. Schematic of two laser LIBS system for CO2 laser enhancement of Nd:YAG induced LIBS plasma emission

A Q-switched Nd:YAG laser (Big Sky Laser Model CRF200; 50 mJ/pulse, 5 Hz, 5 ns pulse length, M2 = 5) was focused by a 10 cm focal length lens onto the target. The focused spot size was measured (by burn patterns) to be about 1 mm in diameter. The plasma produced on the target emitted LIBS emission into a 2π steradian solid-angle cone, which was collected using a 5 cm diameter lens (focal length of 10 cm) focused onto a fiber optic cable. The 300 micron core fiber optic cable transmitted the light to a spectrometer (Ocean Optics: Model HR2000; 200nm – 1100 nm) where it was detected by a linear photodiode array. An order-sorting filter in the spectrometer reduced second order features by a factor of 100 fold. The LIBS spectra from the spectrometer was then transferred to a notebook computer.

A high-power Q-switched CO2 Transverse Electrode Atmospheric (TEA) laser (Lumonics Model 920; 1.3 J/pulse, 5 Hz) was used to produce 10.6 μm laser pulses that were then routed using mirrors onto the same LIBS emission target area. The pulse length of the CO2 laser pulse had an initial TEA laser spike of 100 ns length followed by a nitrogen-fed tail about 5 μs long. The CO2 laser output beam size was controlled using a lens to have a diameter on target between 3 mm to 15 mm. For a 6.5 mm diameter beam, the energy density was about 40 mJ/mm2. The timing of the laser pulses was controlled with digital time-delay generators (SRS Model DG535), and the laser pulses were detected using fast Si photodiodes (Nd:YAG laser) and photon-drag detectors (CO2 laser). A second high speed Si detector (not shown in Fig. 1) was placed within a few cm of the plasma and used to observe the total UV-visible emission from the LIBS plasma; this signal gave an indication of the intensity and temporal length of the plasma. The laser signals and respective timing overlap was displayed on a high-speed digital oscilloscope (LeCroy Model 940). The timing uncertainty (jitter) of the lasers was about 20 ns for the Q-switched YAG laser and about 500 ns for the TEA CO2 laser. Timing errors of the gate electronics were negligible, on the order of a ns.

It should be added that the pulse-to-pulse stability of the YAG laser pulse energy was about 2% and about 5% for the CO2 laser. The spatial profile of the YAG laser was approximately a lower order top-hat mode and appeared stable in mode pattern. However, the Lumonics laser may have had a less stable spatial mode pattern due to small changes in the gain characteristics within the TEA discharge on a pulse-to-pulse basis.

The angular separation of the Nd:YAG beam and CO2 beam was about 30 degrees, with the LIBS collection optics aligned at an angle of about 20 degrees from that of the Nd:YAG beam. The Nd:YAG laser beam was at near normal incidence to the sample.

Most of the data was taken using a high purity alumina target from CoorsTek (AD-998), which was 99.82% Al2O3. Listed impurities were: Na2O 0.08%, MgO 0.07%, Fe2O3 0.01%, and SiO2 0.02%.

3. LIBS measurements: Nd:YAG laser alone

As can be seen, several LIBS lines are evident including two strong lines at 394.40 and 396.15 nm, most probably due to Al I (where the normal convention is used of I = neutral, II = singly ionized) [13

13. CRC Handbook of Chemistry and Physics, Table of Atomic Spectra (2004).

]. These lines are just barely resolved by the portable spectrometer used for these initial results. Other Al I lines are observed at 308.22 and 309.27 nm (not resolved), 226.91 nm, 237.31 nm and 281.62 nm. There is also a weak Al II line at 422.68 nm. An Al II line at 281.62 nm is barely observable [13

13. CRC Handbook of Chemistry and Physics, Table of Atomic Spectra (2004).

]. There is a trio of weak Mg I lines at 382.93, 383.23 and 383.83 nm which are not resolved. The broad line centered at 486.68 may be due to H I, although the line is broad and may be more molecular in nature. The line that at 517 nm is composed of three Mg I lines at 516.73, 517.27 and 518.36 nm. The strong line at 589 nm is actually the Na I doublet at 588.995 and 589.59 nm. The absolute error in the spectrometer wavelength reading was about 1 nm.

Fig. 2. LIBS signal from ceramic alumina target for Nd:YAG laser initiated plasma; emission lines are tentatively identified and listed in nm.

4. Enhanced LIBS measurements: Nd:YAG laser and CO2 laser

In order to ascertain the effect of the Nd:YAG laser power on the LIBS signal and make sure that the emission is linear with the plasma formed, Fig. 4 shows the LIBS signal of several of the prominent lines as a function of the Nd:YAG laser energy. Taking into account the errors (1 nm) in the spectrometer wavelength readings, we identified the emission lines in Fig. 4 as probably due to Al (237 nm), Mg II (279 nm), Mg (383 nm), Al (394 nm), and H (656 nm), although the latter line at 656 nm is rather broad and may be more molecular in nature.

As can be seen in Fig. 4, a threshold in the plasma formation or LIBS emission is observed at an energy of about 10 mJ (energy density of about 10 mJ/mm2 for a beam area of 1 mm2). The emission intensity increases almost linearly with ND: YAG laser energy density above the threshold. This indicates that increasing the LIBS signal should be possible with increasing Nd:YAG laser power and that we were not saturating the Nd:YAG laser LIBS emission due to other possible effects such as quenching or target film ablation.

The influence of the CO2 laser energy on the dual-laser LIBS emission was investigated for several neutral and singly ionized emission lines. Figures 5–7 show plots of the enhancement factor of the LIBS signal as a function of the CO2 laser energy density for different selected emission elements and lines, where we have defined the LIBS enhancement factor as the ratio of the enhanced LIBS signal compared to the corresponding Nd:YAG LIBS signal after appropriate background (detector dark current) correction. It is important to note that, while the absolute accuracy in the line wavelengths was 1 nm, the identification of identical lines between the spectra in Fig. 2 and that in Fig. 3 for calculation of the enhancement ratio was much better, within about 0.1 nm, because the experimental geometry was sufficiently similar to allow the spectra to be directly compared.

Fig. 3. LIBS emission signal for Nd:YAG laser initiated plasma and CO2 laser enhancement
Fig. 4. LIBS signal for several emission lines (listed in nm) as a function of Nd:YAG laser energy (using Nd:YAG laser alone).
Fig. 5. LIBS enhancement ratio for Al LIBS lines (listed in nm) as a function of CO2 laser intensity.
Fig. 6. LIBS enhancement ratio for Al II LIBS lines (listed in nm) as a function of CO2 laser intensity.
Fig. 7. LIBS enhancement ratio for Mg II LIBS lines (listed in nm) as a function of CO2 laser intensity.

Here, the Nd:YAG laser energy density was 50 mJ/mm2 and the CO2 laser was controlled using attenuators, aperture control, and lens focus to vary the energy density at the plasma location. As can be seen, the enhancement of the LIBS lines increases with CO2 laser power, but only up to an energy threshold near 100 mJ/mm2 where the onset of plasma formation due to the CO2 laser alone was observed. The enhancement factor was found to be on the order of 25 to 300 times, depending upon the emission lines observed. For an alumina ceramic substrate the Al emission lines at 308 nm and Fe impurity line at 278 nm showed an increase of 60× and 119×, respectively. In addition, the enhancement of the LIBS lines was seen to vary, depending on which species was being observed, illustrating the need for additional study to determine the physical interactions, including ionization state coupling, on the plasma interactions. Overall, our data seems to indicate that the enhancement factor may be greater for the ionized states.

5. Timing overlap of two LIBS laser pulses

We investigated the timing overlap of the Nd:YAG laser and CO2 laser pulse. For reference, Fig. 8 shows a plot of the scope trace of the CO2 laser pulse (blue bottom trace) and the Nd:YAG laser pulse (upper green trace) for the case where the CO2 laser pulse preceded the Nd:YAG plasma formation. In this case, no enhancement in the LIBS signal was observed.

Fig. 8. Oscilloscope trace of Si detector (upper trace: Nd:YAG and plasma total emission) and CO2 laser pulse (lower trace) when CO2 laser leads Nd:YAG laser pulse. No enhancement in the LIBS emission was observed.

It should be noted that the upper Nd:YAG trace is from the Si photodetector which registers scattered Nd:YAG incident laser light and the UV and visible emission from the plasma. Subtracting the 5 ns Nd:YAG pulse, the plasma emission can be seen in the upper trace to have a lifetime (1/e) of about 1 μs.

When the CO2 laser pulse was delayed to more closely overlap the Nd:YAG induced plasma, then an enhanced LIBS emission was observed and the plasma emission was increased. This is seen in Fig. 9 which shows the same two laser pulses but with the CO2 laser delayed to about 1/2 μs after the initial Nd:YAG pulse.

The enhancement in the LIBS lines was measured as a function of the delay of the CO2 laser pulse compared to the YAG laser pulse, and the results are shown in Fig. 10 for several of the LIBS lines. Significant enhancement occurred when there was between 0 to 0.5 μs delay of the CO2 pulse. However, as indicated earlier, there was timing jitter in controlling the CO2 laser pulse on the order of 0.5 μs. As can be seen, there are some differences in the enhancement for different emission lines, and further studies are being conducted to better quantify our results. It is important to note that the long tail of the CO2 pulse is probably contributing to the enhancement effect, but further controlled studies are required using a short TEA CO2 laser pulse with no tail to better determine how much of the enhancement is due to the CO2 laser pulse heating the plasma to a higher temperature and how much of the enhancement is due to extending the lifetime of the plasma via the longer CO2 pulse. We are planning to conduct these studies and will report on them in the future.

Fig. 9. Trace of Si detector and CO2 detector for optimal overlap and LIBS enhancement.
Fig. 10. Amplitude of LIBS emission lines (listed in nm) as a function of delay between ND:YAG and CO2 laser pulses

6. Discussions and mechanisms

As the delay experiments show, the enhancement is only in effect when the CO2 laser has significant overlap with the Nd:YAG plasma, but can prolong the existence of the plasma significantly as shown in Fig. 10. In fact, the delay experiments show a convolution of the Nd:YAG and the CO2 pulse profiles with a width of several microseconds, corresponding to a significant fraction of the CO2 pulse duration.

In summary, we report for the first time the enhancement of LIBS intensities via coincident CO2 laser irradiation with resulting increases of emission intensities of greater than two orders of magnitude in some cases. In addition, we are in the process of investigating and quantifying this enhancement factor using other target materials and will report on these results at a later date.

Acknowledgments

References and links

1.

K. Song, Y. I. Lee, and J. Sneddon, “Applications of laser-induced breakdown spectrometry,” Appl. Spectrosc. Rev. 32, 183–235 (1997). [CrossRef]

2.

L. J. Radziemski and D. A. Cremers, “Spectrochemical analysis using laser plasma excitation,” L. J. Radziemski and D. A. Cremers, eds., in Laser-Induced Plasma: Physical, Chemical and Biological Applications, (Marcel Dekker, New York, 1989).

3.

A. Miziolek, V. Palleschi, and I. Schechter, eds., Laser Induced Breakdown Spectroscopy, (Cambridge University Press2006). [CrossRef]

4.

C. Lopez-Moreno, S. Palanco, J. J. Laserna, F. DeLucia Jr., A. W. Miziolek, J. Rose, R. A. Walters, and A. I. Whitehouse, “Test of a stand-off laser-induced-breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces,” J. Anal. At. Spectrom. 21, 55 (2006). [CrossRef]

5.

D. N. Stratis, K. L. Eland, and S. M. Angel, “Enhancement of aluminum, titanium, and iron in glass using pre-ablation spark dual-pulse LIBS,” Appl. Spectrosc. 54, 1719 (2000). [CrossRef]

6.

J. Scaffidi, W. Pearman, J. C. Carter, and S. M. Angel, “Observations in collinear femtosecond-nanosecond dual-pulse laser-induced breakdown spectroscopy,” Appl. Spectrosc. 60, 65 (2006). [CrossRef] [PubMed]

7.

J. Scaffidi, J. Pender, W. Pearman, S. R. Goode, B. W. Colston Jr., J. C. Carter, and S. M. Angel, “Dual-pulse laser-induced breakdown spectroscopy with combinations of femtosecond and nanosecond laser pulses,” Appl. Opt. 42, 6099 (2003). [CrossRef] [PubMed]

8.

J. Gonzalez, C. Y. Liu, J. H. Yoo, X. L. Mao, and R. E. Russo, “Double-pulse laser ablation ICP-MS and LIBS,” Spectrochim. Acta. 60, 27 (2005). [CrossRef]

9.

V. I. Babushok, F. C. DeLucia Jr., J. L. Gottfried, C. A. Munson, and A. W. Miziolek, “Double pulse laser ablation and plasma: Laser induced breakdown spectroscopy signal enhancement,” Spectrochimica Acta Part B-Atomic Spectrosc. 61, 999 (2006). [CrossRef]

10.

P. Mukherjee, S. Chen, and S. Witanachchi, “Effect of initial plasma geometry and temperature on dynamic expansion in dual-laser ablation,” Appl. Phys. Lett. 74, 1546 (1999). [CrossRef]

11.

K. Song, Y. Lee, and J. Sneddon, “Recent developments in instrumentation for laser induced breakdown spectroscopy,” Appl. Spectrosc. Rev. 37, 89 (2002). [CrossRef]

12.

S. Y. Chan and N. H. Cheung, “Analysis of solids by laser ablation and resonance-enhanced laser-induced spectroscopy,” Anal. Chem. 72, 2087 (2000). [CrossRef] [PubMed]

13.

CRC Handbook of Chemistry and Physics, Table of Atomic Spectra (2004).

14.

V. N. Rai, F. Y. Yueh, and J. P. Singh, “Optical emission from laser-produced chromium and magnesium plasma under the effect of two sequential laser pulses,” Pramana-J. of Phys. 65, 1075 (2005). [CrossRef]

15.

A. G. Leonov, D. I. Chekhov, and A. N. Staristin, “Mechanisms of resonant laser ionization,” J. Exp. Theor. Phy. 84, 703 (1997). [CrossRef]

16.

J. S. Townsend, Electricity in Gases, (Oxford University Press, New York, 1914); reprinted Wexford College Press (2007).

17.

J. Rahel, M. Sira, P. Stahel, and D. Trunec, “The transition between different discharge regimes in atmospheric pressure air barrier discharge,” Contrib. Plasma Phys. 47, 34 (2007). [CrossRef]

18.

N. Jidenko, C. Jimenez, F. Massines, and J.P. Borra, “Nano-particle size-dependent charging and electro-deposition in dielectric barrier discharges at atmospheric pressure for thin SiOx film deposition,” J. Phys. D 40, 4155 (2007). [CrossRef]

OCIS Codes
(300.6365) Spectroscopy : Spectroscopy, laser induced breakdown

ToC Category:
Spectroscopy

History
Original Manuscript: July 17, 2007
Revised Manuscript: September 9, 2007
Manuscript Accepted: September 12, 2007
Published: September 24, 2007

Citation
Dennis K. Killinger, Susan D. Allen, Robert D. Waterbury, Chris Stefano, and Edwin L. Dottery, "Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse," Opt. Express 15, 12905-12915 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-20-12905


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References

  1. K. Song, Y. I. Lee, and J. Sneddon, "Applications of laser-induced breakdown spectrometry," Appl. Spectrosc. Rev. 32, 183-235 (1997). [CrossRef]
  2. L. J. Radziemski and D. A. Cremers, "Spectrochemical analysis using laser plasma excitation," L. J. Radziemski and D. A. Cremers, eds., in Laser-Induced Plasma: Physical, Chemical and Biological Applications, (Marcel Dekker, New York, 1989).
  3. A. Miziolek, V. Palleschi, and I. Schechter, eds., Laser Induced Breakdown Spectroscopy, (Cambridge University Press 2006).
  4. C. Lopez-Moreno, S. Palanco, J. J. Laserna, F. DeLuciaJr., A. W. Miziolek, J. Rose, R. A. Walters, and A. I. Whitehouse, "Test of a stand-off laser-induced-breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces," J. Anal. At. Spectrom. 21, 55 (2006). [CrossRef]
  5. D. N. Stratis, K. L. Eland, and S. M. Angel, "Enhancement of aluminum, titanium, and iron in glass using pre-ablation spark dual-pulse LIBS," Appl. Spectrosc. 54, 1719 (2000). [CrossRef]
  6. J. Scaffidi, W. Pearman, J. C. Carter, and S. M. Angel, "Observations in collinear femtosecond-nanosecond dual-pulse laser-induced breakdown spectroscopy," Appl. Spectrosc. 60, 65 (2006). [CrossRef]
  7. J. Scaffidi, J. Pender, W. Pearman, S. R. Goode, B. W. Colston, Jr., J. C. Carter, and S. M. Angel, "Dual-pulse laser-induced breakdown spectroscopy with combinations of femtosecond and nanosecond laser pulses," Appl. Opt. 42, 6099 (2003). [CrossRef]
  8. J. Gonzalez, C. Y. Liu, J. H. Yoo, X. L. Mao, and R. E. Russo, "Double-pulse laser ablation ICP-MS and LIBS," Spectrochim. Acta. 60, 27 (2005).
  9. V. I. Babushok, F. C. DeLuciaJr., J. L. Gottfried, C. A. Munson, and A. W. Miziolek, "Double pulse laser ablation and plasma: Laser induced breakdown spectroscopy signal enhancement," Spectrochimica Acta Part B-Atomic Spectrosc. 61, 999 (2006).
  10. P. Mukherjee, S. Chen, and S. Witanachchi, "Effect of initial plasma geometry and temperature on dynamic expansion in dual-laser ablation," Appl. Phys. Lett. 74, 1546 (1999). [CrossRef]
  11. K. Song, Y. Lee, and J. Sneddon, "Recent developments in instrumentation for laser induced breakdown spectroscopy," Appl. Spectrosc. Rev. 37, 89 (2002). [CrossRef]
  12. S. Y. Chan and N. H. Cheung, "Analysis of solids by laser ablation and resonance-enhanced laser-induced spectroscopy," Anal. Chem. 72, 2087 (2000). [CrossRef]
  13. CRC Handbook of Chemistry and Physics, Table of Atomic Spectra (2004).
  14. V. N. Rai, F. Y. Yueh, and J. P. Singh, "Optical emission from laser-produced chromium and magnesium plasma under the effect of two sequential laser pulses," Pramana-J. of Phys. 65, 1075 (2005).
  15. A. G. Leonov, D. I. Chekhov, and A. N. Staristin, "Mechanisms of resonant laser ionization," J. Exp. Theor. Phy. 84, 703 (1997).
  16. J. S. Townsend, Electricity in Gases, (Oxford University Press, New York, 1914); reprinted Wexford College Press (2007).
  17. J. Rahel, M. Sira, P. Stahel, and D. Trunec, "The transition between different discharge regimes in atmospheric pressure air barrier discharge," Contrib. Plasma Phys. 47, 34 (2007).
  18. N. Jidenko, C. Jimenez, F. Massines, and J.P. Borra, "Nano-particle size-dependent charging and electro-deposition in dielectric barrier discharges at atmospheric pressure for thin SiOx film deposition," J. Phys. D 40, 4155 (2007). [CrossRef]

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