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Nanosecond polarization-resolved laser-induced breakdown spectroscopy Yaoming Liu, John S. Penczak, and Robert J. Gordon* Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607-7061, USA *Corresponding author:
[email protected] Received October 26, 2009; revised November 23, 2009; accepted November 24, 2009; posted December 7, 2009 (Doc. ID 119074); published January 11, 2010 It is shown that the continuum emission produced in the ablation of an Al target with nanosecond laser pulses is much more strongly polarized than the discrete line emission. This effect may be utilized to improve the resolution of the laser-induced breakdown spectroscopy spectrum by using a polarizer to filter out the continuum background. The effects of laser fluence and focal position are also reported. It is further shown that the lifetime of the emission closely tracks the intensity spectrum. © 2010 Optical Society of America OCIS codes: 300.6365, 140.3440, 260.5430.
Laser induced breakdown spectroscopy (LIBS) is a powerful analytical tool that provides standoff elemental analysis of a wide variety of materials in solid, liquid, and gaseous states without any sample preparation [1,2]. A typical LIBS spectrum consists of a set of discrete atomic and ionic lines superimposed on a broad continuum [3,4]. Much of the research on LIBS is directed toward understanding the origin of the continuum and finding means of suppressing it in order to increase the signal/background (S/B) ratio. It is generally found that the continuum is much shorter lived than the discrete spectrum. This property allows the background to be suppressed by gating the detector [5,6]. The optimal start and stop times of the detection window are sensitive to the choice of target material [6,7] (which is not necessarily known in advance), the laser properties (wavelength, pulse duration, and focal point), and the specific elements to be detected. A better strategy might be to find a means of filtering out the continuum while minimally affecting the discrete spectrum. Recently we reported that the continuum produced by the ablation of solid targets in air with a femtosecond laser pulse is strongly polarized, with the magnitude of the polarization reaching values in excess of 95% for wavelengths ⬍350 nm, while the lines have little or no polarization [8]. This property was observed for various metals [9,10], semiconductors [8], and dielectrics [10], including crystalline and amorphous materials, and appears to be a general property of solid matter. By placing a polarizer before the detector, it is possible to greatly attenuate the continuum while having little effect on the discrete lines. We refer to this technique as polarization-resolved LIBS, or PRLIBS. Apart from its practical utility, polarization of the plasma continuum is of fundamental interest as a hitherto unexplored property of lasermatter interactions. Extension of the PRLIBS technique to the nanosecond regime could be of great utility, since LIBS measurements are most commonly performed on this time scale. The emission of polarized light implies that the electrons generating the light must be oscillating in some preferred direction. Such directional0146-9592/10/020112-3/$15.00
ity is likely to be quenched by collisions with air molecules. In the femtosecond regime, it plausible that the continuum decays before such collisions intervene. On the nanosecond time scale, however, strong polarization of the emitted light would seem to be less likely. The objective of the present study is to test this expectation. The experimental setup used here is similar to that employed in our recent studies [8–10], apart from the choice of the laser and the target material. Nanosecond pulses were obtained from the second harmonic of a Nd:YAG laser (Continuum Surelite, 532 nm, 4 ns pulse width), while femtosecond pulses were generated with a regeneratively amplified Ti:sapphire laser (Spectra Physics Tsunami oscillator and Spitfire amplifier, 800 nm central wavelength, and 65 fs pulse width after the focusing optics). The laser was focused onto the sample by a 100 mm focal length convex lens. Hand-polished, industrial-grade aluminum (alloy #7075, 91.0% Al, 6.0% Zn, 2.5% Mg) was mounted on a micrometer-precision xyz translation stage in open air. The stage was computer controlled to expose a fresh surface for each incident pulse. The laser was incident 30° from the normal direction of the sample surface, and its polarization plane was parallel to the sample surface (s polarized). The plasma plume was imaged normal to the direction of the laser beam onto the 50-m-wide entrance slit of a spectrograph (Spectrapro 2300i, Princeton Instruments) equipped with a nongated CCD (PIXIS 400, Princeton Instruments) camera. A Glan–Thompson polarizer mounted on a motorized rotation stage was inserted in front of the entrance slit of the spectrograph to measure the polarization of the plasma emission. Further details are provided in [8]. Figure 1 presents the primary and most surprising result of this study. The upper panel shows that the continuum is strongly polarized throughout the wavelength range investigated, with values of the polarization, P, exceeding 90% in some regions. In contrast, the discrete lines are much more weakly polarized and appear as windows in the polarization spectrum. Rotating the polarizer to minimize its transmission produces a well-resolved spectrum © 2010 Optical Society of America
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Fig. 1. (Color online) Plasma emission spectra produced by nanosecond laser ablation of Al at a fluence of 32 J / cm2. The laser focus was set to obtain maximum S/B. (a) PRLIBS spectrum with the polarizer angle set to minimize transmission of polarized light (solid curve) and the polarization spectrum obtained from Malus plots of the polarizer transmission (dots). (b) Comparison of the PRLIBS spectrum and the conventional LIBS spectrum obtained without a polarizer.
(referred to as the PRLIBS spectrum) with little background. It is interesting to compare these results with our earlier findings for femtosecond excitation. As shown in Fig. 2, the discrete lines again appear as minima in the polarization spectrum, but in this case the continuum polarization reaches a maximum value close to 100% and falls off at long wavelengths. Figure 3 shows the dependence of the polarization and PRLIBS spectra on laser fluence. We find that while P decreases with fluence, the S/B ratios for different peaks remain fairly constant, having values ⬃1 – 2 without the polarizer and ⬃5 – 7 with the polarizer. Similar behavior was found in the femtosecond-PRLIBS spectra of Si [8] and Al [9]. Figure 4 shows the effect of the focal point on the PRLIBS spectrum. The upper panel shows the unfiltered spectrum taken with the laser focused above the surface (z = −0.5 and −0.25 mm), on the surface
Fig. 2. (Color online) Plasma emission spectra produced by femtosecond laser ablation of Al at a fluence of 6.4 J / cm2. Other details are the same as in Fig. 1.
Fig. 3. (Color online) Effect of fluence on (a) the nanosecond-PRLIBS spectrum and (b) the polarization spectrum of Al. The fluences correspond to pulse energies of 25, 50, and 100 J.
共z = 0兲, and beneath the surface (z = 0.25 and 0.5 mm). For z 艋 0 the continuum completely obscures the discrete spectrum and saturates the detector. Previous nanosecond-LIBS studies [11,12], as well as our earlier femtosecond-LIBS study [9], also obtained the best S/B for focusing beneath the surface. With the polarizer in place (middle panel), the line spectrum is well resolved, with the largest S/B occurring for z ⬃ 0.25– 0.5 mm. As found previously for femtosecondPRLIBS, the effect of the polarizer is to make the spectrum much less sensitive to focal position, a property that could be useful for standoff detection. Figure 4(c) shows that P is largest for z ⬍ 0 (above the surface), a property that is consistent with the strongest continuum being produced there. From the polarization spectra it is obvious that the discrete and continuum emissions are generated by very different processes. The line spectrum is very likely produced by discrete electronic transitions of atoms and ions in the ablation plume. Polarized emission caused by nonstatistical distributions of magnetic sublevels is readily suppressed by randomizing collisions. The continuum emission, on the other hand, is generally attributed to free–free Bremsstrahlung collisions and bound–free electron recombination reactions [4]. Anisotropy in the velocity distribution of the electrons is transformed into
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Fig. 5. (Color online) Comparison of the decay time and PRLIBS spectrum of Al. The laser fluence was 32 J / cm2, and the focus was set to maximize S/B.
The decline of P with fluence indicates that the electronic anisotropy is diminished at higher plasma temperature. For the discrete emission, the electronically excited species survive multiple collisions before emitting a photon, and any initial polarization is substantially reduced.
Fig. 4. (Color online) Effect of laser focus on (a) the LIBS, (b) PRLIBS, and (c) polarization spectra of Al. The curves are for different focal points of the laser measured in millimeters, with negative values corresponding to the focus above the target. The laser fluence was 8 J / cm2.
polarization of the emitted photons [13]. Such processes are much faster than the radiative and collisional decay of electronically excited atoms and ions, and, indeed, the decay time of the continuum emission is found to be much shorter than that of the discrete spectrum in both the nanosecond [14] and femtosecond [14,15] regimes. This property is illustrated dramatically in Fig. 5, where the decay time is plotted in 1 nm intervals. These data were obtained from exponential fits to the time-resolved fluorescence measured with a photomultiplier tube (R212, Hamamatsu Photonics, 50 ⍀ load) attached to the output of the spectrograph. A plot of the decay time versus wavelength strongly resembles the PRLIBS spectrum, and at some wavelengths it displays even greater spectral resolution. Although it takes 2 orders of magnitude more time to acquire, the decay time spectrum is potentially a useful new spectroscopic tool. The shortest measured decay time of the continuum is 10 ns, which exceeds the pulse width of the laser. This result is surprising in that it suggests that the anisotropy of the electron velocity distribution survives multiple atmospheric collisions. A possible explanation is that the recombination reaction is the rate-limiting step, and randomizing collisions rapidly quench the emission, so that any emitted photons are detected before the anisotropy is lost.
Support by the National Science Foundation (NSF) and University of Illinois at Chicago is gratefully acknowledged. References 1. D. A. Cremers and J. L. Radziemski, Handbook of Laser-Induced Breakdown Spectroscopy (Wiley, 2006). 2. J. P. Singh and S. N. Thakur, Laser-Induced Breakdown Spectroscopy (Elsevier, 2007). 3. T. Fujimoto and A. Iwamae, eds., Plasma Polarization Spectroscopy (Springer, 2007). 4. T. Fujimoto, Plasma Spectroscopy (Oxford U. Press, 2004). 5. L. Dudragne, P. Adam, and J. Amouroux, Appl. Spectrosc. 52, 1321 (1998). 6. B. Le Drogoff, J. Margot, M. Chaker, M. Sabsabi, O. Barthelemy, T. W. Johnston, S. Laville, F. Vidal, and Y. von Kaenel, Spectrochim. Acta Part B 56, 987 (2001). 7. I. Bassiotis, A. Diamantopoulou, A. Giannoudakos, F. Roubani-Kalantzopoulou, and M. Kompitsas, Spectrochim. Acta Part B 56, 671 (2001). 8. Y. Liu, S. Singha, T. E. Witt, and R. J. Gordon, Appl. Phys. Lett. 93, 161502 (2008). 9. J. S. Penczak, Y. Liu, and R. J. Gordon, J. Phys. Chem. A 113, 13310 (2009). 10. Y. Zhao, S. Singha, Y. Liu, and R. J. Gordon, Opt. Lett. 34, 494 (2009). 11. L. St.-Onge, M. Sabsabi, and P. Cielo, J. Anal. At. Spectrom. 12, 997 (1997). 12. J. A. Aguilera, C. Aragón, and F. Peñalba, Appl. Surf. Sci. 127–129, 309 (1998). 13. H. M. Milchberg and J. C. Weisheit, Phys. Rev. A 26, 1023 (1982). 14. J. B. Sirven, B. Bousquet, L. Canioni, and L. Sarger, Spectrochim. Acta Part B 59, 1033 (2004). 15. K. L. Eland, D. N. Stratis, D. M. Gold, S. R. Goode, and M. Angel, Appl. Spectrosc. 55, 286 (2001).
