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Comparative study of laser-induced plasma emission from microjoule picosecond and nanosecond KrF-laser pulses
Spectrochimica Acta Part B 58 (2003) 497–510
Comparative study of laser-induced plasma emission from microjoule picosecond and nanosecond KrF-laser pulses G.W. Rieger1, M. Taschuk, Y.Y. Tsui, R. Fedosejevs* Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Canada T6G 2V4 Received 9 April 2002; accepted 15 January 2003
Abstract In this paper, the emission of laser produced silicon and aluminum plasmas is investigated in the energy range from 0.1 to 100 mJ (0.5–500 Jycm2) using 10 ns and 50 ps KrF laser pulses focused to a 5 mm diameter spot. For energies higher than 3 mJ, there is little difference between 50 ps and 10 ns pulses in the plasma emission both in terms of the intensity of the emission lines and in terms of lifetime of the emission. Differences become significant only at very low fluences approaching the plasma formation threshold, which is significantly lower for 50 ps pulses than for 10 ns pulses. Calculations using a plasma ablation model show that initial plasma conditions are significantly different for 50 ps and 10 ns pulses during irradiation by the laser pulses. However, the dominant process leading to plasma emission at later times is from expansion and cooling of the plasma plume in the form of a blast wave in the ambient air which is primarily dependent on the energy deposited in the plasma and not the pulse length. Calibrations have also been carried out in order to give the emission in absolute numbers of photons emitted and thus facilitate the comparison with modeling and future experiments. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: LIBS; Laser, plasma; Absolute calibration; Silicon; Aluminum; Plasma ablation model
1. Introduction Plasmas produced by short laser pulses of relatively low millijoule energies have attracted considerable interest in the past due to their application in different elemental analysis methods based on laser ablation such as laser-induced breakdown spectroscopy (LIBS) or laser ablation inductively coupled plasma techniques. In addition, such plas*Corresponding author. E-mail address: [email protected] (R. Fedosejevs). 1 Present address: Department of Physics, University of British Columbia, Vancouver, Canada V6T 1Z1.
mas are of interest in other applications such as micromachining. In this work, microplasmas produced by focusing microjoule energy laser pulses to small 5 mm diameter focal spots are studied in order to characterize the emission for LIBS at very low energies. In a previous study w1x, it was shown that it is possible to achieve low detection limits in the parts per million range for trace elements in aluminum alloys using energies in the range of only (100–200) mJ compared to the tens of millijoules typically encountered in LIBS. Due to the lower energies and the smaller focal spots, the damage to the sample surface is greatly reduced
0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547(03)00014-4
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opening up the possibility of performing surface scans with high lateral resolution w1–3x. The effect of laser pulses of different durations have been compared in the past, mostly focusing on the amount of ablated material. However, a few recent studies have also compared the spectral emission of plasmas generated by nanosecond pulses with those generated by picosecond pulses w4x and recently also by femtosecond pulses w5– 7x. These studies indicate that the shorter pulse duration might be more effective for LIBS. Eland et al. have recently reported on two studies, one comparing 1.3 ps and 7 ns pulses w4x and the other comparing 140 fs and 7 ns pulses w5x in LIBS. Unfortunately, different lasers of different wavelengths and significantly different energies were used in these studies (200 mJ, 1064 nm, 7 ns vs. 1 mJ, 570 nm, 1.3 ps and 810 nm, 1 mJ, 140 fs) leading to a much larger plasma in the case of nanosecond ablation and making a direct comparison problematic. However, the authors obtain a better precision in their calibration curves when using the short laser pulses, especially in case of femtosecond ablation. Margetic et al. w6x who investigated the spectra of brass samples using 6 ns and 170 fs pulses of 0.1–1 mJ energy from the same laser also find a better precision with short pulses in their LIBS experiment. In addition, the authors quote the higher precision in sample ablation, lower continuum background coupled with faster plasma dissipation and the possible use of a non-gated detector as advantages due to the use of the shorter pulse duration. Le Drogoff et al. w7x recently published LIBS measurements on aluminum with 100 fs pulses (800 nm), in which the authors compare their results to a previous LIBS study with 8 ns pulses (1064 nm) by Sabsabi and Cielo w8x. The energy density of 20 Jycm2 and the spotsize of 0.6 mm were chosen to approximately match those of Sabsabi and Cielo (21 Jycm2, 0.6 mm spot size). Since the temporal evolution of the plasma was found to be different for femtosecond and nanosecond pulses, Le Drogoff et al. use a shorter delay time of 1 ms and a shorter gate width of 5 ms for their detection times than Sabsabi and Cielo who used a 10 ms delay and a 10 ms gate width. Le Drogoff et al. present detection limits that are approximately a factor of 3–8 higher
than those obtained by Sabsabi and Cielo, except in the case of copper, where the LOD of 7 ppm obtained with 100 fs is slightly below the previous result of 10 ppm obtained with 8 ns pulses. Le Drogoff et al. believe that the full potential of the short pulse duration was not reached in their study and that, after optimization, the femtosecond pulses will yield better analytical results than the nanosecond pulses. Studies characterizing the initiation and evolution of the plasma for picosecond laser pulses w9– 11x have also been reported recently. The dynamics of air breakdown w9x, initiation of plasma formation w10x and the various thresholds from onset of ablation to the formation of a hot electronic plasma w11x are discussed in these references for 35 ps 1064 nm pulses. However, as discussed later, the air breakdown plays a much smaller role for the 248 nm ultraviolet laser pulses used in the present study. In addition, the initial threshold for ablation can occur with little excitation of optical emission and the current study focuses on characterization of the emission which is of interest for LIBS. In this paper, we report on measurements of emission from microplasmas produced by 50 ps (10–10 000 GWycm2) and 10 ns (0.05–50 GWy cm2) KrF laser pulses (248 nm) focused to a spot size of 5 mm using pulse energies in the range of 0.1–100 mJ. The low energies are not only interesting from a fundamental perspective but will also become important in applications in the near future due to the availability of low-cost microchip lasers that are capable of generating short pulses of microjoule energies with high repetition rates. Another important consequence of using very low energies is the fact that the heated plasma is relatively small, cools rapidly, and thus does not emit much continuum radiation. Usually, the detection of spectra is delayed in LIBS in order to avoid the continuum emission from the early hot and dense phase of the plasma immediately after plasma formation. Optimum signal to noise ratios are obtained when the plasma has already considerably cooled off, typically hundreds of nanoseconds up to a few microseconds after the laser pulse. Thus, expensive intensified multi-channel detectors that allow delayed gating of the observation on a nanosecond time scale have become standard in
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Fig. 1. Experimental set-up. MO, UV-microscope objective; L1, achromat (fs125 mm); L2, lens (fs100 mm); D, dichroic mirror (for 248 nm); M, Al-mirror; SPECT, ARC 500 spectrometer with OMA detector; PD, calibrated photodiode; CCD, CCD camera; PMT, photomultiplier tube; IF, Interference filter.
LIBS. An important result of this work is that for low energies, delaying the detection with respect to the laser pulse is no longer necessary allowing the use of ungated CCD cameras as detectors. In order to understand the scaling issues involved, the expected background plasma conditions are estimated using calculations based on a plasma ablation model w12,13x. 2. Experiment The experimental set-up for LIBS, shown in Fig. 1, is described in detail elsewhere w1x so only a brief introduction of the main components is given here. The KrF-excimer laser operates at 248 nm (Excimer 540, Lumonics Corp.) and emits pulses of 10 ns (FWHM) duration when operated at low energies. It can also be seeded with single frequency doubled short pulses from a dye laser (FL 4000, Lambda Physik) yielding 50 ps pulses at energies of up to several millijoules. The laser system is similar to that described in Ref. w14x but with single pulse injection and operating at rela-
tively low gain yielding single high contrast 50 ps output pulses. In order to demonstrate that there was no influence from amplified spontaneous emission or additional weak pulses, test cases were carried out using very high contrast clean single pulses. Low energy test shots were taken with pulses directly from the frequency doubled dye laser with energies of approximately 5 mJ. At higher energies comparisons were performed using the laser system together with a saturable absorber to ensure clean single pulses w15x. The results obtained using these very high contrast pulses did not differ significantly from those obtained using the normal laser system as described above. The data presented here are due to clean single pulses from the normal laser system. The pulse shape was monitored on an oscilloscope for every shot and shots with imperfect pulse shapes were not considered for analysis. The laser pulses are focused on the samples with a microscope objective (Optics For Research, 15 mm working distance, 10=, NAs0.25 or Carl
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Zeiss, 10=, NAs0.2) that is also used for collecting the radiation emitted by the plasma. The samples used for the experiments were commercially polished silicon wafers and hand polished aluminum plates. The silicon samples had a highly smooth and polished surface down to the submicron scale level. The aluminum samples, while visibly shiny, still had micron scale surface roughness. In all experiments presented in this paper only a small fraction of the full laser beam is selected by an aperture of 3 mm diameter and then expanded with a telescope to 10 mm in order to slightly overfill the microscope objective and to provide a relatively homogeneous laser spot. The focal spot size measured in the plane of the target surface in a knife-edge experiment is approximately 5 mm limited by the quality of the laser beam for both objectives used. A spectrometer (Spectra Pro 500, Acton Research) equipped with three gratings and an intensified multichannel array detector (OMA 1455 G, Princeton Instruments) is used for spectrally resolved light detection. A highresolution grating (1200 linesymm) blazed at 500 nm is used in all experiments described here and the width of the entrance slit is 10 mm yielding an instrumental linewidth of 0.09 nm. The gate width of the OMA detector is 200 ns and no gate delay is used in experiments with laser energies below 10 mJ; at higher energies delay times of 100–200 ns are used. A different experimental set up is used for monitoring the emission from the strongest lines of silicon and aluminum in the scaling studies at very low energies. The plasma emission is detected from the side at a close distance using a photomultiplier tube (PMT). The signal levels obtained with the photomultiplier detector had substantially better signal to noise characteristics than the intensified multichannel array detector allowing for better measurements of very weak signals. In the case of silicon, a solar blind PMT (Hamamatsu R821) fitted with an interference filter (289 nm, 10 nm FWHM) is used to detect the Si I 288 nm line emission. Additionally, three 1 mm thick Schott glass WG 280 filters, which cut off radiation below 280 nm, efficiently block all laser radiation in front of the photomultiplier. In the case of aluminum the two strongest aluminum
lines at 394.4 and 396.2 nm are selected by a different interference filter (400 nm, 25 nm FWHM) and are monitored on a PMT (RCA 7265) that has a high sensitivity at 400 nm. A glass window is used in front of the filter to block all UV radiation from the laser. The collection cone angle is approximately 268 in both cases. The photomultiplier signal is monitored on an oscilloscope and stored for further analysis. All experiments were conducted in air. The target can be exactly positioned using a camera alignment system and a high-precision mechanical translation stage w1x. All experimental results presented in this paper are based on single laser pulses on fresh target spots. The energy of each laser pulse is monitored using a calibrated photodiode (Hamamatsu R1193U-02). 3. Results and discussion 3.1. Absolute calibration of emission levels In the past, many studies have been devoted to improve the LIBS technique by investigating the dependence of signal levels on different experimental parameters such as the laser wavelength, the pulse duration and energy, the focusing and observation geometry and the optimum observation (gating) time, but these findings were often valid only for the particular experimental set-up being used and the generalization of results was difficult and mostly qualitative. It is therefore desirable to describe experimental LIBS results independently of the experimental set up, i.e. in absolute units. Spectra, for example, can be calibrated in terms of photons emitted per steradian rather than counts detected and therefore yield information about the plasma generated under certain conditions. As a consequence, results become independent of the detection equipment and emission levels from different experiments can be compared thus allowing one to derive more general conclusions. Even more important is the fact that experiments in absolute units can be compared more directly to plasma models facilitating a physical interpretation of experiments. The calibration method for such quantitative measurements is discussed next in detail.
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Since each optical surface contributes to loss of intensity, which may be wavelength dependent, it is important to perform an absolute calibration for the spectrometer including the imaging system (Fig. 1). A calibrated quartz tungsten lamp with a known blackbody-type radiation spectrum is used as a standard light source. The known spectral radiance of the lamp is compared to the number of counts observed with the OMA system yielding a wavelength dependent calibration curve in terms of photons per counts detected. Parameters such as the spectral bandwidth of the detector, gating times and solid emission and detection angles must be taken into account. It is also important that a well-defined amount of light is imaged by the optical system and that it enters the spectrometer completely. A 200 mm pinhole located at the source point of the imaging system is used to define the emission region of the calibration lamp, and since the optical system used for the calibration measurements had approximately a three times magnification ratio, a slit width of at least 0.6 mm has to be used on the spectrograph. Additionally the intensity is monitored as a function of the slit width to make sure that the light enters the spectrometer completely. Since the tungsten lamp has a poor efficiency in the UV region below 350 nm, relative spectral line intensities of a mercury calibration lamp w16x (Oriel 6035) powered by a stabilized d.c. power supply (Oriel 6060) are used in order to extrapolate the calibration curve down to 250 nm. The calibration process described above yields the absolute number of photons detected but one wants to know the number emitted by the plasma so the detection geometry has to be included. Usually LIB-spectra are observed with narrow slit widths in order to obtain a good spectral resolution. However, the plasma expands rapidly after formation to tens or hundreds of micrometers and consequently, only a fraction of the light enters the spectrometer. This fraction must be determined accurately for all species of interest since their distribution is non-uniform. In this study, significant differences in the distribution of atoms and ions were obtained, a fact that has been observed previously (see, e.g. Refs. w17–19x). The plasma image size in the plane of the entrance slit was
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measured by monitoring the emission of neutral and ion lines from an aluminum plasma while moving the plasma image across the entrance slit of the spectrometer. This was done for several laser energies. The data can be fitted with a Gaussian which minimizes the RMS error due to shot-to-shot variations in the plasma emission. The width of the image size is in the range of 0.06 mm up to 1.1 mm (FWHM) in case of the Al I 396 nm line for a laser pulse energies between 4 and 250 mJ, respectively. The emission from ions (Al II 466 nm) is confined to a much smaller region of 0.45 mm for 250 mJ laser pulse energy, due to the higher plasma temperatures needed for the excitation of ionic lines. Optical opacity of the neutral lines at the higher energies above 100 mJ may also contribute to the larger plasma size observed for the neutral Al lines. However, at energies of a few microjoules optical opacity is not expected to be significant. The photon emission levels measured with the spectrometer setup for the two strongest aluminum lines in the visible region (at 394 and 396 nm) can be compared to results obtained with a PMT– interference filter combination (Section 2). Fig. 2 shows a calibrated spectrum obtained with a 50 ps pulse and the spectrometer setup for an energy of 8 mJ. Note that a gate time of 200 ns is sufficient to collect the plasma emission since e-folding emission decay times are significantly below 100 ns for these energies as will be discussed below. The RCA 7265 photomultiplier–interference filter combination was also intensity calibrated using the tungsten lamp since the quantum efficiency of the PMT has decreased over time. Table 1 shows a comparison between the calibrated PMT results and the results obtained with the OMA system for an aluminum plasma. Each data point shown is due to a single shot. While there is significant shot-to-shot scatter and a slight saturation of the PMT signal for pulses above 5 mJ in energy (as discussed below), the absolute photon numbers obtained with the PMT are on average two times as high as the results obtained with the OMA system. For the present experiments, this is a reasonable agreement. One potential source of difference is that the two detectors view the plasma from different angles. The largest source of error
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Fig. 2. Spectrum of the plasma emission of an aluminum plasma produced with 248 nm, 50 ps single pulses of 8 mJ energy. The spectrum was obtained with the spectrometeryOMA system for which an absolute calibration was performed. The gate width was 200 ns without delay. The entrance slit width was 20 mm yielding an instrumental resolution of 0.18 nm for this case.
is probably the determination of the plasma image size and the fraction of light that enters the spectrometer. Significant improvement might be achieved through simultaneous imaging of the plasma on a CCD camera and the acquisition of the spectra. The silicon emission experiments were performed with the solar blind PMT as detector. The number of photons emitted by the plasma into one steradian was derived from the photomultiplier signal using the manufacturer’s specifications for the quantum efficiency and gain of the PMT, the transmission of the filters, and the solid detection angle in the experiments. No calibration experiment was performed in this case since a degradation of the quantum efficiency for this relatively new solar blind PMT is not expected. While more experiments are needed to quantify and decrease the experimental errors, the results presented in this paper give the absolute photon emission from silicon and aluminum microplasmas
to within an estimated factor of two to three accuracy. 3.2. Comparison of pulse duration—experimental results As discussed in the introduction, many previous studies of scaling of plasma emission with pulse length employed different laser systems with different experimental conditions. In this study the same laser is used for the generation of the two pulse durations of 50 ps and 10 ns and thus allows a quantitative comparison of their use in LIBS experiments. Important experimental parameters, such as the laser wavelength, beam quality, and energy are similar in the present study. The results of these experiments for both, 50 ps and 10 ns laser pulses, are presented in Fig. 3 (silicon) and Fig. 4 (aluminum), in which the time integrated emission signal observed by the photomultiplier
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Table 1 Photon emission per steradian in the 390–400 nm region from an aluminum plasma generated by 248 nm, 50 ps pulses Energy (mJ)
Number of photons (1ysr) OMA system
Number of photons (1ysr) PMT
7.8 6.1 3.6
7.9=107 3.7=107 2.4=107
10.9=107 9.4=107 5.9=107
Results from two different experimental set ups are compared. The OMA data correspond to single-shot spectra similar to that shown in Fig. 2.
detectors is plotted as a function of the laser energy. An example of two typical photomultiplier traces is presented in Fig. 5. At energies above 5 mJ, the signal becomes saturated due to output current limitation of the photomultiplier. There is a gentle onset of saturation with the output pulse stretching in time so that the integrated area still increases with pulse energy. This can be seen from the case of the 10 mJ shot shown in Fig. 5. By carrying out calibration tests it has been found that there is a gentle roll over with the time integrated response still increasing monotonically with illumination
energy with a less than linear dependence on illumination pulse energy. Thus, while the response is nonlinear, the integrated response is still an increasing function of light energy. Below approximately 5 mJ, where the response is linear, the arbitrary units for the emission intensity in Figs. 3 and 4 correspond to emitted photons per steradian as detected by the photomultipliers. Time constants can also be obtained from the photomultiplier traces. In the case of saturated signals, only the exponential part of the signal at later times when the signal has dropped below 0.5 V is fitted with a single exponential function. This
Fig. 3. Time integrated emission of Si I 288 nm as measured with a photomultiplier at an average angle of 778 from the target normal as a function of the laser pulse energy for single shots on a silicon wafer. For signals below approximately 3=107 on the vertical axis the arbitrary units correspond to total photons emitted per steradian. Above approximately 3=107 arbitrary units the signals start to become slightly saturated and the response is no longer linearly related to the emission.
Fig. 4. Time integrated emission of Al I (394.4, 396.2 nm) as measured with a photomultiplier at an average angle of 778 from the target normal as a function of the laser pulse energy for single shots on a polished aluminum target. For signals below approximately 4=107 on the vertical axis the arbitrary units correspond to total photons emitted per steradian. Above approximately 4=107 arbitrary units the signals start to become slightly saturated and the response is no longer linearly related to the emission.
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Fig. 5. Photomultiplier traces of plasma emission from the Silicon I 288 nm line obtained at approximately 2 and 10 mJ incident laser pulse energy. Above a laser pulse energy of 5 mJ saturation of the output signal occurs due to current limitation of the photomultipliers. In the case of a saturated signal only the tail of the pulse at a signal level below 0.5 V is used for the determination of the time constant.
yields the exponential decay time constant. At lower energies, the obtained signal is not saturated and the entire curve can be fitted. The time constant of the strong neutral lines gives an estimate of how long visible radiation is emitted by the plasmas and is thus related to the evolution of the plasma temperature. The time constants of the Si I 288 nm radiation and the Al I 394y396 nm radiation are plotted in Fig. 6 (silicon) and Fig. 7 (aluminum) as a function of energy. It can be seen that at low energies, at approximately 1 mJ and below, there is significant scatter in the emission data and time constant measurements taken for the aluminum targets. It is expected that this scatter arises from the residual micron scale roughness of the aluminum surface leading to variations in energy absorption and plasma heating from shot to shot. It has also been reported w1,2x that aluminum alloys contain micron size precipitate crystals with substantially different composition than the background aluminum matrix alloy. Hitting
Fig. 6. Decay time constant for Si I 288 nm emission as a function of the laser pulses energy for single laser shots on a silicon wafer. The horizontal line corresponds to the temporal resolution limit of the photomultiplier.
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Fig. 7. Decay time constant for Al I (394.4, 396.2 nm) emission as a function of the laser pulses energy for single laser shots on a polished aluminum target. The horizontal line corresponds to the temporal resolution limit of the photomultiplier.
these precipitate crystals could lead to increased energy absorption due to reduced reflectivity and give a different strength of signal compared to pure aluminum. Thus, more variability is expected for the aluminum alloy. The results for silicon targets, which have very smooth surfaces and very pure composition, show significantly less scatter and represent the best data set for the current scaling studies. It is seen that for energies below approximately 3 mJ there are significant differences in the emission levels and time constants between 50 ps pulse and 10 ns pulse excitation. This is not surprising since the energy required to reach the intensity threshold for breakdown and plasma formation is much lower for 50 ps pulses. From the measurements shown in Figs. 3 and 4, the plasma formation thresholds for silicon are approximately 0.1 mJ or 0.5 Jycm2 (10 GWycm2) for 50 ps pulses and 1 mJ or 5 Jycm2 (0.5 GWycm2) for 10 ns pulses. For aluminum, the plasma formation thresholds are somewhat lower than for silicon but the scatter in the aluminum data prohibits a more precise number. The larger scatter in the aluminum data is mainly due to the surface roughness of the target and interference from precipitates on the surface. These plasma formation thresholds are in ´ and the range of previous studies (e.g. Cabalın
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Laserna w20x). For laser energies above approximately 3 mJ, it is observed that the plasma emission and time evolution is similar for 50 ps and 10 ns pulses. It appears that for these energies well above the plasma formation threshold the total energy deposited in the plasma is the most important variable. The plasma emission occurs over similar time periods for both pulse durations (Figs. 6 and 7) and signal levels are also approximately the same (Figs. 3 and 4). These results are also supported by experiments on aluminum alloy targets using the spectrometer and OMA detection system. The spectra obtained with 50 ps and 10 ns pulses in the energy range 50–300 mJ are similar with respect to emission levels of spectral lines, continuum background and observation of spectral lines from minor constituents. Additionally, signal levels and ablation crater diameters were measured as a function of the distance between the focusing lens (microscope objective) and the sample surface for single shots with 50 ps or 10 ns pulse duration on polished aluminum alloy targets. As an example, the signal of the strongest line in neutral aluminum (Al I 396.2 nm) is plotted in Fig. 8a and the diameter of the corresponding ablation craters is plotted in Fig. 8b, both as a function of the lens to sample distance (focal distance) using an energy of approximately 20 mJ. At this energy, no major differences between 50 ps and 10 ns pulses are observed with respect to signal levels and ablation craters, even when the focus is placed tens of micrometers above or below the sample surface. As above, differences in the results of the two pulse durations are observed only at very low energies or low intensities. In Fig. 9 pictures taken with an optical microscope of ablation craters due to single laser shots on aluminum are presented. Fig. 9a and b show ablation craters generated by 10 ns and 50 ps pulses of 3 mJ energy at best focus. Even at such a low energy, the craters are similar in shape and size for the case when the laser is tightly focused. The signal levels obtained are also similar as noted previously (Fig. 3). Fig. 9c and d show ablation craters generated with the focus placed approximately 30–40 mm below the sample surface. The beam spot area on the sample surface has increased by a factor of 16 and,
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threshold leading to observable residual damage on the target surface (melting and vaporization) is below the plasma formation threshold where excitation of emission lines can occur. Thus, the residual damage seen in Fig. 9c may be due to ablation at a low intensity for which little heating and plasma formation occurs to excite significant line emission. 3.3. Comparison of laser pulse durations—theoretical considerations
Fig. 8. Al I 396 nm emission levels (a) and ablation crater diameters (b) as a function of the lens-to-sample distance due to single KrF laser pulses of approximately 20 mJ energy and 50 ps (full circles) or 10 ns (open squares) duration. Maximum emission levels and minimum crater diameters correspond to a lens-to-sample distance close to the focal length of the microscope objective. The 10=, NA 0.25 microscope objective was used for these measurements.
consequently, the average focused intensity has decreased by this amount falling below the plasma threshold for 10 ns pulses but it remains above the threshold for 50 ps pulses. It is seen that the 10 ns pulses are no longer able to ablate the material uniformly showing only partial ablation in a few ‘hot spots’. It is known w11x that the ablation
It has been seen that at energies below 3 mJ significant differences exist in the observed line emission between 50 ps irradiation pulses and 10 ns irradiation pulses. Above approximately 3 mJ both the level of emission and the emission time constants become similar for the same two irradiation conditions. These differences can be understood in terms of two different energy fluence regimes. The first is a threshold energy fluence regime where the emission is a strongly dependent on the amount of heating of the plasma in order to reach the requirements for excitation of the emission lines. The second is a strong plasma regime which corresponds to the electronic plasma regime discussed by Russo et al. w11x. In this regime the heating is more than sufficient to reach conditions for excitation of the emission lines and, in fact, optimum emission conditions occur during the plasma expansion and cooling in the form of a blast wave expanding into the ambient atmosphere. The differences between 50 ps and 10 ns pulses at very low energies can be explained simply by the difference in focused intensity and the plasma heating which can be achieved at a given intensity. Information on the plasma heating and initial conditions in the plasma can be obtained using self-consistent plasma expansion models which balance plasma heating and kinetic energy of expansion with absorption of laser light in the expanding plasma plume via inverse Bremsstrahlung (IB). In such models an exponential electron density profile is assumed extending from close to solid density out into vacuum w12,13x. One such model by Mora w12x is employed here for the calculation of the peak plasma temperature and
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Fig. 9. Optical microscope images of craters generated by single KrF laser pulses of 10 ns and 50 ps duration and approximately 3 mJ energy on aluminum. (a and b) show the ablation craters with the target near the focal point of the lens. (c and d) show the ablation craters with the focus placed 30–40 mm below the sample surface.
the electron density at the 2e-folding absorption point for aluminum. This is the electron density at the point in the expanding plasma plume where absorption has reduced the incident laser intensity by 2e-foldings or to value of 13.5% of its incident value. In the case of the 50 ps pulses a planar expansion model was used since the plasma only expands a small fraction of the spot diameter during the interaction. A spherical expansion model with an exponential scale length of 10 mm was assumed for the 10 ns pulses since a stationary spherical flow will develop within the first nanosecond of the interaction. Calculated results from the plasma model are summarized in Table 2, comparing the two pulse durations. It is seen that 50 ps pulses generate higher initial temperatures and higher initial elec-
tron densities. All interaction electron densities are much higher than that of the air breakdown plasma which has an electron density of ne;1–3=1020 y cm3. Densities in this range were observed by Mao et al. w9x in air breakdown in front of copper targets heated by 35 ps 1064 nm wavelength pulses. The IB heating of this air breakdown plasma is proportional to the product of electron density squared times the laser wavelength squared and thus would be much less for the present 248 nm wavelength laser pulses than the 1064 nm laser pulses in Mao’s reported experiments. From the results given in Table 2 it is seen that absorption and heating occur primarily in the high density plasma near the target surface at electron densities much higher than that of the air breakdown plasma. Thus, plasma shielding from breakdown of the
Table 2 Calculated parameters for an aluminum plasma generated by 248 nm pulses of 50 ps and 10 ns duration focused to a 5 mm diameter spot using the model by Mora w4x Pulse duration
50 ps
Energy (mJ) Intensity (Wycm2) Temperature (eV) Electron density at 2e-folding absorption point (1ycm3)
1 1=1011 14 1=1022
10 ns 100 1=1013 183 1.2=1022
1 5=108 2.4 7=1020
100 5=1010 19 3=1021
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ambient atmosphere is not expected to be important for UV pulses. At the same time, the electron densities are much below the critical electron density w13x of 1.8=1022 ycm3 at which total reflection of the incident laser pulses would occur. Thus the assumption made in the model of IB absorption in the expanding plasma profile is valid for the intensities considered here and the laser light is efficiently absorbed in the plasma expanding from the solid surface. Hence the model predicts complete absorption of the incident laser energy in an exponential electron density profile in front of the target surface for both cases of the 50 ps and 10 ns pulses for the intensities of our experiment. For 50 ps pulses at an incident energy of 1 mJ it is seen that the plasma temperature of 14 eV is much higher than required to heat and excite emission from aluminum. For similar 10 ns pulses the peak plasma temperature is 2.4 eV and is approaching the threshold temperature to excite the aluminum line emission. The actual emission generally occurs at later times as the plasma expands and cools, launching a blast wave expanding into the ambient atmosphere w21,22x. Further collisional excitation and emission occurs within this expanding plasma plume. The expanding blast wave can be seen at very early times for 35 ps laser pulses in the shadowgrams shown in Fig. 1d–f of Ref. w10x by Mao et al. The same blast wave can be seen at later times in the shadowgrams shown in Fig. 3a of Ref. w11x by Russo et al. The expansion and propagation of the blast wave depends primarily on the energy released by the plasma to drive the blast wave and not on the details or time duration of the initial energy release. Thus, emission occurring at times later than 10 ns in the expanding blast wave is almost independent of whether the energy is released from a 50 ps plasma or a 10 ns plasma. These qualitative expectations are borne out by the experimental data where it is seen that at energies above approximately 3 mJ the plasma emission time constant is significantly longer than 10 ns and emission levels become similar for the 50 ps and 10 ns pulses. In both cases the time constant for the emission shrinks to a few nanoseconds as the energy decreases to the threshold for plasma formation.
In summary, for pulse energies above several microjoules it is expected that most of the emission occurs from inside an expanding blast wave after the laser irradiation pulse has ended. The blast wave is driven by the energy absorbed in the plasma which is expected to be similar for both the 50 ps and 10 ns pulses in the regime of intensities reported here. Below 3 mJ energy, the reduced plasma heating for the 10 ns pulses leads to inefficient excitation of emission and thus lower emission levels compared to the 50 ps pulses. At approximately 1 mJ the threshold for breakdown of the target surface is reached for the 10 ns pulses and emission rapidly disappears with decreasing energy for energies below this threshold. Above 3 mJ, the emission time extends to tens of nanoseconds indicating that the emission occurs primarily after the laser absorption and heating has ended and is primarily dependent on the energy deposited independent of pulse duration. 4. Conclusion In this study, a comparison of picosecond and nanosecond laser pulses in LIBS applications is presented for microjoule pulse energies. The plasma emissions obtained with pulse durations of 50 ps and 10 ns at energies above approximately 3 mJ are found to be similar. These energies are significantly above the plasma formation threshold and the observed emission time constants are of the order of 30 ns or longer in these cases. Although initial plasma conditions during the time of irradiation of the laser pulses are quite different, after 10 ns the energy absorbed by the plasma for both laser pulse lengths will create a similar blast wave expanding into the background air. A large part of the observed emission occurs in this expanding blast wave which primarily depends on the energy absorbed. However, as one approaches the energy threshold for plasma formation for 10 ns pulses (;1 mJ) the emission rapidly diminishes for 10 ns pulses. For the case of 50 ps pulses significant plasma emission is still observed for 1 mJ pulses since the much higher intensities are still significantly above the plasma breakdown threshold and significant plasma heating occurs for these ultrashort pulses.
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The results presented in this work were obtained using the same laser system and diagnostic setup for both, picosecond and nanosecond pulses, which is important in a comparative study. This is in contrast to previous studies w4–6x where different laser systems were employed for different pulse durations. For both picosecond and nanosecond pulses a reduction in continuum emission is observed for the low energy microjoule pulses employed in the present experiments as compared to typical results reported in the literature for millijoule and higher energy pulses. This reduction is primarily a function of pulse energy and occurs independent of pulse duration. The results presented in this work have shown that for tightly focused microjoule level pulses well above the plasma breakdown threshold the pulse energy is one of the dominant parameters in determining the plasma emission even though initial conditions during the ablation and plasma formation phase are significantly different for nanosecond and picosecond pulses. It is clear that more work needs to be done exploring the signal to noise levels and limits of detection and comparing theory and experiment for picosecond vs. nanosecond pulses. These issues will be the subject of future investigations.
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Acknowledgments
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The authors gratefully acknowledge financial support for this research by the Natural Sciences and Engineering Research Council and MPB Technologies Inc.
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References w16x w1x G.W. Rieger, M. Taschuk, Y.Y. Tsui, R. Fedosejevs, Laser-induced breakdown spectroscopy for microanalysis using sub-millijoule UV laser pulses, Appl. Spect. 56 (2002) 689–698. w2x C. Geertsen, J.L. Lacour, P. Mauchien, L. Pierrad, Evaluation of laser ablation optical emission spectrometry for microanalysis in aluminum samples, Spectrochim. Acta Part B 51 (1996) 1403–1416. w3x M. Taschuk, G.W. Rieger, Y.Y. Tsui, R. Fedosejevs, Lateral and depth resolution of laser-induced breakdown spectroscopy at low energies, Phys. Can. 57 (2001) 74. w4x K.L. Eland, D.N. Stratis, L. Tianshu, M.A. Berg, S.R. Goode, S.M. Angel, Some comparisons of LIBS meas-
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urements using nanosecond and picosecond laser pulses, Appl. Spect. 55 (2001) 279–285. K.L. Eland, D.N. Stratis, D.M. Gold, S.R. Goode, S.M. Angel, Energy dependence of emission intensity and temperature in a LIBS plasma using femtosecond excitation, Appl. Spect. 55 (2001) 286–291. V. Margetic, A. Pakulev, A. Stockhaus, M. Bolshov, K. ¨ Niemax, R. Hergenroder, A comparison of nanosecond and femtosecond laser-induced plasma spectroscopy of brass samples, Spectrochim. Acta Part B 55 (2000) 1771–1785. B. Le Drogoff, J. Margot, M. Chaker, M. Sabsabi, O. ´ Barthelemy, T.W. Johnston, S. Laville, F. Vidal, Y. von Kaenel, Temporal characterization of femtosecond laser pulses induced plasma for spectrochemical analysis of aluminum alloys, Spectrochim. Acta Part B 56 (2001) 987–1002. M. Sabsabi, P. Cielo, Quantitative analysis of aluminum alloys by laser-induced breakdown spectroscopy and plasma characterization, Appl. Spect. 49 (1995) 499–507. S.S. Mao, X. Mao, R. Grief, R.E. Russo, Dynamics of an air breakdown plasma on a solid surface during picosecond laser ablation, Appl. Phys. Lett. 76 (2000) 31–33. S.S. Mao, X. Mao, R. Grief, R.E. Russo, Initiation of an early-stage plasma during picosecond laser ablation of solids, Appl. Phys. Lett. 77 (2000) 2464–2466. R.E. Russo, X. Mao, S.S. Mao, The physics of laser ablation in microchemical analysis, Anal. Chem. 74 (2002) 70A–77A. P. Mora, Theoretical model of absorption of laser light by a plasma, Phys. Fluids 25 (1982) 1051–1056. T.P. Hughes, Plasmas and Laser Light, Wiley, London, 1975. R. Fedosejevs, R. Bobkowski, J.N. Broughton, B. Harwood, keV X-ray source based on high repetition rate excimer laser-produced plasmas, SPIE Proc. 1671 (1992) 373–382. H. Nishioka, H. Kuranishi, K. Ueda, H. Takuma, UV saturable absorber for short pulse KrF laser systems, Opt. Lett. 14 (1989) 692–694. J. Reader, C.J. Sansonetti, J.M. Bridges, Irradiances of spectral lines in mercury pencil lamps, Appl. Opt. 35 (1996) 78–83. R.A. Multari, L.E. Foster, D.A. Cremer, M.J. Ferris, Effects of sampling geometry on elemental emissions in laser-induced breakdown spectroscopy, Appl. Spect. 50 (1996) 1483. ´ J.A. Aguilera, Two-dimensional spatial disC. Aragon, tribution of the time integrated emission from laserproduced plasmas in air at atmospheric pressure, Appl. Spect. 51 (1997) 1632–1638. ´ J.J. Laserna, Diagnostics of silicon plasmas M. Milan, produced by visible nanosecond laser ablation, Spectrochim. Acta Part B 56 (2001) 275–288.
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w20x L.M. Cabalın, ´ J.J. Laserna, Experimental determination of laser-induced breakdown thresholds of metals under nanosecond Q-switched laser operation, Spectrochim. Acta Part B 53 (1998) 723–730. w21x G. Taylor, The formation of a blast wave by a very
intense explosion I. Theoretical Discussion, Proceedings of the Royal Society of London, Series A, Math. Phys. Sci. 201 (1950) 159–174. w22x H.L. Brode, Numerical solutions of spherical blast waves, J. Appl. Phys. 26 (1955) 766–775.
Comparative study of laser-induced plasma emission from microjoule picosecond and nanosecond KrF-laser pulses G.W. Rieger1, M. Taschuk, Y.Y. Tsui, R. Fedosejevs* Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Canada T6G 2V4 Received 9 April 2002; accepted 15 January 2003
Abstract In this paper, the emission of laser produced silicon and aluminum plasmas is investigated in the energy range from 0.1 to 100 mJ (0.5–500 Jycm2) using 10 ns and 50 ps KrF laser pulses focused to a 5 mm diameter spot. For energies higher than 3 mJ, there is little difference between 50 ps and 10 ns pulses in the plasma emission both in terms of the intensity of the emission lines and in terms of lifetime of the emission. Differences become significant only at very low fluences approaching the plasma formation threshold, which is significantly lower for 50 ps pulses than for 10 ns pulses. Calculations using a plasma ablation model show that initial plasma conditions are significantly different for 50 ps and 10 ns pulses during irradiation by the laser pulses. However, the dominant process leading to plasma emission at later times is from expansion and cooling of the plasma plume in the form of a blast wave in the ambient air which is primarily dependent on the energy deposited in the plasma and not the pulse length. Calibrations have also been carried out in order to give the emission in absolute numbers of photons emitted and thus facilitate the comparison with modeling and future experiments. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: LIBS; Laser, plasma; Absolute calibration; Silicon; Aluminum; Plasma ablation model
1. Introduction Plasmas produced by short laser pulses of relatively low millijoule energies have attracted considerable interest in the past due to their application in different elemental analysis methods based on laser ablation such as laser-induced breakdown spectroscopy (LIBS) or laser ablation inductively coupled plasma techniques. In addition, such plas*Corresponding author. E-mail address: [email protected] (R. Fedosejevs). 1 Present address: Department of Physics, University of British Columbia, Vancouver, Canada V6T 1Z1.
mas are of interest in other applications such as micromachining. In this work, microplasmas produced by focusing microjoule energy laser pulses to small 5 mm diameter focal spots are studied in order to characterize the emission for LIBS at very low energies. In a previous study w1x, it was shown that it is possible to achieve low detection limits in the parts per million range for trace elements in aluminum alloys using energies in the range of only (100–200) mJ compared to the tens of millijoules typically encountered in LIBS. Due to the lower energies and the smaller focal spots, the damage to the sample surface is greatly reduced
0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547(03)00014-4
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opening up the possibility of performing surface scans with high lateral resolution w1–3x. The effect of laser pulses of different durations have been compared in the past, mostly focusing on the amount of ablated material. However, a few recent studies have also compared the spectral emission of plasmas generated by nanosecond pulses with those generated by picosecond pulses w4x and recently also by femtosecond pulses w5– 7x. These studies indicate that the shorter pulse duration might be more effective for LIBS. Eland et al. have recently reported on two studies, one comparing 1.3 ps and 7 ns pulses w4x and the other comparing 140 fs and 7 ns pulses w5x in LIBS. Unfortunately, different lasers of different wavelengths and significantly different energies were used in these studies (200 mJ, 1064 nm, 7 ns vs. 1 mJ, 570 nm, 1.3 ps and 810 nm, 1 mJ, 140 fs) leading to a much larger plasma in the case of nanosecond ablation and making a direct comparison problematic. However, the authors obtain a better precision in their calibration curves when using the short laser pulses, especially in case of femtosecond ablation. Margetic et al. w6x who investigated the spectra of brass samples using 6 ns and 170 fs pulses of 0.1–1 mJ energy from the same laser also find a better precision with short pulses in their LIBS experiment. In addition, the authors quote the higher precision in sample ablation, lower continuum background coupled with faster plasma dissipation and the possible use of a non-gated detector as advantages due to the use of the shorter pulse duration. Le Drogoff et al. w7x recently published LIBS measurements on aluminum with 100 fs pulses (800 nm), in which the authors compare their results to a previous LIBS study with 8 ns pulses (1064 nm) by Sabsabi and Cielo w8x. The energy density of 20 Jycm2 and the spotsize of 0.6 mm were chosen to approximately match those of Sabsabi and Cielo (21 Jycm2, 0.6 mm spot size). Since the temporal evolution of the plasma was found to be different for femtosecond and nanosecond pulses, Le Drogoff et al. use a shorter delay time of 1 ms and a shorter gate width of 5 ms for their detection times than Sabsabi and Cielo who used a 10 ms delay and a 10 ms gate width. Le Drogoff et al. present detection limits that are approximately a factor of 3–8 higher
than those obtained by Sabsabi and Cielo, except in the case of copper, where the LOD of 7 ppm obtained with 100 fs is slightly below the previous result of 10 ppm obtained with 8 ns pulses. Le Drogoff et al. believe that the full potential of the short pulse duration was not reached in their study and that, after optimization, the femtosecond pulses will yield better analytical results than the nanosecond pulses. Studies characterizing the initiation and evolution of the plasma for picosecond laser pulses w9– 11x have also been reported recently. The dynamics of air breakdown w9x, initiation of plasma formation w10x and the various thresholds from onset of ablation to the formation of a hot electronic plasma w11x are discussed in these references for 35 ps 1064 nm pulses. However, as discussed later, the air breakdown plays a much smaller role for the 248 nm ultraviolet laser pulses used in the present study. In addition, the initial threshold for ablation can occur with little excitation of optical emission and the current study focuses on characterization of the emission which is of interest for LIBS. In this paper, we report on measurements of emission from microplasmas produced by 50 ps (10–10 000 GWycm2) and 10 ns (0.05–50 GWy cm2) KrF laser pulses (248 nm) focused to a spot size of 5 mm using pulse energies in the range of 0.1–100 mJ. The low energies are not only interesting from a fundamental perspective but will also become important in applications in the near future due to the availability of low-cost microchip lasers that are capable of generating short pulses of microjoule energies with high repetition rates. Another important consequence of using very low energies is the fact that the heated plasma is relatively small, cools rapidly, and thus does not emit much continuum radiation. Usually, the detection of spectra is delayed in LIBS in order to avoid the continuum emission from the early hot and dense phase of the plasma immediately after plasma formation. Optimum signal to noise ratios are obtained when the plasma has already considerably cooled off, typically hundreds of nanoseconds up to a few microseconds after the laser pulse. Thus, expensive intensified multi-channel detectors that allow delayed gating of the observation on a nanosecond time scale have become standard in
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Fig. 1. Experimental set-up. MO, UV-microscope objective; L1, achromat (fs125 mm); L2, lens (fs100 mm); D, dichroic mirror (for 248 nm); M, Al-mirror; SPECT, ARC 500 spectrometer with OMA detector; PD, calibrated photodiode; CCD, CCD camera; PMT, photomultiplier tube; IF, Interference filter.
LIBS. An important result of this work is that for low energies, delaying the detection with respect to the laser pulse is no longer necessary allowing the use of ungated CCD cameras as detectors. In order to understand the scaling issues involved, the expected background plasma conditions are estimated using calculations based on a plasma ablation model w12,13x. 2. Experiment The experimental set-up for LIBS, shown in Fig. 1, is described in detail elsewhere w1x so only a brief introduction of the main components is given here. The KrF-excimer laser operates at 248 nm (Excimer 540, Lumonics Corp.) and emits pulses of 10 ns (FWHM) duration when operated at low energies. It can also be seeded with single frequency doubled short pulses from a dye laser (FL 4000, Lambda Physik) yielding 50 ps pulses at energies of up to several millijoules. The laser system is similar to that described in Ref. w14x but with single pulse injection and operating at rela-
tively low gain yielding single high contrast 50 ps output pulses. In order to demonstrate that there was no influence from amplified spontaneous emission or additional weak pulses, test cases were carried out using very high contrast clean single pulses. Low energy test shots were taken with pulses directly from the frequency doubled dye laser with energies of approximately 5 mJ. At higher energies comparisons were performed using the laser system together with a saturable absorber to ensure clean single pulses w15x. The results obtained using these very high contrast pulses did not differ significantly from those obtained using the normal laser system as described above. The data presented here are due to clean single pulses from the normal laser system. The pulse shape was monitored on an oscilloscope for every shot and shots with imperfect pulse shapes were not considered for analysis. The laser pulses are focused on the samples with a microscope objective (Optics For Research, 15 mm working distance, 10=, NAs0.25 or Carl
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Zeiss, 10=, NAs0.2) that is also used for collecting the radiation emitted by the plasma. The samples used for the experiments were commercially polished silicon wafers and hand polished aluminum plates. The silicon samples had a highly smooth and polished surface down to the submicron scale level. The aluminum samples, while visibly shiny, still had micron scale surface roughness. In all experiments presented in this paper only a small fraction of the full laser beam is selected by an aperture of 3 mm diameter and then expanded with a telescope to 10 mm in order to slightly overfill the microscope objective and to provide a relatively homogeneous laser spot. The focal spot size measured in the plane of the target surface in a knife-edge experiment is approximately 5 mm limited by the quality of the laser beam for both objectives used. A spectrometer (Spectra Pro 500, Acton Research) equipped with three gratings and an intensified multichannel array detector (OMA 1455 G, Princeton Instruments) is used for spectrally resolved light detection. A highresolution grating (1200 linesymm) blazed at 500 nm is used in all experiments described here and the width of the entrance slit is 10 mm yielding an instrumental linewidth of 0.09 nm. The gate width of the OMA detector is 200 ns and no gate delay is used in experiments with laser energies below 10 mJ; at higher energies delay times of 100–200 ns are used. A different experimental set up is used for monitoring the emission from the strongest lines of silicon and aluminum in the scaling studies at very low energies. The plasma emission is detected from the side at a close distance using a photomultiplier tube (PMT). The signal levels obtained with the photomultiplier detector had substantially better signal to noise characteristics than the intensified multichannel array detector allowing for better measurements of very weak signals. In the case of silicon, a solar blind PMT (Hamamatsu R821) fitted with an interference filter (289 nm, 10 nm FWHM) is used to detect the Si I 288 nm line emission. Additionally, three 1 mm thick Schott glass WG 280 filters, which cut off radiation below 280 nm, efficiently block all laser radiation in front of the photomultiplier. In the case of aluminum the two strongest aluminum
lines at 394.4 and 396.2 nm are selected by a different interference filter (400 nm, 25 nm FWHM) and are monitored on a PMT (RCA 7265) that has a high sensitivity at 400 nm. A glass window is used in front of the filter to block all UV radiation from the laser. The collection cone angle is approximately 268 in both cases. The photomultiplier signal is monitored on an oscilloscope and stored for further analysis. All experiments were conducted in air. The target can be exactly positioned using a camera alignment system and a high-precision mechanical translation stage w1x. All experimental results presented in this paper are based on single laser pulses on fresh target spots. The energy of each laser pulse is monitored using a calibrated photodiode (Hamamatsu R1193U-02). 3. Results and discussion 3.1. Absolute calibration of emission levels In the past, many studies have been devoted to improve the LIBS technique by investigating the dependence of signal levels on different experimental parameters such as the laser wavelength, the pulse duration and energy, the focusing and observation geometry and the optimum observation (gating) time, but these findings were often valid only for the particular experimental set-up being used and the generalization of results was difficult and mostly qualitative. It is therefore desirable to describe experimental LIBS results independently of the experimental set up, i.e. in absolute units. Spectra, for example, can be calibrated in terms of photons emitted per steradian rather than counts detected and therefore yield information about the plasma generated under certain conditions. As a consequence, results become independent of the detection equipment and emission levels from different experiments can be compared thus allowing one to derive more general conclusions. Even more important is the fact that experiments in absolute units can be compared more directly to plasma models facilitating a physical interpretation of experiments. The calibration method for such quantitative measurements is discussed next in detail.
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Since each optical surface contributes to loss of intensity, which may be wavelength dependent, it is important to perform an absolute calibration for the spectrometer including the imaging system (Fig. 1). A calibrated quartz tungsten lamp with a known blackbody-type radiation spectrum is used as a standard light source. The known spectral radiance of the lamp is compared to the number of counts observed with the OMA system yielding a wavelength dependent calibration curve in terms of photons per counts detected. Parameters such as the spectral bandwidth of the detector, gating times and solid emission and detection angles must be taken into account. It is also important that a well-defined amount of light is imaged by the optical system and that it enters the spectrometer completely. A 200 mm pinhole located at the source point of the imaging system is used to define the emission region of the calibration lamp, and since the optical system used for the calibration measurements had approximately a three times magnification ratio, a slit width of at least 0.6 mm has to be used on the spectrograph. Additionally the intensity is monitored as a function of the slit width to make sure that the light enters the spectrometer completely. Since the tungsten lamp has a poor efficiency in the UV region below 350 nm, relative spectral line intensities of a mercury calibration lamp w16x (Oriel 6035) powered by a stabilized d.c. power supply (Oriel 6060) are used in order to extrapolate the calibration curve down to 250 nm. The calibration process described above yields the absolute number of photons detected but one wants to know the number emitted by the plasma so the detection geometry has to be included. Usually LIB-spectra are observed with narrow slit widths in order to obtain a good spectral resolution. However, the plasma expands rapidly after formation to tens or hundreds of micrometers and consequently, only a fraction of the light enters the spectrometer. This fraction must be determined accurately for all species of interest since their distribution is non-uniform. In this study, significant differences in the distribution of atoms and ions were obtained, a fact that has been observed previously (see, e.g. Refs. w17–19x). The plasma image size in the plane of the entrance slit was
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measured by monitoring the emission of neutral and ion lines from an aluminum plasma while moving the plasma image across the entrance slit of the spectrometer. This was done for several laser energies. The data can be fitted with a Gaussian which minimizes the RMS error due to shot-to-shot variations in the plasma emission. The width of the image size is in the range of 0.06 mm up to 1.1 mm (FWHM) in case of the Al I 396 nm line for a laser pulse energies between 4 and 250 mJ, respectively. The emission from ions (Al II 466 nm) is confined to a much smaller region of 0.45 mm for 250 mJ laser pulse energy, due to the higher plasma temperatures needed for the excitation of ionic lines. Optical opacity of the neutral lines at the higher energies above 100 mJ may also contribute to the larger plasma size observed for the neutral Al lines. However, at energies of a few microjoules optical opacity is not expected to be significant. The photon emission levels measured with the spectrometer setup for the two strongest aluminum lines in the visible region (at 394 and 396 nm) can be compared to results obtained with a PMT– interference filter combination (Section 2). Fig. 2 shows a calibrated spectrum obtained with a 50 ps pulse and the spectrometer setup for an energy of 8 mJ. Note that a gate time of 200 ns is sufficient to collect the plasma emission since e-folding emission decay times are significantly below 100 ns for these energies as will be discussed below. The RCA 7265 photomultiplier–interference filter combination was also intensity calibrated using the tungsten lamp since the quantum efficiency of the PMT has decreased over time. Table 1 shows a comparison between the calibrated PMT results and the results obtained with the OMA system for an aluminum plasma. Each data point shown is due to a single shot. While there is significant shot-to-shot scatter and a slight saturation of the PMT signal for pulses above 5 mJ in energy (as discussed below), the absolute photon numbers obtained with the PMT are on average two times as high as the results obtained with the OMA system. For the present experiments, this is a reasonable agreement. One potential source of difference is that the two detectors view the plasma from different angles. The largest source of error
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Fig. 2. Spectrum of the plasma emission of an aluminum plasma produced with 248 nm, 50 ps single pulses of 8 mJ energy. The spectrum was obtained with the spectrometeryOMA system for which an absolute calibration was performed. The gate width was 200 ns without delay. The entrance slit width was 20 mm yielding an instrumental resolution of 0.18 nm for this case.
is probably the determination of the plasma image size and the fraction of light that enters the spectrometer. Significant improvement might be achieved through simultaneous imaging of the plasma on a CCD camera and the acquisition of the spectra. The silicon emission experiments were performed with the solar blind PMT as detector. The number of photons emitted by the plasma into one steradian was derived from the photomultiplier signal using the manufacturer’s specifications for the quantum efficiency and gain of the PMT, the transmission of the filters, and the solid detection angle in the experiments. No calibration experiment was performed in this case since a degradation of the quantum efficiency for this relatively new solar blind PMT is not expected. While more experiments are needed to quantify and decrease the experimental errors, the results presented in this paper give the absolute photon emission from silicon and aluminum microplasmas
to within an estimated factor of two to three accuracy. 3.2. Comparison of pulse duration—experimental results As discussed in the introduction, many previous studies of scaling of plasma emission with pulse length employed different laser systems with different experimental conditions. In this study the same laser is used for the generation of the two pulse durations of 50 ps and 10 ns and thus allows a quantitative comparison of their use in LIBS experiments. Important experimental parameters, such as the laser wavelength, beam quality, and energy are similar in the present study. The results of these experiments for both, 50 ps and 10 ns laser pulses, are presented in Fig. 3 (silicon) and Fig. 4 (aluminum), in which the time integrated emission signal observed by the photomultiplier
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Table 1 Photon emission per steradian in the 390–400 nm region from an aluminum plasma generated by 248 nm, 50 ps pulses Energy (mJ)
Number of photons (1ysr) OMA system
Number of photons (1ysr) PMT
7.8 6.1 3.6
7.9=107 3.7=107 2.4=107
10.9=107 9.4=107 5.9=107
Results from two different experimental set ups are compared. The OMA data correspond to single-shot spectra similar to that shown in Fig. 2.
detectors is plotted as a function of the laser energy. An example of two typical photomultiplier traces is presented in Fig. 5. At energies above 5 mJ, the signal becomes saturated due to output current limitation of the photomultiplier. There is a gentle onset of saturation with the output pulse stretching in time so that the integrated area still increases with pulse energy. This can be seen from the case of the 10 mJ shot shown in Fig. 5. By carrying out calibration tests it has been found that there is a gentle roll over with the time integrated response still increasing monotonically with illumination
energy with a less than linear dependence on illumination pulse energy. Thus, while the response is nonlinear, the integrated response is still an increasing function of light energy. Below approximately 5 mJ, where the response is linear, the arbitrary units for the emission intensity in Figs. 3 and 4 correspond to emitted photons per steradian as detected by the photomultipliers. Time constants can also be obtained from the photomultiplier traces. In the case of saturated signals, only the exponential part of the signal at later times when the signal has dropped below 0.5 V is fitted with a single exponential function. This
Fig. 3. Time integrated emission of Si I 288 nm as measured with a photomultiplier at an average angle of 778 from the target normal as a function of the laser pulse energy for single shots on a silicon wafer. For signals below approximately 3=107 on the vertical axis the arbitrary units correspond to total photons emitted per steradian. Above approximately 3=107 arbitrary units the signals start to become slightly saturated and the response is no longer linearly related to the emission.
Fig. 4. Time integrated emission of Al I (394.4, 396.2 nm) as measured with a photomultiplier at an average angle of 778 from the target normal as a function of the laser pulse energy for single shots on a polished aluminum target. For signals below approximately 4=107 on the vertical axis the arbitrary units correspond to total photons emitted per steradian. Above approximately 4=107 arbitrary units the signals start to become slightly saturated and the response is no longer linearly related to the emission.
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Fig. 5. Photomultiplier traces of plasma emission from the Silicon I 288 nm line obtained at approximately 2 and 10 mJ incident laser pulse energy. Above a laser pulse energy of 5 mJ saturation of the output signal occurs due to current limitation of the photomultipliers. In the case of a saturated signal only the tail of the pulse at a signal level below 0.5 V is used for the determination of the time constant.
yields the exponential decay time constant. At lower energies, the obtained signal is not saturated and the entire curve can be fitted. The time constant of the strong neutral lines gives an estimate of how long visible radiation is emitted by the plasmas and is thus related to the evolution of the plasma temperature. The time constants of the Si I 288 nm radiation and the Al I 394y396 nm radiation are plotted in Fig. 6 (silicon) and Fig. 7 (aluminum) as a function of energy. It can be seen that at low energies, at approximately 1 mJ and below, there is significant scatter in the emission data and time constant measurements taken for the aluminum targets. It is expected that this scatter arises from the residual micron scale roughness of the aluminum surface leading to variations in energy absorption and plasma heating from shot to shot. It has also been reported w1,2x that aluminum alloys contain micron size precipitate crystals with substantially different composition than the background aluminum matrix alloy. Hitting
Fig. 6. Decay time constant for Si I 288 nm emission as a function of the laser pulses energy for single laser shots on a silicon wafer. The horizontal line corresponds to the temporal resolution limit of the photomultiplier.
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Fig. 7. Decay time constant for Al I (394.4, 396.2 nm) emission as a function of the laser pulses energy for single laser shots on a polished aluminum target. The horizontal line corresponds to the temporal resolution limit of the photomultiplier.
these precipitate crystals could lead to increased energy absorption due to reduced reflectivity and give a different strength of signal compared to pure aluminum. Thus, more variability is expected for the aluminum alloy. The results for silicon targets, which have very smooth surfaces and very pure composition, show significantly less scatter and represent the best data set for the current scaling studies. It is seen that for energies below approximately 3 mJ there are significant differences in the emission levels and time constants between 50 ps pulse and 10 ns pulse excitation. This is not surprising since the energy required to reach the intensity threshold for breakdown and plasma formation is much lower for 50 ps pulses. From the measurements shown in Figs. 3 and 4, the plasma formation thresholds for silicon are approximately 0.1 mJ or 0.5 Jycm2 (10 GWycm2) for 50 ps pulses and 1 mJ or 5 Jycm2 (0.5 GWycm2) for 10 ns pulses. For aluminum, the plasma formation thresholds are somewhat lower than for silicon but the scatter in the aluminum data prohibits a more precise number. The larger scatter in the aluminum data is mainly due to the surface roughness of the target and interference from precipitates on the surface. These plasma formation thresholds are in ´ and the range of previous studies (e.g. Cabalın
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Laserna w20x). For laser energies above approximately 3 mJ, it is observed that the plasma emission and time evolution is similar for 50 ps and 10 ns pulses. It appears that for these energies well above the plasma formation threshold the total energy deposited in the plasma is the most important variable. The plasma emission occurs over similar time periods for both pulse durations (Figs. 6 and 7) and signal levels are also approximately the same (Figs. 3 and 4). These results are also supported by experiments on aluminum alloy targets using the spectrometer and OMA detection system. The spectra obtained with 50 ps and 10 ns pulses in the energy range 50–300 mJ are similar with respect to emission levels of spectral lines, continuum background and observation of spectral lines from minor constituents. Additionally, signal levels and ablation crater diameters were measured as a function of the distance between the focusing lens (microscope objective) and the sample surface for single shots with 50 ps or 10 ns pulse duration on polished aluminum alloy targets. As an example, the signal of the strongest line in neutral aluminum (Al I 396.2 nm) is plotted in Fig. 8a and the diameter of the corresponding ablation craters is plotted in Fig. 8b, both as a function of the lens to sample distance (focal distance) using an energy of approximately 20 mJ. At this energy, no major differences between 50 ps and 10 ns pulses are observed with respect to signal levels and ablation craters, even when the focus is placed tens of micrometers above or below the sample surface. As above, differences in the results of the two pulse durations are observed only at very low energies or low intensities. In Fig. 9 pictures taken with an optical microscope of ablation craters due to single laser shots on aluminum are presented. Fig. 9a and b show ablation craters generated by 10 ns and 50 ps pulses of 3 mJ energy at best focus. Even at such a low energy, the craters are similar in shape and size for the case when the laser is tightly focused. The signal levels obtained are also similar as noted previously (Fig. 3). Fig. 9c and d show ablation craters generated with the focus placed approximately 30–40 mm below the sample surface. The beam spot area on the sample surface has increased by a factor of 16 and,
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threshold leading to observable residual damage on the target surface (melting and vaporization) is below the plasma formation threshold where excitation of emission lines can occur. Thus, the residual damage seen in Fig. 9c may be due to ablation at a low intensity for which little heating and plasma formation occurs to excite significant line emission. 3.3. Comparison of laser pulse durations—theoretical considerations
Fig. 8. Al I 396 nm emission levels (a) and ablation crater diameters (b) as a function of the lens-to-sample distance due to single KrF laser pulses of approximately 20 mJ energy and 50 ps (full circles) or 10 ns (open squares) duration. Maximum emission levels and minimum crater diameters correspond to a lens-to-sample distance close to the focal length of the microscope objective. The 10=, NA 0.25 microscope objective was used for these measurements.
consequently, the average focused intensity has decreased by this amount falling below the plasma threshold for 10 ns pulses but it remains above the threshold for 50 ps pulses. It is seen that the 10 ns pulses are no longer able to ablate the material uniformly showing only partial ablation in a few ‘hot spots’. It is known w11x that the ablation
It has been seen that at energies below 3 mJ significant differences exist in the observed line emission between 50 ps irradiation pulses and 10 ns irradiation pulses. Above approximately 3 mJ both the level of emission and the emission time constants become similar for the same two irradiation conditions. These differences can be understood in terms of two different energy fluence regimes. The first is a threshold energy fluence regime where the emission is a strongly dependent on the amount of heating of the plasma in order to reach the requirements for excitation of the emission lines. The second is a strong plasma regime which corresponds to the electronic plasma regime discussed by Russo et al. w11x. In this regime the heating is more than sufficient to reach conditions for excitation of the emission lines and, in fact, optimum emission conditions occur during the plasma expansion and cooling in the form of a blast wave expanding into the ambient atmosphere. The differences between 50 ps and 10 ns pulses at very low energies can be explained simply by the difference in focused intensity and the plasma heating which can be achieved at a given intensity. Information on the plasma heating and initial conditions in the plasma can be obtained using self-consistent plasma expansion models which balance plasma heating and kinetic energy of expansion with absorption of laser light in the expanding plasma plume via inverse Bremsstrahlung (IB). In such models an exponential electron density profile is assumed extending from close to solid density out into vacuum w12,13x. One such model by Mora w12x is employed here for the calculation of the peak plasma temperature and
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Fig. 9. Optical microscope images of craters generated by single KrF laser pulses of 10 ns and 50 ps duration and approximately 3 mJ energy on aluminum. (a and b) show the ablation craters with the target near the focal point of the lens. (c and d) show the ablation craters with the focus placed 30–40 mm below the sample surface.
the electron density at the 2e-folding absorption point for aluminum. This is the electron density at the point in the expanding plasma plume where absorption has reduced the incident laser intensity by 2e-foldings or to value of 13.5% of its incident value. In the case of the 50 ps pulses a planar expansion model was used since the plasma only expands a small fraction of the spot diameter during the interaction. A spherical expansion model with an exponential scale length of 10 mm was assumed for the 10 ns pulses since a stationary spherical flow will develop within the first nanosecond of the interaction. Calculated results from the plasma model are summarized in Table 2, comparing the two pulse durations. It is seen that 50 ps pulses generate higher initial temperatures and higher initial elec-
tron densities. All interaction electron densities are much higher than that of the air breakdown plasma which has an electron density of ne;1–3=1020 y cm3. Densities in this range were observed by Mao et al. w9x in air breakdown in front of copper targets heated by 35 ps 1064 nm wavelength pulses. The IB heating of this air breakdown plasma is proportional to the product of electron density squared times the laser wavelength squared and thus would be much less for the present 248 nm wavelength laser pulses than the 1064 nm laser pulses in Mao’s reported experiments. From the results given in Table 2 it is seen that absorption and heating occur primarily in the high density plasma near the target surface at electron densities much higher than that of the air breakdown plasma. Thus, plasma shielding from breakdown of the
Table 2 Calculated parameters for an aluminum plasma generated by 248 nm pulses of 50 ps and 10 ns duration focused to a 5 mm diameter spot using the model by Mora w4x Pulse duration
50 ps
Energy (mJ) Intensity (Wycm2) Temperature (eV) Electron density at 2e-folding absorption point (1ycm3)
1 1=1011 14 1=1022
10 ns 100 1=1013 183 1.2=1022
1 5=108 2.4 7=1020
100 5=1010 19 3=1021
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ambient atmosphere is not expected to be important for UV pulses. At the same time, the electron densities are much below the critical electron density w13x of 1.8=1022 ycm3 at which total reflection of the incident laser pulses would occur. Thus the assumption made in the model of IB absorption in the expanding plasma profile is valid for the intensities considered here and the laser light is efficiently absorbed in the plasma expanding from the solid surface. Hence the model predicts complete absorption of the incident laser energy in an exponential electron density profile in front of the target surface for both cases of the 50 ps and 10 ns pulses for the intensities of our experiment. For 50 ps pulses at an incident energy of 1 mJ it is seen that the plasma temperature of 14 eV is much higher than required to heat and excite emission from aluminum. For similar 10 ns pulses the peak plasma temperature is 2.4 eV and is approaching the threshold temperature to excite the aluminum line emission. The actual emission generally occurs at later times as the plasma expands and cools, launching a blast wave expanding into the ambient atmosphere w21,22x. Further collisional excitation and emission occurs within this expanding plasma plume. The expanding blast wave can be seen at very early times for 35 ps laser pulses in the shadowgrams shown in Fig. 1d–f of Ref. w10x by Mao et al. The same blast wave can be seen at later times in the shadowgrams shown in Fig. 3a of Ref. w11x by Russo et al. The expansion and propagation of the blast wave depends primarily on the energy released by the plasma to drive the blast wave and not on the details or time duration of the initial energy release. Thus, emission occurring at times later than 10 ns in the expanding blast wave is almost independent of whether the energy is released from a 50 ps plasma or a 10 ns plasma. These qualitative expectations are borne out by the experimental data where it is seen that at energies above approximately 3 mJ the plasma emission time constant is significantly longer than 10 ns and emission levels become similar for the 50 ps and 10 ns pulses. In both cases the time constant for the emission shrinks to a few nanoseconds as the energy decreases to the threshold for plasma formation.
In summary, for pulse energies above several microjoules it is expected that most of the emission occurs from inside an expanding blast wave after the laser irradiation pulse has ended. The blast wave is driven by the energy absorbed in the plasma which is expected to be similar for both the 50 ps and 10 ns pulses in the regime of intensities reported here. Below 3 mJ energy, the reduced plasma heating for the 10 ns pulses leads to inefficient excitation of emission and thus lower emission levels compared to the 50 ps pulses. At approximately 1 mJ the threshold for breakdown of the target surface is reached for the 10 ns pulses and emission rapidly disappears with decreasing energy for energies below this threshold. Above 3 mJ, the emission time extends to tens of nanoseconds indicating that the emission occurs primarily after the laser absorption and heating has ended and is primarily dependent on the energy deposited independent of pulse duration. 4. Conclusion In this study, a comparison of picosecond and nanosecond laser pulses in LIBS applications is presented for microjoule pulse energies. The plasma emissions obtained with pulse durations of 50 ps and 10 ns at energies above approximately 3 mJ are found to be similar. These energies are significantly above the plasma formation threshold and the observed emission time constants are of the order of 30 ns or longer in these cases. Although initial plasma conditions during the time of irradiation of the laser pulses are quite different, after 10 ns the energy absorbed by the plasma for both laser pulse lengths will create a similar blast wave expanding into the background air. A large part of the observed emission occurs in this expanding blast wave which primarily depends on the energy absorbed. However, as one approaches the energy threshold for plasma formation for 10 ns pulses (;1 mJ) the emission rapidly diminishes for 10 ns pulses. For the case of 50 ps pulses significant plasma emission is still observed for 1 mJ pulses since the much higher intensities are still significantly above the plasma breakdown threshold and significant plasma heating occurs for these ultrashort pulses.
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The results presented in this work were obtained using the same laser system and diagnostic setup for both, picosecond and nanosecond pulses, which is important in a comparative study. This is in contrast to previous studies w4–6x where different laser systems were employed for different pulse durations. For both picosecond and nanosecond pulses a reduction in continuum emission is observed for the low energy microjoule pulses employed in the present experiments as compared to typical results reported in the literature for millijoule and higher energy pulses. This reduction is primarily a function of pulse energy and occurs independent of pulse duration. The results presented in this work have shown that for tightly focused microjoule level pulses well above the plasma breakdown threshold the pulse energy is one of the dominant parameters in determining the plasma emission even though initial conditions during the ablation and plasma formation phase are significantly different for nanosecond and picosecond pulses. It is clear that more work needs to be done exploring the signal to noise levels and limits of detection and comparing theory and experiment for picosecond vs. nanosecond pulses. These issues will be the subject of future investigations.
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Acknowledgments
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The authors gratefully acknowledge financial support for this research by the Natural Sciences and Engineering Research Council and MPB Technologies Inc.
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