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We report on the delivery of low energy ultra-short (<1 ps) laser pulses for laser induced breakdown spectroscopy (LIBS). Ultra-short pulses have the advantage of high peak irradiance even at very low pulse energies. This opens the possibility to use compact, rare-earth doped fiber lasers in a portable platform for point detection applications using LIBS for elemental analysis. The use of low energy ultra-short pulses minimizes the generation of a broad continuum background in the emission spectrum, which permits the use of non-gated detection schemes using very simple and compact spectrometers. The pulse energies used to produce high-quality LIBS spectra in this investigation are some of the lowest reported and we investigate the threshold pulse requirements for a number of near IR pulse wavelengths (785-1500 nm) and observe that the pulse wavelength has no effects on the threshold for observation of plasma emission or the quality of the emission spectra obtained.
©2007 Optical Society of America
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is emerging as a potentially powerful tool for rapid analysis of an unlimited array of solid, liquid and gaseous materials. For example, the use of an intense laser-induced spark for spectral analysis has been used for determination of chlorine gas [1, 2], halogenated hydrocarbons [2], environmental lead [3], aluminum alloys [4], bacterial spores [5], landmine casings [6], chemical and biological agents [7], and energetic materials [8–10]. The application of LIBS has also been seen in the areas of space exploration [11, 12], materials science [13], biomedicine [14, 15] and archaeology [16–18].
Most applications of LIBS use high energy, nanosecond duration laser pulses, but several studies have revealed that improved performance can be achieved with the use of ultrashort (<100 ps) laser pulses [19–23]. The improved performance is linked, at least in part, to the fact that ultrashort laser pulses can deliver radiant intensity to a material more rapidly than thermal energy can diffuse away from the irradiated volume. The result is rapid conversion of solid material into ionized gas (plasma) with very reproducible threshold intensities. The resulting ablated area is cleaner and free of surface cracking or irregularities, which normally arise in the nanosecond pulse regime due to ejected fragments or molten droplets from within the ablation crater. The latter performance benefit has seen the application of ultrashort laser pulses to not only LIBS but more extensively to the related area of laser-assisted micromachining.
Soon after the development of the mode locked Ti:Sapphire laser and the introduction of chirped-pulse amplification (CPA), it was realized that femtosecond lasers could be used as tools for very efficient micromachining [24]. The generation of sub-micron ablation spots and even sub-diffraction limited feature sizes is possible with careful control of the pulse intensity [25, 26]. By only achieving the breakdown threshold at the most intense cross-sections of the beam, ablation spot sizes can be controlled by variation of the total pulse energy, and therefore, the diameter of the threshold cross-section. Micromachining of glass using 5 nanojoule pulse energies and extremely tight focusing conditions has been demonstrated with this technique [27]. Feature sizes smaller than 100 nm are also possible [28] and will continue to get smaller as ultrashort UV pulses become increasingly easier to generate and control. For example, recent work has investigated the use of ultrashort extreme ultraviolet pulses from a free-electron laser for machining of Si and GaAs with precise control of crater shape and topography [29].
While numerous studies have investigated the use of low energy, ultrashort laser pulses in micromachining applications; only a few reports have investigated the application of these pulses for use in laser-induced breakdown spectroscopy (LIBS). Rieger, et al. used nanosecond and picosecond KrF laser pulses of sub-microjoule pulse energy to thoroughly quantify the emission from micro-plasmas generate on aluminum and silicon samples using tight focusing conditions [30]. In that report it was determined that the threshold for observation of LIBS emission was significantly lower for 50 ps pulses compared to 10 ns pulses of the same wavelength. The work also alluded to the fact that shorter pulses resulted in a LIBS emission spectrum that was nearly free of continuum background commonly encountered in nanosecond LIBS. However the majority of data presented was single wavelength integrated line intensities and complete LIBS spectra demonstrating the absence of the continuum at low pulse energies were not presented. Winefordner and co-workers demonstrated the ability to use 7 μJ, 550 ps pulsed microchip lasers and compact spectrometers for portable LIBS [31, 32]. They demonstrated complete LIBS spectra for a number of different metallic samples using this technique. However it was noticed that significant plasma continuum, though less intense than in ns LIBS, was still present in the spectra collected using a non-gated spectrometer. Microchip lasers have demonstrated great potential to serve as sources for portable LIBS [4, 33, 34], but there are advantages to using even shorter pulses which, with the development of mode-locked fiber lasers, may become available in compact and easy-to-use packages in the near future. Recent reports using femtosecond sources of increasingly lower pulse energy, as low as 100 nJ, demonstrate that the use of such pulses is not only feasible but also produces consistent results and enhanced spatial sampling resolution [35, 36].
In this report we use low energy, ultrashort laser pulses from a Ti:Sapphire regenerative amplifier and optical parametric amplifier (OPA) to extend the characterization of ultra-short pulsed LIBS out further into the NIR spectral region. While the system used for the study was a laboratory system, the low pulse energies used in the study are currently available from more compact mode-locked fiber laser systems, making the use of these pulses in a portable LIBS analysis platform a realizable potential. In this report we also make clear demonstration of the continuum-free LIBS spectra that can be rapidly obtained using ultra-short pulses and non-gated detection schemes, and show that the threshold energies for obtaining these spectra are comparable to those used in earlier low-energy, ultra-short pulse LIBS studies.
2. Experimental
Sub-picosecond, tunable near-infrared laser pulses were generated using an optical parametric amplifier (Spectra-Physics OPA-800) pumped by a 1-kilohertz Ti:Sapphire regenerative amplifier (Spectra-Physics Spitfire). Seed pulses for the amplifier were provided by a mode-locked Ti:Sapphire oscillator (Spectra-Physics Tsunami). Output pulses at 785 nm from the regenerative amplifier were 150 fs in duration with energies of 0.8 mJ. Pulse widths were measured using SHG-FROG (Newport UPM-8-200). Signal and idler pulses from the OPA have orthogonal polarizations and the signal pulses (tunable from 1200-1500 nm) were isolated from the idler pulses using a Glan-Taylor polarizer. Residual fundamental 785 nm was removed from the beam using a narrow-band harmonic separator mirror and sent to a beam dump. When fundamental pulses were to be used, the OPA BBO crystal was removed from the beam path and the power amplifier stage was isolated from the pre-amp and white light seed and used directly. The second harmonic of the signal pulse (SHS) was generated in a 0.6 mm think crystal of BBO-I in order to monitor the wavelength of the signal pulse during wavelength tuning. Monitoring of the OPA wavelength and detection of the LIBS emission was done using an un-cooled and non-gated, portable fiber-coupled 16-bit USB spectrometer (B&W Tek Inc. BRC111A). The NIR pulses from the fundamental (785 nm) or the OPA (1200-1500 nm) were focused using an 8 mm diameter, 0.50 NA aspheric lens onto the surface of a rotating sample.
Focal spot sizes under the tight focusing conditions and high peak intensities present in our experiments could not be determined directly and were inferred from an analysis of the beam quality out of the OPA using an M2 analysis procedure. Beam diameters were measured using a laser beam-profiling camera (Spiricon LBA-100A) equipped with a Silicon detector array. The camera was translated through the minimum spot of a loosely focused beam from the power amplifier stage of the OPA (785 nm, f =100 mm lens) and the beam diameter was recorded as a function of propagation distance. The resulting data are shown in Figure 1, along with a best fit the propagation equation for a real (multimode) laser beam,
where W(z) is the beam waist at any distance z, W0 is the minimum beam waist, M2 is the propagation constant, λ is the laser wavelength and z0 is position of the minimum beam waist in the propagation direction.
The value of M2 extracted from the fit was used to correct the diffraction limited spot size expected from an aspheric lens with focal length f and beam diameter d according to the expression
The minimum possible spot diameters (2w0) determined from this procedure were 7.10 μm, 10.85 μm, 12.66 μm at 785 nm, 1200 nm and 1400 nm respectively and these values are consistent with the widths of the ablated tracks observed on the surface of our samples using a 75X light microscope with a calibrated eyepiece, data not shown.
Plasma emissions from the sample surfaces were collected collinearly to the excitation laser using the same focusing aspheric, reflected off with a 45° narrow-band harmonic separator mirror (Rmax = 532 nm) and launched onto a multimode fiber optic patch cord (0.6 mm core diameter) using a 0.25 NA infinity corrected microscope objective. Excitation laser pulse energy was attenuated using a variable reflective neutral density filter and measured using a high-sensitivity optical power meter with a Germanium sensor head (Thorlabs PM132). To rule out plasma initiation from small 7 ns pedestal pulses from the regenerative amplifier, unseeded output from the amplifier was focused onto the samples under the same conditions used with short-pulse seeding. With no attenuation the pulse energy for the pedestal is 100 μJ and intense enough to initiate plasma, but for the attenuation level (10-2 to 10-3) used in this study, the 7 ns pedestal pulses from the un-seeded regenerative amplifier are not intense enough generate any observable plasma emission. The data acquisition period for each sample totaled 5 seconds; 10 sets of spectra generated from observing the emission for 500 ms (500 laser shots) were averaged to produce a single spectrum.
Fig. 1. Beam waist measurements as a function of the propagation distance across the focus of an amplified Ti:Sapphire laser beam operating at 785 nm and 1 kHz. Data were fit to the propagation equation for determining the value of M2.
For comparison, LIBS measurements using 18 ns pulses were carried out using a traditional single-pulse LIBS apparatus. The laser source was a Q-Switched Nd:YAG laser (Big Sky Laser Technologies, Inc. CFR-200) operating at a repetition rate of 10 Hz and output energy of 200 mJ at 1064 nm. Laser pulses were directed toward the sample with a 1064 nm high-energy harmonic separator and focused onto a slowly rotating copper sample using a 25 mm diameter, 50 mm focal length plano-convex lens. LIBS emission was collected collinearly in the same lens and sent back through the 1064 nm harmonic separator to be launched onto a 1 mm core diameter multimode fiber-optic patch cord and sent to a spectrograph (Acton SP-2300i). Spectra were collected using an intensified CCD camera (Princeton Instrument PI-MAX series) equipped with a Marconi CCD47-10 1024x1024 array. The camera was operated in gated mode using a built-in programmable timing generator (PTG). Typical gating parameters used were 500 ns gate delay and 2000 ns gate width and spectra were taken using single laser shots. Samples of copper metal used in all experiments were obtained from an industrial source. Copper was chosen because its plasma emission has been well characterized, including studies using ultra-short laser pulses for ablation and plasma generation [19–21, 37, 38].
3. Results
Figure 2 shows the pulse energy dependent atomic emission spectra of copper emitted from a laser induced plasma on the surface of a clean copper sheet using 785 nm fundamental 150 fs laser pulses from a Ti:Sapphire regenerative amplifier. Spectra are offset for visual clarity. The three major lines of interest are the copper atomic emission lines at 510.76 nm, 515.36 nm, and 521.94 nm and the spectra were all normalized to the maximum 521.94 nm line. Small artifact peaks that appear in the lowest energy spectra are due to inconsistencies in the dark background correction and only appear significant due to the normalization and give an indication of the signal-to-background ratio achieved with the current pulse characteristics and apparatus. Pulse energies for each spectrum are indicated in Figure 2 and are comparable to the lowest laser pulse energies used for laser induced breakdown spectroscopy [36]. These pulse energies are up to two orders of magnitude greater than energies that have been used in the femtosecond micromachining applications referenced in the introduction. Uncertainties in the pulse energy directly from the regenerative amplifier were ± 3 nJ without OPA pumping. Several previous reports have highlighted the advantages of using sub-nanosecond laser pulses for LIBS [4, 19–22, 30–33, 35, 36, 39]. The major advantages pointed out in those studies were reduced continuum background and improved reproducibility of the signal. The use of long pulses (> 100 ps) requires the use of gated spectral acquisition to temporally discriminate the continuum background that is generated early in the plasma lifetime. Gated detection was not necessary for the measurements performed in this study; the spectra shown in Figure 2 are well resolved and clean of continuum background as long as the pulse energies are kept lower than 10 μJ. At 785 nm a 10 μJ pulse in our apparatus yields a peak irradiance of 168 TW/cm2, great enough that breakdown of the air is easily evident in the absence of a solid sample as indicated by a visible spark and emission spectrum of excited state molecular nitrogen. Also contributing to the continuum background at higher total pulse energy may be a more intense 7 ns pedestal from the regenerative amplifier which, as mentioned in the experimental section, is not intense enough to initiate plasma on its own but may be enough to support existing plasma and foster longer lived emission and encourage continuum generation.
Fig. 2. Atomic emission spectra of Cu obtained by laser-induced breakdown of a clean copper sheet using 785 nm, 150 fs amplified Ti:Sapphire laser pulses. Spectra were collected at different pulse energies as indicated on the figure using an un-gated handheld spectrometer. Spectra were normalized to the peak intensity of the 521.94 nm emission line and offset for clarity.
Improved reproducibility in ultrashort pulse LIBS is related to a cleaner ablation and minimized substrate damage, particularly for the very low pulse energies used in this study. Figure 3 shows the single wavelength peak intensity of the 521.94 atomic emission line of copper extracted from single spectral acquisitions (5 s collection time: 500 ms exposure, 10 averages, 5000 laser shots) using 785 nm, 150 fs laser pulses at 18 different pulse energies.
Fig. 3. Pulse energy dependent atomic emission intensities at 521.94 nm obtained from 18 single spectral acquisitions of the laser-induced plasma emission from copper metal using 785 nm, 150 fs amplified Ti: Sapphire laser pulses.
Each data point corresponds to one spectral acquisition at that pulse energy and the smoothness of the resulting curve reflects the reproducibility of the laser induced plasma generation at these low pulse energies. The 2σ variance in intensity at each point is on the order of ± 100 counts, which is smaller than the width of the data points plotted on the figure.
The results shown in Figure 4 show a comparison of the plasma emission spectra obtained using low energy ultra-short laser pulses versus more traditional Q-Switched Nd:YAG laser pulses. For acquisition of the emission spectra with the nanosecond laser pulses, gated detection was required to eliminate the intense and featureless continuum emission that is generated early in the plasma lifetime. Such gating was not necessary using ultra-short pulses and yet the spectral quality obtained from each method match very closely. Differences in peak intensity ratios can be observed in the spectra and are presumably due to differences in the plasma temperature that is reached using the two methods, but further studies are necessary to quantify the plasma temperatures being achieved in these measurements.
The ultra-short 150 fs laser pulses used for collecting the spectra in Figures 2 and 4 were 785 nm in wavelength, but very similar results are obtained using other NIR wavelengths. Figure 5 shows the emission spectrum of laser-induced plasma on the surface of a clean copper sheet using 150 fs pulses at three different NIR wavelengths. The pulse energies were kept at 2 μJ and all other experimental parameters were kept constant while the OPA was scanned from 1280 nm through 1420 nm. These data indicate that there is very little discernable difference in the spectra obtained using the three different pulse wavelengths and that our apparatus does not introduce artifacts into the spectra at different pulse wavelengths. The energy dependence of the plasma emission was then investigated to observe any differences in the threshold for plasma emission detection at these longer wavelengths. The pulse energy dependent emission spectra of copper using 1200 nm pulses are shown in Figure 6. Uncertainties in the pulse energy were ± 50 nJ using the OPA. As indicated on the figure, the pulse energies required for obtaining clean spectra of the plasma emission are well below 2 μJ, and the threshold for observation of plasma emission is not significantly different than for 785 nm pulses. Similar results were seen when the pulse wavelength was tuned to 1400 nm, as shown in Figure 7. In fact, the threshold fluences for observation of plasma emission using 1400 nm pulses are the lowest of all three wavelengths used.
Fig. 4. Comparison of the atomic emission spectra of Cu obtained by laser-induced breakdown of a clean copper sheet using 150 fs, 5 μJ, 785 nm (top) and 18 ns, 200 mJ, 1064 nm (bottom) laser pulses. Gated acquisition with the ICCD for the 18 ns pulse measurement was accomplished 500 ns following the laser pulse with a gate-width of 2000 ns. Spectra were normalized to the peak intensity of the 521.94 nm emission line and offset for clarity.
A number of previous studies have investigated the mechanism of laser ablation using ultrashort pulses [40–47] and it is understood that the mechanism of ablation will differ depending on laser pulse width, pulse energy, wavelength and properties of the material. The most accepted mechanism for ablation in the short pulse regime is photoionization followed by avalanche ionization [40]. In this process “seed” electrons are generated early in the pulse propagation by photoionization of the valence electrons in the outer layers of the material, reaching a critical density high enough that the liberated free electrons can transfer sufficient energy by electron-phonon interaction to neighboring atoms to liberate more electrons and feed the plasma [42]. The resulting avalanche leads to very rapid expansion of the plasma in the form of a Coulomb explosion. For ultra-short laser pulses this all happens on a very short timescale compared to the heat diffusion time of most known materials and there is no damage to the material outside of the ablated region. The rate of avalanche ionization can be described by Thornber’s model which predicts that the avalanche rate will increase linearly with irradiance in the limit of high electric fields achieved in ultra-short laser pulses [40]. Strong support for the avalanche ionization has been shown based on ablation depth and pulse width dependent breakdown thresholds [43], but an understanding of the seed process that initiates the avalanche is still a complex and evolving field of study.
Fig. 5. Atomic emission spectra of Cu obtained by laser-induced breakdown of a clean copper sheet using 150 fs pulses from a tunable optical parametric amplifier operating at 1280 nm, 1360 nm and 1420 nm. Pulse energies used for each spectrum were 2 μJ corresponding to fluences of 1.90 J/cm2, 1.68 J/cm2, and 1.54 J/cm2 for 1280 nm, 1360 nm, and 1420 nm pulses respectively. Spectra were normalized to the peak intensity of the 521.94 nm emission line and offset for clarity.
The mechanism by which the photoionized seed electrons are generated has been a topic of debate and has implications on how the laser wavelength will affect the ionization threshold and thus the lowest pulse energies that can be utilized in LIBS. In most metals, NIR and visible laser pulses can liberate seed electrons simply by linear absorption of photons with energies above the work function of the metal. In wide-band semiconductors and insulators the presence of narrow-band absorbing impurities in the matrix can act as absorption centers to provide the needed seed electrons for plasma formation. In addition to resonant absorption processes, several other mechanisms have been identified that contribute to the seed electron generation. These mechanisms are especially enhanced at high intensity and in combination with resonant absorption processes can make low energy LIBS spectroscopy with ultra-short pulses a material and pulse wavelength insensitive technique.
Multiphoton ionization is understood to play a role in the generation of seed electrons for ultrashort pulses and this process is expected to become less efficient at longer wavelengths owing to the greater number of photons required to reach the ionization potential. Ionization thresholds will scale approximately as λn, where n is the order of the multiphoton process, i.e. n = 2 being a 2-photon process, etc. and λ is the laser wavelength [48]. In some cases, multiphoton ionization alone, without the subsequent avalanche ionization can also be sufficient to reach critical plasma density, so it is difficult to completely predict the exact mechanism at work in any one case.
Fig. 6. Pulse energy dependent atomic emission spectra of Cu obtained by laser-induced breakdown of a clean copper sheet using 150 fs pulses of 1200 nm light from an optical parametric amplifier. Spectra were normalized to the peak intensity of the 521.94 nm emission line and offset for clarity.
Another process for generation of seed electrons is tunneling ionization, which can occur at the very large electric fields (>100 MV/cm). These electric fields are readily achievable with ultra-short laser pulses and are strong enough to pull outer shell electrons from atoms by the tunneling effect. Keldysh developed a complete treatment of the phenomenon in 1965 [49], and the ionization threshold is found to scale as IP3, where IP is the ionization potential of the free atom. Therefore, tunneling ionization will in general be independent of wavelength and cannot alone describe observed differences in ablation threshold at different wavelengths [45]. Morphology of the damage spot as well as the depth of the ablated area can be used to determine the relative contributions of multiphoton and tunneling ionization mechanisms, but it is reasonable to expect that both multiphoton and tunneling ionization could both contribute to seeding the avalanche ionization and plasma initiation process for many materials when ultra-short pulses are used. Owing to several wavelength independent processes, it is then not surprising that we have observed no difference in LIBS threshold and quality of LIBS spectra using different NIR wavelengths. Despite the low pulse energies, for the pulse width of the laser and irradiances used in this study, the contributions from resonant absorption, multiphoton and tunneling ionization to the seeding process may contribute in such a way that the initiation of the avalanche process and breakdown proceeds through effectively the same mechanism for all of the wavelengths used in this study. In addition, although our studies were carried out on a metallic sample where resonant absorption is the dominant seed generation mechanism, it is expected that similar results will be observed in wide-band materials owing to effective multiphoton and tunneling ionization using ultra-short pulses. It should be mentioned that for longer pulse widths, the competition between the seeding mechanisms is likely to change and larger differences in LIBS threshold as a function of laser wavelength might be observed. However, recent reports [50] have shown little difference in breakdown thresholds for pulse widths from 10 fs up to 20 ps, suggesting that similar breakdown mechanisms are involved for pulses shorter than 20 ps and LIBS results obtained using 100 fs pulses should be similar to results from 10 ps pulses.
Fig. 7. Pulse energy dependent atomic emission spectra of Cu obtained by laser-induced breakdown of a clean copper sheet using 150 fs pulses of 1400 nm light from an optical parametric amplifier. Spectra were normalized to the peak intensity of the 521.94 nm emission line and offset for clarity.
4. Conclusion
We have demonstrated the use of very low energy, ultra-short NIR laser pulses for laser-induced breakdown spectroscopy (LIBS). The quality of the spectra obtained using these pulses is equivalent to that achieved using traditional nanosecond laser pulses of much higher pulse energy. Gated detection is not required for LIBS using low-energy ultra-short pulses and the plasma emission is highly reproducible from shot-to-shot at repetition rates up to 1 kilohertz. Spectral shape and threshold for observation of plasma emission are independent of pulse wavelength over the entire range of NIR wavelengths studied, suggesting the possibility that compact and rugged ultra-fast pulsed fiber lasers which have fundamental emission wavelengths from 785 nm to 1550 nm may be used as sources for portable LIBS spectroscopy applications. Furthermore, the ability to use non-gated detection could also lead to an overall simpler and inexpensive system which is more amenable to field use. The metallic sample investigated in this study has high reflectance in the NIR and large contributions of seed electrons to the plasma avalanche come from linear absorption of the laser pulse. However, similar results are expected from semiconductors and dielectrics because of the efficient multiphoton and tunneling ionization processes that are possible with ultra-short pulses. Continuing work is focused on the time-resolved characterization of the plasma emission, determination of plasma temperatures with low-energy pulses and the characterization of different materials. Further developments will involve studies using compact ultra-fast fiber sources and the development of integrated systems using these sources and compact spectrometers in a portable LIBS platform.
Acknowledgments
This research was supported by the U.S. Army Research Laboratory under Contract No. W911QX-04-C-0129. Views and conclusions contained in this report are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Government, U.S. Army Research Laboratory or the University of Maryland, Baltimore County. The authors would like to thank Andy Mott for the use of the Spiricon LBA-100A beam profiler. Thanks also to Alex Newburg for helpful discussions of beam quality analysis and spot size determination.
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