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Abstract
We present the spatial and temporal characterization of the copper (Cu) plasma produced by the femtosecond laser filaments. The filaments of various lengths and intensities were generated with the aid of three different focusing lenses. Further, the filamentation induced breakdown spectroscopy (FIBS) measurements were carried out for each filament at three different positions along the length of the filament. The filaments were spatially characterized by estimating the plasma temperature and electron density. Our investigation has demonstrated that the centre of the filament is the best to obtain a maximum signal. Both the spectral line intensity and their persistence time are highest for the center of the filament. The enhanced persistence and the scalability of the spectral line intensity tested across different focusing geometries can boost the application of this technique in various fields.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
An intense laser pulse when focused onto a sample surface creates a microplasma at the focal volume. The detection and analysis of electromagnetic radiation (UV-VIS-IR) radiated from the excited ionic, atomic, or molecular species within the plasma provides the details of the chemical constituents of the analyte and this analytical technique is called as Laser-induced breakdown spectroscopy (LIBS). It is a versatile spectroscopic method that provides an elemental composition of the sample considered for the interrogation. The spectral information combined with multivariate analysis can be used for material identification [1]. As an analytical technique, it offers multi-elemental detection in one shot. Minimum/no sample preparation and the standoff measurements are unique features of the LIBS technique. These key attributes, especially the standoff detection capability enable the identification of hazardous materials in hostile environments where the physical access to the sample is not viable but optical access can be envisaged [2–5]. The conventional standoff LIBS (ST-LIBS) technique based on ns laser pulses has been reported in the literature for various applications such as the identification of explosives, contaminants, and nuclear materials, etc [3–8]. However, it has limitations for achieving the maximum standoff distances owing to the atmospheric turbulence and diffraction effects [9–13]. This hurdle can be overcome by utilizing the advantage of complex but favorable nonlinear dynamics of ultrafast femtosecond (fs) laser propagation in ambient air, which produces the filaments and can be exploited for probing samples for long-range applications. The inherent property of the filaments to preserve the spatio-temporal characteristics of the excitation fs pulse and the ability to pre-compensate the fs pulses to overcome dispersion accrued due to long-range propagation makes it a desirable excitation source for LIBS applications. Also, it has got significant attention of the research community owing to the wide range of applications viz., harmonic generation [14], filament induced breakdown spectroscopy (FIBS) [15–19], LIDAR [20], THz generation [21], and others [22].
FIBS is a promising spectroscopic method where the high intensities of fs filaments in the range of 1013 W/cm2 is more than sufficient to ablate the samples even at longer standoff distances [23,24]. The fs filaments can propagate and deliver energies over very long distances without any diffraction effects in contrast to the conventional nanosecond (ns) pulses and makes it very desirable for remote sensing applications [25]. These unique features of the fs laser pulse propagation have demonstrated over the range of a few kilometers [26,27]. Most of the FIBS studies have been focused on the capability of standoff measurements and the recognition of atomic and molecular spectra. It has been applied for the remote identification of the materials in different applications such as military/defense [10,28], archeology [19], biology [15–17], nuclear technology [18] and environment monitoring [29], etc. [30–33].
Although it has been reported that the FIBS is a promising spectroscopic tool for the standoff detection of materials, their characteristic properties are still yet to be completely understood. The enhancement of the spectral line intensity for the remote FIBS is reported with spatio-temporally chirped pulses [34]. Harilal et al. demonstrated that temperature clamping in FIBS and it is attributed to the intensity clamping in a filament. They have also investigated the effect of loosely focused (2- 4 m focal length) and freely propagated filaments (no focusing optics used) on the evolution of the aluminum oxide (ALO) molecular species [35,36]. Fs laser ablation studies of steel and titanium alloys with tightly focused fs pulses, sharply focused and unfocused filaments have been demonstrated that the ablation efficiency is more for the tightly focused fs pulses followed by sharply focused filaments [37]. The longitudinal variation of the filament intensity in ambient air cannot be directly measured due to the high filament intensities but can be spectroscopically estimated. Xu et al. characterized the lead plasma produced by the loosely focused filaments by measuring the plasma temperature and electron density [38]. Skrodzki et al. have demonstrated the transition from single to multiple filament regime with the shadowgraphy technique and quantified the laser coupling efficiency to the metal target [39]. The coupling efficiency has been estimated as a function of input energy by keeping the other parameters constant. It is worth considering that the properties of the laser-induced plasma greatly depend on the irradiation conditions. In order to improve the figures of merit of this technique, it is worth looking into the collection of the signal from different parts of the filament, which has received less attention in the literature.
In this study, we report the spatial-temporal characterization/dynamics of the FIBS measurements generated by different filaments. Cu plasma is produced by the filaments generated with three focusing geometries. The plasma produced by each filament is characterized at different positions along its spatial length and LIBS signal is collected to examine which part of the filament can yield the highest spectral intensity. An appropriate focusing geometry resulting in the enhanced spectral intensity of a specific signature emission can help in improving the figures of merit of this technique. Our studies demonstrate (a) the filament conditions required to enhance persistence of the emission lines, and (b) the flexibility with which FIBS can be used as a stand-off as well as a lab-based technique, an essential factor for improving the figures of merit of this technique.
2. Experimental details
For the present investigation, the fs filaments were generated by focusing the laser pulses from a chirped-pulse amplified (CPA) Ti: sapphire laser system (Libra, 4 W, M/s Coherent Co.) delivering ∼ 50 fs at the repetition rate of 1 kHz and excitation wavelength centered at 800 nm [40]. A typical schematic of the FIBS setup is shown in Fig. 1(a). The filaments of three different intensities (lengths) were produced by focusing the laser pulses of 2 mJ energy using plano-convex lenses of focal lengths 50, 100, and 200 cm respectively. The images of the three filaments are shown in Fig. 1(c). The filament was imaged using a Nikon T4i camera, the images show the visible extent of the filaments, and however, the emission cannot be compared between filaments of different focal lengths since the exposure of the camera sensor was set for optimal observation of the particular filament. The FIBS experiment has been performed on the Cu target in an ambient atmosphere. The sample was kept on the X-Y translation stage which is perpendicular to the incident laser beam propagation as shown in Fig. 1(b). The Cu sample was exposed to the filament at three different positions along its length by moving the lens along the laser propagation direction as shown in Fig. 1(b). To maintain reproducibility in experimental data, the lens was placed on a rail which can be moved in both + z and –z directions. The optical emissions from the filament-induced Cu plasma were acquired by the collection optics (Andor, ME OPT-007) which is positioned at 45° relative to the incident laser beam and kept at a distance of 18 cm from the Cu target as shown in Fig. 1(a). The collection optics and Cu target were held fixed at a position on the optical bench for constant collection efficiency. The spectral emissions from the collection system were guided to the Mechelle spectrometer through an optical fiber (core diameter 600 µm, NA-0.22, M/s Ocean optics - QP600-2-SR-BX). The Echelle spectrograph covers the spectral region of 220 -880 nm with a resolution of 0.1 nm at 500 nm and is coupled to an ICCD (Andor i-star DH334T-18U-E3). A TTL pulse from the synchronized delay generator (SDG Elite - Coherent) of the fs laser system was used to trigger the ICCD.
Fig. 1. a) Typical schematic of the FIBS setup. M1 and M2 are mirrors, L is the focusing lens, and C is the collection system. b) FIBS measurements at three different positions of the filament. LP, CP, and TP correspond to the exposure of the Cu target by keeping at leading, central, and trailing parts of the filament respectively. c) Images of the filaments captured by a camera.
Two types of FIBS spectra were acquired for spatial and temporal characterization.
- a) Time-integrated spectra recorded with a gate width of 2 µs.
- b) Time-resolved spectra acquired for the temporal characterization in the time window 0.02-1.82 µs, by increasing the delay in steps of 0.2 µs with a constant gate width of 0.2 µs.
The initial delay (20 ns) and gain (1200) was fixed for these two datasets and each recorded spectrum corresponds to an accumulation of 1000 laser pulses. Ten such spectral data sets were acquired at each position.
3. Results and discussion
Filamentation is a result of a dynamic interplay between the self-focusing of the laser beam due to the optical Kerr effect and the defocusing of the plasma generated through the multi-photon or tunnel ionization process of air molecules. In principle, the fs laser pulse self-focuses when its power exceeds the ‘critical power’ (Pcr)
where no and n2 are the linear and non-linear refractive index respectively, λ is the laser wavelength.By considering the excitation laser wavelength 800 nm the estimated Pcr in the air is 3 GW [41]. The power utilized in the current experiment is 40 GW which exceeds the Pcr corresponding to an intensity of 8GW/cm2 at free propagation. Further, at filamentation which is induced using a focusing element the intensity estimated using the FIBS spectrum of air, we find that for filament from 50 cm lens it is estimated to be ∼30 TW/cm2, and for 100 cm it is ∼15 TW/cm2. The length of the observable filaments from these lenses is 3 cm, 5 cm, and 15 cm for filaments from 50 cm, 100 cm, and 200 cm lenses respectively.
3.1 Description of the spectral lines
The FIBS spectra acquired for the Cu target irradiated by the three parts of the filament for the 50 cm focal length lens are shown in Fig. 2. It shows the spectral lines in the window of 323-590 nm. All the prominent spectral lines were indicated at their characteristic wavelengths.
Fig. 2. The LIBS spectra were acquired at three different parts of the filament produced by the 50 cm focal length lens. a, b, and c correspond to the leading edge, central position, and the trailing edge of the filament respectively.
The spectral transitions at 324.79 and 372.46 nm are strongly self-absorbed irrespective of the irradiating conditions. The emission intensities of the spectral lines have shown significant changes when the Cu plasma is produced by the different parts of the filament. Most of the emission features are neutral transition lines of the Cu plasma. Similar observations were noticed in the previous studies where the FIBS spectra predominantly provide neutral emissions due to the absence of plasma reheating [42,43]. The signal strength is observed maximum for the sample irradiated by the central part of the filament and it is almost double the intensity to that exposed by the leading and trailing part of the filament as shown in Fig. 2. The results were almost similar (spectra not shown here) when the sample was interrogated by the filaments produced by the other two lenses of focal lengths 100 cm and 200 cm.
The prominent spectral lines viz., 465.11, 510.55, 515.32, and 521.82 nm were fitted for the Lorentzian function and extracted the area (considered as the intensity for the further analysis), width, and height of the peaks. The Cu spectral line intensities (521.82 nm) measured for three different focusing lenses are presented in Fig. 3. LP, CP, and TP in Fig. 3 correspond to the Cu target exposed to the leading part/edge, central position, and the trailing part of the filaments respectively.
Fig. 3. The bar plot of the intensities 521.82 nm spectral line. The figure in the inset represents the zoomed view of the intensities obtained for a 200 cm focal length lens
The error bar represents the standard deviation obtained from the ten trials at each position. It is evident from Fig. 3, the spectral line intensities are higher when the Cu target is at the central part of the filament than either end, and it is observed for three different lenses and remaining spectral lines also (not shown in the figure). It is also noticed that the spectral intensities at the trailing edge of the filaments are always higher in comparison to the leading edge of the filaments. This is in line with the reports that the trailing edge of the filament has enhanced intensities in comparison with the leading stage of the filament [44].
The significant changes in emission line intensities along the filament channel can be linked to variation in plasma properties [43] as given in Eq. (2)
As seen from Eq. (2), the intensity of a particular spectral transition/line depends on the temperature of the plasma and the number density of emitting species. Both of these parameters are crucial to characterize the plasma and are highly transient. Hence, the density and temperature measurements were performed at different positions along the filament channel by exploiting optical emission spectroscopy (OES) which is a non-intrusive and reasonably accurate technique for determining the plasma parameters.
3.2 Measurement of plasma temperature
The plasma temperature is estimated from the Boltzmann plot method [45]. The following spectral lines at 465.11, 510.55, 515.32, and 521.82 nm are utilized for estimating the plasma temperature [45]. All the remaining spectroscopic parameters are taken from the literature [45]. The temperatures estimated for the three of the lenses are shown in Fig. 4(a). It has been clearly visualized that the measured plasma temperature is varying with the different parts of the filament channel.
Fig. 4. a) The estimated plasma temperature for the different parts of the three filaments. The boxes in the figure help to visualize the variation of the plasma temperature at three different points along the length of the filament. b) A linear fit of the plasma temperature with the focal length of the lenses
The variation is ∼9200-10700, 8300-9800, and 7500-9100 K for the filaments produced with the 50, 100, and 500 cm focal length lenses respectively. It has been found that the estimated plasma temperatures are linearly decreasing with increasing the focal length of the lens as shown in Fig. 4(b) which can be understood as follows. The filament intensity decreases with increasing the focal length of the lens (laser pulse energy utilized in the experiment is constant). In the case of higher focal length lenses, the laser spot size is larger which in turn means the photon density is low and, the interaction is weak in comparison to focusing with short focal length lenses where tighter focusing leads to smaller spot size and high photon density. Hence, it results in the formation of the filament with less temperature and vice-versa. The plasma generated with the interaction of the leading edge of the filament with the Cu target has shown minimum temperature compared to the other two positions. Further, the central part has shown maximum temperature and similar observations were noticed for the remaining filaments of different lengths, generated with different focusing lenses. These results suggest that the intensity measurements can be correlated with corresponding plasma temperature. According to the Boltzmann distribution, the increase in temperature leads to the excite more number of species/atoms into a particular excited energy level and populates it. Further, the de-excitation of the atoms results in higher intensity for that respective spectral transition. The same observation is noticed in our present investigation where the temperatures are highest for the ablation with the central part of the filament and resulted in the emission of spectral lines with maximum intensity. A similar performance is noticed for all the spectral features and also for all the three focusing lenses. The electron density measurements were performed in the next sections.
3.3 Estimation of electron density
The electron density measurements have been performed for all the filaments. Stark broadening method is employed for the estimation of the electron density as given in Eq. (3).
Where Ne - electron density, ‘ω’ - impact width parameter, and Δλ1/2- FWHM of the respective spectral line.Out of the five persistent spectral lines (324.82, 327.46, 510.17, 515.32, and 521.82 nm), the first two are self-absorbed which cannot be considered for the electron density estimation. Further, only 510.57 nm line has the highest possible energy separation among all hence used for the measurement of electron density and McWhirter criterion (same is reported in the literature). The corresponding electron impact width parameter was obtained from the literature [45,46]. The electron densities estimated for all the filaments are shown in Fig. 5.
Fig. 5. The electron densities measured at three different parts of the filaments for the three lenses
It has been found that the electron densities are of the same order (1017/ cm3) irrespective of the ablation condition. It can also be observed that the variation in electron density between the ends of the filament to the center of the filament reduces with increasing focal length, this is due to increased intensity clamping as we progress towards a higher focal length [47,48]. The energy in the core of the filament is continually replenished by the surrounding energy reservoir of the filament while the peak intensity in the filament is restricted by the plasma formation, also resulting in the prolonged propagation of the filament.
Both the electron density (Fig. 5) and temperature measurements (Fig. 4) are performed with the FIBS spectral data collected in time-integrated mode and are highly dynamic. Thus the numerical values given in Fig. 4 and Fig. 5 are should be considered as the average temperature and density during the evolution of the plasma lifetime. However, the change in spectral line intensity is significant compared to the electron density. It can be possible due to the variation in plasma persistence time. Further to explore this observation, the time evolution studies were performed which is elaborated in the next section.
3.4 Time evolution studies
The temporal evolution of the FIBS spectra was recorded and analyzed in the time window of 0.02-1.82 µs. The kinetic evolution of the FIBS spectra (510 - 524 nm region) acquired at the central part of filament produced with the 50 cm lens is illustrated in Fig. 6(a). After the formation of the plasma, as time progresses, the plasma decays and expands into the surrounding atmosphere [49]. As a result, the intensity of spectral lines gradually decreases with the delay time as shown in Fig. 6(a). Maximum intensities were observed at the initial dealy, and it decayed continuously with increasing time. Figure 6(b) shows the decay of the 521.82 nm spectral transition. The intestines are fitted for the exponential decay function and the plasma persistence times are obtained. Symbols represent the actual data and a solid line corresponds to the exponential fit. The error bar represents the standard deviation of the ten trials at each position. The spectra acquired with the central part of the filament persists for a longer time compared to the other two positions as shown in Fig. 6(b).
Fig. 6. a) The time evolution Cu FIBS spectra. The spectra were recorded at the central part of the filament in the time window of 0.02-1.82 µs. b) The kinetic evolution of the Cu-521.82 nm spectral line for the Cu target exposed by three different parts of the filament. Both the figures correspond to the filament produced by a 50 cm focal length lens.
The intensity decay for the Cu-521.82 nm is almost similar for the leading and trailing edge of the filament. The intensity is drastically decreased to 44% of the initial intensity in the first 0.2 µs for the central part of the filament and further decays slowly to zero in the 1 µs. In the case of irradiation with the leading and trailing edge of the filaments, it decays to 30% of the initial intensity. The intensities are approximately the same and approach zero in the time window of 1.22 - 1.82 µs irrespective of the focusing conditions. Similarly, the analysis has been performed for other two filaments and the decay constants are obtained. Figure 7(a) represents the decay constants estimated for the Cu -521.82 nm line for the different filaments.
Fig. 7. a) Representation of the decay constants /persistence time obtained for the 521.82 nm line for three positions of each filament. b) Correlation between the persistence time and spectral line intensity
A similar trend has been observed for the other spectral lines which are not presented here. It has been noticed that the plasma persists for a longer time when the target is exposed with the filaments generated by a lower focal length lens. The persistence time is found to decrease linearly with increasing focal length. This could be attributed to the decrease in fluence/irradiance with an increase in the focal length of the lens. Also, it is observed that plasma persists for a longer time when irradiated with the central part of filament in contrast to the other two ablation conditions. The higher persistence results in the emission with a maximum intensity which exactly resembles intensity measurement as shown in Fig. 7(b). It is clear from Fig. 7(b) that the higher plasma persistence time results in higher spectral line intensity and vice-versa.
4. Conclusions
We have demonstrated the characterization of the Cu plasma generated by fs filaments with three different intensities (focusing conditions). Further, measurements were carried out by exposing the Cu target at three different positions of each filament. It is revealed that the intensities are strong at the central part of the filament compared to either end. These observations are attributed to the changes in the plasma properties where the plasma temperature is maximum for the central part of the filaments. Moreover, it is noticed that the plasma persistence is longer when generated with the central part of the filament. The enhanced persistence of the emission lines at the central part of the filament for the three different focusing geometries confirm the scalability of the spectral emissions. This coupled with optimal collection of the signal from the central part of the filament enables us to minimize the input laser power requirement which in turn improves the figures of merit of this technique. We envision that this can open up the way to several applications such as material ablation and spectral analysis at standoff distances using optimal focusing of fs pulses.
Funding
Defence Research and Development Organisation.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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