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We investigated the role of spot size on plume morphology during ultrafast laser ablation of metal targets. Our results show that the spatial features of fs LA plumes are strongly dependent on the focal spot size. Two-dimensional self-emission images showed that the shape of the ultrafast laser ablation plumes changes from spherical to cylindrical with an increasing spot size from 100 to 600 μm. The changes in plume morphology and internal structures are related to ion emission dynamics from the plasma, where broader angular ion distribution and faster ions are noticed for the smallest spot size used. The present results clearly show that the morphological changes in the plume with spot size are independent of laser pulse width.
© 2015 Optical Society of America
1. Introduction
Recent advancements in ultrafast laser technology have led to a new field of laser-matter interaction called ultrafast laser ablation (ULA). Even though the only differences between the conventional nanosecond (ns) laser-based ablation and ULA is the duration of pulse width, the mechanisms leading to energy absorption and target ablation are entirely different for both cases [1–3]. Whereas ionization, sample heating, and vaporization all occur during the duration of the laser pulse in ns LA, fs laser pulses are so short compared to typical relaxation times (e.g., electron-to-ion energy transfer time, electron heat conduction time, etc.) that these phenomena do not occur until after the end of the laser pulse. Because of this, there have been significant advantages in using ultrashort pulses for LA for many applications, which include a greatly reduced thermal damage and heat-affected zone (HAZ), reduced continuum emission, generation of atomic and nanoparticle plume, lessened elemental fractionation and matrix effects, etc [1–3].
Understanding the expansion dynamics of a LA plume is ultimately important for most of its applications. The factors affecting the plume propagation, morphology and emission features are laser-wavelength [4], spot size [5,6], target geometry and composition, temporal pulse shape [7], and environmental aspects (background gas, B-field, etc.) [8,9]. The role of laser wavelength on absorption properties are well documented for short pulse ns LA [10]; however, its role is limited in ULA [4]. Previous reports showed that the divergence of the LA plumes are affected by the laser spatial profiles: plumes with low divergence can be generated using a Gaussian laser profile compared to top-hat laser profiles [11]. The role of the ns laser impact area on plume hydrodynamics has been reported widely [12–16]. In ns LA, larger spot sizes lead to plume-sharpening effects caused by ion acceleration due to enhanced laser-plasma coupling [15]. Changes in ion angular distribution caused by a rectangular beam ‘flip over’ have been reported for both ns [16] and fs [17] LA. The changes in ablation and hydrodynamics features with varying spot size will influence nearly all applications of LA viz. laser-induced breakdown spectroscopy [18], LA-inductively coupled mass spectrometry [19], pulsed laser deposition [20], laser patterning [21], light sources [22], particle sources [23], particle generation [24] etc.
In this letter, we report the expansion dynamics of ULA plumes with varying spot sizes. The present results show that the plume morphology of ULA changes dramatically with changes in spot size, changing from nearly spherical to highly collimated when the spot size increased from 100 μm to 600 μm. Plume imaging, ion angular distributions, and spatially resolved spectroscopy are used to characterize spatial and temporal properties of the ULA plume expansion in vacuum. Previous studies documented that fs LA plumes are highly collimated in comparison with ns LA, which was explained as pressure confinement due to strong overheating of the laser impact zone caused by low optical penetration depth and greater energy per unit volume [2,3,25]. We present results demonstrating that a nearly spherical plume expansion is also possible with ULA for small laser focus spots. We show that these observations are consistent with prior results and models showing that plume hydrodynamic expansion in vacuum is governed by initial pressure gradients in the plume, and that this behavior is similar for both ns LA and ULA, despite the significant differences in plasma formation dynamics.
2. Experimental details
For producing plasmas, pulses from a Ti:Sapp. amplifier system (40 fs, ~800 nm, p-polarized,) were focused on a planar Al target placed in a vacuum chamber (base pressure of ~10−5 Torr) using a f = 40 cm plano-convex lens. The laser impact area on the target surface was varied by translating the focusing lens. Constant laser energy of ~5 mJ was used. The focal spot size at the target surface was measured using optical imaging of the crater and compared to an estimation of focal spot sizes using OSLO optical design software [26]. The target was translated to provide a fresh surface for each shot to avoid errors associated with local heating and drilling. Plume imaging was accomplished with an intensified CCD (ICCD) camera placed orthogonal to the laser beam. The ion emission was monitored using a negatively biased Faraday cup (FC) placed at a distance 14 cm from the target surface and the ion current was captured using a 1-GHz oscilloscope. Emission spectroscopy of the plasma was carried out using a 0.5-m spectrograph and the dispersed spectra were collected using an ICCD detector.
3. Results and discussion
Fast gated photography is one of the most versatile plasma diagnostic tools for understanding plume hydrodynamics and morphology and it provides 2D snapshots of the 3D plume expansion. Images of ULA plumes recorded using gated photography in vacuum with different spot sizes are given in Fig. 1 . All images were obtained using a gate time of 200 ns and recorded 100 ns after the onset of plasma formation for avoiding early bright emission. Constant laser energy of ~5 mJ was used. The spot size was varied by translating the focusing lens, with position “0” corresponding to the best focal position. Positive distances (in cm) given in Fig. 1 correspond to when the focal point is above the target; conversely, the negative values represent the focal point inside the target. The measured spot size diameter using crater analysis at the best focal plane was ~100 μm while it increased to ~600 μm when the lens is moved by ± 2.5 cm. The gated images clearly show dramatic changes in the plume morphology with spot size during ULA. The aspect ratio obtained from the images in Fig. 2 showed a value of ~1 at the best focal position and increased significantly to a value ~3.7 when the spot size increased by a factor of 7. Hence the fs LA plume geometry is found to be nearly symmetrical or spherical at or near the best focal plane and deviates from spherical to cylindrical symmetry with increasing lens-target distance either side of the best focal position.
Fig. 1 Fast gated images of the ULA plumes are given for various spot sizes. The images were obtained with a 100-ns delay and a 200-ns integration time after the onset of plasma generation. Each image is obtained from different laser shots and images are normalized to a constant intensity (given in the scale bar). The best focal position is marked as 0. Negative or positive lens-target distances (given in cm) correspond to the best focal plane position behind or in front of the target, respectively.
Fig. 2 The aspect ratio obtained from the ICCD images. The morphology becomes nearly spherical at the best focal spot and becomes more cylindrical with increasing spot size.
Time-resolved images recorded for representative smaller (~100 μm) and larger (~600 μm) focal spots are given in Fig. 3 . The differences in plume morphology are evident at earlier as well as later times of the evolution. Emission intensity as a function of distance along the direction of plume expansion is also given in Fig. 3 at certain times after the evolution of the plume. Plume splitting is evident in both cases due to three propagating phases. The smaller spot size exhibits (1) a faster moving component that propagates with a velocity ~6.7 × 106 cm/s and disappears at later times, (2) a slower propagating component with a velocity ~1.6 × 106 cm/s that persists for longer times, and (3) a nearly stagnant emission region at the laser impact area. For the larger spot size, the faster component propagates with a velocity ~2.5 × 106 cm/s, and the slower propagating emission is visible, but not well resolved. The nearly stagnant emission zone at the focal spot, which is observed for both focal spot diameters, could be contributed by nanoparticle ejection during fs laser ablation [27]. Compared to the smaller spot size LA, the plume emission is found to be more collimated and homogenous with the use of large spot size. However, the plume speed is found to be higher with the use of smaller spot size.
Fig. 3 Time-resolved images of self-emission from the plasma. The top row gives the evolution of the plume at the best focal position (100 μm) and the bottom row gives the plume images when the lens is moved 2.5 cm away from the target (600-μm diameter). Intensity counts obtained from the ICCD images along the plume expansion direction (z-axis) is given in the right side for time indices (a) 125 ns and (b) 300 ns.
The splitting of plume structures seen with both larger and smaller spot sizes could be related to emission from various species in the plume propagating with different velocities. Triple plume structures due to emission by ions, neutrals, and nanoparticles propagating with decreasing velocity have been reported recently by Anoop et al. [28] by employing monochromatic imaging during ULA of Cu with a spot size ~120 μm. Their velocity measurements of various populations showed that the ionic component expands nearly three times faster than the neutrals, while nanoparticles move much slower, and these values are consistent with three-fold structures noticed in the present studies. For the smaller spot size, the three phases remain separated throughout the expansion; however, for the larger spot size a more homogenized emission is apparent due to overlapping of the ion and neutral emission regions. Typically in laser-produced plasmas, the ions with higher charge states attain the highest velocity because of space charge effects, while neutrals and molecules are slower propagating species. This can be understood by considering the plume expansion in the initial stages where electrons strive to overtake the ions, resulting in a space charge field that accelerates ions resulting in higher velocities for highly charged ionic states.
To further study how the spot size influences the fastest moving plume component due to ionic emission, we evaluated the velocity and angular distribution of ions detected using a FC. For collecting the ions, the FC was positioned at ~14 cm away from the target surface and rotated with respect to the target normal. For obtaining the ion velocities and flux information normal to the target, the angle of incidence of the laser beam with respect to the target was set at 45 degrees for the ion analysis. The estimated ion flux and maximum probable ion velocities for 100-μm and 600-μm spot sizes are given in Fig. 4 . The angular distribution of the ion flux showed a narrower profile for the largest spot size used, while a significantly broader profile is observed for the smaller spot size. This is in agreement with the self-emission images where we observed spherical plume morphology for smaller laser spot sizes and cylindrical geometry for larger spot sizes. It can also be seen in Fig. 4(b) that the ions produced using the smaller spot size propagated with a significantly higher velocity compared to ions produced using the larger spot. The estimated ion velocities for both smaller and larger spot sizes normal to the target surface are also found to be in good agreement with plume expansion front velocities estimated using gated images, confirming that emission from the plume front positions is mainly contributed by ions. Previous reports employing ns LA showed that the ions of the highest ionization state dominate in the direction normal to the target, their concentration falls sharply away from the normal, and excited neutrals have the most angular spread [29]. This can be understood by considering the charge density at outer angular regions of the plume is effectively diminished by recombination. Moreover, for ns LA, the laser-plasma coupling is significant and hence a large ion population is accelerated in the plume front due to efficient reheating [22]. Present results employing ultrafast lasers showed that ions emanating from the 100-µm impact zone possess nearly 3 × higher velocities compared 600-µm spot sizes although they have broader ion flux angular distribution.
Fig. 4 (a) Angular distribution of ion flux for 100- and 600-μm laser focal spots estimated using a FC positioned 14 cm from the target are given. (b) The peak ion velocities at various angles with respect to target normal are given.
In order to resolve differences in atomic and ionic emission in the multi-structure plumes, visible emission spectroscopic studies have been carried out for the 100-μm and 600-μm spot sizes. Spectra were recorded with a 100-ns delay and 1-μs integration time; spectra obtained at 0.5 and 2 mm from the surface are given in Fig. 5 . The spectral features recorded for smaller and larger spot sizes showed significant differences in line intensities, ion fractions, and continuum emission. The continuum and ion emission are found to be significantly higher for the 100-μm laser spot for both distances studied (0.5 mm and 2 mm), while the spectra obtained using the 600-μm laser spot showed predominantly atomic emission regardless of the observation point. The inset to Fig. 5 shows the ion to neutral intensity ratio (Al II 358 nm/Al I 396 nm) as a function of laser spot size, showing that smaller spot sizes generate significantly higher ionization than the larger spot sizes. Because the ion/neutral intensity ratio is directly correlated with plasma temperature, we also conclude that the plume temperature is higher for smaller spot sizes.
Fig. 5 The spectral emission details obtained at (a) 0.5 mm and (b) 2 mm from the target surface for LPP produced with varying spot sizes. The spectra are recorded with a 100-ns delay and a 1-μs integration time. The inset in (b) provides the Al II to Al I emission intensity ratios for the emission lines at 358 nm and 396 nm.
Some of the possible reasons for the observation of drastic changes in plume hydrodynamic features with varying spot sizes are differences in focusing beam shape at various lens-target positions [30], or changes in laser peak intensity with respect to spot size that may lead to changes in physics of ultrafast laser ablation (laser-target coupling, ablation mechanisms, penetration length, initial plume pressure, etc.). Because we observed similar morphological changes with respect to a converging or diverging beam focus (either side of the best focal conditions), we rule out the role of incoming beam divergence on plume morphology. It is well known that the plume hydrodynamics in ns LA is affected by differences in laser-target and laser-plasma coupling with varying spot sizes. Previous reports showed that cylindrical and sharpened plumes were observed for ns LA with larger spot sizes while lower aspect ratio plumes were recorded with the use of smaller spot sizes [15,22]. However, in the case of fs LA, the laser-plasma coupling is negligible ruling this out as the source of the difference in plume hydrodynamics. The presence of an ambient gas medium can also lead to collimation effects in both ns and fs laser plasmas due to the ‘plasma pipe’ effect [8,31]. The present experiments are performed in vacuum and hence the pipe effect is not anticipated.
For ULA of metals, the material removal occurs through several competing mechanisms—Coulomb explosion, phase explosion, fragmentation, thermal vaporization etc.—and all of them depend strongly on laser intensity at the target surface. For example, the Coulomb explosion dominates at low laser intensities near to the ablation threshold while thermal vaporization dominates at sufficiently high laser intensities above the ablation threshold [32]. Experimental and modeling studies showed that two distinct ablation regimes exist for ULA of metals separated by laser fluence at the impact zone [33,34]. In the lower fluence regime (F ≤ 1.5 J/cm2), the ablation rate increases slowly where density of the hot electrons are negligible and the energy transfer occurs only within the area characterized by optical penetration depth. In the second ablation region when F ≥ 1.5 J/cm2 the ablation rate increases steeply with laser fluence due to large electron heat diffusion length [33]. The fluences used in the present experiment (2-60 J/cm2) are all significantly higher than the fs ablation threshold of metals [35], regardless of spot size used, and therefore belong to the second ablation regime where the energy distribution is contributed by electron heat diffusion length. In this regime, a steep rise in ablation rate is expected with increasing fluence.
The kinetic energies of ions in a plasma are directly related to initial temperature of the plasma. Most of the ions are generated during the initial times of plasma evolution, when the temperatures are higher, while excited atoms are mostly created through three-body recombination. According to the ion-to-neutral intensity ratio given in Fig. 5(b) (inset), the plume temperature is significantly higher with the use of the smaller spot size compared to the larger laser impact zone. In contrast to ns LA, in which laser-plasma coupling leads to plasma heating and high initial temperatures, for fs LA the initial temperature is determined by laser-sample coupling. For ULA with smaller spot sizes with higher laser fluences, significant overheating of the absorbing volume takes place and the material is transformed into the gas phase, quickly leading to a higher ionization fraction and higher ion kinetic energies. At larger spot sizes, the degree of overheating is lessened because of the increase in heat losses in the materials due to the larger impact area, and hence material ejection takes place primarily in the form of atoms and clusters.
For ns LA, the expansion dynamics and plume morphology of laser ablation plumes in vacuum is governed by the initial pressure gradients generated in the plasma. According to Anisimov [16], the expansion of ns laser-generated ellipsoidal plumes goes faster along the axis with smaller initial dimensions. Previous reports also showed that for ns LA, the smaller spots sizes lead to reduced plasma scale lengths, while for larger laser spot the scale length of the plasma becomes higher [22]. These characteristic features reported for ns LA are consistent for ULA, despite the differences in the plasma formation mechanisms. The measurements of higher ion velocity and higher initial plasma temperature for the smaller spot sizes indicate that the collimation and later expansion features of ULA are also regulated by the initial pressure gradients generated at the laser impact zone. It has to be mentioned that for a fixed spot size, the collimation properties of fs LA (angle of particle ejection) may be slightly different from ns LA because of changes in angular distribution of ions. The changes in ion angular distributions could affect the confinement properties of the plume when the plume expansion happens at ambient pressure levels [8]. However, the spherical to cylindrical plume morphological changes with spot size can be observed during spot size scan regardless of the laser pulse width.
4. Conclusions
In this letter we show that the laser spot size has a strong influence on the dynamics of ULA. The self-emission plasma images showed gradual morphological changes from spherical to cylindrical when the spot size increased from ~100 μm to ~600 μm. Ion analysis data showed that the emitted ions propagate with significantly higher velocities and broader angular distributions for plasmas generated by a smaller spot size, while a narrower angular distribution is observed for plasmas generated by larger spot sizes. The spectral studies showed that the ULA produces higher ion populations and higher initial plasma temperatures with smaller spot sizes relative to larger spot sizes. The observed results for ULA are consistent with previous measurements on ns LA, in which changes in plume morphology with spot size are explained on the basis of initial pressure gradients, which is somewhat surprising given the large differences in ablation mechanisms between ns LA and fs LA.
Acknowledgment
This work was supported by the DOE/NNSA Office of Nonproliferation and Verification Research and Development (NA-22). The Pacific Northwest National Laboratory is operated for the U.S. Department of Energy (DOE) by the Battelle Memorial Institute under Contract No. DE-AC05-76RL01830.
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