INTRODUCTION
Generation of collimated laser ablated plasma plume is of great interest because of its importance in several fundamental and applied research areas. It can play a crucial role in the development of extreme ultraviolet lithography source, thin film deposition, synthesis of nanoparticles, metallic atomic beam source for accelerators, and probing neutral atomic beam in plasma environment (Chrisey & Hubler, Reference Chrisey and Hubler1994; Geohegan et al., Reference Geohegan, Puretzky, Duscher and Pennycook1998; Doria et al., Reference Doria, Lorusso, Belloni, Nassisi, Torrisi and Gammino2004; Fazio et al., Reference Fazio, Neri, Ossi, Santo and Trusso2009; Hoffman, Reference Hoffman2009; Huber et al., Reference Huber, Samm, Schweer and Mertens2005; Masnavi et al., Reference Masnavi, Nakajima, Sasaki, Hotta and Horioka2006; Nardi et al., Reference Nardi, Maron and Hoffmann2009; Sizyuk et al., Reference Sizyuk, Hassanein and Sizyuk2007; Wolowski et al., Reference Wolowski, Badziak, Czarnecka, Parys, Pisarek, Rosinski, Turan and Yerci2007; Wang et al., Reference Wang, Xu, Zhou, Chu and Fu2007). Several attempts have been made to optimize the expanding laser produced plasma plume by varying experimental factors like, ambient gas, focal spot size, laser pulse width, irradiance, and wavelength of ablating laser (Key et al., Reference Key, Toner, Goldsack, Kilkenny, Veats, Cunningham and Lewis1983; Bulgakova et al., Reference Bulgakova, Bulgakov and Bobrenok2000; Amoruso et al., Reference Amoruso, Bruzzese, Spinelli, Velotta, Vitiello and Wang2003; Harilal, Reference Harilal2007; Beilis, Reference Beilis2007; Laska et al., Reference Laska, Jungwirth, Krasa, Krousky, Pfeifer, Rohlena, Velyhan, Ullschmied, Gammino, Torrisi, Badziak, Parys, Rosinski, Ryc and Wolowskim2008; Rafique et al., Reference Rafique, Khaleeq-Ur-Rahman, Riaz, Jalil and Farid2008). General observation is that the lateral velocity of the plume species increases with the increase in the laser fluence. Besides this, the plume expansion becomes cylindrical in shape with the increase in the laser spot size.
Plasma plume formed by laser-blow-off (LBO) technique (Adrian et al., Reference Adrian, Bohandy, Kim, Jette and Thomson1987; Bakos et al., Reference Bakos, Földes, Ignácz, Kedves and Szigeti1992; Veiko et al., Reference Veiko, Shakhno, Smirnov, Miaskovski and Nikishin2006; George et al., Reference George, Kumar, Singh and Nampoori2009; Kumar et al., Reference Kumar, Singh, Prahlad and Joshi2010), where the laser beam interacts with a thin film of target material supported on a thick transparent substrate, is used to generate short bursts of high intensity, neutral atomic/ionic beams. This technique is extensively used for neutral atomic beam injection in plasma as a diagnostic tool, impurity transport studies for the high temperature Tokamak plasma (Huber et al., Reference Huber, Samm, Schweer and Mertens2005) and as a source of metallic atomic beam for accelerators (Doria et al., Reference Doria, Lorusso, Belloni, Nassisi, Torrisi and Gammino2004).
In LBO scheme, the size, shape, and divergence of the expanding plume are highly dependent on the thickness of the film, laser spot size, and laser fluence. Numerous experimental and theoretical studied have been made to address the effect of film thickness and laser fluence on the LBO generated beam (Adrian et al., Reference Adrian, Bohandy, Kim, Jette and Thomson1987; Baseman & Froberg, Reference Baseman and Froberg1989; Singh et al., Reference Singh, Kumar, Patel and Subramanian2007). However, little attention has been paid toward investigating the role of the intensity profile of laser beam on the geometrical aspect of the LBO plume (Schultze & Wagner, Reference Schultze and Wagner1991) and its dynamics, which are highly relevant in thin film deposition, neutral beam injection in plasma environment, and other LBO induced beam applications.
In view of the above, we have conducted a systematic experiment to understand the expansion dynamics of the LBO plume formed by two different laser systems having different intensity profiles viz Gaussian and top-hat. The LiF-C target is selected due to its application in Tokamak plasma diagnostics. Neutral atomic beam of lithium atom is advantageous for injection as a diagnostic tool due to its low atomic number (power radiated due to emission is small) and simplicity of its emission spectra (all the emission lines from ground states lie in visible region). In this report, emphasis is given to the comparison of the shape, size, directionality and angular divergence of the LBO plume observed with two different laser profiles by fast time resolved imaging spectroscopy.
EXPERIMENTAL SET-UP
A detailed description of experimental set-up has been reported in our previous papers (Singh et al., Reference Singh, Kumar, Patel and Subramanian2007). Only the main features and additional parts are briefly summarized. The experiment has been carried out in a cylindrical stainless steel chamber, evacuated to a base pressure <2 × 10−5 Torr. The target is composed of uniform layers of 0.05 mm LiF and 0.5 mm thick carbon film, deposited on a 1.2 mm thick quartz substrate.
Nd: YAG (λ = 1064 nm) lasers having two different intensity profiles, top-hat laser beam profile (referred as “THP”) of 8 ns pulse width and Gaussian beam profile (referred as “GP”) of 5 ns, are used in the present study. Intensity profiles for both lasers are shown in Figure 1. The spot size of the laser beam is ~1 mm diameter at the target. By adjusting the operating parameters of the laser, fluence ~20 J/cm2 is set at the target surface for both lasers. An intensified charge-coupled device (ICCD) camera (4 Picos, Stanford Computer Optics, Inc., Berkeley, CA) having variable gain and gating on time, has been used to record the time resolved images of the plume luminescence in the spectral range of 350–750 nm. In the present experiment, gate opening (integration) time is set at 4 ns. Temporal evolution of the LBO plume has been obtained by varying the time delay (from 100 to 4000 ns) between the laser pulse and the opening time of ICCD gate. Five images are recorded under similar experimental conditions. These images are found to be nearly identical in shape and the reproducibility of the emission intensity is better than 5%. A mesh image of known dimensions has been recorded in order to map the geometrical parameters of the plume. Length and full-width at half maximum of the plume are estimated by segmentation algorithm using MATLAB (Natick, MA). For better visibility, gray images have been converted into pseudo-color images using jet color map.
(Color online) Recorded intensity profiles of laser; Gaussian beam profile, “GP” and top hat profile, “THP.”
In order to measure the distributions of ablated ions across the expansion axis, an electrical ion probe is mounted in front of the plume propagation direction. The ion probe is constructed with tungsten wire of 0.4 mm diameter. The length of the probe, which is exposed to the plasma and separation between the probe and target plate are set as 3 mm and 45 mm, respectively. In order to get the better spatial resolution (in transverse direction), orientation of the probe is aligned along the axis of plume expansion. The probe assembly is mounted on a linear motion feed-through, which enables the positioning of probe perpendicular to the plume expansion axis. A negative bias voltage (22 V) is applied to the probe to measure the ion current in the saturation limit. A 2 µF capacitor is used to decouple the measuring circuit from the applied bias voltage. The ion signal across the 50 Ω resistance is recorded on a fast digital oscilloscope.
RESULTS AND DISCUSSION
Fast imaging of the electronically excited plume species, driven by collisional processes between electrons, ions, and neutrals generated by laser-film interaction provides the two-dimensional snap shot of the expanding LBO plume. Expansion dynamics of the species as well as geometrical aspect of the expanding plume (e.g., local structure, directionality, and divergence) can be studied by observing these emissions as a function of time. Typical ICCD images of expanding plume formed by THP and GP lasers in vacuum and at various time delays are shown in Figure 2. Each image represents spectrally integrated emission intensity in the region 350–750 nm emitted from different plume species and is normalized to the maximum intensity. A visual examination of the plume images reveals following interesting effects.
(Color online) The sequence of images of expanding LBO plume in vacuum formed by THP and GP laser at different time delays. The integration time of ICCD was fixed as 4 ns. Color bar shows the normalized intensity in arbitrary unit.
For both laser beam profiles, plume expands linearly, and their intensities diminish gradually with time. The plume expansion in vacuum under the influence of pressure gradient inside ablated plume is treated as adiabatic expansion (Singh & Narayan, Reference Singh and Narayan1990), where the thermal energy of plume species is rapidly converted into the kinetic energy. This leads to decrease in electron temperature and density with time and hence decrease in electron impact process (George et al., Reference George, Kumar, Singh and Nampoori2009). Therefore, considerable reduction in the emission intensity of the plume with increase in the time delay can be anticipated. Moreover, linear dependence of the plume front position with time delay confirms free expansion of the plume (Fig. 3). Average translational velocities of 1.8 × 106 and 1.1 × 106 cm/s for the plume by THP and GP lasers, respectively, are obtained from the slopes of the curve.
Observed plume length versus time plot for the THP and GP laser generated plumes. The solid lines represent the linear fit for the experimental data.
However, it is observed that the lifetime of the emissive plume, directionality (divergence) and shape are highly dependent on the intensity profile of the laser beam. In case of THP, the plume is ellipsoidal in shape, i.e., the velocity component along the expansion axis is larger than the lateral one. The recorded images for THP show a non-uniform intensity pattern parallel to the direction of the plume expansion and are more intense at the leading edge of the plume and at points closer to the target. During the expansion, emission intensity decreases rapidly and finally becomes highly diffused after >1500 ns and is almost beyond the detection limit of the ICCD.
On the other hand, plume formed by GP laser expands linearly with smaller lateral velocity in comparison to that observed with THP laser. In this case, plume has nearly cylindrical shape. Moreover, the lifetime of the emissive plume is found to be significantly larger and is clearly visible up to t > 4000 ns. Unlike THP produced plume, it has a uniform intensity distribution; of course, it has bright patches at leading and trailing edges of the plasma. Another noteworthy observation is that the overall integrated intensities in the vertical section (Δz = 0.5 mm) of plumes produced by THP and GP laser are nearly the same (within 5% uncertainty) at any fixed location of the plume. However, due to confined geometry, images with GP laser look brighter as compared to THP-plume.
The difference in lateral expansion for these cases is clearly visible in plume width versus time plot as shown in Figure 4. In case of THP laser, width of the plume gradually increases with time. On the other hand, plume width with GP laser slowly increases up to t = 2500 ns and attains nearly constant value with further increase in the time delay. Figure 4 shows that the transverse velocity of THP plume is higher than the transverse velocity of GP plume in the overlapping region; and after t > 2500 ns, GP plume follows one-dimensional motion with negligible transverse velocity component.
Variation of plume width as a function of time delay for the both THP and GP laser.
Visible inspection of Figure 2 reveals that there is a significant difference in the divergence of the expanding plume formed by THP and GP lasers. The divergence of plume can be estimated by measuring the diameter of the plume at two separate points di and df separated by a distance x and using the relation,
In this regard, we have selected one set of plume formation at t = 800 ns; and considered the portion of the plume up to the maximum acquired diameter (nearly up to half of the total length of the plume). Diameter of the plume at different locations of the plume and separation between them are used to estimate the divergence of plume for both laser systems. The estimated values of divergence of plumes formed by THP and GP lasers at t = 800 ns are 1.5 rad and 40 mrad, respectively. This indicates that transverse expansion in case of THP produced plume is larger in comparison to GP-plume by a factor of ~40. The measured divergence (40 mrad) with GP laser further reduces at later stages (t >2500 ns) where the plume width is almost invariant with time. This is an important finding in the sense that GP produces low divergence and long lived highly collimated plume. The persistence of low divergence for the longer times is highly suitable for producing collimated atomic/ionic beams.
Further, while comparing the directionality of THP and GP laser induced plumes; it is worthwhile to see the angular dependence of ejected species in the respective plumes extracted from the recorded images (Amoruso et al., Reference Amoruso, Sambri and Wang2006). The recorded image is divided into radial slices (of specific width) and the emission intensity is integrated along each axis. A typical intensity distribution (normalized to its maximum) of LBO plumes formed by THP and GP laser, respectively, at a time delay, t = 800 ns is shown in Figure 5. It should be noted that, the difference in angular distributions for THP and GP is not as prominent as observed in visible inspection of the plume images, shown in Figure 2. This discrepancy is due to the limitation of adopted method, where the major portion of the angular slice is lying in the intense portion of the plume (close to the target), especially at higher angles, which gives wrong information at these angles. In spite of this, some features are still comparable for these two cases and we have observed some differences as well. For both cases, one can see that distribution of species is highly peaked in the forward direction and shows isotropic behavior. Almost similar distributions are observed for different time delays. The intensity distribution profile of THP plume appears to have two types of distributions (1) a narrow one lying near the central position and (2) a broader one appearing as a shoulder. On the other hand, species in the GP-plume lie in the narrow angular region and vary smoothly with the angle of incidence.
Angular dependence of emission intensity of the LBO plumes produced by THP and GP laser at 800 ns time delay.
Since the ICCD images provide the information about the excited plume species, it is worthwhile to verify the above results with other diagnostic techniques. In this regard, the ion distributions across the expansion axis have been measured using electrical ion probe. The total intensity of ions at any probe position is obtained by the area under the temporal profile of ion current. The normalized (with maximum intensity) intensity variation of ejected ions as a function of radial distance for both THP and GP lasers are shown in Figure 6. Figure 6 clearly shows that the ion distribution is peaked in forward direction for both the laser systems and also the ions formed by GP laser have narrow distribution as compared to that of THP laser. Thus, the ion probe results further support the fact that the plume formed by GP laser is of low divergence.
Ion distributions of the LBO plumes as function of radial distance for the both THP and GP laser. The distance between the ion probe and target plate is set as 45 mm.
It is mentioned earlier that THP plume having large divergence in comparison to plume formed by GP laser. For similar power density range, Singh and Narayan (Reference Singh and Narayan1990) and others (Anisimov & Luk'yanchuk, Reference Anisimov and Luk'yanchuk2002) have modeled the plume formation and its expansion for the conventional solid ablation. According to their proposed models, the pressure driven acceleration is larger along the direction where the plume has smaller size. Since at the initial stage, plume size along the expansion direction is small. Therefore, the velocity component along the expansion axis is much larger than the lateral one and hence the plume acquires an ellipsoidal shape. We have also observed that plume shape during expansion is elliptical as in case of THP, which is in close agreement with the proposed model. However, plume shape is close to cylindrical for GP plume. The plume size in the transverse direction varies negligibly with time (especially at higher time delays), which could not be fully predicted by the above model. This difference in the divergence of both plumes during their expansion should be related to the difference in plume formation (discussed below).
The GP-plume moves with smaller velocity as compared to the THP-plume, which can be understood in terms of a qualitative mechanism for the sequence of the processes in the formation of the LBO plume (Adrian et al., Reference Adrian, Bohandy, Kim, Jette and Thomson1987) e.g., melting of the film after the laser strikes the front surface, propagation of the melt front toward the back surface and propelling of the material due to increased vapor pressure. In case of thin film target (skin depth attaining time <laser pulse duration), all these events are completed before the termination of laser pulse. Therefore, the propelled material further interacts with the laser and forms the plasma plume. The time required for the melt through (melt front reaches the back surface), plays an important role in the removal process, which should depend on the power density of the laser, thickness and thermal properties of the target film.
In case of THP laser, uniform heating of target film within the irradiated region is expected. For the considered power density, the super-heated front surface having high vapor pressure causes explosive rupture of the vacuum-film interface. Therefore, the propelled material is propagated in the forward direction with significant transverse velocity component. On the other hand, in case of GP produced plume, a non-uniform heating/melting of the film favors bubble formation in between the substrate-film interface (Broer & Vriens, Reference Broer and Vriens1983). With increase in vapor pressure, ultimately an opening may be formed to release the material rather than the occurrence of explosive rupture. Since peak intensity of the GP laser is higher than the intensity of the THP, a smaller region of the film near the Gaussian peak will reach melt through rapidly and propulsion may start through a small area. This will reduce the buildup of vapor pressure. Therefore, the material removal will take place through a small orifice similar to gas expansion through a smaller size nozzle but with less stagnation pressure, which may result in a relatively low velocity collimated plume with GP.
SUMMARY
The reported results clearly show the dependence of characteristic expansion of the LBO plume on the laser beam intensity profile. The geometrical shape, velocity, and directionality of the plumes formed by the GP and THP laser are significantly different. One of the important observations reported in the present work is very low divergence plume produced by GP in comparison to the plume generated by THP. The results obtained by the ion probe also support the above observations. The present observations are explained on the basis of the existing models; however more theoretical work is required to understand it quantitatively. Nonetheless we feel that present observations will be of significant importance in shaping laser generated plasma plumes and understanding and controlling the geometrical aspect of the LBO generated atomic/ionic beam.
