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Si and Cu targets were ablated by a capillary-discharge 46.9-nm pumped laser beam, focused by a toroidal mirror at grazing incidence. The peak power density of the focal spot was ~2 × 107 W/cm2. Clear ablation patterns on the surfaces of Si and Cu targets were obtained, with shapes consistent with simulations. A YAG:Ce scintillator (cerium-doped YAG crystal) was used to image the variations of the laser spots. We discuss the shape and damage mechanics of the measured patterns. Melting of the target material was observed in the ablation region on Cu, but not on Si.
© 2015 Optical Society of America
1. Introduction
Interactions between laser light and matter produce a wealth of experimentally observable physical phenomena. Of particular interest is the ablation of diverse materials. Current cutting-edge research uses lasers with short wavelengths and high photon energies as radiation sources to study interactions experimentally. Solid targets are used for their stability and ease of preparation.
Initial research focused on investigating variations in the external morphology of ablation patterns [1]. Subsequently, the depth and surface area of patterns were measured and different kinds of target gradually came into use [2–4]. Ablation rates and thresholds were also calculated from the information contained in ablation patterns [5,6]. Improved detection techniques allowed more physical information to be derived from the ablation patterns; also, radiation sources have not been confined to lasers. Tetsuya Makimura et al. used X-rays to ablate a SiO2 target [7]. Atomic species ejected from a silica surface during the irradiation were detected, and they also showed that the ablation process was non-thermal.
Spherical mirrors with an Sc/Si multilayer coating or an Ir coating are used at normal incidence with a 46.9-nm laser in most interaction experiments. These mirrors serve to reduce optical aberration and increase the power density of the focused beam. However, this way of focusing the X-ray laser has two drawbacks. Firstly, their reflectivity for 46.9-nm laser light is only 15%-40%. In addition, damage to the mirror caused by the laser and the ejection of the capillary discharge plasma cannot be ignored. Rocca’s group performed an experiment to investigate this damage specifically [8] and concluded that it affected the lifetime of the mirror. Secondly, the spherical mirror focuses the beam onto a rather small region, so that the ablation patterns caused by the laser beam and the background light cannot be distinguished. Therefore, new ways to optimize the focusing a 46.9-nm laser would be sought to address the problems caused by spherical mirrors.
XUV light can also be focused by a toroidal mirror. In this work, a toroidal mirror was used to focus a 46.9-nm laser beam, at grazing incidence, with a reflectivity of up to 90%. The toroidal mirror was not coated and therefore it was damaged negligibly by the laser and ejected plasma. Also, thanks to optical aberration caused by the toroidal mirror, the ablation patterns produced by the laser and the background light could be discriminated easily.
2. Experimental setup
The radiation source used in this experiment was a 46.9-nm X-ray laser with a pulse width of 1.7 ns and an energy of 50 μJ. The beam was produced from a 35-cm-long Ne-like Ar capillary plasma excited with a fast current pulse. Besides the A laser lines of 46.9 nm wavelength derived from the Ar plasma, the C line at 69.8 nm and the E line at 72.6 nm were also simultaneously generated. Our experiments were performed under the condition where the 46.9-nm laser was much stronger than the other two lasers. Details of the X-ray laser are described elsewhere [9,10].
The vacuum chamber provided the ablation environment. During the experiment, the chamber pressure was ~10−5 Pa. The toroidal mirror, made of fused silica with no coating, was placed inside the chamber to focus the X-ray laser. The curvature of the meridional plane was 3048 mm and the curvature of the sagittal surface was 47.6 mm.
A detailed description of the focal beam path was described in a previous publication [11]; we here only give a general outline in Fig. 1. The toroidal mirror was fixed at the bottom of the chamber to focus the laser, while the target was fastened on a 2D stage along the beam propagation direction and also perpendicularly to it. An XRD (X-ray diode) was also placed on the beam path, behind the target. Based on the curvature of the toroidal mirror and the size of the vacuum chamber, an incidence angle of 83° was chosen. According to the focal-distance formula, the focal length was ~195mm. At the start of the experiment, the target was moved away from the beam path using the 2D-stage. Then the XRD was used to measure the relative energy of the X-ray laser. Once the laser output became stable, the target was returned to the beam path and irradiated with the X-ray laser.
3. Focal spot variation simulated by ZEMAX and measured by a YAG:Ce scintillator
Because X-rays are invisible, the beam path was also simulated to determine the focal spot and location of the target. To improve the simulation accuracy, a YAG:Ce scintillator (cerium-doped YAG crystal) was used to display the real shape of the laser spots. The visible fluorescence induced by the X-ray beam on the YAG:Ce crystal was detected. The observation beam path is as shown in Fig. 1, except that the mirror and XRD are removed and the target is replaced by the scintillator. The scintillator is located 1000 mm away from the laser output port. A high-resolution camera was used to capture the image of the visible light. Two spots with and without the laser are shown in Fig. 2(a) and 2(b). Figure 2(a) shows that the shape of the laser spot has a complicated structure with an intensity minimum at the center. On the other hand, the background light shown in Fig. 2(b) is uniform. Based on the shape of the actual laser-induced spot, the laser spot was simulated as in Fig. 2(c) using the ZEMAX software. The color spectrum in Fig. 2(c) expresses the relative power density.
Fig. 2 The real and simulated laser beams and the background light: (a) Laser beam; (b) Background light; (c) Simulated light source spot.
On the basis of the beam-path parameters of Fig. 1, a focal spot was simulated using ZEMAX software and is shown in Fig. 3. Because of the grazing incidence, the power density of the focal spot was affected by optical aberration. In spite of this, the maximal power density of the focal spot was 300 times higher than the original spot and most of the light energy was focused onto a very small area. The area of the detector in Fig. 2(c) is 11 times larger than the one in Fig. 3, which helps to display the spot clearly. The focal length returned by the ZEMAX simulation is close to the result of the focal-length formula.
The real focal spot was observed on the scintillator. The target, which could be moved along the beam path in Fig. 1, was replaced by the scintillator and the camera captured the resulting image. Figure 4 shows the fluorescence induced by the laser spots on the scintillator (left-hand side) and the corresponding ZEMAX simulations (right-hand side), at different distances from the focus. Figure 4(b) corresponds to the focal position, whereas Fig. 4(a) and 4(c) correspond to imaging planes located 3 cm upstream or downstream, respectively. The shapes of the real laser spots are consistent with the simulated ones.
Fig. 4 Intensity patterns observed on the scintillator (left) and the corresponding ZEMAX simulations (right): (a) 3cm upstream from the focal plane; (b) at the focal plane; (c) 3cm downstream from the focal plane.
4. Ablations results and discussion
The ablation experiment parameters were based on the ZEMAX simulations and the scintillator experiments. A Si target located at the focal spot in the beam path (Fig. 1) was ablated. A scanning electron microscope (SEM) image of the ablation patterns is shown in Fig. 5, magnified by a factor of 100. Pattern 1 in Fig. 2(b) was induced by 200 shots of background light, and 2 in Fig. 2(a) by 200 shots of both the X-ray laser beam plus the background light. The darker region in the middle of pattern 2 is similar to the patterns visible in Fig. 4(b), which are supposed to be induced by the 46.9-nm laser. No melting of the Si was observed in the ablated region. Based on the area of the ablation pattern (the darker region in the middle of pattern 2), the power density of the focused laser is calculated to be ~2 × 107 W/cm2.
Ablation patterns on a Si target produced by different numbers of laser shots are shown in Fig. 6. The patterns labeled 1 to 6 were induced, respectively, by 1, 2, 3, 6, 12, and 25 laser shots. The juxtaposition of these ablation patterns helps to convey an impression of the relative faintness of the patterns produced with just a few shots. The observation of damage produced by even a single shot (pattern 1 in Fig. 6) indicates that the fluence of the laser is higher than the ablation threshold for Si.
Depth and phase information within the ablation area was obtained with an atomic-force microscope (AFM). Figure 7 shows the depth morphology of the ablated area, and Fig. 8 the corresponding phase image, which explains the physical properties of the surface. These images show that the physical properties of the Si surface are transformed distinctly after the ablation, while the morphology of the ablation area is barely changed. This effect provides a detection method, for use in interaction experiments that is more effective and more accurate in cases where the ablation pattern cannot be distinguished clearly enough in terms of depth alone.
The interaction of the X-ray laser with a Cu target was also investigated under the same experimental conditions. The SEM image in Fig. 9 was induced by 200 shots of the 46.9-nm laser. The shape of the pattern on the Cu target is similar to that on the Si target. However, melting of copper was observed in the ablation region. Figure 10 shows a magnified image of such a melted region on the Cu surface and the corresponding energy spectrum. The energy spectrum demonstrates that the melted part consists mainly of Cu with only a very small proportion of elemental sulfur and oxygen, originating from crude Cu. The melted parts of the ablation region form several small spherical features and a few bulkier ones, as shown in Fig. 10(a). The copper seems to have melted and then resolidified. The size of the visible nanoparticles was measured to range from tens to hundreds of nanometers. Most particles are regularly spherical, and some of the irregular-shaped ones shown in Fig. 10(a) are supposed to have formed through successive multiple shots. Craters are also visible in the ablation region, as shown in Fig. 11. According to our analysis, these craters were formed as a result of copper emitted by thermal evaporation.
Fig. 10 Magnified SEM image showing (a) a melted region on the Cu surface and (b) the energy spectrum indicating the component of the Cu surface.
The attenuation depths in Si and Cu for energies of 26.5 eV are approximately 0.2 and 0.01 μm, respectively, and their respective melting points are 1410° and 1083°. According to Ishino et al. [12], variations in the attenuation depth and melting point are explained by differences in the surface modifications. This may explain the diversity in surface behavior on Si and Cu targets, in particular regarding the melting effect seen only on Cu.
The melted droplets seen within the ablation region of the Cu target are presumed to be laser-induced nanoparticles (nanoclusters) [13–15]. This effect can be observed as a result of the expansion of vapor produced by the laser ablation. The size distribution of the nanoparticles may be affected by the optical-energy fluence, and the pulse width and repetition rate, among other parameters. Especially in our ablation process, the material experienced not only heating, melting, boiling, and evaporation (the processes that occur in nanosecond laser ablation), but also spallative ablation and other ablation process seen with a soft X-ray laser. Therefore, the size distribution of the nanoparticles depends on the complicated ablation process.
5. Conclusion
In conclusion, Si and Cu targets were ablated by an X-ray laser of 46.9-nm wavelength and ~50-μJ energy, focused by a toroidal mirror at grazing incidence. The peak power density of the focal laser was calculated to be 2 × 107 W/cm2. Clear patterns are observed on the Si target, where the ablation process is supposed to be non-thermal. Also, single-shot damage on Si shows that the fluence of the 46.9-nm laser is higher than the ablation threshold. Similar patterns were observed on the Cu target and displayed evidence of melting. Laser-induced nanoparticles were also detected in the ablation region of Cu and were presumed to be a product of the complicated ablation process. Future experiments will investigate different targets for use in ablation experiment, and more advanced detection technology will probe the related ablation phenomena.
Acknowledgment
This project was supported by the National Natural Science Foundation of China (No. 61275139).
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