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Abstract
Concentric rings are observed on the front surface when a laser pulse focuses on the rear surface of fused silica. The spatial distribution of concentric rings presents a certain regularity, and the width and depth of the ring gradually increase from inside to outside. These features of the concentric rings are affected by the laser wavelength and energy. Their formation mechanism is explored by means of combining a time-resolved pump-probe shadowgraph and surface damage morphology. Concentric rings are formed during the irradiation process of one nanosecond pulses, whose radius and depth characteristics may record the relationship between the early stage of laser-induced damage structure and the evolution of early plasma.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Laser-induced damage (LID) for optical material is an important factor restricting the development of high-power laser systems [1,2]. Studying the physical mechanism of optical material damage and the basic laws of damage growth is helpful to improve the optical processing technology and increase the service life of optical material to develop more-powerful laser systems. As an optical element with excellent properties, fused silica is widely used in laser systems, and many researchers have studied its LID [3–5]. The surface or subsurface defects during grinding and polishing [6,7] nonlinear effects [8,9] and surface contaminants [10,11] can cause damage, and the damage growth is correlated to the laser energy and wavelength [12,13].
A time-resolved pump-probe shadowgraph is a conventional method to study LID. However, because of the limited repeatability of pulses, it is difficult to capture the early dynamic characteristics of LID accurately, whereas LID always happen at the preliminary stage of the incident nanosecond laser pulse. Concentric ring damage induced by a laser pulse has been found on a sample surface [14–21], which may record some early LID information. Qiu et al. [16] used a 1064 nm nanosecond laser to ablate fused silica. The concentric ring appeared on the surface and was believed to be caused by the erosion of loose polishing layer by high-temperature and high-pressure plasma. However, there was no such phenomenon for 532 nm laser ablation. Sun et al. [17,18] used a nanosecond laser with a wavelength of 1064 nm to ablate the SiO2 thin layer. The concentric ring structure was attributed to the interference of the laser beam between the film interfaces and the shock wave front. Chambonneau et al. [19–21] found that the distribution of concentric rings corresponded to the intensity spikes of the incident pulse and considered that the formation of the rings was related to multiple longitudinal modes laser pulses. In our work, the concentric ring damage is not observed on the sample surface which laser focuses on directly. However, this structure appears on the front surface of the sample when the laser focuses on the rear surface. In addition, the concentric rings exhibit some characteristics that differ from other reported results. The spatial distribution of the rings presents a certain regularity, and its spacing and depth increase with the radius increasing. The influence of laser parameters (energy and wavelength) and focusing conditions (focus point in the front or the rear surface) on the ring characteristics has been explored, and the cause of ring formation is conjectured.
2. Experiments
The fused silica sample (100 × 7 × 2 mm) was fixed on a movable platform and ablated by the focused nanosecond laser pulse. An Nd:YAG laser instrument (Innolas Spitlight 600) emitted laser pulses in the pump lasers with 355 or 532 nm wavelength, 10 Hz repetition rate, and 8 and 14 ns pulse durations (full width at half maximum, FWHM), respectively. The probe laser was from a titanium sapphire femtosecond laser instrument, which operated at 50 fs pulse duration, 800 nm central wavelength, and 10 Hz repetition rate. The temporal profiles of these two laser pulses are shown in Fig. 1. The pump laser passed through a lens and focused on the sample surface. The spot size was approximate 100 µm. The probe laser penetrated the ablation plume and entered the charge-coupled device (CCD) camera (Megaplus ES3200). A mechanical shutter was placed in the probe and pump light paths to limit the passing pump or probe laser number. The pump laser, probe laser, CCD, and mechanical shutter were controlled asynchronously or synchronously by a digital pulse delay generator (DG645, Stanford Research System). Two photoelectric detectors with a high response time were used to transform the pump and probe light into electric signals, which were connected to a digital oscilloscope (Wave Surface XS). The actual delay time of pump light and probe light can be calculated from the peak value difference between the two electric signals. A microscope (Nikon eclipse LV100) was used to capture the damage sample surfaces. A stylus profiler (DektakXT, vertical resolution of 1 Å, repeatability of 5 Å) recorded the 2D profile morphology of the ablation craters. More-detailed information about this pump-probe shadowgraph setup can be found in previous work [22].
3. Results and discussion
3.1 Surface damage on fused silica
A nanosecond pump laser was focused on the front and rear surfaces of fused silica. In Fig. 2, several typical damage characteristics can be observed: 1) the ablation crater located at the center of the damage region, 2) the longitudinal cracks that extend from the ablation center to the outside, and 3) the lamellar damages are around the center (chips) [23]. When the laser focuses on the front surface, most of the energy is not transmitted to the rear surface; thus, there is no damage on the rear surface. The image in Fig. 2(c) is similar to that in Fig. 2(a), equivalent to viewing the ablation crater from the bottom. The concentric ring shown in Fig. 2(b) only appears on the front surface upon the rear surface irradiation. According to the propagation law of the Gaussian beam, laser has a higher energy fluence on the focal spot. In Fig. 2(b), when the laser beam spot is about 100 µm, several ablative phenomena can be observed. An obvious ablation crater with wide of about 55 µm appears. The lengths of some cracks extend to about 400 µm. On the plane away from the focus, the beam diameter is larger and more dispersed in energy, which is conducive to causing ablation damage on a larger range and preserving them. When the laser ablates the rear surface, the beam spot on the front surface is about 340 µm, the maximum diameter of ring damage structure is approximately 200 µm. Because of the low fluence, the center damage is small, and the surrounding ring structure is not destroyed. In the case of Fig. 2(a) and (d), the laser focuses on these two surfaces, the beam diameter is smaller, and the surface absorbs a lot of energy, causing serious damage, destroying the ring structure and making it difficult to observe.
Fig. 2. Damage morphology observed from different surfaces of fused silica, 355 nm laser wavelength and 25 mJ laser energy. Front surface (a) and rear surface (c) upon laser focus on the front surface. Front surface (b) and rear surface (d) upon laser focus on the rear surface.
3.2 Concentric ring features
In Fig. 3, there are some irregular pits distributed in the ring center. With the increase of radius, the ring structure becomes sparser. The outermost ring is the most severely damaged, and some cracks along it can be observed. Figure 4 shows the profile of the concentric ring measured by a stylus profiler. The maximum damage diameter of the outermost concentric ring on the front surface is approximately 240 µm, and the maximum depth is approximately 0.65 µm. Furthermore, there are many small grooves (nanometer-level depth) distributed symmetrically from the crater center to the ring edge. This depth distribution indicates that the distribution of light intensity on the surface is not a Gaussian distribution, the defects in the outer ring absorb more energy. This may because the truncation effect of the lens aperture [24], the Gaussian beam evolve into a form of higher energy at the center and the edges, and makes the concentric rings deeper at the center and outer rings.
Fig. 3. Concentric ring morphology on the front surface for lasers with different parameters striking the rear surface. 355 nm wavelength, 25 mJ energy (a) and 90 mJ energy (b).
Fig. 4. Two-dimensional morphology of crater on the front surface for laser with 355 nm wavelength and 90 mJ energy striking the rear surface
In addition to the focusing conditions, the laser parameters also affect the concentric ring. Under various experimental conditions, samples, which were ablated with more possibility, were selected. For example, when the laser energy was 90 mJ, the number of rings for most results was 19-22. 15 samples were selected to test and these experimental data statistics were calculated under each condition, respectively. Table 1 records some features of the rings under different wavelengths and energies. Compared with a 532 nm wavelength, the maximum diameter of the rings increased by approximately 20 µm, and the maximum depth increased by approximately 50 nm at a 355 nm wavelength, but there was no significant difference in the ring number. The change is more obvious when the laser energy is increased, under 90 mJ laser irradiation, the maximum diameter can increase by approximately 40 µm, the maximum depth increases approximately 300 nm, and the number of rings increases significantly from 6-8 to 19-22. In addition, the distance between the four grooves in the outermost layer of the ring structure is recorded in the Table 1. Use (i) to (iii) to mark the spacing from outside to inside. At the same wavelength, increasing the laser energy will make the spacing increase (i), but the spacings (ii) and (iii) are almost unchanged. At the same energy, the spacing for 532 nm laser irradiation is greater than that for 355 nm laser irradiation. These results indicate that the ring structure may be generated by optical interference, and laser-induced plasma is also involved in the generation process in some way. Nanosecond laser ablation occur during the early stage of one whole pulse, and the rest of one pulse is modulated by the resulting plasma plume to form concentric rings. Due to the combined action of the shock front and laser ablation, the outermost ring is ablated deeper. Therefore, its depth and width are affected by both laser wavelength and energy. Because the fluctuation of breakdown time and laser energy in each experiment, the standard deviations of the depth and spacing of the outermost ring are larger, as shown in Table 1.
Table 1. Concentric ring features under different laser parameters
3.3 Time-resolved shadowgraph
The evolutions of plasma induced by a nanosecond laser on the front surface and on the rear surface, as shown in Fig. 5, can be recorded by using a time-resolved shadowgraph. In Fig. 5(a) and (b), the two surfaces are broken down when the focus point at the rear surface. There are one semispherical shock wave and one pear-shaped shock wave (the wave front has been penetrated by the ejection particles) in the left and right air regions, respectively. The plasma luminance at the front surface is stronger than that at the rear surface. when the focus point at the front surface, only the front surface is ablated and produce shock waves. Based on the point explosion model, [25] the relationship between the shock wave radius and the time is calculated, and it is shown in Fig. 6. The point explosion model describes the evolution process of shock wave with a certain initial energy in the target environment. The experimental data points are consistent with the theoretical simulation curves. The difference between these curves can be used to distinguish the initial energy value of the plasma plume under different conditions. At the same time a larger radius indicates that the plasma absorbs more energy during the laser pulse and expands more rapidly. Figure 6 shows the radius of the shock wave front increases significantly as the incident laser energy increases from 25 to 90 mJ. While the influence of the laser wavelength is relatively unremarkable, the plasma expansion induced by the 355 nm laser is slightly faster than that induced by the 532 nm laser. The influence of laser parameters on the evolution of the plasma plume is consistent with that on the maximum diameter of the ring structure. The plasma expands outward in a hemispherical shape and forms an optical interface at the front, which is conducive to the emergence of the interference phenomenon and leaves an interference ring on the surface. This can explain the damaged surface to exhibit a zigzag shape, rather than a ladder shape that gradually becomes shallower as the radius increases. When the incident laser energy increases to 90 mJ, the plasma expands more quickly, and the density of the shock front is higher, which makes the ring structure more pronounced, with larger maximum radius and more rings.
Fig. 5. Time-resolved shadowgraph images of nanosecond laser (355 nm wavelength, 25 mJ energy) ablation of the rear and front surfaces of fused silica. (a), (b) laser focus on the rear surface and (c), (d) laser focus on the front surface
Fig. 6. Relationship between shock wave front radius and delay time under different laser parameter combination
4. Conclusion
When one pulsed nanosecond laser focuses on the rear surface of fused silica, concentric rings can be observed on its front surface. These rings are made up of many grooves. As the radius increases, the width and depth of the groove gradually increase. The diameter and depth of these rings reaches sub-millimeter level and sub-micron level, respectively. The formation of this structure is determined by the combination of focusing conditions and plasma plumes. The shock wave front induced by the early nanosecond laser pulse forms a hemispherical optical interface, the rest part of one laser pulse causes interference inside the optical interface and form concentric rings on the sample surface. During laser ablation process, the larger beam spot is conducive to preserve these minor structures. The number, radius, and depth of the concentric rings may reflect the important information of the early ablation process, such as the early plasma evolution and the initial damage structure. Concentric ring damage morphology may help us understand the underlying mechanism of LID.
Funding
National Natural Science Foundation of China (41573016, 11972313).
Acknowledgements
The whole experimental work was finished in Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology, Research Center of Laser Fusion, CAEP.
Disclosures
The authors declare no conflicts of interest.
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