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OPTICS LETTERS / Vol. 38, No. 15 / August 1, 2013
Combining wet etching and real-time damage event imaging to reveal the most dangerous laser damage initiator in fused silica Guohang Hu, Yuanan Zhao,* Xiaofeng Liu, Dawei Li, Qiling Xiao, Kui Yi, and Jianda Shao Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China *Corresponding author:
[email protected] Received May 10, 2013; revised June 18, 2013; accepted June 25, 2013; posted June 26, 2013 (Doc. ID 190011); published July 18, 2013 A reliable method, combining a wet etch process and real-time damage event imaging during a raster scan laser damage test, has been developed to directly determine the most dangerous precursor inducing low-density laser damage at 355 nm in fused silica. It is revealed that ∼16% of laser damage sites were initiated at the place of the scratches, ∼49% initiated at the digs, and ∼35% initiated at invisible defects. The morphologies of dangerous scratches and digs were compared with those of moderate ones. It is found that local sharp variation at the edge, twist, or inside of a subsurface defect is the most dangerous laser damage precursor. © 2013 Optical Society of America OCIS codes: (140.3330) Laser damage; (140.3440) Laser-induced breakdown; (140.3380) Laser materials. http://dx.doi.org/10.1364/OL.38.002632
In fused silica material, laser intensities in excess of 1011 W∕cm2 are required in order to achieve intrinsic breakdown [1]. However, absorbing defects in the redeposition layer and subsurface defects hidden under the redeposition layer can generate laser damage at more than 2 orders of magnitude lower intensities [2–4]. The redeposition layer with a thickness of 100 nm can be removed by a wet etch process [5,6]. The subsurface defects, commonly identified as scratches and digs, were acknowledged as the leading factor lowering laser damage resistance of fused silica. The creation of subsurface defects can be thought of as the repeated indentation of mechanically loaded hard abrasives sliding on the surface of a brittle substrate during grinding and polishing processes [7,8]. These subsurface defects were minimized in practice by using a controlled sequence of successively gentler grinding and polishing steps, making sure that each step removed enough material to eliminate damage produced by the previous step [9]. But, even so, subsurface defects still existed in the fused silica. Many studies have been conducted to understand the mechanism of subsurface defects’ inducing laser damage in fused silica. Before the year 2005, most researchers acknowledged that subsurface defects reduced laser damage resistance by providing sites for light-absorbing contaminants to reside [3,10]. Recently, many experimental results disclosed that a clean fracture, without contaminants, had the potential to induce laser damage. Bercegol et al. demonstrated the strong relation between laser damage initiation and a clean fracture, which was created deliberately on the surface of fused silica [11]. They also developed a theoretical model to explain the fracture-induced laser damage mechanism by introducing an absorbing layer on the fracture surface [12,13]. Laurence et al. and Miller et al. proved that the contaminant-free fracture, which was deliberately created on the surface, was especially bright in photoluminescence (PL) images captured by a time-resolved PL technique, and they confirmed that the clean fracture itself could absorb laser beam energy and lead to laser damage [14–16]. 0146-9592/13/152632-04$15.00/0
Although many studies have revealed the strong relation of laser damage initiation and deliberately created fractures, no direct evidence has proved the process of subsurface defects’ inducing laser damage initiation. In this Letter, a reliable method, combining a wet etch process and real-time damage event imaging during a raster scan laser damage test, was developed to directly determine the absorber that was inducing low-density laser damage. A wet etch process was applied to expose the subsurface defect to the surface, and then the subsurface defect, such as scratches and digs, could be detected by the on-line imaging system. The wet etch process has become an attractive tool for inspecting subsurface defects and/or increasing the laser-induced damage threshold. An optimized etching process has being widely used in high-power laser systems for improving the laser damage resistance of fused silica. Therefore, in this Letter, a wet etch process exposed the subsurface defect, on the one hand, and imitated the posttreatment of fused silica, on the other hand. The fused silica samples, which were customarily polished by CeO2 , were etched to the depth of ∼10 μm in a 1% hydrofluoric acid and 15% ammonium fluoride (NH4 F) solution in deionized water for 6.6 h at 23°C. After etching, the raster scan laser damage test was implemented on the etched surface to find the low-density laser damage events. The laser damage test facility is shown in Fig. 1. This facility can be separated into four main parts:
Fig. 1. Schematic of laser damage test experimental bench used for fused silica. © 2013 Optical Society of America
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the laser source, yielding an intense laser pulse; the beam delivery system to focus the laser pulse onto samples; a beam analyzer; and a damage detector for accurate metrology of laser damage. The experiments were performed using a Nd:YAG laser that operated at 355 nm with a frequency of 10 Hz. The laser had a Gaussian temporal profile with pulse duration of about 8 ns. The laser fluence was adjusted by using an energy attenuator, which consisted of a half-wave plate and a polarizer. The beam was focused by focusing optics to the bulk of the sample. The focal length was 5000 mm, and the effective area of the spot on the sample was 0.23 mm2 , measured by a laser beam analyzer. The surface exposure site was illuminated by a white light, and a surface detection system [17,18] provided high-resolution images of the exposure area on the back surface of the sample immediately before and after laser pulses. The resolution of the surface detection system was ∼10 μm. The raster scan procedure was implemented to expose lower fluence damage and disclose the absorber with low density. The sample was scanned over a 5 cm2 area at a given fluence. The spatial overlap between two successive shots was 90%. Typically 5–7 areas were raster scanned for the requirements of different fluence levels. The automated damage detection system was enabled by the sample imaging system. The sample imaging system provided high-resolution images of the sample surface. The image was captured after each pulse irradiation while the optic was moving. The damage detection routines found a damage event by comparing an image of the exposed area prior to the laser pulse with an image of the same area taken after the laser pulse. Each captured site before and after laser irradiation was subtracted from a reference image. Then defect tables, recording the position and size of damage sites, could be created before and after the laser shot. The number, location, and diameter in these two tables were compared. If a matching defect was not found or if the defect grew significantly, the defect was considered a new one, and the damage occurrence was determined. When a damage event was detected during the raster scan process, images captured before and after the occurrence of the damage event were extracted to determine the initiator. As seen in Fig. 2, the upper and bottom images were captured after the (N − 1)th and Nth laser pulse irradiation. The box represents the same area in consecutive images captured while the optic was moving. It is seen that the damage site in the bottom image was initiated by a surface defect revealed in the upper image.
Fig. 2. Images captured after (a) (N − 1)th pulse and (b) Nth pulse irradiation. The box represents the same area in consecutive images captured while the optic was moving.
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Fig. 3. Damage density versus laser fluence detected on the six samples.
Six 100 mm × 100 mm fused silica samples were etched to a depth of about 10 μm and then committed to the raster scan laser damage test with automated damage detection system. Figure 3 showed the results of damage density versus laser fluence detected on these samples. The density of damage sites was 0–0.8 cm−2 under laser irradiation with a fluence of 13–23 J∕cm2 (8 ns), and the density did not linearly increase with the fluence. 45 damage sites were revealed in these 6 samples. All of the damage initiators in these samples were determined by a detailed analysis of the images captured before exposure to laser irradiation that could lead to laser damage. It is found that there were three kinds of initiators: scratches, digs, and invisible defects, as shown in Fig. 4. Figures 4(a) and (b) show that damage sites were initiated separately by a dig and a scratch, while Fig. 4(c) shows that a damage site was initiated by an invisible defect. The size of scratches and digs was larger than 10 μm, corresponding to the resolution of the on-line imaging system. The respective proportion of damage sites induced by scratches, digs, and invisible defects was 16%, 49%, and 35%. It is seen that both scratches and digs, exposed by the wet etching process, could lead to laser-induced damage. Moreover, the dangerous digs had a higher density than the dangerous scratches. The invisible defect might be a subsurface defect hidden at a depth deeper than ∼10 μm, which was not revealed by the wet etch process, or a surface defect smaller than 10 μm, which could not be imaged by the detection system. It is hard to determine the property of invisible defects. In this Letter, more attention has been paid to the visible defects, such as scratches and digs. In the following, all dangerous subsurface defects and those moderate ones that appeared during the raster
Fig. 4. Consecutive images captured during laser irradiation while the optic was moving. It revealed that laser damage sites were induced by (a) a dig, (b) a scratch, and (c) an invisible defect.
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scan test were analyzed in detail by the on-line imaging system. The on-line imaging system was implemented to reveal the characteristics of visible defects and their response during exposure to laser irradiation. Statistics analysis of the raster scan test area showed that the density of scratches was 60–200 cm−2 , and only seven scratches on these six samples led to laser damage under laser exposure with a fluence of 13–23 J∕cm2 . Therefore, most of scratches were moderate; only a few of them were dangerous. Comparing the moderate and dangerous scratches could provide useful information to help find the dominant factor inducing laser damage. Most of the scratches appeared as a dotted line consisting of circular-shaped pits or elliptical-shaped pits, as shown in Fig. 5(a). Although this kind of scratch had a high density, only one of them generated laser damage, as shown in Fig. 5(b). This dangerous scratch consisted of elliptical pits, and the damage was generated at the edge of an elliptical pit. The position of damage initiation is marked with a circle in Fig. 5. Figure 5(c) shows a faint scratch appearing as a continuous straight line. 17 continuousline scratches were found in the test area, and 2 of them generated damage sites. These two damage sites were initiated at the periphery at a distance of ∼10 μm from the scratch. Figure 5(d) shows a scratch that appeared as an indentation with black pits inside. Three scratches of this type were found in the test area, and two of them generated damage sites. The damage was initiated at the inside black pit. Compared with the gray pit in the scratch, the black pit was much deeper, or had an uneven surface. Figure 5(e) shows a scratch that appeared as a bent line with a sharp immediate change of direction. Only one bent-line scratch was found in the tested area. The damage was initiated at the twist. Figure 5(f) shows a trailing indentation scratch consisting of banana-shaped pits. Three trailing indentation scratches were found in the test area, and one of them generated laser damage. The damage was initiated at the edge of a banana-shaped pit. As is seen, all of the dangerous scratches had a special shape. The black pit in the indentation was a local rough area. The twist in the bent-line scratch had a joint or a bend. The edges of the continuous-line and trailing indentation scratches had an abrupt discontinuity between the scratch and the adjacent surface. In summary,
Fig. 5. On-line detected images of (a) dotted-line scratch consisting of circular-shaped pits, (b) dotted-line scratch consisting of elliptical-shaped pits, (c) continuous straight line scratch, (d) indentation, (e) bent-line scratch, and (f) trailing indentation scratch. The position of damage initiation is marked with a circle.
Fig. 6. On-line detected images of (a) small point digs, (b) shadow digs, (c) circular digs, and (d) polygon digs.
local sharp variation was considered the primary cause of laser damage initiation by the scratch. Statistical analysis of the raster scan test area showed that the density of digs was 400–2000 cm−2 , and only 22 digs led to laser damage under laser exposure with a fluence of 13–23 J∕cm2 . Most of digs appeared as the morphologies shown in Figs. 6(a) and 6(b). The digs shown in Fig. 6(a) were small points. The density of small points was 400–2000 cm−2 . 18 damage sites were initiated by this kind of dig. But these points were too small to discern their shape. Therefore, some larger digs, as shown in Figs. 6(c) and 6(d), were chosen to analyze in detail to determine the special characteristics of dangerous digs. The digs, shown in Fig. 6(c), were relatively large circular-shaped pits. The density of these kinds of digs was 0–100 cm−2 . None of these circular digs would induce any damage site. The dig shown in Fig. 6(d) appeared as a polygon. Three polygon digs were found in the raster scan test area, and one of them produced laser damage. The digs shown in Fig. 6(b) appeared as shadow digs; sometimes there was a black point at their center. Three damage sites were initiated by shadow digs. Two of them were initiated at the edge, and the other one was initiated at the center. As is seen, polygon and shadow digs had an abrupt discontinuity between the dig and the adjacent surface. The black point at the center of a shadow dig was a local uneven area. In summary, local sharp variation was also the major factor of inducing laser damage by the dig. Figure 3 not only provides the information of damage precursor and initiation position, but also shows the growth under a subsequent laser pulse after the initiation of damage site. As is seen, the increased size of the damage site after a subsequent laser pulse was much larger than the initiation size. So the absorption of the initiated damage site was much higher than that of the precursor that initiated the damage site. Based on the above discussion, real-time imaging revealed that the low-density laser damage sites were initiated at specific scratches and digs that were significantly different from the regular ones. These dangerous scratches and digs had local sharp variation around or inside them, and the damage site was initiated at the place of local sharp variation. So the local sharp variation was acknowledged as the most dangerous precursor in fused silica. Bercegol et al. deliberately created scratches on the surface of fused silica and measured the damage resistance of these scratches. They demonstrated that the fracture could lead to laser damage and that plastic deformation could not produce laser damage [11]. So the regular scratches or digs, exposed by the wet etch process, were acknowledged as plastic deformation, which would not induce laser damage. The local sharp variation was acknowledged as a local fracture existing in the
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dangerous scratches or digs. Laurence et al. and Miller et al. proved that the fracture in the periphery of the indentation, which was created with a Vicker’s indenter with 0.5 N load, was bright in a PL image captured by a time-resolved PL technique [14,19]. This proved that the fracture had the ability of light absorption. Therefore, it is concluded that the local sharp variation in the dangerous subsurface defects could absorb the laser beam and lead to laser damage. An energy-dispersive spectrometer microanalysis technique was applied to measure the chemical composition of the melted zone, fracture periphery of damage sites. The chemical composition of the fracture periphery stood for the chemical composition of a local fracture in the dangerous scratch. The average value of Si:O atom ratio in the melted zone of damage sites was about 32.1∶67.9; that in the fracture zone was about 27.2∶72.8; and that in the undamaged area was about 27.3∶72.7. It is concluded that the oxygen content was lowest in the melted zone, compared with the undamaged area. Oxygen loss in the melted zone proved that the chemical composition and molecular structure were modified during the occurrence of laser damage. Rapid material heating and melting during laser-induced breakdown would lead to forming the oxygen-deficiency centers [20]. These defects could be the precursor for absorbing the laser energy and leading to damage growth. The energy-dispersive spectrometer analysis proved that oxygen content did not change in the fracture zone, so the oxygen-deficiency center defect should not exist in the fracture. The absorption defect in the fracture might be the nonbridging oxygen hole centers (NBOHCs, oxygen dangling bonds). The NBOHCs can be formed in regions exposed to plastic deformation and cracking [20]. However plastic deformation, such as the regular scratch, did not generate any laser damage. Laurence et al. proved that fast PL was directly linked to laser damage. But the NBOHC had slow PL. So it is hard to ascribe the fracture absorption to the NBOHC. Kucheyev and Demos found that the PL bands of damage sites centered on 1.9, 2.2, 2.7, and 4.3 eV. Bands centered on 1.9, 2.7, and 4.3 eV were attributed to NBOHCs (1.9 eV) and oxygendeficiency centers (2.7 and 4.3 eV) [20,21]. The band centered on 2.2 eV was not clear about its chemical composition and molecular structure; additional work was needed to understand the relation of this defect and fracture absorption.
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