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Abstract
We investigate the interest of combined process of reactive ion etching (RIE) and dynamic chemical etching (DCE) as a final step after polishing to improve the laser damage resistance of fused silica optics at the wavelength of 355 nm. The investigation is carried out on the polished fused silica optics by changing the RIE depth while keeping the DCE depth fixed. We evidence that the combined etching process can effectively remove the damage precursors on the fused silica surface and thus improve its laser-induced damage threshold exceeding the level of the deep HF-etched surface. The effects of the combined etching depth on the surface roughness and surface error are also studied systematically. We show that the combined shallow etching can achieve better overall surface quality. Deeper etching will cause surface quality degradation of the fused silica optics, which is believed to be associated with the chemical etching during the combined process. Given that HF acid processing will degrade the surface quality of fused silica optics, the combined shallow etching appears as a pertinent alternative to HF-based deep etching.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For optical applications in the field of high-energy density science such as Inertial Confinement Fusion (ICF), high-purity synthetic fused silica is widely used for windows, lenses, shield sheet, and so forth [1, 2]. A serious problem is the performance degradation of these optical components during high-power UV laser irradiation because laser-induced damage (LID) can readily occur on the optical surface [3]. Defects (for example, sub-surface damage or SSD), which are generally introduced during the polishing and/or grinding process, have been demonstrated as the key damage precursors [4, 5]. In the past, various methods of surface modification to remove or mitigate these precursors have been developed and investigated including HF acid etching [5–7], ion beam etching (IBE) [8, 9], magneto-rheological polishing (MRF) [10] and reactive ion etching (RIE) [11–14]. Among these, HF acid etching has been proved as a most effective technique for dramatically improving the laser damage performance of fused silica optics [6, 15, 16]. However, structural defects such as scratches and pits in the subsurface layer are geometrically enlarged and extended since a relatively large etching amount (even > 20 μm) has to be required to completely remove the damage precursors [17]. In this case, the surface roughness and surface error are degraded [18, 19]. Moreover, light intensification due to the reflections and interferences near the rough surface will decrease the effective laser-induced damage threshold [17].
In our recent work [20], a two-step combined process of RIE and dynamic chemical etching (DCE) techniques was employed to significantly improve the damage performance of fused silica optics. By optimally matching the removal depths of RIE and DCE, a near-perfect surface with excellent damage performance and relatively low surface roughness (<0.7 nm) was created. The thorough elimination of the RIE-induced deterioration layer by the DCE retreatment process is believed to be a key factor for obtaining the optimized matching relationship of RIE and DCE depths. However, some important effects have not been discussed and included in our previous research. For example, more removal amount of RIE needs to be considered if the depth of SSD imbedding in the fused silica surface changes. In this case, we have to increase the RIE depth to completely remove the SSD deep below the fused silica surface. Thus, the effects of combined process of RIE and DCE on laser-induced damage resistance and surface quality of fused silica optics with different RIE depths urgently need to be studied.
In this work, a detailed research was implemented on the damage performance and surface quality of polished fused silica optics treated by the combined process of RIE and DCE. Different RIE depth was chosen during the RIE processing. Samples preparation and combined etching process are detailed in paragraph 2.1. We measured the surface roughness and surface error with an interferometer and an atomic force microscope. The surface chemical-structure defects were analyzed with a fluorescence spectrophotometer. Results of these surface characterizations and their damage probabilities are presented in Section 3. Finally, we discuss and conclude (Sections 4 and 5) about the performances obtained with the combined etching of RIE and DCE in comparison with single HF acid etching on fused silica optics.
2. Experimental
2.1 Samples preparation and etching processes
Five square fused silica samples (Corning 7980, 50 mm wide and 5 mm thick) used for this study were manufactured by the same vendor (Z&Z Optoelectronics Tech. Co., Ltd) using the conventional polishing process. CeO2 with diameter of 0.4 µm is used as the abrasive particle. Before etching, all the samples (marked as A, B, C1, C2, and C3) were cleaned with deionized water and allowed to air dry. Sample A is an original sample without any treatment. Sample B was treated only by DCE process with the etching depth of 20 μm, which has been demonstrated to be a required removal amount for achieving high damage threshold of fused silica [4, 7]. Samples C1, C2, and C3 were all treated by the combined process.
Combined etching process in this study involves two main processes: RIE and DCE, as shown in Fig. 1. RIE was first implemented on one side of the sample C1, C2, and C3 to remove the SSD anisotropically. Considered the SSD depth of fused silica could be deep to 20 μm [6], the RIE depths for these samples were chosen to be 1 μm, 5 μm, and 15 μm respectively. The RIE-treated side was regarded as the rear surface for damage test. Then DCE was adopted on both sides of these samples, which includes three steps: (1) a weak alkali cleaning process to remove trace oil and dirt; (2) an inorganic acid cleaning process to dissolve the possible contaminants (metal impurities and/or fluorocarbon deposits) induced by the RIE process, and (3) a buffered HF-based etching process to remove and passivate the unstable chemical-structure on the surface of the samples. For all the samples treated by the combined etching process, the etching depth of HF acid was controlled to be about 3 μm. The detailed parameters of all the etching treatments are shown in Table 1.
Fig. 1 Flow diagram of the combined etching process for the fused silica samples. Figures 1(a)-(d) show the anisotropic RIE pretreatment on the fused silica surface to remove the SSD step by step. Figures 1(e)-(g) show the isotropic DCE retreatment on the RIE-pretreated fused silica surface to passivate the unstable chemical-structure.
Table 1. Parameters of the samples treated by combined etching process.
The RIE process was conducted in a parallel-plate discharge etcher using a CHF3-Ar mixture gas with the etching rate of ~2 μm/h. Before the etching, all the samples were water spray rinsed using deionized water and allowed to air dry. During the etching, all the samples were put flatly on the surface of the sample stage. After the etching, the vacuum chamber of the etcher was immediately filled with high-pure argon for 2 minutes and then filled with air. The detailed parameters of the RIE treatment are shown in Table 2.
Table 2. Detail parameters of RIE process.
The whole DCE process was carried out under megasonic condition using a Teflon-lined, multi-frequency megasonic transducer (multiMEGt 430 kHz, 1.3 MHz) in which the samples were constantly mounted with their edges held still. During the alkalescent cleaning process, the samples were all submerged in a Micro-90 (a kind of alkalescent cleaning agent) and high pure H2O mixed solution with a volume ratio of 1: 10. The processing time was 32 min. Subsequently, the samples were removed and submerged in deionized water for rinsing. During the inorganic acid cleaning process, the samples were all submerged in a mixed inorganic acid consisting of 70 wt. % HNO3 and 40 wt. % H2O2 with a volume ratio of 2: 1. The processing time was 80 min. After that, the samples were also removed and submerged in deionized water for rinsing. Similarly, during the buffered HF-based etching process, the samples were all submerged in an HF acid etchant consisting of 49 wt. % HF and 30 wt. % NH4F with a volume ratio of 1: 4 and followed by the same rinsing procedure. The HF etching rate was controlled to be 100 nm/min. After the DCE treatment, the samples were water spray rinsed using deionized water and allowed to air dry. The cleaning, etching, and drying processes were totally implemented in a Class 100 clean room.
2.2 Surface quality characterizations
A white light interferometer and an Atomic Force Microscope (AFM) were employed to measure the surface roughness of the samples. The white light interferometer was used for measuring the surface roughness at sub-millimeter scale (0.7 × 0.5 mm) and 5 testing sites were randomly chosen on the surface of each sample. AFM was used for measuring the surface roughness at micrometer scale (5 × 5 µm) and 3 testing sites were randomly chosen on the surface of each sample. The surface error was also obtained by measuring the peak-valley (P-V) values of the samples. The testing area is the full aperture of the samples. The wavelength of the laser used for the surface error measurement is 632.8 nm.
2.3 Surface fluorescence spectra analysis
Laser-induced fluorescence (FL) was used for examining the chemical-structure defects on the sample surfaces with a fluorescence spectrometer furnished with a photomultiplier used as a detector and a Xe lamp used as an excitation source. The slit width for excitation and emission was 7 nm and 4 nm respectively. The emission spectra were excited with a peak centered at 5.4 eV (~230 nm) for all the samples. To make the fluorescence data more reliable, the location and direction of each sample were strictly fixed during the measurement.
2.4 Laser-induced damage threshold test
Laser induced damage threshold (LIDT) of the samples was assessed using small beam test in a laser damage test facility with a frequency-tripled Nd-YAG laser. The wave length, pulse width, and laser repetition rate were 355 nm, 7 ns (FWHM), and 1 Hz, respectively. The spatial beam distribution was Gaussian profile with diameter of 1.2 mm. The relationship between damage threshold and probability was fitted linearly. Then we obtained the 0% and 100% probabilities damage thresholds by extrapolating the fitting lines. The uncertainty of the energy density in the test system is 4.5%. Before the test, all the samples were rinsed using deionized water and allowed to air dry.
Given that the laser damage test will destroy the optical surface and make the testing area unusable for subsequent experiments, R-on-1 strategy was used for testing the average LIDT of the RIE-treated samples. With increasing laser power, damage threshold was obtained statistically by laser-irradiating the same position of the sample surfaces with the same time span and power increment (about 0.5 J/cm2). 1-on-1 strategy was used for testing the samples treated by the combined etching process since R-on-1 strategy has laser-preconditioning effect which can cause threshold saturation. The 1-on-1 test consists of one laser shot, at one test site. Twenty testing sites for every laser power were chosen randomly on every sample surface. The description of the testing strategy in high-power laser system can be found elsewhere [21, 22].
3. Results
3.1 Samples pretreated by reactive ion etching
3.1.1 Laser-induced damage threshold test
The average LIDT results of the samples pretreated by RIE are shown in Fig. 2. The results show that the LIDTs of the sample surfaces both increased significantly when the RIE depth were 1 µm (sample C1) and 5 µm (sample C2), respectively. However, when the etching depth further increased to 15 µm (sample C3), the LIDT decreased greatly instead, which was even lower than that of the unetched sample (sample A). It suggested that the high-density precursors with low damage threshold were remarkably removed by the RIE pretreatment but the resulting surfaces were not chemically stable especially under high etching depth condition. It can also be noted that the deviations of the measured LIDTs of the samples A and C1 were higher than those of the samples C2 and C3, which suggested that the best RIE depth for effectively removing the SSD of the fused silica samples was about 5 µm. According to our previous study [11], we speculate that the degradation of surface quality and the increase of oxygen-deficiency center (ODC) defects at high RIE depth are the main factors limiting the further increase of the damage threshold of the fused silica optics. We thus conducted the surface roughness measurement and FL spectra analysis for the RIE-treated samples.
3.1.2 Surface roughness measurement
The surface roughness measurement with the spatial scale of sub-micron (0.7 × 0.5 mm) was performed before and after the RIE pretreatment respectively. Five testing sites were chosen randomly on the surface of each sample in order to achieve better statistics for the measurement. The root mean squared (RMS) roughness, which is an average value of peaks and valleys of the fused silica surface profile is shown in Fig. 3. The RMS value of the unetched sample surface was 1.07 nm. After the sample was exposed in CHF3-Ar plasma with the etching depth of 1 µm, the RMS value decreased to 0.65 nm. However, when the etching depth increased to 5 and 15 µm, the RMS values increased to about 1.06 and 1.28 nm respectively. The results indicated that the RIE pretreatment with low etching depth (1 µm) could smooth the optical surface of fused silica, but RIE with high etching depth (5 and 15 µm) would degrade the surface quality and increase the surface roughness of the optics.
3.1.3 Surface fluorescence spectra analysis
Surface FL measurement was performed to analyze the chemical-structure defects (such as ODC) on the sample surfaces after RIE. Figure 4 presents the FL emission spectrum for all the RIE-pretreated samples and the unetched sample. Two characteristic peaks centered at 381 nm (3.25 eV) [23] and 407 nm (3.05 eV) [24] were detected, which were attributed to typical structural defects: ODC and O2- respectively. The FL spectra showed that the ODC and O2- intensity increased dramatically with increased RIE depth suggesting that the RIE pretreatment would induce chemical-structure defects on fused silica surface.
3.2 Samples treated by combined etching process
3.2.1 Laser-induced damage threshold test
The LIDT test was carried out with the same laser parameters after the samples were treated by the combined etching process. Figure 5 presents the damage probabilities of the samples treated with different etching processes. It is evident from this figure that the LIDT of zero probability of the 20-μm DCE-treated sample (sample B) dramatically increased comparing with that of the untreated sample (sample A) (from 22.7 J/cm2 to 32.8 J/cm2). For the samples treated by the combined etching process, several interesting phenomena were observed. First, compared with the 20-μm DCE-etched sample (sample B), all the samples (samples C1, C2 and C3) treated by the combined etching process had a prominent increase in LIDT of zero probability. Second, as the etching depth of the DCE retreatment was fixed to be 3 µm, the 5-μm RIE-pretreated and 3-μm DCE-retreated sample (sample C2) had a higher LIDT of zero probability than the 1-μm RIE-pretreated and 3-μm DCE-retreated sample (sample C1). In contrast, for the 15-μm RIE-pretreated and 3-μm DCE-retreated sample (sample C3), the damage threshold of zero probability presented an obvious decrease (from 61.1 J/cm2 to 47.1 J/cm2) whereas that of 100% probability presented an unusual increase. The results indicated that the combined etching process can effectively remove the damage precursors on the fused silica surface and thus improve their laser-induced damage threshold exceeding the level of deep HF-etched surface.
Fig. 5 Damage probability of the samples treated with different etching processes. Sample A: black squares - reference, sample B: red circles - 20 μm DCE, sample C1: blue triangles - 1 μm RIE and 3 μm DCE, sample C2: green stars - 5 μm RIE and 3 μm DCE, sample C3: pink diamonds - 15 μm RIE and 3 μm DCE.
3.2.2 Surface roughness and error measurements
The surface roughness (RMS) of the samples at sub-millimeter scale was studied with the white light interferometer for samples A, B, C1, C2, and C3 (see Fig. 6). The roughness of the samples threated by the combined etching process dramatically increased with the RIE depth. The result suggested that the 3-μm DCE retreatment on the RIE-pretreated sample surfaces could strongly degrade the optical surface quality and increase the surface roughness, especially when the RIE depth was very large (i.e., 5 μm and 15 μm in this work). As expected, the unetched sample had the lowest surface roughness and there was also an obvious increase in surface roughness for the 20-μm DCE-treated sample.
The surface roughness of the samples at micron scale was subsequently studied by AFM, as shown in Fig. 7. The detecting area was 5 × 5 μm square. The surface roughness of the 20 μm DCE-treated sample was 0.877 nm, which was higher than that of all the samples treated by the combined etching process as well as that of the unetched sample. Sample C2 had the lowest value of surface roughness whereas the difference among C1, C2, and C3 was not obvious.
Fig. 7 Surface roughness and morphology of the samples treated with different etching processes (micron scale).
The surface error of the samples with different etching processes was measured by means of the interferometry, as shown in Fig. 8. The P-V values of the 20-μm DCE-treated and unetched samples were 2.133λ and 0.168λ, respectively. When the sample was pretreated by 1 μm RIE and retreated by 3 μm DCE, the P-V value fell below 0.3λ. There was an obvious increase in P-V value when the RIE depth increased to 5 and 15 μm. Especially, the P-V value of sample C3 was even higher than that of sample B.
To enhance the understanding of the effect of the combined process on surface error of the samples, we get the 3D plotting and histogram by extracting the data from the measurement results of surface error, as shown in Fig. 9. The colors in the 3D plots represent the P-V values of the sample surfaces, with black and green denoting high P-V values while red and blue denoting relatively low P-V values. The unetched sample had the lowest surface error since nearly no green or black color could be found in the 3D plot. It is quite clear from this figure that the surface error of sample C1 was obviously lower than that of samples C2, C3 and B, and the largest P-V values of sample C2, C3 and B were mainly located in the green region. The histograms presented in Fig. 9 give us a more detailed statistic densities of the P-V values at different positions of the sample surfaces. For sample C1, the P-V values in most regions were below 0.05λ and the P-V values with largest density were about 0.03λ. But for other samples, a large number of regions had the P-V values exceeding 0.2λ, even 0.5λ. Especially for sample C3, the P-V values with the largest density were even in a wide range of 0.2 to 0.6λ.
Fig. 9 3D plot and histogram of the surface error for all the samples treated with different etching processes.
3.2.3 Surface fluorescence spectra analysis
Figure 10 presents the FL emission spectrum for all the etched samples. The two characteristic peaks (ODC centered at 381 nm and O2- centered at 407 nm), which had been observed in Fig. 4, were revealed again. Some changes occurred when the RIE-treated samples were retreated by 3 μm DCE. First, compared with the 20-μm DCE-treated sample, the samples treated by the combined process had a much lower intensity for the two characteristic peaks. Second, unlike the sharp profile of the characteristic peaks in Fig. 4, the characteristic peaks in Fig. 10 became much gentler. Third, deeper etching caused a redshift of the whole FL spectrum. The results indicated that the DCE retreatment could passivate the unstable chemical-structure on the RIE-treated sample surfaces. However, deeper RIE treatment could cause the deterioration of the bonding structure, which was probably disadvantageous to the improvement of the damage resistance of fused silica optics.
4. Discussion
The combined process of reactive ion etching and dynamics chemical etching considerably improved the optical performance of fused silica optics. Their LIDTs and surface qualities were influenced by the total removal depth of RIE and DCE and these effects were obtained by changing the RIE depth while keeping the DCE depth fixed. This large improvement of LIDT of the samples could be achieved at a relatively low etching depth. In our previous study in 2016 [20], we proved that the maximum LIDT of 0% probability was obtained when the optical surface was treated by 1 μm RIE and 3 μm DCE. Comparing with the unetched sample, there was a 2.4 times improvement (from 20.9 to 50.8 J/cm2) in LIDT of zero probability. In this study, LIDT improvement of the optical surface of fused silica was also observed. The LIDT of the sample treated by 5 μm RIE and 3 μm DCE (sample C2) was 2.7 times higher than that of the unetched sample B (with the LIDT of zero probability improved from 22.6 to 61.3 J/cm2). This best level of LIDT was even higher than that of the sample treated by 1 μm RIE and 3 μm DCE (sample C1). Actually, the prior polishing quality of the fused silica samples could influence the required RIE depth. For these polished samples, 5-μm RIE pretreatment may be more efficient to remove the fractured defects in the subsurface layer of fused silica. The obtained results also seem to be telling us that the imbedding depth of the SSD in these polished sample surfaces is 1 to 5 μm. Laser damage test was also performed on the RIE-treated surface (see Fig. 2). The testing results revealed that it was not enough to improve the laser damage resistance by single RIE treatment due to the etching process inevitably involving the chemical-structure deterioration. But RIE is indeed very important during the combined etching process and can’t be replaced by HF etching completely because such excellent increase in LIDT achieved by the combined etching (either sample C1, C2 or C3) is nearly impossibly achieved by the single DCE (sample B). The chemical structure on the sample surfaces could be influenced by the RIE depth since there were obvious intensity rise (for RIE) and redshift (for combined etching) of the FL spectrum with increasing the RIE depth, especially when the RIE depth was increased to 15 μm (see Figs. 4 and 10). Hence, the LIDT of the sample C3 presented a slight decrease. These effects have to be studied in more details.
Different surface quality could be obtained by changing the total removal depth of RIE and DCE. Deep etching could dramatically degrade the surface quality of the samples. For example, the surface roughness at sub-millimeter scale and the surface error was obviously increased when the optical surface was treated by 15 μm RIE and 3 μm DCE (see Fig. 6 and Fig. 8). But all these evolutions had no adverse effect on the LIDT of neither zero nor 100% probability (see Fig. 5). It suggests that the nature of the surface damage of fused silica which has been treated by the combined etching process is not strongly related to the surface quality, which is also a very significant point for ICF optics. Comparing with our previous researches [25], it is important to notice that the values of the surface roughness and P-V for all the samples in this present research were considerably high. We believe that the surface quality degradation of these samples etched by DCE or RIE/DCE can be due to the chemical etching during the combined process. For example, chemical products will be probably left on the RIE-treated sample surface since RIE is a physical-chemical process using CHF3 as a reactive gas with fused silica, which might result in the degradation of surface roughness and uniformity. The same reason can apply to the DCE process. Hence, the chemical etching of either RIE or DCE during the combined process might be a crucial factor influencing the final surface quality after combined etching. We finally used Metropro (an analysis software of Zygo company) to observe the microscopies of the samples B and C3, whose etching depths were much closer. The analyzed area was 0.7 mm × 0.5 mm. From the observed results in Fig. 11, it is important to notice that a large number of scratches were revealed on the surface of the 20-μm DCE-treated sample (sample B). In contrast, there were no any scratches on the surface of the sample treated with 15 μm RIE and 3 μm DCE (sample C3). The results further indicate that the combined etching process can effectively remove the fractured SSD on the fused silica surface without leaving any trace. Excellent surface quality and laser-induced damage resistance of fused silica optics can be obtained by choosing appropriate RIE and DCE depths, which is also closely related to the prior finishing process of the optics. Due to the serious degradation of the surface quality of fused silica optics associated with the HF-based treatment, the combined etching process needs to be more comprehensively studied in order to make it a pertinent alternative to HF-based deep etching.
Fig. 11 Microscopy observation of the samples treated with different etching processes. a - sample C3: 15 μm RIE and 3 μm DCE, b - sample B: 20 μm DCE. Three testing positions for each sample were presented.
5. Conclusions
We investigated the combined process of RIE and DCE as a potential technique for improving the laser-induced damage resistance of fused silica optics. The combined etching process give much better results than the DCE process for the LIDT on fused silica optics, especially when the matching relationship of RIE and DCE depth reaches an optimal level. The FL measurements lead us to believe that the limitation of the LIDT’s increase in the combined deep etching process is in all probability attributed to the chemical-structure deterioration of the fused silica surface. More comprehensive studies are necessary to improve our understanding on the chemical structures influenced by combined etching. There is a significant evolution in surface quality when changing the RIE depth while keep the DCE depth fixed. The surface quality, in term of roughness and surface error, is deteriorated at deep etching on fused silica surface. But this evolution has no adverse effect on LIDT of the UV optics. The observed results convince us that chemical etching during the combined process is crucial for the final surface quality after combined etching.
Finally, considering that high LIDT and good surface quality can be achieved with small removal amount comparing to the use of HF-based deep etching, we believe that combined shallow etching of RIE and DCE is a promising technique for the manufacture of fused silica UV optics used for high-power laser facilities.
Funding
National Natural Science Foundation of China (NSFC) (61705204 and 61705206).
Acknowledgments
The authors would like to gratefully acknowledge the help of Jin Huang Team for laser damage testing and Bin Heng for the large number of observations and characterizations. We also acknowledge Shufan Chen for helpful discussions.
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