Layer by layer exposure of subsurface defects and laser-induced damage mechanism of fused silica

Journal Pre-proofs Full Length Article layer by layer exposure of subsurface defects and laser-induced damage mechanism of fused silica Bo Li, Chunyuan Hou, Chengxiang Tian, Jianlei Guo, Xia Xiang, Xiaolong Jiang, Haijun Wang, Wei Liao, Xiaodong Yuan, Xiaodong Jiang, Xiaotao Zu PII: DOI: Reference:

S0169-4332(19)34003-6 https://doi.org/10.1016/j.apsusc.2019.145186 APSUSC 145186

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

17 July 2019 30 November 2019 24 December 2019

Please cite this article as: B. Li, C. Hou, C. Tian, J. Guo, X. Xiang, X. Jiang, H. Wang, W. Liao, X. Yuan, X. Jiang, X. Zu, layer by layer exposure of subsurface defects and laser-induced damage mechanism of fused silica, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145186

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layer by layer exposure of subsurface defects and laser-induced damage mechanism of fused silica Bo Lia,b, Chunyuan Houa, Chengxiang Tiana, Jianlei Guoa, Xia Xianga,*, Xiaolong Jiangb,*, Haijun Wangb, Wei Liaob, Xiaodong Yuanb, Xiaodong Jiangb, Xiaotao Zua a

School of Physics, University of Electronic Science and Technology of China, Chengdu

610054, China b

Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang

621900, China

Abstract: Inert ion beams with large incident angle were used for layer by layer etching of chemical-mechanical polished fused silica. The evolution of the impurities, subsurface defects, surface roughness, and surface molecular structure after ion beam etching and their effects on the laser-induced damage threshold (LIDT) were investigated to understand the laser damage mechanism of fused silica. The impurity elements are mainly distributed at a depth of 0~200 nm of the sample surface, which will strongly absorb ultraviolet (UV) laser energy. The result is consistent with the measurement of weak photothermal absorption. After 500 nm removal of the fused silica surface, the deposition layer of polishing powder is removed, and the subsurface defects are exposed. Correspondingly, the amount and size of the defects reach the maximum, and result in the greastest surface roughness and deterioration of surface quality, which enhances the local optical field. As the removal depth increases, the passivation and removal of subsurface defects lead to a decrease in surface roughness and improvement of surface quality. In addition, the density of structural defects and the Si-O-Si bond angle will decrease after ion beam etching, which yields a densified and strengthened surface of fused silica. The results indicate that metallic impurities are the key factors that limit the improvement of LIDT. Moreover, subsurface defects restrict the further enhancement of LIDT. Therefore, ion beam etching can be developed to remove subsurface damage for improving the laser resistant capacity of fused silica.

*

Corresponding authors. E-mail addresses: [email protected] (X. Xiang), [email protected] (X.L. Jiang). 1

Keywords: fused silica; ion beam etching; impurity element; subsurface defect; laser damage mechanism 1. Introduction High peak power laser systems have been developed worldwide for pursuing inertial confinement fusion (ICF) and studying material properties under extreme conditions. A few examples are the National Ignition Facility (NIF) [1] in United States, Laser MegaJoule (LMJ) [2] in France, High Power laser Energy Research facility (HiPER) [3] in Europe and SG-III laser facility in China [4] Thousands of large-aperture optics with high precision surfaces are required to transmit the laser beams in these facilities. Among these, fused silica has been widely used as a manufacturing material in switch and vacuum windows, wedged focus lens, diffraction grating, and debris shields and so on [5]. It features extremely low thermal conductivity, super-strong thermal-shock resistance, low dielectric loss, high deformation (~1370 K) and softening (~2000 K) temperatures, high chemical stability, and broad optical transmission spectra. However, laser-induced damage has severely restricted the performance and lifetime of fused silica optics, and seriously limited the maximum output power of the laser facilities [6]. Fundamental aspects of damage in the bulk of fused silica induced by femtosecond to nanosecond laser pulse has been investigated experimentally and theoretically [7,8]. The nanosecond laser pulse is widely used in the high power laser facilities, which will damage fused silica due to collisional and multiphoton ionization, and plasma formation. Generally, the damage can easily occur on the surface at lower fluence far below the intrinsic damage threshold of bulk fused silica in ultraviolet laser light [8]. Hence, in the past decades, efforts have been made to improve the laser-induced damage threshold (LIDT) of fused silica optics in the past decades. The low damage threshold on the surface of fused silica is closely related to the subsurface damage produced during the chemical-mechanical polishing process. The subsurface damage layer located beneath the polished surface generally consists of a deposition layer of polishing powder (Beilby layer), defect layer and deformed layer [9-12]. Generally, throughout the deposition layer, the distribution of highly absorptive impurities (Ce, Fe, etc.) coming from the polishing slurries is random. Some of the impurities become embedded and hidden deep inside the defect layer by entering the open subsurface via cracks and scratches. Under the ultraviolet laser irradiation, the impurity defects can absorb incident laser, resulting in very high local temperatures, stress, and eventual permanent damage to the fused silica [13,14]. The subsurface defects, including cracks and scratches in the defect layer 2

introduced by the initial rough grinding process, may decrease the laser damage threshold by reducing the mechanical strength and enhancement of the local optical field [15,16]. Besides, the composition and the structure in the subsurface damage layer have been changed during the grinding and polishing processes. The molecular structure and point defects such as non-bridging oxygen hole centers (NBOHCs) and oxygen deficient centers (ODCs) are also related to the laser damage of fused silica [17-19]. Therefore, the metallic impurities, defects, and structural changes are consisder damage precursors responsible for the majority of laser damage of fused silica. For fused silica optics, its hard and brittle characteristics make the manufacturing of defect-free surfaces a great challenge, even though the surface fabrication level is close to optimum. Thus, it is of great importance to find ways to minimize or eliminate the damage precursors. Post-processing techniques improve the laser damage resistance of fused silica. Methods such as ultraviolet (UV) or carbon dioxide (CO2) laser conditioning, hydrofluoric (HF) acid etching, magnetorheological polishing (MRF), plasma etching, as well as ion beam etching (IBE) [11, 20-26] are used to remove or mitigate the subsurface damage produced during the grinding and polishing. Laser conditioning can mitigate the subsurface defects and heal the micro-structure with lower energy laser irradiation. However, the residual stress and surface figure error induced by the temperature gradient in the laser-heated zone lead to wavefront distortion, which substantially limits the improvement of laser damage resistance. HF acid etching is an effective post-processing treatment for removing the surface impurities and blunting the subsurface defects. But depth etching will worsen the surface quality due to isotropic etching of subsurface defects. Moreover, the reaction products will redeposit on the surface of fused silica, which becomes the new precursor to lead to laser damage. MRF polishing yields a high-quality surface with low roughness and few subsurface defects. However, the contact method involves the contact of hard abrasive particles with the surface. Nanoscale damage and contamination, such as the presence of Fe would be induced, resulting in a low damage threshold. As an alternative, plasma etching is also a promising method for treating the fused silica surface. However, scattered micro-pits and contamination from carbon tetrafluoride (CF4) gas will be induced; consequently, this will impair the damage threshold improvement. Ion beam etching based on the physical sputtering effect is an efficient mean to remove material at an atomic scale. Compared with the above post-processing methods, ion beam etching with the characteristics of anisotropy and controllable etching depth can remove the subsurface damage layer. Especially for the large incident angle of ion beams, the subsurface 3

defects such as scratches and pits are gradually exposed and removed without longitudinal replication and lateral expansion. As a non-contact technique, ion beam etching has advantages in controlling destructive defects such as residual stress, contamination, and damage. Because of these advantages, ion beam etching has been used to polish fused silica surfaces and achieve good effects in improving the damage threshold. Most previous works focused on the evolution of subsurface defects and removing the damage precursors and improving surface quality. However, few investigation has been conducted for ion beam etching with large incident angle, where the distributions of the impurities and the subsurface defects can be obtained. Meanwhile, the evolutions of the structural defects and the molecular structure have been paid little attention. Moreover, the damage mechanism of fused silica and the mechanism of ion beam etching on the improvement of the laser damage performance of fused silica is not clearly understood and requires systematical investigation. In this work, ion beam etching was used to gradually remove the fused silica surface with a large incident angle. The surface impurities, subsurface defects, micro-structures, and laser damage thresholds were systematically measured and analyzed. Then the mechanism of ion beam etching on the laser damage resistance was comprehensively clarified. This work provides a greater understanding of the laser-induced damage mechanism on the fused silica surface, which is of great practical significance to optimize the grinding and polishing process. Further research can build on this research and develop a new engineering technique to remove the subsurface damage layer and improve the laser damage threshold of fused silica. 2. Experimental Fused silica samples (Corning 7980, 50 mm × 50 mm × 5 mm) manufactured via the same grinding and polishing process with submicrometer CeO2 slurry from the same vendor were used in the experiment. Before ion beam etching, all the samples were first ultrasonically cleaned in deionized water and then dried with absolute alcohol to remove the surface contamination. The IBE experiment was performed using ion beam surface treatment equipment, and the surface was bombarded with Ar ions at a working pressure of 2.3×10-2 Pa. Within the experiments, the incident ion energy and the beam current density were kept at 800 eV and 0.83 mA/cm2, respectively. The incident angle of ion beams to the substrate surface was set at 70o to avoid longitudinal replication and lateral expansion of the defects. The etching rate of Ar ions on fused silica was ~23.6 nm/min. The beam spot area of the extracted ion beam 4

was 600 mm × 60 mm, which is used to process large aperture optics. The beam inhomogeneity of the ion source was less than ±5% in the range of 500 mm along the axis direction. During the ion beam etching process, the neutralizer provided negatively charged electrons to counter positively charged ion beams, and the emission electron current was set as 360 mA. In addition, silica shield plates were employed to avoid metallic contamination sputtered from the chamber. Surface impurities were detected by Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). 30 keV bismuth ion (Bi+) was used as the sputtering source. The sputtering area is 500 μm × 500 μm and the analysis area is 200 μm × 200 μm. Photothermal absorption of the fused silica surface was measured utilizing photo-thermal common-path interferometry. A condocal fluorescence microscopy was applied to observe the surface defects. A white light interferometer (Zygo NewView 7200, USA) was used to measure the surface roughness of samples. Five sites were selected randomly to calculate the average roughness for each sample. Photoluminescence (PL) spectra were obtained using a Hitachi F-7000 fluorescence spectrometer. A gated photomultiplier was used as a detector and a Xe discharge lamp as an excitation source. The excitation wavelength was 240 nm and the slit width for exciting and emission was 20 nm and 20 nm, respectively. Fourier transform infrared spectra (FTIR) were measured using a Nicolet 5700 spectrometer with an attenuated total reflection mode. The measurements were carried out in a range of 450 cm-1~1500 cm-1, with 32 scans and a resolution of 0.96 cm-1. The laser R-on-1 test protocol [27] was used to measure the laser damage threshold of fused silica surfaces. We incrementally ramped the laser fluence (~0.5 J/cm2) at a repetition frequency of two shots per fluence until the damage occurred. Subsequently, another new site was shot again likewise. The experiment was carried out using a single longitudinal mode Q-switched Nd: YAG laser with a wavelength of 355 nm and a pulse duration of 6.4 ns. Fig. 1(a) shows the incident pulse graph. During the test, the laser beam is focused on the exit surface of the sample to achieve high fluence. The beam was a spatial and temporal near-Gaussian distribution with a 1/e2 area of 0.6 mm2, as shown in Fig. 1(b). Under the laser irradiation, a charge-coupled device (CCD) camera monitored the damage, defined as the visible modification of the sample surface. 3. Results and discussions

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3.1. Surface contamination analysis The depth profiles of impurity elements of fused silica surface were measured by TOF-SIMS before and after ion beam etching. The results of their distribution analysis are shown in Fig. 2. We found that a large number of impurity elements are distributed in the deposition layer of the un-etched sample. This distribution is especially true for Ce, Al, Fe, Ca, Mg, and K. The maximum concentrations of the impurity elements are located on the surface of the silica and decrease with the increasing detection depth. Under the irradiation of intense laser, these metallic impurities will make the fused silica optics highly absorptive to laser energy, leading to local temperature instant heating up to produce unsteady and non-uniform temperature distribution. When the local temperature reaches a certain extent, the material may suffer from modification, softening, melting and even boiling, which will certainly incur laser-induced damage. Meanwhile, the great temperature gap inside silica materials will give rise to internal thermal stress, and correspondingly lead to the negative effects of obvious wavefront distortion and weak mechanical strength. In addition, the metallic impurities will change the initial band-gap structure of fused silica and also trigger new photon excitation, which will substantially affect the laser-induced nonlinear excitation of dielectric silica optics (e.g., multi-photon and avalanche ionizations), making the optical material more susceptible to laser damage. After 200 nm etching of the fused silica surface, the impurity elements of B, Na, Mg, Al, Ti, Zn, Mo and La are almost completely removed, and the concentrations of other impurity elements are significantly reduced. The maximum concentrations of impurity elements decrease by at least one order of magnitude compared with the un-etched sample surface. With the further increasing etching depth, the concentrations of impurity elements decrease slightly then stabilize when the etching depth is greater than 500 nm. This effect may be due to the exposure and removal of a small number of metallic impurities embedded in the subsurface defects such as cracks and scratches. The above results indicate that the impurity elements are mainly distributed in the depth of 0~200 nm of the sample surface. The effect of metal impurities on the laser damage resistance depends on the species, size and spatial position of impurities. The Ce impurities distributed 150 nm beneath the fused silica surface plays the dominant role to degrade the 6

laser damage resistance, followed by Fe, Al and other metal impurities[28]. Therefore, ion beam etching can be used to etch fused silica and easily remove the metallic elements introduced by the polishing process. This method can contribute to improving the laser damage resistance of fused silica. 3.2. Photothermal weak absorption analysis Absorptive defects are considered to be one of the most critical damage precursors to laser damage due to the deposition of laser energy on the surface of fused silica optics. To further understand the influence of these defects on overall damage performance, the photo-thermal common-path interferometry was used to measure the weak photothermal absorption of the surface. The continuous pump laser wavelength is 355 nm and a He-Ne laser is used as a probe beam.The photothermal technique will detect a small absorption because of its high sensitivity. Fig. 3(a)~(h) illustrates the photothermal weak absorption images of fused silica surface with a 3 mm×3 mm region at different etching depths. The average photothermal weak absorption intensity was calculated and as a function of etching depth is shown in Fig. 3(i). The photothermal weak absorption intensity of the un-etched sample surface is the highest, with an average absorption intensity of 0.0959. This is because of many absorptive defects distributed in the surface of fused silica. After ion beam etching at 200 nm, most of the absorptive defects have been removed. Therefore, the weak absorption intensity dramatically decreases to 0.0367. With the further etching of fused silica, the intensity of weak absorption decreases slowly, and then gradually reaches a constant (~0.01) when the etching depth exceeds 500 nm, owing to the gradual removal of a small number of absorptive defects in the area of the fused silica surface. In general, the weak photothermal absorption decreases with the increasing etching depth. Sun and Xu et al. also observed the decrease of weak absorption of fused silica surface after ion beam etching, but they did not obtain the depth distribution of the weak absorption[29,30]. The change of the photothermal weak absorption with the etching depth is consistent with the concentrations of impurity elements (as shown in Fig. 1), which indicates that the impurity elements are closely related to the absorptive defects. Ion beam etching could be used as a post-processing 7

technique to remove the absorptive defects, reduce the thermal impact and enhance the laser damage resistance. 3.3. Surface defect analysis To investigate the distribution of subsurface defects, ion beam etching was applied to remove the fused silica surface layer by layer, and then the surface defects were detected using the confocal fluorescence microscope. The scattering images of fused silica surfaces at different etching depths are shown in Fig. 4, in which the defects are indicated by the white points. Based on the Scattering images, the distribution of defect size on fused silica surface was evaluated by performing a statistical analysis of defects number with respect to different defect size ranges at various etching depths and the results are the corresponding insets in Fig. 4. Besides, the total defect number and defect area percentage of fused silica surfaces at different depths were also calculated and the results are shown in Fig. 5, which was introduced to characterize the subsurface damage of fused silica. There are some punctate defects with size 0-25 μm distributed on the un-etched sample surface. The defect area percentage of the un-etched fused silica surface is 0.0903 %. Because of the existence of subsurface defects beneath the deposition layer, as the etching depth goes up to 500 nm, a large number of punctate defects and scratch defects appear on the surface of fused silica. The defect size and the corresponding number significantly increase, and the defect area percentage of fused silica surface increases to the maximum value of 0.5371 % due to the exposure of subsurface defects hidden by the deposition layer. It indicates that the deposition layer has been completely removed, and the subsurface defects are completely exposed at this depth. When the etching depth goes up to 1000 nm, massive passivation and removal of subsurface defects result in a sharp reduction in the number and size of surface defects, and the defect area percentage of fused silica surface. With the continuous increase of etching depth, the number and size of surface defects, and the defect area percentage of fused silica surface gradually decreases and then stabilizes. This may be because the residual defects are gradually removed by ion beam etching.

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Under UV laser irradiation, the sub/-surface defects will modulate the incident laser, resulting in enhancement of the local light field and reduction of the mechanical strength, which is one of the major reason of laser-induced damage of fused silica associated with the type, size, quantity and depth of the defects[31]. Sun et al. used the confocal fluorescence microscope to measure fluorescence image to obtain the number and the size of subsurface defects in fused silica after reaction ion etching. However, the type and the depth distribution of subsurface defects are not clear, which is due to the downward replication of the defects induced by reaction ion etching with 0o incident angle and the 10 μm detecting depth of the fluorescence image[32]. In this work, ion beam etching with large incident angle was used to remove fused silica surface layer by layer to observed two kinds of defects (punctate defects and scratch defects) which are mainly distributed in the size range of 0~5 μm and the depth range of 0~3 μm in the surface of fused silica. The number of the defects increases to the maximum value at 500 nm and then decreases with the increasing etching depth. Ion beam etching with large incident angle can expose the subsurface defects in fused silica, which can then be effectively removed to improve the laser damage resistance. 3.4. Surface roughness analysis A white light interferometer was used to investigate the surface roughness of fused silica at different etching depths. Figure 6 shows the root mean square (RMS) roughness of the fused silica surface at different etching depths. The un-etched surface of fused silica is relatively smooth with a surface roughness of 0.689 nm because of the deposition layer of polishing powder covered on the fused silica surface. After 500 nm etching, the surface roughness increases to a maximum value of 0.841 nm.This roughness is probably due to the complete removal of the deposition layer and exposure of subsurface defects such as scratches and pits. Accordingly, the surface quality declines drastically. With the continuous increase of etching depth, the surface quality improves owing to the gradual removal of subsurface defects. Therefore, the surface roughness decreases to 0.663 nm at an etching depth of 3000 nm, which is smaller than that of the un-etched sample, resulting in a smooth surface. However, when the etching depth increases to 5000 nm, the surface roughness 9

slightly increases, and the surface quality deteriorates as a result of a small deposition of etching products on the fused silica surface. The variation in surface roughness with etching depth agrees well with the measurement results of the subsurface defect as shown in Fig. 5 and the result of Ref. [33]. It indicates that the subsurface defects have a significant effect on the surface roughness of fused silica. The surface quality can be improved by ion beam etching with large incident angle due to the removal of sub/-surface defects of fused silica. 3.5. Structural defect analysis Fused silica is usually finished vis the polishing and grinding processes using metallic oxides as polishing compounds. As a result, the metallic impurities (Ce, Fe, Al, etc.) will inevitably remain in the surface of finished optics [21]. Besides, the physical force involved at the microscopic scale will lead to the breaking of the Si-O-Si bond and formation of defects such as ODCs and NBOHCs in the surface of fused silica [34]. Therefore, these structural defects (metallic impurities, ODCs and NBOHCs, etc.) are considered as one of the important precursors to induce laser damage of fused silica, which will introduce sub-band gap structure in fused silica surface. Under the laser irradiation, these defects also strongly absorb the laser energy to produce free electrons, so that the temperature of the defect center rises rapidly and finally induces avalanche ionization, which will lead to destruction of the material structure. To investigate the structural defects of fused silica surface before and after ion beam etching, Fluorescence spectrometry was used to analyse the defect evolution at different etching depths. Figure 7(a) shows the PL spectra of un-etched and etched sample surfaces. For better understanding the emission induced by the defects, the spectrum of the unetched sample was fitted into three Gaussian peaks centered at ~336 nm (peak 1), ~443 nm (peak 2) and ~621 nm (peak 3), associated with impurity ions, ODCs and NBOHCs, respectively [18]. The results are shown in Fig. 7(b).Subsequent to ion beam etching, there is no new emission peak in the PL spectra, indicating that ion beam etching does not introduce a new structural defect. However, the intensities of the emission peaks listed in Table 1 change with the etching depths. The results indicate that the densities of the defects have changed after ion 10

beam etching. At the etching depth of 500 nm, the intensity of the emission peak of impurity ions significantly decreases owing to the complete removal of the deposition layer, which is consistent with the measured results of TOF-SIMS and photothermal weak absorption (as shown in Fig. 2 and 3). With the continuous increase of etching depth, the passivation and removal of subsurface defects, and removal of impurity ions embedded subsurface defects, such as scratches and pits, lead to a gradual reduction of the intensities of emission peaks. When the etching depth is up to 5000 nm, the emission peak of impurity ions disappears. At the same time, the intensity of the NBOHC emission peak decreases, and the intensity of the ODC emission peak reaches its lowest value. The intensity variation of the emission peaks with the etching depths indicates that ion beam etching can be used to eliminate or mitigate the structural defects of the fused silica surface to enhance the laser damage resistance. 3.6. Evolution of surface molecular structures During the polishing and grinding processes, physical forces change the surface molecular structures of fused silica surface. Infrared (IR) spectroscopy has been used to investigate the structural changes in the fused silica surface. Fig. 8(a) shows the infrared absorption spectra of un-etched and etched fused silica surface with different etching depths, which can be fitted into four Gaussian peaks centered at ~788.33 cm-1 (peak 1), ~983.28 cm-1 (peak 2), ~1073.36 cm-1 (peak 3) and ~1182.42 cm-1 (peak 4), corresponding to Si-O-Si bending vibration (TO2), Si-OH bridging (υ1), transverse optical (TO3) and longitudinal optical (LO3) components for Si-O-Si asymmetric stretching vibrations, respectively [35,36]. The Gaussian components of infrared absorption spectra for un-etched and etched fused silica surface are shown in Fig. 8(b)~(i), and the corresponding peak positions of Gaussian bands are listed in Table 2. The shift of peak positions of the infrared absorption spectra indicates that the surface molecular structures of fused silica surface are different from the bulk material. The structural change of fused silica is associated with the changed Si-O-Si bond angle, which will result in the variation of fictive temperature accompanied by the variation of material density [36].

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The TO3 stretching band has been extensively connected with the Si-O-Si bond angle and density of fused silica. Based on the central force model, the wavenumber of the peak position of TO3 stretching band decreases with the increasing etching depth, indicating that the Si-O-Si bond angle decreases. The relations between the fictive temperature, density and the peak position of the asymmetric stretching vibration have been reported and are generally written using the following equations [37-39]: r ( g / cm3 ) = 9.39 ´10-6 Tf ( oC) + 2.1902 uP = 2221.2 + 6.1086 ˄(104 / Tf ) - 0.097385(104 / Tf )2

uP = 1376.8 + 0.8544uS (TO)

(1) (2) (3)

whereˈρ and Tf are the surface density and the fictive temperature of fused silica, respectively, υP is the peak positions of the overtone for Si-O-Si asymmetric stretching vibration in the infrared transmission spectrum, and υS (TO) is the peak position of the transverse optical mode for Si-O-Si asymmetric stretching vibration in infrared absorption spectra. From the formula (1)~(3), with the increasing etching depth, the TO3 stretching band shifts towards the lower frequency, leading to the increase of the density and the fictive temperature of fused silica surface. This above may be due to the densification effect of material surface induced by physical force during the polishing and grinding processes. Therefore, a densified and strengthened surface of fused silica will be obtained. Hydrostatic pressure, compressive stress and ion implantation for fused silica reveal a similar trend. This pattern is detectable by the shifts in the infrared structure bands [40, 41]. According to Ref[42], the intensity of the infrared spectrum depends on the polarization of the molecular structure. Because there are a lot of structural defects composed of the broken Si-O bonds in the un-etched sample surface, the polarizability of the molecular structure is large. With the increasing etching depth, the intensity of the infrared spectrum gradually decrease, which indicates that the polarizability of the molecular structure decreases and the structure 12

defects are gradually removed. The result is consistent with the measured result of PL spectra. The above results imply that ion beam etching will remove the deposition layer and subsurface defect layer, leading to a strengthened surface. Therefore, the material is not easy to fracture under the thermal stress caused by laser irradiation. 3.7. Damage evaluation The LIDT of fused silica at different etching depths was measured and twenty points were selected randomly to calculate the average LIDT for each sample. The results are shown in Fig. 9. The LIDT of un-etched fused silica is 19.55 J/cm2. At the etching depth of 200 nm, most of the metallic impurities distributed in the deposition layer have been removed. The thermal impact caused by the impurity elements is eliminated effectively. Therefore, the LIDT significantly increases to 27.14 J/cm2, with an enhancement ratio of 38.82%. However, the LIDT decreases when the etching depth reaches 500 nm due to the exposure of subsurface defects. At this depth, the size and number of the defects reach their maximum, resulting in the worst surface roughness and deterioration of surface quality, which will enhance the local optical field. With the continuous increase of etching depth, the subsurface defects are gradually passivated and removed, resulting in a decrease in surface roughness and improvement in surface quality. Meanwhile, the metallic impurities embedded in subsurface defects are cleaned. Also, structural defects such as ODCs and NBOHCs are gradually removed. As a result, the LIDT increases again and keeps almost stable at ~29.69 J/cm2 after 3000 nm etching of fused silica surface, with further enhancement ratio of 17.12% compared with that of fused silica surface with etching depth of 500 nm. As the etching depth goes up to 5000 nm, the LIDT slightly decreases owing to a small amount of etching products deposition. Laser damage tests reveal that the metallic impurities play an important role in limiting the improvement of LIDT and that the subsurface defects restrict the further increase in LIDT. The maximum increase ratio of LIDT is 51.87 % because ion beam etching with large incident angle is easy to remove the metallic impurities and subsurface defects in fused silica surface. The similar effect was 13

reported in Ref. [33]. Therefore, ion beam etching with large incident angle can be used to expose and obtain the distributions of metallic impurities and subsurface defects which can also be effectively removed to improve the laser resistant capacity of fused silica. 4. Conclusions Fused silica surface was gradually removed layer by layer using inert argon ion beams with a large incidence angle of 70o. The distributions of impurity elements and subsurface defects, and the evolution of surface roughness and surface molecular structure, as well as LIDT after ion beam etching, were systematically investigated. The impurity elements are mainly distributed in the depth of 0~200 nm of the sample surface, which will strongly absorb laser energy, resulting in very high local temperature and stress. The result is in good agreement with the measurement of weak photothermal absorption. With the increasing etching depth, the number of the defect first increases to maximum value at 500 nm and then stabilizes, which is consistent with the measured result of surface roughness. The size of subsurface defects are in the range of 0~5 μm and the defects distribute in the depth of 0~3 μm, which will enhance the local optical field. The concentration of structural defects decrease with the increasing etching depth, thus reducing the damage probability of fused silica. After ion beam etching, the Si-O-Si bond angle decreases and the surface density increases. Therefore, a densified and strengthened surface will be obtained, which is not easy to fracture under the thermal stress caused by laser irradiation. Ion beam etching with large incident angle can be used to remove the metal impurities to significantly improve the LIDT, and remove the subsurface defects to futher improve the LIDT of fused silica. Furthmore, the reduction of structural defect and the strengthened surface are also beneficial for the improvement of LIDT. Acknowledgments This work is financially supported by the Key Project of National Natural Science Foundation of China-China Academy of Engineering Physics joint Foundation (NSAF, Grant No. U1830204). References 14

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Table 1 The intensities of PL emission peaks of fused silica surfaces at different etching depths. Etched depth (nm)

Fluorescence intensity (a.u.) Peak 1

Peak 2

Peak 3

0

774.12

1071.35

74.38

500

438.14

807.41

72.31

1000

339.80

440.15

71.13

2000

306.20

391.03

71.76

3000

277.21

391.98

76.64

5000

70.74

355.89

29.75

Table 2 Peak positions of infrared absorption spectra of fused silica surfaces at different etching depths. Group vibration model

Etched -1

depth (nm)

TO2 (cm )

υ1 (cm-1)

TO3 (cm-1)

LO3 (cm-1)

0

788.33

983.28

1073.36

1182.42

100

787.00

978.01

1070.11

1178.08

200

786.76

975.26

1068.28

1176.84

500

787.19

974.99

1067.15

1178.99

1000

786.59

972.54

1066.00

1180.51

2000

786.73

972.19

1065.61

1181.48

3000

786.60

970.30

1063.84

1181.42

5000

786.44

965.09

1057.67

1180.78

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Fig. 1. The incident pulse graph (a) and the beam distribution image (b)

Fig. 2. Depth profiles of impurity elements in fused silica surface detected by TOF-SIMS at different etching depths: (a) 0 nm; (b) 200 nm; (c) 500 nm; (d) 1000 nm; (e) 2000 nm.

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Fig. 3. Photothermal weak absorption images and intensities of fused silica surfaces at different etching depths

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Fig. 4. Scattering images of fused silica surfaces at various etching depths: (a) 0 nm; (b) 500 nm; (c) 1000 nm; (d) 2000 nm; (e) 3000 nm; (f) 5000 nm; insets are the corresponding defect number with respect to different defect size ranges

Fig. 5. Defect number and defect area percentage of fused silica surfaces at different etching depths.

Fig. 6. RMS surface roughness of fused silica at different etching depths.

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Fig. 7. (a) PL spectra of un-etched and etched fused silica surfaces at different etching depths; (b) Gaussian components of PL spectrum for un-etched fused silica surface.

Fig. 8. Infrared absorption spectra corresponding to Gaussian components for un-etched and etched fused silica surfaces at different etching depths.

Fig.9. Relationship between LIDT and etching depth. 22

Author Contribution Statement Bo Li: Conceptualization, Methodology, Investigation, Writing-Original Draft. Chunyuan Hou: Investigation. Chengxiang Tian: Investigation. Jianlei Guo: Investigation. Xia Xiang: Supervision, Conceptualization, Writing-review & editing. Xiaolong Jiang: Investigation, Formal analysis. Haijun Wang: Investigation, resources. Wei Liao: Supervision, resources. Xiaodong Yuan: Supervision, Resources. Xiaodong Jiang: Conceptualization, Supervision. Xiaotao Zu: Supervision, Funding acquistion.

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1) Inert ion beam etching with large incident angle can be used to expose and remove the subsurface damage layer by layer to understand the laser damage mechanism and improve the laser-induced damage threshold (LIDT) of fused silica. 2) The metallic impurities are the key factors to limit the improvement of LIDT and the subsurface defects restrict the further enhancement of LIDT. 3) Ion beam etching can be used to eliminate or mitigate the structural defects of fused silica surface. 4) The Si-O-Si bond angles will decrease, and a densified and strengthened surface of fused silica will be obtained after ion beam etching.

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Declaration of interests Ĝ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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