Advanced nanoporous SiO₂ coatings for ultraviolet laser-resistant high-reflection and anti-reflection optics

Dielectric coatings capable of withstanding high-power lasers are essential for contemporary laser facilities. For instance, thousands of dielectric coatings are required to support inertial confinement fusion (ICF) lasers^1,2, such as the National Ignition Facility (NIF) in the USA³, the Laser Megajoule (LMJ) facility in France⁴, and the Shen Guang (SG) series facilities in China⁵. The achievable performance of dielectric coatings directly affects the output of an entire laser system. Among various factors, the laser-induced damage threshold (LIDT)⁶ of coatings is critical for determining the output power of a laser system. Decades of research have focused on enhancing the LIDT of coatings. Studies have shown that defects, such as subsurface defects⁷, nodular defects⁸, and substrate pits⁹, often lead to significantly lower LIDT for coating materials compared to those of bulk materials. Substantial progress has been made in minimizing defect densities and improving the laser resistance of coatings through design, deposition, and post-treatment strategies, including E-field optimization^10,11, nanolaminate design^12,13, and deformation planarization^14,15. Although the complete elimination of defects remains challenging, the selection of appropriate coating materials provides an effective approach to enhance LIDT.

SiO₂ is the preferred coating material for ultraviolet (UV) to near-infrared (NIR) lasers because of its wide bandgap and low absorption loss. Typically, SiO₂ serves as a low-refractive-index (low-n) layer material and is alternately deposited with high-n layer materials to achieve a specific spectral performance¹⁶. The LIDT of multilayer coatings is typically limited by the high-n layer material, which has a relatively low optical bandgap, making it prone to damage under laser irradiation¹⁷. If both high-n and low-n layers can be fabricated from SiO₂-based materials, such as dense SiO₂ and porous SiO₂ layers^18,19, the LIDT of the coating is expected to be significantly enhanced.

Numerous strategies have been proposed for obtaining low-n SiO₂ layers. Sol-gel processes enable the fabrication of nanoporous SiO₂ layers with adjustable porosities, resulting in high LIDT. This method is often used to produce anti-reflection (AR) coatings with few layers^20–22. However, cracking becomes more prevalent when preparing coatings with multiple layers, making the fabrication of high-reflection (HR) coatings challenging²³. The glancing angle deposition method, which tunes n values by varying the glancing angle, has been employed not only for preparing AR coatings^24–26, but also for fabricating HR coatings²⁷, polarizers²⁸, and zero-order waveplates²⁹. However, the non-uniformity of the coating poses challenges for large-scale optics³⁰. Chemical etching of nanolaminates or mixed coatings can yield porous SiO₂ coatings³¹. This technique has been successfully employed to prepare both single-wavelength³² and broadband³³ AR coatings with high LIDT values. Despite the progress in porous coatings, methods suitable for preparing multilayer and large-size porous SiO₂ for laser applications are yet to be developed.

This study proposes an advanced method that combines plasma-ion-assisted electron-beam co-evaporation and chemical etching to fabricate a laser-resistant porous SiO₂ multilayer coating. Fig. 1 shows a schematic illustration of the multilayer coating structure. Coating fabrication involves two key processes: co-evaporation deposition and chemical etching, both of which are suitable for large-size coatings. During deposition, a multilayer coating composed of alternating Al₂O₃–SiO₂ mixture and SiO₂ layers was prepared. The Al₂O₃–SiO₂ layers were deposited using dual electron-beam co-evaporation. Subsequently, the Al₂O₃ within the Al₂O₃–SiO₂ mixture layers was selectively dissolved during the chemical etching process, yielding porous SiO₂ layers with low-n values. The SiO₂ layers remained unchanged after chemical etching and served as high-n layers in the new multilayer structure. The proposed strategy enabled the fabrication of HR and AR coatings with good spectral performance and high LIDT values. Notably, the AR coating exhibited an LIDT of 46.9 J/cm² at 355 nm, surpassing that of the substrate (41.1 J/cm²). Thus, the proposed approach holds significant promise for widespread adoption in advanced laser systems and offers a cost-effective solution for multilayer coatings with superior performance.

Fig. 1  Schematic of the coating structure illustrating a multilayer coating composed of nanoporous SiO₂ layers.

Porous SiO₂ layers were prepared by etching Al₂O₃–SiO₂ mixture layers with an H₃PO₄ solution. The influence of the Al element ratio in the Al₂O₃–SiO₂ mixture layers on the properties of the porous SiO₂ layers was investigated. Fig. 2 shows the transmittance spectra of the as-deposited single-layer Al₂O₃–SiO₂ mixture coatings, denoted as A-SL_X (where X represents the measured Al ratio in the Al and Si elements), along with the transmittance spectra of the porous single-layer SiO₂ coatings obtained after etching, denoted as P-SL_X-H₃PO₄-T (where T refers to the duration of the chemical etching process). For A-SL_51% (with an Al element ratio of approximately 51%), a significant change in the transmittance spectrum was observed when the etching duration was less than 3 h. Notably, the transmittance curves for 3 h and 4 h of etching overlapped closely, suggesting that the chemical reaction reached saturation within the initial 3 h. For A-SL_57%, the chemical reaction nearly reached completion after 1 h of etching. Upon further increasing the Al element ratio to 65% to 77%, the spectral properties of the porous coating stabilized after 0.5 h of etching. During etching, Al₂O₃ reacts with H₃PO₄ to form soluble aluminum phosphate, while the SiO₂ network remains relatively stable and does not participate in the reaction. This results in the selective dissolution of Al₂O₃ from the Al₂O₃–SiO₂ mixture coatings, leading to pore formation. The removal rate of Al₂O₃ is co-limited by the chemical reaction rate and the solution diffusion rate, with the pore size generated during etching significantly influencing the latter³¹. As experimentally observed, the accelerated removal rate of Al₂O₃ with increasing Al element ratio can be attributed to the proliferation of pore channels within the coating. The greater abundance of pores in coatings with appropriately higher Al content facilitates infiltration of the etchant into the coating, thereby enhancing the efficiency of the chemical reaction and subsequent removal process.

Fig. 2  Transmittance spectra of the as-deposited single-layer Al₂O₃–SiO₂ mixture coatings, denoted as A-SL_X, and the porous single-layer SiO₂ coatings, denoted as P-SL_X-H₃PO₄-T.

The refractive index dispersion curves and layer thicknesses of A-SL_X and P-SL_X-H₃PO₄-T were fitted according to the transmittance spectra. As shown in Fig. 3a, b, an increase in the Al element ratio was correlated with an increase in the n value (at a wavelength of 355 nm) of the mixture coating, while both the n value and thickness of the porous coating decreased. Notably, when the Al element ratio reached 80% or higher, a marked reduction in the layer thickness after etching was observed. Scanning electron microscopy (SEM) images shown in Fig. 3c reveal an obvious microporous structure at an elevated Al content. The surface morphologies of the Al₂O₃–SiO₂ mixture coatings and porous coatings were characterized using atomic force microscopy (AFM), and the root-mean-square (RMS) surface roughness of the coatings was computed using AFM software, as shown in Fig. 3d. The RMS values of the Al₂O₃–SiO₂ mixture coatings decreased with increasing Al element ratio. The RMS value of the porous coating was slightly higher than that of the corresponding mixture coating. Notably, the RMS value of P-SL_80%-H₃PO₄-3h was significantly higher than that of the other samples. This may be attributed to the unstable SiO₂ grid structure that remains after a substantial amount of Al₂O₃ is removed, leading to an additional loss of SiO₂ material during etching and cleaning, ultimately resulting in a reduced amount of SiO₂ being retained on the substrate surface.

Fig. 3  Properties of the as-deposited single-layer Al₂O₃–SiO₂ mixture coatings A-SL_X and the porous single-layer SiO₂ coatings P-SL_X-H₃PO₄-T. a n value (at a wavelength of 355 nm). b Thickness. c Scanning electron microscopy (SEM) images of P-SL_77%-H₃PO₄-4h and P-SL_80%-H₃PO₄-3h. d Atomic force microscopy (AFM) images along with root-mean-square (RMS) values.

In addition to the H₃PO₄ solution, an HNO₃ solution was used to prepare porous single-layer SiO₂ coatings, denoted as P-SL_X-HNO₃-T. Once the sample was removed from the acid solution and exposed to air, surface cracks developed on the coating during solvent evaporation (Fig. 4a). This phenomenon may be associated with the high capillary pressure gradient during the drying process³⁴. Notably, the etching time did not significantly affect the appearance of surface cracks, probably because most of the Al₂O₃ was removed after 1 h of etching, rendering subsequent increases in etching duration ineffective in altering the coating structure. X-ray photoelectron spectroscopy (XPS) results are shown in Fig. 4b; evidently, the Al element content decreased to approximately 2.5% after 1 h of etching. To address the surface-cracking issue, a surface modification process was carried out after etching, using a mixed solution of n-hexane and hexamethyldisilazane (HMDS). As illustrated in Fig. 4c, d, this surface-modification technique effectively prevented the formation of cracks in the porous SiO₂ coating. However, this process may generate residual organic substances and decrease the LIDT of the coating. According to the experimental results reported by Chen et al.³⁵, after modifying the sol-gel SiO₂ coating using an ammonia-HMDS atmosphere, the LIDT of the coating decreased from 24.3 J/cm² to 19.4 J/cm². The transmittance spectra, refractive index dispersion curves, and layer thicknesses of the coatings are shown in Fig. 4e-g, respectively. After 1 h of etching, the n value decreased from 1.564 to 1.299, accompanied by a thickness reduction of approximately 7%, from 747 nm to 693 nm. Increasing the etching time to 12 h resulted in a further slight decrease in the n value to 1.288, while the thickness decreased to 683 nm. However, extending the etching time to 36 h did not significantly alter the n value, though the thickness continued to decrease, reaching 674 nm. Compared with H₃PO₄, HNO₃ exhibited a higher removal efficiency for Al₂O₃ in thicker mixture coatings, likely due to differences in both the reaction rate and product solubility. Specifically, HNO₃ possesses stronger acidity, etches effectively at room temperature without heating, and produces highly soluble reaction species, while by-products with limited solubility formed in H₃PO₄ may slightly retard the reaction progress³⁶.

Fig. 4  Properties of the as-deposited single-layer Al₂O₃–SiO₂ mixture coatings, denoted as A-SL_73%, and the porous single-layer SiO₂ coatings, denoted as P-SL_73%-HNO₃-T. a Optical microscopy images of surface cracks of P-SL_73%-HNO₃-T. b Elemental percentage profiles of P-SL_73%-HNO₃-1h. c Photograph and d optical microscopy images of P-SL_73%-HNO₃-12h after surface modification. e Transmittance spectra. f Refractive index dispersion curve. g Thickness.

Etching with the HNO₃ solution achieved a faster etching rate than using the H₃PO₄ solution without heating, making it more applicable for large-size or multilayer Al₂O₃–SiO₂ mixture coatings. An Al₂O₃–SiO₂ mixture coating with an Al element ratio of 73% was deposited on a fused silica substrate measuring 200 mm × 120 mm × 30 mm. The sample was then etched in a 36% HNO₃ solution for 12 h. Fig. 5a shows an image of the etched sample. Transmittance measurements were performed at 15 points uniformly distributed across the coating, and the locations of the test points, fitted n values (at a wavelength of 355 nm), and fitted layer thicknesses are shown in Fig. 5b-d, respectively. The refractive index and thickness uniformity of the porous coating were better than ±1%, and the thickness uniformity could be further improved using a shadowing mask.

Fig. 5  Properties of the porous SiO₂ coating on a large-size fused silica substrate. a Photograph of the large-size optics. b Schematic of spectral measurement locations. c Refractive index uniformity and d thickness uniformity of the porous coating.

An HR coating with the structure of substrate|H_S(HL_HR)²⁵H|air was designed to achieve a reflectivity exceeding 99% at 355 nm with an intended incidence angle of 38°. Here, H_S and H represent the SiO₂ layers deposited under different process parameters, with thicknesses of 141.8 nm and 70.9 nm, respectively. L_HR refers to a nanoporous SiO₂ layer with a thickness of 83.6 nm. Compared with NO₃⁻, SO₄²⁻ has a lower UV absorption cross-section^37,38. When acid solution residues are present in porous coatings, SO₄²⁻ residues exhibit less absorption, potentially resulting in a smaller negative impact on the LIDT of the porous coatings. Consequently, H₂SO₄ was chosen as the acid solution for comparison with the coatings prepared using HNO₃. Both types of HR coatings were etched for 36 h in their respective acid solutions to ensure complete removal of all Al elements from the Al₂O₃–SiO₂ layers. The as-deposited HR coatings are denoted as A-HR, while the porous HR coatings etched with HNO₃ and H₂SO₄ are denoted as P-HR-HNO₃-36h and P-HR-H₂SO₄-36h, respectively.

Fig. 6a shows the reflectivity spectra of the porous coatings measured at a 38° incidence angle under s-polarized light. At 355 nm, the reflectivity of P-HR-HNO₃-36h and P-HR-H₂SO₄-36h was 98.5% and 99.3%, respectively. These spectra were measured in the central region of the samples, indicating that a sufficient chemical reaction occurred in this area. This is because the pure SiO₂ layer deposited under the process conditions adopted in this work has a relatively low packing density, providing pathways for the acid solution to penetrate from the surface toward the substrate. As shown in Fig. 6b, the absorption of A-HR decreased with increasing laser irradiation time, stabilizing at approximately 115 ppm after 150 s. In contrast, the absorption of both porous coatings remained relatively constant with the laser irradiation time, approaching approximately 10 ppm, similar to that of the fused silica substrate. Unlike traditional HR coatings, this all-silica structure avoids high-n materials with relatively low optical bandgaps, such as HfO₂, resulting in significantly lower absorption under UV laser irradiation. Laser damage probability was evaluated in the 1-on-1 mode in accordance with ISO 21254 guidelines. An s-polarized 3ω Nd:YAG laser, operating at a pulse width of 8.2 ns, served as the light source, with damage detection performed via CCD imaging. Fig. 6c illustrates the laser damage probabilities of the two porous coatings as a function of laser fluence at an angle of 38°. Notably, P-HR-H₂SO₄-36h exhibited a lower damage probability than P-HR-HNO₃-36h under identical laser fluences, indicating superior laser resistance. Focused-ion-beam scanning electron microscopy (FIB-SEM) images shown in Fig. 6d-l reveal the typical laser-induced damage morphologies of both porous coatings. Two distinct damage types were observed: flat-bottom pits and nodule-related pits. Unlike conventional HR coatings, no evidence of material melting was observed¹². Additionally, microcracks were present in most damage pits, suggesting a relationship between laser-induced damage and coating stress. Initially, the damage appeared as flat-bottom pits, while nodule-related pits manifested at higher irradiation fluences. Cross-sectional analysis confirmed that the flat-bottom pits were associated with the porous SiO₂ layer, as shown in Fig. 6h, i.

Fig. 6  Properties of A-HR, P-HR-HNO₃-36h, and P-HR-H₂SO₄-36h. a Reflectance spectra. b Absorption versus testing time. c Laser-induced damage probability curves. d-g SEM images of surface damage morphologies of P-HR-HNO₃-36h. h-l SEM images of surface and cross-sectional damage morphologies of P-HR-H₂SO₄-36h.

The damage probability curves of the two porous coatings were further analyzed using the model developed by Krol et al³⁹. Based on the observed damage morphologies, two types of defects were identified: absorptive defects and nodules, each characterized by different LIDT values (Ti) and area densities (Di, integrated over the thickness). As summarized in Table 1, P-HR-H₂SO₄-36h exhibited a slightly higher LIDT and lower defect densities for both defect types than P-HR-HNO₃-36h. Flat-bottom pits originating from the porous SiO₂ layer were first observed under low-fluence laser irradiation, indicating that the absorptive defects within this layer are the primary factor limiting the LIDT of the HR coatings, while its defect density and LIDT can be influenced by the etching process.

The as-deposited coating A-HR and porous coating P-HR-H₂SO₄-36h, which demonstrated higher LIDT, were further examined. Fig. 7a presents images of the two coatings. Elemental composition was analyzed at the center region of the sample using XPS and Energy-dispersive spectroscopy (EDS). In the XPS analysis, 2 keV Ar⁺ ions were employed to etch P-HR-H₂SO₄-36h to a depth of approximately 1.8 μm, with the energy spectrum recorded every 90 s. During this process, the Al element content remained consistently low, and no distinct peak was observed in the Al 2p spectra; only background noise was present, as shown in Fig. 7b, c. Similarly, EDS detected no Al element in any layer of P-HR-H₂SO₄-36h, as illustrated in Fig. 7d. These results confirm the complete removal of Al₂O₃ from each Al₂O₃–SiO₂ layer by chemical etching in the H₂SO₄ solution, resulting in an all-silica UV HR coating. The surface roughness increased slightly, from 3.6 nm to 3.9 nm, after etching (Fig. 7e). Fig. 7f shows the change in surface profile: A-HR displayed a convex profile, while P-HR-H₂SO₄-36h exhibited a concave profile. This shift towards a concave shape suggests that the porous SiO₂ layer was under tensile stress, which could explain the microcracks observed in the laser damage morphologies. Under multi-pulse laser irradiation, the cyclic accumulation of thermal and mechanical stress may exacerbate coating fracture, leading to more severe damage. Given that coating stress is related to porosity and pore radius⁴⁰, it can be mitigated by adjusting the co-evaporation parameters or implementing an appropriate annealing protocol⁴¹ to suit various laser application scenarios.

Fig. 7  Properties of A-HR and P-HR-H₂SO₄-36h. a Photographs. b Elemental percentage profiles of P-HR-H₂SO₄-36h. c X-ray photoelectron spectroscopy (XPS) spectra of Al 2p for P-HR-H₂SO₄-36h. d Cross-sectional morphologies and energy-dispersive spectroscopy (EDS) characterized chemical composition. e AFM characterized morphologies and RMS surface roughness values. f Surface profile figures.

An AR coating with the structure of substrate|L_AR|air was designed to achieve a reflectivity below 0.15% at 355 nm. Here, L_AR refers to a nanoporous SiO₂ layer with a designed thickness of 70 nm. Considering that the porous HR coating prepared by etching in H₂SO₄ solution exhibited superior spectral performance and a higher LIDT than that prepared by etching in HNO₃ solution, H₂SO₄ solution was used to prepare porous AR coatings with an etching duration of 4 h. The as-deposited and porous coatings are denoted as A-AR and P-AR-H₂SO₄-4h, respectively.

The transmittance and reflectance spectra of A-AR and P-AR-H₂SO₄-4h were compared with those of the fused silica substrate, as shown in Fig. 8a, c, respectively. The n value and thickness of A-AR were fitted to be 1.581 and 80.9 nm, while those of P-AR-H₂SO₄-4h were fitted to be 1.255 and 72.2 nm, respectively. P-AR-H₂SO₄-4h exhibited a notable anti-reflective effect with a reflectivity of approximately 0.08% at 355 nm, as shown in Fig. 8c. The RMS roughness values of A-AR and P-AR-H₂SO₄-4h were both approximately 0.9 nm, as shown in Fig. 8d. Fig. 8e shows the absorption of the coatings at 355 nm over testing time. The average absorption of A-AR was approximately 22 ppm, which decreased to approximately 10 ppm after etching, similar to that of the fused silica substrate.

Fig. 8  Properties of A-AR and P-AR-H₂SO₄-4h. a Transmittance spectra. b Schematic of the etching process. c Reflectance spectra. d AFM characterized morphologies and their RMS surface roughness values. e Absorption versus testing time. f Laser-induced damage probability curves. SEM images of surface damage morphologies of g-j the silica substrate and k-n P-AR-H₂SO₄-4h.

The 1-on-1 LIDT was determined using a 3ω Nd:YAG laser (355 nm, 8.2 ns) as the light source, with the laser incident at an angle of 23°, the minimum achievable angle on our measurement platform. Fig. 8f shows the laser damage probabilities as a function of laser fluence for the porous coating and fused silica substrate; the linear fit of the LIDT for P-AR-H₂SO₄-4h was 46.9 J/cm², surpassing the substrate’s LIDT of 41.1 J/cm². The improved laser resistance of the porous AR coating compared to that of the substrate may be attributed to its lower absorption, as well as its nanoporous structure which likely facilitates more efficient energy relaxation. The damage morphologies of the fused silica substrate and P-AR-H₂SO₄-4h are shown in Fig. 8g-n. Typical damage features of the substrate included pinpoint pits (Fig. 8g), ripple-like damage (Fig. 8h), and craters (Fig. 8i). In contrast, P-AR-H₂SO₄-4h exhibited similar damage morphologies but with isolated flat-bottom pits (Fig. 8k) observed under near-LIDT laser irradiation. Under identical laser fluences, the density of craters in P-AR-H₂SO₄-4h was lower than that in the substrate (Fig. 8j, n), demonstrating that P-AR-H₂SO₄-4h provided better laser resistance than the fused silica substrate.

All-silica laser-resistant coatings, including UV AR and HR coatings, were fabricated using a combination of plasma-ion-assisted electron-beam co-evaporation of Al₂O₃–SiO₂ mixtures and selective chemical etching. This unique all-silica coating method exhibited several key characteristics. First, complete removal of Al₂O₃ from the mixture was achieved by optimizing the Al₂O₃–SiO₂ ratio and etching time, resulting in the formation of a low-n porous SiO₂ layer. Notably, the absorption of this coating at 355 nm was comparable to that of the fused silica substrate. Second, the rapid etching rate provided by HNO₃ or H₂SO₄ solutions enabled the preparation of large-size single-layer or multilayer coatings. The UV AR and HR coatings produced using the proposed technique exhibited good spectral properties and laser resistance at 355 nm. Specifically, the LIDT of the AR coating exceeded that of the fused silica substrate. Moreover, this cost-effective approach eliminated the need for expensive laser coating materials such as hafnium. In practical applications, porous coatings are prone to adsorbing moisture⁴² and organic contamination^43–45, which adversely affect the optical stability and LIDT of the coatings. This issue may be addressed in the future by investigating anti-wetting and anti-fouling measures such as surface hydrophobic modification⁴⁶ and non-destructive cleaning⁴⁷. Therefore, this method holds great promise for widespread applications in advanced laser systems.

Seven single-layer Al₂O₃–SiO₂ mixture coatings were deposited on double-side-polished fused silica substrates using a plasma-ion-assisted electron-beam co-evaporation process. Prior to loading into the vacuum chamber, the substrates underwent ultrasonic cleaning. Subsequently, the chamber was heated to 50 ℃ and evacuated to a base pressure below 9 × 10⁻⁴ Pa before deposition. Al₂O₃–SiO₂ mixtures were deposited via the co-evaporation of Al₂O₃ and SiO₂, with the deposition rates monitored by individual quartz crystal monitors positioned near each source. Plasma ion assistance was provided by an advanced plasma source (APS), operating in an Ar/O₂ atmosphere at a pressure of approximately 4.9 × 10⁻² Pa. After deposition, the Al₂O₃–SiO₂ mixtures were chemically etched in an acidic solution. The etched samples were rinsed with deionized water for at least 1 min and then soaked in anhydrous ethanol for at least 5 min. The detailed deposition and etching parameters of the Al₂O₃–SiO₂ mixtures are listed in Table 2. Notably, the samples labeled P-SL_73%-HNO₃-T were immersed in a mixed solution of n-hexane and HMDS at a volume ratio of 4:1 for 2 h following chemical etching to prevent surface cracking.

As previously mentioned, an HR coating with a substrate|H_S(HL_HR)²⁵H|air structure and an AR coating with a substrate|L_AR|air structure were designed. The detailed design information is listed in Table 3. Notably, the L_AR layer was deposited thicker than the designed thickness of 70 nm to compensate for the thickness reduction after etching. Single-side-polished fused silica was used as substrate for the reflectance measurements of the AR coatings, while double-side-polished fused silica was employed for all other measurements.

The substrates for the HR coatings underwent ultrasonic cleaning, while those for the LIDT testing and AR coatings were cleaned using a buffered HF solution⁴⁸. The detailed deposition and etching parameters of the HR and AR coatings are listed in Tables 3 and 4, respectively. The HR and AR coatings were etched for 36 h and 4 h, respectively, to achieve complete removal of Al₂O₃ from the mixture layer. The cleaning procedure following the chemical etching was consistent with that used for the P-SL_X-H₃PO₄-T samples. Notably, this fabrication technique for nanoporous coating demonstrates good reproducibility. The HR and AR coatings, designed based on the parameters of the single-layer nanoporous coating, yield spectral measurement results that are all in line with design expectations.

The transmittance spectrum was measured using a UV–VIS–NIR spectrometer (Perkin Elmer Lambda 1050), and the reflectance spectrum was derived from the transmittance data, neglecting absorption. Based on the iterative genetic algorithm simulation method, the refractive index dispersion curves and layer thicknesses were fitted according to the measured transmittance spectra⁴⁹. The spectra used for fitting ranged from λ₁ (300 nm) to λ₂ (800 nm). The variance (T_v) between the fitted and measured transmittance values served as the evaluation function, with a target variance of 10⁻⁹.

where T_M(λ) and T_S(λ) represent the fitted and measured transmittance values at wavelength λ, respectively. The surface profile of the coating was characterized using an optical interferometer (ZYGO Mark Ⅲ-GPI) in a controlled environment with a temperature of 23 ± 1.5 ℃ and relative humidity of 45% ± 5%. The absorption at 355 nm was measured using a custom system based on the surface thermal lensing technique⁵⁰. The RMS roughness was measured using AFM (Veeco Dimension 3100) over a 5 μm × 5 μm area in tapping mode for each coating. The post-etch surface morphologies were examined using an optical microscope (Carl Zeiss Axioscope 5). The chemical compositions of the coatings before and after chemical etching were analyzed by XPS (Thermo Scientific K-Alpha). The LIDT was tested in 1-on-1 mode in accordance with ISO 21254 guidelines, using an s-polarized 3ω Nd:YAG laser with an 8.2 ns pulse width as the light source. The effective beam size was approximately 0.22 mm², and 15 sites were tested for each laser fluence. Surface and cross-sectional damage morphologies were characterized by FIB-SEM (Carl Zeiss AURIGA CrossBeam). The elemental composition of one layer of the multilayer coating was analyzed using EDS (Oxford X-Max).

This work was supported by the Program of Shanghai Academic Research Leader (23XD1424100), CAS Project for Young Scientists in Basic Research (YSBR-081), Shanghai Leading Talent Program, and National Natural Science Foundation of China (61975215). The authors express their appreciation to Dr. Jian Sun and Mr. Longsheng Wang for their assistance in sample preparation, to Prof. Dawei Li for absorption measurements, and to Prof. Yuan’an Zhao and Mrs. Ziyuan Xu for LIDT measurements.