Design and Implementation of Three-Layer Mesoporous Silica Coating for Tri-wavelength Broadband Antireflection by Block Copolymer Assisted Sol–Gel Method

A three-layer tri-wavelength broadband antireflective (AR) coating has been successfully fabricated on quartz substrate via a sol–gel route using acid-catalyzed silica sols. An ethylene oxide-propylene oxide-ethylene oxide triblock copolymer is used as a template to prepare ordered mesoporous SiO2 films. Assisted by Filmstar thin film design software, film thickness for each layer is optimized based on actual optical constants of the three mesoporous silica films. The three layers generate a reasonable refractive index gradient from the air, and thus the obtained AR coating possesses high transmittance of 99.24%, 99.66%, and 99.64% at 351 nm, 527 nm, and 1053 nm, respectively. The mesoporous SiO2 films with tough skeletons despite different porosity endow the coating with good abrasion-resistance, and 1H, 1H, 2H, 2H-Perfluorodecyltriethoxysilane is further used to modify the surface of the AR coating, which can improve the experimental stability of the coating. This work provides beneficial references for AR coating production of the sol–gel technique.


Introduction
Antireflective (AR) coatings, which can suppress undesired interfacial Fresnel reflections, have a wide range of promising applications in optical devices, solar cells, and laser systems [1][2][3][4][5]. In a high-power laser system that involves harmonic converter crystals to convert 1053 nm laser to 351 nm laser, there exist simultaneously laser beams at 351, 527, and 1,053 nm passing through an optical component [6][7][8]. To reduce the energy loss, broadband AR coatings exhibiting high transmittance simultaneously at the three wavelengths are extremely necessary. In principle, assembling multilayer coatings is more likely to achieve the multiple targets of film design. Notably, the peak position must be tailored in specific wavelengths by precisely matching the refractive index and thickness of each layer in the multilayer coating system, implying that the fabrication of these broadband AR coatings faces greater challenges compared with the common broadband AR coatings.
According to the theoretical design, for the broadband AR coatings applied onto fused quartz, the refractive index of the top layer should be even lower than 1.20 to obtain the best optical performance, and its certain values will affect the optimized results of other layers. Generally, standard film materials with such low refractive index are very scarce, and there is no guarantee to achieve high performance for multilayer AR coatings prepared by physical techniques. Fortunately, the sol-gel technique may be a better approach to fabricating broadband AR coatings because the structure and properties of gel materials can be easily tailored in the process of sol synthesis. Xu et al. reported a double-layer tri-wavelength broadband AR coating built from refractive controlled MgF 2 films, with the transmittance of 99.54%, 98.65%, and 98.58% at 351 nm, 527 nm, and 1053 nm, respectively [6]. Further, Xu's group prepared another three-layer tri-wavelength broadband AR coating based on in situ surface assembly of MgF 2 -SiO 2 hybrid sols, achieving transmittance of 99.60%, 99.61%, and 99.99% at the three wavelengths [9]. It is a pity that, however, the environmental stability of the broadband AR coatings was not investigated. Besides, the use of hydrofluoric acid is unfriendly to the environment. If only silica material is usable, the refractive index of film can be adjusted from 1.22 to 1.44 by mixing the base-catalyzed silica sol and acid-catalyzed silica sol in different proportions, which is Thomas's sol-gel method [10]. Jiang's group adopted this method to obtain the bottom layer and the middle layer, and successfully fabricated a broadband AR coating with hexamethyldisilazane-modified silica film as the top layer [7]. Nevertheless, this method involved ammonia removal by refluxing the sol so it was not easy for large-scale sol preparation.
Acid-catalyzed silica templated by triblock copolymer may be an alternative choice for the fabrication of multilayer optical coatings, due to its ordered mesoporous structure with low refractive index being obtained by burning the template out. Besides, acid-catalyzed silica sol is very stable compared with base-catalyzed silica sol. In this study, EO 106 PO 70 EO 106 triblock copolymer (Pluronic F127) was used as a pore former considering its compatibility with silica systems. The mesoporous silica obtained through this route has been proved to be a potential film material because of the ordered mesoporous structure, high laser-induced damage threshold (LIDT), abrasion-resistant, low cost, and easy large volume production [11][12][13]. Herein, silica films with different contents of F127 were employed to construct the three-layer broadband AR coating. For the AR coating applied onto fused quartz, the results of the theory design show that a wide region of high performance at the specific wavelengths can be achieved only if the refractive index of the top layer is significantly lower than 1.2. Meanwhile, the refractive index of the bottom layer should be adjusted from 1.3-1.4, and that of the middle layer must be in the range of 1.2-1.3. Through adjusting the template content, the silica sols for the bottom and middle layers were easily synthesized by introducing a prehydrolysis step of tetraethyl orthosilicate (TEOS) under acidic conditions. As for the top layer, the prehydrolysis step and ratio of reactants were modified to prepare a stable sol with the further increase of F127.
In this work, a three-layer tri-wavelength AR coating was designed and fabricated by using mesoporous silica films. On the basis of film design implemented by the thin film design software Filmstar, the amount of the template F127 in each sol was carefully controlled for the regulation of film refractive index. Film optical constants obtained from single-layer analysis were imported into Filmstar in order to further optimize film thicknesses in the multilayer structure. Finally, a three-layer tri-wavelength broadband AR coating was successfully prepared, and surface modification was also implemented by 1H, 1H, 2H, 2H-Perfluorodecyltriethoxysilane (PFDS) treatment to improve film durability [14]. The AR coating exhibits excellent optical performance with transmittance values of 99.24%, 99.66%, and 99.64% at 351 nm, 527 nm, and 1053 nm, respectively. Abrasionresistance test shows that the prepared multilayer AR coating can satisfy the application demand in practical situations.

Preparation of SiO 2 Sols
In order to prepare F127-containing sols used for the bottom and middle layers, TEOS was mixed with hydrochloric acid, anhydrous ethanol, and deionized water with molar ratios of TEOS: EtOH: H 2 O: HCl = 1:6:2:0.005. The solution was left to react under stirring at 60˚C for 2 h. A second solution was prepared by dissolving different content of F127 in the mixture of ethanol and hydrochloric acid aqueous solution, and then slowly added to the former stock solution of prehydrolyzed silica precursor. The molar composition of the final sols was TEOS: EtOH: H 2 O: HCl: F127 = 1:30:5:0.02:x (x = 0.0015 and 0.005). The sol was stirred for 24 h and aged for 5 days at room temperature for film preparation.
By following the above route, mesoporous SiO 2 with ultralow refractive index seems to be achievable by further increasing the F127/SiO 2 ratio. Unfortunately, the consequent high viscosity of sol and probable pore collapse during film heat treatment will seriously hamper film optical performance. According to the literature [12], the situation can be refined if the prehydrolysis of TEOS is carried out at room temperature by increasing the quantities of acid. Half of the anhydrous ethanol was firstly mixed with TEOS. Subsequently, hydrochloric acid and distilled water were added, followed by stirring for 1 h. F127 and the remaining half of the ethanol were mixed and stirred at a temperature of 40˚C until the polymer was completely dissolved. The mixture was then added drop by drop into the prehydrolyzed solution of TEOS with stirring for 2 h. The final molar ratio was TEOS: EtOH: H 2 O: HCl: F127 = 1:38:4.5:0.45:0.00995. The sol was aged for 10 d and then used to prepare the top layer of the threelayer coating. For the sake of clarity, both the SiO 2 sols and the corresponding mesoporous films were noted as SF015, SF05, and SF0995 according to the F127 content in solution, respectively.

Preparation of Broadband AR Coating
Fused quartz substrates were successively rinsed with water, acetone, and ethanol. During deposition of the multilayer AR coating, the prepared sols (SF015, SF05, and SF0995) were sequentially dip-coated onto the well-cleaned quartz substrates and film thickness was controlled by adjusting the withdrawal rate. Each fresh layer was preheated at 150˚C for 10 min to accelerate solvent evaporation before the next layer was deposited. To stiffen the silica network, the triplelayer coating was placed in saturated ammonia vapor for 1 h. A final heat treatment of the multilayer structure at 350˚C for 2 h concluded the deposition process.
The modifier solution was obtained by mixing PFDS and anhydrous ethanol at a volume ratio of 1:2. The surface modification process was accomplished by dipping the triple-layer coating in the modifier solution for 2 min and withdrawing out at a rate of 200 μm/s. Finally, the modified AR coating was dried in an oven at 150˚C for 1.5 h to remove the unreacted material from the surface. To test the experimental stability of AR coating, the modified three-layer coating was transferred to a vacuum chamber in a vacuum chamber of 10 -3 Pa and exposed to polydimethylsiloxane (PDMS) vapor at room temperature for two months.

Characterization
Transmittance spectra were measured using an ultraviolet/ visible/near-infrared (UV-Vis/NIR) spectrometer (Shimadzu, UV3600 plus, Japan) over the wavelength range of 300-1600 nm. Transmission electron microscopy (TEM) of the FEI Talos F200X G2, USA, was used to investigate the pore structure of the mesoporous coating. The nitrogen adsorption-desorption measurements were performed on a Micromeritics ASAP 2460 automated physical adsorption instrument, USA. The samples were degassed at 120˚C for at least 12 h under vacuum conditions to remove surface contaminants prior to testing. The specific surface areas of the samples were calculated using the BET equation and the pore size distributions were obtained from the isotherm adsorption branch by using the BJH model [15,16]. The surface morphology of the coating was characterized by atomic force microscopy (AFM) of Bruker Dimension Icon, Germany. In addition, a TESCAN MIRA LMS scanning electron microscope (SEM) was used to investigate the cross-section image of the three-layer AR coating. Prior to SEM observations, the samples were coated with a layer of gold by ion sputtering. The mechanical property of the AR coating was assessed by a linear abraser (DaZhong Instruments, China). The coating was rubbed for 30 cycles via CS-10F Wearasers and the controlled normal stress on the coating surface was about 20 kPa.

Computer-Aided Design of Tri-wavelength Three-Layer AR Coating
The design of single-or double-layer AR coatings at one working wavelength can be steadily achieved on a basis of fundamental theoretical techniques such as vector method and optical admittance loci [17]. Nevertheless, it is usually difficult to design multilayer broadband AR coatings, owing to the thickness and refractive index requirements of each layer for multiple targets. Alternatively, computer-aid design is the preferred method because it is fast and straightforward. The three-layer tri-wavelength AR coating was designed with the aid of thin film design software Filmstar in this work.
In view of the estimated reduction of film refractive index by the presence of pores, a gradient index model was selected to construct the three-layer AR coating with the targeted transmittance of 100% at 351, 527, and 1053 nm. According to the index model, the refractive index of the top layer is the lowest and its value has impact on the optimized refractive indices of the inner layers. In the design, the lowest refractive index was manually set to a specific value in a reasonable range. As shown in Table 1, with the refractive index value of the top layer decreasing from 1.20 to 1.11, the optimum refractive indices of the bottom and middle layers and film thicknesses of all the three layers were automatically obtained through the optimization procedure. Unquestionably, the top layer with a sufficiently low refractive index will endow the three-layer coating with excellent optical performance. As the refractive index of the top layer was selected as 1.11, the transmittances at the three working wavelengths can exceed 99.9% and the average transmittance in the range of 300-1100 nm attains 99.66%. It should be noted that the prerequisite for theoretic results listed in  Table 1 is ignorance of refractive index dispersion, which contributes to easily obtaining the preliminary design best suited for a given application. The matching requirements of layer refractive indices provide a quick guide for the preparation of the sols and films.

Microstructure Analysis of Mesoporous SiO 2 Films
Generally, the hydrolysis of the TEOS precursor is faster than the polycondensation under low pH conditions, which results in finer ramified polymeric siloxane chains. During the synthesis process of mesoporous silica films, further polycondensation of the siloxane chains forms the inorganic walls of the mesoporous framework [13]. Figure 1 shows TEM photographs of the three film materials used in this study. It can be seen that no obvious mesopores are observed in the film SF015. With increasing the F127/TEOS molar ratio to 0.005 for the film SF05, a well-ordered mesopore structure appears in the image. Further increasing the F127 content to 0.00995 for the film SF0995, the oriented pore arrangement becomes relatively disordered to some extent. The pore structures of the three mesoporous silica gel materials were further investigated through nitrogen adsorption/desorption tests. The nitrogen adsorption-desorption curves and the pore size distribution plots of the samples are illustrated in Fig. 2. It is obvious that the sample SF05 exhibits typical type-IV curves with H2 hysteresis loops, associated with the adsorption and desorption behavior of cage-like mesoporous structures. In the case of SF0995, despite a little shape change, the H2 hysteresis loop still exists, suggesting that cage-like mesopores are maintained. With increasing the template content from 0.005 to 0.00995, the mean pore size significantly increases from about 5.0 nm of SF05 to 10.0 nm of SF0995. In contrast, the isotherm obtained for sample SF015 is of type I, being characteristic of a microporous solid [18]. The unclosed adsorption and desorption isotherms may be attributed to a large volume of very small pores. The pore structure information obtained from the nitrogen adsorption/desorption tests is well consistent with the TEM results. The tunable pore structures are the foundation of the regulation of film optical properties.

Optical Properties of Mesporous SiO 2 Films
Accurate analysis of the optical properties of film materials is the basis of the rational design of multilayer AR coatings. For optical films deposited on transparent substrates, fitting transmittance data can conveniently acquire the refractive index n and extinction coefficient k of film. According to the strict theoretical formula [19], a fitting program in Matlab was built to analyze film optical properties in the range from ultraviolet to near-infrared. The Cauchy equations [20] can be well suited to model the nearly transparent mesoporous SiO 2 films in the whole current spectral region. The fitting parameters such as film thickness and optical constants are automatically adjusted to find the calculated spectrum closest to the measured data by combing the genetic algorithm and the lsqcurvefit algorithm.
The calculated curves of the single-layer mesoporous SiO 2 films in the spectral region of 300-1600 nm are shown in Fig. 3a, which are in good agreement with the measured transmittance. The achieved optical constants of the films are presented in Fig. 3b. The curves of the extinction coefficients of the mesoporous SiO 2 films are very close to each other, and the dispersion curve of film SF05 is representatively shown in the figure. The degree of index dispersion of film SF0995 is almost negligible and the refractive index of film SF0995 at 550 nm is 1.117, manifesting that the film is suitable for use as the top layer of the three-layer AR coating. The refractive indices of the other two films present a relatively evident dispersion only in the ultraviolet region, and the refractive indices of films SF05 and SF015 at 550 nm are 1.248 and 1.345, respectively. The optical constants of the films SF05 and SF015 are very close to the requirements of the inner layers for the three-layer AR coating. Moreover, the film thickness can also be determined through transmittance spectrum fitting. As such, the dipping rate of the substrate can be nicely controlled until the thickness of the singlelayer film meets the need of the designed AR coating.

Assembly of Three-Layer Tri-wavelength Broadband AR Coating
Once the specific materials are selected, accurate control of each layer thickness is very important for the successful preparation of multilayer optical coatings. By importing the refractive indices and extinction coefficients of the selected single-layer films into the design software Filmstar, the optical performance of the AR coating is further improved and the optimized thicknesses for the top, middle, and bottom layers are 124.2, 104.3, and 85.6 nm, respectively. Note that the thickness values deviate from the results corresponding to the model design with the lowest refractive index of the top layer (listed in Table 1), which is attributable to the introduction of actual optical constants of the films including dispersion. Combined with the analysis of the transmittance spectrum, three primary dip rates can be obtained based on the optimized thickness of each layer. During practical deposition of the AR coating, It should be noted that, compared with the single-layer film on the quartz substrates, film formation will be slightly affected by the surface of the previous one, with the number of layers increasing. For this reason, a further readjustment of the dipping rate for each layer is usually necessary for the correct production of multilayer AR coatings.
Based on the design model and the control strategies in the deposition cycle, the three-layer tri-wavelength AR coating is successfully realized. The optimized pulling speeds for the bottom, middle, and top layers are 1600, 700, and 700 μm/s, respectively. Figure 4 illustrates the experimental and theoretical transmittance spectra of the three-layer AR coating as well as the transmittance of the substrate. For comparison, the transmittance of the theory data derived from the best model design using the top layer with a refractive index of 1.11 (listed in Table 1) is also shown. The transmittance of the design based on actual film optical constants is inferior to that of the design ignoring index dispersion. It is unfortunate that refractive index dispersion is an intrinsic property, especially for low-porosity materials. Moreover, on the whole, the theoretical transmittance of the design including dispersion is closer to the experimental data. As can be seen from Fig. 4, the prepared three-layer AR coating shows the transmittance of 99.24%, 99.66%, and 99.64% respectively, at 351, 527, and 1053 nm, and an average transmittance of 99.19% from 300 to 1200 nm. It should be admitted that the AR performance is slightly lower than the best results in the previous reports [7,9]. Nevertheless, compared with the transmittance of the glass substrate of about 93.0%, the enhancement in transmittance is still prominent at such a broadband region.  Fig. 4, the transmittance data of the design including dispersion are greater overall than the measured ones in the working wavelength region of the three-layer AR coating, which probably results from the rough surface and interfaces in the multilayer. Figure 5 shows the AFM images and root-mean-square roughness (Rq) values of the bottom layer, middle layer, top layer, and tri-layer coating, respectively. The bottom and middle layers have smoother surfaces with low Rq values of about 0.25 and 1.34 nm, respectively, whereas the top layer has a relatively larger Rq value of 3.40 nm. Fortunately, the Rq value of the surface of the three-layer AR coating is only slightly larger than that of the top layer, indicating that the accumulative effect between the multiple layers is very limited. Hence, the surface and interface scattering of the three-layer coating has a rather weak impact on the transmittance data even in the ultraviolet region [7,21]. Besides, the attainable transmittance at 351 nm is lower than those at the other two designed wavelengths both in the measured and theoretical spectra. Combined with the analysis of the surface morphology and the optical properties of the single-layer films, the relatively poor performance of the AR coating in the ultraviolet region probably arises from the mismatching of the layer refractive indices with different dispersion characteristics.
Film thickness for each layer in the three-layer AR coating was directly confirmed by cross-sectional SEM as shown in Fig. 6. It can be seen that the thickness of each layer is basically consistent with the designed value. Since film thickness is nicely controlled, the small deviations between the experimental and theoretical transmittance spectra especially in the near-infrared region might be related to the less sharp interface structure arising from the infiltration between two adjacent layers. Despite this, from the view of the experiment results, the linear siloxane polymeric chains formed under acidic conditions prevent the formation of serious infiltration during the dip-coating process [7]. Although the precise control requirement for each layer in a multilayer structure will increase the complexity of the coating process, the prepared three-layer AR coating attains the desired targets of the model design, and the soft sol-gel route is a practical and valuable way for the production of multilayer AR coatings.

Durability Performance
The mechanical property is a key issue of AR coatings for practical applications [22]. As mentioned earlier, heat treatment can bring out the further polycondensation of the linear siloxane chains in the F127-containing SiO 2 films, and the walls of the pores are comparable to the dense microstructure of the bulk material. Thanks to the tough skeletons of the mesoporous SiO 2 layers in the AR coating, repeated rubbing via a CS-10F wearaser with normal stress on the contact surface of about 20 kPa only leaves subtle scratches on the AR coating surface as shown in Fig. 7a. The transmittance spectra of the AR coating before and after rubbing test are depicted in Fig. 7b. As expected, the maximum decrease at the three designed wavelengths is 0.22% at 1053 nm, and the average transmittance displays a decrease of only 0.26% in the region of 300-1200 nm. This result suggests that the AR coating assembled using the mesoporous SiO 2 films possesses a high abrasion resistance.
In high-power laser systems, many nonvolatile organic matters can transform into vapors under the large negative pressure of a high vacuum. The adsorption of contaminants results in the increase of film refractive index and then a decrease in film optical performance. For this reason, the surface modification step using PFDS treatment was introduced to conclude the fabrication procedure of the AR coating. The transmittance curves of the AR coating before and after the contamination test are shown in Fig. 7c. After exposure to PDMS vapor in 10 -3 Pa vacuum for 15 d, the transmittances at 1053 nm and 527 nm decrease by 0.24% and 0.22% respectively, and that at 351 nm remains almost no change. Meanwhile, the three peaks present a small shift compared with the curve of the as-prepared coating. Further increasing the test time to 60 d, the transmittance at 351 nm decreases significantly (about 0.65%). The average transmittance in the region of 300-1200 nm decreases by 0.43% in the whole contamination test. Actually, the amount of PDMS in the vacuum chamber is adequate for vaporization and the simulated environment of the contamination test is rather harsh. Thus, it is reasonable to assume that the threelayer AR coating can meet the stability requirements under practical conditions.  Durability tests of the three-layer tri-wavelength AR coating. a Microscope photograph of coating surface after rubbing with a 3H pencil, b corresponding transmittance spectra of the AR coating shown in a, c Change in transmittance of the AR coating before and after before and after contamination

Conclusions
To summarize, a three-layer tri-wavelength broadband AR coating has been designed and optimized with the aid of thin film design software Filmstar. The mesoporous SiO 2 films with tunable refractive index have been proposed to realize the optimized design model. The transmittance of this three-layer AR coating achieves 99.24%, 99.66%, and 99.64% at 351 nm, 527 nm, and 1053 nm, respectively, and the average transmittance in the region of 300-1200 nm reaches to 99.19%. The tough skeletons of the mesoporous SiO 2 films bring good abrasion-resistance to the three-layer AR coating. Moreover, the coating shows fairly good vacuum stability under moderate experimental conditions. The practical fabrication method of the three-layer AR coating with good antireflection and durability performance can provide beneficial references for expanding the principal application area of sol-gel deposition.