Sol-Gel Preparation of Laser Damage Resistant and Moisture-Proof

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Sol-Gel Preparation of Laser Damage Resistant and Moisture-Proof Antireflective Coatings for KDP Crystals Xiaodong Wang, Huiyue Zhao, Yuanyuan Cao, Yanyan Niu, and Jun Shen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01762 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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Sol-Gel Preparation of Laser Damage Resistant and Moisture-Proof Antireflective Coatings for KDP Crystals Xiaodong Wang†*, Huiyue Zhao†, Yuanyuan Cao†, Yanyan Niu‡ and Jun Shen†* †

Shanghai Key Laboratory of Special Artificial Microstructure Materials and

Technology & School of Physics Science and Engineering, Tongji University, Shanghai 200092, P. R. China ‡

College of Science, University of Shanghai for Science and Technology, Shanghai 200093, P. R. China

ABSTRACT: Surface fogging induced degradation has been a bottleneck problem in potassium dihydrogen phosphate (KDP) crystals due to they are grown from aqueous solution. In this paper, we developed a facile method to prepare a double-layer antireflective coating with moisture-proof and laser damage resistant properties for KDP crystals. The bottom layer was a poly-siloxane coating with dense structure and silanol side groups, while the top layer was a hexamethyl-disilazane (HMDS) modified nano-porous SiO2 coating. Both of the sols were non-alkaline and non-aqueous to make sure those are harmless to KDP crystals. The double-layer coated KDP crystal exhibited a maximum transmittance of 99.9% with an average increase of transmitted light of 6–7% over the wavelength range between 351 and

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1053 nm. After exposure in a 55% relative humidity environment for 6 months, the double-layer HMDS_SiO2/PS coating coated KDP crystal displayed nearly the same optical transmittance as the original one, whereas the single-layer HMDS_SiO2 coated KDP had a transmittance loss of ~5%. Moreover, the laser-induced damage threshold of the double-layer coating on KDP crystal reached 11.5 J/cm2 (355 nm, 3 ns). This multifunctional antireflective coating not only can be used for KDP crystals, but also can be applied to thermal-sensitive polymeric substrates.

KEYWORDS: KDP crystals; moisture-proof; antireflective coating; sol-gel

1. Introduction

Potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate crystals are the most important nonlinear optical crystals used for frequency conversion in high power laser optics1-2. As a deliquescent material, the KDP is easily to absorb moisture in the air, causing fogging or even corrosion of the surfaces, with a subsequent scattering and degradation of the optical performance. Therefore, a moisture-proof protective coating would be desperately needed to prevent the degradation in its optical performance in humid environment. In addition, high transmission at the conversion wavelength, high laser damage threshold and environmental stability of the coatings are also essential properties that the protective coatings must have if it is used on KDP crystals in high power laser system. Sol-gel antireflective (AR) coatings possess laser-induced damage thresholds (LIDT) 2–3 times greater than other AR coatings, making them especially suitable for


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high power laser components3-8. Sol-gel SiO2 AR coatings have been extensively applied for the fused-silica glasses. Generally, they are applied by dip-coating followed by ammonia-curing to crosslink and harden the coating9. When applied to KDP crystals, these coatings cannot be ammonia-cured due to sensitivity of the KDP crystal to water and ammonia. The porous nature of the coating allows water moisture to penetrate and react with the crystal surface, which results in the formation of etch pits on crystals10-11. Furthermore, for the wet chemical coating methods, the coating substrates have to contact with a liquid sol and the as-prepared coatings normally experience a thermal annealing process to cure the coatings. However, the KDP crystals are acidic and have a crystal phase transition temperature of ~150 °C12, which limits the synthesis materials of the sol and the thermal annealing temperature for the coatings. Thus, an annealing temperature below 150 °C is requisite and the sol for coating must be non-alkaline and non-aqueous to avoid damaging the KDP crystals. Numerous efforts13-17 have been made to develop a moisture-proof protective coating for KDP crystals. Murahara et al13-16 developed a hard protective waterproof coating by photo-oxidation of silicone oil. Zhang et al17 fabricated a dense ladder-like alkylene-bridged polymethylsiloxane through the hydrosilylation reaction of diene and polymethylhydrosiloxane. This kind of protective coatings are normally prepared from dense and nonporous materials, thereby blocking diffusion of water moisture via their hard and compact structure13 or special lamellar structure17. Unfortunately, although the dense structure could greatly improve the moisture-resistant property of the coatings, the refractive indices are higher than that required for a perfect


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antireflection coating on KDP crystals. A second layer of colloidal silica AR coating has to be applied on top of the protective coating to realize excellent optical performance, which complicates the coating preparation process. And also, the fabrication process of the protective coatings cost either expensive raw materials17 or scarce experimental setup13, as well as long preparation time. Another strategy is to chemically modify the silica coatings with hydrophobic functional groups. This type of hydrophobic coatings could be obtained by functionalizing the silica sol with hydrophobic silanes18-21 or directly reacting the silica coating with a reactive hydrophobic silane in the vapor state4, 22-23. Previous studies4, 18-23 have reported that silica AR coatings with super-hydrophobicity and environment-resistance have been successfully prepared on fused-silica or glass substrates. However, reaction happens in the modification of silica often leads to the formation of by-products such as NH3, HCl or H2O in the sol or coatings, which increases the possibility to damage the crystals if they are applied on the KDP crystals. In this work, hexamethyl-disilazane (HMDS), which contains long chains of hydrophobic methyl groups, was introduced into colloidal SiO2 sol to form a very hydrophobic trimethylsilyl outer shell that prevents the penetration of water vapor. Meanwhile, the reaction by-product ammonia, which is harmful to the KDP crystal, was removed from the sol through refluxing process. A poly-siloxane coating with dense microstructure was used as the bottom layer to resist moisture for protecting the KDP crystals. After cured at low temperature, the HMDS modified colloidal silica sol was coated as the top antireflection layer. The final double-layer coatings on KDP crystals shown superior


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moisture-proof, high transmission and high laser-induced damage threshold properties. 2. Experimental Section 2. 1. Synthesis of colloidal silica sol and hexamethyl-disilazane modified silica sol Colloidal silica sol was synthesized via Stöber method24, through alkaline hydrolyzation and condensation of tetraethoxysilane (TEOS) with ammonia (NH3·H2O) in ethanol (EtOH) solution. The molar ratio of TEOS, NH3·H2O and EtOH was 1: 2.5: 40. A solution of ammonia and half of the prescribed amount of EtOH was added into a solution made up of TEOS and the other half of the prescribed amount of EtOH under magnetic stirring. The mixture was then stirred for 2 h at 20 °C and further aged at 20 °C for 8 days. Finally, the sol was refluxed for 24 h to remove the ammonia. The resultant sol was named as standard SiO2 sol. As a modifier, hexamethyl-disilazane (HMDS) was either added into the above SiO2 sol and aged at 20 °C for 7 days, or added into the SiO2 sol before refluxing and aged at 20 °C for 15 days. The resultant sols were named as HMSA sol and HMSB sol, respectively. The preparation procedure for HMDS modified silica sols is shown in Figure S1. The molar ratio of HMDS to TEOS was varied from 0.5 to 3. For HMSB sol, the sol was refluxed after the HMDS modification to remove the catalyst ammonia and generated ammonia during modification. Thus, the HMSB sol was non-alkaline (Figure S1) so as to be harmless to the acidic KDP crystals. 2. 2. Synthesis of poly-siloxane sol


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The poly-siloxane (PS) sol was prepared from a glass resin (SM902, (CH3SiO1.5)n) that was provided by Laiyang Shunming Chemical Co., Ltd. The raw material was supplied as a soluble silicone pre-polymer in ethanol solution, and then diluted with anhydrous isopropanol to a concentration of 8%. The final product was a colorless and transparent solution. 2. 3. Deposition of protective and antireflective coatings All the sols were carefully filtered through 0.22 µm organic fluoride filters prior to use. Fused-silica glass (FSG) and silicon substrates were firstly cleaned in acetone and ethanol mixed solution, and then blow-dried with nitrogen gas. After annealing at 100 °C for 10 min, the substrates were ready to use. KDP crystals (50×50×5 mm3) were cleaned with toluene and then blow-dried with nitrogen gas. Then the coating layers were deposited on FSGs/KDP crystals using a dip coater (CHEMAT Dip Master-200). The thickness of the coatings was controlled by the withdrawal speed (0–12 inch/min) to match the desired value. All of the coated substrates were finally cured at 80 °C for 24 h before use. 2. 4. Characterization of coatings Transmittance and reflectance spectra were recorded using a CARY 5000 UV-VIS-NIR dual beam spectrophotometer equipped with polarizer, in the wavelength region of 300–1200 nm. The refractive indices and thicknesses of the coatings were calculated through simultaneously fitting the transmittance and reflectance spectra obtained via calculations and measurements. The coating surfaces were characterized using a Philips XL30 FEG field-emission scanning electron microscope (FE-SEM) and a LEICA DM4000 M optical microscope. The ACS Paragon Plus Environment

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microstructure of the standard SiO2 sol and HMSB sol was investigated by a JEOL JEM-2011 transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectra of silica coatings were performed in the range of 400–4000 cm-1 with a Bruker Tensor 27 spectrometer. The laser-induced damage measurement was tested in the “R-on-1” regime according to ISO-standard 11254-1, using a Q-switched Nd: YAG laser at 355 nm with a pulse width of 8 ns. The measurement was performed on 100 different site-locations (10×10 array) with an interval of 2 mm on each sample. Each site was irradiated by a series of laser pulses with gradually increased energy until damage occurred. The initial laser energy density was set to 10 J/cm2 and the increment was ~1 J/cm2. Damage was judged if any visible change of the sample surface was observed by a real-time CCD camera. A damage probability was then obtained for each laser energy density. Based on these results, the LIDT was determined as the maximum energy density of the incident pulse at which there was zero probability of laser damage. All of the fluence were scaled to a pulse width of 3 ns by a scaling factor of τ0.5. 3. Results and Discussion 3. 1. Structure of HMDS modified colloidal silica sol Figure 1 illustrates the reaction mechanism for the HMDS modified silica sol. As shown in Figure 1, in the presence of ammonia, TEOS is hydrolyzed and condensed to form colloidal silica spheres affording the resultant coatings high porosity and low refractive index. HDMS has an imido group and two trimethylsilyl groups. A substitution reaction occurs between trimethylsilyl groups of HMDS and OH groups


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on the surface of SiO2 spheres, follows by the formation of ammonia as by-products. Consequently, the silica spheres will be covalently linked by HMDS, which avoids the self-condensation between OH groups on adjacent SiO2 spheres and also creates a hydrophobic trimethylsilyl surface with very low surface energy25. The introduction of hydrophobic trimethylsilyl groups can also improve the environmental stability of the silica AR coating because they can prevent the penetration of water vapor from the working environment. Finally, a refluxing was conducted to remove the ammonia that existed as catalyst and the reaction by-product.

Figure 1. Schematic representation of the reaction mechanism for the HMDS modified silica sol.

To better reveal the microstructure evolution of the silica nanoparticles after HMDS modification, a TEM characterization of the standard silica sol and HMSB sol has been done. As can be seen in Figure 2, both of the sols show a cross-like network microstructure with an approximate average diameter of ~10 nm. The silica ACS Paragon Plus Environment

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nanoparticles in the standard silica sol prepared by Stöber method are monodisperse, with a partial conglomeration due to their high surface energy and small particle size. While in the HMDS modified silica sol, the silica particles are uniformly dispersed and show fuzzy boundary, indicating the well-controlled growth and modification of the silica particles. Meanwhile, the network microstructure of the HMDS modified silica sol get denser. However, the particles are still monodisperse and show almost no aggregation. This result agrees very well with above modification mechanism analysis that the surface of the SiO2 particles are covered by trimethylsilyl groups with low surface energy.

Figure 2. TEM images of standard SiO2 sol (a) and HMDS modified SiO2 sol (b). Figure 3 shows the FTIR spectra of coatings deposited from pure SiO2 sol and HMDS modified sols. All spectra show a very strong absorption peak at 1082 cm-1 and a medium intensity band around 796 cm-1, which are assigned to the asymmetric and symmetric stretching modes of the –Si–O–Si– bond, respectively26. In the spectra of HMDS modified SiO2 coatings, three new absorption bands, which correspond to the Si–CH3 bending and stretching vibrations27-28, are revealed at 1254, 866 and 847 cm-1, respectively. With increasing molar ratio of HMDS/TEOS, the intensity of ACS Paragon Plus Environment

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absorption band of Si–CH3 increases, whereas the absorption peak at 968 cm-1 which ascribed to Si–OH weakens. As can be noticed from the slope variance for the intensity of the two absorption bands, such variation tendency starts to saturate when the molar ratio of HMDS/TEOS exceeds 2:1. Hence, a molar ratio of 2:1 was typically used in this work to get a significant trimethylsilyl coverage. In conclusion, FTIR spectra confirmed that nonpolar methyl groups were successfully grafted on the SiO2 coating by the trimethylsilyl functionalization of the colloidal silica sol.

Figure 3. FTIR spectra of unmodified and HMDS modified SiO2 coatings. 3. 2. Surface morphology and optical parameters of the coatings The surface morphology of HMDS_SiO2 coating and PS coating were characterized by FE-SEM and shown in Figure 4. From Figure 4 (a), we can see that the HMDS_SiO2 coating consists of nanometer-sized silica colloidal particles as also shown in TEM analysis. The SiO2 particle packed network results in a very high porosity and comparatively rough surface of HMDS_SiO2 coating. Therefore, a refractive index of 1.18 (@633nm) is obtained for the HMDS_SiO2 coating (as shown


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in Figure 4 (c)). In contrast, the PS coating shows an extremely dense structure and smooth surface without obvious pinholes or cracks (as shown in Figure 4 (b)), which gives a refractive index of 1.42(@633nm). The extinction coefficients k are quite small (~10-4), which indicates the light absorption is almost negligible and will not affect the transmittance if they are coated on KDP crystal. Likewise, the refractive indices and extinction coefficients of these two kinds of coatings both decrease exponentially with increasing wavelengths, indicating a normal dispersion behavior.

Figure 4. SEM photographs of (a) HMDS_SiO2 coating and (b) PS coating, (c) optical constants of HMDS_SiO2 coating and PS coating. 3. 3. Optical performance of the AR coatings In high power laser system, KDP is used as a frequency conversion crystal to realize the frequency conversion from fundamental frequency to second harmonic frequency, and finally to the third harmonic frequency. That is why the antireflection wavelengths of the AR coatings are selected as 1053, 527 or 351nm. A simple


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double-layer AR filter structure was designed to realize simultaneous antireflection at 527 and 1053 nm, as shown in Figure 5(a). We selected the PS protective coating and HMDS_SiO2

coating

as

the

high-refractive-index

bottom

layer

and

low-refractive-index top layer, respectively. Thus, the corresponding optical parameters were employed to simulate the transmittance spectrum using an optical coating design program of Essential Macleod. Both of the targeted transmittance at 527 and 1053nm were set as 100%. Then the optimization procedure would give the best parameters of this AR structure through simulated thermal annealing method29. After that, the optimized coating thicknesses d1, d2 were 122, 146 nm. The calculated AR filter gives the best optical transmittance of 99.0% and 99.3% at 527 and 1053nm respectively, as shown in Figure 6(a). According to the simulation results, the desired thicknesses of bottom and top layers were obtained through adjusting dip-coating withdrawal rates. The double-layer AR coating then can be achieved by sequentially coating the bottom and top layer on the KDP crystal. The withdrawal rates for bottom and top layer are 2.0 inch/min and 5.5 inch/min, respectively. Figure 6(a) displays the experimental transmission curve of the as-prepared double-layer HMDS_SiO2/PS coating coated KDP crystal. Compared with the bare KDP, the coated KDP exhibits a maximum transmittance of 99.9% with an average increase of transmitted light of 6–7% over the wavelength range between 351 and 1053 nm. The transmittance at 527nm reaches 99.3%, while that at 1053nm is only 97.5%, which is due to 1053nm is close to the infrared absorption edge of the KDP crystal2,

30

. As can be noticed, there are small deviations between the


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experimental and theoretical transmittance spectra in the short and long wavelength regions, which are attributed to the extinction coefficients of the KDP substrate are difficult to be determined accurately for simulation, as well as the deviation of coating thickness from the theoretical design. To clearly see the interface, the same double-layer coating was deposited on a Si wafer and the cross-section was pre-sputtered with gold to improve the poor conductivity of coating materials for SEM observation. Figure 5(b) shows the cross-sectional SEM photograph of double-layer HMDS_SiO2/PS coating. The interface between HMDS_SiO2 coating layer and PS coating layer can be identified easily from the photograph, which providing the feasibility to measure the thickness of each coating layer. The measured thicknesses of top layer and bottom layer were 141 and 126 nm, respectively, which were slightly different from those of the designed thicknesses. Therefore, the thickness difference should be responsible for the deviation of the experimental and theoretical transmittance spectra. Likewise, the clear interface of the two layers suggests that the PS coating is dense enough so that the SiO2 nanoparticles in top layer cannot diffuse into the bottom layer. Moreover, the cross-sectional image also shows that the bottom protective layer is dense, while the top coating layer is composed of loose accumulated large-size particles, giving the coating high porosity and thus low refractive index. These results also agree well with the surface morphology and TEM analysis.


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Figure 5. Schematic illustration (a) and cross-sectional SEM photograph (b) of double-layer HMDS_SiO2/PS coating. A single-layer AR coating with central wavelength at around 351nm was also dip-coated on KDP crystal using HMSB sol. As shown by the blue short-dotted curve in Figure 6(b), this quarter-wave AR coating coated KDP possesses an excellent transmittance of 99.3% at 351 nm, which is increased by 8.1% compared with that of the bare KDP crystal. It should be noted that an optimal antireflection at 527 or 1053 nm could also be realized through controlling the thickness of this quarter-wave AR coating, which is associated with the withdrawal rate of dip-coating.

Figure 6. Transmittance spectra of KDP crystal coated with double-layer HMDS_SiO2/PS coating (a) and single-layer HMDS_SiO2 coating (b).


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3. 4. Moisture-proof performance of the coatings The moisture-proof performance of the coatings on KDP crystals was tested in a humid environment, where the humidity was higher than that in the practical application environment. Figure 6 presents the transmission variance of the single-layer HMDS_SiO2 and double-layer HMDS_SiO2/PS coated KDP crystals in a environment with a 55% relative humidity (RH) for different periods. As can be clearly noticed in Figure 6(a) and (b), the transmission spectra of both single-layer HMDS_SiO2 and double-layer HMDS_SiO2/PS coated KDP crystals have almost no change after 3 months exposure in the environment of 55%RH, exhibiting quite stable optical performance. Much to our surprise, the single-layer HMDS_SiO2 coated KDP displays an apparent transmittance degradation of ~5% after being exposed in the same humid environment for 6 months. What is worse, the optical transmittance is even lower than that of original bare KDP crystal in the wavelength region above 550 nm, which is probably due to fogging or deliquescence happened on the sol-gel coating coated KDP surface in the environment of 55%RH. Meanwhile, the double-layer HMDS_SiO2/PS coated KDP still possesses nearly the same optical transmittance as the original one. These results illustrates that the moisture-proof ability provided by double-layer HMDS_SiO2/PS coating is obviously much better than that by single-layer HMDS_SiO2 coating. Figure 7 displays the optical micrographs of the coated KDP crystal surface before and after exposure in a humid environment. As shown in Figure 7(a) and (c), the as-prepared single-layer HMDS_SiO2 coating and double-layer HMDS_SiO2/PS


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coating both exhibit an extremely homogenous surface. However, many etch pits (as shown in the red circle parts of Figure 7 (b)), generally several micrometers and a few hundred nanometers deep, developed at the KDP surface beneath the porous HMDS_SiO2 coating after 6 months exposure in a humid environment. The geometries of the pits depend on the crystal surface orientation1, 31. This phenomenon provides further evidence that the surface etching should be responsible for the degradation of optical property. On the other hand, almost no change appears on the surface of the double-layer HMDS_SiO2/PS coating coated KDP after exposure in a humid environment for 6 months (Figure 7(d)). Based on the above analysis, we can conclude that the double-layer HMDS_SiO2/PS coating had an excellent moisture-proof performance. Although the surface of the colloidal SiO2 particles are grafted with trimethylsilyl groups with low surface energy, a single-layer HMDS_SiO2 coating is still not enough to withstand the penetration of moisture for prolonged periods of time. Thus, 6 months exposure of single-layer HMDS_SiO2 coated KDP results in a large decline of its optical property. For double-layer HMDS_SiO2/PS coating, the PS coating layer plays an important role in moisture barrier performance thanks to its extremely dense structure that derived from high molecular weight polysiloxane32. In addition, the double-layer HMDS_SiO2/PS coating exhibits a better abrasion resistance than the single-layer HMDS_SiO2 coating. (Figure S2)


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Figure 7. Optical micrographs for the surface of coated KDP crystal: (a) single-layer HMDS_SiO2 coated KDP and (b) single-layer HMDS_SiO2 coated KDP after exposure in a humid environment for 6 months; (c) HMDS_SiO2/PS coated KDP crystal and (d) HMDS_SiO2/PS coated KDP crystal after exposure in a humid environment for 6 months. 3. 5. Laser-induced damage threshold Laser damage resistance of optical coatings is a crucial factor influencing its application in high power laser system, especially for laser frequency converters such as KDP crystals. Table 1 lists the different coatings coated FSG or KDP crystal at 355 nm with a pulse width of 3 ns. The measured LIDT of single-layer HMDS_SiO2 coated KDP crystal is 11.4 J/cm2, whereas that of the same coating coated FSG is 14.3 J/cm2. For double-layer HMDS_SiO2/PS coated substrates, it also shows a same behavior. The double-layer coating coated KDP possesses a LIDT of 11.5 J/cm2, whereas that coated FSG achieves a higher LIDT of 14.5 J/cm2. Such difference in


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LIDT between coatings on different types of substrates lies in the LIDT of the coating also greatly depends on the surface quality of the substrate33. Therefore, the LIDT of the coating on KDP crystal could be even greater than 11.5 J/cm2 if the surface of the KDP crystal could be specially treated and improved. Nevertheless, these LIDTs are still much higher than that of PVD coatings. In addition, by comparing single-layer HMDS_SiO2 coated substrates and double-layer HMDS_SiO2/PS coated substrates, we can find that the single-layer coating coated substrate shows nearly the same LIDT as the double-layer coating coated substrate no matter which kind of substrates they were coated on. It is generally recognized that the laser damage initiates at the interface between two materials34. Although one more interface exists in the double-layer coating coated substrate, it does not influence the LIDT of the coated FSG or KDP crystal. This finding further demonstrates that the LIDTs depend significantly on the substrate surface quality in our sol-gel coating coated optical components, rather than the coating itself. Table 1. LIDT values of different coatings coated FSG or KDP crystals (355 nm, 3ns) Type of coating structure

LIDT (J/cm2)

HMDS_SiO2/FSG

14.3

HMDS_SiO2/KDP

11.4

HMDS_SiO2/ PS/ FSG

14.5

HMDS_SiO2/ PS/ KDP

11.5

4. Conclusion In summary, a laser damage resistant and moisture-proof antireflective coating for KDP crystal was prepared using poly-siloxane (PS) coating as a bottom layer and


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HMDS_SiO2 coating as a top layer, respectively. The dense structure of PS along with its silanol side groups could effectively work for moisture barrier. HMDS was used to modify the colloidal SiO2 sol to make the surface of silica nanoparticles covered with trimethylsilyl groups. Meanwhile, the modified sol was also refluxed to be non-alkaline and non-aqueous, so that it was entirely harmless to the KDP crystal. The final HMDS_SiO2/PS coating coated KDP crystal exhibits superior moisture-proof and antireflective properties, as well as a high LIDT of 11.5 J/cm2 (355nm, 3ns). A special treatment to KDP crystal is needed to improve the surface quality of the substrate and further increase the LIDT of the coated KDP crystals. The excellent AR performance, facile and low-temperature preparation, along with its environmental stability also affords this broadband AR coatings potential application for thermal-sensitive polymeric substrates. ASSOCIATED CONTENT

Supporting Information

The following files are available free of charge on the ACS Publications website at DOI:.

Preparation procedure for hexamethyl-disilazane (HMDS) modified silica sols; abrasion

resistance

of

single-layer

HMDS_SiO2

and

double-layer

HMDS_SiO2/poly-siloxane (PS) coating coated glass before and after abrasion. (PDF) AUTHOR INFORMATION Corresponding Authors ACS Paragon Plus Environment

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*E-mail: [email protected] (X. Wang); [email protected] (J. Shen)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT

Financial support by the National Key Research and Development Program of China (2017YFA0204600), National Natural Science Foundation of China (U1230113 and 11304228), “Chen Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (14CG19), and Fundamental Research Funds for the Central Universities from Tongji University is highly appreciated. We kindly thank Dr. Haohao Hui and Dr. Qinghua Zhang from Chengdu Fine Optical Engineering Research Center for providing us the KDP crystals and the LIDT measurements.

REFERENCES

(1)

De Yoreo, J. J.; Burnham, A. K.; Whitman, P. K. Developing KH2PO4 and

KD2PO4 Crystals for the World's Most Power Laser. Inter. Mater. Rev. 2002, 47, 113

–152.


Page 20 of 34

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(2)

Xu, M.; Wang, Z.; Wang, B.; Liu, B.; Sun, X.; Xu, X. Study on Optical

Property of Rapid Growth KDP and DKDP Crystals. Chin. Opt. Lett. 2012, 10, S11602–311605.

(3)

Thomas, I. M. High Laser Damage Threshold Porous Silica Antireflective

Coating. Appl. Opt. 1986, 25, 1481–1483.

(4)

Wang, X.; Shen, J. A Review of Contamination-Resistant Antireflective

Sol–Gel Coatings. J. Sol-Gel Sci. Technol. 2012, 61, 206–212.

(5)

Wang, X.; Wu, G.; Zhou, B.; Shen, J. Improvement on Laser-Induced

Damage Threshold of Sol-Gel ZrO2 Coatings by Crystal Structure Tuning. Opt.

Express 2012, 20, 24482–24487.

(6)

Baisden, P. A.; Atherton, L. J.; Hawley, R. A.; Land, T. A. Large Optics for

the National Ignition Facility. Fusion Sci. Technol. 2016, 69, 295–351.

(7)

Tian, B.; Wang, X.; Niu, Y.; Zhang, J.; Zhang, Q.; Zhang, Z.; Wu, G.; Zhou,

B.; Shen, J. Preparation and Stress Evolution of Sol–Gel SiO2 Antireflective Coatings for Small-Size Anisotropic Lithium Triborate Crystals. AIP Adv. 2016, 6, 045208.

(8)

VanBlarcom, D.; Monticelli, M.; Davalos, G.; Miller, P.; Cross, D.; Carr, C.

W. Damage Performance of Sol-Gel Anti-Reflective Coatings for the National Ignition Facility. Optical Interference Coatings 2016, WD.10.

(9)

Belleville, P. F.; Floch, H. G. Ammonia Hardening of Porous Silica

Antireflective Coatings. Proc. SPIE 1994, 2288, 25–32.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(10)

Wheeler, E. K.; Whitman, P. K.; Land, T. A.; De Yoreo, J.; Thorsness, C. B.;

McWhirter, J. H.; Hanna, M. L.; Miller, E. L. Investigation of Etch Pits on KDP Crystals with Porous Sol–Gel Coatings. Appl. Phys. A 2002, 74, 813–823.

(11)

Wheeler, E. K.; Fluck, R.; Woods, B.; Whitman, P. K. Effect of Surface

Degradation on Optical Performance of Potassium Dihydrogen Phosphate Optics.

Appl. Opt. 2003, 42, 5545–5550.

(12)

Zhang, L.; Xu, M.; Liu, B.; Zhu, L.; Wang, B.; Zhou, H.; Liu, F.; Sun, X.

New Annealing Method to Improve KD2PO4 Crystal Quality: Learning from High Temperature Phase Transition. CrystEngComm 2015, 17, 4705–4711.

(13)

Murahara, M.; Sato, N.; Ikadai, A. Hard Protective Waterproof Coating for

High-Power Laser Optical Elements. Opt. Lett. 2005, 30, 3416–3418.

(14)

Murahara, M.; Yabe, T.; Uchida, S.; Yoshida, K.; Okamoto, Y.

Anti-Reflective and Waterproof Hard Coating for High Power Laser Optical Elements. AIP Conference Proceedings 2006, 830, 457–465.

(15)

Murahara, M.; Sato, Y.; Funatsu, T.; Jitsuno, T.; Okamoto, Y. Formation of a

Photo-Oxidized Protective Thin Film with Waterproof and High-Power Laser Tolerance Properties. Proc. SPIE 2008, 7132, 71320K–71320K–8.

(16)

Sato, Y.; Matsunaga, E.; Okamoto, Y.; Murahara, M. Photo-Oxidized

Coating on the Optical Materials for High Power Laser Resistance and Waterproof.

MRS Proceedings 2012, 1401.


Page 22 of 34

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(17)

Zhang, C.; Zhang, C.; Ding, R.; Cui, X.; Wang, J.; Zhang, Q.; Xu, Y. New

Water Vapor Barrier Film Based on Lamellar Aliphatic-Monoamine-Bridged Polysilsesquioxane. ACS Appl. Mater. Interfaces 2016, 8, 14766–14775.

(18)

Yang, Y.; Zhang, Q.; Xu, Q.; Li, W.; Wei, W.; Peng, B. Preparation and

Optical Property of a Novel Hydrophobic Anti-Reflection Coating for KDP Crystal.

Mater. Lett. 2008, 62, 2960–2962.

(19)

Zhang, X.; Zhang, Y.; Ye, H.; Xiao, B.; Yan, L.; Lv, H.; Jiang, B. Sol–Gel

Preparation of Antireflective Coatings at 351 nm with Different Thickness and Improved Moisture-Resistance. J. Sol-Gel Sci. Technol. 2011, 58, 340–344.

(20)

Zhang, X.-X.; Xia, B.-B.; Ye, H.-P.; Zhang, Y.-L.; Xiao, B.; Yan, L.-H.; Lv,

H.-B.; Jiang, B. One-Step Sol–Gel Preparation of PDMS–Silica ORMOSILs as Environment-Resistant and Crack-Free Thick Antireflective Coatings. J. Mater.

Chem. 2012, 22, 13132.

(21)

Zhang, Q.; Wei, Y.; Yang, W.; Hui, H.; Deng, X.; Wang, J.; Xu, Q.; Shen, J.

Improvement on Contamination Resistance to Volatile Organics and Moisture of Sol–Gel Silica Antireflective Coating for 351 nm Laser System by Structural Modulation with Fluorinated Compounds. RSC Adv. 2015, 5, 4529–4536.

(22)

Thomas, I. M.; Burnham, A. K.; Ertel, J. R.; Frieders, S. C. Method for

Reducing the Effect of Environmental Contamination of Sol-Gel Optical Coatings.

Proc. SPIE 1999, 3492, 220–229.


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(23)

Pareek, R.; Kumbhare, M. N.; Mukherjee, C.; Joshi, A. S.; Gupta, P. D.

Effect of Oil Vapor Contamination on the Performance of Porous Silica Sol-Gel Antireflection-Coated Optics in Vacuum Spatial Filters of High-Power Neodymium Glass Laser. Opt. Eng. 2008, 47, 023801–023801–5.

(24)

Stöber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica

Spheres in the Micron Size Range. J. Colloid Interf. Sci. 1968, 26, 62–69.

(25)

Zhang, X. X.; Cai, S.; You, D.; Yan, L. H.; Lv, H. B.; Yuan, X. D.; Jiang, B.

Template-Free Sol-Gel Preparation of Superhydrophobic ORMOSIL Films for Double-Wavelength Broadband Antireflective Coatings. Adv. Funct. Mater. 2013, 23, 4361–4365.

(26)

Innocenzi, P. Infrared Spectroscopy of Sol–Gel Derived Silica-Based Films:

A Spectra-Microstructure Overview. J. Non-Cryst. Solids 2003, 316, 309–319.

(27)

Wright, N.; Hunter, M. J. Organosilicon Polymers. Iii. Infrared Spectra of the

Methylpolysiloxanes. J. Am. Chem. Soc. 1947, 69, 803–809.

(28)

Zhang, X.; Zheng, F.; Ye, L.; Xiong, P.; Yan, L.; Yang, W.; Jiang, B. A

One-Pot Sol–Gel Process to Prepare a Superhydrophobic and Environment-Resistant Thin Film from ORMOSIL Nanoparticles. RSC Adv. 2014, 4, 9838–9841.

(29)

Wang, X.; Shen, J. Sol–Gel Derived Durable Antireflective Coating for Solar

Glass. J. Sol-Gel Sci. Technol. 2010, 53, 322–327.


Page 24 of 34

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(30)

Pritula, I. M.; Puzikov, V. M.; Salo, V. I.; Kolybayeva, M. I.; Garnov, S. V.;

Voznyi, V. L. Laser Damage Threshold and Optical Absorption of KDP Crystal Dependent on Their Growth Conditions. Proc. SPIE 1999, 3578, 501–508.

(31)

Wheeler, E. K.; McWhirter, J. T.; Whitman, P. K.; Thorsness, C. B.; De

Yoreo, J. J.; Thomas, I. M.; Hester, M. Scatter Loss from Environmental Degradation of KDP Crystals. Proc. SPIE 1999, 3902, 451–459.

(32)

Wang, X.; Tian, B.; Niu, Y.; Wu, G.; Zhou, B.; Shen, J. Preparation of

Moisture-Proof Protective Coatings for KDP/DKDP Crystals. Rare Metal Mat. Eng. 2016, 45, 370–374.

(33)

Chai, Y.; Zhu, M.; Yi, K.; Qi, H.; Wang, H.; Sun, W.; Yu, Z.; Bai, Z.; Zhao,

Y.; Shao, J. Coupling Effect of Subsurface Defect and Coating Layer on the Laser Induced Damage Threshold of Dielectric Coating. Proc. SPIE 2014, 9237, 92370P.

(34)

Xing, H.; Zhu, M.; Chai, Y.; Yi, K.; Sun, J.; Cui, Y.; Shao, J. Improving

Laser Damage Resistance of 355 nm High-Reflective Coatings by Co-Evaporated Interfaces. Opt. Lett. 2016, 41, 1253–1256.


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Table of Contents/Abstract Graphics


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Figure 1. Schematic representation of the reaction mechanism for the HMDS modified silica sol. 179x128mm (300 x 300 DPI)


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Figure 2. TEM images of standard SiO2 sol (a) and HMDS modified SiO2 sol (b). 39x19mm (300 x 300 DPI)


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Figure 3. FTIR spectra of unmodified and HMDS modified SiO2 coatings. 170x129mm (300 x 300 DPI)


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Figure 4. SEM photographs of (a) HMDS_SiO2 coating and (b) PS coating, (c) optical constants of HMDS_SiO2 coating and PS coating. 69x61mm (300 x 300 DPI)


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Figure 5. Schematic illustration (a) and cross-sectional SEM photograph (b) of double-layer HMDS_SiO2/PS coating. 24x7mm (300 x 300 DPI)


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Figure 6. Transmittance spectra of KDP crystal coated with double-layer HMDS_SiO2/PS coating (a) and single-layer HMDS_SiO2 coating (b). 55x20mm (300 x 300 DPI)


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Figure 7. Optical micrographs for the surface of coated KDP crystal: (a) single-layer HMDS_SiO2 coated KDP and (b) single-layer HMDS_SiO2 coated KDP after exposure in a humid environment for 6 months; (c) HMDS_SiO2/PS coated KDP crystal and (d) HMDS_SiO2/PS coated KDP crystal after exposure in a humid environment for 6 months. 59x44mm (300 x 300 DPI)


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Table of Contents 41x20mm (300 x 300 DPI)


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Sol-Gel Preparation of Laser Damage Resistant and Moisture-Proof

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