KDP类晶体功能光学膜的界面、应力、抗激光损伤及稳定性研究

KDP-type crystals, as frequency converters, are essential optical components for high power laser systems. However, it is easy to be deliquesced and fogged in the humid environment, which results in the reduction of working life of the crystals. A moisture-proof coating is therefore, desperately needed to protect the KDP-type crystals. Combined with the particularity of KDP-type crystals, we will start from the fundamental research of sol properties and coating structures, to study the relationship between the sol synthesis, control of the microstructure, micro-macro properties and the final properties of the coatings. This project will mainly focus on the low-temperature curing process and their effects on infiltration, inter-diffusion behavior between different coating layers and the interfaces; stress evolution of the moisture-proof coating layer and antireflective coating layer, and also the control methods of stress release between the coating layer and the crystal substrate, as well as between the different coating layers. To increase the laser-induced damage thresholds of the coatings, we will also investigate the methods to improve the surface/interface of coating layers, as well as those to reduce the defects and stress of coatings. Through the microstructure control and surface functionalization, the surface of the SiO2 particles could be passivated and the structure of the coating would be stabilized. The sol-gel AR coatings with excellent AR efficiency, high environmental stability and laser resistance then could be successfully prepared to achieve the new multifunction for KDP-type crystals, which could provide key technology for frequency conversion crystals for high power lasers.

KDP类晶体是高功率激光系统中必不可少的激光频率转换光学元件，但由于其在潮湿环境中极易潮解发雾，严重影响了晶体器件的使用寿命，通常需要在晶体表面镀制保护膜来保证它的稳定工作。本项目结合KDP类晶体的特殊性，从溶胶特性和膜层结构的基础性研究入手，探索溶胶合成、微结构控制及其微观、宏观特性与膜层最终性能之间的规律性关系。重点研究晶体表面膜层的低温固化工艺及不同工艺下膜层间的相互渗透、扩散行为和界面层；防潮膜层与减反膜层的应力演变规律及晶体基板与膜层、膜层与膜层间应力释放的有效控制方法。研究薄膜的表面/界面的改善以及膜层的缺陷和应力的降低方法，以提高其激光损伤阈值。通过对减反膜层的微结构调控和表面改性功能化，“钝化”氧化硅颗粒表面从而提高膜层的稳定性。获得高光学性能、耐环境稳定性和高抗激光损伤能力的工艺参数，实现KDP类晶体光学膜的新型复合功能，为高功率激光器倍频晶体元件提供必要的关键技术。

KDP类晶体是高功率激光系统中实现倍频转换的核心光学元件，但是其遇到水汽极易潮解发雾，通常需要在表面镀制防潮保护膜。此外，高透过率、高激光损伤阈值和耐环境稳定性也是用于KDP类晶体的保护膜必须具备的特性。本项目从致密的有机硅基防潮保护膜和多孔氧化硅减反膜出发，以溶胶改性修饰为主要手段，研究了适用于KDP类晶体的功能光学膜的溶胶合成工艺。通过含氟有机硅烷和聚合物前体对溶胶的改性，合成了两种致密防潮膜溶胶。氟硅烷改性的氧化硅膜表现出优异的防潮性能，镀有该薄膜的基板在高湿环境中放置3个月后的平均透过率仅下降了0.07%。深入探讨了薄膜微结构和表面化学成分对其防潮性的影响，以及二者之间的制约性关系。薄膜的防潮性能随孔体积的减小或表面能的降低显著提升。采用多种气相处理方法对氧化硅薄膜进行后处理，在嫁接疏水基团的同时使薄膜表面及内部“硬化”，有效提高了其耐环境稳定性、抗紫外损伤以及化学稳定性。氨气和六甲基二硅胺烷（HMDS）气氛联合处理的薄膜在真空油污环境和相对湿度大于90%的高湿环境中均表现出优异的耐环境稳定性。紫外辐照的低温固化方法既固化了膜层，又不影响薄膜的基本特性，特别适合用于对温度敏感的KDP类晶体。通过季铵盐对氧化硅溶胶的改性，使薄膜的抗菌率高达98.3%，赋予薄膜防霉、抗菌和增透的多功能一体化。针对溶胶-凝胶膜干燥过程的应力变化，通过溶剂替换方法有效控制了溶胶-凝胶薄膜的应力突变。在前述工作基础上，以聚硅氧烷致密薄膜作为底层防潮膜，HMDS改性后的SiO2薄膜作为顶层低折射率减反射膜，设计制备了适用于KDP晶体的折射率梯度双层减反膜系。镀制该双层膜的KDP晶体峰值透过率可达99.9%，在527 nm处的透过率达99.3%。置于高湿环境6个月后光学性能无明显变化，体现了极佳的耐环境稳定性。同时，其激光损伤阈值可达11.5 J/cm2（351 nm, 3 ns），可满足高功率激光装置的实际应用需求。本项目的研究成果为KDP类晶体的功能光学膜的制备与应用提供了必要的科学依据和工艺技术参考。