研究熔石英光学元件亚表面纳米杂质粒子的特征和分布规律

The nanoscale impurities on the subsurface of fused silica, absorbing the light, can initiate breakdown at fluences or intensities far below the intrinsic damage threshold，which remains a key limitation for large aperture, high-power laser systems. It is nesscessary to investigate the nature and distribution of potential impurities for indentifying the origin of damage. The threshold equation associated with the physical properties and nonlinear absorption coefficient has been development.According to damage test,the nonlinear absorption coefficients of different impuritities have been investigated during thermal explosion. The absorbtivity and damage threshold in a specific area on the suface of fused silica has been detected,respectively, with the technology of photothermal deflection and dark-field imaging.Furthmore, considering the Mie theory and threshold equation, the complex index and size of nanoscale impurities in the specific area can be analyzed. Furthermore,by the detection of species of impurities from SIMS, the nature of potential impurities can be acknowledged. The statistical behaviors of laser damage on the surface of fused silica have been test under the irraddiation of different fluences。Based on binomial distribution, the damage probability versus laser fluence can be fit. In order to obtain the distribution of nanoscale impurities, a model for extracting the damage density from the curve of damage probability has been present. The research for this project will provide a critical acknowlede of the nature and distribution of the most hazardous nanoscale impurities and also a important direction for improvement of damage resistance.

熔石英光学元件亚表面的纳米杂质粒子能够吸收激光能量，在远低于本征损伤阈值的激光辐照条件下引起损伤的发生，因而成为限制大口径高功率激光系统发展的一个关键因素。为了探索特定区域损伤的起源，有必要研究潜在纳米杂质粒子的特征和分布规律。建立联系杂质粒子物理特性和非线性吸收系数的阈值方程，结合损伤实验分析杂质粒子在热爆炸过程中吸收系数的非线性变化规律。采用光热偏转和暗场成像技术分别对熔石英光学元件表面特定区域的吸收和损伤阈值进行测量，并基于Mie理论和阈值方程分析该区域潜在杂质粒子的复折射率和尺寸，进而通过SIMS分析损伤区域的杂质种类以揭示潜在的杂质特征。测试不同能量密度辐照下熔石英光学元件表面损伤统计规律,建立通过损伤几率提取损伤密度的理论模型以获取各种杂质粒子的分布规律。本项目的研究有助于认识最易诱导损伤的杂质粒子的特征和分布，同时为提高光学元件的抗损伤能力提供了指导。

能源短缺问题一直备受各国关注，如今能源消耗日益紧张，探求一种新的能源便显得尤其重要，人们纷纷注意到一种清洁、绿色的能源——核聚变能。为了达到实现聚变的条件，激光器要持续运行在高达几个兆焦耳的能量负载下。在如此之高通量的激光辐照下，装置中的光学元件，尤其是运行在紫外频段的终端组件中的熔石英光学元件极易发生激光诱导损伤。然而，光学元件在加工和制备过程中其亚表面会引入大量的杂质粒子，这些杂质粒子的存在会使得光学元件表面的损伤阈值远低于其本征损伤阈值。而且激光诱导损伤的尺寸在后续激光脉冲的辐照下将以指数形式持续的增长，最终导致激光装置输出光束的质量下降、能量降低。因此，提高熔石英光学元件在紫外纳秒激光辐照下的激光诱导损伤阈值是实现大型高功率固体激光装置高效、稳定运行的关键之一。本项目基于Mie 理论和热传导方程，结合ICP-OES和SIMS对熔石英亚表面杂质粒子的主要成分测量，建立了计算吸收性杂质粒子诱导熔石英光学元件表面损伤概率的模型。通过该模型分析了潜在纳米杂质粒子的分布特征和尺寸特性。揭示了最易造成光学元件损伤的杂质粒子的种类、尺寸以及尺寸分布，有助于在光学元件的加工过程中排除这些杂质粒子的引入，以提高光学元件的抗损伤性能，具有重要的应用价值。