光学薄膜材料极紫外辐射损伤机理研究

With the rapid development of Free-electron-laser (FEL) at the short wavelength regime, the study of the damage mechanism of the ultrashort powerful EUV & X-ray pulses on the optics has become a hot research topic recently. Due to the deficiency of short-wavelength intense light sources, the investigations of the radiation damage on the optical thin-films are rarely explored. For meeting the requirements of developing X-ray FEL in China, this project is planned to setup a wavelength-adjustable EUV micro-focusing light source for radiation damage researches based on the laser plasma source and Schwarzschild focusing system. By using this source, the monolayer films with different materials and different thickness,which are often used in FEL facility, are planned to be investigated at selected wavelengths for determining the damage threshold and the damage mechanism. Concerning the multilayer mirrors used in the FEL facility, it is also planned to study how to improve its damage threshold by adopting different material combinations and by using spacing layers having different materials. With the help of the finite element method, the damage mechanisms for the multilayer optics are planned to be figured out by comparing with the experimental results. The purpose of this project is to provide complemental information in the EUV damage research field. Furthermore, this research has great significance for developing the ability of designing the special beamline and optics in FEL field, as well as the future advanced experiments performed with FEL.

随着短波长自由电子激光技术的发展，超快、超强极紫外和X射线波段自由电子激光对薄膜光学元件辐射损伤问题正成为研究热点。由于高强度短波长光源的缺乏，薄膜材料的极紫外辐射损伤研究开展地极为有限。针对我国计划发展的X射线自由电子激光试验研究，本项目拟结合激光等离子体光源和Schwarzschild微聚焦光学系统，在实验室条件下建立波长可调的高强度极紫外微聚焦源，并利用此光源开展单层膜和多层膜光学元件的极紫外辐射损伤研究。分析不同材料、不同厚度的单层膜在不同入射波长条件下的损伤阈值以及损伤机理。针对自由电子激光实验中所使用的多层膜光学元件，研究不同材料对组合以及间隔层对于提高多层膜抗辐射损伤的能力，结合有限元等方法分析多层膜的损伤机理。本项目的开展将弥补国际上该研究领域的空白，对我国自由电子激光的光束线设计、光学元件的开发，以及将来利用自由电子激光开展科学实验研究都具有十分重要的意义。

我国正在大力发展短波长自由电子激光及其相关技术，其中自由电子激光对光学元件的辐照损伤是影响光源亮度提升以及光源品质的核心问题之一。针对该科学问题，本项目开展了极紫外波段单脉冲辐照损伤薄膜光学元件的实验和理论研究。依托实验室自身力量，课题组成功研发了光、机、电、控一体的高能量密度极紫外辐照损伤测试平台，该平台包括了激光等离子体光源、改进型Schwarzschild物镜、在线监测、真空以及LabVIEW控制系统等。在13.5nm波段，该装置可以产生能量密度高达2.3 J/cm^2的纳秒量级的极紫外光脉冲，可以针对大多数的薄膜光学元件开展EUV波段辐照损伤研究，整个系统的性能达到了国际领先水平。.. 利用该极紫外辐照损伤测试平台，本项目对Au单层膜、Mo/Si周期多层膜、PMMA和CaF2块体等材料开展了辐照损伤研究。对于Au和Mo/Si薄膜，实验发现其损伤机制主要为热熔融损伤，损伤阈值分别为1.37+/-0.12 J/cm^2和1.02+/- 0.9 J/cm^2。PMMA材料的损伤机制为光化学反应，通过测量PMMA损伤坑的形貌我们可以准确获得聚焦光的强度分布信息。对于CaF2材料，实验中发现其损伤阈值为1.39+/-0.13 J/cm^2，虽然该材料的损伤机制也是属于热熔融损伤，但是实验发现其损伤阈值可能与CaF2的晶相有关。.. 基于该损伤测试平台，将来我们可以进一步针对我国自由电子激光用光学元件开展辐照损伤研究，通过实验获得实际的损伤阈值，分析提高和改善光学元件极损伤阈值的方法，为我国自由电子激光的顺利开展提供技术支持。