Damage Characteristics of 500 ps Laser-conditioned Multilayer Dielectric High-Reflectivity Coatings under Picosecond Laser Irradiation

The development of picosecond (ps) petawatt laser systems proposes higher requirements on the laser damage resistance of multilayer dielectric (MLD) high-reflectivity (HR) coatings. Focusing on electron beam-deposited HfO2/SiO2 HR coatings, this study investigates the effects of 500 ps laser conditioning (LC) on the 1 ps, 10 ps, and 20 ps laser-induced damage resistance. Results demonstrate that conditioned samples exhibit negligible laser-induced damage threshold (LIDT) variations under 1 ps laser irradiation, while achieving 8% and 9% enhancements under 10 ps and 20 ps laser irradiation, respectively. The damage morphologies and defect density remain statistically unchanged before and after LC. Under 1 ps to 20 ps laser irradiation, the damage initiation processes in HR coatings involves the combined effects of electric field intensity (EFI) and defects. Although LC does not eliminate defects, it “passivates” defects-induced damage capability under 10 ps and 20 ps laser irradiation. The effect of LC on the 1 ps LIDTs of the sample is not significant, which should be related to the higher defect density induced by the 1 ps laser damage and the coupled non-linear ionization processes resulting from it. Based on the ionization rate equation and the two-temperature model, the mechanism of the LIDT improvement under 10 ps and 20 ps laser irradiation is analyzed: LC optimizes the distribution of localized defect energy levels, reducing the transition probability of the electron under 10 ps and 20 ps laser irradiation, thereby improving the coatings’ damage resistance. This study provides new insights for enhancing the ps laser damage resistance of MLD HR coatings.

皮秒（picosecond, ps）拍瓦（petawatt）激光系统的发展对多层介质（multilayer dielectric, MLD）高反射率（high-reflectivity, HR）薄膜的抗激光损伤性能提出了更高要求。本研究以电子束沉积的HfO₂/SiO₂高反射率薄膜为研究对象，探讨了500 ps激光预处理（laser conditioning, LC）对1 ps、10 ps及20 ps激光诱导抗损伤性能的影响。实验结果表明，经激光预处理后的样品在1 ps激光辐照下的激光损伤阈值（laser-induced damage threshold, LIDT）变化可忽略不计，而在10 ps和20 ps激光辐照下的LIDT分别提升了8%和9%。激光预处理前后，样品的损伤形貌与缺陷密度在统计学上无显著变化。在1 ps至20 ps的激光辐照条件下，高反射率薄膜的损伤起始过程由电场强度（electric field intensity, EFI）与缺陷共同作用导致。尽管激光预处理无法消除缺陷，但可“钝化”10 ps与20 ps激光辐照下由缺陷引发的损伤效应。激光预处理对样品1 ps激光损伤阈值的影响并不显著，这应与1 ps激光损伤诱导的更高缺陷密度及其伴随的耦合非线性电离过程相关。基于电离速率方程与双温模型（two-temperature model），本研究分析了10 ps与20 ps激光辐照下LIDT提升的机制：激光预处理优化了局域缺陷能级的分布，降低了10 ps与20 ps激光辐照下电子的跃迁概率，从而提升了薄膜的抗激光损伤性能。本研究为提升多层介质高反射率薄膜的皮秒激光抗损伤性能提供了新的研究思路。