Effect of subsurface impurity defects on laser damage resistance of beam splitter coatings

The laser-induced damage threshold (LIDT) of plat laser beam splitter (PLBS) coatings is closely related to the subsurface absorption defects of the substrate. Herein, a two-step deposition temperature method is proposed to understand the effect of substrate subsurface impurity defects on the LIDT of PLBS coatings. First, BK7 substrates are heat-treated at three different temperatures. The surface morphology and subsurface impurity defect distribution of the substrate before and after the heat treatment are compared. Then, a PLBS coating consisting of alternating HfO2–Al2O3 mixture and SiO2 layers is designed to achieve a beam-splitting ratio (transmittance to reflectance, s-polarized light) of approximately 50: 50 at 1053 nm and an angle of incidence of 45°, and it is prepared under four different deposition processes. The experimental and simulation results show that the subsurface impurity defects of the substrate migrate to the surface and accumulate on the surface during the heat treatment, and become absorption defect sources or nodule defect seeds in the coating, reducing the LIDT of the coating. The higher the heat-treatment temperature, the more evident the migration and accumulation of impurity defects. A lower deposition temperature (at which the coating can be fully oxidized) helps to improve the LIDT of the PLBS coating. When the deposition temperature is 140 ℃, the LIDT (s-polarized light, wavelength: 1064 nm, pulse width: 9 ns, incident angle: 45°) of the PLBS coating is 26.2 J/cm2, which is approximately 6.7 times that of the PLBS coating deposited at 200 ℃. We believe that the investigation into the laser damage mechanism of PLBS coatings will help to improve the LIDT of coatings with partial or high transmittance at laser wavelengths.

平板激光分束器（plat laser beam splitter, PLBS）涂层的激光诱导损伤阈值（laser-induced damage threshold, LIDT）与基底的亚表面吸收缺陷（subsurface absorption defects）密切相关。本文提出一种两步沉积温度法（two-step deposition temperature method），以探究基底亚表面杂质缺陷（subsurface impurity defects）对PLBS涂层LIDT的影响。首先，对BK7基底进行三种不同温度的热处理，比较热处理前后基底的表面形貌和亚表面杂质缺陷分布。随后，设计由HfO₂–Al₂O₃混合物层与SiO₂层交替组成的PLBS涂层，以在1053 nm波长、45°入射角下实现约50:50的分束比（beam-splitting ratio，透射率与反射率之比，s偏振光），并在四种不同沉积工艺下制备该涂层。实验与模拟结果表明，热处理过程中基底的亚表面杂质缺陷会迁移至表面并在表面积累，成为涂层中的吸收缺陷源（absorption defect sources）或结节缺陷种子（nodule defect seeds），从而降低涂层的LIDT。热处理温度越高，杂质缺陷的迁移与积累越显著。较低的沉积温度（涂层可完全氧化的温度）有助于提高PLBS涂层的LIDT。