缺陷诱导光学薄膜光场增强损伤分析

高损伤阈值的光学薄膜是高功率激光系统的关键器件。众多研究显示，纳米量级的缺陷是光学薄膜激光损伤的主要诱因，是制约光学薄膜向高损伤阈值发展的主要因素。基于有限差分时域方法分析了纳米大小的缺陷诱导SiO₂光学薄膜的局部光场增强导致的激光损伤。结果显示:缺陷的存在使SiO₂单层薄膜的光场分布发生了变化，无缺陷的SiO₂薄膜峰值光场位于膜层表面，而有缺陷的SiO₂薄膜峰值光场位于缺陷与薄膜的边界处，光场增强了约2.3倍；同时缺陷诱导的光场局部增强不仅依赖于缺陷与膜层之间的相对折射率，而且也依赖于缺陷的大小、缺陷在膜层中的分布深度，以及入射激光波长。缺陷与膜层的相对折射率越大，缺陷的直径越大，缺陷在膜层中的深度越小，入射激光波长越短，光场增强越大。研究结果显示光学薄膜中纳米大小的缺陷诱导的光场增强不可忽视，在研究光学薄膜的激光损伤过程中应予以考虑。

Analyses of light field enhancement damage induced by defects in optical thin films

Optical thin films with high laser-induced damage thresholds are the pivotal components in high power laser systems. A lot of studies have shown that nano defects locating in optical thin films are the main factors which can induce the decreasing of the laser-induced damage thresholds. And, defects have been proved to be the main restrictions in developing optical thin films with the high laser-induced damage thresholds accordingly. Based on the finite-difference time-domain (FDTD) algorithm, the laser-induced damage caused by the local light field enhancement due to the nano defect lodging in SiO₂ single layer thin film was analyzed. The modeling results showed that the nano defect caused the changes of light field distribution in SiO₂ single layer thin films. For the SiO₂ thin film without defect, the peak of light field located on the surface of film layer, whereas the maximum of light field lodged on the boundary between the defect and the film layer when the defect existed in the SiO₂ thin film. Moreover, a light field intensification as large as 2.3X in the SiO₂ thin film with defect occurred in contrast to the condition of absence of defect. In addition, the local light field intensification induced by defect in optical thin film depended on not only the relative refractive index between the defect and the film layer, but also the defect diameter, the embedded depth of the defect and the wavelength of incident laser. And the larger the relative refractive index between the defect and the film layer, the larger the defect diameter, the shallower the embedded depth of the defect lodging in the film, and the shorter the wavelength of the incident laser are, the stronger the laser field intensifications are. This study can clearly indicate that the light field intensification induced by a nano-defect cannot be neglected, and such nano-defect should be taken into consideration when investigating the laser induced damage of optical thin-films.