Cheng Xinbin, Tuniyazi Abudusalamu, Wei Zeyong, Zhang Jinlong, Ding Tao, Jiao Hongfei, Ma Bin, Li Hongqiang, Li Tongbao, Wang Zhanshan
Opt Express. 2015 Apr 6;23(7):8609-19. doi: 10.1364/OE.23.008609.
Although the finite-difference time-domain (FDTD) technique has been prevailingly used to calculate the electric field intensity (EFI) enhancement at nodular defects in high-reflection (HR) coatings, the physical insight as to how the nodular features contribute to the intensified EFI is not explicitly revealed yet, which in turn limits the solutions that improve the laser-induced damage threshold (LIDT) of nodules by decreasing the EFI enhancement. Here, a simplified model is proposed to describe the intensified EFI in nodules: 1) the nodule works as a microlens and its focal length can be predicted using a simple formula, 2) the portion of incident light that penetrates through the HR coating can be estimated by knowing the angular dependent transmittance (ADT) of the nodule, 3) strong EFI enhancement is created when the focal point is within the nodule and simultaneously a certain portion of light penetrates to the focal position. In the light of the proposed model, a broadband HR coating was used to reduce the EFI enhancement at the seed by a factor about 10, which leads to a 20 times increment of the LIDT. This work therefore not only deepens the physical understanding of EFI enhancement at nodules but also provides a new way to increase the LIDT of multilayer reflective optics.
尽管时域有限差分（FDTD）技术已被广泛用于计算高反射（HR）涂层中结节缺陷处的电场强度（EFI）增强，但关于结节特征如何导致EFI增强的物理见解尚未明确揭示，这反过来限制了通过降低EFI增强来提高结节激光损伤阈值（LIDT）的解决方案。在此，提出了一个简化模型来描述结节中增强的EFI：1）结节起微透镜的作用，其焦距可用一个简单公式预测；2）通过了解结节的角度相关透射率（ADT），可以估计穿透HR涂层的入射光部分；3）当焦点在结节内且同时有一定比例的光穿透到焦点位置时，会产生强烈的EFI增强。根据所提出的模型，使用宽带HR涂层将种子处的EFI增强降低了约10倍，这导致LIDT增加了20倍。因此，这项工作不仅加深了对结节处EFI增强的物理理解，还提供了一种提高多层反射光学元件LIDT的新方法。
