摘要:
在近红外反射类激光薄膜中，节瘤缺陷是引起薄膜激光损伤的主要因素。为了提高激光薄膜的损伤阈值，对节瘤缺陷及其损伤特性进行研究具有重要意义。从“真实”节瘤缺陷和“人工”节瘤缺陷两个方面介绍节瘤缺陷的研究进展。基于“真实”节瘤缺陷的研究，建立了节瘤缺陷的结构特征，形成了节瘤缺陷损伤特性和损伤机制的初步认识，利用时域有限差分法(FDTD)模拟了电场增强，初步解释了节瘤缺陷的损伤机制，发明了抑制节瘤缺陷种子源的方法和激光预处理技术，减少了节瘤缺陷，提高了薄膜损伤阈值。但是“真实”节瘤缺陷的性质，如种子源尺寸、吸收性以及位置深度等，都难以控制和预测，难以开展节瘤缺陷损伤特性的系统和量化研究，致使关于节瘤缺陷损伤的科学认识尚有不足。基于“人工”节瘤缺陷的研究，可以实现节瘤缺陷损伤特性的系统、量化甚至单一因素研究，极大地提高了实验研究的效率和可靠性，获得了一系列定量损伤规律。“人工”节瘤缺陷的高度受控性使实验研究与理论模拟的可靠对比成为可能，“人工”节瘤缺陷的损伤形貌和FDTD 电场模拟的直接比较实验不仅验证了时域有限差分法(FDTD)模拟电场的正确性，也进一步明确了电场增强是诱导节瘤缺陷损伤的主要机制。对节瘤缺陷的损伤机制有了更为深刻的认识后，人们开始调控节瘤缺陷的电场增强效应提高节瘤缺陷的损伤阈值，发展了宽角度反射薄膜技术和节瘤缺陷平坦化技术，抑制电场增强，提高损伤阈值。这扩展了控制节瘤缺陷的思路和方法，从原来单一的去除节瘤缺陷到调控节瘤缺陷，为进一步提高薄膜的损伤阈值开辟了新的方向和途径。
Abstract:
In near infrared high reflectors,nodular defects are the main factor inducing the laser damage of optical coatings. In order to improve the laser-induced damage threshold (LIDT)of the optical coatings,it is necessary to study the damage charac-teristics of nodules.In this paper,the studies on laser-induced damage of nodular defects is reviewed from two aspects:the real nodules and the artificial nodules.For real nodules,the geometries of nodular defects were established and the preliminary under-standing of the damage mechanisms of nodular defects was achieved.The finite-difference time-domain (FDTD)technique has been successfully used to simulate the electric-field intensity enhancement in nodules,which helps to explain the damage mecha-nism of nodules.The experimental methods of eliminating the seeds and laser conditioning have been proposed to control nodular defects and to improve LIDT.However,the properties of real nodules,such as the diameters,absorption and lodging depth of seeds,are quite diverse and very difficult to predict,control and reproduce.This makes the systematical and quantitative study of nodular damage quite challenging,and the understanding of the nodular damage mechanisms is still insufficient.Artificial nodules offered the opportunity to study their damage behaviors systematically,quantitatively or even in a single factor manner,which greatly improved the efficiency and reliability of the experimental studies.In addition,a direct comparison between the experimen-tal results and theoretical simulation results could be achieved using the well-controlled artificial nodules.The damage morpholo-gies of artificial nodules were almost the same with the FDTD simulated electric-field intensity distributions,which not only dem-onstrates the correctness of the simulated results but also further illustrates that the electric field enhancement is the main mecha-nism of inducing nodular damage.After having a deeper understanding of the nodular damage mechanisms,the broadband coating and planarization technique have been used to suppress the electric field enhancement and to improve the LIDT of nodules.This extends the ideas and methods dealing with nodular defects,from solely removing nodular defects to controlling nodular defects, which opens up new directions for improving the LIDT of laser coatings.
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