Visualization of nanometer scale defects responsible for optical loss and laser induced breakdown in binary coating materials for the UV spectral region

目前，紫外光学元件被认为是光子学和激光技术中许多创新应用的重要关键部件。在许多其他领域中，大面积的紫外激光材料加工，激光医学，特别是半导体光刻对光学紫外组件的质量提出了苛刻的挑战。器件的光损耗和激光功率处理能力对于激光概念及其应用的成功至关重要。激光诱导的光学元件击穿发生在较低的辐照强度下，这是基于光电离损伤理论所预测的。几十年来，人们普遍认为光学损伤阈值的降低和光损失的增加是由缺陷引起的，如纳米级颗粒、空洞、界面以及化学计量缺陷。这些缺陷可以嵌入薄涂层中，在层或基板界面处，或在基板表面以下。尽管这一理论在光学学界得到了广泛的接受，但这些缺陷要么是无法直接观察到的，要么是无法直接验证它们对损伤行为的影响。在提出的研究工作中，计划确定和研究紫外激光应用的光学涂层中光学损耗和激光诱导击穿的主要贡献。如前所述，这一挑战的原因是建议的缺陷尺寸为几纳米。本提案中用于识别导致光学击穿的纳米缺陷的方法是双轨的：一方面，应将相应的人工缺陷纳入最先进的光学涂层中以重现光学损伤行为，另一方面，应开发光学探针技术，以获得纳米尺度上的光学性质的拓扑结构。

Nowadays UV optics are considered as important key components for many innovative applications in photonics andlaser technology. Among many others, the large areas of UV laser material processing, laser medicine or particularly,semiconductor lithography impose demanding challenges onto the quality of optical UV-components. Both the opticalloss and the laser power handling capability of the components are of outmost importance for the success of a laserconcept and its applications. Laser induced breakdown of optical components occurs at much lower irradiationintensity as predicted by theories based on damage due to photoionization. Since decades, it is a well-acceptedinterpretation that the reduction of optical damage threshold, and also the increase of optical losses are caused bydefects, such as nanometer-sized particles, voids, and the interfaces as well as stoichiometric deficiencies. Thesedefects could be embedded in the thin coating layer, at layer- or substrate interfaces, or below the substrate surface.Despite the broad acceptance of this theory in the optical community, mostly these suggested defects either remainedinvisible for direct observation, or it was not possible to verify their influence on the damage behavior directly.In the proposed research endeavor, it is planned to identify and study the dominant contributions to optical losses andlaser induced breakdown in optical coatings for UV-laser applications. As mentioned before, the reason for thischallenge is the suggested defect size of a few nanometers. The approach within this proposal for identifying nanodefectsresponsible for optical breakdown, is double-tracked: On the one hand, corresponding artificial defects shall beincorporated in state of the art optical coatings to reproduce optical damage behavior, and on the other hand an opticalprobe techniques shall be developed, to obtain topographies of optical properties on the nanometer scale.