Theory and control technique of optical loss in multilayer coatings used in femtosecond laser cavities

飞秒增强腔(ECs)的发展利用了光谱学方面的许多突破性成果，包括瞬时吸收光谱、直接频率梳状光谱和拉曼光谱等许多研究领域。除了提高宽带吸收光谱的灵敏度外，还需要高强度来驱动非线性过程，例如在多兆赫重复频率下产生高次谐波。宽带、低损耗的色散镜是这些系统的关键部件，限制了泵浦激光器的性能。腔镜的损耗直接影响到检测的精细度，限制了对痕量原子的检测灵敏度。此外，除了飞秒增强腔外，宽光谱、低损耗和良好控制的色散的独特组合在高功率飞秒振荡器技术领域具有重要意义。因此，宽带、低损耗的色散反射镜是飞秒激光器和光学镀膜界最重要的研究领域之一，ECS的光学带宽主要受群延迟色散(GDD)的限制，但也受到光学损耗的显著影响。提出的研究项目的目标集中在对限制超短脉冲应用的介质薄膜系统的带宽和光学质量的机理的基本了解上。预期的研究成果将为宽带色散镜GDD的详细控制铺平道路。在计划中的补充研究战略中，EC的精巧将通过从根本上了解光学损耗的潜在机制，并开发新的技术来减少宽带色散镜的光学损耗。为了了解超短脉冲宽度下的散射和吸收的基本机制，本研究项目将从两个主要方面着手：一方面是材料的本征光学性质与微结构的结合，另一方面是介质多层膜系统中辐射和缺陷的相互作用。应通过使用有或没有嵌入人工缺陷的模型涂层、使用涂层材料混合物和创新的沉积技术来实现这一理解。导出的散射和吸收模型将被用于开发用于色散反射镜的创新的多层膜设计。缺陷缓解方法将在项目期内得到验证。通过将这种缓解方法与新型涂层设计相结合，将开发出具有更低光学损耗的超快应用的多层反射镜。

The development of femtosecond enhancement cavities (ECs) has leveraged a number of breakthrough achievements in spectroscopy, including transient absorption -, direct frequency comb -, and Raman spectroscopy among many other research areas. In addition to boosting the sensitivity of broadband absorption spectroscopy, high intensities are required to drive nonlinear processes such as high harmonic generation at multi-megahertz repetition rates. Broadband, low-loss dispersive mirrors are the key components of these systems, limiting the performance of the pump laser. The loss of the cavity mirrors directly affects the finesse, limiting the sensitivity for the detection of trace amounts of atoms. Moreover, beyond femtosecond enhancement cavities, the unique combination of broad spectra, low losses and well-controlled dispersion is of high importance in the field of high-power femtosecond oscillator technology. Therefore, broadband, low-loss dispersive mirrors are one of the most prominent research fields in the femtosecond laser and optical coating community.The optical bandwidth of ECs is mainly limited by the group delay dispersion (GDD), but also significantly by optical losses. The targets of the proposed research project are concentrated on a fundamental understanding of the mechanisms limiting the bandwidth and the optical quality of dielectric thin film systems for ultrashort pulse applications. The expected research results will pave the way towards a detailed control of the GDD of broadband dispersive mirror. In the planned complementary research strategy, the finesse of EC will be addressed by fundamentally understanding the underlying mechanisms of optical losses, and developing novel techniques to reduce them for broadband dispersive mirrors.For attaining an understanding of the fundamental mechanisms of scattering and absorption in the ultrashort pulse duration regime, two major aspects will be addressed within this research project: Material intrinsic optical properties in combination with the microstructure on the one hand side, and the interaction of radiation and imperfections in the dielectric multilayer system on the other hand side. The understanding shall be attained using model coatings with and without embedded artificial defects, using coating material mixtures and an innovative deposition technology. The derived models for scattering and absorption shall be applied to develop innovative multilayer coating designs for dispersive mirrors. A defect mitigation approach will be validated within the project period. With a combination of this mitigation approach and the novel coatings designs, multilayer mirrors fur ultrafast applications with reduced optical loss will be developed.