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

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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.