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

Abstract

Optical thin films are used in many high-end applications including quantum sensing, atto-second physics, and inertial confinement fusion. The optics in these systems are often limited by defects within the coating. These defects may lead not only to increases in optical losses, but also a reduction in the laser induced damage threshold (LIDT). Understanding how defects influence the optical parameters of the coating is critical to improve the quality of coatings. In this project, in cooperation with the Tongji University, we aim to improve understanding of the interaction between ultra-short pulse irradiation and two defect types. The first are extrinsic defects, specifically, particles on the substrate overcoated during deposition, while the second are ones which, among other effects, lead to changes in the electronic structure of the coating to include electronic states which are located between the valence and conduction bands. These are often caused by ternary elements contaminated, or variations in the relative chemical composition of the coating. By preparing samples with known fused silica particles, direct investigations of their influence are performed. The use of oblique angle ion etching is investigated as a process to reduce the scattering losses of the coating. While particles were effectively removed, this resulted in an increase in scattering losses, so instead the effectiveness of etching to increase the LIDT were examined. This was demonstrated as a sample prepared with 0.5 μm particles, which was etched for 1 μm and reached similar LIDT values of a coating on a substrate without particles. Using this experience, a GDD mirror was coated as a demonstrator optic for real life applications. While electronic defects can lead to an increase in linear absorption, they also significantly influence the nonlinear absorption, and as a result, the LIDT under ultra-short pulse irradiation. A measurement system based on laser calorimetric absorption (LCA) was developed to allow for the measurement of nonlinear absorption. This development included extensive work investigating methods to improve both the efficiency and effectiveness of the measurement. Existing models from nonlinear absorption are expanded to account for the presence of electronic defect states. These models were successfully correlated with nonlinear absorption measurements to demonstrate the influence of inter band structure, giving new insights into the nature of potential ultra-short pulse damage interactions. Two peer-reviewed papers were published in Applied Optics presenting some of the project results. In particular, the removal of particles through ion etching was presented in 2023, while a model for defect driven nonlinear absorption was published in 2026. The results were also presented at several international conferences and published in associated proceedings. Additionally, three students completed their master’s thesis within the remit of this project, and the work is expected to form the basis of the thesis of the involved PhD student.