Adjoint-based optimization of dielectric coatings for refractory metals to achieve broadband spectral reflection

Abstract

Refractory metals, which include niobium, tantalum, molybdenum, and tungsten, are critical components in applications in extreme environments due to their attractive thermomechanical properties. However, their low reflectivity below 1500 nm has prompted researchers to focus on increasing their reflection at shorter wavelengths. In this study, we applied an adjoint-based optimization technique to improve the spectral reflectivity of refractory metals in the broadband spectrum (300–3000 nm). An optimized periodic multilayer consisting of SiO2/TiO2 is selected as a starting point for the process. Then, the adjoint-based method is implemented to enhance the reflection of the surfaces. This approach involves an iterative procedure that guarantees improvement in every iteration. In every iteration, both the direct and adjoint solutions of Maxwell’s equations are computed to predict the scattering characteristics of a particular microstructure on a surface and measure its effectiveness. The results of our study indicate that the final designs not only increase reflectivity to over 90% but also have thermomechanical benefits that make them suitable for use in harsh environments. We also explored the effect of initial geometry on the results. Overall, our study shows that the adjoint-based optimization technique is an effective method for creating high-performing broadband reflectors with refractory metal substrates coated with dielectric multilayers.