Fabrication-Aware Design of a Hybrid Metasurface-Bragg Mirror for Low-Noise Precision Optics
High-reflectivity coatings for precision interferometry must simultaneously minimize optical losses and thermally driven displacement noise. Dielectric Bragg mirrors provide robust high reflectance but rely on thick multilayer coatings, whereas metasurface mirrors provide high reflectance and low noise but are sensitive to fabrication-induced deviations. A fabrication-aware hybrid mirror concept is introduced that combines a resonant single-layer metasurface, an etch-stop layer, an antiresonant spacer, and a reduced Bragg reflector. Fabrication effects, including geometric tolerances and line-edge roughness, are explicitly considered in the optical design. Full-wave electromagnetic simulations are combined with a truncated-Gaussian Monte Carlo analysis to determine the performance distribution under the assumed fabrication conditions. The ideal metasurface design exceeds $99.999\%$ reflectance. After roughness-aware reoptimization, the modeled non-reflected power remains below $2.88\cdot10^{-4}$ at $95\%$ fabrication yield. For the cryogenic ETpathfinder gravitational-wave testbed, three Bragg layer pairs reduce the non-reflected power of the complete stack to approximately $6.4\,\mathrm{ppm}$. The estimated thermal displacement-noise amplitude spectral density is $8.4\cdot10^{-21}\,\mathrm{m\,Hz^{-1/2}}$ at $100\,\mathrm{Hz}$. The architecture connects fabrication robustness, optical performance, and thermal-noise reduction within a single design framework.