Robust meta-surface designs for ultra-high reflectivity in precision interferometry

Metasurfaces enable precise light manipulation, like fostering reflections close to unity, through resonance mechanisms. While traditional Bragg mirrors enable very high reflectivity they limit the achievable thermal noise. Meta-material-mirrors (MMM) can overcome the noise limitations but suffer from limited reflectivity. This trade-off is crucial for next-generation cryogenic gravitational wave detectors, such as the Einstein Telescope, which need high reflectivity and low thermal noise test mass coatings to achieve dramatic sensitivity. Hence, we are proposing a new combined design unifying the advantages from both approaches - composed of an MMM, a Fabry–Pérot spacer, and a Bragg mirror – achieving extremely high reflectance and low thermal noise. We are evaluating different 1D and 2D design approaches to achieve MMM robust to fabrication tolerances while offering broad, high reflection at 1550 nm. A key focus is on bandwidth, manufacturability, and thermal noise. This systematic analysis provides a pathway to promising MMM for production via e.g. character projection electron beam lithography, paving the way for high-performance mirrors in gravitational wave astronomy and beyond.