Designing a low-loss high reflectivity mirror for gravitational waves detectors by combining a dielectric metasurface and a multilayer stack

The design of new low-mechanical-loss, high reflectivity mirrors is crucial in the development of the next generation of gravitational-wave observatories. Currently, the state-of-the-art amorphous multilayer reflective coatings which are deposited at the surface of the test masses in interferometric gravitational-wave detectors present the limiting factor in detector sensitivity due to their thermal noise. These coatings require a large number of thin layers to achieve ultra-high reflectivity. However, the thermal noise generated by this type of stack increases with the number of layers used. These dielectric mirrors represent a very mature technology, with current research producing only incremental improvements, highlighting the need for new technical solutions that can address this specific issue. Here, we provide insights into the expected performance of mirrors that combine a resonant metasurface with a multilayer stack. The suggested mirror design ensures the high reflectivity required for interferometric gravitational wave detectors, while using fewer layers of properly selected materials. It allows to reduce the total thickness of the material with the poorest thermal-noise performance, namely TiO2:Ta2O5, by a factor of more than 3, making it a promising option for reducing thermal noise as well