Substoichiometric silica in a multimaterial highly reflective coating

Gravitational-wave detectors impose exceptionally rigorous demands on optical coatings, which include high reflectivity, low thermal noise, low absorption, low scatter, and minimal defect counts. Simultaneously satisfying the thermal noise and absorption requirements is particularly challenging. One promising approach is to use three or more materials in a single coating. In this work we characterize such a multimaterial coating made using low index silica (SiO₂), high index tantala (Ta₂O₅), and high index substoichiometric silica (SiO). We find that by using a multi-material structure, we achieve a more than 1000-fold reduction in optical absorption relative to a mirror made solely of SiO and SiO₂. Exploration of the SiO_x deposition parameter space may allow for further optimization and we present one possible optimized coating design that may reduce coating thermal noise by 20% relative to the coatings used in current detectors.