Optical losses in SiNx and SiOxNy coatings deposited by plasma-enhanced chemical vapor deposition for gravitational wave detectors

Gravitational wave detectors (GWDs) rely heavily on low mechanical and optical loss mirror coatings to detect cosmic events happening in the Universe. This work discusses optical losses through light absorption and scattering mechanisms in silicon nitride (SiNx) and silicon oxynitride (SiOxNy) thin films deposited by plasma-enhanced chemical vapor deposition technique. We report an efficient and repeatable procedure to tune the refractive index of both SiNx and SiOxNy thin films, whilst preserving a low optical absorption and scattering in the range of ppm. Finally, we demonstrate the design, fabrication and characterization of a SiNx/SiOxNy multi-layer stack composed of 20-layers with a total thickness of 3.9µm, achieving a reflectance above 99% (and a low absorbance of 0.259%) in the near infrared region, which is a promising step towards meeting the optical requirements of third generation of GWDs.