Optical losses in SiN_x and SiO_xN_y 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 (SiO_xN_y ) thin films deposited by the plasma-enhanced chemical vapor deposition technique. We report an efficient and repeatable procedure to tune the refractive index of both SiN_x and SiO_xN_y thin films, while preserving a low optical absorption and scattering in the range of ppm. Finally, we demonstrate the design, fabrication, and characterization of an SiN_x /SiO_xN_y multi-layer stack composed of 20 layers with a total thickness of 3.9 µm, achieving a reflectance above 99% (and a low absorbance of 0.259%) in the near-infrared region, which is a promising step toward meeting the optical requirements of the third generation of GWDs.