Optical losses in SiN _x and SiO _x N _y coatings deposited by plasma-enhanced chemical vapor deposition forgravitational 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 (<i>S</i><i>i</i><i>N</i>_<i>x</i>) and silicon oxynitride (<i>S</i><i>i</i><i>O</i>_<i>x</i><i>N</i>_<i>y</i>) 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 <i>S</i><i>i</i><i>N</i>_<i>x</i> and <i>S</i><i>i</i><i>O</i>_<i>x</i><i>N</i>_<i>y</i> 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 <i>S</i><i>i</i><i>N</i>_<i>x</i>/<i>S</i><i>i</i><i>O</i>_<i>x</i><i>N</i>_<i>y</i> 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.