Graded-index anti-reflection coatings with high-power laser-induced damage resistance and high transmittance based on a SiO_xN_y layer deposited by PECVD

To address the issue that conventional anti-reflection coatings (ARCs) are vulnerable in high-power laser systems, this paper presents a design method for a graded-index ARC with laser-induced damage resistance and high transmittance, based on the SiO_xN_y layer prepared by PECVD techniques. Three graded-index ARCs are designed and prepared; the minimum average transmittance is 99.01 % of the graded-index ARCs is 0.21 % lower than that of the conventional ARCs. The laser damage resistance is predicted by calculating the electric field intensity distribution of all ARCs. The ARCs consisted of amorphous and nonstoichiometric SiO_xN_y films formed on glass substrates by PECVD techniques. Through determining the crystallite state, relative atomic concentration, Si-centred tetrahedral phase Si-Si_4-(ν+η)O_νN_η, refractive indices, and extinction coefficients of each layer, the microscopic mechanism of the effects of chemical bond properties on the refractive index is discussed in detail. The designed and measured transmittances of all graded-index ARCs were compared; the average transmittance maximum deviation of the single-sided ARCs over the visible spectrum is 0.1 %, which shows good agreement. The measured laser-induced damage threshold value of the 3rd graded-index ARCs is 7.15 J/cm², and the relative change improves to 161.90 %, compared to the conventional ARCs of 2.73 J/cm². The damage spot morphologies of high-power laser irradiation test demonstrate that the spot diameter was reduced by close to 85.33 % to conventional ARCs. These results are discussed in detail and are expected to serve as a reference for the preparation of high-power laser protection optical films formed via PECVD.