Sol-gel deposition of highly reflective multilayer coatings from PVP-ZrO2 hybrid systems

A series of ZrO2 and polyvinylpyrrolidone (PVP) doped-ZrO2 sols have been synthesized by the controlled hydrolysis and condensation of zirconium n-propoxide, with diethanolamine (DEA) as the chelating agent. PVP-ZrO2 hybrid monolayers and highly reflective (HR) multilayer coatings were then prepared by spin-coating the stable sols on K9 glass substrates. The fabrication of the HR coatings involves alternate depositions of PVP-ZrO2 (as the high-index material) and SiO2 (as the low-index material) layers. The microstructures of the sols, the structural and optical properties, as well as the laser damage resistance of the films were thoroughly characterized. By virtue of the small angle X-ray scattering (SAXS) analysis, the sol-gel processing parameters were successfully optimized, endowing the as-prepared films with good optical properties and high laser damage resistance. SAXS studies also offer the possibility to a good knowledge of the microstructure of the sols. In our hybrid system, the unhydrolyzed DEA ligands, which become anchored in the inorganic network, may considerably weaken the effects of PVP on the formation and growth of ZrO2 particles. Hence, the incorporation of a proper amount of PVP gives rise to negligible change of the sol structure and only a slight variation in refractive index and laser damage resistance of the films. However, at the slight expense of the refractive index and laser damage resistance, the incorporation of PVP can easily alleviate the stress incompatibility between different layers and facilitate the fabrication of the multilayer coatings. As the mass fraction of PVP reaches 15%-20%, a perfect match between different layers can be achieved, which ensures the successful deposition of 21-layer, nearly full-reflection coatings with the minimum transmittance of about 1.6%-2.1% (at 1064nm) and high laser damage threshold of 16.4-18.2J/cm(2) (at 1064nm, Ins pulse duration and R/1 mode).