Facile Fabrication of Graded-Refractive-Index SiO2 Antireflective Films with Enhanced Laser Damage Resistance

The graded-refractive-index SiO2 antireflective (AR) films were prepared using the sol-gel method. A monolayer with a porous nanonetwork structure was designed as the mixed volume ratio of 70:30 between the alkali- and acid-catalyzed SiO2 sols. As the top layer, the hexamethylisilazane (HMDS) modified alkali catalyzed SiO2 sols induced SiO2 nanoparticles transforming from hydrophilic to hydrophobic nature. In comparison with the structures of monolayer and bilayer films, the AR properties and laser damage resistance were investigated in detail. By designing the nanostructures of the bottom and top layers, the graded-refractive-index SiO2 film was obtained, consequently enhancing the AR performance. In the bilayer film, the transmittance reached similar to 98%@532 nm and 97%@1064 nm, respectively, corresponding to the obvious reduction of reflectivity. The assembly of the bilayer SiO2 film increased the laser-induced damage threshold by similar to 30% from 25.3 J/cm(2 )(monolayer) to 33.1 J/cm(2) (bilayer). The inducement of laser damage was further revealed by the analysis of morphological changes induced by laser damage, attributing to the initiator absorption of environmental contaminations. Therefore, the SiO2 film not only provided a graded-refractive-index structure with good AR performance, but also had high laser damage resistance. The unique graded-refractive-index structure of SiO2 AR films offers a new strategy for improving the performance of optical components in high-power laser systems.