SiO₂ antireflective films prepared using a Sol-gel method are an important feature of high-power laser facilities, and a significant demand exists for third-harmonic porous SiO₂ antireflective films in terminal optical components. Due to the porous nature of SiO₂ antireflective films, their properties are easily affected by organic pollutants and moisture water molecules during facility operations. To ensure the basic properties of third-harmonic SiO₂ antireflective films, further improving the stability of the films to reduce the replacement frequency of the third-harmonic components and increasing the overall operational efficiency of the facilities are necessary.

SiO₂ Sol was prepared using tetraethyl orthosilicate as a precursor, ethanol as a solvent, and ammonia as a catalyst. SiO₂ antireflective films with enhanced transmittance at the third harmonic were then obtained by dip-coating following SiO₂ Sol dilution (Fig. 1). An optimized third-harmonic porous SiO₂ antireflective film (3AR+5MR) was prepared by surface cladding with a small-particle silica Sol containing methyl groups, and its properties were compared with those of a third-harmonic SiO₂ antireflective film (3AR) prepared by traditional chemical atmospheric treatment using ammonia and hexamethyldisilazane. The films were analyzed based on their optical performance, laser damage threshold performance, and environmental stability.

Results and discussions The initial optical performance of the 3AR+5MR film shows that the peak transmittance is greater than 99.5% at 370 nm, which is close to that of the 3AR film (Fig. 2), and the uniformity of the films is good (Fig. 4). Although a thin layer is present on the surface of the 3AR+5MR film, it still maintains the characteristics of high porosity and achieves efficient antireflection, whereas the surface pore size tends to be more uniform (Fig. 3). The water contact angle of the 3AR+5MR film reaches nearly 120°, and the change trend affected by water vapor is relatively slow because the interface layer of the 3AR+5MR film protects the porous film from the effects of water vapor better than that of the 3AR film (Fig. 5). The 3AR+5MR film is more stable than the 3AR film in terms of antipollution and moisture resistance, and the degradation of the various properties is slower (Figs. 6 and 7). The surface cladding layer can reduce the effects of organic gas molecules and water vapor molecule intrusion on the optical properties of the film. The surface roughness of the 3AR+5MR film is approximately 9 nm, which is comparable to that of the 3AR film (Figs. 8 and 9). Analysis of the laser damage performance shows that the initial zero probability laser-induced damage threshold measured by a 1-on-1 method is 18.9 J/cm² (3AR+5MR, 355 nm, 8.8 ns) and 19 J/cm² (3AR, 355 nm, 8.8 ns) (Fig. 10). Simultaneously, the preparation process time of the 3AR+5MR film is shorter than that of the 3AR film, which can effectively improve the production capacity of SiO₂ antireflective films for large optical components.

A high demand exists for third-harmonic SiO₂ antireflective films applied to large-aperture optical elements in high-power laser facilities. In this study, a modified third-harmonic SiO₂ antireflective film with excellent properties and environmental stability was fabricated using a surface-cladding process. Results show that the preparation efficiency of the SiO₂ antireflective films is significantly improved, which helps to increase the batch production capacity of SiO₂ antireflective films for large optical components whenever a new large high-power laser facility is constructed.