SiO2/HfO2 Laser Film with Enhanced Protection and Antireflection for Sapphire Infrared Windows at High Temperatures

It is indispensable to achieve antireflection and protection of the sapphire (i.e., Al2O3 single crystal) infrared window, especially in extreme environments such as high temperature and high-energy laser irradiation. Here, double-layered SiO2/HfO2 films were deposited on sapphire by RF magnetron sputtering with subsequent annealing. The annealing of HfO2 films in pure oxygen facilitates crystallization, as well as an obvious increase of the O/Hf ratio. The influences of the HfO2 interlayer on the film microstructure, adhesive strength, film stress, laser-damage resistance, flexure strength, and infrared (IR) transmittance at various temperatures were investigated. The introduction of the HfO2 interlayer leads to the densification of the SiO2 protective layer, an improvement in adhesive strength, and an apparent reduction in film stress. A higher flexure strength at high temperature is observed for sapphire coated with a SiO2 film, while the double-layer SiO2 (670 nm)/HfO2 (650 nm) film is superior to the single-layer SiO2 film in terms of IR high-temperature transmittance and laser-damage resistance.