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., Al 2 O 3 single crystal) infrared window, especially in extreme environments such as high temperature and high-energy laser irradiation. Here, double-layered SiO 2 /HfO 2 films were deposited on sapphire by RF magnetron sputtering with subsequent annealing. The annealing of HfO 2 films in pure oxygen facilitates crystallization, as well as an obvious increase of the O/Hf ratio. The influences of the HfO 2 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 HfO 2 interlayer leads to the densification of the SiO 2 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 SiO 2 film, while the double-layer SiO 2 (670 nm)/HfO 2 (650 nm) film is superior to the single-layer SiO 2 film in terms of IR high-temperature transmittance and laser-damage resistance.