Contaminant-free layer-by-layer annealing to improve the properties of HfO2/SiO2 optical films for the fs laser

The low laser-induced damage threshold (LIDT) of HfO 2 /SiO 2 films is an important factor in limiting the further development of high repetition rate femtosecond (fs) laser systems. Conventional whole-layer annealing can effectively improve the properties of SiO 2 films, but it is difficult to improve the properties of HfO 2 films located in the intermediate layer and is also prone to introduce contaminants. In this study, an innovative magnetron sputtering-vacuum tube furnace combined system was presented to deposit and anneal the HfO 2 /SiO 2 films without contaminant. The layer-by-layer annealing optimizes the stoichiometric ratio, stress, and surface morphology of HfO 2 /SiO 2 films. The fs laser damage test at a high repetition rate indicated that the LIDT of the layer-by-layer annealed films reached 1.15 J/cm 2 and 1.99 J/cm 2 at 515 nm and 1030 nm, which were about 28% and 25% higher than those of the un-annealed films, respectively. This work provides a way to effectively avoid the contaminant introduction during annealing and improve the damage threshold of bilayer films by layer-by-layer annealing in the magnetron sputtering-vacuum tube furnace combined system.