From microstructure engineering to withstanding electric fields: interface passivation and oxygen vacancy repairing for high-performance femtosecond laser mirrors

Ultra-intense ultrashort laser pulses are pivotal for frontier sciences, yet their development is hindered by the inherent difficulty in fabricating mirrors that simultaneously achieve broad bandwidth, high reflectance, and a high laser-induced damage threshold (LIDT). The pursuit of high optical performance with multilayer structures inevitably introduces defects and discontinuous interfaces, which promote electric field enhancement and thus deteriorate femtosecond laser resistance. Here, we address this challenge by applying post-deposition thermal annealing in air to Ta 2 O 5 /SiO 2 multilayer reflective films fabricated by ion beam assisted electron beam evaporation. This process effectively enhances the femtosecond LIDT without sacrificing spectral performance, with its efficacy governed by a critical temperature. Mechanistically, the oxidation behavior during air annealing reduces oxygen vacancies within the films, thereby suppressing light absorption pathways induced by defects. Simultaneously, thermal diffusion promotes atomic migration, leading to film densification and passivation of the originally discontinuous Ta 2 O 5 -SiO 2 interfaces, which enhances the resistance to strong electric fields. The high-reflection film exhibits reinforced structure and performance, along with an approximately 47% increase in the LIDT. This work establishes air annealing as a convenient and effective strategy for manufacturing robust optical components critical for high-power laser systems.