Effect of Annealing Treatment on Laser-Induced Damage Characteristics of Non-Quarter-Wave HfO2/SiO2 Antireflection Films

High-power laser systems impose stringent requirements on the laser-induced damage threshold of optical thin films. Post-deposition thermal annealing has been demonstrated as an effective approach to enhancing the LIDT of HfO2-based coatings; however, its effects on antireflection coatings—particularly those employing non-quarter-wave designs that offer industrial advantages such as fewer layers and lower fabrication cost—remain insufficiently investigated. In this work, non-quarter-wave HfO2/SiO2 multilayer antireflection films were fabricated by ion beam-assisted electron beam evaporation, and the effects of thermal annealing on their laser-induced damage characteristics were systematically investigated. Experimental results show that the effect of thermal annealing between 300 and 700 °C on the laser damage resistance of the antireflection coating is not monotonic: the laser-induced damage threshold rises from 27.6 J/cm2 in the as-deposited state to 36.7 J/cm2 at 500 °C, an improvement of approximately 33.1%, and then falls at 600 °C and 700 °C, while remaining above the as-deposited value throughout. This non-monotonic behavior originates from a competition between two contributions. On the one hand, the repair of oxygen vacancies, the densification of the film, and the growth of the crystallites continuously reduce the population of initiating absorbers and improve heat dissipation. On the other hand, the penalties arising from the exhaustion of the free volume and from the accumulation of tensile stress accelerate in the upper part of the temperature range, and their influence on the threshold progressively outweighs the gain produced by the former. Annealing at 500 °C is therefore the optimum state determined by this competition.