Near-ultraviolet absorption-annealing effects in HfO2 thin films subjected to continuous-wave laser irradiation at 355 nm

Hafnium oxide is the most frequently used high-index material in multilayer thin-film coatings for high-power laser applications ranging from near-infrared to near-ultraviolet. Absorption in this high-index material is also known to be responsible for nanosecond-pulse laser-damage initiation in multilayers. In this work, modification of the near-ultraviolet absorption of HfO2 monolayer films subjected to irradiation by continuous-wave (cw) 355-nm or 351-nm laser light focused to produce power densities of the order of similar to 100 kW/cm(2) is studied. Up to a 70% reduction in absorption is found in the areas subjected to irradiation. Temporal behavior of absorption is characterized by a rapid initial drop on the few-tens-of-seconds time scale, followed by a longer-term decline to a steady-state level. Absorption maps generated by photothermal heterodyne imaging confirm the permanent character of the observed effect. Nanosecond-pulse, 351-nm and 600-fs, 1053-nm laser-damage tests performed on these cw laser-irradiated areas confirm reduction of absorption by measuring up to 25% higher damage thresholds. We discuss possible mechanisms responsible for near-ultraviolet absorption annealing and damage-threshold improvement resulting from irradiation by near-ultraviolet cw laser light.