High temperature annealing effects on spectral, microstructural and laser damage resistance properties of sputtered HfO2 and HfO2-SiO2 mixture-based UV mirrors

Hafnium oxide is widely used as high refractive index material for making high power laser coatings for UV spectral range. Thermal annealing is one of the most efficient post-processing techniques for improving optical coating stress, its absorption, and resistance to laser radiation. In this article we investigate high temperature annealing effects on optical, microstructural, surface and laser radiation resistance properties of pure hafnia and two different hafnia-silica mixture-based sputtered dielectric mirrors for 266 nm wavelength. Three distinct annealing temperatures were selected for different coatings, leading to amorphous and polycrystalline phases of high refractive index layers. Annealing increased reflectance of all mirrors. Annealing at high temperatures that triggered crystallization considerably increased LIDT of all coatings. In addition, substantial increase in transmittance at shorter wavelength range was registered. Our results suggest that high temperature annealing above crystallization onset of HfO2 and HfO2-SiO2 mixture based dielectric mirrors has a great potential of increasing their damage resistance for UV laser pulses.