Effect of annealing on the properties of HfO2-Al2O3 mixture coatings for picosecond laser applications

HfO2-Al2O3 mixture coatings with different Al2O3 doping contents are deposited on fused silica substrate using an electron-beam co-evaporation process. The effects of the material composition and thermal annealing temperature on the chemical states, crystal structure, transmittance spectra, refractive index, optical band gap, and picosecond laser-induced damage threshold (LIDT) properties of these HfO2-Al2O3 mixture coatings are investigated. Characterization measurements confirmed that the properties of the mixture coatings may be improved by increasing the doped Al2O3 content and thermal annealing. The experimental results show that, firstly, with the increase of the doped Al2O3 content, the optical band gap and the LIDTs of the HfO2-Al2O3 mixture coating increase monotonically. Secondly, the highest LIDT of 7.1 J/cm(2) is obtained at an annealing temperature of 500 degrees C, which is similar to 20 % higher than that of the as-deposited coating. The S-M-2 coating annealed at 500 degrees C shows a similar LIDT and a higher refractive index as compared to the pure Al2O3, which provides significantly support for the preparation of reflective coatings with a higher picosecond damage threshold.