Annealing effects on microstructure and laser-induced damage threshold of quasi-rugate filters

Ta₂O₅/SiO₂ quasi-rugate filters with high damage thresholds were deposited by ion-beam sputtering and then annealed at temperature of 200-800°C. The relations between microstructure, optical properties, chemical composition, weak absorption, and laser-induced damage threshold (LIDT) were studied. It was found that the transmittance spectra shifted to short wavelength as the annealing temperature increased. Three evolution courses of the films in the annealing process were analyzed by Atomic Force microscopy (AFM), Zygo interferometer measurement and Focused Ion Beam microscope (FIB). The decreased weak absorption during annealing process was found with significant effect on the LIDT. As the annealing temperature increased to 600°C, the weak absorption of films decreased from 39.99 to 7.2 ppm and the 50%-LIDTs increased from 59.32 to 158.87J/cm². Distinct damage micrographs of the films annealed at different temperature were obtained. A combination of substoichiometric defect and structural defect dominant description was used to illustrate the aggravation of laser-induced damage.