Optical resistance of sapphire

Abstract

Single-crystals of α-Al₂O₃ were subjected to multi-stage polishing with diamond powder of successively decreasing grain size. After each stage of polishing, the infrared reflection spectra were recorded and the nanosecond laser resistance at 1.06 μm was measured. The laser induced damage data were also obtained for natural face, annealed and ion-etched surfaces. It was shown that (i) disorder of the crystal structure affects the optical resistance, resulting in a decrease of damage threshold, and (ii) the surface layer disturbed by grinding retains a “memory” of the initial structure. The multi-pulse sub-threshold laser irradiation which leads, finally, to macroscopic damage of a sample, stimulates the process of structural ordering, together with the formation of “dangerous” local defects.