Influence of overcoat on laser-induced damage threshold of 532 and 800 nm TiO2/SiO2 high reflectors

TiO2/SiO2 high reflectors with and without a SiO2 overcoat are deposited by electron-beam evaporation. The film properties are characterized by visible spectrometry measures, structure analysis, roughness and laser-induced damage threshold (LIDT) tests, surface defects and damage morphology observation. The effects of overcoats on LIDT at 532 nm, 8 ns and 800 nm, 220 ps laser pulses are investigated. The relations between film structure, roughness, surface defects, electric field and LIDT are discussed. It is found that overcoats can increase the LIDT at these two laser wavelengths. The reduction of peak temperature, the low defects density and roughness, the low intrinsic absorption of SiO2 and its amorphous structure are the main reasons for LIDT improvement by overcoats.