PHOTOACOUSTIC STUDIES OF LASER DAMAGE IN OXIDE THIN-FILMS

Laser damage thresholds of ZrO2, TiO2 and HfO2 films of optical thickn ess lambda, (lambda = 248 nm) evaporated on quartz glass substrates we re investigated with the photoacoustic probe beam deflection technique in a 1-on-1 irradiation mode for a range of fluences from below to ab ove the thin-film laser damage threshold. In addition, irradiated spot s were investigated systematically by a video imaging difference techn ique. It is demonstrated that the photoacoustic technique allows a mor e sensitive and precise determination than optical inspection of the o nset of damage. An exponential dependence of damage thresholds on the apparent band gap of the respective thin-film material, as determined by optical absorption spectroscopy, was found. Results for amorphous a nd polycrystalline films on BK7 glass and SQ1 quartz substrates are co mpared and the influence of a SiO2 protective coating on the laser-dam age threshold is investigated.