Photothermal microscopy of defects and laser damage morphology in Al2O3/SiO2 dielectric mirror coatings for 248 nm

The cw photothermal displacement technique (PDT) has been shown to be a useful tool for the characterisation of optical coatings with high lateral resolution combined with an ultrahigh sensitivity. By micrometer resolved PDT-measurements on Al2O3/SiO2 multilayer coatings we found that the non-damaged thin film systems contain a great amount of photothermal inhomogeneities (defects) with lateral sizes ranging from several mu m that are not visible by optical microscopy. In most cases these areas of strongly enhanced displacement response originate from microdelamination, decreased thermal impedance at the film interface or absorption centers. Thermal inhomogeneities in the film normally play a minor role. The measured damage thresholds of various mirrors correlate with both the defect concentration and the PDT background signal. Furthermore, the damage morphology has been investigated by photothermal measurements and photothermal inages were compared to results from scanning electron mi croscopy. Areas with enhanced thermal impedance in the surroundings of some damage spots are attributed to lacal microdelamination within the film systems, indicating the onset of laser damage (pre-damage)