Laterally and depth resolved photothermal absorption measurements on ZrO2 and MgF2 single-layer films

Photothermal displacement microscopy was used for the characterization of ZrO2 and MgF2 single-layer thin films with respect to inhomogeneities of the absorption at lambda=514 nm. Images are presented for lambda/2, lambda, and 2lambda films on BK7 glass and SQ1 quartz substrates. Applying modulation frequencies ranging from 1 kHz to 10OkHz a lateral resolution of several microns could be obtained. We found that size and density of the absorption inhomogeneities as well as absorptivity strongly depend on the fabrication process and weakly on substrate material, no dependence on thin film thickness was found. It is shown that the apparent defect density varies with the modulation frequency demonstrating the capability of the photothermal method to discriminate between absorptions in various depth of the thin film system. Defect densities derived from the photothermal measurements are related to results from total integrated light scattering (TIS) experiments performed on various samples at lambda=632nm. TIS amplitudes for MgF2 films on glass substrates were about one order of magnitude smaller than those from films on quartz. The latter revealed strong large scale (1 mm) variations of the light scattering amplitude what is in accordance with the absorption measurements.