Photothermal absorption microscopy of defects in ZrO2 and MgF2 single-layer films

Photothermal displacement microscopy has been used for the characterization of ZrO2 and MgF2 single-layer thin films to detect absorption inhomnogeneities at lambda = 514 nm. Images are presented for films of lambda/2,lambda, and 2 lambda optical thickness deposited on BK7 glass and SQ1 quartz substrates. Applying modulation frequencies ranging from 1 to 100 kHz a lateral resolution of several micrometers was obtained. We found that the size and density of the absorption inhomnogeneities as well as absorptance depend strongly on the deposition process and weakly on the substrate material. No dependence on thin film thickness was found. The apparent defect density varies with the modulation frequency demonstrating the capability of the photothermal method to localize absorption in various depths of the thin film. Defect densities derived from the photothermal measurements are compared with results from total integrated light scattering (TIS) experiments performed on various samples at l ambda = 632 nm. TIS intensifies for MgF2 films on glass substrates were about one order of magnitude smaller than those from films on quartz. The latter revealed strong long-range (1-mm) variations of the light scattering intensity . This finding is in accordance with the absorption measurements.