The laser-induced damage threshold (LIDT) is a commonly used method for testing optical thin films, where the sample is exposed to laser radiation of a defined intensity and observed for laser-induced damage. Sensitive evaluation of the damage is essential for the experiment and different approaches are used for this purpose. This work introduces a new approach to LIDT evaluation based on the second harmonic generation (SHG) principle for the detection of defects in thin films. The process of SHG is very sensitive to changes in the symmetry of the crystal lattice of the material, which can be very well exploited for the observation of defects and various changes in thin films. We developed a setup able to track the polarization-dependent SHG from the samples with micrometer resolution for a broad range of incident angles in both reflection and transmission regimes. We use this setup to study the polarization and angular dependence of SHG in pristine and irradiation-affected areas on various thin films (Si3N4, TiO2,...). Our measurements show that SHG makes it possible to reliably detect spots with subtle laser-induced changes in the material that are hardly detectable or even undetectable by scanning electron microscopy or other commonly used methods.
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