To account for changes induced in optical coatings by the fs-laser irradiation at fluences lower than the laser induced damage threshold (LIDT) fluence, a general term, incubation, is used. The goal of this research project was to try to understand the physical and chemical modification in optical coatings generically described as incubation. Based on these findings, better dielectric layers for fs-laser mirrors, beam-splitters, etc. could be designed and manufactured, which will save money during the operation of large fs-laser installations such as ELI-NP.

The first step was to obtain high quality dielectrics layers that could be laser irradiated and then investigated. Thin films of HfO2 and ZrO2 were deposited on quartz and Ag covered Si wafers using the pulsed laser deposition technique. After characterizations using advanced techniques such as Rutherford backscattering spectrometry, X-ray grazing incidence diffraction, X-ray reflectivity, X-ray photoelectron spectroscopy, the LIDT values for a 25fs long laser pulse at 100 consecutive pulses were determined. Once the LIDT values were measured, the deposited films were irradiated at subthreshold fluence values for 100 pulses.

Two techniques were found to be sensitive enough to observe changes during the incubation period. First, the electron paramagnetic resonance (EPR) investigations of dielectric films turned out to be one of the most sensitive technique to oxygen content and hence to the deposition conditions or irradiation effects.

In Fig. 1 the EPR signals acquired from a HfO2 film before (black) and after fs-laser irradiation (red) are displayed. One could note that after fs-laser irradiation at a fluence below the LIDT, the signal intensity and width remained almost unchanged; however, two new very narrow bands appeared. These bands are associated with new defects induced by the fs-laser irradiation.

The other sensitive technique for monitoring the effect of fs-laser irradiation on dielectric layers is the measurement of the leakage current. In Fig. 2 there is an example of I-V curves measured on an as prepared sample and in a location that was irradiated with a fs-laser at a fluence equal to 0.7 of the LIDT value. One could notice that the leakage current after fs laser irradiation is around one order of magnitude higher than that measured initially.

In conclusions, we found two techniques to monitor the effect of the fs-laser irradiations at fluences below the LIDT value. It is clear that such irradiations can induce defects that alter the structure and the leakage current. Based on these results we optimized the deposition process and succeeded to obtain films exhibiting LIDT values similar to those reported in the scientific literature.

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