Influence of coating thickness on laser-induced damage characteristics of anti-reflection coatings irradiated by 1064 nm nanosecond laser pulses.

The influence of coating thickness on laser-induced damage (LID) characteristics of anti-reflection (AR) coatings irradiated by 1064 nm nanosecond laser pulses was investigated. Two HfO₂/SiO₂ AR coatings with different physical thicknesses, 0.7 and 2.7 μm, were prepared and tested. To study the effect of coating thickness on a laser-induced damage threshold (LIDT) in isolation, electric field intensities (EFIs) at the substrate-coating interface were kept the same by using proper AR designs. Moreover, 2 nm artificial gold particles with a density of 10  mm^-2 were implanted into the substrate-coating interface to achieve reliable experimental results. An optical microscope (OM) and a scanning electron microscope (SEM) were used for an online LIDT test and offline LID morphology observation, respectively. The typical LID morphology of thicker AR coatings was flat bottom craters with diameters of 20-50 μm, which can be easily observed by an online OM. For thinner AR coatings, hemispherical craters with diameters down to 1 μm were found as typical LID morphology by a SEM. However, these tiny craters could not be observed by an online OM. Moreover, such tiny craters did not grow with subsequent pulses, so they did not degrade the functional laser damage resistance of the thin AR coatings. When identified with an online OM, the LIDT of thinner AR coatings is found to be about two times higher than the thicker ones, and large delamination was mainly found as the LID morphology of AR coatings with high fluence. When observed with a SEM, the LIDT of thin AR coatings with tiny craters was over 60% lower than the LIDT of thick AR coatings, which agrees with the model that less energy is required to form smaller LID craters of thinner coatings.