Influence of defects in HfO₂ film on absorptance and LIDT measurements

The absorption is a main cause of laser damage in optical coatings. A group of HfO₂ films were deposited under the oxygen pressure of 5 mPa ~ 43 mPa. When pressure was less than 20 mPa, the films with higher absorptance had lower laser-induced damage threshold (LIDT); however, when pressure was larger than 20 mPa, this correlation did not exist anymore. Defects might be the cause of such phenomenon. Defect models were established to mimic the absorptance test and the LIDT test. The influence of defect size, defect density, defect absorptive coefficient on absorptance and LIDT was studied, and the mechanism was analyzed. It is found that those defects with 1 000 times higher absorptive coefficient can significantly decrease LIDT to as large as 1 000 times, but do not influence the absorptance test at all. This might explain that some films with high absorptance have even higher LIDT, and some films with low absorptance have even lower LIDT.