激光弱吸收是导致光学薄膜损伤的重要原因.在(5～43)mPa的氧分压下制备并测试了一组HfO2薄膜.实验发现,当氧分压小于20 mPa时,薄膜弱吸收越大,损伤阈值越低;当氧分压大于20 mPa时,薄膜的损伤阈值与弱吸收并不一一对应,具有较高弱吸收的薄膜可能同时具有较高的损伤阈值.建立了缺陷模型,采用有限元法模拟了缺陷对弱吸收测量和损伤阈值测量的影响,分析了缺陷尺寸、密度、吸收系数对弱吸收和损伤阈值的影响.研究结果显示,吸收系数高于薄膜1 000倍的缺陷可以降低薄膜的损伤阈值1 000倍,却并不影响薄膜的弱吸收.缺陷对HfO2薄膜的激光弱吸收与损伤阈值测试有完全不同的影响,是导致某些薄膜弱吸收与损伤阈值背离的原因.

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.