Electric field enhancement effect and damage characteristics of nodular defect in 45° high-reflection coating

Nodular defect is one of the most common defects that affect the laser damage resistance of optical thin films. It has been an important research object in the field of high power laser thin films at home and abroad. 45° multilayer mirrors with a central wavelength of 1064 nm were designed and fabricated. Through numerical simulations and experiments, the electric field enhancement effect (EFEE) caused by nodular defect and its influence on the laser damage resistance of the film were studied. The results show that when the 1064 nm laser is incident obliquely from right to left at 45°, the EFEE mainly appears in the surface layer of the nodular defect and the middle of its left profile. The EFEE increases with the size of nodule defect. In the experiment, mono-disperse SiO₂ microspheres were sprayed on the surface of clean substrates as the artificial nodule seeds, the multilayer high reflection coatings were prepared by electron beam evaporation, and the laser damage resistance of the samples was tested by R-on-1 method. By comprehensively analyzing the experiment results, it is concluded that the damage threshold reduction of coatings is due to the nodular defects and micro-defects in the coatings, and the damage threshold of coatings decreases with the increase of nodular defect size. (authors)