Improve the LIDT of high-reflection coatings by planarizing nodular defects

Nodular defects planarization was investigated to improve the laser-induced damage threshold (LIDT) of high-reflection coatings. The monodisperse SiO₂ microspheres were deposited on the substrate surface by spin coating process. In the dual ion beam sputtering system, the artificial nodules were grown from these engineered seeds in 1064 nm HfO₂/SiO₂ multilayer coatings. After a series of coating and etching steps, the SiO₂ microspheres were smoothed by a single SiO₂ planarization layer. The relation between the thickness of the planarization layer and the size of the microspheres has been investigated. When the planarization layer (etching layer) thickness was slightly larger than the diameter of the seeds, the seeds could be completely planarized to obtain smooth thin films. Furthermore, the three-dimensional finite-difference time-domain code (FDTD) was used to simulate the electric-field intensity distributions in the artificial nodular defects. The comparison between the electric-field intensity distributions and the nodular morphologies of the non-planarized nodular defects and partially planarized nodular defects indicates that the nodular defects planarization has changed the geometry of nodular defects and effectively suppressed the electric field enhancement in nodular defects. Finally, nodular defects with different thickness planarization layers were tested by raster scan damage tests. For the nodular defects with an adequate planarization layer, the laser damage threshold test results show that the ejection fluences has been greatly raised, which has verified that nodular defect planarization could improve the damage resistance of thin films.