Research On The Electric Field Enhancement Effect And Damage Characteristics Of Nodular Defect In HfO₂/SiO₂ High-reflection Coating

In this paper,we studied the electric field enhancement effect(EFEE)of nodular defect(ND)and the law of electric field enhancement of ND under different conditions is explored.Based on the preparation technology of electron beam evaporation thin film,the effect of ND on laser induced damage threshold(LIDT)of HfO2/SiO2 high reflection(HR)coating was studied by means of artificial planting ND.First,we use Finite difference time domain(FDTD)method to simulate the EFEE in ND.According to the defect geometry model of ND by electron beam evaporating,the EFEE of ND under different conditions was simulated comprehensively.Compared with the literature,the simulation results have some similar and different conclusions,which are as follows:The EFEE of ND increased with the increase of seed size,and the growth trend of EFEE tended to be flat when the seed size reached 1500 nm.Compared with the ND on the surface of the substrate,the ND in the coating tend to have a larger EFEE.It reaches the maximum when ND is in the middle of coating layer,which is twice as large as on the surface of substrate.When the seed is HfO2 and SiO2,HfO2 has a greater EFEE when the seed size is 2000 nm,which is about twice that of SiO2.Comparing different wavelengths,the shorter the wavelength,the larger the EFEE of ND is;When the laser is oblique incident,the EFEE appears at the profile of the ND,and the larger the incident angle is,the closer the position of the electric field enhancement area is to the upper of ND.In addition,we also simulate the EFEE of ND under other conditions and obtain very reliable results.Secondly,the micro and sub-micron(300 nm-2000 nm)SiO2 micro-sphere seeds were used to generate the artificial ND.ND damage experiments were carried out by the R-on-1 damage test method.The results show that under the irradiation of 1064 nm@10 ns laser,the higher the defect density,the lower the LIDT.The larger the size of ND,the smaller the LIDT is.ND located on the surface of the substrate are more likely to cause coating damage than ND located in the middle of coating layer.When the laser 45° incidence HR coating,the LIDT decreases with the increase of seed size.Through observation of the damage morphology,it is found that the damage position of the ND is consistent with the corresponding electric field enhancement position of the ND.355 nm HR coating damage experiments shows that its LIDT is also affected by ND.The triple-frequency(355 nm@8 ns)HR coating damage experiment shows that the damage morphology is caused by absorptive of coating,but the ND also has some effect to the damage of coating.By comparing the EFEE and damage characteristics of ND,we can see that there is some deviation between them,the law of coating damage caused by the ND is not completely consistent with its EFEE,but from the damage morphology it can be concluded that the EFEE of ND is the reason that caused of the coating damage.This study focuses on the nodular defect which is harmful to the coating,studies the EFEE and damage characteristics of ND,Provided some help for the subsequent control of nodular defects,and provided strong support for the development of coating component with high laser damage threshold.