Study On UV Damage Mechanism Of KDP Crystal Sol-gel Anti-reflection Coating

In intense laser systems,most optical components need to use surface coating to improve optical transmittance and reduce energy loss in the transmission process.SiO2 coating prepared by sol-gel method are widely used because of its excellent anti-reflection characteristics and high laser damage threshold.The refractive index of SiO2 coating prepared by sol-gel method can be adjusted by the porosity of dispersed SiO2 particles,but because of the existence of pores,the coating has poor mechanical properties.When its coated on the surface of the optical element its adhesion and hardness is much lower than the dielectric coating;At the same time,the higher porosity makes the water and organic pollutants in the environment more easily adsorbed on the surface of the coating to fill the pores,especially working in the UV laser irradiation and vacuum environment.Sol-gel coating after hundreds of times of laser irradiation will appear a large area of damage phenomenon,seriously restricting the service life of the coating.Therefore,it is a mainly problem to improve the service life of optical coating in intense laser systems.In this paper,the basic theory of inertial confinement fusion and UV damage of KDP crystal coating is briefly described.The coating mode and damage theory of KDP crystal coating are systematically described,and the research content of this paper is proposed based on the research status of modification methods of KDP crystal coating.The experimental methods and principles used in this paper are briefly explained.In terms of experimental content,this paper firstly studies the fatigue damage of KDP crystal sol-gel antireflective coating.Using scanning near-field optical microscopy and other characterization methods,it is obtained that the damage of AR coating on the surface of KDP crystal is mainly caused by surface spalling.With the increase of laser pulse intensity,the accumulation of tensile stress leads to the damage of AR coating.The damage extension and interconnection of the coating were preliminatively observed to form a large area of spalling phenomenon.The splitting mechanism of Si-O-Si(TO3)at the modal peaks of 1064 cm-1 and 1096 cm-1 was found,which indicated that the irradiation damage process was progressive.Combined with a small aperture Gaussian pulse laser,surface analysis methods such as ToF-SIMS were used to study the UV damage dynamics of coating,including:Time-of-Flight Secondary Ion Mass Spectrometry(ToF-SIMS)and scanning electron microscopy(SEM)were used to study the 3D chemical composition and damage processes of coating in detail.By analyzing the transformation process of Si-O-Si bond to Si-O-K bond,the reason of coating densification caused by UV laser irradiation is explained.In addition,the dynamic evolution law of laser irradiated coating is obtained by compared with unirradiated coating,.The defect analysis methods such as positron annihilation lifetime spectroscopy and X-ray photoelectron spectroscopy were used to characterize the defects during the fatigue damage process.Scanning electron microscopy(SEM)and slow positron beam(SPB)techniques were used to study the macroscopic and microscopic defects in the coating damage process.The formation of non-bridging oxygen(NBO)and the variation of O/Si ratio along the coating depth were analyzed by X-ray photoelectron spectroscopy(XPS).The damage factors of coating were investigated by using a small aperture damage simulation platform.The main factors affecting coating damage were studied by controlling laser energy density,such as oil pollution degree,environmental atmosphere and other conditions.The anti-fatigue properties of sol-gel coating prepared with various coating parameters were verified by using small aperture coating damage platform.On the basis of traditional hydrophobic coating modified by HMDS,hydrochloric acid/ammonia strengthened coating were prepared and their properties were verified.It mainly includes:the 29Si structure of the modified coating was characterized by solid state nuclear magnetic resonance and the modification process was verified;A small aperture damage simulation platform was used to modify the damage resistance of the coating at a flux of 6 J/cm2 of 3000.The results show that both modification methods can improve the life of the coating,and the ammonia strengthening method has the most obvious effect.Finally,the work of this paper is summarized and the future research is prospected.