Theoretical Study Of Several Irradiation Effects On Silica Optical Materials

Various silica optical materials are used as the optical components in high-power laser ignition devices,such as fused silica lenses,porous silica films,etc.However,the lifetime and performance of silica optical materials can be reduced by irradiation of intense laser and transient high-energy particles and rays（neutron particle beam,gamma rays,etc.）radiation from the fusion target.Irradiation-induced performance degradation and damage of the silica optical components limit the development of high-power solid-state laser systems.Based on the finite difference,molecular dynamics and first principles methods,we systematically studied the irradiation effects of triple frequency ultraviolet laser,high-energy neutron beam and gamma ray on silica optical materials（fused silica and porous silica film prepared by sol-gel method）in this dissertation.The main research contents and conclusions of this dissertation are as follows:1.Study on oxygen deficiency and physical property degradation of fused silica induced by ultraviolet laser irradiation.The effects of oxygen deficiency induced by triple frequency（3ω）ultraviolet laser（wavelength 355 nm）irradiation on the structural characteristics,thermal,mechanical and optical properties of fused silica are studied.The influence of the oxygen/silicon ratio on the structural characteristics and physical properties is analyzed.The results show that:（1）Oxygen deficiency leads to the fracture and formation of the ring structure.A significant reduction in bond angle distribution and structure density is found.Moreover,oxygen deficiency decreases the average coordination number of silicon atoms,and the content of oxygen-deficiency-related defects increases significantly;（2）Oxygen deficiency reduces the elastic modulus and thermal conductivity,leading to the degradation of these properties;（3）In terms of optical properties,oxygen deficiency leads to 3ωabsorption and the absorption intensity increased with the aggravation of oxygen deficiency.The electronic structure of fused silica with oxygen deficiency shows that the neutral oxygen vacancy defects can cause a significant reduction in the band gap of fused silica and strong absorption of 3ωultraviolet laser.A potential 3ωabsorption source of fused silica has been found in our research.Moreover,our research show that oxygen ion implantation can repair oxygen deficiency on the fused silica surface and increase the damage threshold.2.Study on thermal effect and damage of fused silica irradiated by ultraviolet laser.The thermal effect of ultraviolet laser irradiation on fused silica caused by neutral oxygen vacancy absorption(decrease in the ratio of O/Si,R_O-Si)and the corresponding3ωdamage mechanism is studied by using an improved absorption front model.The results show that:（1）When the laser pulse width is 5 ns and the incident laser energy reaches 8.5 J/cm²,the surface temperature of fused silica rises rapidly（more than 10000K）,producing obvious absorption wavefront and damage.The damaged area is a semi-ellipsoidal crater with a surface radius of 900 nm and a depth radius of 400 nm;（2）By adjusting the laser parameters,it is found that the reduction of laser pulse width can significantly reduce the damage threshold of fused silica.Moreover,the increase of laser energy density can increase the damaged area;（3）Lower ambient temperature can effectively increase the damage threshold and control the surface damage radius,but the change of damage pit depth is relatively small.（4）The damage threshold of fused silica decreases with the decrease of R_O-Si.The damage threshold of defect-free fused silica is90 J/cm².As R_O-Si drops to 1.60,the damage threshold is only 7 J/cm²;Excess oxygen can maintain the damage threshold of fused silica above 45 J/cm².Our research show that3ωabsorption caused by oxygen vacancy defects can cause extremely high temperatures on the fused silica surface and reduce the damage threshold,and oxygen ion implantation can effectively repair and improve the damage threshold of fused silica.3.Study on neutron irradiation effect of porous silica film.Neutron irradiation-induced microstructure modification and changes of thermal,mechanical and optical properties of silica film are studied by the neutron irradiation model.The results show that:（1）The microstructure of silica film is changed by neutron irradiation,with the increasing coordination defects and structure density;（2）Neutron irradiation destroys the tight packing of Si O₂ tetrahedron,resulting in the decrease of elastic modulus.The thermal conductivity decreases with the increase of irradiation defects;（3）The band gap of silica film is reduced by neutron irradiation.The increase of refractive index and the appearance of obvious 3ωabsorption leads to the decrease of 3ωtransmission;（4）The neutron irradiation effects with different porosity silica show that the silica film with low porosity has less structural modification and degradation of properties,showing better resistance to neutron irradiation.Our research find the neutron irradiation degradation law of the silica film,indicating that the neutron irradiation will significantly reduce the performance of silica film and reduce the antireflection ability to3ωultraviolet laser.4.Study on gamma irradiation effect of porous silica film.The effects of gamma irradiation on the microstructure,thermal,mechanical and optical properties of silica film are studied with the gamma irradiation model.The results show that:（1）Similar to the neutron irradiation effect,the microstructure of silica film changes significantly under gamma irradiation.The number of defects increases and the physical properties are degraded,but the degradation effect of gamma irradiation is less than that of neutron irradiation;（2）Low porosity silica film has less structural change and less degradation of properties after gamma irradiation,showing better resistance to gamma irradiation;（3）The increase of neutron irradiation dose leads to the decrease of elastic modulus and the increase of Poisson’s ratio,which indicates that the structure and basic properties of silica film are changed and softened.However,the increase of gamma radiation dose only lead to the decrease of elastic modulus,while Poisson’s ratio remains basically unchanged.The increase of neutron and gamma radiation dose reduces the thermal conductivity.The decrease of thermal conductivity under gamma irradiation has a convergence value,while neutron irradiation does not converge;（4）With the increase of irradiation dose,neutron irradiation increases the refractive index and 3ωabsorption,resulting in a serious decline in 3ωtransmission of silica film.However,gamma irradiation has little effect on the refractive index and maintains a high 3ωtransmittance and low 3ωoptical absorption in a certain dose range.Our research show that there is a safe dose threshold for gamma irradiation.