【作者】 黎波；

【导师】 向霞；

【作者基本信息】 电子科技大学 ， 凝聚态物理， 2019， 博士

【摘要】 熔石英材料具有优良的光学、力学及机械性能,主要用于制作紫外光学元件,被广泛地应用于高功率激光系统,其负载能力很大程度上取决于熔石英元件的抗激光损伤能力。然而,熔石英光学元件在紫外激光辐照下容易损伤,严重降低了光学元件的使用寿命和高功率激光装置的负载能力,成为制约惯性约束聚变发展的瓶颈。提升熔石英元件的负载能力已成为目前迫切需要解决的问题之一。从损伤机制来看,通常提升熔石英抗激光损伤能力的途径从以下两个方面出发:(1)减少甚至去除熔石英表面的缺陷,降低其损伤的概率;(2)施加压应力,强化表面,同时降低或抵消激光辐照转换而来的热致应力,使其低于光学材料断裂的临界应力。目前关于熔石英表面缺陷和表面应力与激光损伤的关系的报道还很少。本论文基于离子束与材料相互作用的两个主要效应:溅射和注入,研究了基于溅射的离子束刻蚀去除熔石英表面缺陷,改善表面质量,提升熔石英元件激光损伤阈值;研究了离子注入在熔石英表面形成压应力层,强化表面,进一步提升熔石英抗激光损伤性能。论文主要内容及研究结果如下:1.研究了离子束刻蚀参数对熔石英表面离位损伤、刻蚀速率、表面质量及激光损伤阈值的影响。结果表明,离子能量越大及入射角越小,离位损伤越严重,损伤深度越深;刻蚀速率随着离子能量的增加而增加,随着束流的增加呈线性增加,随着离子入射角度的增加先增加后减小。离子束刻蚀可以去除金属杂质,钝化或去除表面缺陷,改善表面质量,提升熔石英的激光损伤阈值。本论文优化的离子束参数为:能量800 eV、束流300 mA及离子入射角度范围为0~o~30~o及≥70~o。2.采用大入射角离子束对熔石英表面进行逐层去除,研究了刻蚀后熔石英表面金属杂质和缺陷、表面分子结构变化及其对激光损伤阈值的影响;同时分析了熔石英表面激光损伤机理。结果表明,金属杂质元素主要分布在熔石英表面0~200nm的深度范围;刻蚀500 nm后,暴露出大量的亚表面缺陷,缺陷的尺寸和数量最大,表面质量恶化;随着刻蚀深度进一步增加,亚表面缺陷逐步去除,表面质量得到改善;随着刻蚀深度的增加,结构型缺陷密度和Si-O-Si键角减小而表面密度增加;熔石英表面金属杂质对紫外激光具有很强的吸收,是制约激光损伤阈值提升的关键因素;此外,亚表面缺陷会导致光场调制,限制了激光损伤阈值的进一步提升。3.针对离子束刻蚀和HF酸刻蚀各自的局限性,提出了复合刻蚀方法,即大角度离子束刻蚀+动态HF酸刻蚀。离子束刻蚀能够有效地去除熔石英表面金属杂质及亚表面缺陷,减少了HF酸对熔石英的刻蚀去除量,避免了缺陷的复制扩展,反应产物的沉积,面型的恶化。HF酸刻蚀能够去除离子束刻蚀引起的原子尺度的损伤层,并改善离子束刻蚀后的面型,提高刻蚀效率。采用70~o离子束刻蚀熔石英表面2μm后,再采用HF酸刻蚀3μm,熔石英元件的激光损伤阈值为35.54 J/cm~2,提升了102.74%。4.研究了Ar~+、He~+、N~+离子注入对熔石英的激光损伤行为的影响。结果表明,离子注入的同时伴随着溅射效应,钝化表面缺陷,粗糙度略微降低;离子注入过程中,缺陷的复合起主要作用导致结构型缺陷密度减小;离子注入均导致熔石英表面Si-O-Si键角减小,密度增加,形成了压应力层,从而增强了熔石英表面的力学性能。相较于He~+离子注入,Ar~+离子注入后熔石英表面粗糙度较小,压应力较大,激光损伤阈值提升较多。对于N~+离子注入,会与键断裂的硅氧结合,形成氮硅化物,结构型缺陷密度最小,硬度最大,激光损伤阈值提升幅度最大,但是增加了光学击穿的可能性。5.采用不同能量、注量氩离子注入熔石英表面,研究了氩离子注入对熔石英表面形貌、微结构、光学和力学性能以及激光损伤性能的影响规律。结果表明,离子注入的挤压作用可以弥合熔石英表面的微裂纹;离子注入的同时伴随着溅射效应,钝化表面缺陷,粗糙度略微降低,但是,当注量超过1×10~177 ions/cm~2时,由于氩气泡的出现,粗糙度增加,表面质量恶化;离子注入过程中,结构缺陷的产生与复合相互竞争,导致缺陷密度随着离子能量和注量的增加,先减小后增加,在能量为10 keV、注量为1×10~177 ions/cm~2时,缺陷密度及种类最少;氩离子注入导致熔石英表面Si-O-Si键角减小,密度增加,形成了压应力层,从而增强了熔石英表面的力学性能;氩离子注入提升熔石英激光损伤阈值是缺陷密度的减少、表面质量的改善、压应力的形成等多种因素综合作用的结果,熔石英表面压应力对紫外激光辐照下损伤增长也有一定的抑制作用。更多还原

【Abstract】 Fused silica is an excellent optical,thermal and mechanical material,which is extensively used to fabricate UV optics for high-power laser systems.The laser damage resistance of fused silica optics is a very important factor to determine the output capability of high power laser facilities.However,fused silica is easy to be damaged under UV laser irradiation,which seriously reduces the lifetime of optics and the damage capability of high power laser facilities.It has become the bottleneck to restrict the development of the inertial confinement fusion(ICF).Therefore,it is an urgent problem to improve the damage resistance capability of fused silica optics.From the damage mechanism,the ways to improve the laser damage resistance of fused silica are generally from the following two aspects:One is to eliminate and reduce the defects on the surface of fused silica to reduce the damage probability;The other is to increase the mechanical properties by pre-added stress field to restrain or reduce the thermal stress field generated by laser irradiation,so that it is lower than the critical fracture stress of optical material.At present,there are few reports on the correlation between laser damage and surface defects and stress of fused silica.In this work,based on the two main effects of ion beam interaction with materials:sputtering and implantation,the ion beam etching based on sputtering was investigated to remove the surface defects to improve the surface quality and the laser-induced damage threshold of fused silica;The ion implantation was investigated to form the compressive stress to strengthen the surface and further improve the laser damage resistance of fused silica.The main contents and research results are as follows:1.The effects of ion beam etching parameters on the irradiation damage,etching rate,surface quality and laser damage threshold of fused silica were investigated.The results indicate that the larger ion energy and the smaller incident angle have the more serious damage and the lager damage depth.The etching rate of fused silica increases with the increasing ion energy.With the increasing beam current,the etching rate increases linearly.The etching rate firstly increases and then decreases with the increasing ion incident angle.Ion beam etching can be used to remove the metal impurities,passivate or remove the surface defects to improve the surface quality and laser damage threshold of fused silica.The optimized ion beam parameters are 800 eV,300 mA,and 0~o~30~o and≥70~o.2.The inert ion beam with large incident angle was used to remove the surface of fused silica layer by layer.The evolutions of the metal impurities,defects and molecular structure of the surface of fused silica after etching and its influence on the laser damage threshold were investigated.Then the laser damage mechanism on the surface of fused silica was analyzed.The results indicate that the impurity elements mostly distribute in the depth of 0~200 nm of fused silica surface.After 500 nm removal of fused silica,the surface quality is deteriorated,which is due to the exposure of subsurface defects with the maximum amount and size.With the continuous increasing of removal depth,the subsurface defects are gradually removed and the surface quality is improved.During the ion beam etching,the concentration of structural defects and Si-O-Si bond angle decrease,and the surface density increase with the increasing etching depth.The metal impurities on the surface of fused silica will strongly absorb UV laser,which is the key factors to limit the improvements of laser damage threshold.In addition,the subsurface defects will lead to light field modulation,which restricts the further enhancement of laser damage threshold.3.A combined etching method,i.e.large angle ion beam etching combined with dynamic HF acid etching,is proposed in this work considering that both ion beam etching and HF acid etching have their own limitations.Ion beam etching can efficiently remove the surface impurities and subsurface defects of fused silica,reduce the removal amount of HF acid etch,and thus avoid the replication and expansion of defects,deposition of reduction products and deterioration of reflected wavefront.HF acid etching can remove the atomic scale damage layer caused by ion beam etching to improve the reflected wavefront.After 2μm etching of fused silica by ion beam with incident angle of 70~o followed by 3μm etching by HF acid,the laser damage threshold of fused silica is 35.54 J/cm~2 with enhancement ratio of 102.74%.4.The effects of Ar~+,He~+,N~+ion implantations on the laser damage behavior of fused silica were studied.The results show that the surface roughness is slightly decreased,which may be due to the sputtering effect induced by ion implantation to passivate and remove the surface defects.During the ion implantation,the recombination of defects plays a major role in reducing the density of structural defects.Ion implantation results in the reduction of Si-O-Si bond angle,increase of density,and formation of a compressive stress layer,so the mechanical properties of fused silica surface is improved.Compared with the He~+ion implantation,Ar~+ion implanted fused silica has smaller surface roughness and larger compressive stress,resulting in higher laser damage threshold.For N~+ion implantation,the N ions will react with silicon and oxygen bond to form nitrogen silicide.The densities of structural defects are smallest,and the hardness and the laser damage threshold are largest,but the possibility of optical breakdown is increased.5.The effects of Ar~+ion implantation on the surface morphology,microstructure,optical and mechanical properties,and laser damage performance of fused silica were systematically investigated with different ion energies and fluences.The results show that the microcracks on the surface of fused silica can be healed by ion implantation.The surface roughness slightly decreases by the passivation and removal of surface defects resulted from surface sputtering accompanied with ion implantation.However,when the fluence exceeds 1×10~177 ions/cm~2,the roughness is increased and the surface quality is deteriorated due to the introduction of Ar bubbles.During the implantation process,the densities of structural defects decrease firstly and then increase with the increasing ion energy and fluence,and this depends on the competition between the formation and recombination of defects.When the ion energy is 10 keV and the fluence is 1×10~177 ions/cm~2,the number and the type of structural defects are the least.Ar ion implantation leads to the strengthening the surface of fused silica by reduction of Si-O-Si bond angle,surface densification and generation of compressive stress layer.The enhancement of laser damage threshold of fused silica by Ar~+ion implantation is result of various factors including the reduction of defect density,the improvement of surface quality as well as the formation of compressive stress.The compressive stress in the surface of fused silica can also mitigate the damage growth under UV laser irradiation.更多还原