Improvement Laser-induced Damage Threshold Of Fused Silica By Ion Beam Surface Modification

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.