【摘要】 真空环境中高功率激光装置光学元件表面的有机物污染是限制其负载能力的原因之一。针对装置中常见的有机物污染和三倍频激光溶胶凝胶减反膜，通过精确控制真空环境中污染源的挥发扩散，制备了有机物质量面密度不同的元件表面，定量研究了有机污染物质量面密度对溶胶凝胶减反膜光学性能及损伤特性的影响规律。实验结果表明：样品表面粗糙度、透过率、损伤阈值等的变化量均与有机物质量面密度成正相关。有机污染物沉积量较少时，由于膜层孔隙被填充，膜层的表面粗糙度略有减小；随着沉积量增加，有机物附着影响表面形貌，粗糙度显著增加。溶胶凝胶减反膜在351 nm波长处的光学透过率随着有机物质量面密度的增加而逐渐降低，这与有机物分子改变溶胶凝胶膜孔隙填充比有关。样品表面的激光损伤阈值变化量和损伤面积随着有机物质量面密度的增加而增加，而且不同有机物沉积量的光学表面的损伤形貌存在显著差异。基于实验结果讨论了有机物影响溶胶凝胶减反膜性能的机理，并探讨了高功率激光系统的洁净度控制方法。更多还原

【Abstract】 Objective Optical products are often coated with a sol-gel antireflective film that has a porous structure with a tendency to adsorb organic contamination easily.This reduces their performance in terms of transmittance and laser-induced damaged threshold （LIDT）.Hence,organic contamination volatilized from non-metallic materials under a high vacuum environment poses a significant limitation on the output capacity of high-power laser systems.In this study,we investigate the influences of the surface deposition of organic contaminations on the surface morphology and roughness,transmittance and LIDT at 355 nm for 3ωsol-gel antireflective film quantitatively.Based on the experimental results,we discuss the mechanism by which the organic contamination influences the performance of sol-gel antireflective coating.Finally,we propose two measures to reduce organic contamination in high-power laser systems,which serves as a basis for improving the cleanliness and output capacity.Methods The experimental section was divided into three parts:sample preparation,optical properties characterization,and laser-induced damage test and characterization.First,an experimental vacuum device was set up to simulate the deposition of organic contamination released from non-metallic materials onto optical elements surface in high-power laser systems.The deposition of the organic contamination on the sol-gel antireflective coating was precisely controlled using a heating evaporation chamber.The volatilization of organics was measured using a high precision balance,and the surface mass density of organic deposition was calculated by the principle of angle factor equal division.The second step was to characterize and compare the optical properties of samples before and after contamination.The surface morphology of sample was observed using scanning electron microscope （SEM）,the surface roughness was determined using atomic force microscopy （AFM） and the transmittance was measured using LAMBDA900 UV spectrometer.The last step was the damage test and morphology characterization.Nanosecond laser damage testing platform was used to measure the LIDT at 355 nm.The damage morphology of the coating was characterized using an optical microscope.Results and Discussions We prepared four groups of samples with different surface deposition qualities using the experimental vacuum contamination device.Based on SEM images of samples before and after contamination,we found that a“mist”was formed on the coating after contamination,which reduced the resolution of the surface topography （Fig.4）.It proves that organic molecules in the evaporation chamber were volatilized,diffused and deposited on the sol-gel antireflective coating.When the surface deposition quantity was low,such as in Group 1,the surface roughness decreased from 2.785 nm to 2.453 nm.This may be due to the small amount of organic contamination molecules filling in the pores of the sol-gel antireflective film.As the amount of organic contamination deposited on the sample surface increased,the surface roughness also gradually increased.Here,the organic molecules not only filled the pores,but also accumulated on the film surface （Fig.5 and Fig.6）.With the increase in deposited mass,the transmittance and LIDT at 355 nm of sol-gel antireflective coating decreased gradually[Fig.7,Fig.8,Fig.9（b）].When the surface mass density of organic deposition in Group 1 was 1.665 mg/m²,the sample was not damaged.This indicates that the NVR cleanliness class A/10 meets the cleanliness requirements of high-power laser systems[Fig.9（b）].The damage morphology was observed under an optical microscope.In Group 1,the low-fluence damage morphology was all subsurface-defects induced by substrate damage,leading to film protrusion or rupture that presents as a white-color morphology.The damage morphology of Group4 samples with the higher surface deposition was significantly different from that of Group 1,indicating that organics played an important role in the damage initiation.When the amount of deposited organic contamination was high,both the film and the substrate of the sample surface were damaged.The morphology of the damage site was irregular and the edge showed thermal ablation contours（Fig.10）.With increasing laser fluence and constant deposition quantity on the sample surface,the area of damage sites increased gradually.For Groups 1 to 3,the area of the damage sites of the sample before and after contamination hardly changed,whereas in Group 4 it was significantly larger after contamination.The deposition quantity of organic contamination on the sample surface in Group 4 was high,therefore,the absorption of laser energy by the organics caused strong thermal effect,leading to a larger damage site with smoother contours （Fig.11）.Conclusions In this study,an organic mixture of C₃₀H₆₂ and C₂₄H₃₈O₄ with mass ratio of 17∶3,a common contamination in high-power laser systems,was selected as the contamination source.The influence of different contamination quantities on the properties of sol-gel antireflective coating was studied quantitatively.The interaction between organic molecules and porous structures influences the optical properties,where the size of organic molecules,refractive index and deposition quantity determine the change in properties.The extent of damage was related to the absorption of organic molecules,deposition quantity of organic contamination and surface defects.First,organic contamination deposited on the coating increased the surface roughness of the film and changed the morphology of the film.As organic molecules filled the pores of the film,the equivalent refractive index was increased,which reduced the transmittance and enhanced the intensity of stray light.Second,the organics on the surface of the thin film had stronger absorption characteristics of laser energy than the substrates.Both these factors can easily lead to laser damage.Finally,to reduce the influence of organic contamination in the high-power laser system on the performance of sol-gel antireflective film,we proposed two measures to improve the cleanliness of the high-power laser systems.更多还原