【作者】 郭培涛；

【导师】 薛亦渝； 夏志林；

【作者基本信息】 武汉理工大学 ， 车辆工程， 2011， 博士

【摘要】 光学薄膜是激光系统的重要元件,溶胶-凝胶法制备的多孔光学薄膜在长脉冲高功率激光器中已经得到了广泛应用,但是其损伤阈值却成了长脉冲激光器向更大功率发展的瓶颈。因此研究长脉冲激光辐照下多孔薄膜的激光损伤特性与机制,对提高长脉冲激光器的功率非常重要。本课题首先采用多种工艺制备了一批具有一定光学特性的多孔薄膜,采用差热-热重法分析硅溶胶制定了多孔氧化硅薄膜的干燥烧结工艺,场发射扫描电子显微镜表征了多孔薄膜的显微结构,分光光度计表征了多孔薄膜的透射特性,热-电法测试了典型多孔薄膜的吸收特性,纳米压痕仪测试了部分多孔薄膜的显微力学性能；采用1064nm,12ns, Nd:YAG激光器,1-on-1模式测试了多孔薄膜的损伤阈值,采用光学显微镜及场发生扫描电子显微镜表征了损伤斑形貌；建立两结果微滴模型和四结果微滴模型分析了薄膜发生损伤后的等离子体雾化及膨胀过程,然后结合雾化和膨胀机理模拟了等离子体爆炸过程中各种参数的演化。研究发现：通过调整氨水比例和DMF的含量,可以得到不同颗粒尺寸和堆积方式的薄膜,有良好的增透效果,其薄膜损伤阈值强烈依赖与薄膜内氧化硅颗粒尺寸的大小和堆积方式；玻璃基多孔氧化铝薄膜中的孔呈六方排列,通过改变实验参数可以得到不同结构的多孔薄膜,其损伤阈值很小,随着氧化电压的增大,硫酸电解液制备的多孔氧化铝膜抗激光损伤能力略有增强。三种电解液工艺下制备的氧化铝多孔膜损伤斑明显不同,草酸氧化铝多孔膜损伤斑点外缘有类似于“爆炸”时形成的“菊花”状散射波痕迹,由中心向外辐射；硫酸氧化铝多孔膜损伤斑点外缘没有散射波痕迹,而是分布有与斑点内大小相同的损伤坑；磷酸氧化铝多孔膜损伤斑点内的损伤坑尺寸不一致,同时有大块已损伤膜被剥离,损伤斑外缘同样没有散射波痕迹和轻微的损伤坑,损伤斑外延是在力的作用下发生的脆性断裂。多孔薄膜在长脉冲激光辐照作用下,薄膜内缺陷或者杂质首先吸收激光能量,形成高压高密度等离子体球体,等离子球体继续吸收激光能量获得雾化动力,在雾化过程中多结果微滴雾化机制越倾向于占主导优势；可以通过减小微滴喷溅初期的电子密度、电子温度、离子温度以及延缓微滴喷溅发生的时间,降低其雾化程度,控制微滴对激光能量的吸收和利用,降低材料激光损伤过程后期的等离子体微滴爆炸的剧烈程度；微滴膨胀过程中,随半径的增加外壳层膨胀速度在初始阶段迅速增加,随后趋于稳定。微滴膨胀过程中,电子密度不断下降,当电子密度为临界电子密度的3倍时,等离子体吸收出现共振吸收峰。综合考虑裂解雾化和膨胀的影响,等离子体爆炸开始时间越早,则后续演化越复杂。更多还原

【Abstract】 Optical thin films are important components of laser systems. The damage threshold of optical thin film is the bottleneck of laser systerm develops into higher power. The optical thin films prepared by sol-gel are widely used in long pulse superpower laser. In a word, it is important to study the rule and mechanism of long-pulse-laser induced damage of porous films.Several methods are experimented to prepare porous films, including porous silica films by the base catalyst sol-gel, porous alumina films by anodic oxidation combined with physical vapor deposited aluminum on glass substrate. The heat treatment process of porous silica films designed according to the TG-DTA analysis of silica sol. The morphologies of porous films and laser induced damages are characterized by scanning electron microscope. The transmittance and nano-mechanical property of porous films is characterized by spectrophotometer and nano-indentation respectively. The electric-thermal method is use to measure the absorption of porous films on the wavelength of 1064nm. The Nd:YAG laser (1064nm,12ns) and optical microscope is used to measure the laser induced damage threshold (LIDT) and damage site of porous films under 1-on-1 mode. Two-result-drop-mode and four-result-drop-mode are built to analyze the explosion evolvement of plasma after the laser induced damage happened.It is found that:the particle size, packing mode, transmittance and LIDT of porous silica films are relate to the ammonia and dimethyl formamide (DMF) content of silica sol. The pores of the porous alumina film on glass substrate arrayed in hexagon. The LIDTs of porous alumina films on glass substrate are very small. But LIDTs increased slightly with the increase of voltage, which prepared in sulphuric acid electrolyte.The laser-induced damage morphology is different associated with the electrolyte used in anodic oxidation. The external damage of porous alumina prepared in oxalic acid electrolyte scattering from the center are similar to a chrysanthemum-like scattering wave extend outward from the center. The external damage of porous alumina prepared in sulphuric acid electrolyte has the same damage spots at the center and parts of the films are peeled off from the substrate. The external damage of porous alumina prepared in phosphoric acid electrolyte is brittle fracture induced by force.Porous films under a long pulse laser, defects or impurities in the films absorb laser energy formed high-pressure and high density plasma droplets which continue to absorb the laser energy, four-result-droplet-mode dominate the atomization process. It is possible to reduce the degree of atomization, laser energy absorption and utilization and the intensity of the explosion by reducing the initial electron density, electron temperature, ion temperature and delay time of droplet splashing occurs. During the droplet expansion process, with the increase of the radius of droplets the outer shell expansion velocity increased rapidly in the initial stage, and then stabilized. The electron density decreasing, when the electronic density is 3 times as the critical electron density, the plasma resonance absorption peaks occur. Take the expansion and atomization account, the evolvement will be more complicated if the burst occurred in early stage.更多还原