| 题名 | 超短脉冲激光微加工透明介质机制与建模 |
| 作者 | 孙明营 |
| 学位类别 | 博士 |
| 答辩日期 | 2013 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 朱健强 |
| 关键词 | 超短脉冲激光 激光击穿 激光损伤 激光微加工 透明介质 |
| 其他题名 | Mechanism and Modeling of Micromachining Transparent Dielectrics with Ultrashort Pulsed Laser |
| 中文摘要 | 随着超短脉冲激光技术的迅猛发展，超短脉冲激光烧蚀技术被广泛地应用于透明介质的微加工，其优势在于两个方面：一是基于光子的非线性吸收特性，激光烧蚀能够实现微米甚至纳米尺度的材料改性或去除；二是激光能量沉积与材料去除在时间上的分离，极大地减少甚至消除了破坏性的热效应。超短激光脉冲在透明介质微加工应用中主要有表面烧蚀加工和内部烧蚀改性两个方面：前者包括切割、钻孔和微结构制造等，后者包括直写光波导、微焊接和光存储等。 超短脉冲激光烧蚀介质表面将形成烧蚀弹坑以及材料损伤。激光损伤对微加工样品质量有着至关重要的影响，例如折射率改变、热改性区域和微裂纹。另外，严重影响激光加工质量的问题还包括样品加工通道侧壁反射的激光脉冲引起的二次烧蚀。高重复频率超短激光脉冲在透明介质内部改性主要利用了非线性吸收特性和热积累效应。目前，实验上已经对透明介质的内部改性进行了广泛的研究，但是激光脉冲的能量沉积与热积累效应之间的相互影响仍需细致的研究，从而深入地理解介质材料内部改性的机制。 本论文围绕着皮秒脉冲激光表面烧蚀和内部改性透明介质的过程和机理两个方面，主要做了以下几个方面的工作： 1. 研究了皮秒激光脉冲在玻璃切割过程中的烧蚀和损伤的动力学过程。首先，提出了一个关于多个皮秒激光脉冲烧蚀透明介质的新颖的数值模型，模拟研究了皮秒脉冲激光切割玻璃样品时的烧蚀过程和损伤特性。通过对数值模拟结果与实验结果的比较，揭示了激光损伤机理。激光束传输效应对烧蚀弹坑结构和损伤区域的演化起着决定性的作用，例如干涉、衍射和折射等。在烧蚀弹坑附近的介质中形成了三种损伤区域：一个厚度为微米量级的薄层和两种不同类型的针状尖峰区域。其次，通过比较不同脉冲空间重叠率时的激光损伤实验结果，证实了高自由电子密度造成的损伤区域的存在，并研究了其与热损伤区域之间的关系。第三，实验研究了表面烧蚀切割实验中表面微裂纹和背面烧蚀破坏。大体积热影响区域产生的应力是表面微裂纹的主要诱因，热影响区域的宽度决定了微裂纹的长度；提出了无表面微裂纹的切割方法。残余激光脉冲在样品背面的反射和干涉导致的激光强度局部增强是背面烧蚀破坏的主要原因，而背面烧蚀弹坑的形貌主要取决于激光束的衍射效应。最后，利用该数值模型模拟了多个飞秒激光脉冲烧蚀熔融石英的过程，研究了烧蚀饱和效应。 2. 利用菲涅尔-德鲁德（Fresnel-Drude）模型，研究了超短脉冲激光加工透明介质中的超快反射特性和二次烧蚀效应。模拟分析了s偏振和p偏振的超短脉冲激光激发的介质表面平均反射率与入射角度的关系，与低激光通量时的菲涅尔反射率-入射角度曲线显著不同。以第一次和第二次的入射角度作为二维变量，我们得到了主烧蚀过程中的反射脉冲引起的二次烧蚀分布图。激光诱致的高密度等离子体在超快反射率的演化中起着决定性的作用。相对于主烧蚀过程，反射脉冲引起的二次烧蚀将在弹坑表面产生新的损伤和微裂纹。 3. 数值模拟研究了高重复频率皮秒激光脉冲在块状硼酸盐玻璃中的内部改性机理。首先，提出了一个新颖的数值模型。模拟研究了激光诱致自由电子密度的演化和激光能量的非线性沉积过程以及介质体内的温度分布。讨论了电子损伤和热损伤两种机制在内部改性中的相对作用。根据形貌，内部改性区域分为内层和外层结构，轮廓平滑的外层区域为热扩散导致的热熔融区域；通过考虑热积累和热电离的作用，发现泪滴状的内层结构为高密度自由电子诱致的损伤区域。其次，通过改变脉冲形状或控制双脉冲序列的时间间隔，分析了自由电子密度的激发、弛豫和积累，实现了对能量沉积过程和内部改性区域的控制。通过比较相邻脉冲的时间间隔、等离子体弛豫时间和热扩散时间，我们讨论了两种不同的自由电子密度积累机制。最后，采用随脉冲数目变化的沉积能量分布作为热源，发现并研究了空间不同位置处温度积累的饱和效应。 |
| 英文摘要 | With the rapid development of ultrashort laser technology, ultrashort pulsed laser ablation has been widely used in micromachining of transparent dielectrics because of two major advantages: Ultrashort pulsed laser ablation can provide a localized material modification or removal in the micro/nanometer scale through nonlinear absorption of photons; the separation of energy deposition and material modification/removal in the time domain significantly reduces the destructive thermal and thermo-mechanical effects. Micromachining of transparent dielectrics with ultrashort pulsed laser includes suface ablation (e.g. cutting, drilling and micro-structure fabrication) and internal ablation/modification (e.g. waveguide writing, micro-welding and optical storage). Surface ablation by ultrashort pulsed laser consists of two characteristic aspects, namely, ablation crater and material damage beneath the crater surface. Laser damage, such as refractive index modifications and micro-cracks, is crucial for the quality of the brittle dielectric product. Meanwhile, the secondary ablation induced by the ultrafast-reflected pulse is one of the issues, which have significant influences on the processing quality. For internal modification by high-repetition-rate ultrashort pulsed laser, nonlinear absoption and heat accumulation are the main processes. Till now, the internal modification in bulk transparent dielectrics has been beautifully demonstrated in experiments, but more work is necessary to understand the meachanism, which includes the relationship between nonlinear energy absorption and heat accumulation. In this dissertation, suface ablation and internal modification of transparent dielectrics with picosecond laser pulses are investigated in the following aspects: 1. We study the dynamics of laser ablation and damage in the glass cutting with picosecond pulsed laser. Firstly, we present a novel numerical model for laser ablation and damage in glass including beam propagation and nonlinear absorption of multiple incident picosecond laser pulses. The laser ablation process and the damage characteristics in the glass cutting process were numerically studied and the numerical results were in good agreements with our experimental observations, thereby revealing the damage mechanism induced by laser ablation. Beam propagation effects such as interference, diffraction and refraction, play a major role in the evolution of the crater structure and the damage region. There are three different damage regions, a thin layer and two different kinds of spikes. Secondly, the electronic damage mechanism was verified and distinguished from heat modification by comparing the experimental results with different spatial overlaps of pulses. Thirdly, the surface microcracks and the rear side damage were experimently investigated. The strains in the large-volume heat-affected zones result in the formation of surface microcracks and the width of heat-affected zone determines the microcrack length. The reflection and interference of the residual laser pulse from the surface ablation leads to the local intensity enhancement and thereby provides the probility of the rear-side ablation. The morphology and shape of the ablated crater on the rear side depend on the diffraction pattern of the laser beam. At last, using the presented model, laser ablation of fused silica by multiple femtosecond pulses was also simulated and studied. 2. Ultrafast reflection and secondary ablation induced by ultra-short laser pulses have been theoretically investigated with a Fresnel-Drude model in laser processing of transparent dielectrics. The angular dependence of reflectivity on the ultrafast-laser-excited surface for s- and p-polarized light is significantly different from the usual Fresnel curve in the low-fluence limit. A map of the secondary ablation induced by the reflected pulse is shown along the angles of the first and second incidence. It indicates that the laser-induced plasma plays a major role in the high-reflectivity and thereby the secondary ablation, which may generate damage or micro-cracks underneath the crater wall. 3. We numerically study the mechanism of the internal modification in bulk borosilicate glass by high repetition rate picosecond laser pulses. Firstly, we present a numerical model of internal modification. Laser-induced free-electron dynamics, nonlinear energy deposition and the temperature distribution are studied. The simulated modification regions have good agreements with the experimental results, thereby revealing the roles of electronic and thermal damage in the internal modification. While the contour-smooth outer zone is the molten region, the damage induced by high-density free-electrons is dominant in the inner-structure formation because of the combined effect of heat accumulation and thermal ionization. Secondly, electronic excitation, relaxation and accumulation in the interaction volume are analyzed using different pulse shapes and a double-pulse train. The deposited energy distribution and the internal modification size are controlled by shaping laser pulse and varying the time delay of the double-pulse. Two kinds of electronic accumulation mechanisms are revealed when we compare the time interval of successive pulses with the plasma relaxation time and the thermal diffusion time. At last, with the pulse-number-dependent heat source, the saturation effect of the temperature accumulation was found and discussed. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15765]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 孙明营. 超短脉冲激光微加工透明介质机制与建模[D]. 中国科学院上海光学精密机械研究所. 2013. |
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