飞秒激光作用下透明材料的烧蚀机理及其超快动力学研究
文献类型：学位论文
| 作者 | 李晓溪 |
| 学位类别 | 博士 |
| 答辩日期 | 2005 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 徐至展 |
| 关键词 | 飞秒激光 透明材料 烧蚀机理 超快探测 |
| 其他题名 | Study on ablation mechanism and ultra-fast dynamical property of transparent dielectrics induced by femtosecond laser |
| 中文摘要 | 光与物质相互作用是光物理学的一个重要研究领域。研究超短脉冲激光与透明介质的相互作用，对于认识飞秒激光作用下材料的烧蚀机理、促进飞秒激光微加工应用等方面具有重要意义。本文主要以飞秒激光为辐照源，较系统地研究了80Onm飞秒激光作用下石英玻璃、氧化铝及氟化铿等透明介质的破坏闭值和烧蚀规律。利用光学参量放大（OPA）激光系统研究了透明材料的烧蚀闭值对波长的依赖关系。同时，成功地建立了泵浦一探针超快实验探测平台，研究了透明介质烧蚀过程中电子激发、等离子体的演化、材料烧蚀等超快动力学过程，讨论了材料烧蚀的物理机制。另外，还发展了雪崩击穿的物理模型，利用该物理模型对实验结果作出了合理的解释。本论文取得了创新性的研究成果，归纳如下：1．首先，本文研究了800nln和400lun飞秒激光作用下材料的烧蚀规律，发现烧蚀深度与激光强度近似成对数关系，烧蚀体积随激光强度线性增加；同时发现短波长的激光可以提高晶体材料微加工的质量。其次，本文实验研究了800nLIn飞秒激光作用下石英玻璃、A1203和LIF晶体的破坏闭值与激光脉宽的依赖关系。实验证实在超短脉冲范围内，即T＜IPs，材料的烧蚀闭值随脉宽的减小而降低。实验结果表明，飞秒激光作用下，电子激发仍然以碰撞电离为主，光致电离为碰撞电离提供了种子源。在理论方面，本文发展了雪崩击穿模型。将量子方法与经典近似相结合，研究了导带电子的光吸收，根据double-nux模型和Keldysh理论分别计算了雪崩速率和光致电离速率。数值计算了材料中导带电子数密度的演化。考虑了导带电子的扩散，研究了材料中电子密度、激光能量沉积密度的分布，成功解释了材料的破坏阂值与脉冲宽度的依赖关系，烧蚀深度、烧蚀体积与激光强度的依赖关系。2．利用光学参量放大（OPA）激光系统，获得了从紫外到中红外波段（26Onm-2O00nm），透明材料的烧蚀闭值随波长变化的依赖关系。实验表明，当激光波长小于800nm，材料的烧蚀闭值随波长的减小而降低；当激光波长在100Onm到2000nm范围内，材料的烧蚀闭值变化不大。利用单导带模型较好地解释了长波长的实验结果；对于短波长的实验结果，在考虑导带的子能带间跃迁引起的导带电子的光吸收后，实验与理论结果符合得较好。3．采用长脉宽激光激发，短脉宽激光探测的泵浦一探测技术，详细地研究了材料烧蚀过程中电子的激发过程。实验结果表明，透明材料反射率的上升主要与两个过程密切相关，一是在泵浦激光脉冲作用时间内导电电子数密度的快速增加，二是激光脉冲过后的超快非热相变过程。利用矩阵光学与强场激光作用下介质的介电常数、折射率及消光系数与自由载流子密度的依赖关系，在本文建立的破坏物理模型基础上，对实验结果进行了数值模拟计算，两者吻合的较好，证明该物理模型是有效的。4．对飞秒激光作用下材料烧蚀过程中反射率时间演化曲线的实验结果分析表明：随着激光脉冲到达材料的表面，材料的导带电子数密度迅速上升，并形成稠密等离子体，从而引起材料的破坏。材料破坏是由超快熔化和微爆炸两种机制共同作用的结果，对于强电一声耦合作用材料，超快熔化的破坏机制在烧蚀过程中起主导作用；而对于弱电-声耦合材料，微爆炸机制则能够对材料的烧蚀过程做出较好的解释。 |
| 英文摘要 | The interaction between light and materials has been studied intensively in the recent fifteen years, especially the ultra-short laser pulse induced damage in the optical materials. It can help us to make clear the physical mechanism of laser-induced breakdown and develop the application of femtosecond (fs) lasers in micromachining. With an fs laser at 800nm, the breakdown threshold and the ablation mechanism of transparent materials were studied, such as silica, sapphire, lithium fluoride and so on. By means of an optical parametric amplification (OPA) laser system the dependence of the materials' breakdown threshold on the wavelength of irradiating laser were presented. At the same time, a pump-probe ultra-fast detecting system was built up, by which the electronic excitation, the development of plasma and the ablation process of materials were studied and the mechanism of ablation was discussed. Meanwhile, the avalanche breakdown model was consummated and our experimental results were interpreted with the model. The calculation results based on the model agreed well with the experimental results. The main work and results are as follows: 1. Firstly, the ablated pits irradiated by laser pulses at 800nm and 400nm were observed respectively. It was found that the ablated depth of materials keeps a logarithmic relationship with the laser fluence, and the ablated volume of material increases linearly with the accretion of laser intensity. Meanwhile it was validated that the ablation quality can be improved by using lasers at shorter wavelengths. Secondly, we reported the dependence of damage threshold in fused silica, AI2O3, and LiF crystal on the pulse duration in the range of 40-800fs at 800nm and 400 nm. The results showed that the ablation threshold of materials became smaller step by step with the shortening of pulse duration. The fact indicated that the impact ionization plays a dominant role in the production of conduction band electron while the multiphoton ionization provides seeds for it. Finaly, we made our efforts in theoretical study. Avalanche breakdown model and micro-explosion have been proposed to explain the experimental phenomenon involved in fs-laser micromachining. However, the previous theory only calculated the conduction band electron (CBE) production and the ablation laws and the ultra-fast dynamics of electrons can't be understood very well. In our theoretical model, the impact ionization rates and the photoionization rates were calculated based on double-flux model and Keldysh theory, respectively. Then the development of CBE and the distribution of laser energy in materials were calculated numerically. Based on the model the damage thresholds, ablation depths and ablation volumes, all agreed well with the experimental results. 2. By means of an OPA laser system, whereby laser pulses at wavelengths ranging from UV to IR were obtained, we got the dependence of the breakdown threshold of materials on the wavelength of laser. The experimental results indicated that the threshold fluence increased rapidly with the shortening of laser wavelengths as 260nm< X <800nm, and changes slowly in the range of 1000nm< A <2000nm. We developed a coupled dynamic theory to study the distribution of laser intensity, CBEs generation and laser energy deposition. While the wavelength of incident laser is longer than 800nm, we can explain our experimental results well with the single conduction band model. While the wavelength of laser is shorter than 800nm, the calculation results based on multi-conduction-band model agree well with our experiment. Adopting the pump-probe technology with a wide pulse duration laser as irradiating source and a short pulse duration laser as probe light, we studied the electron exciting process in the ablation of materials in detail. The experimental results showed that the reflectivity of materials varied with the incident laser in two phases. During the laser pulse duration, the reflectivity of materials rises rapidly with the extending CBEs. After the pulse duration, the reflectivity of materials increases slowly with the development of plasma via a non-thermal phase transfer. By means of matrix optics and relationship between the dielectric constant and the density of CBEs, we calculated the development of reflectivity while irradiated by fs laser and the results agree well with our experimental results. Analyzing the development of optical materials' reflectivity in the ablation process, we found that there were two different physical mechanisms in the ablation of materials: ultra-fast melting process and micro-explosion. When the laser irradiates the material in which the couple function between the electrons and the phonons are very strong, the energy absorbed by the electrons from the laser pulse can be transferred to the crystal lattice rapidly. Hence the ultra-fast melting process happens on the focus in the material. When the materials in which the couple function between the electrons and the phonons is weak are irradiated by fs laser pulses, the electrons excited by the high energy laser can't transfer their energy to the crystal lattice in time. Hence the electrons diffuse quickly and the micro-explosion happenes. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15799]  |
| 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 | 李晓溪. 飞秒激光作用下透明材料的烧蚀机理及其超快动力学研究[D]. 中国科学院上海光学精密机械研究所. 2005. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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