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题名	单束飞秒激光诱导自组织准周期微纳结构及其形成机理
作者	宋娟
学位类别	博士
答辩日期	2008
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 ; 邱建荣
关键词	飞秒激光，单束，准周期，自组织，微结构，透明介质
其他题名	Self-organized quasi-periodic micro-/nanostructures induced by a single femtosecond laser beam and the mechanisms
中文摘要	飞秒激光对透明介质的破坏是基于多光子电离过程和碰撞电离过程的，因此飞秒激光能对透明介质进行三维可选区加工。许多的功能微器件都是由周期性的微结构构成的，比如光子晶体、微纳光栅等。传统的周期微结构加工方法有飞秒激光直写法和多束飞秒激光干涉法。超乎意料的是，借助超显微技术，最近研究人员发现仅用单束飞秒激光即可诱导可控的自组织周期微纳结构。深入研究单束飞秒激光诱导的自组织周期微纳结构不仅在探索高速微纳加工方法方面具有重要意义，还为强场物理、等离子体物理、非线性光学和材料科学等基础学科的理论研究提供了丰富素材，促进其进一步发展。 本文选择了光学和电子学应用领域比较重要的SrTiO3晶体和ZnO晶体作为研究对象，发现单束飞秒激光在材料表面和内部诱导出的一些新的自组织准周期微纳结构，分析了其形成机理，探讨了其可能的应用领域。本文的工作以及所取得的主要创新性成果有以下几个方面： 1．S. Kanehira等人用紧聚焦的单束飞秒激光在硼硅酸盐玻璃内部持续辐照时，发现沿着激光传输方向自组织生长出一串孔洞结构。我们的实验结果显示，在SrTiO3晶体中同样能形成该结构，说明该现象具有普适性。与S. Kanehira等人的报道明显不同的是，我们发现即使激光诱导的结构不接触样品表面，该自组织准周期结构仍然能够形成。我们还研究了自组织准周期点串结构的可控性，发现增加激光能量能够大幅度增加点串结构的长度，而选择合适的聚焦深度除了可增加点串长度外还可明显改善点串结构尺寸和周期的均匀性。由于SrTiO3晶体本身具有很高的线性折射率(2.3)，与形成的孔洞折射率有很大差别，因此在光子晶体应用方面，SrTiO3晶体中诱导这种自组织准周期结构相比在硼硅酸盐玻璃中具有更为重要的意义。 2．对于上面提到的普遍现象，虽然S. Kanehira等人提出了一种形成机理，但该机理成立的前提之一是，飞秒激光诱导的结构接触样品底部是该现象形成的关键，但是我们实验发现该前提并不必要。分析发现自组织点串结构形成的关键在于高数值孔径物镜的运用。考虑到采用高数值孔径的干物镜将光束从空气聚焦到样品内部时，各种非线性效应和空气样品折射率不匹配引起的界面球差都非常显著，我们用数值方法模拟了光从空气聚焦到样品内部的传输过程，模拟出的激光能流密度的轴向分布和横向分布分别与实验观测到的结构的侧视图和顶视图有很好的对应关系。通过比较同时考虑界面球差和非线性效应的情况和只考虑界面球差的情况，发现空气和样品折射率不匹配引起的界面球差才是自组织准周期点串结构形成的主要原因。 3．首次在实验上发现，当用紧聚焦的飞秒激光在样品内部沿着垂直于激光传输方向的直线扫描时，样品内部形成了一个规则的自组织平面微光栅结构。有趣的是，调整单位扫描长度上的辐照脉冲个数可以改变微光栅的条纹取向。我们发展了一个现象学模型来解释微光栅的形成以及条纹方向的可控性。这种一维扫描飞秒激光焦点即可诱导二维自组织微光栅的现象为我们提供了一种在透明介质内高速加工大面积微光栅的新思路。 4．单束飞秒激光辐照ZnO晶体表面诱导了位于激光烧蚀坑中心的三维同心圆环结构和位于圆环结构外环区域的纳米光栅结构。三维同心圆环结构鲜有报道，它的出现通常伴随着一个深的锥形坑的形成。因此我们认为它是由入射光与其在烧蚀坑壁上的反射光干涉产生的周期光强调制引起的，我们利用干涉理论模拟了光强分布，模拟结果与实验观测定性吻合。纳米光栅是一种与激光偏振方向垂直的准周期纳米条纹结构，其周期随着激光脉冲能量的增加和辐照时间的增加均稍有增大。纳米光栅更容易在平坦的地方出现，它在烧蚀坑内的生长具有区域选择性。考虑到材料的有效介电函数随激发出的自由载流子密度的变化，利用纳米等离子体表面的局域场增强模型较好地解释了纳米条纹周期的变化规律，但是该模型仍然无法解释纳米光栅的区域选择性生长现象，有待进一步研究。
英文摘要	Femtosecond laser inducing of damages in transparent dielectrics is based on the multiphoton ionization and the avalanche ionization, so femtosecond laser processing of transparent dielectrics is three-dimensional and region-selective. Many functional micro-devices consist of periodic microstructures, such as photonic crystals, microgratings and etc. Traditionally, fabrication of periodic microstructures using a femtosecond laser can be implemented either by the three-dimensional direct-writing method or by the holographic interference method. Unexpectedly, with the help of the super-microscopy technologies, researchers found that, a single femtosecond laser beam could also induce controllable self-organized quasi-periodic nanostructures. In-depth study of the single laser beam induced periodic microstructures is important not only for the exploring of high-speed micromachining method, but also for the development of fundamental sciences such as material science, plasma physics, high-field physics, nonlinear optics and etc. In this thesis, SrTiO3 crystal and ZnO crystal, which are both important in the application field of optics and electronics, were chosen for our research in the frontier area of single laser beam inducing of self-organized periodic microstructures. Some new kinds of structures were found and corresponding mechanisms were proposed. Moreover, possible applications of these new phenomena were discussed. The main research results are summarized as follows: 1. S. Kanehira et al. reported that a self-organized void string could be induced along the laser propagation direction by a tightly focused laser beam. Our experimental results showed that this kind of structure could also be induced in SrTiO3 crystal, indicating the generality of the phenomenon. A conclusion prominently different from those of S. Kanehira was obtained by us that even if the laser-modified zone wasn’t in contact with the sample surface, the self-organized void array still could be induced. Furthermore, the controllability of the void array was also studied. It was found that the increase of pulse energy notably elongated the void array by up to 5 times, and selection of appropriate focusing depth was not only important for the increase of the void array length but also helpful for improvement of the uniformity in void size and void spacing. Since the refractive index of SrTiO3 (2.3) is apparently higher that of the voids, fabrication of self-organized void array in SrTiO3 crystal is more valuable for the application in photonic crystals than that in borosilicate glass. 2. For the general phenomenon mentioned above, S. Kanehira et al. proposed a formation mechanism based on their experimental conclusion that the condition of the void array formation was the contact of the laser-induced filament with the bottom surface of the sample. However, we found that the condition was not necessary. The analysis showed that the key point for void array formation was the use of high-numerical-aperture (high NA) lens. Taking into account the fact that, in the case of tightly focusing the laser beam from air into sample, not only various nonlinear effects but also the interface spherical aberration resulting from air-sample refractive index mismatch were both prominent, we numerically simulated the propagation process of femtosecond laser pulses from air to sample. The simulated axial and transverse distributions of laser fluence were separately in good agreement with the side view and top view of the void array. By comparing the case with both the spherical aberration effect and the nonlinear effects considered and the case with only consideration of the spherical aberration, we proposed that the interface spherical aberration was the main reason for void array formation. 3. We reported for the first time that a regular self-organized micrograting could be induced in bulk of the SrTiO3 crystal by scanning the laser focus along the line perpendicular to the laser propagation direction. Interestingly, the groove orientation of this kind of micrograting could be controlled by irradiation pulse number per unit scanning length. We developed a phenomenological model to explain the micrograting formation and the controllability of the groove orientation. This new phenomenon of self-formation of two-dimensional microgratings just through one-dimensional scanning of laser focus provides us a new idea in high-speed processing of large-scale microgratings in transparent dielectrics. 4. We found that femtosecond laser irradiation on the surface of ZnO crystal induced the three-dimensional concentric circles at the center of the ablation crater and the nanogratings in the annular shaped region outside the crater center. The three-dimensional concentric circles were always present along with a deep and cone-like crater developed. We proposed that they might be induced by the periodic intensity modulation caused by the interference of the incident light with the light reflected by the crater walls. Numerical simulation based on the interference theory was performed, and qualitative agreement between the experimental results and the simulation results was obtained. The nanograting consisted of quasi-periodic nanoripples perpendicular to the laser polarization direction, and its period slightly increased with increasing both pulse energy and pulse number. The nanograting was more easily induced in the flatter region of the ablation crater and its growth in the ablation crater was not uniform but region-selective. Taking into account the change of the effective dielectrics function with the density of the excited electrons, the model of local field enhancement near under-dense nanoplasmas could qualitatively explain the changing rules of the nanograting period, but it still could not explain the region-selective growth of the nanogratings in the ablation crater. More experimental and theoretical work is needed to further clarify the underlying mechanism.
语种	中文
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/15229]
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	宋娟. 单束飞秒激光诱导自组织准周期微纳结构及其形成机理[D]. 中国科学院上海光学精密机械研究所. 2008.

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