深亚波长尺度超快激光诱导固体电离的机制研究

Since the early days of the laser science, laser-induced solid damage has been intensively studied for over four decades, not only because of its broad industrial applications, but also because of the rich physics. In the last decade, along with the application of ultrafast lasers in the field of laser ablation, the vavious deep-subwavelength structures induced by ultrafast lasers have attracted considerable interest of researchers. However, by far the mechanisms of ultrafast laser-induced deep-subwavelength structures is still unclear, which is due to the complexity of the ionization process related to the structure formation occurring at a ultra-short time scale and a deep-subwavelength spatial scale: all kinds of high nonlinear ionization mechanisms, such as multi-photon ionization, tunnelling ionization, impact ionization, and certain complex ionization mechanisms, intertwine and make the experimental and theoritical studies become very difficult; particularly, in the previous studies towards the ionization mechanisms, researchers always focus on the effects of the time scale of ultra-short laser pulses, and ignore the effects of the spatial scale of laser-induced structures. Actually, taking into account the typical deep-subwavelength characteristics induced by ultrafast laser pulses, it is clear that the effects of the spatial scale on the ionization mechanisms are worth focusing on. In the project, we will delve into the effects of the spatial scale on the ultrafast laser-induced ionization of solids, study the change of the ionization mechansisms of solids due to the change of the characteristics of light field and ionization at the deep-subwavelength scale, in particular the appearance of a variety of singular nano-optics effects and the spatial localization of the ionization process, and thus deepen our understanding about the physical nature of the ionization mechanisms of solids and the formation of deep-subwavelength structures induced by ultrafast laser.

激光诱导固体破坏一直是激光与物质相互作用领域的一个研究重点，蕴涵着丰富的物理意义。近十年随着超快激光在烧蚀领域的应用，其诱导的各种深亚波长结构引起广泛关注。但到目前为止，这些奇异结构的形成机制并未完全清楚。这是由于其形成涉及在超短时间尺度下局域在深亚波长空间的复杂固体电离过程：多种高度非线性电离机制，如多光子、隧穿、碰撞电离，及一些复合电离机制交织在一起，给相关研究带来很大困难；特别是，在之前研究中对上述电离机制，一般只探讨脉冲时间尺度效应的影响，而忽略结构空间尺度效应的影响。事实上，考虑超快激光诱导的典型深亚波长特征，空间尺度效应值得重点关注。在本课题中，我们将深入研究空间尺度效应对超快激光诱导固体电离的影响，探讨深亚波长尺度下由于光场及电离特性的改变，特别是纳米光学效应的出现及电离的空间尺寸限域，所导致的电离机制的改变，从而加深对超快激光诱导固体电离及深亚波长结构形成的物理本质的理解。

在本项目中，我们围绕超快激光诱导固体破坏过程中的基本电离机制及动力学过程进行深入研究，重点基于超快激光诱导固体表面亚波长结构的现象，探讨深亚波长结构产生条件下由于光场及电离特性改变，特别是纳米光学效应的出现，所导致的电离及烧蚀机制的改变，进而加深对超快激光诱导固体电离及深亚波长结构形成物理本质的理解。基于本核心研究目标，我们系统开展超快激光诱导固体电离及烧蚀的稳态和瞬态实验及数值研究，获得若干重要创新性成果：1）考察了ZnO飞秒激光诱导结构由近亚波长至深亚波长尺度演化的特性，探讨结构特征尺寸对超快激光诱导瞬态强场建立、材料超快电离及烧蚀过程的重要影响，并基于深亚波长尺度等离激元理论及固体强场电离机制，提出深亚波长尺度下超快激光诱导固体电离及非热烧蚀的、与准静态表面等离激元激发相关的物理模型，揭示飞秒激光烧蚀中“深亚波长空间尺度”与“超短时间尺度”两要素间的物理关联；2）围绕超快激光诱导亚波长光栅的分裂现象，探讨纳米光学效应，特别是等离激元效应，导致超快激光光场在材料表面重新分布并诱导产生局域强场电离形成深亚波长结构的微观机制；3）基于高精度超快激光固体强场电离瞬态实验，观察到近破坏阈值超短脉冲辐照熔融石英过程中光致及碰撞两种电离机制的作用过程及特征时间尺度，揭示超快激光诱导熔融石英破坏的主导电离机制；4）通过飞秒激光结构化表面强场电离瞬态实验，证实深亚波长结构对超快激光强场电离过程的显著增强效果，为超快激光诱导深亚波长结构现象的微观电离机制及动力学过程分析提供直接实验依据；5）开发固体强场电离电磁场数值模拟程序，开展熔融石英及ZnO近破坏阈值超快激光诱导固体电离数值模拟，证实深亚波长尺度下纳米光学机制对超快激光固体强场电离过程的重要影响，为超快激光诱导深亚波长周期结构起源提供直接理论依据；6）发展一种具有亚皮秒时间及亚微米空间分辨能力的超快高分辨多帧瞬态成像探测方法，并建立一套完整的探测系统。