Ultrashort-pulse laser-induced nanostructuring of nonlinear materials (UP-LINN)

光学材料的纳米结构在从生物医学和化学纤维传感器到数据存储、仿生表面或分子成像的各种新应用中越来越引起人们的兴趣。目前，诸如全息光刻或光场支撑的纳米球自组装等特定的制备方法在世界范围内得到了广泛的研究。另一种非常有前途的方法是自发形成激光诱导的周期性表面结构(LIPSS)，这种结构几十年前就被发现了，可以在金属表面和介质材料中观察到。对于特殊情况，可以很好地理解相关的物理机制，例如，入射波和散射波在材料中的干涉形成初始图案，电子-声子耦合和电子扩散(如铜的情况)。然而，最近的研究表明，在高强度超短脉冲激光照射下，自组织波纹产生的基本机制有很大的不同。特别是，一种特征尺寸远低于光学波长的特殊类型的LIPSS的形成，即所谓的纳米颗粒，到目前为止还知之甚少。关于纳米粒子产生的机制和动力学的系统研究，特别是关于初始阶段和非线性激发路径的研究，仍然缺乏。对于像氧化锌这样具有大非线性系数的透明材料，几乎没有可靠的数据报道。因此，拟议项目的一个主要目标是通过纳入这些新的方面并与来自经过充分研究的材料(例如铜)的参考数据进行比较来推广LIPSS的生成模型。通常，LIPSS结构表现为具有一定程度的随机失真的2D栅格，例如与周期的偏差。人们期望，对基本机制的改进将为更好地控制飞秒激光脉冲产生器的时空控制开辟道路。我们建议系统地研究在选定的透明材料中的LIPSS的形成，特别是具有高空间和时间分辨率的未掺杂和掺杂的氧化锌纳米层。结果将与铜等金属层的情况进行比较。通过结合两个研究所在超短脉冲整形和诊断、薄膜制备和高分辨率分析方面的专门知识，该项目将解决以下主要目标：1.研究介质和金属材料中超短脉冲LIPSS形成的基本机制，特别是动力学、偏振相关性和非线性激发通道，以详细探索纳米脉冲形成的本质，并通过纳入这些特殊机制来进一步推广LIPSS模型。2.在晶体结构、掺杂和初始散射特性方面对非线性薄膜(特别是氧化锌)进行调整，以通过LIPSS的空间、光谱和偏振控制来生成特定的功能纳米结构。3.确定和发展适当的表征技术和优值系数，以适当地描述表面形貌，包括有序度。在这一过程中，本研究的目的是扩展关于激光材料相互作用的知识基础，即固体组成-结构-性质之间的显著相关性，以及解释、概括和控制LIPSS效应。因此，迈向用于传感器应用的具有功能纳米结构的非线性材料的第一步将成为可能。

Nanostructuring of optical materials is of increasing interest for various new applications ranging from biomedical and chemical fiber sensors to data storage, bio-mimetic surfaces or molecular imaging. Presently, selected fabrication methods like holographic lithography or light-field supported self-assembling of nanospheres are world-wide investigated in extenso. An alternative, very promising method is the spontaneous formation of laser-induced periodic surface structures (LIPSS) which are known for a few decades and can be observed in metal surfaces as well as dielectric materials. For special cases, the responsible physical mechanisms are well understood, e.g. the initial pattern formation by the interference of the incident wave and a wave scattered in the material, electron-phonon coupling and electron diffusion (like in the case of copper). Recent investigations, however, indicated significantly different basic mechanisms of self-organized ripple generation under highly intense ultrashort-pulse laser irradiation. In particular, the formation of a special type of LIPSS with feature sizes far below the optical wavelength, so-called nanoripples , is only poorly understood up to now. A systematic study of the mechanisms and dynamics of nanoripple generation, in particular with respect to initial stage and nonlinear excitation paths, is still missing. For transparent materials with large nonlinear coefficients like ZnO, only few reliable data were reported. Therefore, one main target of the proposed project is to generalize the model of LIPSS generation by including these new aspects and comparing to reference data from well-studied materials (like, e.g., copper). Typically, LIPSS structures appear as 2D gratings with a certain degree of random distortions such as deviations from periodicity. It is expected that an improved knowledge about the elementary mechanisms opens the road to a better spatio-temporal control of femtosecond LIPSS. We propose a systematic investigation of the LIPSS formation in selected transparent materials, in particular undoped and doped ZnO nanolayers, with high spatial and temporal resolution. The results will be compared to the case of metallic layers like copper. By combining specific know-how of both institutes in shaping and diagnostics of ultrashort pulses, layer fabrication and high-resolution analytics, the following main objectives of the project will be tackled: 1. Study of basic mechanisms of ultrashort-pulse LIPSS formation in dielectric and metallic materials with particular emphasis on the dynamics, polarization dependence and nonlinear excitation channels to explore the nature of nanoripple formation in detail and to further generalize the LIPSS model by including these particular mechanisms. 2. Tailoring of nonlinear thin films (in particular ZnO) with respect to crystalline structure, doping and initial scattering characteristics to enable the generation of defined functional nanostructures via a spatial, spectral and polarization control of LIPSS. 3. Identification and development of appropriate characterization techniques and figures of merit to properly describe the surface topography including the degree of order. Following this course, this study is aimed to extend the knowledge basis concerning lasermaterial interaction, significant correlations between composition-structure-properties of solids, as well as to explain, generalize and control the LIPSS-effect. Thus, first steps towards nonlinear materials with functional nanostructures for sensor applications will be enabled.