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

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，光学材料的纳米结构越来越受到各种新应用的关注。目前，国内外对全息光刻或光场自组装等纳米微球的制备方法进行了广泛的研究。另一种非常有前途的方法是自发形成激光诱导周期性表面结构（LIPSS），这种方法几十年来一直为人所知，可以在金属表面和介电材料中观察到。在特殊情况下，相关的物理机制被很好地理解，例如，入射波的干涉和散射在材料中的波的初始图案形成，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，在强超短脉冲激光照射下，自组织纹波产生的基本机制有很大的不同。特别是，一种特殊类型的LIPSS的形成，其特征尺寸远低于光学波长，即所谓的纳米波纹，到目前为止还知之甚少。对纳米微晶产生的机制和动力学的系统研究，特别是在初始阶段和非线性激励路径方面，仍然缺乏。对于ZnO等非线性系数较大的透明材料，可靠的数据报道较少。因此，拟议项目的一个主要目标是通过包括这些新方面并与充分研究的材料（如铜）的参考数据进行比较来推广LIPSS生成模型。通常，LIPSS结构呈现为二维光栅，具有一定程度的随机畸变，如周期性偏差。期望对基本机制的进一步了解为更好地控制飞秒LIPSS的时空打开道路。我们提出了一个系统的研究LIPSS形成在选定的透明材料，特别是未掺杂和掺杂氧化锌纳米层，具有高的空间和时间分辨率。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成形和诊断、层制造和高分辨率分析方面的具体知识，该项目将实现以下主要目标：研究介电和金属材料中超短脉冲LIPSS形成的基本机制，重点研究动力学、极化依赖性和非线性激发通道，详细探索纳米波形成的本质，并通过包含这些特殊机制进一步推广LIPSS模型。2. 根据晶体结构、掺杂和初始散射特性对非线性薄膜（特别是ZnO）进行裁剪，从而通过对LIPSS的空间、光谱和极化控制来生成定义的功能纳米结构。3. 识别和发展适当的表征技术和数字，以适当地描述表面形貌，包括有序程度。在此过程中，本研究旨在扩展关于激光与材料相互作用的知识基础，固体成分-结构-性能之间的显著相关性，以及解释，推广和控制lipss效应。因此，将使具有传感器应用功能纳米结构的非线性材料迈出第一步。

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.