Self-organized structures in ultrashort pulsed laser processing

在超短激光脉冲（USP）激光材料加工中，既无法定量预测加工结果，也无法控制潜在的自组织结构形成等效应。它们的控制产生和避免是生产技术的重要课题。一方面，它们导致分辨率的降低和表面粗糙度的增加；另一方面，表面结构的变化可以导致有用的性质，如独特的疏水性。因此，本课题将开发并应用多物理过程模型来模拟金属的USP加工。作为中间目标，单脉冲将用于分析是否有可能利用物质热流的瞬态描述和状态方程完全确定温度、压力和密度。此外，本文还研究了散裂和相爆是否可以用流体力学（CFD）方法完全解释。该项目的目的是利用多脉冲探索自组织结构如锥形突起（CLP）和激光诱导周期表面结构（LIPSS）的形成。研究了由表面粗糙度和流体动力学引起的强度变化对其形成的影响。计划中的项目将分三个阶段进行。在第一个项目中，由光子技术研究所（LPT）开发的多物理USP过程模型将与慕尼黑应用科学大学（LHM）激光中心开发的吸收和热输入模型相结合。此外，状态方程和电子材料参数将建模。通过求解麦克斯韦方程组来计算局部强度的模型将被开发和包括在内。模型的发展将不断伴随着实验的比较和迭代模型的改进。这导致了对热动力学和过程动力学的非常详细和全面的研究，这是该项目的基础。第二阶段着重于单脉冲处理。利用仿真、泵-探针椭偏和高速摄像技术，将验证模型描述的一致性，并期望改进经验过程的理解。第三阶段涉及多脉冲处理。通过模拟，原位泵探针显微镜和SEM和LSM分析了多脉冲处理的经验过程，提高了对多脉冲处理的理解。重点是流体动力学效应对CLP-和lipss形成的影响，以及由表面粗糙度和等离子体效应引起的非均匀周期性能量耦合到工件的额外影响。

In laser material processing with ultrashort laser pulses (USP) it is neither possible to quantitatively predict processing results nor to control potentially occurring effects like the formation of self-organized structures. Their controlled generation and avoidance is of high interest for production technology. On the one hand they cause a reduction in resolution and an increase in surface roughness; on the other hand changes in the surface structure can lead to useful properties like distinctive hydrophobicity. Therefore, a multi-physical process model to simulate USP processing of metals will be developed and applied in this research project. As intermediate aim, single pulses will be used to analyse if it is possible to completely determine temperature, pressure and density using a transient description of the material heat flows and the equation of states. Furthermore, it will be studied if spallation and phase explosion can be fully explained by a fluid dynamic (CFD) approach. The aim of the project is to explore the formation of self-organized structures like cone-like protrusions (CLP) and laser-induced periodic surface structures (LIPSS) by using multiple-pulses. The influence of intensity variations caused by surface roughness and fluid dynamics on their formation will be studied.The planned project will be organised in three phases. In the first one, a multi-physical USP process model, developed at the Institute of Photonic Technologies (LPT), will be coupled with a model for absorption and heat input, developed at the Lasercenter at the Munich University of Applied Sciences (LHM). Additionally, the equation of state and electronic material parameters will be modelled. A model to calculate the local intensity by solving the Maxwell’s equations will be developed and included. The development of the model will be continuously accompanied by experiments for comparison and iterative model improvement. This leads to a very detailed and comprehensive study of the thermo- and process dynamics, which builds the basic of the project.The second phase focuses on single pulse processing. Using simulations, pump-probe ellipsometry and high-speed videography, the consistency of the model descriptions will be verified and an improvement of the empirical process understanding is expected. The third phase addresses multi-pulse processing. Using simulations, in-situ pump-probe microscopy and SEM and LSM analyses the empirical process understanding of multi-pulse processing will be improved. The focus is kept on the influence of fluid dynamic effects on CLP- and LIPSS-formation and the additional impact of inhomogeneous, periodic energy coupling into the workpiece, caused by interference effects of the surface roughness and by plasmonic effects.