Temporal and spectral resolved ultrafast ellipsometry and simulation of transient states of matter during laser ablation

直接和诱导利用超短激光脉冲的材料和薄膜系统的（受限）激光烧蚀使得能够在各种工业应用中进行高精度材料处理，硬化玻璃显示器的切割或薄膜太阳能电池和OLED的选择性图案化从文献中已知许多研究如何优化各种材料的直接烧蚀的烧蚀效率。取决于材料，fs-或ps-脉冲显示出更高的烧蚀效率，但对这种行为没有明确的解释。模拟结果和瞬态反射率测量结果表明，电子扩散长度（决定热沉积）和吸收率的变化都与脉冲持续时间有关。通过剥离层系统的完整盘来去除材料。这使得非常精确和非常节能的激光加工远远低于热力学limit.Our驱动烧蚀机制的调查表明，冲击波，这是在超快的时间尺度上产生的，启动的材料传输或删除在纳秒范围内。冲击波可以通过超快加热和膨胀在固相和/或液相中产生，而不是在气相或等离子体相中产生，如纳秒脉冲所观察到的。相变和光学性质（反射和吸收）这些研究是该项目的一部分，将通过测量复折射率来进行利用泵浦-探测椭圆偏振法结合多种物理场和多种材料的多尺度建模。因此，我们需要建立一个在超快时间尺度上的过程模型，然后用这个模型来回答直接烧蚀和诱导烧蚀的基本应用问题，例如最佳激光参数的问题（脉冲宽度、波长、能量密度、光束直径）对于各种材料的最有效处理，最大重复率的问题，脉冲宽度和烧蚀率之间的关系，或光束直径对诱导的激光烧蚀的影响。通过这种方式，该项目的结果将用于优化工业相关的激光工艺。

The direct and induced (confined) laser ablation of materials and thin film systems with ultrashort laser pulses enables high-precision material processing in a variety of industrial applications such as drilling of injection nozzles, cutting of hardened glass displays or selective patterning of thin film solar cells and OLEDs (organic light emitting diodes).From literature numerous studies are known how to optimize the ablation efficiency of direct ablation of various materials. Depending on the material, fs- or ps-pulses show a higher ablation efficiency, but there is no clear explanation for this behaviour. Our simulations and transient reflectivity measurements indicate that both the electron diffusion length, which defines the heat deposition, and the change in absorbance during the irradiation depend on the pulse duration.For the induced ablation, the irradiated material is confined by a transparent layer. The material is removed by a lift-off of an intact disc of the layer system. This enables very precise and very energy efficient laser processing far below the thermodynamic limit.Our investigations of the driving ablation mechanisms suggest that shock waves, which are generated on ultrafast time scales, initiate the material transport or removal in the nanosecond range. The shock waves could be generated in the solid and / or liquid phase by ultrafast heating and expansion, and not in the gas or plasma phase, as observed for nanosecond pulses.In order to get a better understanding of the transient behaviour during the direct and induced ablation, detailed time- and space resolved investigations of the heating, the phase transitions and the optical properties (reflection and absorption) of the irradiated material are necessary especially in the low ps time regime.These investigations are part of the project and will be performed by measuring the complex refractive index with pump-probe ellipsometry in combination with multi-physics and multi-scale modelling for various materials. Thus, a well-founded model of processes on ultrafast time scales shall be developed.The model shall then be used to answer fundamental application-related questions of the direct and induced ablation, such as the question of the optimal laser parameters (pulse width, wave length, fluence, beam diameter) for most efficient processing of various materials, the problem of the maximum repetition rate, the connection between pulse duration and ablation rate, or the influence of the beam diameter on the induced laser ablation. In this way, the results of the project will be used for the optimization of industrially relevant laser processes.