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

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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.