Previously the influence of picosecond laser spot size on ablation depth and threshold fluence on copper has been experimentally investigated. In order to have a comprehensive understanding of the corresponding mechanisms for high laser fluence cases, in which the laser fluence is several folds larger than the optimal ablation fluence, an axisymmetric 2D model, combining modified Two Temperature Model (TTM) and hydrodynamics, was developed. Since the dense vapor and plasma shielding effects have a significant impact on the absorbed energy of incident laser on the material surface, especially for the situation where the laser fluence is higher than vaporization threshold, several assumptions were made. It was roughly supposed that when the lattice temperature reaches 0.9Tc (Tc denotes the critical temperature of copper) dense vapor forms above the surface of the material due to the homogenous nucleation within the superheated melted layer, and once the surface temperature exceeds Tc stronger absorption of incident laser by plasma starts to play a crucial role. As the optical thickness of both dense vapor and plasma were supposed to be constants, correspondingly, the transmittance of both layers were approximately evaluated. Furthermore, as generally supposed in the literature, considerable energy loss caused by homogenous nucleation was also taken into account once the surface temperature of the lattice increased to 0.9Tc, and the melted material was treated as weakly compressible laminar flow with low Mach number (Ma<0.3). The numerical results indicate that the kinetic energy of the evaporated material increases when the laser spot size decreases, which could be a possible mechanism of the deeper ablation depth per pulse observed in the experiments with smaller laser spot sizes. Due to the occurrence of phase explosion, the surface temperature keeps constantly around 0.9Tc, and the intensive evaporation could remain the temperature at Tc until the establishment of equilibrium of both subsystems. Finally, the calculated evaporated mass has the same order of magnitude as the corresponding experimental data.
Yiming Zhang, Beat Neuenschwander, and Valerio Romano, "Numerical study of the influence of picosecond laser spot size on laser ablation of metal for high laser fluence cases," Proc. SPIE 10091, Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXII, 100910S (Presented at SPIE LASE: February 01, 2017; Published: 28 February 2017); https://doi.org/10.1117/12.2253629.
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