Ablation shape determination under ultrafast laser pulse irradiation

Based on the time when collisions govern the evolution of the phenomena, modeling of ultrafast laser-dielectrics interactions can be divided into three stages: 1) femtosecond pulse absorption through photon-electron interactions, including electron heating, excitation and generation; 2) electron-ion interactions, including energy transport, phase change and plasma generation; and 3) plasma expansion, shock wave propagation and radiation during plasma-environment interactions. This paper reports our ongoing efforts to investigate ablation threshold fluence, depth, and shape during femtosecond laser ablation of dielectrics through the Coulomb explosion and electrostatic ablation. A novel plasma model with quantum treatments is developed to account for significantly varying optical properties. The model is used to successfully predict two uncommon phenomena that were experimentally observed: 1) a flat-bottom crater shape created by a Gaussian beam and 2) repeatable nanoscale structures achieved by pulse train technology. By combining the plasma model and improved two-temperature model, the widely-used assumptions for threshold fluence, ablation depth, and shape in the plasma model based on free electron density are validated by the comparison study and experimental data.