Influence of Impact Ionization on the Laser Ablation of Silicon Surface by Ultrashort Pulses in the Near and Mid-IR Range

We investigated the influence of impact ionization on the laser ablation of silicon using ultrashort pulses in the near and mid-infrared ranges. Using electron microscopy and numerical modeling of the ionization process based on dynamical rate equations, we demonstrated that the transition to the mid-IR range significantly changes the mechanisms of plasma generation. In the near-IR range, photoionization (both multiphoton and tunneling) predominates, while the contribution of avalanche ionization becomes dominant for pulse durations exceeding 500 fs. Consequently, the smallest size of microstructures on the silicon surface during laser ablation is achieved for picosecond pulses, where the electron concentration in the generated plasma is low and laser beam defocusing is negligible. In contrast, in the mid-IR range, impact ionization becomes the dominant mechanism for increasing the electron density of the plasma, enabling high spatial resolution during the ablation process at the shortest pulse durations, as impact ionization proves to be more effective. The results of this study contribute to a better understanding of the mechanisms of laser radiation interaction with material surfaces and can be applied to optimize laser processing techniques in various fields.