Coulomb explosion and early plasma generation during femtosecond laser ablation of silicon at high laser fluence

In this paper, a two-dimensional hydrodynamic model is presented to investigate the femtosecond laser ablation of silicon in air. Fast electron/ion ejection and the resultant early stage plasma dynamics are studied at laser fluence much higher than the ablation threshold (above 10 J cm(-2)). It is revealed that the early plasma splits into two portions during its expansion, comprised of fast and slow particles, respectively. During the ablation process, ultra-fast ion ejection (before 0.2 ps) from the silicon surface appears to occur before thermal ejection starts. By investigating the ion expansion speed, electric field distribution, and velocity distribution of different ions, the occurrence of Coulomb explosion (CE) is demonstrated in the ablation of silicon at high laser fluence, due to the intense electron emission at the early stage. It leads to a fast ion ejection from the target surface, increasing the material removal rate at the early stage. In contrast, CE is absent in the ablation of copper, although a double-layer effect exists.