Direct observation of structure-assisted filament splitting during ultrafast multiple-pulse laser ablation

Laser-induced plasma evolution in fused silica through multipulse laser ablation was studied using pump-probe technology. Filament splitting was observed in the early stage of plasma evolution (before ~300 fs). This phenomenon can be attributed to competition between laser divergent propagation induced by a pre-pulse-induced crater and the nonlinear self-focusing effect. This effect was validated through simulation results. With the increasing pulse number, the appearance of filament peak electron density was postponed. Furthermore, a second peak in the filament and peak position separation were observed because of an optical path difference between the lasers propagating from the crater center and edge. The experimental results revealed the influence of a prepulse-induced structure on the energy distribution of subsequent pulses, which are essential for understanding the mechanism of laser–material interactions, particularly in ultrafast multiple-pulse laser ablation.