Femtosecond laser pulse-train induced breakdown in fused silica: The role of seed electrons

Femtosecond laser pulse-train induced breakdown in fused silica is investigated theoretically, with a focus on the role of ultrafast seed electrons during the pulse-train excitation. Material breakdown threshold is investigated by a model, which consists of both the excitation model and an improved optical model by including the optical absorption of self-trapped excitons (STEs). It is found that the evolution of a single train induced breakdown threshold is governed by the interplay of three competing sources of seed electrons initiating an electronic avalanche: residual conduction-band electrons left by the previous pulse, photoionization of atoms in dense media and photoionization of STEs by subsequent pulses. The third source provides a key to the understanding of some potential and existing problems involved, and leads to many pulse-separation independent phenomena (e.g. surface damage/ablation size) for pulse-train processing when it becomes dominant, and can contribute to the repeatable processing. For a single train of two or several femtosecond pulses, the third source can become dominant and sustained at large pulse-separations only when the first-pulse energy is over a critical value, ∼65-75% of the single-pulse breakdown threshold. Our calculations are in agreement with the experimental data.