The effect of emitted electrons during femtosecond laser-metal interactions: A physical explanation for coulomb explosion in metals

Recent experiments in the literature have observed Coulomb explosion (CE) in metals under femtosecond (fs) laser irradiation. This is different from the previous common belief that CE will be strongly inhibited in metals due to the existence of a large number of free electrons with good mobility and the associated screening effect. It is still not well understood why CE can occur in metals. CE requires a sufficiently high outwards pointing electric field in the metal near-surface region. Using a physics-based model, this study shows that during the early stage of fs laser-metal interactions, the emitted electrons due to fs laser irradiation are still very close to the metal target surface, whose effects also need to be considered. The emitted electrons will generate an additional outwards pointing electric field in the target near-surface region, and will also exert a repulsive force on the electrons flowing from the deeper region of the target towards its surface. These effects are helpful to the development of a large outwards pointing electric field in the target near-surface region. The model calculation considering the effects of emitted electrons shows that the electric field at around the target surface can exceed the CE threshold under the studied conditions. The study has provided a physical explanation for why CE can occur in metals under fs laser irradiation.