In this study, a femtosecond laser beam is delivered to metal wire targets to generate suprathermal electron jets reaching energies of several hundreds of keV. During the process, it is observed that the mirror-imaging distribution of the beam focus with respect to the surface of the target displays highly asymmetric features and different dynamic responses. Especially, the exterior focus exhibits an extraordinary polarity reversal of the macroscopic current, while the interior focus behaves ordinarily. The former is attributed to the strong field at the focal point outside the surface, causing the secondary ionization and driving electrons back to the target, thereby reshaping the distribution of these high-energy hot electrons and the morphology of plasma jets. A numerical model is proposed to simulate the experimental observation and interpret the unexpected phenomenon. Furthermore, the particle-in-cell algorithm is also implemented to verify the results and present more details. This study seeks to emphasize the role of focal position in regulating the photoemission process, which may offer a fresh perspective for research in laser–material interactions and dynamics.
Liu et al. (Fri,) studied this question.