Figure 2

Short pulse probe beam shadowgraphic microscope images of the evolution of filaments and ionization from expanding energetic electrons within the thin liquid sheet (video provided in the Supplemental Data). Images are integrated over the ~80 fs duration of the probe, and times are ±100 fs. (A) Preceeding the arrival of the main pulse, the target surface is perturbed by prepulse on the picosecond timescale, indicated by the arrow. (B) A filament forms along the target surface in the direction of the laser pulse due to energetic electrons accelerated from coupling between the pulse and surface plasma waves. (C) The rapidly expanding hot electron population induces field ionization in the cold electron population of the liquid leading to a growing dark ring which surrounds the laser point where the laser is incident onto the target. (D) Filaments within the expanding ionization front grow due to anisotropy in the temperature distribution and are reinforced by magnetic fields initiated by the hot electron current. (E) Growth of the ionization front stagnates after 1.14 ps. (F) Recombination occurs as the plasma cools, shrinking the darkened patch. (G) The plasma cools over a relatively long timescale (greater than 100 ps). (H) Filaments persist at late times likely due to non-uniform heating caused by the radial instability.