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Schematic diagram of experimental setup. SPP, spiral phase plate; QWP, quarter-wave plate. To reverse the sign of the total angular momentum J, the SPP and QWP are inverted. A circularly polarized optical vortex pulse is focused onto a metal target by a focusing lens (NA = 0.08, 0.09, 0.13, and 0.18). The optical vortex pulse energy in this experiment was controlled in the range 0.075–0.3 mJ, which is less than one-sixth that used in our previous experiments.
