Multiphoton ionization of potassium atoms with a sequence of two counter-rotating circularly polarized femtosecond laser pulses produces vortex-shaped photoelectron momentum distributions in the polarization plane describing Archimedean spirals. The pulse sequences are produced by polarization shaping and the three-dimensional photoelectron distributions are tomographically reconstructed from velocity map imaging measurements. We show that perturbative ionization leads to electron vortices with $c_6$ rotational symmetry. A change from $c_6$ to $c_4$ rotational symmetry of the vortices is demonstrated for nonperturbative interaction.

• Received 26 August 2016

DOI:https://doi.org/10.1103/PhysRevLett.118.053003