Interference in photoelectron momentum distributions from strong-field ionization is often interpreted by means of electron trajectories. We show that Coulomb focusing forces certain trajectories through focal points and therefore leads to phase changes (analogous to the Gouy phase in optical beams). In semiclassical modeling, these focal-point effects are encoded in a prefactor of the exponentiated action which affects both the weight and the phase of each trajectory. We demonstrate a scheme based on clustering algorithms that enable a simple classification of electron orbits and, hence, the correct implementation of the semiclassical propagator for various laser-field geometries.
 Simon Brenneckeet alPhys. Rev. Lett.129, 213202, 2022  Simon Brenneckeet alPhys. Rev. Lett. 124, 153202, 2020