Persistence of Coulomb focusing during ionization in the strong-field regime

We consider the recollision process occurring during the ionization of a neutral atom or molecule by a very strong linearly polarized near-infrared laser field. We find that Coulomb focusing persists even when the Coulomb field of the ion is barely noticeable compared with the laser field. We focus on the consequences of Coulomb focusing for trajectories that maximize the energy exchange upon recollision and thus determine the high-harmonic cutoff. Delayed recollisions proliferate in the strong-field regime and bring back energy slightly above the 3.17Up high-harmonic cutoff (where Up is the pondermotive energy), in stark contradiction with the strong-field approximation. We investigate the nonlinear-dynamical phase-space structures which underlie this dynamics. It is found that the energetic delayed recollisions are organized by a reduced number of periodic orbits and their invariant manifolds.