Can one turn off Coulomb focusing?

TL;DR

This paper demonstrates that Coulomb focusing persists even under weak Coulomb fields, leading to delayed recollisions that challenge the predictions of the Strong Field Approximation, and explores the underlying nonlinear phase space structures.

Contribution

It reveals the persistence of Coulomb focusing at low Coulomb field strengths and links this to specific phase space structures, providing new insights into high-harmonic generation.

Findings

Delayed recollisions occur even with barely noticeable Coulomb fields.

Energy slightly exceeds the 3.17 U_p cutoff due to these recollisions.

Energetic recollisions are organized by periodic orbits and invariant manifolds.

Abstract

We find that Coulomb focusing persists even when the Coulomb field is barely noticeable compared with the laser field. Delayed recollisions proliferate in this regime and bring back energy slightly above the 3.17 U_p high-harmonic cutoff, 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.