Nonperturbative transverse mode coupling in high-order harmonic generation

This paper is a joint publication with the paper by Vimal et al. [Phys. Rev. Lett. 131, 203402 (2023)]. Highorder harmonic generation (HHG) is an extreme nonlinear optical phenomenon at the heart of modern ultrafast physics. When HHG is driven by two noncollinear beams, one being significantly weaker than the other, the yields of the harmonic emission channels obey a perturbative power law with respect to the drivers' intensity ratio, even though HHG has a nonperturbative efficiency in general. Here, we drive HHG with two different transverse modes and show this power law to be directly imprinted in the spatial structure of the harmonic beamlets. As the driving beam intensities are brought closer, changes in the beamlet profiles reflect a transition from a perturbative to a nonperturbative behavior, understood by generalizing the photon-pathway formalism introduced in the companion paper. This work provides a visual method to study the efficiency of nonlinear optical processes and paves the way for finer all-optical shaping of extreme ultraviolet light.