Transverse phase matching of high-order harmonic generation in single-layer graphene.

The efficiency of high-harmonic generation (HHG) from a macroscopic sample is strongly linked to the proper phase matching of the contributions from the microscopic emitters. We develop a combined micro+macroscopic theoretical model that allows us to distinguish the relevance of high-order harmonic phase matching in single-layer graphene. For a Gaussian driving beam, our simulations show that the relevant HHG emission is spatially constrained to a phase-matched ring around the beam axis. This remarkable finding is a direct consequence of the non-perturbative behavior of HHG in graphene-whose harmonic efficiency scaling is similar to that already observed in gases- and bridges the gap between the microscopic and macroscopic HHG in single-layer graphene.