Helicity flip of high-harmonic photons in Haldane nanoribbons

TL;DR

This paper explores how high-harmonic photons emitted from Haldane nanoribbons can undergo helicity flips due to phase differences in Bloch states, revealing new polarization effects in finite two-dimensional systems.

Contribution

It demonstrates that thin Haldane nanoribbons exhibit elliptically polarized high harmonics with helicity flips, a phenomenon not previously reported in bulk systems.

Findings

Harmonics are elliptically polarized despite thin geometry.

Helicity flips occur at specific harmonic orders.

Phase differences in Bloch states cause the helicity change.

Abstract

Recent studies in high-harmonic spectroscopy of condensed matter mainly focused on the bulk of the system under consideration. In this work, we investigate the response of thin, hexagonal nanoribbons to an intense laser pulse that is linearly polarized along the ribbon. Such nanoribbons are prime examples of two-dimensional systems that are bulk-like in one direction and finite in the other direction. Despite the atomically thin scale in the direction perpendicular to the linearly polarized driving laser field, the emitted harmonics are elliptically polarized if an alternating onsite potential and Haldane hopping is taken into account. For given hoppings, we find a sudden change of the helicity for a certain harmonic order. The origin of this flip is traced back to phase differences between the components of Bloch states.