Momentum alignment and the optical valley Hall effect in low-dimensional Dirac materials

Abstract

This is the author accepted manuscript. The final version is available from Springer via the DOI in this recordWe study the momentum alignment of photoexcited carriers and the optical control of valley population in gapless and gapped two-dimensional Dirac materials. The trigonal warping effect leads to the spatial separation of charge carriers belonging to different valleys upon linearly polarized high-frequency photoexcitation. Valley separation in gapped materials can be detected by measuring the degree of circular polarization of band-edge photoluminescence at different sides of the sample or light spot (optical valley Hall effect). We demonstrate that the celebrated Rashba effect, caused by substrate-induced system asymmetry, leads to a strong anisotropy in the low-energy part of the spectrum. This results in optical valley separation by a linearly polarized excitation at much lower frequencies compared to the high-energy trigonal warping regime. We also show that the momentum alignment phenomenon explains the giant enhancement of near-band-edge interband optical transitions in narrow-gap carbon nanotubes and graphene nanoribbons independent of the mechanism of the gap formation. These enhanced transitions can be used in terahertz emitters based on low-dimensional Dirac materials.European Union FP7European Union Horizon 2020URC