Valley separation of photoexcited carriers in bilayer graphene

TL;DR

This paper investigates how photoexcited carriers in bilayer graphene exhibit valley-dependent behavior due to anisotropic band structure and optical selection rules, enabling potential valleytronic applications in the terahertz range.

Contribution

It derives the angular generation density of carriers, reveals valley separation effects, and proposes experimental setups for optovalleytronic devices in bilayer graphene.

Findings

Valley separation occurs due to anisotropic band structure at low energies.

Gapped bilayer graphene shows valley-dependent optical selection rules.

Predicted optical valley Hall effect with circularly-polarized light.

Abstract

We derive the angular generation density of photoexcited carriers in gapless and gapped Bernal bilayer graphene. Exploiting the strong anisotropy of the band structure of bilayer graphene at low energies due to trigonal warping, we show that charge carriers belonging to different valleys propagate to different sides of the light spot upon photoexcitation. Importantly, in this low-energy regime, inter-valley electron-phonon scattering is suppressed, thereby protecting the valley index. This optically induced valley polarisation can be further enhanced via momentum alignment associated with linearly-polarised light. We then consider gapped bilayer graphene (for example with the gap induced by external top- and back-gates) and show that it exhibits valley-dependent optical selection rules with circularly-polarised light analogous to other gapped Dirac materials, such as transition metal dichalcogenides. Consequently, gapped bilayer graphene can be exploited to optically detect valley polarisation. Thus, we predict an optical valley Hall effect - the emission of two different circular polarisations from different sides of the light spot, upon linearly-polarised excitation. We also propose two realistic experimental setups in gapless and gapped bilayer graphene as a basis for novel optovalleytronic devices operating in the elusive terahertz regime.