Valley separation of photoexcited carriers in bilayer graphene

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 polarization can be further enhanced via momentum alignment associated with linearly-polarized 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-polarized light analogous to other gapped Dirac materials, such as transition metal dichalcogenides. Consequently, gapped bilayer graphene can be exploited to optically detect valley polarization. Thus, we predict an optical valley Hall effect - the emission of two different circular polarizations from different sides of the light spot, upon linearly-polarized 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.