Valley-polarization in biased bilayer graphene using circularly polarized light

TL;DR

This paper provides a theoretical analysis of how circularly polarized light can induce nearly perfect valley-polarization in biased bilayer graphene, highlighting optimal conditions and factors affecting polarization.

Contribution

It identifies the optimal pulse frequency for valley-polarization and clarifies the physical origin of polarization in biased bilayer graphene.

Findings

Nearly perfect valley-polarization achievable with proper bias and pulse frequency

Optimal pulse frequency is when photon energy equals the interlayer potential difference

Intervalley scattering and thermal effects reduce valley-polarization

Abstract

Achieving a population imbalance between the two inequivalent valleys is a critical first step for any valleytronic device. A valley-polarization can be induced in biased bilayer graphene using circularly polarized light. In this paper, we present a detailed theoretical study of valley-polarization in biased bilayer graphene. We show that a nearly perfect valley-polarization can be achieved with the proper choices of external bias and pulse frequency. We find that the optimal pulse frequency $\omega$ is given by $\hbar\omega=2a,$ where $2a$ is the potential energy difference between the graphene layers. We also find that the valley-polarization originates not from the Dirac points themselves, but rather from a ring of states surrounding each. Intervalley scattering is found to greatly reduce the valley-polarization for high frequency pulses. Thermal populations are found to significantly reduce the valley-polarization for small biases. This work provides insight into the origin of valley-polarization in bilayer graphene and will aid experimentalists seeking to study valley-polarization in the lab.