Exciton absorption, band structure, and optical emission in biased bilayer graphene

TL;DR

This study uses advanced computational methods to analyze how electric fields influence exciton behavior and optical properties in biased bilayer graphene, revealing significant field-dependent changes in exciton states and emission characteristics.

Contribution

The paper provides a detailed ab initio analysis of exciton band structures and optical properties in biased bilayer graphene under varying electric fields, highlighting the field-dependent exciton dynamics.

Findings

Exciton energies and dipole orientations vary significantly with electric field strength.

Low fields favor dark ground state excitons, high fields favor bright excitons.

Radiative lifetimes and emission properties are strongly temperature-dependent.

Abstract

Biased bilayer graphene (BBG) is a variable band gap semiconductor, with a strongly field-dependent band gap of up to $300 \, \text{meV}$, making it of particular interest for graphene-based nano-electronic and -photonic devices. The optical properties of BBG are dominated by strongly bound excitons. We perform ab initio density-functional-theory+Bethe-Salpeter-equation modelling of excitons in BBG and calculate the exciton band structures and optical matrix elements for field strengths in the range $30-300 \, \text{mV}/{\unicode{x212B}}$. The exciton properties prove to have a strong field dependence, with both energy ordering and dipole alignment varying significantly between the low and high field regions. Namely, at low fields we find a mostly dark ground state exciton, as opposed to high fields, where the lowest exciton is bright. Also, excitons preferentially align with a dipole moment opposite the field, due to the field-induced charge transfer in the ground state of BBG. However, in stronger fields, this alignment becomes energetically less favorable. Additionally, the bright excitons show particle- and light-like bands similar to monolayer transition metal dichalchogenides. Finally, we model the radiative lifetimes and emission properties of BBG, which prove to be strongly dependent on temperature in addition to field strength.