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

Biased bilayer graphene (BBG) is a semiconductor with a strongly field-dependent band gap of up to $300\phantom{\rule{4pt}{0ex}}\text{meV}$, making it of particular interest for graphene-based nanoelectronic and nanophotonic devices. The optical properties of BBG are dominated by strongly bound excitons. We perform ab initio density-functional-theory$+$Bethe-Salpeter-equation modeling of excitons in BBG and calculate the exciton band structures and optical matrix elements for field strengths in the range $30--300\phantom{\rule{4pt}{0ex}}\text{mV/\AA{}}$. 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 regimes. 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 lightlike 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.