  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.
