  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.
