▎ 摘　　要

Trilayer graphene is receiving an increasing level of attention due to its stacking-dependent magnetoelectric and optoelectric properties and its more robust ferromagnetism relative to monolayer and bilayer variants. Additionally, rhombohedral stacked trilayer graphene presents the possibility of easily opening a gap via either an external electric field perpendicular to the layers or the application of external strain. In this paper, we consider an external electric field to open a band gap in rhombohedral trilayer graphene and study the excitonic optical response of the system. This is done via the combination of a tight-binding model with the Bethe???Salpeter equation solved semianalytically and requiring only a simple numerical quadrature. We then discuss the valleydependent optical selection rules followed by the computation of the excitonic linear optical conductivity for the case of a rhombohedral graphene trilayer encapsulated in hexagonal boron nitride. The tunability of the excitonic resonances via an external field is also discussed together with the increasing localization of the excitonic states as the field increases.