Ab initio exciton-polaritons using an interacting Green's function formalism

We develop an ab initio Green's function formalism to describe exciton-polaritons, hybrid states of excitons and photons. Our approach considers the exchange interaction, which is typically assumed to be instantaneous, to instead have explicit time dependence. The result is a dynamical, retarded Bethe-Salpeter equation (rBSE). Using this rBSE, we study the renormalization of exciton and photon properties in bulk materials MgO, pentacene, and CdS. We find our predicted polariton dispersion to have excellent agreement with available experimental data for CdS. Near the light cone, we find that the retarded exchange interaction manifests as attractive, leading to a dramatic localization of the exciton Bohr radius and an increase in the exciton dipole moment by over an order of magnitude. We further investigate the relative photon and exciton character of our exciton-polariton states and find strong multi-particle contributions for states near the light cone. We discuss the implications of these findings for computed excitonic properties and the importance of understanding the correct vacuum state.