Exciton-polaritons and exciton localization from a first-principles interacting Green's function formalism

Exciton-polaritons -- hybrid states of photons and excitons -- offer unique avenues for controlling electronic, optical, and chemical properties of materials. However, their modeling is mostly limited to formalisms that wash out atomistic details and many-body physics critical to describing real systems. Here, we present an ab initio Green's function formalism based on the Bethe-Salpeter equation (BSE) wherein exciton-polaritons naturally emerge through an attractive, dynamical electron-hole exchange interaction. In MgO and crystalline pentacene, this attractive interaction dramatically reduces exciton Bohr radii and increases, by one order of magnitude, transition dipole moments of exciton-polaritons. Our calculations are in good agreement with experimental polariton dispersions in wurtzite CdS, and allow one to capture how electronic and collective excitations in materials are qualitatively modified through polaritonic effects.