First principles demonstration of strong-light matter coupling and cavity exciton-polaritons via the polaritonic Bethe-Salpeter equation

The strong-coupling limit of light-matter interactions gives rise to hybrid neutral excited states known as exciton-polaritons, which can be realized conveniently when materials are embedded in optical cavities. In addition to a significant renormalization of the exciton dispersion, previous work on intrinsic, free-space exciton-polaritons outside of cavities suggests that this strong coupling regime should also be accompanied by dramatic modifications to the exciton wavefunction [1]. However, existing theories of cavity exciton-polaritons often rely on simple phenomenological models which treat the excitons as an effective, frequency-dependent screening and thus fail to capture the rich interplay between many-body effects and atomistic features. To capture both the cavity-mediated enhancement in light-matter coupling strength and its effect on the excitonic portion of polaritons, we employ a generalization of the polaritonic Bethe-Salpeter equation (pBSE) using a fully-relativistic, retarded electron-hole exchange interaction mediated by a cavity photon propagator. We present ab initio calculations for a monolayer MoTe2 embedded in a prototypical planar Fabry-Pérot cavity, showcasing significant changes in the exciton-polariton dispersions and wavefunctions. We also discuss the extension of the pBSE in which we deduce the photon propagator from more exotic cavity geometries, such as nanophotonic metamaterials, allowing us to obtain quantitative predictions of exciton-polaritons in realistic cavities. Our results show how to realize significant modifications in the spatial, valley, and spin components of the underlying excitons in experimentally accessible cavity geometries.