Cavity Control of Excitons in Two-Dimensional Materials.

We propose a robust and efficient way of controlling the optical spectra of two-dimensional materials and van der Waals heterostructures by quantum cavity embedding. The cavity light-matter coupling leads to the formation of exciton-polaritons, a superposition of photons and excitons. Our first-principles study demonstrates a reordering and mixing of bright and dark excitons spectral features and in the case of a type II van-der-Waals heterostructure an inversion of intra- and interlayer excitonic resonances. We further show that the cavity light-matter coupling strongly depends on the dielectric environment and can be controlled by encapsulating the active two-dimensional (2D) crystal in another dielectric material. Our theoretical calculations are based on a newly developed nonperturbative many-body framework to solve the coupled electron-photon Schrödinger equation in a quantum-electrodynamical extension of the Bethe-Salpeter approach. This approach enables the ab initio simulations of exciton-polariton states and their dispersion from weak to strong cavity light-matter coupling regimes. Our method is then extended to treat van der Waals heterostructures and encapsulated 2D materials using a simplified Mott-Wannier description of the excitons that can be applied to very large systems beyond reach for fully ab initio approaches.