Microscopic theory of exciton–polariton model involving multiple molecules: Macroscopic quantum electrodynamics formulation and essence of direct intermolecular interactions

Cavity quantum electrodynamics (CQED) and its extensions are widely used for the description of exciton–polariton systems. However, the exciton–polariton models based on CQED vary greatly within different contexts. One of the most significant discrepancies among these CQED models is whether one should include direct intermolecular interactions in the CQED Hamiltonian. To answer this question, in this article, we derive an effective dissipative CQED model including free-space dipole–dipole interactions (CQED-DDI) from a microscopic Hamiltonian based on macroscopic quantum electrodynamics. Dissipative CQED-DDI successfully captures the nature of vacuum fluctuations in dielectric media and separates them into free-space effects and dielectric-induced effects. The former include spontaneous emissions, dephasings, and dipole–dipole interactions in free space; the latter include exciton–polariton interactions and photonic losses due to dielectric media. We apply dissipative CQED-DDI to investigate the exciton–polariton dynamics (the population dynamics of molecules above a plasmonic surface) and compare the results with those based on the methods proposed by several previous studies. We find that direct intermolecular interactions are a crucial element when employing CQED-like models to study exciton–polariton systems involving multiple molecules.