Influence of the Chemical Structure on Molecular Light Emission in Strongly Localized Plasmonic Fields

The coupling between a molecular emitter and an optical cavity is often addressed theoretically with the molecule regarded as a point dipole, thus lacking any information on chemical structure. This approximation usually works well because the spatial extent of the electromagnetic fields considered is typically spread over a larger volume than the size of the molecule. However, in extreme plasmonic structures as those used in state-of-the-art nanophotonics, the local electric field is much more confined, producing an inhomogeneous spatial distribution of photonic states reaching 1 nm or less, comparable or even smaller than the molecular size. In such a situation, it is necessary to consider the spatial distribution of the electronic transitions in the molecule to properly describe plasmon-exciton coupling. By introducing the concept of electronic transition current density to describe the excitonic emission from a single molecule, we are able to account for the inhomogeneity of the plasmonic field in the process of light emission and analyze its properties. With the use of this formalism, we address the modification of light emission from a molecule placed at a subnanometer distance from an atomic-scale feature of a plasmonic structure, indicating the failure of the point-dipole approximation and the importance of considering the spatial distribution of both photonic and electronic states.