Electromagnetic enhancement generated by Ap term of cavity quantum electrodynamics demonstrated by single coupled systems between plasmon and molecular exciton

In non-relativistic quantum electrodynamics, an electromagnetic (EM) interaction between a photon and a molecular exciton can be expressed by a Ap term and A^2 term, where A and p are the operators of the vector potential of the EM field and the momentum of the exciton, respectively. We developed a method for investigating the contribution of the Ap and A^2 terms to EM enhancement, which occurs in coupled systems composed of a plasmon polariton and a molecular exciton. The spectral shapes of the Ap and A^2 terms terms, and the EM enhancement were experimentally obtained from absorption, Rayleigh scattering, and ultrafast surface enhanced fluorescence (ultrafast SEF) of the systems, respectively. The relationships between them reveal that the absorption spectra correctly reproduce EM enhancement, indicating that ultrafast SEF can be described as a two-step process using the Ap term. Furthermore, we demonstrate that the origin of spectral deviation between Rayleigh scattering and EM enhancement is subradiant plasmon resonance, which spectra are visualized in the absorption not in Rayleigh scattering, with numerical calculation based on electromagnetism.