Enhanced photoluminescence of strongly coupled single molecule-plasmonic nanocavity: analysis of spectral modifications using nonlocal response theory.

Plasmonic nanocavities with highly localized fields in their nanogaps significantly enhance light-matter interactions at the nanoscale, surpassing the diffraction limit. Strong coupling between a plasmonic nanocavity and a molecule forms hybrid upper and lower branch states, resulting in Rabi splitting (RS) in optical spectra. However, scattering and absorption spectra often fail to unambiguously distinguish whether the double peaks arise from energy transparency or RS. In contrast, photoluminescence (PL) clearly reveals the quantum state of a molecule coupled with a plasmon by filtering out background fields. This paper presents a theoretical framework based on nonlocal response theory to calculate the PL of a single molecule coupled with arbitrary metallic nanostructures. Our theory provides an analytical approach to design the spatial arrangement of metallic nanostructures and molecular orbitals and to calculate the PL in strongly coupled systems, addressing limitations in previous studies. Using this framework, we investigated a coupled system comprising a gold nanoplate dimer and a planar porphyrin tape. By modifying porphyrin units to modulate coupling strength, we explored the molecular quantum state coupled with the nanocavity through PL analysis. We elucidated the spectral features of absorption, excitation, and PL in weak and strong coupling regimes and evaluated the dependence of coupling strength on the molecular position and orientation within the nanogap. Our results demonstrate that the quantum state of a molecule in an optically forbidden transition can be excited by the highly localized field in the nanogap. This work advances the fundamental understanding of light-matter interactions at the nanoscale and provides a foundation for the development of future nanophotonic devices.