Optical Spectra of Plasmon-Exciton Core-Shell Nanoparticles: An Anisotropic Classical Model Eliminates Discrepancies with Experiments

The optical properties of the hybrid core--shell nanostructures composed of a metallic core and an organic shell of molecular J-aggregates are determined by the electromagnetic coupling between plasmons localized at the surface of the metallic core and Frenkel excitons in the shell. In cases of strong and ultra-strong plasmon--exciton coupling, the use of the traditional isotropic classical oscillator model to describe the J-aggregate permittivity may lead to drastic discrepancies between theoretical predictions and the available experimental spectra of hybrid nanoparticles. We show that these discrepancies are not caused by limitations of the classical oscillator model itself, but by considering the organic shell as an optically isotropic material. By assuming a tangential orientation of the classical oscillators of the molecular J-aggregates in a shell, we obtain excellent agreement with experimental extinction spectra of TDBC-coated gold nanorods, which cannot be treated with the conventional isotropic shell model. Our results extend the understanding of the physical effects in the optics of metal--organic nanoparticles and suggest an approach for the theoretical description of such hybrid systems.