  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.
