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We report on the study of the optical properties of hybrid systems made from molecular aggregates (J-aggregates) and metallic nanoparticles (Au and Ag). Both entities have outstanding optical properties that have been extensively addressed both through experimental and theoretical efforts. The J-aggregates were chosen for their linear and non-linear excitonic response that shows among other intriguing properties, superradiant emission, ultrafast optical switching, and electroluminescence. These J-aggregates form spontaneously on the surface of Au and Ag nanoparticles, used primarily to excite the organic shell through optical near-field interaction. Steady state as well as ultrafast spectroscopic measurements was used to characterize the nature of the interaction between the two constituents as well as to follow the dynamics of the exciton in the aggregate upon near-field excitation from the particle. For both Au and Ag, Mie scattering theory calculations of the hybrid ground state absorption spectra account for the experimental observations and reflect the coherent coupling between the excitonic states in the aggregate and the nanoparticles electronic transition dipoles. Furthermore we show that the dipole-dipole coupling strength between the individual molecules in the aggregate is increased by ~30% on the surface of the particle compared to its value in solution. The different dynamics of these nanosystems have been probed using femtosecond spectroscopy and reveals contrasted relaxation pathways for the exciton when interacting with Ag with respect to Au as well as a delay dependent excitonic coherent length.
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