Interactions Between Localized Surface Plasmons and Molecular Resonances

Licentiate thesis, 2013

Molecular plasmonics is the study of interactions between plasmonic nanostructures and molecules. It has been basis for fundamental understanding of light-matter interactions and development of many technological applications, such as biological and chemical sensing, plasmon-enhanced spectroscopies, optical switches, and plasmon-enhanced energy harvesting. When the plasmon energy of a metal nanostructure is degenerate with the absorption energy of a nearby molecule, the interaction becomes resonant. Strong and confined electromagnetic field induced by the metal nanoparticle polarizes the molecule. This consecutively modifies the electron oscillations in the metal nanostructures. As a result, optical responses of both the molecules and the plasmonic nanostructures are changed by the interaction of the molecule and plasmons. Organic chromophores interacting with plasmonic nanostructures constitutes an important part in the molecular plasmonics field. Rhodamine 6G is one of the organic chromophores that has been widely studied with the interaction of silver nanostructures in the context of single molecule surface-enhanced Raman spectroscopy. In this thesis, spectral dips in the Rayleigh scattering of single silver nanoparticles interacting with Rhodamine 6G have been shown for the first time. This was achieved by using a novel way of adsorbing Rhodamine 6G on silver surface. Similar observations have been reported between single plasmonic nanoparticles and chromophores. However, the mechanism behind was only explained by strong coupling or plasmon resonant energy transfer. Mie theory calculations suggest that surface-enhanced absorption significantly contributes to these spectral modifications as well as the coupling. The strength of molecule-plasmon interactions is strongly affected by the properties of plasmonic nanoparticles and chromophores. By decreasing the radiative damping of plasmonic particles, and using J-aggregates of a cyanine dye, which exhibit high oscillator strength and narrow transition linewidth, it is possible to approach strong coupling regime. In this thesis we observed 50% transparency in the scattering spectra of single silver nanorods. To our knowledge, this is the strongest modification reported up to date at single particle level.