Resonant Interactions Between Nanoparticle Plasmons and Molecular Excitons

Doktorsavhandling, 2014

Molecular plasmonics involves the study and applications of plasmonic metal nanostructures interacting with molecules. It has been a basis for development of fundamental understandings of light-matter interactions as well as of new technologies, including biological and chemical sensors and plasmon-enhanced spectroscopies. Plasmonic nanoparticles can focus light to subwavelength volumes resulting in strong induced electromagnetic fields. When a molecule is placed in such a “hot spot”, its properties can change dramatically due to modifications of the photon density of states. This thesis focuses on studies of coupled molecule-plasmon systems in which the plasmon energy coincides with a molecular exciton absorption band. Colloidal silver nanorods and silver nanoprisms were coupled to molecular J-aggregates and studied at the single particle level. J-aggregates are attractive for studies of resonant plasmon-molecule interactions because of their narrow linewidths and high oscillator strengths. Depending on the damping rates, quality factors and mode volumes of the nanoparticle plasmon resonance, it was shown that it is possible to reach different interaction regimes, ranging from weak to strong coupling, characterized by distinct spectral profiles. Rhodamine 6G (R6G) is a dye molecule which interactions with plasmons has been studied extensively in the context of surface-enhanced Raman spectroscopy. Thiolated R6G and ordinary R6G was adsorbed on silver nanoparticles and the scattering and absorption properties of the composite systems were compared. The use of thiol linker leads to higher molecular coverage per particle, which resulted in pronounced spectral dips in the plasmon scattering spectra qualitatively similar to the J-aggregate case. The simulations, however, showed that surface-enhanced absorption was the main mechanism behind the spectral changes in this case. In addition to confining light in small volumes, plasmonic nanoparticles can also direct light under special circumstances. The last part of the thesis deals with directional scattering from bimetallic plasmonic nanoparticle antennas. It is shown that light can be scattered in different directions determined by its wavelength, that is, the plasmonic antenna constitute a nanoscale “color router”.