Plasmon on metal nanoparticles can efficiently confine, amplify light at the nanoscale. When a metallic nanoparticle is coupled to another nanostructure or film, the intensified electric field within the gap of coupled plasmonic systems can drive a range of physical and chemical processes at interfaces. However, how field enhancement in the plasmon-coupled systems dictates the physical and chemical processes at the interfaces has not yet been completely understood.

 

In this dissertation, the physical, chemical, and optical interaction in plasmonic interfaces has been investigated extensively using the nanoparticle on mirror (NPoM) plasmonic nanocavities as a model system. We have explored light-matter interaction in various NPoM plasmonic nanocavity systems, a very specific bottom-up approach where a metallic nanoparticle is placed above a metallic film (mirror) with a dielectric spacer layer. First, we have studied the plasmon-molecular interaction by correlating elastic dark field scattering (DF) and inelastic scattering, such as photoluminescence (PL) and surface enhanced Raman scattering (SERS) in a plasmonic nanocavity. We have reported for the first time that plasmon-vibration interaction can induce laser-plasmon detuning-dependent plasmon resonance linewidth broadening, indicating energy transfer from the plasmon field to collective vibrational modes. The linewidth broadening accompanied by large enhancement of Raman scattering signal is observed as the laser-plasmon blue-detuning approaches the CH vibrational frequency of the molecular systems integrated in a gold nanorod-on-mirror nanocavities. The experimental observations are explained based on the molecular optomechanics theory that predicts dynamical backaction amplification of the vibrational modes and high sensitivity of Raman scattering when the plasmon resonance overlaps with the Raman emission frequency.

 

Also, the ability to precisely tune plasmon resonances is critical for advancing nanophotonic and sensing technologies. In the following work, we exploit the photothermal effect to achieve picometer-level tunability of plasmon resonances in nanorod-on-mirror (RoM) nanocavities, using polyelectrolyte (PE) layers as dielectric spacers. The plasmon-induced thermal response of these soft materials allows real-time adjustment of the nanocavity, unlike stable inorganic spacers like aluminum oxide. It is shown that this precise tuning capability enables the exploration of various photophysical processes, including the excitation of higher-order plasmon modes, optomechanical enhancement of surface-enhanced Raman scattering signals, molecular diffusion out of the nanocavities, and the transition to charge transfer plasmons. Finally, the near-field interaction in plasmonic nanocavities is investigated using s-SNOM (scattering-type near-field optical microscopy), which reveals that the high dielectric constant of the polyelectrolytes improves the optical contrast of near-field imaging.