Interaction of molecular excited states and plasmons in metal nanostructures can give rise to new hybrid eigenstates in the so-called strong coupling regime causing new modifications in optical responses. Plasmonic nanostructures can localize EM fields in extremely small volumes and produce large vacuum electric fields resulting in strong interaction even at a single molecular level. Specifically molecular vibrational modes can interact strongly with such fields leading to new possibilities in controlling chemistry. Achieving a large amplitude of EM field and thereby strong coupling in the nanostructures requires the proper choice of material and geometry of the nanostructures. Extending the semiclassical theory of Gersten and Nitzan, we study the onset of strong coupling of single molecules placed near metal nanostructures of several geometries (such as nanosphere, nanospheroid, and spherical nanoparticle dimers): we analyze effects of geometric parameters of such systems by characterizing the coupling strength, relaxation rates, and the interplay between them. We observe significant effects of these parameters on the strong coupling condition. To find the proper choice of material for the nanostructures in vibrational strong coupling, we also study few selected materials including n-doped semiconductors with a Nanoparticle-on-Mirror cavity geometry for which we calculate the vacuum electric field amplitude, mode volume, and damping rates. We find that these observables are sensitive to nanoparticle-molecule distance, emitter orientation, gap size, nanoparticle surface curvature, and the choice of materials- all in accordance with experiments.

Electromagnetic field and strong coupling maps for single molecules in plasmonic cavities

ACS Spring 2022

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