Plasmonics Platforms for Strong Coupling

Nanophotonic and plasmonic architectures present a compelling platform for investigating strong coupling phenomena. These nanofabricated optical structures enable precise tailoring of electromagnetic modes on sub-wavelength scales, leading to significant coupling interactions within near-field "hot spots". Recent advancements from our laboratory have showcased ultrastrong coupling to vibrational modes using innovative plasmonic cavity designs. The sub-wavelength localization of these platforms facilitates strong coupling to fewer molecules compared to traditional Fabry–Pérot (FP) cavities, potentially limiting contributions from "dark states" in polariton chemistry.<br/> <br/>A notable feature of plasmonic-based cavities is their compatibility with established surface science techniques, such as confocal molecular Raman spectroscopy. With these tools, our recent studies have uncovered unique temperature-dependent dehydration behavior of transition metal salt films on plasmonic substrates, suggesting resonant thermal energy transfer when vibrational strong coupling is present. This discovery hints at the potential of pronounced thermal gradients within the near-field of plasmonic substrates.<br/> <br/>Furthermore, our research delves into the design of cavities that support sub-radiant modes that exhibit higher q-factors, leading to stronger coupling interactions with molecular samples. Preliminary results indicate that these sub-radiant cavity modes with quadripolar field symmetry achieve very strong coupling with molecular samples, while offering a modification of the usual optical selection rules for bright and dark (N-1) polariton states.