Plasmonic substrates modify phase transitions via vibrational strong coupling

Vibrational strong coupling (VSC) has been explored recently as a means to alter chemical reactions. When molecules exchange energy with an optical cavity faster than losses to the environment, the strong coupling regime is reached, primarily characterized by Rabi splitting of the original eigenstates into upper and lower polariton modes, which are hybridized light-matter states. Usually, Fabry–Pérot (F.P.) cavities are used for coupling due to their high Q-factors; however, the chemical effects that can be expected are intrinsically limited by diploe misorientation, relatively small coupling per molecule, and the large number of dark modes. To better understand the limitations this imposes for modifying chemical properties, we have developed plasmonic substrates as an alternative optical platform for achieving VSC with surface-deposited molecules. Plasmonic substrates were designed to provide angle-independent coupling to ensembles of molecules, which may help decrease the number of unperturbed molecules during VSC. We are currently monitoring how the plasmonic substrates change the dehydration temperature Copper sulfate pentahydrate. This analyte is advantageous because it undergoes four dehydration states that can be monitored as a function of temperature using Raman spectroscopy. To do date, we have observed the first direct evidence of modified phase transition temperatures due to VSC. Furthermore, confocal Raman mapping of the “open cavity” platform allows us to probe where in space the highest degree of modification takes place. It appears that the dehydration occurs at lower temperatures in the optical “hotspots” of the coupled plasmonic geometry. This work has the potential to be generalized to broad range of molecular and plasmonic systems, opening new pathways in plasmon-mediated chemical reactions and how they can be impacted by strong coupling.