Motional narrowing of molecular vibrations strongly coupled to surface lattice resonances

We report on strong coupling between light in open cavities formed by arrays of gold microparticles and the carbonyl molecular vibration in poly(methyl methacrilate). These arrays sustain surface lattice resonances (SLRs), which are optical modes generated by the enhanced radiative coupling of the localized resonances in single scatterers by the diffraction in the array. Using the high-coherence beam of a free-electron laser, we measure the vibrational strong coupling and find a Rabi splitting into an upper and lower vibropolariton band. We observe the narrowing of the lower polariton band when the SLR is resonant with the molecular vibration. This behavior of the lower polariton linewidth can be modeled taking into account motional narrowing due to the spatial averaging of delocalized molecular vibrations in the strong coupling regime. The presence of motional narrowing in vibropolaritons in plasmonic arrays gives insight into the strong coupling of molecular vibrations, and introduces a different platform to investigate polaritonic chemistry, where reactions are not governed by individual molecules but by collective coupled vibrations.