Emergence of Multiple Polaritonic States of Water-in-Salt in Cavity.

Water-in-salt electrolytes exhibit multiple spectroscopically isolated OH stretching modes owing to spatially and temporally isolated water species. Here we show that these modes couple cooperatively to a Fabry-Perot cavity, yielding well-resolved collective polaritonic states. These characteristics provide an insight to distinguishing the light-matter modes of water which have remained unresolved due to the common assumption of homogeneous broadening. This demonstrates that vibrational polariton formation in aqueous media is dictated by the vibrational heterogeneity that remains spectroscopically distinguishable whenever structural exchange occurs more slowly than the light-matter coupling timescale, establishing water-in-salt electrolytes as an ideal platform for disentangling multi-mode light-matter interactions in molecular liquids.