Infrared nano-imaging of strongly coupled vibrational micro-cavities

Vibrational strong coupling (VSC) has been suggested to modify ground state chemical reactivity and energy transfer pathways in different molecular systems. Many demonstrations of VSC have utilized large ensembles of molecules, with coupling strength proportional to sqrt(N), to achieve VSC at room temperature in Fabry-Perot type cavities in both liquid and solid state. The collective behavior of molecules under VSC in these cavities has been a debated issue, whether all molecules collectively or individually couple to the cavity and questions surrounding the hypothesized delocalized nature of the vibrational wavefunctions in such an environment. Here, we perform infrared nano-imaging and -spectrsocopy in the evanescent field of Fabry-Perot microcavities coupled to molecular thin films. We probe directly nanometer-sized ensembles of molecules collectively coupled to the micron-sized system and their heterogeneity in the vicinity of patterned structures on the cavity surface. We also probe the length scales over which coupling persists in the presence of discontinuities in the gold mirror, which is evidence of the length scale over which vibrational wavefunctions delocalize in such cavities.