  Vibrational strong coupling is emerging as a promising tool to modify
molecular properties, by making use of hybrid light-matter states known as
polaritons. Fabry-Perot cavities filled with organic molecules are typically
used, and the molecular concentration limits the maximum reachable coupling
strength. Developing methods to increase the coupling strength beyond the
molecular concentration limit are highly desirable. In this letter, we
investigate the effect of adding a gold nanorod array into a cavity containing
pure organic molecules, using FT-IR microscopy and numerical modeling.
Incorporation of the plasmonic nanorod array, that acts as artificial
molecules, leads to an order of magnitude increase in the total coupling
strength for the cavity filled with organic molecules. Additionally, we observe
a significant narrowing of the plasmon linewidth inside the cavity. We
anticipate that these results will be a step forward in exploring
vibropolaritonic chemistry and may be used in plasmon based bio-sensors.
