Strong coupling and the C=O vibrational bond

In this technical note we calculate the strength of the expected Rabi splitting for a molecular resonance. By way of an example we focus on the molecular resonance associated with the C=O bond, specifically the stretch resonance at $\sim$1730 cm$^{-1}$. This molecular resonance is common in a wide range of polymeric materials that are convenient for many experiments, because of the ease with which they may be spin cast to form optical micro-cavities, polymers include PVA and PMMA. Two different approaches to modelling the expected extent of the coupling are examined, and the results compared with data from experiments. The approach adopted here indicates how material parameters may be used to assess the potential of a material to exhibit strong coupling, and also enable other useful parameters to be derived, including the molecular dipole moment and the vacuum cavity field strength.