Forming Planarized Solid Films Containing Potassium Ferricyanide for Achieving Vibrational Strong Coupling

Harnessing vibrational strong coupling (VSC) in molecular systems offers a promising route to controlling material properties and chemical dynamics via hybrid light–matter states. While prior studies have demonstrated VSC in polymeric thin films, incorporating strongly absorbing molecular species into solid-state, aqueous-compatible platforms while maintaining suitable optical and morphological properties remains challenging. Here, we report the experimental realization of VSC between the C≡N stretching mode of potassium ferricyanide and a mid-infrared microcavity mode using spin-coated solid films composed of potassium ferricyanide embedded in a polyvinyl alcohol (PVA) matrix. By introducing a polymethyl methacrylate (PMMA) planarization layer, the surface roughness of the resonant layer was substantially reduced, providing a smoother interface for fabrication of the Fabry–Pérot microcavity. Angle-resolved Fourier-transform infrared (FTIR) spectroscopy revealed the characteristic avoided crossing of the upper and lower polariton branches, while simultaneous least-squares fitting of their dispersion using a two-level coupled-oscillator model yielded a Rabi splitting of 6.05 meV. These results demonstrate strong light–matter coupling in a solution-processed solid-state system and establish a platform for studying vibrational polaritons. Our findings highlight the importance of surface engineering in the fabrication of solid-state vibrational microcavities and provide a foundation for further investigations of cavity-modified molecular dynamics.