Coherent Coupling between a Molecular Vibration and Fabry–Perot Optical Cavity to Give Hybridized States in the Strong Coupling Limit

The coherent coupling between an optical transition and a confined optical mode, when sufficiently strong, gives rise to a new pair of mixed modes separated in frequency by the vacuum Rabi splitting. Such systems have been widely investigated for electronic-state transitions such as molecular excitons coupled to surface-plasmons and optical microcavities. However, only very recently have vibrational transitions been considered. Here we experimentally investigate the coupling between a Fabry–Perot cavity and the carbonyl stretch at an infrared frequency near 1730 cm–1 in polymethyl methacrylate. As is requisite for the "strong coupling" regime, the measured vacuum-Rabi-splitting of 132 cm–1 is much larger than the full width of either the cavity resonance (34 cm–1) or the inhomogeneously broadened carbonyl-stretch absorption (24 cm–1). With the assistance of quantitative analysis using transfer-matrix methods, we provide evidence that the mixed-state resonances are relatively immune to inhomogeneous vibrational broadening and demonstrate the ability to extract splittings by convenient angle tuning of the Fabry–Perot cavity to match the vibrational frequency. Opening the field of polaritonic coupling to vibrational species promises to be a rich arena amenable to a wide variety of infrared-active bonds that can be studied both statically (as here) and dynamically with ultrafast methods. Moreover, microfluidic cavities will permit the study of liquids, greatly expanding the range of assessable molecules.