Vibrational Strong Coupling Controlled by Spatial Distribution of Molecules within the Optical Cavity

Similar to excitonic materials interacting with optical cavity fields, vibrational absorbers coupled to resonantly matched optical modes can exhibit new hybridized energy states called cavity polaritons. The delocalized nature of these hybrid polaritonic states can potentially modify a material’s physical and chemical characteristics, with the promise of a significant impact on reaction chemistry. In this study, we investigate the relationship between the spatial distribution of vibrational absorbers and the cavity mode profile in vibrational strong coupling by systematically varying the location of a 245-nm-thick poly­(methyl methacrylate) (PMMA) film within a few-micrometer-thick Fabry–Perot cavity. Angle-tuning the cavity reveals that the first- and second-order cavity resonances couple to molecular absorption lines of PMMA (the CO and C–H stretching bands at 1731 and 2952 cm^–1, respectively), resulting in quantifiable vacuum Rabi splittings in the dispersion response. These splittings, as extracted from experiment, transfer-matrix calculations, and an analytical treatment, display a consistent and strong dependence on the molecular spatial distribution within a cavity. Furthermore, we demonstrate the response of two physically separated molecular layers by measuring and calculating the vacuum Rabi splitting for cavities loaded with single and widely spaced pairs of PMMA layers. The results provide evidence that extended cavity polariton modes sample these separate layers simultaneously and, more broadly, provide guidance for controlling the coupling strength, and potentially chemical reactivity, of a given region through modification of the cavity mode profile or through introducing a remotely located molecular layer.