  The strong coupling of an IR-active molecular transition with an optical mode
of the cavity results in vibrational polaritons, which opens a new way to
control chemical reactivity via confined electromagnetic fields of the cavity.
In this study, we design a voltage-tunable open microcavity and we show the
formation of multiple vibrational polaritons in methyl salicylate. A Rabi
splitting and polariton anticrossing behaviour is observed when the cavity mode
hybridizes with the C=O stretching vibration of methyl salicylate. As this
vibration contributes to the reaction coordinate of the photoinduced proton
transfer process in methyl salicylate, we suggest the coupling might be used to
modulate the photophysical properties of the molecule. Furthermore, the
proposed theoretical model based on coupled harmonic oscillator reveals that
the absorption of uncoupled molecules must also be considered to model the
experimental spectra properly and that simultaneous coupling of multiple
molecular vibrations to the same cavity mode has a significant influence on the
Rabi splitting.
