A macroscopic condensation theory for vibrational strong coupling effects

Recent experiments demonstrated the possibilities of modifying ground-state chemical reaction rates by placing an ensemble of molecules in an optical microcavity. This regime is commonly referred to as Vibrational Strong Coupling (VSC), and typically operates in the absence of any light source (in the dark). VSC causes reaction rate constant modifications, exhibits phase-transition type of behavior for equilibrium constant modifications, and the effect starts only when the collective Rabi splitting surpasses a threshold. Existing theoretical work often focuses on the single excitation pictures, centered around the idea of vibrational polaritons and dark states. We find that due to the many-body nature of VSC, most of the VSC effects can be potentially explained by forming a macroscopic condensation of vibrations. We theoretically demonstrate that by surpassing a critical Rabi splitting, the VSC system starts to macroscopically occupy one condensate state, which we believe could be the common explanation for VSC-induced effects. Despite experimental evidence of modified chemical reaction rates in the vibrational strong coupling regime, there is no unifying theory beyond the few-excitation subspace limit. Here, the authors introduce a regime where cavity-induced interactions among vibrational states lead to condensation of Bogoliubov quasiparticles, opening to the characterization of phase transitions and collective effects.