Boosting Self-interaction of Molecular Vibrations under Ultra-strong Coupling Condition

In this letter, we investigated the modification of oscillator strength of an asymmetric stretching band of CS₂ by strong coupling to an infrared cavity photon. This is achieved by placing liquid CS₂ in a Fabry-Perot resonator and tune the cavity mode position to match with the molecular vibrational transition. Ultra-strong coupling improves the self-interaction of transition dipoles of asymmetric stretching band of CS₂ that resulted in an increase of its own oscillator strength. We experimentally proved this by taking the area ratio of asymmetric stretching and combination band by selectively coupling the former one. A non-linear increase in the oscillator strength of the asymmetric stretching band is observed upon varying the coupling strength. This is explained by a quantum mechanical model that predicts quadratic behavior under ultra-strong coupling condition. These findings will set up a new paradigm for understanding chemical reaction modification by vacuum field coupling.