Understanding the Nature of Vibro-Polaritonic States in Water and Heavy Water

Very recent experiments on vibrational strong coupling tend to modify chemical reactivity, energy transfer, and many other physical properties of the coupled system. This is achieved without external stimuli and is very sensitive to the vibrational envelope. Water is an excellent vibrational oscillator, which is being used for similar experiments. However, the inhomogeneously broad OH/OD stretching vibrational band make it complicated to characterize the vibro-polaritonic states spectroscopically. In this paper, we performed vibrational strong coupling and mapped the evolution of vibro-polaritonic branches from low to high concentration of H₂ O and measured the on-set of strong coupling. The refractive index dispersion is correlated with the cavity tuning experiments. These results are further compared with transfer matrix simulations. Simulated data deviate as noted in the dispersion spectra as the system enters into ultra-strong coupling due to enhanced self-dipolar interactions. Hopfield coefficients calculation shows that even at ±400 cm^-1 detuning, the vibro-polaritonic states still possess hybrid characteristics. We systematically varied the concentration of H₂ O and mapped the weak, intermediate, and strong coupling regimes to understand the role of inhomogeneously broad OH/OD stretching vibrational band. Our finding may help to better understand the role of H₂ O/D₂ O strong coupling in the recent polaritonic chemistry experiments.