Revealing Hidden Vibration-Cavity Polariton Interactions by 2D IR Spectroscopy

We report the first two-dimensional infrared (2D IR) spectroscopy of new molecular photonic materials – vibrational polaritons. The 2D IR technique is unique in its ability to reveal interactions and state-selective resolving of hybrid light-matter excitations and dark states. Comparing to 2D IR of pure molecular systems, 2D IR of vibrational polariton show drastic different spectral features which are definite evidences of dark states and interactions between dark states and polaritons. This finding is different from the prevalent optical model of polariton derived from linear spectroscopy, which only considers the upper and lower polaritons. Furthermore, our results highlight role of vibrational anharmonicities in generating the observed non-linear signals and disproves the notion that vibration-polaritons are free harmonic Bosons. This work reveals polariton physics that are otherwise difficult to be probed by traditional linear spectroscopy and lay foundations for novel nonlinear optics and polariton chemistry of vibrational polariton.