Multiple Rabi Splittings under Ultra-Strong Vibrational Coupling

TL;DR

This paper demonstrates ultra-strong vibrational coupling of molecular vibrations to multiple IR cavity modes, revealing significant Rabi splittings, a vibrational polaritonic bandgap, and a ladder of polaritonic states, with implications for mode-selective chemistry.

Contribution

It provides the first experimental demonstration of multi-mode ultra-strong vibrational coupling with large Rabi splittings and a vibrational bandgap, advancing understanding of vibrational polaritonic systems.

Findings

Rabi splittings up to 24% of vibrational frequencies

Observation of a 60 meV vibrational polaritonic bandgap

Resolution of a vibrational ladder of polaritonic states

Abstract

From the high vibrational dipolar strength offered by molecular liquids, we demonstrate that a molecular vibration can be ultra-strongly coupled to multiple IR cavity modes, with Rabi splittings reaching $24\%$ of the vibration frequencies. As a proof of the ultra-strong coupling regime, our experimental data unambiguously reveal the contributions to the polaritonic dynamics coming from the anti-resonant terms in the interaction energy and from the dipolar self-energy of the molecular vibrations themselves. In particular, we measure the opening of a genuine vibrational polaritonic bandgap of ca. $60$ meV. We also demonstrate that the multimode splitting effect defines a whole vibrational ladder of heavy polaritonic states perfectly resolved. These findings reveal the broad possibilities in the vibrational ultra-strong coupling regime which impact both the optical and the molecular properties of such coupled systems, in particular in the context of mode-selective chemistry.