Strong Coupling of Light to Collective Terahertz Vibrations in Organic Materials

Several years ago, it was shown that strong coupling between an electronic transition in organic molecules and a resonant photonic structure can modify the electronic landscape of the molecules and affect their chemical behavior. Since then, this new concept has evolved into a new field known as polaritonic chemistry, which employs strong coupling as a new tool for controlling material properties and molecular chemistry. An important ingredient in the progress of this field was the recent demonstration of strong coupling of molecular vibrations to mid-infrared resonators, which enabled the modification of chemical processes occurring at the electronic ground-state of materials. Here we demonstrate for the first time strong coupling with collective, intermolecular vibrations occurring in organic materials in the Terahertz frequency region. Using a tunable, open-cavity geometry, we measure the temporal evolution and observe coherent Rabi oscillations, corresponding to a splitting of 68 GHz and approaching the ultra-strong coupling regime. These results take strong light-matter coupling into a new class of materials, including polymers, proteins and other organic materials, in which collective, spatially extended degrees of freedom participate in the dynamics.