  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.
