Strong Coupling of Terahertz Fields to Collective Intermolecular Vibrations*

Several years ago, strong coupling of intra-molecular vibrations to mid-infrared resonators was introduced as a new paradigm in the field of polariton chemistry [1]. Such vibrational strong coupling allows the manipulation of molecular processes occurring at the electronic ground-state, by targeting a specific bond within the molecules [2].
Here, we demonstrate for the first time strong coupling of collective vibrations in organic molecule crystallites, occurring at THz frequencies. In our experiments, we performed THz time-domain spectroscopy on a metallic cavity filled with α-lactose crystallites, which exhibit a sharp absorption peak at 0.53 THz. We observed a Rabi-splitting of ~70 GHz, and directly probe the Rabi-oscillations in the coupled system. Our results take strong coupling and polariton chemistry into a new class of material, including polymers, proteins and other organic materials, in which collective, spatially extended degrees of freedom participate in the dynamics.

[1] A. Shalabney et al., Nat. Commun. 6, 5981 (2015)
[2] A. Thomas et al., Angew. Chemie 55, 11462 (2016)