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In strong coupling light-matter coupling, optical transitions in a material are resonantly coupled to the electromagnetic mode confined within a cavity. During the past decade, strong coupling in molecular system is gaining increasing interest, which is driven by recent observations that the hybridization of the molecular wavefunctions with the photonic ones can be used for modifying chemical reactions and material properties. Until today, research in this evolving new field, which is known as polaritonic chemistry, had focused on the internal degrees of freedom of molecules, involving the coupling of either electronic or vibrational transitions to optical cavities in the visible or mid-IR spectral region. Here, I will present our recent results, demonstrating for the first time vibrational strong coupling with intermolecular THz vibrations in organic materials. In many cases, these excitations correspond to a collective, oscillatory motion of the molecules with respect to one another and, as such, they govern the structural and mechanical properties of organic materials. First, I will discuss THz strong coupling in a tunable cavity containing α-lactose crystallites. By analyzing the field exiting the cavity in the time-domain, we directly observed vacuum Rabi-oscillations with a period of ~15psec, corresponding to the reversible, quantum-coherent energy exchange between the intermolecular bonds in the organic crystal network and the cavity. These results open a new path toward controlling mesoscale and bulk properties of materials and take polaritonic chemistry into a new class of material processes, for which collective, spatially extended degrees of freedom participate in the dynamics.
Finally, I will discuss our recent demonstration of self-hybridization in a monolithic, 1D-photonic crystal made from α-lactose: in such a system, the intermolecular vibrations are strongly coupled to the electromagnetic modes confined by the α-lactose structure itself, without a cavity or any other external photonic structure.
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