Strong coupling between mid-infrared localized plasmons and phonons.

We numerically and experimentally demonstrate strong coupling between the mid-infrared localized surface plasmon resonances supported by plasmonic metamaterials and the phonon vibrational resonances of polymethyl methacrylate (PMMA) molecules. The plasmonic resonances are tuned across the phonon resonance of PMMA molecules at 52 THz to observe the strong coupling, which manifests itself as an anti-crossing feature with two newly formed plasmon-phonon modes. It is also shown that the forbidden energy gap due to mode splitting is proportional to the overlapped optical power between the plasmonic resonance mode and the PMMA molecules, providing an effective approach for manipulating the coupling strength of light-matter interaction.