Plasmon-phonon coupling between mid-infrared chiral metasurfaces and molecular vibrations.

Plasmon-phonon coupling between metamaterials and molecular vibrations provides a new path for studying mid-infrared light-matter interactions and molecular detection. So far, the coupling between the plasmonic resonances of metamaterials and the phonon vibrational modes of molecules has been realized under linearly polarized light. Here, mid-infrared chiral plasmonic metasurfaces with high circular dichroism (CD) in absorption over 0.65 in the frequency range of 50 to 60 THz are demonstrated to strongly interact with the phonon vibrational resonance of polymethyl methacrylate (PMMA) molecules at 52 THz, under both left-handed and right-handed circularly polarized (LCP and RCP) light. The mode splitting features in the absorption spectra of the coupled metasurface-PMMA systems under both circular polarizations are studied in PMMA layers with different thicknesses. The relation between the mode splitting gap and the PMMA thickness is also revealed. The demonstrated results can be applied in areas of chiral molecular sensing, thermal emission, and thermal energy harvesting.