Strong coupling of metamaterial quadrupolar mode with molecular vibration

Metamaterial multipolar mode presents a compelling scheme for exploring fundamental properties and technological applications of light-matter interactions due to its strong near field and high quality factor. In this work, we demonstrate strong coupling and mode hybridization between metamaterial quadrupolar mode and molecular vibration in the mid-infrared. In our fabricated cross-shaped metasurfaces spin-coated with a polydimethylsiloxane (PDMS) film, a quadrupolar resonance with a quality factor of 33 is excited at oblique incidence, whose electric dipolar component efficiently couples to the Si-CH3 vibration with pronounced spectral splitting and anti-crossing behaviors. The coupling strength increases with the PDMS film thickness and reaches the strong coupling regime for thickness above 27 nm. A Rabi splitting of 0.79–1.13 THz is measured in the strong coupling regime. Our results indicate that the use of quadrupolar mode in plasmonic nanostructures provides an effective and convenient approach for the realization of vibrational polaritons, which hold promise for applications in ultrasensitive infrared sensing and photochemistry.