Strong coupling of localized surface plasmons and intermolecular vibration mode at terahertz frequencies.

Organic molecular vibrations, typically occurring in the terahertz (THz) regime, can resonate with a metastructure. A hallmark Rabi splitting occurs when the coupling strength is sufficiently strong. In this work, we observe the strong coupling of localized surface plasmons (LSPs) and intermolecular vibration mode at THz on a metasurface spin-coated with organic molecule α-lactose monohydrate. Excited by transverse-electric THz waves, dispersive localized surface plasmons interact with nondispersive intermolecular vibrations and form two vibro-polariton modes. The angle-resolved transmission spectra of the coupled system are detected by using a terahertz time-domain spectrometer, demonstrating an anti-crossing effect with a clear Rabi splitting. By retrieving the coupling strength and Hopfield coefficients of polariton bands from the measured data, we further verify that these two bands originate from the strong coupling between LSPs and molecular vibration mode. Moreover, we show that it is possible to implement molecular concentration sensing based on this strong coupling effect. This study demonstrates a unique approach to investigate vibro-polaritons at the terahertz regime and provides a testbed for future applications of strong coupling effects in chemical detection and biosensing. [ABSTRACT FROM AUTHOR]