Polarization-dependent terahertz strong coupling in a lactose-covered anisotropic plasmonic metasurface

Polarization-dependent terahertz (THz) strong coupling enables the exploration of anisotropic molecular vibrations, enhances the sensitivity and selectivity of molecular detection, and provides a route to tunable THz photonic devices based on organic materials. Here, we experimentally demonstrate polarization-dependent strong coupling in the terahertz region between the intermolecular vibrational mode (IVM) and the lattice-tuned localized surface plasmons (LLSPs) in an anisotropic plasmonic metasurface covered by an <i>α</i>-lactose monohydrate film. Excited by different polarized terahertz waves, polarization-dependent LLSPs interact with polarization-independent IVM, forming distinct vibro-polariton modes. Angle-resolved transmission spectra reveal significant differences in anti-crossing phenomena and Rabi splitting for incident polarization angles of 0° and 90°. Furthermore, multimode strong coupling is realized when the polarization angle is changed to 45°, where four vibro-polariton bands are observed. This polarization-resolved terahertz strong coupling provides a powerful mechanism for tailoring vibro-polaritons, with potential applications in both polaritonic chemistry and high-precision molecular sensing.