Strong Coupling between Epsilon‐Near‐Pole Plasmons and Phonons on a Calcite Substrate

Vibrational strong coupling, arising from strong interactions between vibrational modes of chemical bonds and photonic modes, demonstrates remarkable potential for promoting mid‐infrared sensing. Herein, single‐crystal calcite (CaCO 3 ) is employed as an epsilon‐near‐pole (ENP) substrate, and the in‐plane anisotropic ENP plasmonic resonances are realized using U‐shaped gold antennas. Such ENP plasmons can be systematically tuned by precisely controlling the antenna dimensions. Notably, pronounced mode splitting and anti‐crossing dispersion are observed between ENP plasmons of the gold antennas and the lattice phonon vibrations of the calcite substrate, with the extracted coupling strength reaching the strong coupling regime. This work shows the possibility of using an ENP substrate for designing mid‐infrared plasmonic antennas and may advance practical applications of ENP plasmons in mid‐infrared sensing technologies.