Calcite Phonon Polaritons for Mid-Infrared Molecular and Refractive-Index Sensing

Mid-infrared thermal emission offers a powerful means of probing molecular vibrational fingerprints, whereas conventional emitters are typically broadband and lack spectral selectivity. Here, we demonstrate that calcite (CaCO3) nanostructures overcome this limitation by hosting high-Q phonon polaritons that enable molecular and refractive index sensing. CaCO3 gratings sustain surface phonon polariton modes with quality factors of 80–100, which strongly hybridize with molecular vibrations and yield a clear Rabi splitting of 2.05 meV, allowing discrimination of overlapping resonances. In parallel, graphene–CaCO3 hybrid heterostructures host gate-tunable plasmon–phonon polaritons with dynamically controlled dispersion and coupling strength, reaching a refractive-index sensitivity of 2.5 μm/RIU with excellent linearity of R2 = 0.9946. Numerical simulations and coupled-oscillator analysis consistently reveal strong confinement and resolvable mode splitting, confirming the hybridization mechanism. These findings provide a promising approach for realizing calcite-based sensors, establishing a foundation for high-sensitivity biosensing platforms.