Surface Phonon Polaritons from Resonant Circular Microcavities on Hydroxyapatite Ceramic Surfaces for Biological Sensing Applications

Low-loss surface phonon polaritons (SPhPs) are attractive alternatives to mid-infrared (IR) plasmonics for subdiffractional light confinement. This study demonstrates the excitation of SPhPs on hydroxyapatite [HAp; Ca₁₀(PO₄)₆(OH)₂] ceraPmics using Au microhole arrays, providing narrow reflectance dips in the Reststrahlen band. By combining micro-IR spectroscopy measurements with theoretical analysis, we elucidate the propagation mechanism of SPhPs induced at the air─HAp interface in Au microholes by investigating their spectral behavior dependence on the incident angle and microhole size. Circular cavity modes in Au microholes play an important role in exciting SPhPs. The SPhPs on the HAp ceramics exhibited quality factors comparable to those of conventional SPhP materials even though HAp is a nonpolar polycrystal rather than a polar crystal. The detection sensitivity of the SPhP resonances was evaluated by depositing ultrathin Al₂O₃ layers on the HAp ceramic surface to induce changes in the refractive index. Finally, the surface-sensing capability of the SPhPs was experimentally verified by selectively attaching bone morphogenetic protein 2 mimetic peptide molecules to the HAp ceramics. The attachment of the peptide molecules caused redshifts in the SPhP resonances and induced surface-enhanced IR absorption (SEIRA) responses related to the peptide molecules. These optical behaviors are similar to the SEIRA activity of surface plasmon polaritons. This study demonstrates that SPhPs on HAp ceramics provide optical functionality for new biological sensing platforms.