Harnessing Phonon Polaritons for Dynamic and Sensitive Hydrogen Detection in the Mid-Infrared.

Phonon polaritons─quasiparticles formed by coupling infrared (IR) photons with optical phonons in polar materials─enable highly confined light-matter interactions with lower losses than those of plasmonic systems. Although they have been successfully exploited for enhanced mid-IR chemical sensing in solid- and liquid-phase environments, their application in gas-phase detection remains largely underexplored. Here, we introduce a low-loss phonon polariton platform based on planar Pd/SiC heterostructures and nanostructured Pd/SiC metasurfaces for enhanced mid-IR gas detection. We investigated the mid-IR optical properties of planar Pd/SiC heterostructures under dynamic gaseous atmospheres, particularly at low H concentrations. We found that the 25 nm Pd layer can serve as a chemical transducer, facilitating dissociative adsorption and intercalation of H into a PdH phase that systematically modulates the mid-IR dielectric function. Even on the unpatterned phonon polaritonic substrate, we demonstrate phonon-enhanced H detection. Furthermore, by leveraging nanostructured Pd/SiC metasurfaces that exhibit localized phonon polariton modes with near-unity absorption, our platform achieves narrowband, highly sensitive, and reversible H detection as a proof-of-concept, outperforming other nanophotonic materials in the mid-IR. This hybrid gas-phase chemical detection platform, driven by phonon polaritons, advances passive optical H sensing beyond the visible spectrum and into the mid-IR, enabling integration with advanced IR spectroscopy for dynamic chemical process monitoring─with broader implications for gas-phase sensing, environmental monitoring, and reaction studies.