Low-Loss Phonon Polaritons for Enhancing Hydrogen Detection in the Mid-Infrared

The field of nanophotonics focuses on confining and concentrating electromagnetic energy to length scales much shorter than the free-space photon wavelength. In the middle-IR (mid-IR) region, which coincides with the molecular fingerprint region (approximately 6-20 μm in wavelength), such sub-diffractional confinement benefits applications like enhanced molecular sensing. Phonon polaritons, quasi-particles comprising an IR photon and an oscillating ionic charge (optic phonon), enable sub-diffractional light-matter interactions in the IR spectrum. [1] These hybridized light-matter modes are advantageous for enhancing molecular sensing in the mid-IR due to their low loss, high photon-confinement, and heightened sensitivity to environmental changes. We have leveraged low-loss phonon polariton material, silicon carbide (SiC), to enhance hydrogen gas sensing in the mid-IR. Hydrogen, crucial for sustainability, is a promising energy carrier in transitioning away from fossil fuels, necessitating compact optical sensors for safe operations and simultaneous IR detection of other green energy-related molecules. However, as a homonuclear diatomic molecule, hydrogen has no dipole moment and is IR inactive. We propose and demonstrate a SiC metasurface platform [2] with palladium (Pd) metal as a hydrogen transducer material [3] for mid-IR sensitive hydrogen detection at room temperature using low-loss phonon polariton modes. These hybridized polar material/metal metasurfaces, fabricated through standard photolithographic processes, exhibit near-unit absorption for unpolarized IR light and high quality-factors (&gt;70) before hydrogen loading. Upon hydrogen loading, Pd undergoes a phase transition from alpha-phase to beta-phase, altering the local dielectric environment of the SiC resonators and shifting the resonance of the narrowband phonon polariton modes. Using this platform, we demonstrate enhanced and reversible hydrogen detection from 0.5% to 5% concentration compared to Pd thin film, expanding current visible-light-based hydrogen sensors to the mid-IR and potentially integrating with other surface-enhanced infrared spectroscopy techniques.<br/>1. Caldwell, J.D., et al., Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons. Nanophotonics, 2015. 4(1): p. 44-68.<br/>2. Lu, G., et al., Collective Phonon-Polaritonic Modes in Silicon Carbide Subarrays. ACS Nano, 2022. 16(1): p. 963-973.<br/>3. Darmadi, I., F.A.A. Nugroho, and C. Langhammer, High-Performance Nanostructured Palladium-Based Hydrogen Sensors-Current Limitations and Strategies for Their Mitigation. ACS Sens, 2020. 5(11): p. 3306-3327.