Metafibers for highly sensitive carbon dioxide sensing

Despite the urgent need for high-sensitivity, fast-response, ambient-temperature CO₂ sensing in environmental, industrial, and biomedical applications, conventional optical platforms suffer from weak light-matter interactions and poor signal contrast. Here, we propose and demonstrate a highly sensitive CO₂ gas sensor based on a metafiber platform integrated with a functional polymer thin film. The sensor consists of a gold nanohole array fabricated on the core of a single-mode fiber jumper using physical vapor deposition and focused ion beam milling, followed by dip-coating of a polyhexamethylene biguanide (PHMB) layer as the CO₂-sensitive material. Finite-element simulations reveal pronounced electric field enhancement at the nanohole edges and a refractive-index sensitivity of ∼600 nm/RIU. Experimental characterizations show a CO₂ sensitivity of 5.77 pm/ppm in the concentration range of 200-600 ppm, with response and recovery times of about 4.5 min and 7 min, respectively. The sensor also exhibits good repeatability and stability over consecutive sensing cycles. This metafiber-based architecture provides a promising and scalable strategy for developing miniaturized, high-performance optical fiber gas sensors.