A gas sensor based on the hyperbolic mode resonance by integrating hafnium dioxide thin-film in an optical structure

Hafnium dioxide (HfO₂) is emerging as a transformative material in the field of optical gas sensing, offering a unique combination of high stability, exceptional dielectric properties, and strong surface adsorption capabilities. This work presents the integration of HfO₂ thin films—fabricated via Atomic Layer Deposition (ALD)—onto planar optical waveguide structures aimed to excite Hyperbolic Mode Resonances (HMR) with a remarkable refractive index (RI) sensitivity of 3347 nm/RIU in the range 1.3098–1.4311 RIU, among the top-performing devices in this type. The device was subjected to a diverse set of gaseous species, including oxygen, ethylene, nitric oxide, methanol, ethanol, and relative humidity. It revealed an outstanding performance for NO with a sensitivity of 1.89 nm/ppm and a limit of detection (LoD) as low as 76.2 ppb when operated at room temperature (23 º C). These results underscore the potential of HfO₂ as a novel sensing material that surpasses conventional metal oxides in both versatility and performance, opening new avenues for applications in environmental monitoring, industrial safety, and biomedical diagnostics, where low-cost, high-sensitivity, and room-temperature operation are critical.