High‐Sensitivity Terahertz Gas Sensing Enabled by Undercut Metal‐Dielectric‐Metal Metamaterial

ABSTRACT Terahertz (THz) metamaterial gas sensors offer a promising platform for refractive‐index‐based detection owing to their extended near‐field interaction volume. However, in conventional metal–dielectric–metal (MDM) architectures, gas–field interactions are restricted by uniform dielectric spacer layers, thus limiting achievable sensitivity. Herein, we propose and demonstrate a THz metamaterial gas sensor based on an MDM absorber incorporating an undercut in the dielectric layer. In contrast to conventional MDM metamaterials, the undercut dielectric enables partial replacement of the dielectric by the target gas, thereby increasing the gas‐replaced volume within the dielectric spacer and amplifying the effective refractive‐index perturbation of the resonant mode. Numerical simulations based on three‐dimensional electromagnetic analysis revealed that refractive‐index sensitivity increases with undercut width. The proposed metamaterial device is fabricated using standard microfabrication processes, and a reflection dip at 0.766 THz is experimentally observed, consistent with simulations. Gas sensing experiments using acetone–nitrogen mixtures exhibited concentration‐dependent resonance shifts, yielding a sensitivity of 0.16 GHz/% (23 THz/ Refractive Index Unit based on the estimated refractive‐index change). These results highlight undercut‐enabled dielectric replacement as an effective strategy for enhancing the sensitivity of THz metamaterial gas sensors.