Design and analysis of a terahertz metamaterial sensor with cross-shaped resonators for alkene detection.

This paper presents the design and performance of a compact terahertz metamaterial absorber with strong angular stability and high sensing capability. The proposed structure, consisting of a fractal resonator fabricated on a polyamide substrate with copper metallization, achieves near-unity absorption (~ 99.99%) at 3.65 THz. The absorber demonstrates excellent angular robustness, maintaining stable resonance under TE polarization up to 60°, and preserving its primary mode under TM polarization despite the emergence of secondary resonances. At normal incidence, the structure also exhibits polarization insensitivity, ensuring consistent performance in practical excitation conditions. Unlike multiband meta-surfaces, which often suffer from complex resonance behavior and reduced resolution, the single-band design ensures a sharp, narrowband absorption peak, offering higher precision for detecting specific substances. Sensing performance is evaluated by monitoring resonance frequency shifts in response to changes in the surrounding refractive index. The absorber achieves a high sensitivity of 714.47 GHz/RIU, a figure of merit (FOM) of 14.29 RIU⁻¹, and a quality factor (Q) of 48.03, outperforming several previously reported terahertz absorbers while maintaining a compact footprint of 50 × 50 μm². The enhanced performance arises from strong electric field confinement and surface current localization within the fractal geometry, which amplifies light-matter interactions. Owing to its combination of high absorption efficiency, angular and polarization stability, and exceptional sensing performance, the proposed metamaterial absorber is a promising candidate for next-generation terahertz applications, including chemical and environmental sensing, and material characterization.