Electromagnetically Coupled Resonant Face-to-Face Double-Layer Metamaterial for Highly Sensitive THz Impedance Spectroscopy

A new electromagnetic coupling mechanism is introduced between two passive terahertz (THz) electric-LC resonator metasurfaces, aimed at maximising THz metamaterial impedance spectroscopy sensitivity, translated into resonance spectral red-shift (ΔF). When two resonant metasurfaces are brought into a face-to-face proximity and exposed to THz radiation, a resonant optical cavity is generated, and an electromagnetic coupling of THz radiation occurs from both metasurfaces. This coupling triggers the enhancement of plasmonic interaction between the incident THz radiation and the metasurfaces. It is found out that an optimal distance between the metasurfaces is a critical parameter for achieving an enhanced coupling of the electrical fields. Once optimally coupled, the double-layer metamaterial becomes an integrated sensor for THz impedance spectroscopy, exhibiting an enhanced resonance with a high Q-factor of 549, without any major requirements for alignment of the two metasurfaces. The double-layer metamaterial sensor achieves a high dielectric sensitivity of 2300 GHz RIU⁻¹ that can detect a variety of inorganic and organic nanoparticles, as well as sugar in ultra-low concentrations. As an example, precise blood sugar tracking is demonstrated defining clear distinctions between hypoglycaemia, normal, borderline high, and hyperglycaemia, all based on ΔF. This novel sensor architecture has potential applications in high-sensitivity biosensing and ultra-low-concentration dielectric detection.