Broadband multi-functional graphene-MoS₂ polarization conversion metasurface for biomolecular refractive index sensing at the THz band

A tunable multifunctional reflective metasurface is proposed and numerically studied for mid-terahertz (THz) polarization conversion and refractive-index sensing. The unit cell comprises two orthogonally arranged elliptical resonators with rectangular cross-slots, integrated with a graphene layer, a MoS \(_2\) layer, a SiO \(_2\) spacer, and a gold ground plane. The electromagnetic characteristics of the structure are investigated using the frequency-domain finite-element method (FEM), with graphene and MoS \(_2\) described by the Kubo and Drude models, respectively. Owing to the hybrid graphene–MoS \(_2\) configuration, the metasurface supports dual-band linear-to-linear (LP-to-LP) polarization conversion in the ranges 1.233–1.409 THz and 2.172–2.215 THz, as well as broadband linear-to-circular (LP-to-CP) polarization conversion from 1.574 to 2.105 THz. The polarization-conversion performance remains high across the operating bands and is maintained for incident angles up to 45 \(^∘\) . In addition, the proposed metasurface demonstrates high-performance refractive-index sensing capability through analyte-induced spectral resonance shifts. Across the biological refractive-index range ( \(n = 1.33\) –1.49), the sensor exhibits average sensitivities of 380.5 GHz/RIU and 748.2 GHz/RIU for the first and second resonance modes, respectively, reaching peak local sensitivities of up to 1.25 THz/RIU and 1.45 THz/RIU for high-index biological analytes. Furthermore, the device achieves maximum figure-of-merit (FOM) values of 6.41 RIU \(⁻¹\) and 6.04 RIU \(⁻¹\) , quality factors ranging from 6.6 to 10.8 (first resonance) and 9.2 to 15.4 (second resonance), and a minimum limit of detection (LOD) of 0.0078 RIU. The enhanced field confinement and dynamic electrostatic tunability make the proposed metasurface a promising platform for biomedical diagnostic applications-such as distinguishing blood components, including proteins, glucose, urea, lipids, and collagen-as well as compact, multifunctional THz polarization control.