Graphene-Enabled Terahertz Metamaterial Absorber for Ultra-Wide Refractive Index Sensing

This study introduces a graphene-assisted terahertz metamaterial absorber operating at a resonance frequency of 8.436 THz. The unit cell incorporates a graphene-based split-ring resonator (SRR) configuration with additional auxiliary splits on the top layer. The structure achieves near-perfect absorption of 99.99% under TM polarization, maintaining absorption above 90% for incident angles up to 75$^{\circ}$. In addition, the proposed design remains polarization-insensitive up to 50$^{\circ}$. The sensor demonstrates a figure of merit (FOM) of $4.22 RIU^{-1}$ and achieves a maximum Q-factor of 20.98, corresponding to a full width at half maximum (FWHM) of 402 GHz. An equivalent circuit model is also developed to verify the resonance behavior. Due to the strong dependence of the resonant frequency on the surrounding refractive index (RI), the absorber is applied as an RI-based sensor. It exhibits a high sensitivity of 1698 GHz/RIU across a wide refractive index range from 1.0 to 2.0. The proposed design holds promise for sensing applications involving gases, oils, solvents, polymers, and high-index dielectric materials, including the biomedical refractive index window (1.30-1.39) for label-free biosensing.