High-sensitivity refractive index sensing based on a dual ultra- narrowband terahertz metasurface absorber

This work presents a dual ultra-narrowband terahertz metasurface absorber for high-sensitivity refractive index (RI) sensing. The proposed structure consists of a periodically arranged array of cross-grooved silicon disks on a silver-backed silica substrate, achieving near-perfect absorption (over 99.5%) at two resonant frequencies. The magnetic dipole resonance mode at 1.2 THz ( Q  = 394) and anapole mode at 1.42 THz ( Q  = 1069) are excited based on electromagnetic field analysis, resulting in perfect absorption for incident electromagnetic waves. The ultra-narrow bandwidths (FWHM = 3.04 GHz and 1.33 GHz, respectively) and strong field confinement are observed in the region of the cross-grooved silicon disks and silica dielectric layer. The metasurface achieves exceptional sensitivities of 114.28 GHz/RIU and 321.43 GHz/RIU, figure of merit (FOM) of 37.59 and 241.67 at resonant frequency 1.2 THz and 1.42 THz, respectively. Structural parameters have also been studied for tuning resonance frequency and absorption efficiency (> 95%). The proposed metasurface is used to detect the concentration of glucose solution (0 ~ 465.8 mM, the corresponding RI = 1.333 ~ 1.345) to verify the sensing applications. The results demonstrate that the anapole resonance mode exhibits higher sensitivity than the magnetic dipole mode, enabling enhanced sensing performance of the metasurface-based sensor through synergistic dual-mode detection. This work presents a practical design strategy for ultra-high- Q multi-band metasurface sensors with exceptional sensitivity and FOM, promising for biosensing and environmental monitoring applications.