Six-band polarization-insensitive terahertz metamaterial absorber based on a square-ring and circular-ring hybrid resonator for refractive index sensing

A six-band polarization-insensitive terahertz metamaterial absorber based on a hybrid square-ring and circular-ring resonator is proposed for refractive-index sensing applications. Numerical simulations were carried out using the finite-difference time-domain method. The absorber consists of a patterned metallic resonator, a dielectric spacer, and a metallic ground plane. The results demonstrate six distinct absorption peaks within 0.4–4.5 THz, five of which exhibit absorption above 95%, while the sixth peak reaches 64.8%. The resonant frequencies can be effectively tuned by varying the structural parameters. Electric- and magnetic-field analyses indicate that the six absorption peaks arise from square-ring-dominated, circular-ring-dominated, and hybrid resonances with distinct field-localization characteristics. Refractive-index sensing analysis shows that the proposed absorber exhibits favorable sensing performance, with a maximum figure of merit of 5.71 at the f 4 resonance peak. In addition, stable polarization-insensitive absorption behavior is achieved owing to the symmetric resonator geometry. Benefiting from its simple structure, stable polarization response under normal incidence, and favorable refractive-index sensing performance, the proposed absorber shows potential for terahertz refractive-index sensing and related integrated terahertz sensing devices.