Two-in-one terahertz metasurface: a widely adjustable narrowband perfect absorber and a highly sensitive biosensor

A dual-function terahertz metasurface is designed under idealized (noise-free, lossless) simulation conditions based on a copper-based resonator. The meta-structure serves not only as a perfect narrowband absorber but also as a highly sensitive refractive index sensor for analytes. The proposed meta-structure features a compact, ultrathin, simple, single-material design based on a periodic array of circular rings and disks. A near-perfect absorption of 99.99% is demonstrated at the resonant frequency of 0.8 THz, which can be configured over the span of 0.53-1.47 THz via geometric parameters and dielectric permittivity adjustments. The meta-absorber design is confirmed to be polarization-insensitive and also wide-angle, preserving the absorption > 90% for a wide range of incident angles up to 45° (TE) and 75° (TM). The meta-absorber functions as a high-performance dual-mode metasensor, simultaneously measuring both the analyte's thickness and refractive index. Under TE polarization, it achieves a refractive index sensitivity of 0.173 THz/RIU (with a FOM of 3.47 RIU⁻¹ and a Q-factor of 12.76) for thick analytes, while also providing a thickness sensitivity of 14 GHz/µm for ultrathin analytes. This study presents a cost-effective and high-performance platform for terahertz sensing applications. Its straightforward fabrication can render it as a competitive alternative to more complex metasurface designs.