High-sensitivity polarization-independent terahertz Taichi-like micro-ring sensors based on toroidal dipole resonance for concentration detection of Aβ protein.

Terahertz (THz) metamaterial sensor is a newly-developing interdisciplinary technology, which combines the essential characteristics of THz spectroscopy and metamaterials, to obtain better sensitivity for trace detection of the different target analytes. Toroidal dipole resonances show great sensing potential due to their suppression of the radiative loss channel. Here, we found a high-quality planar toroidal dipole resonance in the breaking Chinese Taichi-like ring and then designed a novel polarization-independent terahertz toroidal sensor by combining four Taichi-like rings into a cycle unit. The sensor shows high-sensitivity sensing characteristics for the ultrathin analyte and refractive index. The optimized sensitivity of pure analytes under 4 μm coating thickness can numerically reach 258 GHz/RIU in the corresponding ∼1.345 THz frequency domain, which is much higher than that of previously reported sensors. We further fabricated experimentally the sensor and demonstrated its fascinating polarization-independent characteristics. Finally, it was successfully applied to the low-concentration detection (ranging from 0.0001 mg/mL to 10 mg/mL) of Aβ protein associated with Alzheimer's disease. Our high-sensitivity polarization-independent THz toroidal dipole sensor would give access to rich applications in label-free biosensing.