Polarization-Insensitive Terahertz Biosensing Method Based on Toroidal Dipole Metasurface

Terahertz (THz) metasurfaces based on toroidal dipoles exhibit significant potential in biosensing applications. This study presents a polarization-insensitive THz metasurface biosensor designed with a fourfold rotationally symmetric toroidal dipole structure, consisting of a circular ring and four split-ring resonators. By exploiting the low-loss and polarization-insensitive properties of toroidal dipoles, the proposed sensor achieves significant electromagnetic field magnification and high sensitivity of biomolecular detection in the THz regime. Numerical simulation demonstrates a refractive index sensitivity of 721 GHz/RIU, while experimental measurements of cell suspensions indicate a sensitivity of 400 kHz/(cell/mL). The polarization-insensitive response, analyzed through multipole decomposition theory, is attributed to the balanced electromagnetic characteristics of the structure, which ensures consistent excitation of toroidal dipoles and magnetic quadrupoles across different polarization angles. For quantitative analysis, partial least squares (PLS) and support vector regression (SVR) methods are utilized to analyze liver cell lines (HepG2 and Miha), achieving high predictive accuracy with a mean squared error of 0.083. This work circumvents the polarization dependence observed in conventional THz biosensors and proposes an innovative approach for developing high-performance biosensing devices.