Toroidal Dipole-Dominated High-Performance Optical Sensors via Nanoimprinted Tetramer Architectures

Practical high-Q metasurface sensors face two bottlenecks: maximizing analyte near-field coupling while suppressing radiative loss and ensuring compatibility with low-cost large-area fabrication. Here, we present a nanoimprinted tetramer architecture (NTA) sensor fabricated via two-step nanoimprint lithography, consisting of polystyrene nanopillar tetramer arrays atop a PS waveguide and gold mirror, supporting a prominent resonance mode at 810 nm (D₁ mode) exhibiting a pronounced Fano line shape with a high Q of 3124. In contrast, the experimentally measured resonance of the fabricated device exhibits a FWHM of approximately 12 nm, corresponding to a Q factor of about 70. Multipole decomposition confirms that the D₁ mode is governed by toroidal dipoles, with suppressed conventional electric/magnetic multipoles, thereby achieving radiative cancellation via multipole manipulation. Measured in aqueous glycerol solutions, the sensor achieves a refractive index sensitivity of 501 nm/RIU and a figure of merit of 42 RIU^-1. Label-free hemoglobin biosensing yields a sensitivity of 354.7 nm·(μg/L)^-1. Benefiting from tetramer symmetry, the resonance response remains stable across 0°-90° polarization angles. This work develops a scalable low-cost strategy for toroidal-dipole-enhanced high-Q biosensors, promising high-sensitivity detection for medical diagnosis and environmental monitoring.