Application Of Terahertz Metasurface Based On Toroidal Dipole Resonance For Detection Of Aβ Protein

Terahertz(THz)wave,which is between infrared and microwave,has attracted wide attention in biomedical field owing to its outstanding characteristics such as noninvasive,non-ionization,low scattering,high permeability and fingerprint spectrum.However,because of the long wavelength,the detection of trace or ultra-thin analytes is very difficult for terahertz spectroscopy,which severely limits its application in the sensing field.Introducing metamaterials has solved this difficulty.Metamaterials are artificial electromagnetic media capable of achieving physical properties that do not exist in nature.In particular,the surface plasmon resonance stimulated by the interaction between metamaterials and electromagnetic waves results in local field enhancement,which can improve the resolution and sensitivity of bio-detection.Terahertz metamaterial sensor is a newly-developing crossover technology,which combines the advantages of terahertz spectroscopy and metamaterial to provide better sensitivity for trace detection of analytes.Toroidal dipole resonance has an excellent local field enhancement effect and very low radiation loss,which is very promising in the sensing field.We found that the Chinese Taichi-like ring has simple and beautiful symmetry.By introducing symmetry breaking into the Taichi ring,we successfully achieved highquality toroidal dipole resonance in the terahertz band.When a single Taichi-like ring is used as a periodic unit,the polarization-dependent effect on incident terahertz waves is observed.To obtain a metasurface with a polarization-independent effect,four Taichi-like ring with openings perpendicular to each other on the diagonal were combined into a periodic unit.Then we investigated the sensing performance of four Taichi-like rings by using simulations and found that it shows very high sensitivity to the ultra-thin analyte layer.The theoretical sensitivity of the analyte with a coating thickness of 4 μm can reach 258 GHz/RIU at the corresponding resonant frequency of～1.345 THz,which is higher than most reported sensors.The experimental preparation and measurement of the designed sensor were carried out.The measured polarization-independent effect is in good agreement with the simulation.Finally,it was successfully applied to the low-concentration detection(range from 0.0001 mg/ml to 10 mg/ml)of Aβ protein associated with Alzheimer’s disease.A 7.0 GHz resonance frequency redshift for a concentration of 0.0001 mg/ml was observed.For 10 mg/ml,the redshift reached 244.8 GHz.Our designed sensing metasurface has rich applications for label-free biosensing.