Feasibility of Using Terahertz Toroidal Metasurface Sensor for Detection and Quantification of Chlorothalonil in Water

Terahertz (THz) waves have aroused great interest in novel sensing due to the fingerprint characteristics. The use of toroidal metasurface at THz frequencies could enhance interactions between the sample and the electromagnetic waves and thus realize highly sensitive detection. However, it is difficult to implement detection using toroidal metasurface because toroidal dipole (TD) resonance is usually masked by other stronger multipoles. Here, we theoretically and experimentally report the use of a THz metasurface sensor to detect chlorothalonil in water based on the TD resonance for the first time. The metasurface can support a distinct resonance peak located at 0.74 THz, which matches the absorption peak of chlorothalonil. Both the rigorous multipole decomposition and electromagnetic field pattern reveal the excitation of TD resonance in the THz band. The calculated results show that the metasurface sensor is sensitive to the thin thickness of the analyte and can be used to detect the refractive index of the trace analyte with a high sensitivity of 141 GHz/RIU. The measured results demonstrate that the frequency shift increases almost linearly with the increasing of chlorothalonil concentration ranging from 0 to 1000 mg/L, achieving the prediction of chlorothalonil concentration in water. A low concentration of chlorothalonil, as small as 0.001 mg/L, can be successfully detected. Our work enables THz detection and quantification of pesticides in water and promotes the application of THz technology in environmental monitoring.