Optimized bow-tie metasurface and its application in trace detection of lead ion

The strong localized plasmon resonance of metasurfaces makes the resonance frequency extremely sensitive to the dielectric environment, which can be applied to label-free environment detection. In this paper, a bow-tie terahertz metasurface with an optimized ratio of the quality factor to effective mode volume(Q/V_eff) is designed. The unit cell of the proposed structure is composed of a mirror-symmetrical metallic bow tie in the middle and continuous metallic strips on both sides. The width of each metal strip and the length of the bow-tie gap are optimized for the parameter Q/V_eff. When the metal strip width is 25 μm and the gap length is 2 μm, the effective mode volume is 3.6 μm³ and Q/V_eff is 2.2 μm^-3 at 0.7 THz. In the experiment, different concentration of the lead ion solution was dropped on the proposed metasurface. The transmission spectrum was measured by a terahertz time-domain spectroscopy system. The results showed that there is a linear relationship between resonance frequency shift and lead ion solution concentration from 0.1 ng/mL to 20 ng/mL. The detection limit is 0.1 ng/mL. The terahertz metasurface sensor has the advantages of the miniaturized size, easy sample preparation, fast measurement capability and real-time detection, which will be widely used in environmental protection and food safety.