A Novel Particle in Liquid Biosensor Based on Terahertz Spoof Surface Plasmon Polariton Transmission Line

In this paper, we propose a novel terahertz (THz) biosensor that could be used for the highly sensitive detection of micro-particles in liquids, which consists of a one-dimensional (1D) subwavelength metasurface with periodic trapezoidal metal grooves and is shown to be especially suitable to work together with the THz vector network analyzer (VNA). By adopting a high resistivity and high refractive index silicon (Si) prism as an external coupler, tightly confined THz spoof surface plasmon polaritons (SSPPs) could be coupled in this structure. Besides, a tapered gradient SSPP transmission line (TL) is designed in its central region to achieve a further confinement of the electromagnetic field. Furthermore, a microfluidic channel is integrated into this region, allowing a liquid containing particles to flow through this channel crossing the TL to give an interaction volume on the microliter scale. In particular, due to the trapezoidal metal grooves with approximate size to the micro-particle on the surface of the SSPP TL, the flow rate of the particles in the channel would be very slow. Hence, this device would be especially suitable to work together with the broadband high-frequency resolution THz VNA with high sweep time. Numerical simulations demonstrate that the SSPPs propagated along the TL structure with a low insertion loss and are extremely sensitive to the permittivity of a liquid flowing through a microfluidic channel. In addition, its highly sensitive detection of individual subwavelength micro-particles flowing in the liquid is also demonstrated, showing its great potential for the combination of the THz biosensor and lab-on-a-chip microfluidic devices that are suitable for the detection and analysis of biological liquids with proteins and circulating tumor cells.