Terahertz Refractive Index Sensing Based on Gradient Metasurface Coupled Confined Spoof Surface Plasmon Polaritons Mode

We have investigated the sensing performance of the terahertz (THz) spoof surface plasmon polaritons (SSPs) on a metal groove array excited by a one-dimensional (1D) dielectric gradient metasurface (GMS) coupler. Such a compact SSPs coupler overcomes the disadvantages of their metal counterparts consisted of subwavelength metal antennas such as low efficiency and absorption loss. The normal incident terahertz wave is deflected by the dielectric GMS to the predetermined angles in the dielectric substrate which excites the SSPs modes on the metal groove array. Treating the 1D all-dielectric GMS as the excitation coupler, the confined SSPs modes can be generated in a small resonant cavity, leading to the strong light-matter interaction. The designed SSPs dielectric meta-couplers can be used to construct ultra-thin and compact sensing devices. Detailed investigation shows that the reflected amplitudes and phases of terahertz wave exhibit resonant responses to the change of the sample refractive index filled in the gap between the coupler and the metal groove array. The SSPs sensing by GMS coupling with the resonant frequency around 0.46 THz exhibits a high sensitivity of about 517.9 GHz/RIU with the resolution of 0.0001 RIU. A high Q-factor of 262 can also be achieved. In addition, the sensitivity of 410 GHz/RIU can be achieved with the resolution of 0.05 RIU, which can be used to detect large changes in refractive index. The study paves the ways for practical applications of terahertz trace-amount detection of chemical, organic or biomedical materials.