Spoof Plasmonic On-chip Sensor Based on Electromagnetically Induced Transparency (Invited)

Spoof Surface Plasmon Polaritons (SSPPs) are a surface electromagnetic modes of the low-frequency regime with characteristics similar to those of surface plasmons in the optical regime. By altering the geometric parameters of the metal structure,the localized electromagnetic field can be manipulated, enabling its significant role in various fields such as on-chip communication,signal processing,and sensing detection. Herein,we propose a novel strategy to achieve the refractive index sensor on chip. The classical two-side metallic stripes are utilized as SSPP waveguide,and two types of meta-particles with diverse Q-factors and identical resonant frequencies are employed as a bright-dark module along the propagating trajectory of the waveguide structure. Due to the near-field interactions between resonant modes of meta-particles and the evanescent field of waveguide,Electromagnetically Induced Transparency(EIT)effect is achieved on planar SSPP waveguide. The working frequency is 6.03 GHz and Q-factors can be up to 28.575. Due to the characteristics of the electric-field distribution at the EIT-like peak frequency, a compact refractive-index sensor on-chip is developed. The sensing mechanism of the on-chip EIT-like effect primarily relies on the sensitivity of the strongly confined electric-field inside the dark mode resonators to the refractive-index of surrounding medium. This sensitivity can be reflected into the shift of the EIT-like characteristic frequency and changes in the resonance amplitude. The circular analyte is chosen as the study object and positioned above the center of the dark mode resonators. The full-wave simulations are conducted to investigate the response characteristics of the EIT transparency window to variations in parameters such as the refractive index n,tangent loss tan s,thickness th,and radius r of the analytes. The increase of refractive index n contributes to redshift of EIT window,which show the linear relationship and the slope is described as the sensitivity of sensor on-chip. As the thickness t h and radius r of the analytes increase,the relationship between redshift and refractive index still maintains a linear trend. The simulated results have shown that the on-chip sensor exhibits maximum sensitivity of 1.12 GHz/RIU within the refractive index range from 1.26 to 1.79 (corresponding to dielectric constants in the real part of 1.6 to 3.2), and t(h)=1.0 mm, tan sigma =0, r= 1.5 mm, respectively. Correspondingly,the Figure of Merit (FOM)value is calculated to be 5.45. To experimentally validate the proposed on-chip sensor based on the EIT-like effect,the SSPP device is fabricated using the process of printed circuit board,and sensing measurements are performed on three types of edible oils,including linseed oil,sunflower oil and flaxseed oil. Three types of edible oils differ mostly owing to their fat contents,which causes a variation in the related refractive index. Use a syringe with a needle to drop each edible oil to the position of the center of the dark mode resonators, the results have confirmed that the sensor exhibits high sensitivity, label-free detection capability,and experimental flexibility. As the parameters of analytes are t(h)= 0.6 mm, r=1.5 mm, tan s = 0, n= 1.52,1.50, and 1.49 in the simulation,the simulated results agree well with the experimental results. The proposed on-chip sensor based on the EIT-like effect in SSPP waveguide possesses several key characteristics, including simple structural design, easy integration, and high sensing sensitivity. Extendedly, by placing EIT-like modules with different characteristic frequencies along the SSPP waveguide,simultaneous sensing detection of multiple analytes will be achieved. This approach provides a novel perspective for the design of on-chip sensing devices.