Terahertz Spoof Surface Plasmon Polariton Structure for High-Precision Gas Sensor Technology

It is a significant challenge to accurately identify and differentiate sample materials in the gas phase, especially when they have closely similar refractive indices. A promising solution in the THz range is to use plasmonic spoof surface structures configured in an Otto arrangement. Here, this study proposes a multilevel meta-grating structure sensor that achieves a remarkable improvement over the conventional binary-grating structure model. The proposed setup has been meticulously designed to maximize reflectance within the sensor's reflectance spectrum. This has been achieved by precisely adjusting the air gap distance, effectively minimizing the impact of sample material density. An in-depth analysis of gas samples with this structure shows a considerable increase in sensitivity with a small refractive index change up to 11.4 (TH/RIU), and the results are validated by simulating the reflection spectrum using the semi-analytical rigorous coupled wave analysis method. Moreover, the eigenmode solver in the CST Studio software is used to generate dispersion curves. The newly proposed design is particularly suitable for effectively detecting different gases with closely spaced refractive indices, making the proposed structure very useful in high-precision sensors that discern small refractive index changes.