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

Abstract

In the realm of 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 the use of plasmonic spoof surface structures configured in an Otto arrangement. This configuration increases sensitivity by amplifying the trapped fields within the periodic structure and the sample material. This represents a remarkable improvement over the conventional binary-grating structure (CBGS) model with the proposed multilevel meta-grating structure (MMGS) sensor. An in-depth analysis of gas samples shows a remarkable increase in sensitivity to a small refractive index change up to by 11.4 (TH/RIU). The refractive index of gas sample is more sensitive to both pressure and molecular density. 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. The new proposed design proves to be particularly suitable for effectively detecting different gas with closely spaced refractive indices. An investigation of four common and well-known gas shows a minimum resolution of 5 GHz for resonance peak positions. The results are validated by simulating the reflection spectrum using the semi analytical rigorous coupled wave analysis (RCWA) method. Moreover, the eigenmode solver in the CST-Studio software is used to generate dispersion curves. The proposed structure is very useful for high precision sensors with a small refractive index change.