Design of terahertz metasurface methane sensor based on bound states in the continuum

Compared to traditional methane sensors used in mines, the metasurface methane sensor has significant advantages in sensitivity, stability, and other aspects, making it better suited to meet the practical needs of mine production. To address the issue of relatively low sensitivity to refractive index in existing metal terahertz metasurface sensors, a terahertz metasurface methane sensor based on bound states in the continuum was designed. The metasurface structure consisted of a three-layer configuration: metal dielectric metal (MDM), where the metal material was gold, and the dielectric material was polyimide. The upper metal structure was a circle, and by adjusting the size of the opening on the left side, the symmetry of the structure could be altered, which in turn induced quasi bound states in the continuum (QBIC). The analysis results showed that when the left-side opening gap was 5 μm, the modulation depth was maximal at 95.69%. A methane-sensitive membrane material (cryptophane-A) was then applied to the metasurface structure to form the methane sensor. Five different methane volume fractions and five environmental refractive indices were selected to validate the methane sensor's detection performance. The results showed that the sensitivity of the metal terahertz metasurface sensor to refractive index and methane volume fraction were 949 GHz/RIU and 4.4 GHz/%, respectively, and both the refractive index and methane volume fraction exhibited a good linear relationship with the QBIC resonance peak shift. A square ring metal metasurface methane sensor was designed and compared with the circular ring structure. It was found that the circular ring structure outperformed the square ring structure in terms of Q factor, modulation depth, and sensitivity.