This study explores the feasibility and superiority of coupled cavities as electromagnetically induced transparent (EIT) excitation media. The EIT effect is realized by the coupled cavity structure, and its performance in gas sensing is investigated, including its sensitivity, detection limit, and selective recognition ability for different gases. In addition, we analyze the influence of the coupling cavity spacing on the EIT excitation and gas sensing performance to provide new ideas for the development of high-performance optical sensor devices.

The study is based on the coupled-mode theory, and the simulation model of the EIT-like effect of the coupled cavity is constructed using the finite difference in the time domain (FDTD) method for simulation. By optimizing the structure of the double-ring coupled cavity, the EIT-like effect is realized, and the refractive index sensing tests of several gases (hydrogen, oxygen, carbon dioxide, and chlorine) that are conventionally employed in the industry are carried out using this effect. The size, spacing, intrinsic loss, and coupling loss of the coupling cavity are considered in this study, and the EIT-like effect is modulated by changing these parameters and analyzing their effects on the gas sensing performance.

The results show that the coupling cavity can be employed effectively as an excitation medium for the EIT-like effect and that when the EIT is excited, its sensitivity is stable and can reach 205.166 nm/RIU, despite the decrease in sensitivity. The limit of detection (LOD) value reaches 7.46×10^-4 RIU, and it is optimized by a factor of 4.16 with respect to that of the unexcited EIT. This indicates that the coupling cavity structure has significant potential for applications in optical sensing, especially for improving the sensitivity and detection limit of the sensor. The effect of the coupling-cavity spacing on the excitation of the EIT is discussed in this paper. Researchers have observed the generation and variation of EIT-like effects by varying the spacing between the coupling cavities M1 and M2. As the spacing increases, the coupling loss increases, leading to the gradual absorption of the transparent peaks; however, when the spacing reaches a certain value, the EIT-like effect becomes obvious and the resolution and detection limit performance of the sensor are significantly improved. In addition, this study explores the refractive index sensitivity properties of the EIT-like effect under different gas environments. It is found that the change in the refractive index induced by gas molecules affects the waveguide transmission efficiency of the coupled mode, which thereby affects the position of the transparent peak in the EIT-like effect. By analyzing the corresponding transmission spectral line shapes and wavelength shifts of different gases, this study confirms that the coupling-cavity structure has a good selective recognition ability for different gases.

EIT media have the significant potential for application in several fields, including optical communication, optical pulse storage, and quantum information processing. The coupling cavity is an excellent medium for the study of EIT, owing to its advantages of a highly localized optical field, strong tunability, and simple structure. In this study, the feasibility and superiority of coupling cavities as EIT excitation media are investigated based on coupled mode theory. Utilizing FDTD simulations, this study investigates EIT-like excitation in coupled cavities for the selective identification of common gases in industrial applications. Additionally, the impact of the coupling distance on EIT-like excitation, as well as the sensor performance, including refractive index sensitivity and detection limit, is explored. The sensitivity of the EIT excitation is reduced, but it is approximately stable and can reach 205.166 nm/RIU. Moreover, the limit of detection reaches 7.46×10^-4 RIU, which is approximately 4.16 times that of the corresponding unexcited value. The results show that the coupling cavity has important research value in the field of electromagnetically induced transparent sensing and provides new ideas for the further realization of optical devices with easy tuning and ultralow detection limits.