Silicon Racetrack Resonators and EIT: A Pathway to Enhanced Near Infrared Gas Sensing

This work presents the design, fabrication, and experimental validation of a silicon-on-insulator (SOI) optical platform exploiting electromagnetically induced transparency (EIT) for enhanced near-infrared (NIR) refractive index sensing. The proposed device consists of a pair of coupled non-concentric racetrack resonators with optimized geometrical parameters engineered to operate at the telecommunications wavelength of 1.55 μm. By aligning the resonance frequencies of the two racetrack resonators, strong optical coupling is achieved, resulting in eigenfrequency splitting and the formation of a sharp EIT-like transparency window that enhances the spectral response of the device. The coupled structure produces two split resonances centered at 1.544 μm and 1.546 μm, with quality factors reaching 3227 and 3447.15, respectively. The sharp transparency peaks significantly strengthen the light–matter interaction, leading to a sensitivity of 150 nm RIU −1 and a figure of merit (FOM) of 289. To further understand the physical origin of the EIT response, the coupling interaction between the resonators was analyzed using temporal coupled-mode theory (CMT). In addition, a parametric study of the coupling gap was carried out to determine the optimal interaction regime that maximizes resonance sharpness and sensing performance. The gas sensing capability of the platform is investigated numerically for ammonia (NH 3 ) and acetylene (C 2 H 2 ), whose absorption features lie within the NIR region. The interaction with these gases is modeled through refractive index perturbations and absorption-induced losses, predicting measurable spectral shifts and linewidth variations. The device was fabricated using a multi-project wafer (MPW) SOI process and experimentally characterized, showing strong agreement with numerical simulations and confirming the successful realization of the EIT effect. A comparison with previously reported microring-resonator-based sensors further highlights the competitive performance of the proposed design while maintaining a compact footprint and CMOS compatibility. The demonstrated high Q-factor, enhanced FOM, and robust spectral response make this EIT-based coupled racetrack resonator a promising platform for future integrated gas sensing applications in the near-infrared regime.