Sensitivity-Enhanced Parity-Time Symmetric Fabry–Pérot Sensor for Detection of Gas Concentration

In this article, we propose a parity-time (PT)-symmetric Fabry–Pérot microcavity to achieve high-sensitivity detection of gas concentration with refractive index ranging from $10^{-{4}}$ to $10^{-{6}}$ owing to the nonlinear eigenfrequency splitting responses around the exceptional point (EP) of a non-Hermitian system. The transmission peak frequency splitting effect can be enhanced in the PT-symmetric phase of the proposed Fabry–Pérot sensor structure due to perturbation of the system state around the EP induced by the refractive index variation of the gaseous analytes of different concentrations. Calculation results indicate that gas refractive index sensitivity of the proposed sensor reaches $1.81\times 10^{{7}}$ GHz/RIU, which is significantly higher than that of the Fabry–Pérot structure with the same geometric parameters operating in the Hermitian regime. When methane/air mixture is injected into the microcavity of the Fabry–Pérot sensor, the change in methane concentration would lead to the refractive index variation in the gas mixture, and the pair of transmission eigenfrequency peaks would shift accordingly. Calculation results show that the methane concentration detection sensitivity is up to 28.03 GHz/vol.%. Moreover, the gas detection sensitivity can be further improved by reducing the number of periods ${N}$ in the gain/loss doping region. Our proposed EP-induced sensitivity-enhanced PT symmetric Fabry–Pérot sensor possesses several desirable merits such as high sensitivity, compact size, low cost, and good compatibility with functional materials, which make it a promising candidate for potential applications in the detection of low-concentration gas component in mixture and recognition of gaseous analytes based on refractive index measurement.