Abstract

This work presents the novel concept of silicon germanium (${{\rm Si}_{1 - {\rm x}}}{{\rm Ge}_{\rm x}}$) asymmetric rib cross-slot waveguides (ACWGs) as a potential solution for sensing a wide range of atmospheric gases in the mid-infrared (MIR) region. The investigation focuses on the analysis of ${{\rm Si}_{1 - {\rm x}}}{{\rm Ge}_{\rm x}}$ ACWGs, which encompass both vertical and horizontal slots. These waveguides are examined in the context of a polarization diversity scheme, aiming to provide robust confinement in the slot region for fundamental quasi-TE and quasi-TM modes. The fabrication of this WG can be achieved through the utilization of advanced complementary metal-oxide-semiconductor technology. In order to enhance the sensing performance of the proposed WG-based sensors, the width of both horizontal and vertical slots is optimized to maximize the total slot power confinement factor (PCF). For the optimized ACWG structure, our simulated results reveal that the fundamental quasi-TM mode exhibits a higher PCF compared to the fundamental quasi-TE mode. In particular, the PCF values for the fundamental quasi-TM mode are found to be 76.4%, 86.8%, and 88.3% at $\lambda = {3.67}\;{\unicode{x00B5}{\rm m}}$ (methane: ${{\rm CH}_4}$), 4.47 µm (nitrous oxide: ${{\rm N}_2}{\rm O}$), and 4.67 µm (carbon monoxide: CO), respectively. Furthermore, when the propagation loss $(\alpha)$ is equal to 0.5 dB/cm, the corresponding sensitivity values for ${{\rm CH}_4}$, ${{\rm N}_2}{\rm O}$, and CO are $3.77 \times {10^{- 7}}\;{{\rm ppm}^{- 1}}$, $6.98 \times {10^{- 5}}\;{{\rm ppm}^{- 1}}$, and $3.53 \times {10^{- 5}}\;{{\rm ppm}^{- 1}}$, respectively. Additionally, with $\alpha = 0.5\;{\rm dB}/{\rm cm}$ and ${\rm SNR} = {1}\;{\rm dB}$, the minimum detectable concentration (${{\rm C}_{{\min}}}$) of ${{\rm CH}_4}$, ${{\rm N}_2}{\rm O}$, and CO is determined, yielding related values of 0.24 ppm, $1.3 \times {10^{- 3}}\;{\rm ppm}$, and $2.6 \times {10^{- 3}}\;{\rm ppm}$. The simulated results demonstrate better values of PCF, sensitivity, and ${{\rm C}_{{\min}}}$ when compared to previously reported sensors based on vertical slot WG, horizontal slot WG, or cross-slot WG. Thus, the proposed ACWG structure presents a potential avenue for the development of highly efficient MIR photonic gas sensors.

© 2023 Optica Publishing Group

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