High-Performance Germanium-Tin Fabry-Perot Microcavity Strip-Loaded Mid-Infrared Waveguide-Based Optical Sensors

This work proposes and theoretically investigates the efficacy of gratings-based strip-loaded germanium-tin (Ge<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{1} - {x}}$ </tex-math></inline-formula>Snx) waveguides (WGs) as gas sensors for the mid-infrared (MIR) region. Focusing on Ge<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{1} - {x}}$ </tex-math></inline-formula>Snx as a novel material system, we comprehensively examine and optimize WG types, including Fabry-Perot (F-P) microcavity strip-loaded WGs and conventional rib WGs as a reference. In addition, a rigorous analysis is directed toward the study of mode profiles and normalized electric field distribution. Finite element analysis was used to optimize the WG structures and determine the propagation loss and evanescent field ratio (EFR). The effective sensing region in the gratings-based WG encompasses not only the top and side of the WG but also the area between the Ge<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{1} - {x}}$ </tex-math></inline-formula>Snx strips along the direction of light propagation. It results in a significant increase in EFR, which in turn enhances sensitivity and provides a distinctive sensing characteristic compared to conventional rib evanescent wave sensors. Furthermore, the effect on Sn alloying in GeSn is closely analyzed for EFR and power confinement factor (PCF). The operating wavelengths are chosen as 3, 3.47, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.3~\mu $ </tex-math></inline-formula> m based on the absorption peaks of NH3, CH4, and CO2, respectively. The sensitivities obtained with the proposed sensors are approximately <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$31\times $ </tex-math></inline-formula>, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$33\times $ </tex-math></inline-formula>, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$28.4\times $ </tex-math></inline-formula> greater than the values obtained with conventional rib WG sensors at the wavelengths of 3, 3.47, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.3~\mu $ </tex-math></inline-formula>m, respectively. In comparison to earlier reported WG sensors operating at the same wavelength, the proposed sensor based on gratings demonstrates the lowest propagation loss, the highest sensitivity, and the lowest limit of detection (LoD). In addition, the study investigated the impact of fabrication tolerance on the EFR of the optimized WG. The findings indicate that the EFR is marginally affected considering ±10% tolerance. As a result, on-chip gas sensing in the MIR region could be possible with the proposed Ge<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{1} - {x}}$ </tex-math></inline-formula>Snx grating-based WG sensor.