Silicon slot waveguide for detection of methane with 300 ppb detection limit (Invited Paper)

Methane concentrations often exceed the atmospheric background level of approximately 2 ppm across diverse industrial sites and landfills. Accurately registering subtle deviations from this atmospheric baseline, however, proves to be notoriously challenging, especially with chip-sized devices. Laser absorption spectroscopy with photonic integrated circuits (PICs) is emerging as a breakthrough in addressing this challenge. We have been developing optical waveguides [1], [2] for MIR and, recently, we designed and fabricated a silicon slot waveguide for operation at 3270.4 nm, which coincides with a strong absorption peak of methane [3]. The design was optimized for (i) a high air confinement, (ii) low lateral and substrate leakage, and (iii) single-mode operation at the target wavelength. Our slot waveguide employed for methane detection demonstrates a 1-σ detection limit (LOD) as low as 300 ppb. This is 1–2 orders of magnitude lower than the current state-of-the-art, and the first instance where an on-chip device shows the capability to register methane changes below atmospheric background levels. Furthermore, in our effort to make the sensor more applicable in practical settings, we acknowledge and address operational constraints, particularly focusing on the impact of atmospheric humidity on the sensor performance. We studied the impact by exposing the waveguide to 70–75% relative humidity, which yielded a total propagation loss of 13.8 ± 0.4 dB cm–1. Baking the waveguide at 115 ± 5 °C for 10 min and then keeping it in dry N2 atmosphere reduced the loss to the 8.3 ± 0.3 dB cm–1. Removing the water is thus a major concern and may require e.g., a hydrophobic coating to stabilize the loss in the future.

References
[1] M. Vlk et al., “Extraordinary evanescent field confinement waveguide sensor for mid-infrared trace gas spectroscopy,” Light Sci. Appl., vol. 10, no. 1, p. 26, Dec. 2021, doi: 10.1038/s41377-021-00470-4.
[2] J. Salaj et al., “Suspended nanophotonic waveguide for isotope-specific CO2 detection,” Optica, vol. 11, no. 12, p. 1654, Dec. 2024, doi: 10.1364/OPTICA.533710.
[3] H. D. Yallew et al., “Sub-ppm Methane Detection with Mid-Infrared Slot Waveguides,” ACS Photonics, vol. 10, no. 12, pp. 4282–4289, Dec. 2023, doi: 10.1021/acsphotonics.3c01085.