Silicon-on-nitride slot waveguide: A promising platform as mid-IR trace gas sensor

•Silicon-on-nitride slot waveguide-based evanescent field absorption-gas sensing scheme exploiting mid-IR fingerprint regime is proposed.•Numerical demonstration of the detection of NH3 gas in atmosphere operating at 3μm is analyzed.•With optimized waveguide structure, maximum achievable evanescent field fraction becomes ∼43%.•Feasible detection up to 5ppm level with a waveguide length of just 8.7mm.•Various fabrication tolerances are examined in detail and proposed structure has the potential for chip-scale trace gas sensing. We propose design of a chip-scale evanescent field absorption-based trace gas sensor, exploiting silicon-on-nitride (SON) slot waveguide structure at mid-IR wavelength region. As an example, we numerically demonstrate detectability of NH3 gas in the atmosphere at its characteristic absorption wavelength of 3μm. Using the optimum sensor design, detection down to 5ppm including loss is viable in just ∼8.7mm length of the proposed slot waveguide. The achievable evanescent field fraction (η) is upto ∼43%. Our tolerance studies show that η is relatively more critical to the slot width as compared to its height.