Design of chip scale silicon rib slot waveguide for sub-ppm detection of N2O gas at mid-IR band

Abstract

A chip-level silicon rib horizontal double-slot waveguide geometry has been proposed and designed for trace gas sensing in mid-IR regime through the evanescent field absorption based sensing scheme. We have optimized the structure at the 4.47 μm mid-IR band for efficient detection of hazardous N2O gas in the atmosphere at sub-ppm concentration level. Maximum achievable evanescent field fraction (i.e. power confinement (Γ)) in the sensing region is as high as 68.0%, which leads to detection down to nearly 0.2 ppm level with 1.44 cm length of our designed waveguide even after including the waveguide losses. With this optimum sensor design, we have also studied detectability of CO and CH4 gases numerically at the wavelength of 4.6 μm and, 3.67 μm, respectively, and have shown that minimum detectable concentrations for these gases are nearly 0.44 ppm and 36 ppm, respectively. Fabrication tolerances of the structure have shown that sensor has high tolerance to fabrication imperfections.