In this work, the design of a novel universal multigas evanescent field absorption sensor based on a silicon-on-sapphire slot waveguide is proposed for sensing trace gases in the mid-infrared. The design features a small waveguide cross-section of 0.74 \(\upmu\) m \(\times 1.4\) \(\upmu\) m and waveguide length of 0.33 cm. The multigas sensor is optimized for sensing carbon dioxide, nitrous oxide, ammonia, and methane gas by operating the waveguide sensor at their characteristic absorption wavelengths of 2.76, 2.86, 3.00, and 3.315 \(\upmu\) m, respectively, using the same waveguide structure. A high evanescent field ratio of approximately 50 \(\%\) has been obtained for all four gases. The sensitivity of the trace gas sensor is estimated as 4.04 \(\times 10^{-5}\) , 4.17 \(\times 10^{-5}\) , 4.88 \(\times 10^{-5}\) , 118.71 \(\times 10^{-4}\) \(\hbox {ppm}^{-1}\) , and the limits of detection are obtained as 8.80, 8.44, 7.25 and 0.25 ppb for carbon dioxide, nitrous oxide, ammonia, and methane gas molecules, respectively. The universal multigas sensor shows better sensitivity than the sensor based on conventional strip waveguide. The high sensitivity for multigas sensing, low limit of detection, small footprint, and CMOS compatibility of the multigas sensor make it suitable for on-chip applications.
