Mid Sweden University

Carbon dioxide (CO2) is a gas vital for life on Earth. It is also a waste product of human activities and is widely used in agriculture and industry. Its accurate sensing is therefore of great interest. Optical sensors exploiting the mid-infrared light absorption of CO2 provide high selectivity, but their large size and high cost limit their use. In this Letter, we demonstrate CO2 gas sensingat 4.2 μm wavelength using an integrated silicon waveguide, featuring a sensitivity to CO2of 44% that of free-space sensing. The suspended waveguide is fabricated on a silicon-on-insulator substrate by a single-lithography-step process, and we route it into a mid-infrared photonic circuit for on-chip-referenced gas measurements. Its demonstrated performance and its simple and scalable fabrication make our waveguide ideal for integration in miniaturized CO2 sensors for distributed environmental monitoring, personal safety, and medical and high-volume consumer applications. © 2019 Optical Society of America.