Carbon dioxide absorption spectroscopy with a mid-infrared silicon photonic waveguide

Carbon dioxide ( C O 2 ) 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 C O 2 provide high selectivity, but their large size and high cost limit their use. In this Letter, we demonstrate C O 2 gas sensing at 4.2 µm wavelength using an integrated silicon waveguide, featuring a sensitivity to C O 2 of 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 C O 2 sensors for distributed environmental monitoring, personal safety, and medical and high-volume consumer applications.