  Carbon dioxide is a vital gas for life on Earth, a waste product of human
activities, and widely used in agriculture and industry. Its accurate sensing
is therefore of great interest. Optical sensors exploiting the mid-infrared
light absorption of CO$_{\mathrm{2}}$ provide high selectivity, but their large
size and high cost limit their use. Here, we demonstrate CO$_{\mathrm{2}}$ gas
sensing at 4.2 $\mu$m wavelength using an integrated silicon waveguide,
featuring a sensitivity to CO$_{\mathrm{2}}$ 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 CO$_{\mathrm{2}}$ sensors for
distributed environmental monitoring, personal safety, medical, and high-volume
consumer applications.
