Sub-ppt level detection of carbon monoxide and nitrous oxide enabled by mid-infrared doubly resonant photoacoustic spectroscopy

We report highly sensitive detection of carbon monoxide (CO) and nitrous oxide (N 2 O) using doubly resonant photoacoustic spectroscopy paired with a quantum cascade laser (QCL) at 4.57 μm. The butterfly-packaged QCL is used to exploit the CO absorption line at 2190.02 cm −1 and the N 2 O absorption line at 2191.42 cm −1 by scanning the injection current. Leveraging the simultaneous acoustic and optical resonances and adopting a lower photoacoustic detection frequency, we achieve a minimum detection limit of 0.85 part-per-trillion (ppt) for CO over the 500 s averaging time, and 0.7 ppt for N 2 O over the 200 s averaging time. Our approach demonstrates record sensitivity for CO and N 2 O detection compared to state-of-the-art optical gas sensors.