Trace gas detection on a photonic chip with sub-ppb detection levels and isotope discrimination

Spectroscopic techniques, such as laser absorption spectroscopy (LAS), are the gold standard for trace gas detection in environmental research and atmospheric monitoring. Nevertheless, the large size and cost of the LAS instruments limit the scale and contexts in which they can be employed. A promising solution to this challenge is the development of LAS sensors based on photonic integrated circuits. However, the detection limits of the on-chip sensors remain to date in the upper ppm range, i.e. several orders of magnitude higher than that of large instruments based on bulk optics. 

Here we will present an on-chip LAS sensor based on an integrated nanophotonic waveguide capable of detecting CO₂ with a record-low sensitivity of 20 ppb. This result was achieved through a careful waveguide design, aiming at (i) operation in the mid-infrared spectral range, (ii) strong evanescent field interaction between the guided light and the gas sample, and (iii) minimizing spurious etalons that are a major source of spectral noise in on-chip devices. Moreover, with this sensor we demonstrate the first δ¹³C isotope ratio measurement realized with an on-chip device. The achieved precision of the order of 1‰ is comparable with high-end commercial instruments.  

This work is more than an incremental step in on-chip gas sensor development; it is a leap forward for in-situ gas monitoring. Our sensors offer a scalable, efficient solution for autonomous monitoring of remote areas and sensor networks of the future. Additionally, with their minute active sensing volume (microlitre), the on-chip devices open new possibilities for applications where small sample volumes are required such as ice-core samples analysis and microbiological essays