Carbon-dioxide absorption spectroscopy with solar photon counting and integrated lithium niobate micro-ring resonator

We demonstrate a spectroscope using single-photon counters and a chip-integrated lithium niobate micro-ring filter to measure the atmospheric CO₂ absorption spectrum passively. By thermo-optically sweeping the filter over 150 pm and referencing the resulting photon counts to a bypass channel, we sample the absorption spectrum at an ultrahigh-resolution of 6 pm. Incorporating it into a ground-based field system, we characterize the CO₂ absorption through the atmosphere by counting the solar photons across the absorption line around 1572.02 nm, which agrees well with its transmission spectrum at standard atmospheric pressure. Our results highlight the potential of adopting integrated photonics and single-photon counting in remote sensing systems for high detection sensitivity, superior resolution, and significantly reduced size, weight, and power.