On-chip photothermal gas sensor based on a lithium niobate rib waveguide

On-chip optical gas sensors have attracted a lot of attention in many fields such as the internet of things and point-of-care diagnosis due to their compactness and high scalability. However, the current laser-based on-chip absorption gas sensors have limited performance caused by weak gas absorbance and strong interference fringe noise. Here, we report sensitive on-chip photothermal gas detection on an integrated lithium niobate photonic platform. The evanescent wave of the frequency-modulated pump light (2004 nm) on a nanophotonic lithium niobate rib waveguide (length of 91.2 mm) is absorbed by the target gas molecules (carbon dioxide, CO2), leading to the significant refractive index modulation of the rib waveguide due to the photothermal effect. A probe light (1550 nm) transmitted through the same waveguide undergoes the photothermal-induced phase modulation, which is sensitively detected by a heterodyne interferometer. As a proof of concept, we demonstrate the on-chip photothermal detection of CO2 with a minimum detection limit of 870 ppm. This work provides new insight for future on-chip gas sensor development with high sensitivity and robustness.