Sub-parts-per-billion CO₂ Detection based on Dissipative Whispering Gallery Mode Microcavity Sensor

Whispering gallery mode microcavities provide strong light-matter interactions owing to their ultrahigh optical confinement, but the small gas refractive index change limits their ability to sense trace gases. Here we show that gas absorption can be detected using a dissipative sensing mechanism in a non-functionalized whispering gallery mode microcavity. Instead of tracking resonance frequency shifts used in conventional dispersive sensing, our method converts optical absorption into variations in resonance depth through thermally induced dissipation. Quantitative carbon dioxide detection was achieved over a concentration range of 1.5 to 400 parts per million with a correlation coefficients exceeding 0.99. The sensor reached a detection limit of 168 parts per trillion at an integration time of 400 seconds and an accuracy of approximately 0.4%. Continuous monitoring further demonstrated stable operation under ambient conditions. These results establish dissipative microcavity sensing as a promising approach for compact, low-cost, and highly sensitive trace gas detection.