A team of researchers in China and France has developed a compact optical sensor capable of detecting carbon dioxide at concentrations as low as 168 parts per trillion, roughly 2,500 times more sensitive than ambient atmospheric CO₂ levels. The device, described July 4 in Nature Communications, uses a whispering gallery mode microcavity, a tiny glass resonator that traps light in a circulating path, and exploits a sensing mechanism that has been largely overlooked in trace gas detection.
“This is the first demonstration of dissipative sensing in a non-functionalized WGM microcavity for trace gas detection,” said corresponding author Tingdong Cai of Jiangnan Normal University. “Instead of tracking how the resonance frequency shifts, we measure how the resonance depth changes when gas absorbs light.”
Two ways to sense
Most optical sensors based on whispering gallery mode microcavities work by measuring shifts in the resonance frequency, a principle called dispersive sensing. When gas molecules bind to or pass near the microcavity surface, they change the local refractive index, causing the resonant wavelength to drift. The problem is that the refractive index change produced by tiny concentrations of gas is vanishingly small, limiting sensitivity.
The team led by Shujing Ruan, Guangzhen Gao, and Jianing Zhang took a different approach. Their sensor measures dissipative changes, specifically, how much light is lost from the cavity due to optical absorption by CO₂ molecules. When CO₂ absorbs the circulating light, it causes local heating, which changes the resonance depth (the contrast between the resonant and non-resonant transmission). The effect is proportional to gas concentration and, crucially, does not require any chemical coating or functionalization of the cavity surface.
The result is a sensor that combines extreme sensitivity with remarkable simplicity. Over a concentration range of 1.5 to 400 parts per million, the sensor showed a correlation coefficient exceeding 0.99, near-perfect linearity. At 400 seconds of integration time, the detection limit reached 168 parts per trillion. The accuracy was approximately 0.4%.
Why it matters
Carbon dioxide sensing is critical across a wide range of applications, from climate monitoring and industrial safety to indoor air quality and medical diagnostics. Current high-precision CO₂ sensors, non-dispersive infrared (NDIR) analyzers and cavity ring-down spectrometers, are sensitive but bulky, expensive, and power-hungry. A compact, low-cost alternative could enable dense sensor networks for environmental monitoring, smart building ventilation systems, and portable safety devices.
The whispering gallery mode microcavity at the heart of the sensor is a glass structure just tens of microns in diameter, small enough to fit on a chip. It requires no moving parts, no gas cells, and no special coatings. The sensing is all-optical: a near-infrared laser is coupled into the microcavity via a tapered optical fiber, and the transmitted light is analyzed for changes in resonance depth.
“Because the cavity does not need to be functionalized with any chemical coating, the sensor is inherently stable and long-lasting,” said co-author Deyuan Shen. “There is no coating to degrade, no drift from chemical changes on the surface.”
Performance in real-world conditions
The researchers demonstrated continuous monitoring under ambient conditions, showing that the sensor maintained stable operation without temperature or humidity control, a critical requirement for field deployment. The detection limit of 168 ppt is well below ambient CO₂ concentrations (roughly 420 ppm), meaning the sensor could easily distinguish small variations in indoor or outdoor CO₂ levels.
The sensor’s response time, determined by the thermal dynamics of the cavity, was on the order of seconds, fast enough for real-time monitoring but slower than the microsecond response of some electronic sensors. For environmental monitoring applications, where changes occur over minutes to hours, this is more than adequate.
Broader implications
The dissipative sensing mechanism is not limited to CO₂. Any gas with an absorption line in the near-infrared region, methane, water vapor, ammonia, and many volatile organic compounds, could, in principle, be detected using the same approach, simply by tuning the laser wavelength to match the gas’s absorption feature.
“The principle is general,” the authors note. “A non-functionalized WGM microcavity can serve as a universal platform for trace gas detection, with the specificity coming from the laser wavelength rather than from surface chemistry.”
For a field where extreme sensitivity typically comes with extreme complexity, the combination of sub-parts-per-billion detection, chip-scale size, and no functionalization requirements represents a meaningful step toward practical, deployable optical gas sensors.
Source: Ruan S, Gao G, Zhang J, Wang H, Cheng D, Guo J, Ren C, Chen W, Shen D, Cai T. Sub-parts-per-billion CO₂ Detection based on Dissipative Whispering Gallery Mode Microcavity Sensor. Nature Communications (2026). DOI: 10.1038/s41467-026-75218-y