image: Figure | Working principle and performance of the suspended waveguide-enhanced photothermal spectroscopy sensor. a, Schematic of the suspended chalcogenide glass waveguide. The air bottom-cladding enhances light-gas interaction and suppresses thermal dissipation. b, Comparison showing the massive improvement in heat source power and probed temperature change over traditional non-suspended waveguides. c, The sensor achieves a record-breaking noise-equivalent absorption (NEA) of 3.8×10-7cm-1 for acetylene with a large dynamic range.
Credit: Wei Jin et al.
The demand for compact, sensitive, and low-cost gas sensors is rapidly growing for applications ranging from environmental monitoring to wearable healthcare. Traditional on-chip sensors based on direct light absorption have limited sensitivity due to weak interaction between light and gas molecules over short optical distances.
In a new paper published in Light: Science & Applications, a collaborative team of scientists from The Hong Kong Polytechnic University, Zhejiang Lab, Jilin University and Changchun Institute of Optics, Fine Mechanics and Physics, has developed a revolutionary on-chip gas sensing platform. They combined a novel suspended chalcogenide glass waveguide with highly sensitive and background-free photothermal spectroscopy, achieving parts-per-billion level molecular gas detection for the first time on an integrated near-infrared photonic chip.
The key innovation lies in the "suspended" design of the waveguide. By replacing the solid bottom cladding (like silicon dioxide) with air, the team achieved two major enhancements: Optical Enhancement—more light extends into the surrounding gas, increasing absorption and heat generation; and Thermal Enhancement—air's low heat conduction traps the generated heat near the waveguide, amplifying the temperature induced phase modulation. Their theoretical model guided the optimization, resulting in a 45-fold improvement in photothermal efficiency compared to non-suspended waveguides.
The fabricated sensor is remarkably compact (1.2 cm chip length), operates with near-infrared light (compatible with mature telecom technology), and incorporates an on-chip Fabry–Pérot interferometer for photothermal signal readout. It demonstrated exceptional performance: detecting acetylene (C2H2) down to 330 parts-per-billion, a dynamic range spanning over 6 orders of magnitude, and a response time of less than 1 second.
"Our work overcomes the fundamental limitations of weak light-gas interaction and fast heat loss that have plagued on-chip sensors," the scientists stated. "This suspended waveguide platform paves the way for highly sensitive, selective, fast, and highly integrated 'sensor-on-a-chip' devices for real-world applications."
The technology is not limited to near-infrared sensing. The chalcogenide glass material can transmit light across a wide mid-infrared spectrum, allowing future sensors to detect a variety of gases and molecules, including greenhouse gases and volatile organic compounds. Moreover, planar suspended platforms enable the investigation of photothermal, optomechanical, and optoacoustic effect in solid and liquid medium, thereby significantly advancing the fields of interface optics and chemical reaction dynamics.
Article Title
Suspended waveguide-enhanced near-infrared photothermal spectroscopy for ppb-level molecular gas sensing on a chalcogenide chip