Reviewed by Lexie CornerMay 1 2025
A research team from Huazhong University of Science and Technology has developed a novel surface acoustic wave (SAW) sensor based on a passive parity-time (PT) symmetric architecture. This design allows the sensor to operate near exceptional points (EPs), enhancing its sensitivity. The device features coupled resonators and a tin oxide (SnO₂) thin film, carefully engineered to optimize internal energy losses.
The passive PT-symmetric model for SAW sensor. a The schematic diagram of the coupled resonators model and the real part of the eigenfrequency surfaces in the parameter space (ω1 = ω2 = ω0 = 0,γ0 = 0). The solid line denotes an EP line where γ = 2κ. b A schematic diagram of the passive PT-symmetric SAW sensor. The coupled resonators are defined by three Bragg mirrors and coupled to the SAW transmission line. A sensitive layer is deposited onto the resonator I. External disturbance activates the sensitive layer and introduces additional loss γ in the resonator I. Image Credit: AlphaGalileo
SAW sensors are known for their compact size, ease of integration, and compatibility with digital systems. However, improving their selectivity and sensitivity has remained a persistent challenge.
Traditional SAW sensors detect frequency shifts caused by changes in surface layers, but their performance is often limited by the linear nature of this mechanism. In contrast, exceptional points (EPs)—unique degeneracies in non-Hermitian systems—have shown potential for amplifying weak signals in optics and electronics. Until now, however, their application in acoustic systems has been limited by engineering constraints.
Recognizing the growing demand for real-time, high-precision sensing in fields like environmental monitoring and personalized healthcare, the researchers aimed to integrate EPs into SAW sensor design to overcome long-standing limitations.
The result is a next-generation hydrogen sulfide (H₂S) sensor capable of detecting trace gas concentrations as low as 2 ppm, with a response time of under 10 seconds.
At the heart of this breakthrough is a passive PT-symmetric design featuring two acoustically coupled resonators and a SnO₂-coated surface. This configuration enabled the system to exhibit a square-root relationship between frequency shift and perturbation strength near the EP, allowing highly sensitive, nonlinear responses to H₂S levels as low as 0.4 ppm, far beyond what linear SAW sensors can achieve.
Instead of relying on absolute frequency changes, the system tracked differential peak shifts, providing strong temperature stability. It also delivered fast, reversible responses, showing complete recovery after H₂S exposure and high selectivity against common interferents like nitrogen dioxide and ammonia.
To further ensure stability, the team used an asymmetric electrode design to offset frequency drift caused by the SnO₂ film. By operating near—but not exactly at—the EP, the design also avoided excess quantum noise often associated with EP systems.
The sensor’s performance was validated through both physical experiments and COMSOL simulations. It demonstrated compatibility with a range of substrates, including quartz, and lays the groundwork for adapting EP-enhanced sensing strategies to other applications.
This research bridges abstract physics and practical sensing. By leveraging exceptional points, we have fundamentally changed what is possible in gas detection. He emphasized the approach's scalability and its potential to influence a wide array of sensing technologies. We see this as a platform not just a device which can be extended to mechanical, biological, and chemical sensors with transformative results.
Dr. Wei Luo, Study Co-Corresponding Author, Huazhong University of Science and Technology
This technology holds promise across a range of industries. In environmental monitoring, it could serve as a crucial early-warning system for detecting toxic gas leaks in industrial settings. In healthcare, it may enable real-time breath analysis for diagnosing conditions such as metabolic disorders or liver disease.
Thanks to its compatibility with MEMS technology, the sensor can be mass-produced at low cost, ideal for integration into Internet of Things (IoT) systems. Future developments may include adapting the design to detect a broader array of gases and biomarkers, or exploring higher-order exceptional points to further enhance sensitivity.
By merging sensor engineering with advanced physics, this research sets the stage for a new generation of smart, ultra-miniaturized detectors.
Journal Reference:
Lu, X., et al. (2025). Harnessing exceptional points for ultrahigh sensitive acoustic wave sensing. Microsystems & Nanoengineering. doi.org/10.1038/s41378-024-00864-5.