The development of ultrasensitive and rapid trace gas sensing techniques is of significance in diverse scientific and technological fields. Accurately detecting and monitoring trace gases at low concentrations are crucial for a wide range of applications, including environmental monitoring, non-invasive breath analysis, and industrial process control. Trace gas sensors play a vital role in assessing air quality, monitoring greenhouse gas emissions, and studying atmospheric chemistry. Real-time on-site detection of pollutants is essential for effective environmental protection and understanding the influence of human activities on climate change. Non-invasive breath analysis using trace gas sensors provides a promising approach for diagnosing and monitoring various diseases. Non-invasive respiratory diagnosis can be applied to the detection of CO₂ excreted by the skin and the analysis of NH₃ exhaled by the human body, which has medical significance and application prospect. In various industrial processes, monitoring trace gas concentrations is crucial for ensuring product quality, optimizing efficiency, and detecting leaks. Applications include monitoring gas purity in semiconductor manufacturing, detecting leaks in pipelines, and controlling emissions from industrial facilities. High-voltage equipment such as gas-insulated switchgear (GIS) relies on insulating gases like sulfur hexafluoride (SF₆). However, these gases can decompose over time, forming byproducts that compromise equipment performance and pose safety risks. Early detection of these decomposition products by adopting trace gas sensors is critical for maintaining the reliability and safety of power grids.

Laser-based absorption spectroscopy techniques, especially those exploiting the photoacoustic effect, have emerged as powerful tools for trace gas detection, featuring high sensitivity, selectivity, and rapid response time. Among these, quartz-enhanced photoacoustic spectroscopy (QEPAS) has caught significant attention for its compact size, low cost, and robustness against environmental noise. Recent research focuses on three key areas to achieve ultrasensitive and rapid QEPAS sensing, including enhancing sensitivity via acoustic resonators, calibration-free and rapid gas detection with beat frequency QEPAS (BF-QEPAS), and customizing quartz tuning forks for performance enhancement. One major limitation of traditional QEPAS systems is the relatively low sensitivity due to the inherent properties of quartz tuning forks. To this end, researchers focus on integrating acoustic resonators with tuning forks to amplify the generated photoacoustic signals, which leads to significant enhancement in sensitivity and enables the detection of trace gases at even lower concentrations. Early attempts to enhance QEPAS sensitivity involve employing simplified models of acoustic resonators. However, subsequent research highlights the crucial role of acoustic coupling between resonator segments and quartz tuning forks. This understanding leads to the development of optimized segmented resonators, with specific lengths and gaps carefully chosen to maximize acoustic coupling and signal amplification (Fig. 3). To enable dual-channel detection, researchers have developed QEPAS systems incorporating double acoustic micro-resonators (AmRs). These systems utilize two sets of segmented resonators strategically placed on either side of the quartz tuning forks (Fig. 5). While having rapid response time, the dual-channel design often comes with a trade-off in sensitivity compared to systems with single resonator configurations. Further advancements in resonator design have brought about the development of on-beam single-tube micro-resonators. This configuration focuses on maximizing the acoustic coupling between a single resonator and a custom-designed quartz tuning fork with large prong spacing (Fig. 6). By strategically placing the resonator with symmetrical openings around the tuning fork, researchers have realized significant enhancement in sensitivity, surpassing those achieved with bare tuning fork systems by orders of magnitude. In addition to sensitivity enhancement, another crucial aspect is rapid and calibration-free detection. Traditional QEPAS systems often require time-consuming calibration procedures and suffer from slow response time, limiting their applicability in real-time monitoring scenarios. To overcome these limitations, researchers have developed BF-QEPAS, a groundbreaking technique that leverages the transient response of quartz tuning forks to pulsed photoacoustic signals. In BF-QEPAS, rapidly scanned laser wavelengths generate pulsed photoacoustic signals, exciting the quartz tuning fork. By analyzing the transient response of the tuning fork using beat frequency detection, BF-QEPAS enables simultaneous measurement of gas concentration, resonant frequency, and quality factors. Finally, this eliminates the need for pre-calibration steps and significantly reduces response time, making BF-QEPAS ideal for real-time gas monitoring applications. Recognizing the limitations of commercially available quartz tuning forks, we focus on developing customized forks with specific properties tailored for QEPAS performance enhancement. By carefully adjusting the geometry and parameters of the tuning forks, significant improvements in sensitivity, selectivity, and detection limits are yielded. Customizing quartz tuning forks with wider prong spacing allow for the utilization of light sources with lower beam quality. This is particularly advantageous in the case of integrating QEPAS systems with high-power lasers, such as fiber amplifiers, as it enables efficient light transmission via the wider gap between the tuning fork prongs. Combining these customized forks with high-power lasers and optimized micro-resonators leads to highly sensitive detection of specific gases, such as hydrogen sulfide (H₂S). Additionally, we develop techniques for exploiting the overtone vibration modes of quartz tuning forks to enable simultaneous detection of multiple gases. By optimizing the geometry of the tuning fork, distinct fundamental and overtone vibration modes with well-separated resonant frequencies are achieved. Strategically aligning independent laser beams with the respective antinodes of these vibration modes allows for frequency division multiplexing, effectively creating separate detection channels for different target gases (Fig. 8). This approach eliminates the need for multiple tuning forks or complex optical setups, simplifying the sensor design and reducing overall system size.

Driven by the need for higher performance and versatility, recent advancements in QEPAS technology, including optimized acoustic resonators, BF-QEPAS, and custom quartz tuning forks, have significantly advanced trace gas sensing. In the future, integrating novel excitation sources and miniaturization techniques will further enhance QEPAS capabilities, paving the way for wider adoption in environmental monitoring, medical diagnostics, industrial process control, and beyond.