Hydrogen sulfide (H₂S) is a silent threat that is responsible for many incidents of toxic exposure and its indispensable role of H2S has also been recognized in cellular signaling and protection and in regulating numerous biological functions. Hence, the stability and the accuracy of the gas detection devices are not only crucial to save lives, but also required in the multidisciplinary domains for fundamental or applied research. However, for a long time, sensitive and fast H₂S detection remains challenging, especially in the few parts per million (ppm) concentration range or lower.

In a study published in Photoacoustics, Prof. WANG Qiang and ZHANG Hui from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) developed a ultrasensitive H₂S gas sensor based on doubly resonant photoacoustic spectroscopy (PAS), and proposed an intriguing strategy of laser-cavity-molecule locking to increase the sensor response and to enhance the system stability for fast and continuous measurements.

The H2S sensor is implemented in the near-infrared region where optics are mature and commercially available. Its sensitivity is enhanced by using doubly resonant PAS with a PAS effect enhancement factor of 18,000 achieved by the concurrent pump laser power accumulation with an optical resonator and sound energy with an acoustic resonator. Its fast response is realized by specialized laser-cavity-molecule locking. Rather than scanning the entire spectrum, the locking strategy enables the simultaneous locking of laser frequency, cavity mode, and absorption line by two separate feedback loops.

The sensor’s performance was experimentally evaluated by the detection of H₂S/N₂ mixtures under 1 atm and at room temperature. Noise equivalent concentration (NEC), a factor to show the sensitivity, was determined to be 79 ppb at an integration time of 1 s, leading to a normalized noise equivalent absorption (NNEA) coefficient of 8.9×10¹² W·cm^-1·Hz^-1/2. The NEC further reached 10 ppb at an integration time of 200 s. Moreover, the PAS-1f signal remains quite stable. This mainly benefits from the proposed locking strategy, which can compensate for the cavity length slow drifts or variation caused by the heating source.

These values confirmed that the sensitivity of this H₂S sensor is superior to those of the QEPAS-based sensors and CRDS-based sensors reported in the literature, and the NNEA achieves an improvement of 50 times.

This study provides a powerful analysis tool for accurate H₂S measurement in medical diagnosis, air quality assessment, risk prediction in coal mine gas, etc., where H₂S measurement from ppb to ppm level is needed.