Fig. 1. QTF assembled with acoustic micro-resonator^[37]

Fig. 2. Resonance diagrams of sound wave in segmented resonator^[37]. (a) Acoustic intensity distribution when segmented resonator is strongly coupled and approximated as single tube; (b) acoustic intensity distribution when segmented resonator is no acoustic coupling; (c) acoustic intensity distribution when acoustic coupling occurs between QTF and segmented resonator

Fig. 3. Gain of signal-to-noise ratio (SNR) of micro-resonator with respect to bare QTF^[37] (length, inner diameter, and outer diameter of micro-resonator are 4.4 mm, 0.6 mm, and 0.9 mm, with 50 µm gap between tube facet and QTF surface)

Fig. 4. Variations of signal amplitude, quality factor, and signal-to-noise ratio of QEPAS system with distance y_t between top of acoustic micro-resonator and top of QTF^[45]. (a) Detected signal amplitude of QEPAS system; (b) quality factor of QEPAS system; (c) signal-to-noise ratio of QEPAS system

Fig. 5. Schematic of QEPAS system based on double-tube acoustic micro-resonator^[46]

Fig. 6. On-beam single-tube acoustic micro-resonator^[48]. (a) Assembly of on-beam single-tube acoustic micro-resonator; (b)‒(d) geometric parameters of on-beam single-tube acoustic micro-resonator

Fig. 7. Photograph of custom-made tuning fork^[60]

Fig. 8. Illustrations of QTF arm deformations under different vibration modes^[71]. (a) Fundamental flexural mode; (b) first overtone flexural mode; (c) combined fundamental and first overtone flexural modes

Fig. 9. Simultaneous dual gas (C₂H₂ and H₂O) measurements using QEPAS sensor based on combined vibration motion of QTF^[71]

Fig. 10. Simultaneous dual gas continuous monitoring of H₂O (fundamental channel) and CH₄ (1st overtone channel)^[73]

Fig. 11. Setup of simultaneous dual gas measurement using double-tube resonators^[74]

Fig. 12. Continuous measurement results of volume fraction of atmospheric CH₄ near Hou Village Landfill in Taiyuan, China on October 11, 2018^[73]

Fig. 13. Seven-day CO volume fraction at Shanxi University, China^[63]. (a) Measured data by CO sensor based on grooved QTF; (b) corresponding data available from nearby station of the Department of Ecology and Environment of Shanxi Province