Fig. 1. Three in-plane vibration modes of the four-prong QTF.

Fig. 2. The simulation of stress distribution in QEPAS: (a) standard QTF, (b) four-prong QTF.

Fig. 3. The simulation of surface charge density distribution in QEPAS: (a) standard QTF, (b) four-prong QTF.

Fig. 4. The simulation of temperature distribution in LITES: (a) standard QTF, (b) four-prong QTF.

Fig. 5. The simulation of surface charge density distribution in LITES: (a) standard QTF, (b) four-prong QTF.

Fig. 6. Schematic diagram of the C₂H₂ sensor. (a) The sketch of the QEPAS system based on the four-prong QTF. (b) The sketch of the LITES system based on the four-prong QTF. (c) The picture of the four-prong QTF and the standard QTF. AmR: acoustic micro-resonator, TA: transimpedance amplifier, QTF: quartz tuning fork.

Fig. 7. The frequency response characteristic of the standard QTF (orange), the bare four-prong QTF (green), and the four-prong QTF equipped with AmR (blue).

Fig. 8. The modulation current curves of (a) the standard QTF and (b) the four-prong QTF.

Fig. 9. The 2f-QEPAS signal peak as a function of the inner diameter and tube length L of the AmR.

Fig. 10. (a) The 2f signal of the C₂H₂-QEPAS system based on the standard QTF, the four-prong QTF, and the four-prong QTF with AmR. (b) The noise level of the C₂H₂-QEPAS system based on the standard QTF, (c) the four-prong QTF, (d) and the four-prong QTF with AmR.

Fig. 11. The concentration response of the C₂H₂-QEPAS system based on the four-prong QTF with AmR. (a) 2f signals at different concentrations. (b) The linear relationship between 2f signal peak and C₂H₂ concentration.

Fig. 12. The Allan deviation analysis of the C₂H₂-QEPAS system based on the four-prong QTF with AmR.

Fig. 13. (a) 2f signals of the C₂H₂-LITES system based on the standard QTF and the four-prong QTF, the noise level of C₂H₂-LITES system based on (b) the standard QTF, and (c) the four-prong QTF.

Fig. 14. The concentration response of the C₂H₂-LITES system based on the four-prong QTF. (a) 2f signals at different concentrations. (b) The linear relationship between 2f signal peak and C₂H₂ concentration.

Fig. 15. The Allan deviation analysis of the C₂H₂-LITES system based on the four-prong QTF.