Fig. 1. Photographic image and resonant characteristics of QTF. (a) Photograph of QTF; (b) vibration characteristic curve of QTF; (c) simulation analysis on fundamental vibration mode of QTF

Fig. 2. Experimental setup and experimental results of the LITES system proposed by He Ying et al.^[25] (a) Schematic diagram of the experimental setup; (b) 2f signal and noise of TDLAS and LITES system. Reprinted with permission from Ref. [25] © The Optical Society.

Fig. 3. Experimental setup and results of the LITES system proposed by Zhang Qinduan et al.^[29] (a) Schematic diagram of the LITES system; (b) signals before and after applying QTF-self-difference technique

Fig. 4. Experimental setup and experimental results of the LITES system proposed by Wei Tingting et al..^[31] (a) Schematic diagram of the experimental setup; (b) 2f signals measured by QTF at 700 Torr and 8 Torr. Reprinted with permission from Ref. [31] © The Optical Society.

Fig. 5. Experimental setup and experimental results of multi-QTF LITES system proposed by Ma Yufei et al..^[32] (a) Schematic diagram of LITES based on two corroded QTFs; (b) the measured 2f signal amplitude. Reprinted with permission from Ref. [32] © Elsevier.

Fig. 6. Schematic diagram of experimental setup and experimental results of QEPAS-LITES system proposed by Ma Yufei et al..^[34] (a) Schematic diagram of the experimental setup; (b) 2f signals measured at 25 °C and atmospheric pressure; (c) 2f signals measured at T₂=67.5 °C; (d) 2f signals measured at P₂=500 Torr. Reprinted with permission from Ref. [34] © Elsevier.

Fig. 7. Schematic diagram of the experimental device and experimental results of the S-QEDS system proposed by Qiao Shunda et al..^[35] (a) Schematic diagram of the experimental setup; (b) 2f signal of the S-QEDS, LITES and QEPAS system. Reprinted with permission from Ref. [35] © The Optical Society.

Fig. 8. Schematic diagram of LITES system and experimental results based on customized QTF. (a) Schematic diagram of custom QTF^[36]; (b) schematic diagram of the LITES sensor^[37]; (c) 2f signal of LITES sensor systems^[37]. Reprinted with permission from Ref. [37] © The Optical Society.

Fig. 9. Schematic diagram and SEM of QTF with PDMS and rGO coating and corresponding experimental results. (a) Schematic diagram of QTF with PDMS and rGO coating^[43]; Reprinted with permission from Ref. [43] © The Optical Society. (b) SEM image of QTF with PDMS and rGO coating^[41]; (c) the 2f signal of 300×10⁻⁶ NH₃ detected by different QTFs^[41]; (d) the amplitude and SNR of 2f signals in figure (c)^[41]

Fig. 10. Photo of on-site measurement of CH₄ leakage and curve of CH₄ concentration varying with time^[45]. (a-c) Photographs of the long-distance gas leakage measurement on Jilin University campus; (d) measured CH₄ concentration curve as a function of measurement time