Fig. 1. Simulation results of commercial QTF. (a) Surface charge density distribution; (b) stress distribution

Fig. 2. Simulation results of T-head QTF. (a) Surface charge density distribution; (b) stress distribution

Fig. 3. QEPAS sensing system. (a) H₂O-QEPAS detection system; (b) actual illustration of two QTFs

Fig. 4. Frequency response curves of two QTFs. (a) Commercial QTF; (b) T-head QTF

Fig. 5. Modulation depth curves of QEPAS sensing system. (a) Commercial QTF; (b) T-head QTF

Fig. 6. QTF's position optimization results. (a) Commercial QTF; (b) T-head QTF

Fig. 7. 2f signal detected by QEPAS system with different QTFs