Fig. 1. Sound pressure distribution of the selected cutoff line on the surface of the non-resonant PAC.

Fig. 2. Schematic diagram of the CH₄-NR-QEPAS sensor.

Fig. 3. Frequency response curve and physical diagram of the self-designed T-head QTF.

Fig. 4. Modulation depth of the NR-QEPAS sensor.

Fig. 5. Comparison of the 2f signal for left, middle, and right holes.

Fig. 6. Comparison among QEPAS, NR-QEPAS, and off-beam QEPAS.

Fig. 7. (a) 2f signal of CH₄-NR-QEPAS sensor at different gas concentrations. (b) Linear fitting of the peak value of the 2f signal. (c) The noise of CH₄-NR-QEPAS sensor obtained under pure N₂.

Fig. 8. Allan deviation of the CH₄-NR-QEPAS sensor.