## 2 Experiments and Results  
The schematic diagram of the QETAS experimental setup is depicted in Figure 1. A 1.37 \(\upmu\)m near-infrared fiber-coupled distributed feedback (DFB) diode laser was employed as the excitation source to detect water vapor (H\({}_{2}\)O) in ambient air. The temperature and current of the diode laser can be controlled by a high-stability laser diode controller (ILX Lightwave). A two-channel arbitrary waveform function generator (Tektronix AFG3102) was used to produce a ramp signal with the frequency of 10 mHz and a sine signal with the frequency of \(f_{0}\)/2, where \(f_{0}\) corresponds to the resonance frequency of the QTF. The 2\(f\) wavelength modulation technique has the advantage of tunability and modulation capabilities of DFB lasers, improving the sensor performance [32]. The QTF was fixed on an XYZ linear stage and positioned in a cap, not shown in Figure 1. The acoustic and thermal waves induced by the laser radiation drove the QTF to vibrate. The electric signals generated by the piezoelectric effect in quartz was processed by a custom transimpedance preamplifier with a feedback resistance of 10 M\(\Omega\) and then were fed to a lock-in amplifier (Standford SR830) to demodulate the signal in 2\(f\) mode. A Labview(tm) program was used to control the experimental system and retrieve the gas concentrations from the obtained 2\(f\) signal [24]. The experiment was conducted at a local atmospheric pressure of \(\sim\)933 hPa (700 Torr) and room temperature of \(\sim\)25 \({}^{\circ}\)C.  
As proof of concept, a pigtailed laser diode with the beam M\({}^{2}\) factor of 1.258 was used as the excitation source. Compared to previous publications on QEPAS and QEPTS, a non-focusing light source was firstly used in the experiment. The beam quality of the laser was analyzed by a dual scanning slit beam profiler (Thorlabs BP209-IR/M) with a beam diameter error \(<\)10%. The beam diameter as a function of laser head position is plotted in Figure 2. The distance between the laser head and the slit of the beam profiler changed from 5 mm to up to \(\sim\)1 m. The laser temperature and injection current were set to 23 \({}^{\circ}\)C and 40 mA, respectively, corresponding to the emission power of 6.7 mW. When the position of the beam profiler was 955 mm, the beam diameter reached \(\sim\)10 mm, corresponding to a beam spot about 100 times that used in the conventional QEPAS. The edge of the beam profile was defined as the position where the intensity fell to 1/e\({}^{2}\) of the maximum value at the center. Figure 1: Experimental setup of quartz-enhanced photothermal-acoustic spectroscopy. \(\Sigma\): Adder, PC: Personal computer, Preamp: Pre-amplifier; QTF: Quartz tuning fork. RS232: A standard protocol used for serial communication. Figure 2: Laser beam quality analysis at a distance of up to 1 m. The beam diameter error of 10% was determined by the beam profiler. In order to investigate the photothermal-acoustic response of the QTF surface to the laser beam, the normalized QEPTAS signal amplitude, as a function of \(x_{0}\), was researched. Figure 3a shows the diagram of QEPTAS. The QTF was fixed at the position of 150 \(\upmu\)m from the laser head; \(y_{0}\) of the laser spot was set to 0.7 mm from the QTF opening; \(x_{0}\) of the laser spot was changed from 0 to 800 \(\upmu\)m by use of an XYZ translation stage with the precision of 10 \(\upmu\)m, as shown in Figure 3b. The prong length of the QTF, defined as the distance from the QTF opening to the x-axis, was measured to be \(\sim\)3.6 mm. The prong spacing of the QTF was measured to be \(\sim\)0.3 mm. The QTF prong width, defined as the distance from Line A to Line C, was measured to be \(\sim\)0.6 mm. QEPTAS obtained the maximum signal amplitude when the laser spot was positioned at the center of the QTF prong spacing. When the position of the laser spot moved horizontally to the edge of the QTF prong (Line A), the signal amplitude decreased by \(\sim\)23%.