## 2 Experimental Section  
### Beam Shaping for Strip Laser Spot  
In order to obtain a uniform strip laser spot, a Powell lens with a prism fan angle of \(10^{\circ}\) was coupled to the QEPAS sensing system as a beam shaping element. Differing from plano-convex cylindrical lens, Powell lens has a complex two-dimensional aspheric top surface. A slew of spherical aberrations is generated when the laser beam passes through the prism, then the optical path is redistributedwhich reduces the beam distribution in the center and increases it in the edges, thus forming a uniform strip laser spot. Therefore, the divergent light in the center of the Gaussian beam can be effectively eliminated after beam shaping by the Powell lens.  
The beam propagation path in a Powell lens is schematically simulated by TRACEPRO software as shown in Figure 1a-c. When a collimated laser beam passes through the arc-shaped edge at the top of the Powell lens, the incident beam is refracted. The refraction angle is mainly determined by the angle formed by the two prism surfaces. Finally, the second refraction occurs when it passes through the exit surface. The entire outgoing beam forms a fan-shaped light curtain.  
In terms of lens structure, such a Powell lens has a cylindrical surface defined by a base curve in the shape of an angle with a rounded apex. The curvature radius of the cylindrical surface thus decreases in the center and increases smoothly towards both ends. As described in the previous papers [31], the cylindrical surface can be fitted to a base curve by the following Equation:  
\[\mathrm{z}=\frac{\mathrm{cr}^{2}}{1+\sqrt{1+(1+\mathrm{k})\mathrm{c}^{2} \mathrm{r}^{2}}} \tag{1}\]  
where \(\mathrm{c}\) is the curvature constant, \(\mathrm{r}\) is the radial coordinate in the length direction of the lens, and \(\mathrm{k}\) is the conic coefficient (the conic coefficient value is less than \(-1\) for hyperbola, \(-1\) for parabola, \(-1\)-\(0\) for ellipses, and \(0\) for spheres). When the curvature radius and the conic coefficient value of the surface change, the curve shape of the partial top surface is different.  
In addition, the prism fan angle is also a key parameter of the Powell lens of which the width is a function of the refractive index of material and the angle formed by the two prism surfaces. The fan angle represents the divergence of outgoing beam after passing through Powell lenses with different curvature radius and cone coefficients. As the divergence angle increases, a longer linear effective length can be obtained within a certain working distance. In our experiments, a Powell lens with a divergence angle of \(10^{\circ}\) was used in order to obtain a stripe spot with appropriate size.  
For the purpose of optimal shaping result, a Powell lens with a transmittance of 94% was placed 2 cm behind the fiber collimator. In addition, a strip slit was used to filter the lateral divergent light and the distance \(L\) between the Powell lens and the slit was 5.4 cm. Since the spot diameter of laser beam entering the Powell lens is less than 250 \(\mathrm{\SIUnitSymbolMicro m}\), the outgoing laser beam was not a perfect strip laser, but an elliptical beam. As shown in Figure 2a, the final elliptical spot was observed by a Pyrocam IIIHR Beam Profiling Camera. The major axis length and minor axis length of the elliptical spot was 2.5 mm and 280 \(\mathrm{\SIUnitSymbolMicro m}\), hence it could match the prong spacing (300 \(\mathrm{\SIUnitSymbolMicro m}\)) of a standard QTF. In comparison, the  
Figure 1: (**a–c**) Diagram of the beam propagation path for passing through the Powell lens, \(L\) is the distance from Powell lens to slit.  
original circular spot observed at the same position which was 8.7 cm from the fiber collimator without beam shaping was shown in Figure 2b.The diameter of the circular spot is 247 \(\upmu\)m which means that the spot area is only 1/10 of the elliptical spot area, resulting in a significant reduction of the excited gas molecules.