## 3 Experimental Setup and Measurements  
The experimental setup used in this work is schematically illustrated in Figure 2a. The pump laser (pulse duration close to 0.75 ns with a repetition rate of 1 kHz) had an optical wavelength of 1064 nm and was focused into a stretched ellipse close to a line generation source: the minor axis of the ellipse is \(\sim\)20 um along the \(x\)-axis and its major axis is \(\sim\)500 um along the \(y\)-axis. The \(z\)-axis, as depicted in Figure 2b,c, is normal to the free surfaces of the plate, its origin being in the middle of the plate thickness. Another continuous laser (with a wavelength of 532 nm) was focused to a circular spot (diameter \(\sim\)20 um) in the vicinity of the line source and was used for ultrasonic wave detection thanks to the beam deflection technique. Note that this technique is often used for laser ultrasound probe [39; 40; 41; 42; 43; 44]. The calibration of the coincidence of the generation line source and the detection circular spot was controlled by a rotational mirror. The aluminum plate was attached on a three-dimensional stage. With this experimental setup, two different scans were conducted: (1) to characterize the sample thickness, the rotational mirror position was fixed, the distance between the position of the pump laser and that of the probe laser was fixed at \(d=20\) um, and the sample was moved along the \(x\)-axis (see Figure 2b); (2) to perform spatiotemporal measurements (B-scan), the sample position was fixed and the mirror rotated in order to move the generation from the position \(x=0\) mm, where generation and detection are superimposed, to the position \(x=L\), with a scanning step of \(d=20\) um (see Figure 2c). Note that, in Configuration (1), as shown in Figure 2b, the distance \(d\) between the pump and the probe was mandatory for the detection of the ZGV resonance since the beam-deflection technique is sensitive to the radial gradient of the normal displacement (which is zero at \(x=0\), i.e., \(\partial u_{z}^{\text{ZGV}}(x=0)/\partial x=0\), where \(u_{z}^{\text{ZGV}}\) stands for the part of the normal displacement that is due to the ZGV resonance). Before the B-scan measurements, the sample was polished to improve the reflection of the probe beam, but the sample thickness was thus changed. If the thickness varies along the measured area, this may result in enhancing the probed attenuation due to thickness inhomogeneity. This unsought effect should be reduced in order to measure the ultrasonic attenuation associated mainly to material properties. To do so, a characterization of the thickness was therefore performed to find a location of a constant thickness of the plate. The experimental characterization of the thickness changes of the plate was carried out by looking at the changes of the ZGV frequency along the \(x\)-axis: all other parameters being unchanged, the relative variation of the thickness is equal to the opposite of the relative variation of the ZGV resonance frequency [12]. It seems important to underline here that the polishing of the sample, which can modify the film thickness homogeneity, is generally not mandatory at all in laser ultrasonics. We did so because of the detection technique we used. However, commercially available interferometers could indeed allow optical detection of the laser-generated elastic waves on rough surfaces. In Figure 3a, a typical signal measured when the generation and detection are at a distance of \(d=20\) um is shown. The low frequency part of the signal corresponds to the first anti-symmetric mode \(A_{0}\), which is dominant. By zooming in on the time signal (inset), we can see a higher frequency oscillation, which corresponds to the ZGV resonance. When this signal is Fourier transformed (see Figure 3b), a resonance with quality factor of \(Q_{Al}=61\) (full width at half maximum) shows up around 40 MHz. The quality factor of the local ZGV resonance in an aluminum plate can be used for estimating the local attenuation using the method proposed by [PERSON] et al.