## 3 Results  
The experiments are carried out firstly on a plate with an artificial notch, as shown in Figure 4, using the top generation configuration. A scan of the probe laser from 'P\({}_{\text{A0}}\)' toward the position of the projection of the notch (the blue stripe in Figure 4) at the upper surface along the \(x\)-axis in a scanning step of 0.5 mm is achieved, and the result is shown in Figure 5a. Because of the surrounding water, the amplitude of the A\({}_{0}\) mode decays very fast. Its amplitude is significantly attenuated to be lower than 1/5 of the incident A\({}_{0}\) mode after a propagating distance of \(\sim\)17 mm. It can also be seen that after propagating a short distance, part of the A\({}_{0}\) mode is converted into a quasi-Scholte wave, as shown in Figure 5a. The color range is adjusted to a small degree, to show the A\({}_{0}\) mode and Scholte wave with  
Figure 4: Aluminum plate sample with an artificial notch at the lower surface; the blue stripe indicates the position of probe laser on the \(y\)-axis. a small amplitude; therefore, the A\({}_{0}\) mode with high amplitude (\(\sim\)1.6 V) at the beginning is saturated in this scale. However, even in a very small display range, neither the S\({}_{0}\) mode, nor the reflection of A\({}_{0}\) mode from the notch, can be observed. Furthermore, due to the strong dispersions of both A\({}_{0}\) mode and the converted Scholte wave, it is difficult to distinguish these waves in the time domain. For these reasons, the A\({}_{0}\) mode is not suitable for long-range detection in underwater plates. Then, the experiment was repeated using the lateral generation configuration with the same scanning step, and the results are shown in Figure 5b. As expected, the generated S\({}_{0}\) mode at 1 MHz\(\cdot\)mm is almost non-dispersive, and can propagate a significantly longer distance than the A\({}_{0}\) mode. When the S\({}_{0}\) mode propagates to the defect, part of the S\({}_{0}\) mode is reflected (however, the amplitude is too weak to display in Figure 5b), and part of the S\({}_{0}\) mode is transmmitted (marked by 'tS\({}_{0}\)' in Figure 5b). Additionally, mode conversion takes place at the defect, and part of the S\({}_{0}\) mode is converted into the A\({}_{0}\) mode in this _fd_ range. The converted A\({}_{0}\) mode from the S\({}_{0}\) mode is reflected and transmitted at the defect, marked by 'S\({}_{0}\)-rA\({}_{0}\)' and 'S\({}_{0}\)-tA\({}_{0}\)', respectively, as shown in Figure 5b. The reflected 'S\({}_{0}\)-rA\({}_{0}\)' wave can reveal the presence of a defect. Furthermore, it is worth pointing out that as the plate thickness changes at the notch, the displacement distribution of the incident wave changes. The S\({}_{0}\) mode couples with the other modes reflected between the edges of notch, leading to a short resonance at the notch, as shown in the dotted frame in Figure 5b. The short resonance with high out-of-plane displacement is also of benefit for defect localization in underwater plate. Afterwards, acoustic wave signals obtained at 5 different locations during the approach of the probe to the vicinity of the defect projection on the upper sample surface (4 locations far away,  
Figure 5: (a) B-scan image using the top generation configuration on the underwater plate; (b) B-scan image of lateral generation configuration in the underwater plate; (c) time domain signal of different positions using the lateral generation configuration. and 1 location in the vicinity of the defect projection on the upper sample surface) are shown in Figure 5c. Generally, when the pulsed laser irradiating the side of plate, the laser spot cannot be perfectly symmetric with respect to the middle plane of the sample side surface, so the A\({}_{0}\) mode is excited along with the S\({}_{0}\) mode. Even so, the surrounding water can efficiently eliminate any unwanted A\({}_{0}\) modes.