## 3 Results and Discussion  
Figure 2a shows scanning electron microscopy images of the artificial void defects fabricated by the laser machining method. For each distance of approximately 6 mm, four side-by-side defects with diameters of 578, 498, 395, and 288 um, and a depth of 1.2 mm, were formed inside an XLPE plate with a thickness of approximately 10 mm. Figure 2b shows the two-dimensional amplitude image of the THz waves transmitted through the sample. Because we optimized the transmission signal in the defect-free bulk XLPE plate, the change in the transmitted signal resulting from the void defects appears as dips, as shown by the dark circles in Figure 2b. The image pinpoints the location of the void defects. Figure 2c also shows the cross-sectional profile along the \(x\)-axis at the center of the amplitude dips for the four void defects. The results show that not only is identification of a very small defect with a 288-um diameter possible, but also that extracting information regarding the shape and size of each defect is possible using the CW-THz imaging system. The value of the full width at half maximum (FWHM) and the depth of signal amplitude for four artificial void defects with various diameters, as shown in Figure 3, show steadily rising curves as the diameter increases. As the diameter of the defect increases, so does the depth of transmission amplitude, and the easier it is to distinguish the defects from the surrounding XLPE material. Even in the smallest defect, with a diameter of 288 um, the value of amplitude depth, approximately 0.8 nA, is quite distinguishable against the adjacent area, showing a deviation of up to 0.5 nA. The values of FWHM also exhibit a similar trend with respect to the diameter of the defects. To verify the resolution of the CW-THz imaging system used in the experiment, samples with two adjacent void defects with various diameters and gap widths were produced, as shown in Figure 4a. The samples were designed to have diameters of 600, 500, 400, and 300 um and gap widths of 500, 400, 300, and 200 um, respectively. The actual measured values of the diameter and gap width are shown in Figure 4a,b, which shows the two-dimensional scanning image of the sample and contains all four structures (denoted S1, S2, S3, and S4). The cross-sectional profile of the transmitted amplitude signal along the \(x\)-axis at the center of the dips more clearly reveals the shape of the artificial void defects, as shown in Figure 4c. Figure 3: Full width at half maximum (FWHM) (red circles) and the depth of signal amplitude through the artificial void defects with various diameters (blue squares). The double y axes are used for comparing the two data sets, i.e., the FWHM values and amplitude depths. For the double-void defect with a diameter of approximately 600 \(\upmu\)m, S1, the two defects seem to be clearly distinguishable. In addition, in the case of 500 \(\upmu\)m, S2, at first glance, it looks like only one dip is observable but there seems to be another small dip on the right shoulder of the main dip. On the contrary, in the case of two structures with diameters of approximately 400 and 300 \(\upmu\)m, it is difficult to distinguish the two adjacent void defects. This means that the CW-THz imaging system using a frequency of 1 THz offers a sufficiently distinct resolution up to approximately 500 \(\upmu\)m for two adjacent defects. Three-dimensional plots showing the signal amplitude around the double-void defects and the results of double Gaussian fitting of the cross-sectional profiles along the center of defects for S1 and S2 are shown in Figure 5. For the sake of convenient data analysis, the \(y\)-axis is flipped so that the signals from the defects become peaks. To model the contrast profile of the defect points, we represent the double-void defect signals as the sum of cylindrically symmetric Gaussian peaks, based on the double Gaussian fitting method.