## 2 Materials and Methods  
CW-THz radiation was generated by an optical heterodyning method using two distributed feedback (DFB) diode lasers (laser 1 and laser 2 with wavelengths \(\lambda_{1}\) and \(\lambda_{2}\), respectively; Figure 1a) at 1550 nm. The beat signals, which happen to be the frequency difference, \(\Delta\lambda=\lambda_{1}-\lambda_{2}\), between the two lasers, are divided into two parts and illuminate both the transmitter and receiver simultaneously. The photomixer that serves as the THz transmitter is based on an InGaAs/InP semiconductor structure with a bandgap of approximately 1.5 \(\upmu\)m. When applying a bias voltage to the metal electrodes deposited on the surface of the THz transmitter, the beat signals generate a photocurrent that oscillates at its beat frequency and is then converted into THz radiation. The generated THz waves are focused at the center of the four-THz-lens system, at which point the samples are placed, as shown in Figure 1a,b. Both the transmitted THz waves and the beat signals illuminate the photomixer acting as the THz receiver. The beat signals generate photo-carriers, and the amplitude of the electric field of incident THz waves induces the resulting photocurrent. Measurements using the THz receiver, therefore, depend on the phase difference between the beat signal and the  
Figure 1: (**a**) Schematic of continuous wave (CW)–terahertz (THz) imaging system, which consists of two distributed feedback (DFB) diode lasers, a fiber splitter, two photomixers that serve as the THz transmitter and receiver, four THz lenses, and a twoaxis motorized sample stage. (**b**) Photograph of the CW–THz imaging system. (**c**) Photograph of the sample mount and the sample with artificial void defects inside. incident THz wave. This means that the THz technique shown in Figure 1 is a coherent detection method that has the advantages of very high efficiency and sensitivity. In our experiments, the void defects inside the XLPE plates cause a change in the optical path length, compared with the area filled with the XLPE material. The change in phase caused by the difference in optical path length between the defects, and the XLPE material surrounding them corresponds to the change in the amplitude of the measured transmission of the THz waves in the coherent detection scheme. For example, if there is a void defect with 0.3 mm depth in XLPE whose refractive index is around 1.5 at THz frequencies, the difference of optical path lengths between the transmission signals passing through the void defect point and the surrounding material without any void defect is 0.15 mm and the phase difference is \(\pi\) at the frequency of 1 THz. In that case, the direction of photocurrent induced by the THz waves incident on the surface of the THz receive becomes completely reversed and thus the magnitude values of transmitted THz electric field are the same despite that the signs are opposite. Therefore, it is possible to extract a scan image of the samples by measuring the signal size of the transmitted THz waves at each point while moving the XLPE block containing the internal defects along two axes. To realize an XLPE block sample with artificial void defects inside, we first cut a XLPE plate with a thickness of approximately 10 mm into two pieces of half the total thickness each. On the inner surface of one of the two pieces, artificial void defects of cylindrical shape specifically designed for our experiments were created using a femtosecond laser micromachining method, which is based on laser ablation by amplified femtosecond laser pulses [22]. Ultrashort laser pulses enable the fabrication of machined structures with high quality, as shown in Figure 2a, because the duration of laser pulses less than the conduction time of materials reduces the heat effect and minimizes debris. When assembling the two split pieces together, the fabricated void defects were placed inside the XLPE block, as shown in Figure 1c.