Terahertz imaging of 100  µm scale subsurface defects in both depth and lateral directions in silicone rubber

High-voltage power cables are essential for long-distance energy transmission, where insulation reliability is especially critical at joints and terminations. Polymeric materials, such as silicone rubber (SiR), are widely used, but they remain vulnerable to internal defects, such as air voids and moisture, which can degrade performance and cause failure. Conventional non-destructive evaluation methods, including ultrasonic testing, X-ray imaging, and partial discharge measurements, have limitations in resolution, safety, or field applicability. Terahertz (THz) imaging has emerged as a promising alternative, offering non-contact, high-resolution inspection with sensitivity to moisture. Previous THz studies on polymeric insulators have demonstrated detection of 100 µm scale defects in either the depth or lateral direction, while the defect size along the other dimension typically remained in the millimeter range. To the best of our knowledge, the simultaneous detection of subsurface defects on the order of 100 µm in both directions within polymeric insulators has not yet been achieved. This study demonstrates the feasibility of visualizing internal defects within bulk SiR insulation using a solid immersion lens-based THz time-domain spectroscopy system. Void defects as small as 100 µm in both depth and lateral directions were clearly resolved near the surface. In addition, void defects with a thickness of 100 µm and lateral diameters of several hundred micrometers were detectable at depths up to 1 mm. Furthermore, a layered defect comprising a void and a metallic inclusion was imaged, revealing the inclusion clearly resolved beyond the void, a result unattainable using ultrasonic inspection due to high reflectivity at air interfaces.