Da-Hye Choi

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.

The complex dynamics of terahertz (THz) wave scattering by subwavelength-scale structures remain largely unexplored. This article examines the spectral scattering characteristics of subwavelength-sized spherical particles probed by tightly focused THz waves through numerical simulations and experimental techniques. The simulations reveal that the scattering intensity for lower Mie resonance modes (magnetic dipole and electric dipole modes) remains largely unaffected when THz waves are focused down to 0.3 λ, whereas higher Mie resonance modes experience a significant decrease in intensity as the beam size is reduced. Experimentally, scattering effects were observed by imaging two subwavelength-sized dielectric spheres, each with a diameter of 480 μm and refractive indices of 2 and 6, around 0.3 THz, where the magnetic dipole mode of the particle with refractive index of 2 was detected. Experimental results confirm that the image contrast of the spheres is predominantly influenced by scattering effects near the Mie resonance frequency, even with low refractive index materials. This work represents a significant advancement in the detection and characterization of subwavelength-sized structures within the THz region. Furthermore, the findings extend beyond THz frequency ranges and have potential applications in fields such as sensing, imaging, and sizing across optical, infrared, and millimeter wave ranges.

This study focuses on the spatial and spectral beam characteristics in a terahertz (THz) broadband sub-wavelength imaging system using a solid immersion lens (SIL). Previously, we demonstrated a broadband sub-wavelength THz imaging system by integrating a SIL with a THz time-domain spectrometer (TDS). Key parameters that influence beam characteristics and, consequently, imaging performance, such as SIL misalignment tolerances and beam propagation from the SIL, constitute the primary focus of this investigation. Numerical simulations demonstrate that the system can tolerate millimeter-level transverse and longitudinal SIL position displacements, underscoring its robustness for sub-wavelength imaging in a wide frequency range. Additionally, numerical simulations of beam propagation characteristics reveal that the system achieves sub-wavelength imaging resolution up to 1 mm from the SIL at 0.5 THz, highlighting its potential for non-destructive testing of subsurface structures. These findings gain experimental validation through imaging stacked utility knife blades with sub-wavelength structures ranging from 0.2 to 2 THz.

Terahertz (THz) imaging techniques are attractive for a wide range of applications, such as non-destructive testing, biological sensing, and security imaging. We investigate practical issues in THz imaging systems based on a solid immersion lens (SIL). The system stability in terms of longitudinal misalignment of the SIL is experimentally verified by showing that the diffraction-limited sub-wavelength beam size (0.7 λ) is maintained as long as the SIL is axially located within the depth-of-focus (~13 λ) of the objective lens. The origin of the fringe patterns, which are undesirable but inevitable in THz imaging systems that use continuous waves, is analytically studied, and a method for minimizing the interference patterns is proposed. By combining two THz images obtained at different axial positions of the object and separated by λ/4, the interference patterns are significantly reduced, and the information hidden under the interference patterns is unveiled. The broad applicability of the proposed method is demonstrated by imaging objects with different surface profiles. Our work proves that the resolution of conventional THz imaging systems can easily be enhanced by simply inserting a SIL in front of the object with high tolerance in the longitudinal misalignment and provides a method enabling THz imaging for objects with different surface profiles.

Nondestructive THz imaging technology is a promising technology in several industrial applications such as process monitoring and quality control. To further popularize the application of this technique in the industry, we developed photonics-based devices such as a dual-mode laser, uni-travelling carrier photodiode, and Schottky-barrier diode that enable the realization of a compact, low-cost, multifunctional continuous wave (CW) THz system. To develop CW THz reflective imaging systems with these devices, a telecentric f-θ lens and novel two-dimensional (2D) beam scanner were developed. A high-speed, low-cost 2D reflector with a 4-sided polygon mirror, a brushless DC (BLDC) motor rotating at approximately 2,990 revolutions per minute (RPM) and a stepping motor was designed to ensure that a THz image sized 500 × 400 pixels could be scanned in approximately 2 s. However, it is difficult to maintain a constant speed in high-speed rotating BLDC motors. The measured maximum deviation is approximately 0.7%, which results in distorted images. To compensate for this distortion, a sync signal reflector was designed to monitor the real-time RPM of the BLDC motor. Using the novel 2D beam scanner and sync signal reflector, the distorted images could be corrected simply, enabling the development of a 2D THz reflective imaging system using a single emitter and detector; this system could exhibit a higher speed and was more cost effective than the existing system. Although further improvements to photonics-based THz technologies are necessary, we believe that these efforts will begin an era of THz technologies as a widely used industrial technique.