Il-Min Lee

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.

Terahertz (THz) technology offers unique see-through imaging capability for various non-destructive inspection applications. In this work, we implemented a broadband continuous-wave THz imaging system to study technical issues related to the see-through imaging, including frequency-dependent resolution, material loss, and interference-induced artifacts. The interference-induced false contrast and artifacts were observed, which were suppressed by broadband imaging techniques adopting the homodyne signal amplification by interferometric setup to overcome the material loss.

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.