The terahertz(THz) wave occupies a middle ground between microwaves and infrared light waves. It can provide comparable imaging resolution of sub-millimeter order to the conventional camera and penetrate dielectric materials such as fog, paper, plastics, and clothes. Compared with the conventional camera, which identifies the targets using different colors (wavelength), the THz band imaging results can provide much different information, such as the target's distance, velocity, roughness, and dielectric constant, if the phase information can be appropriately processed. In this case, the THz "camera" can be expected as the next-generation sensor that can contribute to many application scenarios in Society 5.0, including security, smart factories, automotive driving, and remote sensing. Furthermore, the increased information extended the dimension for target recognition, which can lead to the next step of AI technologies. However, due to the complicated and expensive THz band devices, it is a relatively new and underexplored sensing technique that has not been practically applied yet, especially when phase information is required.

 During the previous studies, the applicant has demonstrated that the single resonant tunneling diode (RTD) can operate as both the THz band transmitter and detector simultaneously with only low-frequency (< MHz) bias voltages, which leads to a simple THz band imager for 2D imaging purposes. More importantly, it was also found that the RTD signal can retrieve the phase information directly without using an expensive THz band mixer. However, due to its self-mixing feature, the RTD signal did not operate as perfectly as the conventional mixer signal. The mechanism of the RTD needs to be further investigated to retrieve accurate phase information. Besides, the previous RTD imaging system was still impractical due to the mechanical modulation. The system is also expected to be further simplified by introducing a properly designed electronic modulator.

In this study, the applicant aims to fully manipulate the RTD signal to realize real-time pseudo-3D THz imaging and target identification enabled with the phase information. The improved RTD imaging system will be further combined with the advanced signal processing technologies, and both the interferometric and polarimetric imaging techniques will be introduced to the THz band imaging purposes so that the 3D displacement and material of the imaging targets can be identified. The core concept of the proposed system aims to use a simple and low-cost THz transceiver for realizing advanced sensing functions, which can bring the THz sensing techniques from the laboratory to real applications.