“…Compared with the established imaging instruments and techniques, novel imaging systems in higher frequency bands, such as the millimeter wave (MMW, 30-100 GHz), sub-terahertz (sub-THz, 0.1-0.3 THz), and THz (0.3 THz-10 THz) bands, have attracted significant attention, as these frequencies lead to considerably improved spatial resolution due to their higher frequencies and wider bandwidths. For example, THz band imaging systems have demonstrated submillimeterlevel resolution and transparency for electrical insulators such as paper, glass, and polymers [2], [3], [4], [5]. More importantly, these frequency bands can be used for wireless communication and industrial applications since they do not harm the human body.…”