# Line Beam Scanning-Based Ultra-Fast THz Imaging Platform
## 2 Platform Design and Construction  
The optical system of the line beam scanning-based ultra-fast THz imaging platform shown in Figure 1A is rather simple, only composed of a source/sensor chip in Figure 1B, an elliptical cylindrical reflector in Figure 1C, and a scanning mirror in Figure 1A. The source/sensor chip not only emits THz spherical wavefront generated by the emitting antenna, but also detects the THz signals reflected from the target with the receiving antenna: The THz signal with the central frequency of 340 GHz, the bandwidth of 20 GHz, and the peak output power of 0.5 mW, is transmitted from the 4-Tx channels and generates a narrow beam line based on the optical system, and the reflected THz signal from the detecting sample is collected by all the 16-Rx channels. In our case, the THz wave is generated from microwave through frequency multiplication, in which the efficiencies are rather low in both emitting and receiving, much lower than the efficiencies of the THz generation based on photoconductive antenna [23; 24; 25], but a rather narrow wavelength band can often be maintained, which can hardly be achieved using photoconductive antenna. Moreover, an elliptical cylindrical reflector was used to reshape the spherical wavefront from the source into the cylindrical wavefront to generate the line beam, as well as to collect the reflected THz signals to the THz sensors. For the elliptical cylindrical reflector, two elliptical foci were conjugated, as the source/sensor chip was located at one focus, the line beam was well generated at another elliptical focus. Moreover, the scanning mirror can realize the line beam scanning with a rather large range and high speed. Considering the practical applications of the THz imaging platform, the distance between the working plane and the scanning mirror was set as 3 m, and the scanning range should reach 2 m \(\times\) 0.8 m at the working plane. In order to optimize the optical system, ray tracing algorithm [32] was implemented: after inputting the parameters of the scanning mirror and the elliptical cylindrical reflector, as well as their positions, the THz intensity distribution can be estimated using multiple rays, and according to the numerically computed THz intensity distribution, the configuration of the elliptical cylindrical reflector was finally obtained: The optimized elliptical cylindrical reflector has the radius of the curvature of 554.1 mm and the Conic factor of \(-\)0.724. According to the optimized optical system, the intensity distribution at the working plane was then computed as shown in Figure 1D. It is shown that the length of the line beam can reach 0.8 m as provided by the central intensity distribution along the x-axis; besides, according to the central intensity distribution along the y-axis, the full width at half maximum (FWHM) reached 12 mm, indicating the resolution of the THz imaging platform. Via numerical simulations, it is proved that the designed optical system can achieve fast safe screening. Next, the mechanical system of the line beam scanning-based ultra-fast THz imaging platform shown in Figure 2A was designed and constructed not only to assemble the optical system, but also to realize line beam scanning by controlling the scanning mirror. In order to realize lightweight design, super-hard aluminum alloy 7075 was used as the materials of both the mechanical system and the mirrors (including the elliptical cylindrical reflector and the scanning mirror); moreover, honeycomb hole array were drilled at the back side of the scanning mirror as shown in Figure 2B to further decrease its weight to 17 kg, which is 34% lighter than without hole drilling.