# Full-Field Super-Resolution Terahertz Imaging Based on Rotating Coherent Scattering Microscopy
## 3 Simulation and Experimental Results  
Numerical simulations were first performed based on Equation (4). A simple sample containing two point scatterers and a complicated sample with several microspheres were selected. For simplification, the magnification of the imaging system was assumed to be a unit. Figure 3 shows the results for the two point scatterers with the distance of 150 \(\upmu\)m. The wavelength of the incident THz wave was 119 \(\upmu\)m, and the NA of the imaging system was 0.6, which yielded a theoretical resolution of 162.6 \(\upmu\)m (\(d=0.82\lambda\)/NA). Therefore, the scatterer sample could not be distinguished when normally incident by the plane wave, as shown in Figure 3a. When the sample was illuminated by an evanescent wave with a 1.17 \(k_{0}\) transverse wave vector through the TIR, the cross-section of the corresponding image, as shown in Figure 3b, and the two scatterers could be distinguished. It is worth noting that the peaks of the scatterers under high transverse wave vector illumination were 250 \(\upmu\)m apart from each other, which indicates that in this case, the imaging resolution was enhanced, and at the same time, the measured distance between two distinct intensity peaks in the image plane was larger than the actual distance between two point scatterers and was an interference effect depending on the phase delay, which is like a kind of aberration under a single illumination angle. Therefore, the ROCS method needs incoherent superposition with different images under as many illuminating directions as possible to obtain an aberration-free result. Figure 4 shows the simulated imaging results of the multiple microsphere samples. The radius of each microsphere was 25 \(\upmu\)m, which were assumed to be transparent to the THz wave such as the HRFZ-Si material with an absorption coefficient of 0.04 cm\({}^{-1}\)[41], and the other parameters were the same as the previous one. Figure 4a-e depicts five of the direct intensity imaging results under a different illumination direction (represented by the white arrow) every 20\({}^{\circ}\) of rotation. According to the principle of ROCS, the intensity of 18 images was superposed to obtain the final result shown in Figure 4f. Compared with the direct intensity imaging result shown in Figure 4g with the normal incident mode, the imaging resolution was greatly improved. From the profile curve in Figure 4f, the two microspheres could be resolved based on the two distinct peaks, even with the low  
Figure 3: Simulated images of two point scatterers with a spacing of 150 \(\upmu\)m under (**a**) illumination with the normal incident plane wave and (**b**) illumination with an evanescent wave of 1.17 \(k_{0}\) transverse wave-vector through TIR. contrast. It is worth noting that more acquisition at different angles will benefit the final superimposed image. However, it will increase the complexity of the setup and acquisition time. Usually, there can be a compromise between the acquisition time and the quality of the final intensity image. Then, an experimental verification was performed. The experimental setup is shown in Figure 5. The THz source was an infrared-pumped methanol laser (295-FIRL-HP) with an output frequency of 2.52 THz. The THz wave is a monochromatic coherent light with a small divergence angle about 0.63\({}^{\circ}\), which is a longitudinally polarized Gaussian beam. The power can reach up to 400 mW by adjusting the pump power and the THz laser cavity length. In order to guarantee the optical elements are centered, we adjusted the center of all of the optical elements to the same height with the help of the liquid crystal sheets. The beam passed through a pair of off-axis parabolic mirrors with focal lengths of 50.8 mm and 152.4 mm to expand the radius of the beam from 9 mm to 27 mm.