# Full-Field Super-Resolution Terahertz Imaging Based on Rotating Coherent Scattering Microscopy
## 3 Simulation and Experimental Results
Next, a TPX lens with the diameter of 50.8 mm and focal length of 50 mm converged the parallel beam at the position of 50 mm + 35.9 mm (calculated according to Equation (1)) in front of the hemispherical lens, designed and fabricated with HRZF-Si, whose refractive index \(n\) was 3.418 under illumination with a 2.52 THz wave, and the radius \(r\) was 25.4 mm. Then, the spherical beam refracted into the hemispherical lens and became the plane beam. The total internal reflect occurred on the planar surface of the Si-air plane. Here, the hemispherical lens was fixed by a rotator mount and the sample was fixed to the center of the flat surface of the hemispherical lens with alcohol under van der Waals interaction. When rotating the rotator to an angle of 70\({}^{\circ}\) between the plane and the optical axis, the normal angle of the incident beam was about 20\({}^{\circ}\). At this time, the frequency shift was 1.17 \(k_{0}\). A magnified imaging system with an aperture of about 70 mm and a numerical aperture of about 0.6 was placed at a distance of more than 40 mm from the sample to the THz camera with a microbolometer area-array detector. The pixel size of the detector was 35 \(\mu\)m \(\times\) 35 \(\mu\)m, and arranged into a rectangle with 384 \(\times\) 288 pixels. Figure 4: **(a–e)** Five simulated imaging results with the high transverse wave vector illumination at 20\({}^{\circ}\), 40\({}^{\circ}\), 60\({}^{\circ}\), 80\({}^{\circ}\), and 340\({}^{\circ}\) among the total 18 images, (**f**) the superimposed final image with 18 different intensity images, and (**g**) imaging result with the normal-incident illumination. Here, a HRZF-Si 2D grating with the period of 150 \(\upmu\)m (1.26\(\lambda\)), made by deep silicon etching technology, was chosen as the sample. Based on the coherent light imaging, the theoretical resolution is 162.6 \(\upmu\)m, so the imaging system cannot resolve the detailed information of this grating when the THz beam illuminates the sample with normal incident mode. Through the evanescent wave illumination, according to the basic principle of the frequency shifting shown in Figure 2, this can be calculated so that it is possible to move the detail information between 55.1 \(\upmu\)m and 171.2 \(\upmu\)m into the passband of the system. In the actual experiment, we chose to rotate the sample and not the incident wave for the convenience of operation to achieve illumination of the sample by the evanescent beam from different directions. By rotating the sample, we recorded the direct intensity imaging results under the evanescent wave illumination at 20\({}^{\circ}\) intervals. Figure 6a-c shows three of the direct intensity imaging method results for illumination at 60\({}^{\circ}\), 210\({}^{\circ}\), and 300\({}^{\circ}\), which were the angles between the evanescent wave and the sample. The black arrow is the sample orientation and the red arrow is the evanescent wave illumination direction. The interference fringes can be clearly seen, which are related to the cosine item in Equation (6). Then, the images were rotated by taking the black arrow as the baseline, superimposed, and normalized to obtain the final intensity image in Figure 6d, where it can be seen that the sample details containing the concave-convex structure were resolved from the profile curve. The magnification of the imaging system was calculated to be 1.63 by dividing the grating period in the imaging plane by the actual period of the grating. Figure 5: Schematic of the configuration of the THz-ROCS system. Figure 6: (**a–c**) Intensity images at the three rotation angles with a total of 18 angles under the evanescent wave illumination recorded by the microbolometer array detector and (**d**) the final intensity image through the superimposition of their rotated images. It can be seen that the final image quality of Figure 6d was not so good. This is related to the type of sample used, and whether the material has a good transmission property to the THz wave.