# Full-Field Super-Resolution Terahertz Imaging Based on Rotating Coherent Scattering Microscopy
## 4 Conclusions  
Among the full-field THz imaging techniques based on high-power continuous-wave THz sources, direct intensity imaging exhibits advantages due to there being no need for complicated computational reconstruction, while the resolution as a key issue still needs to be improved because of the long wavelength of the THz waves. In this paper, ROCS microscopy was applied to THz imaging for the first time. A specific hemispherical lens was proposed to produce the evanescent wave illumination and to also support the sample. The multi-angle evanescent wave illumination was adopted, and the final result was obtained with the principle of the ROCS method to improve the lateral imaging resolution. a microscopic imaging setup based on a 2.52 THz high-power coherent THz source and an uncooled microbolometer array detector was built, and 2D grating with a period of 150 (1.26\(\lambda\)) was successfully resolved. The observed resolution breaks the theoretical diffraction-limited resolution given by [PERSON]'s formula, which is determined by the wavelength and the numerical aperture of the coherent imaging system. Next, some resolution test targets were employed to calibrate the imaging system and experimentally achieve the diffraction-limited resolution of \(\lambda/2\). The proposed method achieved THz super-resolution imaging under evanescent wave illumination, and provides a new way for THz full-field microscopic imaging.  
Since the pixel size of the THz detector is close to the wavelength, compared with the visible and the infrared band, the current commercial THz image-plane systems are mainly telescopic. In this case, the working distance on the object side is short, usually less than 50 mm. This leads to the difficulty of inserting other optical elements such as hemispherical lens, etc. The hemispherical lens can also be reversely used as a solid immersion lens to further enhance the NA. If possible, the maximum frequency shifting distance generated by the evanescent wave illumination is \(nk_{0}\), and the maximum NA of the system can be improved to \(nk_{0}\). Therefore, the resolution by the evanescent wave illumination can ultimately be \(1/2n\) times that of the diffraction limit.  
D.M.: Methodology, Software, Writing--original draft. J.Z.: Methodology, Data curation, Writing--review & editing. D.W.: Methodology, Data curation, Writing--review & editing, Supervision, Funding acquisition. H.L.: Formal analysis. L.R.: Formal analysis. Y.W.: Formal analysis. S.L.: Formal analysis. All authors have read and agreed to the published version of the manuscript.  
This researched was funded by the National Natural Science Foundation of China (6220106005, 62075001, 62175004); the Science Foundation of Education Commission of Beijing (KZ202010005008); and the Natural Science Foundation of Beijing Municipality (4222061, 4222063).  
Data Availability Statement: Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.  
The authors declare no conflict of interest.