# Full-Field Super-Resolution Terahertz Imaging Based on Rotating Coherent Scattering Microscopy
###### Abstract
The sample and the 2D photosensitive element are in the object-image conjugate plane of the lens or lens group to satisfy the imaging condition [30]. In the aspect of far-field and full-field direct intensity imaging, assuming the wavelength of the THz wave fixed, when the 2D photosensitive elements are used to record the intensity image, the size of each pixel is often smaller than the wavelength, and therefore the imaging resolution depends on the numerical aperture (NA) of the microscopic objective according to the well-known [PERSON]'s theoretical resolution formula, which is also related to the recording distance and the effective aperture of the microscopic objective. Microscopic imaging systems with higher NA have a higher resolution. There are usually two methods to improve the spatial resolution: the first is to expand the NA to include more high frequency information in the passband and the second assumes that the NA is fixed and illuminates the sample by the tilted plane wave or the evanescent wave along the surface so that the part of the spatial spectrum of the sample with higher spatial frequencies is shifted into the passband of the imaging system. In the aspect of improving the NA, since it is equal to be \(n\)sin\(\alpha\), where \(\alpha\) is the aperture angle of the objective accepting light and \(n\) is the object-space refractive index, it can be effectively enhanced by immersion methods. The related advanced work on visible light could inspire research and be helpful for design in the THz band. In 2009, [PERSON] et al. reported near-field high resolution by self-assembled nanoscale spherical lenses [31]. In 2011, [PERSON] et al. used microspheres for the first time to improve the resolution of a far-field white light microscopy imaging system [32]. Furthermore, these studies used solid-state microspheres rather than high-refractive index matching liquid media to enhance the resolution of the imaging system. However, the properties of the materials, objects, media, etc. need to be considered, so these methods cannot be directly applied to the THz band. Although it is not practical to find high RI matching liquids in the THz band, recently, we introduced the solid immersion method into the full-field THz direct intensity imaging method and analyzed the imaging properties of a hemispherical solid immersion lens (h-SIL) by ray tracing simulation. The secondary reflection produced an additional inverted image with a small field curvature. By blocking the direct refracted image, high-fidelity imaging was achieved [33]. In another aspect of the super-resolution imaging realized by the tilted illumination shifting frequency, significant progress has been made in visible light. In 2013, [PERSON] et al. achieved total internal reflection (TIR) through a triangular prism to generate evanescent waves. Super-resolution imaging was achieved by illuminating a three-directional striped sample deposited directly on the TIR interface with the evanescent waves [34]. On the other hand, fluorescence microscopy imaging methods have gained a lot of research and applications. To suppress the background noise, the fluorescence excitation region is confined to the propagation direction of the evanescent waves by TIR, thus enhancing the contrast of imaging [35]. This has also led to the study of the scattered direct intensity imaging method of non-fluorescent objects illuminated by evanescent waves using TIR fluorescence microscopy (TIRFM) [36; 37]. In 2013, [PERSON] et al. used an inverse TIR fluorescence system to achieve super-resolution images of label-free microspheres [38]. Compared with the prismatic illumination scheme, the objective illumination scheme is able to obtain different angular illumination directions without moving the sample, just simply by loading different grating on the spatial light modulator (SLM).