The unique physical properties of terahertz (THz) waves, such as their low photon energy, characteristic spectra, and penetration, provide THz technology with essential application value in basic science and applied science. In biomedical science, traditional THz imaging techniques have been used to detect neural tissue responses, water content distribution in tissues, and bone tissue defects. However, the traditional THz imaging techniques can not satisfy the requirements of single-cell imaging and molecular-level pathological analysis as their spatial resolution is low. In material research, traditional THz imaging techniques have been employed to study the optoelectronic responses of two-dimensional materials, two-dimensional material devices, and quantum well devices. However, the traditional THz imaging techniques are insufficient in detecting the carrier's distribution and electron transportation since the wavelength range of the THz band is 30-3000 μm. Moreover, due to the diffraction limit, the resolution of the conventional THz imaging is on the millimeter scale (λ_1THz=300 μm) and thus cannot meet the requirement of the rapid development of scientific research towards the nano-scale. Therefore, THz microscopy with high spatial and temporal resolutions needs to be developed as soon as possible to explore scientific issues at the micro- and nano-scale.

Near-field THz imaging techniques are important methods to improve the spatial and temporal resolutions of THz imaging in experiments. The near-field coupling system that captures the information contained in evanescent waves can be used to create super-resolution images. The high-frequency signals in the evanescent waves can be used to reconstruct surface information, including surface structure, carrier concentration, and phase evaluation.

Aperture probes and scattering probes are the common techniques used in near-field THz imaging. The basic principle of near-field THz imaging with aperture probes is to create subwavelength THz radiation sources or subwavelength THz detectors with micropores. Physical apertures, dynamic apertures, and spoof surface plasmon polaritons are mature solutions for the fostering of near-field THz imaging systems with aperture probes (Figs. 2-7). The spatial resolution and the cut-off frequency are both related to the structure of the aperture probe and the diameter of the aperture. As the cut-off frequency and the coupling efficiency reach the limit, the imaging quality and the spatial resolution of the aperture probe cannot be further improved. Scattering probe microscopy requires a scanning tunnel microscope (STM) and an atomic force microscope (AFM) to provide near-field conditions for the tip-sample system (Figs. 8-12). The distance between the tip and the sample is much smaller than the wavelength of the THz signal. When the THz signal is incident on the tip and the sample, the polarization of the tip and the sample generates the near-field scattering signal. Information on the sample surface can be reconstructed as the tip scans the surface of the sample two-dimensionally.

This paper summarizes the basic principle of near-field THz imaging and demonstrates the development history and technical routes of various near-field THz imaging techniques. It analyzes the characteristics of those near-field THz imaging techniques and discusses their temporal and spatial resolutions, spectral resolution, imaging quality, signal-to-noise ratio, and application scenarios. Finally, the paper suggests the future development of super-resolution THz imaging.