## 1 Introduction  
Since the photon energy of terahertz (THz) radiation (1 THz \(\sim\) 4 meV) largely coincides with that of the biomolecular low-frequency motions including vibration, rotation and translation of the molecular skeleton and that of the weak intermolecular interactions including hydrogen-bond and van der Waals attraction, _etc._, THz technology has its unique advantages in spectroscopy detection and spectral imaging of biological systems, semiconductors, composite materials, _etc._[1]. Due to the diffraction limit, however, the spatial resolution of conventional THz imaging is limited to about \(\lambda\)/2, where \(\lambda\) is the wavelength of THz wave, with \(\lambda\approx\) 300 \(\upmu\)m at 1 THz, which limited THz imaging of micro/nano-scale objects such as semiconductor devices, cells and biomacromolecules, _etc._  
Recently, scanning near-field optical microscopy has been widely used in high-resolution imaging, spectroscopy detection and nanotechnology [2, 3]. There are modalities of two scanning near-field microscopy based on this goal: transmission-type and scattering-type [4]. The transmission-type microscopy uses a sub-wavelength aperture to restrict the size of the light source or to receive evanescent wave signals of the sample in a distance far less than the excitation wavelength, either of which may realize sub-wavelength resolution. Unfortunately, it failed in effectively recognizing the microscopic characteristics of materials since its low transmission and low frequency filtering properties limit the optical resolution to about \(\lambda\)/10 [5, 6, 7]. While the scattering-type scanning near-field optical microscopy (S-SNOM) which is based on a metal nanotip to confine free-space propagating light into the enhanced electric field around the apex of the nanotip, and to simultaneously scatter the near-field signal of sample to the far-field, has obtained fingerprint characteristics of molecules in a nanometer spatial resolution [8, 9]. The spatial resolution and detection sensitivity of the S-SNOM is mainly determined by the shape, geometric size, dielectric properties of the nanotip and substrate materials. The relevant factors of the nanotip-substrate system on S-SNOM operated in visible and infrared regions were investigated in-depth both experimentally and theoretically [10, 11]. With the development of effective THz sources and THz detectors, the first S-SNOM setup for the THz region was built in 2003 and a spatial resolution of 150 nm was achieved [12]. [PERSON] _et al._[13] applied a tapered photoconductive nanotip to obtain a 10 \(\upmu\)m resolution for imaging the metallic ring resonator at 2.0 THz. [PERSON] _et al._[3] performed the THz near-field spectral imaging of a semiconductor transistor which precisely distinguished the interior nano-scale parts in a single semiconductor transistor such as the drain, source and grid electrodes with an imaging resolution of 40 nm, and found the S-SNOM in THz region was more sensitivity for detecting mobile carriers in a semiconductor than that in the infrared region. About the enhancement mechanism of the THz near-field microscopy, [PERSON] _et al._[14] used the aperture-based THz near-field microscopy to study the interaction between the probe and a sample under the THz radiation and found there were two near field regions in which the sample had different effects on the probe. [PERSON] _et al._[15] found the high dielectric constant sintered barium-ferrite (BaFe\({}_{12}\)O\({}_{19}\)) ceramic probe had approximately same scattering efficiency and field enhancement and faster temporal response comparing to the metal probe. Although great efforts have been made for the THz near-field microscopy, the imaging of low refraction index materials like organic molecules and biomolecules is still a challenge. Moreover, the wavelength of THz waves is much longer than those of visible and infrared waves, which means the nanotip will scatter lower near-field signals in THz region in line with the scattering theory.