Fig. 1. The research process of the nanotip of THz s-SNOM. The research process involves two aspects: the influence of the nanotip geometry and the incident field on the near-field scattering signal.

Fig. 2. Two approximation methods of near-field nanotips. (a) The near-field nanotip is simplified as a sphere. (b) The near-field nanotip is simplified as a cone. θ is the cone angle of the nanotip, r₀ is the circumscribed circle radius, and ξ = θ.

Fig. 3. The evolution trend of the localized field enhancement at the nanotip apex and the FWHM of the focus diameter are influenced by the nanotip cone angle. (a) Field enhancement at the nanotip apex on two material surfaces. (b) Focusing spot diameter at the nanotip apex on two material surfaces.

Fig. 4. The evolution trend of the near-field induced dipole moment of the overall nanotip is influenced by nanotip apex-sample surface distance.

Fig. 5. The evolution trend of the near-field localized field enhancement at the nanotip apex and the full width at half-maximum (FWHM) of the focus diameter are influenced by the nanotip apex radius. (a) Field enhancement distribution at the nanotip apex on two material surfaces. (b) Dipole moment distribution of the overall nanotip on two material surfaces.

Fig. 6. The evolution trend of the near-field localized field enhancement at the nanotip apex and the FWHM of the focus diameter are influenced by the incident field polarization status. (a) Field enhancement distribution at the nanotip apex on two material surfaces. (b) The evolution trend of the focusing spot diameter at the nanotip apex with the incident field polarization.

Fig. 7. Numerical simulation of THz near-field one-dimensional scanning. A tip with apex radius of r = 0.5 nm and length of 17 µm is placed above a sample consisting of Au on the left side (x < 0 nm) and Al₂O₃ on the right (x > 0 nm) side. (a) Near-field localized field enhancement evolution curve and corresponding derived curve. (b) Electric near-field distribution in x = −0.5 nm. The red curve represents the Gaussian fitting curve of the near-field enhancement. (c) Electric field near-field distribution below the tip apex for different tip positions.