Objective Terahertz scattering-type scanning near-field optical microscopy (THz s-SNOM) is a powerful technique for overcoming the diffraction limit of conventional far-field optics and enabling nanoscale characterization of terahertz electromagnetic fields. Its working principle is based on the strong interaction between a micro-/nanoscale metallic tip and the sample surface under terahertz-wave illumination, which gives rise to highly localized electric-field enhancement at the tip apex due to the lightning-rod effect and near-field coupling. The scattered near-field signal, which carries rich subwavelength information about the sample, is subsequently collected in the far field, enabling terahertz imaging with nanometer-scale spatial resolution. Since the spatial distribution of the electric field in the tip–sample nanogap plays a key role in near-field signal generation, imaging contrast, and spatial resolution, a systematic understanding of the mechanisms governing local field enhancement is of fundamental importance. In particular, both the geometric parameters of the metallic probe and the excitation conditions of the incident terahertz wave critically affect the electromagnetic response of the tip–sample system. In this work, the influences of key structural parameters of the tip and the incident terahertz-wave parameters on the localized electric-field distribution are systematically investigated, aiming to provide theoretical guidance for probe design optimization and experimental parameter selection in THz s-SNOM systems.

Results and Discussions With fixed tip structural parameters and incident terahertz-wave conditions, the tip-sample distance d was set to 5 nm, 10 nm, and 20 nm, respectively, to investigate the spatial electric-field distribution properties near the tip at different separations. A smaller tip-sample distance leads to a more pronounced enhancement of the electric field in the near-field coupling region, with the maximum electric-field intensity consistently occurring at the tip apex. In addition, an asymmetric distribution is observed, where the field-enhanced region on the side facing the incident terahertz-wave direction is significantly larger than that on the opposite side. The electric-field intensity curves along the z-axis from the coordinate origin to the tip apex, under the same incident terahertz wave, with tip-sample distances of d=5 nm, 10 nm, and 20 nm, and tip radii of r=5 nm, 10 nm, and 20 nm, respectively. By fixing the tip-sample distance d at 5 nm, 10 nm, and 20 nm, the variation of the maximum electric-field intensity is obtained as the tip radius increases from 5 nm to 20 nm. The results indicate that reducing the tip radius and decreasing the tip–sample separation both significantly enhance the maximum electric-field intensity. This phenomenon can be primarily attributed to the stronger “lightning-rod effect” associated with sharper tip geometries, as well as the intensified near-field interaction within a smaller gap. Both the tip radius and the tip-sample distance have a pronounced influence on the maximum electric-field intensity, thereby affecting the interaction between the terahertz wave and the sample, as well as the performance of near-field imaging. The tip radius r and the tip–sample distance d are both fixed at 10 nm, while the cone angle θ is varied among 20°, 40°, 60°, and 80° to obtain the electric-field intensity distributions along the z-axis. The corresponding field enhancement factors are extracted and fitted using a curve-fitting approach to analyze the influence of θ on the electric-field distribution and field enhancement characteristics. Similarly, the incidence angle of the terahertz wave is varied from 10° to 80° to investigate its influence on the electric-field distribution, and a fitting formula is used to describe the relationship between the incidence angle and the field enhancement factor.

Conclusions  The interaction between the incident terahertz wave and the tip–sample system induces a redistribution of surface charges on the metallic tip, leading to a spatial distribution of the localized electric field. 1) The maximum electric-field intensity is consistently located at the tip apex and gradually decays toward the sample surface. 2) A smaller tip radius results in more pronounced charge accumulation and significantly enhanced localized electric-field intensity due to the intensified lightning-rod effect. 3) When the tip radius r and tip-sample distance d are kept constant, reducing the cone angle of the probe promotes charge concentration at the tip apex and enhances the local electric-field strength. 4) The influence of the incident terahertz wave on the near-field intensity strongly depends on the magnitude of its component perpendicular to the sample surface; a stronger perpendicular component leads to higher electric-field enhancement near the tip apex. These behaviors remain consistent within the investigated frequency range of 0.5–3.0 THz. In summary, the present study provides a comprehensive understanding of the relationship between probe geometry, excitation conditions, and near-field enhancement properties in THz s-SNOM systems. The results offer valuable theoretical guidance for the geometric optimization of probes and the improvement of experimental performance, and might facilitate the design of high-resolution terahertz near-field imaging systems.