Terahertz imaging technology has broad application prospects in several fields, such as biological detection, medical treatment, data communication, and non-destructive security. However, the long wavelength of terahertz wave, limited by diffraction limitations, only allows the resolution of traditional terahertz imaging technology to be of the wavelength order, which cannot accurately image material details such as biological cell tissues and chip internal circuits. Therefore, the development of terahertz super-resolution imaging technology is particularly important. However, the probes prepared based on traditional micro/nano processing technology present drawbacks such as complex processing and high transmission losses. Based on this, this study proposes a design method for a tapered opening near-field probe based on a hollow circular waveguide. This conical probe can surpass the cut-off wavelength limit to achieve sub-wavelength focusing. In addition, the designed probe is processed using 3D printing technology. Subsequently, a 0.1 THz near-field scanning imaging system is built based on a high-precision 3D scanning platform, and the imaging effect of the processed probe is experimentally tested. The experimental results show that the probe can achieve sub-wavelength super-resolution focusing and has high power transmittance, which verifies the feasibility of 3D-printed terahertz near-field probes.

First, the structure and size parameters of the tapered probe with a gradient opening are designed. The probe structure is simulated and optimized using CST Studio2019 software. The power density distribution map is examined (Fig. 2). To accurately study the focusing effect of the tapered probe with gradient opening on terahertz waves, we numerically characterize the focused terahertz spot: plotting a linear power density distribution curve in the x-direction at the exit port of the probe tip, and considering the x-width at 2/2 of the peak value as the focusing width of the terahertz spot (Fig. 3). After optimizing the optimal structural parameters of the probe through simulation, a digital 3D model diagram of the probe is exported. After slicing it in the software, the slicing file is input to a 3D printer for printing. After printing, the surface of the printed product is coated with metal. After the sample production is completed, we build a two-dimensional near-field scanning imaging system to experimentally verify the imaging ability of the prepared probe sample (Fig. 6) and evaluate its imaging resolution.

The simulation results of the probe reveal that the designed probe structure bypasses the influence of the cut-off wavelength. The incident terahertz frequency is 0.1 THz, and terahertz focusing can be achieved when the probe tip radius is 0.2 mm. The x-direction focusing width is 0.779 mm, and the y-direction focusing width is 0.4 mm, according to the physical aperture size. In theory, the conical probe structure can achieve a close resolution of 1/4 wavelength, which is super-resolution imaging. Simultaneously, we simulate the focusing spot size of probes with the same size at 0.1 THz and 0.2 THz (Fig. 6). The linear power density of the focusing spot at 0.2 THz is roughly 6 times that of the focusing spot at 0.1 THz; the diameter of the focusing spot at 0.1 THz is approximately 0.779 mm; and the one at 0.2 THz is approximately 0.706 mm, with a difference of approximately 0.073 mm. The higher the frequency, the smaller the diameter of the focusing spot, and the higher the resolution. To demonstrate that this probe structure can achieve higher precision terahertz focusing, a proportional reduction is made on the basis of the original design model, and the cone tip aperture of the probe is set to 1 μm. This can also form a focused spot at the exit port of the probe. To ensure the printing quality of the near-field probe, we expand the tip radius of the probe to 0.4 mm, and the focusing width of the terahertz spot in the x direction is 1.186 mm, with an power transmittance of 3.16%. In experiments, the imaging resolution of the probe with a tip radius of 0.4 mm is 1.5 mm, and the imaging resolution reaches 1/2 wavelength. The experimental results are consistent with the simulation results.

This study proposes a design method for a tapered opening near-field probe based on a hollow circular waveguide. Through simulation analysis, this probe inherits the advantages of a high signal-to-noise ratio, a wide band, and no cut-off frequency of gap probes and can achieve sub-wavelength super-resolution focusing. Theoretically, it can achieve micron-level high-resolution imaging. To verify the feasibility of this method, we prepare a terahertz near-field probe structure with a tip radius of 0.4 mm based on 3D printing technology and built a terahertz near-field scanning imaging system for experiments. The experimental results demonstrate that the probe can achieve sub-wavelength super-resolution focusing, achieving an imaging resolution of 1.5 mm at 0.1 THz, reaching 1/2 wavelength, and achieving terahertz super-resolution imaging. The experimental results are consistent with the simulation results, proving the feasibility of a tapered near-field probe based on a hollow circular waveguide.