Author: Zhang Siyuan
In 2024, Chengdu Miji Technology Co., Ltd. successfully delivered a terahertz near-field optical microscopy system to Henan University of Technology. The system performs reflection-mode terahertz near-field imaging. Its main body consists of a solid-state source (97.8 GHz, 80 mW), a detector, and a dedicated near-field AFM host.
Fig. 1 Schematic of near-field imaging
Fig. 2 THz s-SNOM system
System overview:
The system is a scattering-type terahertz near-field scanning imaging system, comprising an AFM optimized for scattering near-field operation, a sample scanner with XY closed-loop and Z open-loop scanners, and an integrated parabolic mirror mounted on a three-axis motorized stage that focuses the external light source onto the AFM tip. The system effectively extracts near-field signals and suppresses background noise: it supports simultaneous per-pixel demodulation at the 1st to 3rd harmonics of the tip frequency for two independent signals (mechanical/AFM and s-SNOM signals).
Terahertz near-field imaging:
Terahertz near-field microscopy is a high-resolution imaging technique using terahertz waves (frequency range roughly 0.1–10 THz, wavelengths from 30 μm to 3 mm). Conventional terahertz imaging is limited by the optical diffraction limit, with resolution typically comparable to the wavelength. By exploiting the tip-enhancement effect, however, THz s-SNOM breaks through this limit, confining terahertz waves into a nanoscale volume and achieving nanoscale super-resolution imaging while preserving the unique spectral characteristics of the terahertz band. Through surface plasmons, tip field enhancement, and resonant structures, weak terahertz signals can be effectively amplified. This technique holds great promise in materials science, biomedicine, physical chemistry, semiconductor inspection, and other fields.
The terahertz scattering near-field imaging system launched by Miji Technology adopts a high signal-to-noise-ratio terahertz system, enabling near-field imaging up to the 3rd order while simultaneously acquiring AFM topography data of the sample surface. Measurements show that the system can achieve nanoscale resol
ution while maintaining an excellent signal-to-noise ratio of 200 at the first order and 80 at the second order
Fig. 3 Measured resolution and signal-to-noise ratio of the system
Fig. 4 System user interface
While pursuing near-field imaging quality, the system also offers maximum user-friendliness in software: a workflow-based operation interface that is clean and intuitive, allowing researchers or staff who are not yet familiar with near-field imaging to get up to speed in a short time.
The atomic force microscope (AFM) in the near-field system adopts an open architecture designed specifically for near-field optical applications. Unlike most conventional AFMs on the market, an AFM for near-field optics must provide multi-channel demodulation outputs while maintaining spatial openness. The open design ensures that external light sources (such as visible light, infrared, and terahertz waves) can be precisely focused onto the tip position. Mainstream near-field AFM equipment on the market is dominated by German brands (such as the Nearspec series and Bruker IR series); although their performance is excellent, they are expensive and difficult to integrate efficiently with customers' own terahertz equipment. To address this situation, our near-field AFM retains the open architecture while offering a variety of specially shaped off-axis focusing mirrors with different focal lengths, making it compatible with a wider range of terahertz sources and significantly improving the compatibility and practicality of the equipment.
System specifications:
| Terahertz source frequency | 97.8 GHz |
| AFM near-field single scan range | 80 μm × 80 μm × 5 μm (XYZ) |
| AFM scanning precision | 0.2 nm × 0.2 nm × 0.6 nm (XYZ) |
| Optical CCD camera | 5 megapixels |
| Detector frequency band | 73.8 GHz – 110 GHz |
| Near-field imaging resolution | 80 nm |
| Near-field resolution SNR | 200 (1st order); 80 (2nd order) |
| Observable near-field orders | 4 |
| Maximum sample size on sample stage | 40 mm × 50 mm × 15 mm |