Chongfan Technology
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24
2025
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02
Terahertz 3D Imaging System Based on Metasurfaces
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The composite material fuselage of aircraft and other aviation vehicles may develop micro-cracks or delamination due to fatigue damage during long-term service. These structural damages may lead to significant safety incidents during high-speed flight of the aircraft. Compared to ultrasonic and microwave detection systems, terahertz waves have higher resolution and the radiation risk is much lower than that of X-rays. Therefore, terahertz technology shows great application potential for non-contact detection in fields such as aerospace and precision manufacturing, and has been listed by the U.S. government as one of the "Top Ten Technologies Changing the Future World." However, the current terahertz three-dimensional imaging technology still faces many technical bottlenecks, including low signal-to-noise ratio, varying lateral resolution with depth, poor longitudinal resolution, limited depth of field, and slow imaging speed, which restricts its large-scale application in industrial inspection.
The aforementioned technical bottlenecks mainly stem from the limitations of the core devices in existing terahertz imaging systems, such as high loss of focusing lenses, variation of spot size with wavelength, small Rayleigh range of focused beams, and narrow operating frequency band. Currently, terahertz imaging systems mainly rely on three-dimensional scanning or dynamic adjustment of focal length based on phase change or temperature change materials to achieve three-dimensional imaging of the internal structure of the object to be tested. However, the three-dimensional scanning process is time-consuming and difficult to meet practical application needs, while the focal length adjustment speed of phase change or temperature change materials is slow, and the switchable focal length modes are very limited. To address these challenges, metasurface arrays provide new ideas for solving many of the above problems. Metasurfaces are a type of artificial electromagnetic material composed of sub-wavelength scale metal or dielectric arrays, which can precisely control the amplitude, phase, and polarization of electromagnetic waves.
Recently, the University of Adelaide in AustraliaDr. You Xiaolongand his collaborators proposed a terahertz three-dimensional imaging system based on dispersive metasurfaces. This research achieved a passive metasurface array that can focus incident terahertz waves of different frequencies at different depths, thereby collecting reflection signals at different frequencies through a single two-dimensional scan of the target object, obtaining information at different depths of the object, and achieving efficient three-dimensional imaging of the object's interior.
Based on in-depth theoretical research, the team successfully broke through the characteristic of terahertz spots changing with the operating frequency in the three-dimensional imaging system, keeping the spot size of different frequencies unchanged, thus overcoming the limitation of lateral resolution being affected by depth in traditional imaging systems. In addition, the 3 dB focusing efficiency of this dispersive metasurface has a relative bandwidth of up to 100%, laying a solid foundation for achieving large depth of field and high signal-to-noise ratio imaging. Verified by a series of experiments, the three-dimensional imaging system constructed with this dispersive metasurface as the core device is currently the only technical solution that simultaneously meets the following four key technical index requirements, effectively overcoming the technical challenges that have long plagued the field of terahertz three-dimensional imaging:
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High signal-to-noise ratio imaging
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High lateral resolution (0.5 lp/mm) consistent at different depths
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High longitudinal resolution (0.5 mm or 0.5λ)0) at 275 GHz
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Large depth of field (68.1 mm or 62.4λ)0) at 275 GHz
This achievement was published inLaser & Photonics Reviews, titled3D Terahertz Coherent Imaging and Chromatic Metasurface. The completion units of this work are the Terahertz Engineering Laboratory of the University of Adelaide and the Royal Melbourne Institute of Technology (RMIT). The University of AdelaideDr. You Xiaolongis the first author and corresponding author of the paper. The University of AdelaideProfessor Withawat Withayachumnankuland Royal Melbourne Institute of TechnologyDr. Rajour Tanyi AkoandProfessor Sharath Srirammade significant contributions to the research work.
Dispersive metasurfaces (Chromatic Metasurface) are a type of metasurface that can accurately control the focal position of electromagnetic waves of different wavelengths (or frequencies). In traditional optical systems, dispersion effects usually cause light of different wavelengths to focus at different positions, resulting in chromatic aberration issues, while dispersive metasurfaces can utilize this characteristic to achieve wavelength-dependent beam control.
Metasurfaces are a type of two-dimensional material composed of artificial structural units at sub-wavelength scales, designed to accurately control the amplitude, phase, and polarization of electromagnetic waves. Dispersive metasurfaces further utilize the frequency dependence of these structures, allowing electromagnetic waves of different frequencies to propagate in space in a predetermined manner. For example, dispersive metasurfaces can focus electromagnetic waves of different frequencies at different depths, thereby obtaining three-dimensional information from a single two-dimensional scan of the object to be tested, greatly improving imaging efficiency.
The researchers in this paper adopted a passive terahertz metasurface based on the dispersion effect. By optimizing the design of the metasurface structure, they focused terahertz waves of different frequencies at different depths and cleverly designed to keep the spot size unchanged, successfully overcoming the limitation of lateral resolution being affected by depth in traditional imaging systems. In addition, this dispersive metasurface has a 3 dB focusing efficiency relative bandwidth of up to 100%, while achieving high signal-to-noise ratio imaging, large depth of field, high longitudinal resolution, and other characteristics. This groundbreaking research provides new solutions for terahertz three-dimensional imaging, spectral imaging, high-speed wireless communication, and other fields, and is expected to promote the development of high-performance electromagnetic wave control technology.
Design Ideas and Working Principles
Through in-depth theoretical analysis, it was found that when the phase distribution of the metasurface array remains unchanged across the entire operating frequency band (as shown inFigure 1a), incident terahertz waves of different frequencies will focus at different depths, forming as shown inFigure 1bThe dispersion effect shown. Further research indicates that through clever design, the size of light spots at different frequencies can remain constant across the entire working frequency band. These characteristics provide new ideas for the field of three-dimensional imaging: placing the object to be measured within the focusing range of the dispersion metasurface and performing a single two-dimensional scan along the xy plane to obtain the frequency domain reflection signal of the object, converting it to the time domain through inverse Fourier transform, and observing the reflection images of the object at different moments, thus analyzing the internal structure of the object at different depths. In addition, by expanding the bandwidth of the dispersion metasurface, the depth of field and longitudinal resolution of three-dimensional imaging can be further increased, while reducing the transmission loss of the metasurface can effectively improve the imaging signal-to-noise ratio. At the same time, since the size of the focused light spot does not change with frequency, the lateral resolution of three-dimensional imaging remains consistent at different depths, simplifying subsequent image processing.
The above analysis indicates that overcoming the technical difficulties of low signal-to-noise ratio, insufficient depth of field, poor longitudinal resolution, and varying lateral resolution with depth in existing three-dimensional imaging systems lies in designing a low-loss, wide-bandwidth dispersion metasurface with a phase distribution that does not change with frequency. To achieve these design goals, the team found thatFigure 2aThe traditional metal aperture resonator ring structure shown can achieve high-efficiency transmission (with transmission loss below 3 dB) in the wide frequency band from 175 to 525 GHz by adjusting the size of the resonator ring, as shown inFigure 2b). In addition, as shown inFigure 2c, the transmission phase curves of different array elements remain parallel within the wide frequency band, which meets the requirement for the metasurface array to maintain a constant relative phase distribution at different frequencies. Based on this aperture resonator ring array element, the team completed the metasurface array design shown inFigure 2d, which has a focal length of 50 mm at the design frequency of 275 GHz. It is worth noting that in existing metasurface designs based on aperture resonator rings, there has been no research utilizing the characteristic of parallel transmission phase curves to construct metasurface arrays for three-dimensional imaging.
In three-dimensional imaging, to ensure that the lateral resolution of imaging at different depths remains consistent, it is required that the size of the light spot of the dispersion metasurface remains stable across the entire working frequency band. As shown inFigure 5a, the calculation results indicate that within the frequency band of 175 – 525 GHz, the FWHM of the focused light spot of this metasurface is always 1.5 mm. Experimental measurement results (as shown inFigure 5b) verify the stability of the light spot size in the frequency band of 220 – 330 GHz and are highly consistent with numerical calculations (as shown inFigure 5c). In addition, as shown inFigure 4b, the size of the light spot of this dispersion metasurface is slightly larger than the diffraction limit, indicating that its application in three-dimensional imaging can achieve high lateral resolution. Further experimental measurements indicate that the lateral resolution of the three-dimensional imaging system constructed by this metasurface reaches 1.50 lp/mm.
Since the proposed metasurface is intended for constructing three-dimensional imaging systems, maintaining consistent spatial resolution at different depths and sufficient signal-to-noise ratio is crucial for imaging quality. Therefore, the working bandwidth of this metasurface is defined as the frequency band that simultaneously meets the following two conditions:
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The size of the focused light spot remains constant, ensuring that the lateral resolution of the three-dimensional imaging system remains consistent at different depths;
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The focusing efficiency exceeds 50% to ensure that the three-dimensional imaging system has a high signal-to-noise ratio.
Based on this, the working bandwidth of the proposed dispersion metasurface covers 175–525 GHz, with a relative bandwidth of 100%, and its focusing efficiency is shown inFigure 6As shown, in addition, based on calculations and experimental validation of optical coherence tomography theory, the longitudinal resolution of this three-dimensional imaging system can reach 0.5 mm (0.5λ0 at 275 GHz), effectively resolving the structures at adjacent depths within the object.
Dispersion metasurface for terahertz three-dimensional imaging system
To verify the performance of the proposed metasurface in practical high-resolution three-dimensional imaging, the team conducted imaging experiments on the following target objects: two layers of stacked paper containing different text information, headphones packaged in a paper box and charging case. The experimental measurements were conducted in the frequency band of 220–330 GHz.
It is worth noting that in the terahertz three-dimensional imaging system constructed based on the dispersion metasurface, the frequency domain reflection signal of the object to be measured simultaneously contains focused reflection signals and defocused reflection signals, where the defocused echoes can lead to imaging blur. To improve image contrast, subsequent signal processing uses short-time Fourier transform (STFT) to separate the frequency components at different moments and suppress the defocused reflection signals, thereby optimizing imaging quality.
As shown inFigure 7, two sheets of paper containing different handwritten text information are stacked along the terahertz wave propagation direction (z-axis) with a spacing of 13 mm. Terahertz waves can penetrate thin ink layers and paper, but due to the different complex dielectric constants of ink and paper in the terahertz frequency band, they exhibit different reflection characteristics to the incident terahertz waves. This reflection contrast can be used to extract the text information on each sheet of paper, as shown in, 7e. This three-dimensional imaging system can effectively utilize the frequency domain reflection signal amplitude of the two stacked sheets of paper to construct terahertz imaging and accurately extract the text information on each layer of paper. Given that canvas was expensive during the European Middle Ages, the reuse of canvas has led to some famous paintings hiding other oil paintings underneath. This three-dimensional imaging system is expected to be used to detect hidden oil paintings, expanding its application in the field of cultural heritage preservation.In another experiment, the terahertz three-dimensional imaging system performed terahertz detection imaging on headphones hidden inside a thick paper box and charging case to explore its potential applications in high-resolution public safety inspections. As shown in
Figure 8图8As shown, focused terahertz waves can penetrate various non-conductive layers, including thick paper packaging, multilayer plastics, PCBs, and charging case batteries, thereby accurately detecting hidden earphones. In addition, the metal components inside the charging case, such asFigure 8bthe metal hinge and two small Size chips, can also be effectively detected by focused terahertz waves. This experiment verified the high lateral and longitudinal resolution of the three-dimensional imaging system, indicating its ability to effectively detect multilayer concealed structures inside complex packaging, demonstrating its application potential in fine security screening.
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