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Abstract
The terahertz imagers that have already developed are usually with low frame rate and high cost, making them ineffective for moving target detection and fast imaging. Here, we experimentally report a high-frame-rate, high-resolution and low-cost real-aperture imager developed by combining the terahertz rainbow spectrum manipulation metasurface and quasi-optical method. As a wideband THz signal incidents on such metasurface (called as THz virtual prism), terahertz rainbow spectrum can be formed in one dimension just like a light beam passing through an optical prism, and complete real-aperture scanning imaging within 100 microseconds. By using high-speed displacement platform to realize another dimensional scanning, this imager will easily achieve high-frame-rate and high-resolution imaging with simple algorithm. The feasibility of this new THz imager was verified by imaging Siemens star and concealed targets, respectively. The results show that this imager has important potential applications in fields such as THz remote sensing, imaging radar, body scanner, etc.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Terahertz frequencies are located between the microwave and far infrared spectrum in the electromagnetic spectrum. THz waves offer high resolution, security and the property of being able to ‘see through’ obscuring materials such as clothing, cardboard, and wood with relatively less loss, and were observed and applicated in biology and biomedicine, remote sensing or radar imaging, moving target detection, body security and wireless communications [1–7]. Benefit from the advantages of THz waves, THz body imaging has attracted much attention because of the urgent need for anti-terrorism [8–10].
Unfortunately, due to the shortage of the phase shifters in THz band, it is difficult, even impossible, to realize electrical beam steering by phased arrays as done in microwave band. Thus, most of THz imaging systems acquire body imaging by using the mechanical steering or scanning scheme [11–13], which have low imaging frame rate.
In 2011, the American Jet Propulsion Laboratory developed an active imaging system with a center frequency of 0.675THz based on the accumulation of a series of research work. This system uses the combination of quasi-optical mechanical scanning and frequency-modulated continuous wave to realize three dimensional real-aperture focusing imaging [11]. In 2017, J. Gao carried out the research of an imaging system in the 340GHz frequency band with 4 transmitters and 16 receivers array, and the imaging frame rate can reach 4Hz. The horizontal dimension of the system adopts MIMO array, but in the vertical direction, the plane swing mirror mechanical scanning method is adopted, which limits the imaging frame rate and has complex imaging algorithm [12]. In 2019, Hao Tu reported a real aperture terahertz personnel screening system which uses a single transmitter and receiver pair together with a quasi-optical system to realize body imaging. But the whole imaging time is up to 2.9s limited by quasi-optical scanning system [13]. To reduce the imaging time or increasing imaging frame rate, the frequency-controlled beam scanning antenna based on the leaky wave system, may be one of the alternatives [14–18].
In the visible band, a low-cost dispersive prism can separate the white light into different light spectrum constituent as rainbow. The light spectrum with different wavelengths are discriminated with each other by colors and steered toward various directions to form a rainbow [19]. But, in terahertz band, there is no natural counterpart of THz prism to spatially separate the THz spectrum for the lack of natural materials or the fabrication difficulties of existing materials.
To realize frequency scanning in terahertz frequency range, Basu et al. proposed a dielectric waveguide-based antenna with metal strips on its top side at 212GHz [14]. At 97-103GHz, Zandieh et al. reported a dielectric waveguide with periodic corrugations as the leaky structures [15]. In the frequency range 130-150GHz, Cullens et al. presented a micro-fabricated slotted-waveguide frequency scanning antenna with poor scanning linearity [16]. Y. Ghasempour and H. Saeidi also proposed two leaky-wave antenna with wide angle scanning range to realize wireless link discovery and rapid beam alignment/tracking at THz frequencies [17,18]. In addition, a leaky wave antenna has been realized in the frequency band 330–500 GHz, and the sweeping angle range is 102° [19]. However, the leaky wave antenna based on the waveguide system is not very suitable for terahertz imaging applications in which the transmitter and the receiver are usually share a same antenna.
In the past few years, metamaterials called artificial electromagnetic media and their 2D counterparts metasurfaces demonstrate flexible and effective control capabilities for electromagnetic wave’s polarization, amplitude, phase, propagation mode and other characteristics, and provide new possibilities for fast manipulating THz waves [20–27]. Most of the research works are focused on terahertz absorber [28], THz high-efficiency phase and amplitude modulators [29,30], terahertz holographic imaging [31], the abnormal reflections and transmissions of THz waves [32,33], and low-loss polarization converters [34]. Benefit from the advantages of metasurfaces, it is possible to manipulate the THz spectrum by designing a special metasurface structure, or constructing a virtual THz prism. Based on the periodical metasurface, we have proposed some frequency-controlled beam scanning antennas [35–37], but it is difficult to realize high resolution imaging when beams steer the direction with the frequency change.
Here, based on the previous research works on frequency sweeping antennas, we propose a novel terahertz rainbow spectrum imager by combining the quasi-optical method with terahertz manipulation metasurface to realize high frame rate and high resolution imaging with simple algorithm and low cost. The proof-of-concept experiment has been implemented and confirmed the feasibility for high-frame-rate and high-resolution imaging by the metasurface. Benefit from the optical path design and optimization based on the quasi-optical theory, the proposed terahertz rainbow spectrum imager can produce 2D imaging of concealed targets with high-frame-rate and high resolution with simple algorithm. In addition, with its low cost, the proposed THz rainbow spectrum imager has important applications in the fields of security inspection or body scanner, biomedicine, and non-destructive testing. It can also be extended for potential applications in THz remote sensing, imaging radar, communication, and other fields.
2. Terahertz rainbow spectrum imager working principle
2.1 Terahertz rainbow spectrum manipulation principle
In visible band, A shaped glass prism can produce optical rainbow because the refractive index of frequency components is different. As a white light passes through glass prism, different colors of light corresponding to different wavelengths are spatially separated as a rainbow shown in Fig. 1(a). In terahertz band, the terahertz rainbow can be acquired by the novel constructed metasurface with a planar 2-D periodic structure shown in Fig. 1(b). The metasurface is formed by metal layer, dielectric layer and periodic metal gap.
Fig. 1. The schematic diagram of the optical rainbow and terahertz rainbow manipulation. (a) Parallel rainbow light generated by white light passing through a glass prism. (b)The generation of THz rainbow spectrum manipulated by reflect metasurface. (c) The schematic diagram of the THz rainbow spectrum manipulation system.
As the THz wave incidents on the metasurface, the Floquet mode containing diffraction waves and surface waves will be excited due to the response of the periodical unit cells. The surface waves with phase velocity slower than the speed of light in free space, are constrained near the metasurface and cannot be radiated toward the free space. Meanwhile, the diffraction waves with the phase velocity faster than the speed of light in free space are radiated toward the free space. Additionally, the diffraction waves’ phase velocities along the metasurface surface are dependent on the frequencies, therefore, their space radiation directions will vary with frequencies. Finally, the diffraction waves or spatial spectrum distribution can be manipulated by the metasurface and the rainbow spectrum can be formed.
For the reflective metasurface formed by metal layer, dielectric layer and periodic metal gap, assuming that the XOY plane is periodic surface, and XOZ plane is the incident plane shown in Fig. 1(c). The diffraction waves with different modes called as harmonics are generated based on the Floquet theorem [38]. The diffraction angle of the mth-order harmonic is θm, and it can be also derived from the Floquet theorem as
2.2 Working principle of the terahertz rainbow spectrum imager
Benefiting from the terahertz rainbow, as shown in Fig. 2, a novel terahertz rainbow spectrum imager with high frame rate and high resolution can be developed by combining with the quasi-optical theory. In order to achieve two-dimensional scanning, one dimension of this imager is based on the frequency sweeping, and the other dimension is mechanical scanning. Through the manipulation of the terahertz rainbow spectrum and mechanical scanning, a 2-D image of M×N pixels can be obtained. In this imager, the quasi-optical theory is used to optimize the sweeping beam resolution and design the imaging range along frequency-scanning direction. The frequency scanning time which represents the total imaging time along the frequency-sweeping dimension is less than hundreds of microseconds. This means that tens of thousands of acquisitions can be completed in the mechanical scanning dimension within one second. Therefore, by using high-speed displacement platform to realize mechanical dimensional scanning, the imager will easily achieve high frame rate and high-resolution imaging.
As shown in Fig. 3, the terahertz rainbow spectrum imager is composed of a feed, a receiver horn, lens 1, a reflective metasurface manipulating the terahertz rainbow spectrum, and lens 2. The transmitting and receiving antenna are a pair of pyramid cones with the same structure, and separated by a beam splitter made of polyethylene film. As the incident terahertz wave from lens 1 illuminates the reflective metasurface, the terahertz rainbow spectrum will be generated and radiate towards the lens 2. The lens 2 is used to convert terahertz rainbow into parallel rainbow with small beam radius on the plane of the target. By moving the lifting platform, and repeating the process of terahertz rainbow spectrum scanning, a two-dimensional object image can be achieved.
Assuming the distance between metasurface and lens 2 is din, the frequency scanning angle range of the metasurface is θ, and the focus length of lens2 is f, if the metasurface is placed on the focal point of the lens, namely din=f, then the distance between lens2 and target plane is f, and the beam waist on target plane satisfies following equation
where win is the beam waist of the diffraction wave from the frequency scanning device, and according to the Eq. (2), the imaging resolution can be derived asThe vertical scanning range Lv is depend on the moving distance of the lifting platform. The horizontal scanning imaging range Lh is
3. Design of terahertz rainbow spectrum imager
3.1 Imaging capabilities of imager
Based on the working principle of the terahertz rainbow spectrum imager, the main capabilities of the imager are shown in Table 1. The center frequency is chosen as 200GHz, and bandwidth is 40GHz. The imaging area is 0.1m×0.4m. The resolution is better than 5mm. Based on the above theory, the main imaging indicators, such as imaging resolution and horizontal imaging range, depend heavily on the capabilities of the frequency scanning device itself. Benefit from the advantage of frequency-controlled beam scanning which can achieve a frequency sweeping within 100 microseconds, and regardless of the moving speed of the scanning platform, the imager only needs to complete frequency scanning at 81 positions along vertical direction to acquire an image, which takes 8.1 milliseconds. In actual experiments, limited by the moving speed of the scanning platform, the imager can’t achieve such a high frame rate. But, by adding a rotating oscillating mirror and lens, the imaging distance, range, and frame rate can be further increased, and the frame rate of imaging a human target can exceed 20 Hz. Because the experiment carried out in this paper is only to verify the feasibility of the proposed imaging method, considering the fast scanning capability of the frequency sweep itself, the rotating mirror device is not selected to achieve high-frame-rate imaging, but a mobile scanning platform is used to verify the imaging method.
Table 1. The imaging capabilities of imager
To meet the requirements of the imaging resolution and imaging area, the frequency-controlled beam scanning device must cover 18.3° scanning range which has better linear relationship with different frequency components or rainbow spectrum, and the outgoing beam waist of the frequency scanning device is 28 mm.
3.2 Device design and fabrication
Based on the imaging area requirements of the imager shown in Table 1, a terahertz rainbow spectrum manipulation device was designed by choosing −2nd order mode (m=−2) as main diffraction wave mode in the frequency band 180GHz-220GHz. Figure 4(a) shows the top view of the unit including two subcells. The metasurface’s dielectric layer is Rogers RT/duroid 5880 with the standard thickness of 0.254 mm. The incident angle and the X direction’s unit periodic length has been successfully optimized to be θ0=55° and Dx=2.25mm by taking into account the scanning relationship linearity shown in Eq. (1) and the higher mode suppression problems. The unit length Dy along Y direction is chosen as 0.8mm to avoid the beam blocking. Further, the dimensions of the subcells were optimized using the commercially available Ansys HFSS. Obviously, based on the simulated results of the transfer phases for subcell 1 and subcell 2 over the frequency band 180GHz-220GHz, the value of d was easily computed. Figure 4(a) gives the optimized parameters values of unit cell.
Fig. 4. (a) The top view of the unit including two subcells. (b) The schematic diagram of the experimental setup.
To measure the rainbow spectrum characteristics of the proposed metasurface, an experimental setup was developed, as schematically shown in Fig. 4(b). The setup mainly consists of three sections, the terahertz rainbow spectrum manipulation system, the computer controlled near field scanning platform and the THz transmitter. The terahertz rainbow spectrum characteristics is measured by the THz receiver’s probe which is set on a motorized platform (600mm in length) to get the diffraction field distribution along rainbow direction. The THz transmitter couples the THz signal into the feed horn of terahertz rainbow spectrum manipulation system. The measured field distribution of the terahertz rainbow spectrum over the frequency band 180GHz-220GHz are transferred to the computer and processed. During data processing, the field extrapolation method was used to obtain the far field radiation pattern based on the measured amplitude and phase distribution by the scanning platform.
Figure 5 shows the terahertz rainbow spectrum radiation patterns over the frequency band 180GHz-220GHz, based on the measured data and the near-far field transformation. It can be seen that the terahertz rainbow spectrum scanning range covers from −41.4° to −23.1° over the frequency the band 180GHz-220GHz. Additionally, the measured scanning angles of the terahertz rainbow spectrum are agree well with the theoretical and simulation results and have high scanning linearity. Moreover, the measured diffraction efficiency of the −2nd order diffraction wave is up to 88% and average above 85% over the whole band between such angle range.
The reflective metasurface was fabricated using laser fine etching. The high-quality and low-power laser beam is focused into the material surface with a very small spot and high power density, so it can vaporize the material instantly to form the hole, gap and slot. The laser fine etching technology has a number of advantages: non-contact processing, high degree of flexibility, fast processing speed, non-noise, small zone affected by the heat and good focus performance. The reflective metasurface used in this paper is 150mm×150mm in size which is limited by the beam waist of the frequency scanning device and the incident angle of the feed, and contains tens of thousands of unit structures. It is widely known that the laser precision micromachining is greatly affected by the environment and working time, so we choose the row-to-row processing technology, namely the next row would begin to process after finishing the previous row. This method can ensure that the rainbow spectrum characteristics of the metasurface are less affected by the machining accuracy.
3.3 Rainbow imaging system simulation
Shown in Fig. 6, the terahertz rainbow spectrum beam generated by the reflective metasurface illuminates the lens 2 with an angle, and the terahertz rainbow imager focuses on target plane with a small spot to realize high resolution. Therefore, it is very necessary to verify the focusing effect (beam radius, etc.). Based on the simulation software GRASP, the focusing effect of the terahertz rainbow imager on target plane can be easily obtained. Figure 6 shows the results obtained by GRASP simulation. Specifically, Fig. 6(a) is the focusing effect of 0.2THz beam which propagates along the optical axis of the lens. The focusing effect of diffraction beams which are from the metasurface and propagate along an angle with the optical axis of the lens is shown in Fig. 6(b)–6(d).
Fig. 6. The focusing effect of the scanning beam on target plane. (a) The focusing effect of 0.2THz beam on target plane. (b) The focusing effect of diffraction beams from the metasurface with the angle difference 1° between its propagation direction and the optical axis of the lens. (c) The focusing effect when the angle difference between propagation direction of diffraction wave from the metasurface and the optical axis of the lens is 5°. (d) The focusing effect of diffraction beams from the metasurface with the angle difference 9° between its propagation direction and the optical axis of the lens.
In view of the scanning angle range of the metasurface, the selected incident angles are four representative values which are 0°, 1°, 5°, and 9°. From Fig. 6, it can be found that, as the oblique incidence angle of the Gaussian beam increases, the beam would spread along the direction of increasing angle, which means that the terahertz rainbow spectrum beam would spread out along the rainbow direction to form an approximately ellipse. Fortunately, despite the rainbow spectrum beam radius increases in the rainbow direction, the resolution in rainbow direction still meets the demands of the imaging. Additionally, the terahertz rainbow covering range is larger than 0.1m by analyzing the above simulated results, and meets the requirement of the imaging range in the rainbow direction.
3.4 Experimental verification
To verify the feasibility and superiority of the terahertz rainbow spectrum imager, an imaging experiment prototype was built shown in Fig. 7. The resolution of the imager is verified by imaging a Siemens star. The radius of the concentric rings of the Siemens star is 5cm and 6cm, respectively. The inner ring is divided into 16 parts as shown in Fig. 8(a). Due to the short frequency sweep time, total imaging time is dependent on the moving time of the displacement platform. Vertical step of the motor is set to 2mm, and 51 frequency points are set to form rainbow spectrum scanning. The imaging results are shown in Fig. 8(b). In Fig. 8(b), the colors from red to purple represent different frequencies whose range is 180GHz-220GHz. Figure 9 shows the processed result. From the imaging results of Siemens, the imaging resolution is better than 5mm. The area around the center of the circle cannot be clearly differentiated because it is less than one resolution, but the other areas, including the outer ring and the 16 equally divided areas, are all well distinguished.
Fig. 8. (a)The imaging scene of the Siemens star. (b) The terahertz rainbow imaging results of the Siemens star (raw data).
Further, the imaging experiment of a knife was carried out. Its optical picture and terahertz imaging results are shown in Fig. 10(a) –10(b). The metal knife is 23cm long with the handle of 2cm-2.5cm. Its full contour, shape and different edge have clearly been shown in Fig. 10(a) –10(b). From the experimental results, we can verify the feasibility and superiority of the terahertz rainbow spectrum imager. Moreover, by controlling the frequency sweeping time along the horizontal direction, the imager can achieve high frame rate.
Fig. 10. Imaging results of a knife. (a) The optical picture of the knife. (b) Terahertz imaging results.
4. Conclusion
In summary, to realize high resolution imaging of frequency-controlled beam scanning antenna, we proposed a terahertz rainbow spectrum imager by combing the frequency scanning antenna and quasi-optical theorem. The experimental results show that the frequency scanning antenna can manipulate the incident THz signal from 180GHz to 220 GHz in an angular scope of about 18.3° with excellent scanning linearity and high transferring efficiency. For 2D imager, the data are acquired by rainbow spectrum scanning and high speed mechanical scanning. The proof-of-concept experiment of the imager has been implemented and confirmed that the developed terahertz rainbow imager can produce 2D imaging of concealed targets with high-frame-rate and high resolution. Benefit from the high frame rate, low cost and simple algorithm processing, the imager has important applications in the fields of security inspection or body scanner, biomedicine, and non-destructive testing. It can also be extended for THz remote sensing, imaging radar, and other fields.
Funding
National Key Research and Development Program of China (2017YFA0701004, 2018YFF01013004); National Natural Science Foundation of China (61988102, 62101534, 61731020, 61671432); Project of Equipment Pre-Research (WJ2019G00019); Key-Area Research and Development Program of Guangdong Province (2019B010157001, 2020B0101110001); Key Program of Scientific and Technological Innovation from Chinese Academy of Sciences (KGFZD-135-18-029).
Acknowledgments
The authors would like to thank the editors and reviewers for their efforts to help the improvement and publication of this work.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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