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Abstract
In this paper, a broadband low-scattering metasurface is proposed by using a combination of phase cancellation and absorption mechanisms. The metasurface is composed of two structural layers. One layer adopts the geometric phase cell that can obtain a different reflection phase by changing its orientation. Through the random phase distribution design, electromagnetic diffusion can be realized to reduce the backward scattering energy. The other layer is made of a resistive frequency selective surface (RFSS) that can absorb the incident wave by converting it into Ohmic loss. The above two physical mechanisms respectively play the great roles at two distinct frequency bands, and finally make our metasurface achieve the RCS reduction over a wide frequency band ranging from 13 to 31.5 GHz. Both simulation and experimental results are in good agreement, which fully demonstrates our design method. The analysis of the scattering patterns, electric-field distribution and power loss density are given to explain the hybrid RCS-reduction mechanism of our metasurface.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Over the last decades, metasurface has attracted much attention in physics and engineering communities owing to their exotic properties of manipulating electromagnetic (EM) waves [1–3]. As a two-dimensional metamaterial, it can engineer light-matter interactions on an ultrathin-surface within the sub-wavelength scale, thereby inspiring many intriguing applications, such as wave-front control [4,5], invisible cloak [6,7] and vortex beam generation [8,9]. Among the proposed applications, low-scattering metasurface has been always a hot topic for civil and military reasons, which can effectively reduce the radar cross section (RCS) of the target.
For the low-scattering metasurface, a 10dB RCS reduction level is a general design requirement, which means that at least 90 percent of the backward reflected energy is suppressed. So far, there have been two main approaches to realize low-scattering property based on metasurfaces. One approach is based on the absorbing-type metasurface. In 2008, the perfect absorber was first proposed and experimentally validated by Landy et al. [10]. The integrated design of two resonators can convert the incident EM wave into Ohmic loss with the electric and magnetic resonances, achieving near unity absorptivity. However, limited by the intrinsic resonance property of metasurface, the perfect absorption is typically restricted at a narrow band, and broadband impedance matching is hard to achieve. To attain broadband absorptivity, the frequency selective surface (FSS) technology has been developed to realize thin and wideband absorbers. By using the resistive FSS or the metallic FSS loaded with lumped resistors, strong absorption performance can be achieved over a broadband frequency range [11–15]. Geometry shaping is another widely used method for RCS reduction. The traditional geometry design of the target can reduce RCS by redirecting the scattering waves to other non-threatening directions, but it may destroy the aerodynamic layout. The phase-cancellation metasurface provides a novel route to achieve the similar functionality without altering the geometry of the target. By tailoring the reflected phase of each meta-atom, the scattered waves can be dispersed into several directions or guided to the predesigned direction. As early as 2007, the artificial magnetic conductors (AMC) and perfect electric conductors (PEC) combined in a checkerboard configuration was reported for RCS reduction due to the destructive interference arising from the 180° reflected phase difference between the above two elements [16]. However, the bandwidth of RCS reduction is very limited due to the narrow in-phase reflection bandwidth of the AMC. Subsequently, many efforts have been made to broaden the RCS reduction bandwidth with the phase cancellation metasurface [16–31]. In [20], a hexagonal checkerboard metasurface composed of two anti-phase elements, has been proposed to realize low-scattering property by dispersing the backward scattering wave into six main directions, which can obtain the 10 dB RCS reduction with about 61% fractional bandwidth. In [21], the same group also increased the RCS reduction bandwidth to 83% by using two properly selected AMCs in a blended checkerboard architecture. Recently, the concept of coding metasurfaces has been also used for wideband RCS reduction [27]. In the latest work, the 10 dB RCS reduction bandwidth of about 150% was proposed based on the optimized multi-element phase cancellation, but it is not a planar metasurface due to the use of meta-atoms with different thickness [31].
Previous research mainly focused on the optimized design of the phase elements and their phase distribution to achieve wideband RCS reduction. In Refs [32–36], the metasurfaces adopted the lossy element instead of the traditional metallic element, and then both the phase-cancellation and absorbing mechanisms contribute together to RCS reduction. In these metasurfaces, only a little of the incident wave energy is diffused and most of them are absorbed. In Refs [37,38], dual-band and low-scattering metasurfaces were reported by using the above hybrid physical mechanism, which can simultaneously absorb the incoming wave through the introducing lumped resistors at S-band and control the backward scattering direction at X-Ku band. In this work, we employ the RFSS layer into the traditional phase cancellation metasurface, which can sharply increase the RCS reduction bandwidth. The original phase cancellation metasurface mainly takes responsibility for the RCS reduction in the lower frequency band of 13-21.5 GHz, while the introducing RFSS can absorb most of the incident wave energy in the higher frequency band of 21.5-31.5 GHz. Both simulated and experimental results demonstrate that our metasurface combining the above hybrid mechanisms can finally achieve the 10 dB RCS reduction between 13 and 31.5 GHz with a fractional bandwidth of 84%. The angular response is validated up to 40 degrees, where there is still strong low-scattering performance under both TE and TM modes. The physical mechanism is explained by examining the scattering patterns, electric field distribution and surface power loss density.
2. Structure design and numerical simulation
Figure 1(a) shows the geometrical model of our metasurface and its meta-atom. It is composed of two layers of the structured patterns. The top-layer one employs the C-shape split resonant rings (CSRRs) as the geometric phase cell to modulate the reflected phase by rotating its orientation, and it is printed on a F4B substrate with relative permittivity 2.65 and a thickness of h1 = 1.2 mm. The periodic square ring patches are adopted as the RFSS to absorb the incoming wave, which is fabricated on a FR-4 substrate with relative permittivity of 4.3 and a thickness of h2 = 1 mm. Both two layers of substrates are bonded together by a glue film (t = 0.1 mm). The period of the unit cell p1 = 8mm, and the geometrical parameters of the geometric phase cell are respectively set as r = 2.6 mm, w = 0.3 mm, θ = 58°. For the RFSS layer, the sheet resistance R = 170 ohm/sq, the period of p2 is set to be 4 mm and other parameters of the squire loop are l = 3.6 mm and s = 2 mm.
Fig. 1 Schematic of the proposed metasurface and its reflection characteristics (a) 3D-view of the metasurface and its meta-atom. (b) Reflection magnitudes of the co- and cross- polarized components for the meta-atom under RCP (LCP) incidence. (c) Cross-polarized reflection magnitudes and phases of the meta-atom with different rotation angles of the CSRR under RCP incidence. (d) Absorptivity of the meta-atom with different rotation angles of the CSRR under LP incidence.
To investigate the reflection characteristics of this meta-atom, numerical simulation is performed by using the commercial software CST Microwave Studio. Periodic boundary condition is applied to emulate the infinite array. For the top-layer geometric phase cell, it is illuminated by a right-handed or left-handed circularly polarized (RCP or LCP) wave. It is seen in Fig. 1(b) that the incident CP wave is converted to cross-polarized CP reflected wave from 13.3 to 21.5 GHz. When there is no RFSS layer, the outgoing CP reflected wave has very high efficiency with polarization conversion ratio (PCR) larger than 15 dB. After introducing the RFSS layer, the reflection loss is obviously increased in the whole frequency band of interest, especially in the range of 21.9-30.6 GHz, which is due to the resistive loss of RFSS. Compared with the simulated result of the meta-atom without the RFSS layer, both the cross- and co-polarized components of the reflected wave are almost below −10 dB between 21.9 and 30.6 GHz, indicating that most of the incident wave is absorbed by the RFSS layer. In order to further analyze the phase modulation property of the proposed meta-atom in the frequency band of 13.3-21.5 GHz, the reflection phases of the CSRRs with different rotation angles are given in Fig. 1(c) under the incidence of RCP wave. It is found that the variation of the rotation angle has almost no influence on the magnitude of cross-polarized component of the outgoing wave, and near-parallel phase response from –π to π is obtained when the rotation angle is varied from 0° to 180° with a step of 45°. In addition, the influence of the geometric phase cell with the varying rotation angles on the absorptivity spectra of the RFSS is also discussed under the illumination of a linearly polarized (LP) wave. As Fig. 1(d) shows, the strong absorption band is clearly revealed between 21.9 and 32.8 GHz, where the absorptivity reaches about 80% for the different rotating angles of the geometric phase cell. In the lower frequency band of 13.3-21.5 GHz, the absorption is relatively weak, and RCS reduction mainly relies on the phase cancellation of the scattering wave through the surface phase modulation.
Figure 2(a) shows the simulated RCS reduction performance of the proposed metasurface under normal LP incidence. We employ 10 × 10 super cells to construct the whole metasurface with each super cell composed of 2 × 2 unit cells. For the RFSS layer, the four CSRRs have the same orientation in one super cell and four kinds of phase cells with 0°, 45°, 90° and 135° orientations are adopted to generate the random phase distribution. The dimension of the designed metasurface is 180 mm × 180 mm, and relative to a metallic plate with the same size, its RCS reduction performance can be obtained. It is clearly seen in Fig. 2 that the metasurface can achieve 10 dB RCS reduction over a wide frequency band ranging from 13 to 31.5 GHz. When there is no RFSS layer, the 10 dB RCS reduction band is sharply shrunk within 14.5-22.5 GHz, except for some frequency points where the RCS reduction is about 8 dB. If the geometric phase layer is removed, the obvious RCS reduction is only obtained in the higher frequency band of 18-30 GHz. That indicates the above two RCS-reduction frequency band are mainly caused by the geometric phase layer and RFSS layer, respectively. To better understand how our metasurface works, we adopted the calculation method well described in [35] to discuss the distribution of absorption and scattering cancellation in our metasurface under normal incidence. The absorption is the weighted average of four types of super cells with different orientations of CSRR, while the scattering cancellation part can be obtained by 1 - absorption - reflection. As Fig. 2(b) shows, in the lower frequency band ranging from 13 to 21.5 GHz, the absorption is relatively weak and most of the incident energy (64.2%) is diffused into various directions, resulting in the reduction of backscattering energy. In the higher frequency band of 21.5-31.5 GHz, the absorption mechanism plays a dominant role with more than 80% incident energy absorbed by the RFSS layer. Only 9.1% of the incident energy is reflected back along the normal direction at the whole RCS reduction band.
Fig. 2 (a) Simulated RCS reduction performance of the designed metasurface at three different cases and (b) The ratio of the absorption, diffusion and reflection of our metasurface under normal incidence.
In order to get an insight into RCS reduction mechanism, the 3D scattering patterns of our metasurface under normal incidence are displayed in Fig. 3, which also includes a metallic plate with the same size as a comparison. As Figs. 3(a)-3(c) show, the strong backward scattering waves are obviously generated along normal at 14, 20 and 27 GHz due to the mirror reflection of the metallic plate. When employing our metasurface instead, the incident EM energy is redirected to many directions at 14 GHz, resulting in the EM diffusion phenomenon, as seen in Fig. 3(d). Therefore, the monostatic RCS is sharply reduced. At the frequency of 20 GHz, although there is still EM diffusion phenomenon in Fig. 3(e), the scattering energy along the normal is obviously larger than that in other directions. At this case, using the absorption mechanism of the RFSS layer can further suppress the monostatic RCS in the low level. As Fig. 3(f) shows, the scattering pattern of the metasurface performs like that of the metallic plate shown in Fig. 3(c) in the higher frequency of 27 GHz, but its backward scattering energy is obviously reduced. It is obvious that the different physical mechanism dominates the RCS reduction performance of the metasurface at different frequency bands. The phase cancellation mechanism mainly contributes to the RCS reduction at the lower frequency band, while the absorption mechanism plays a great role in the RCS reduction at the higher frequency band where there is almost no diffusion effect. The introduction of the RFSS layer has improved the RCS reduction bandwidth of the phase cancellation metasurface.
Fig. 3 Simulated 3D scattering patterns of (a-c) the metallic plate and (d-f) the metasurface at the frequencies of 14 GHz, 20 GHz, and 27 GHz.
To better understand the RCS-reduction mechanism of our metasurface, the electric field distributions at 14 GHz, 20 GHz and 27 GHz have been illustrated in Figs. 4(a)-4(c), respectively. It is seen that there is strong electric field energy distributed on the CSRR at 14 GHz, and its field intensity is gradually reduced at 20 GHz and is even weakened to be ignored at 27 GHz. The existence of the strong electric field energy is associated with the resonance of the CSRR, which indicates that the geometric phase cell mainly operates at the lower frequency band, reducing the monostatic RCS by phase cancellation. In contrast, the power loss density distributions at the above three frequency points have different results. From Figs. 5(a)-5(c), it is obvious that the power loss is mainly focused on the RFSS layer, and its density is gradually enhanced with the increase of the frequency. We can deduce that the RFSS layer achieves the strong absorption in the higher frequency band where most of the incident wave is converted into Ohmic loss of the resistive film. At the middle frequency of 20 GHz, both the phase cancellation and absorption mechanisms contribute together to the RCS reduction of our metasurface.
Fig. 4 Simulated electric field distribution of the proposed unit cell on the top metallic layer at (a) 14 GHz, (b) 20 GHz and (c) 27 GHz.
Fig. 5 Simulated power loss density of the unit cell on xoz plane at (a) 14 GHz, (b) 20 GHz and (c) 27 GHz.
3. Experimental verification
In order to experimentally verify the RCS reduction performance of the proposed metasurface, a design example is fabricated and tested. In this design, the geometric phase layer is fabricated by printed circuited board technology, while the RFSS layer is fabricated by conductive ink using silk-screen printing technique. We measured the RCS reduction performance of the sample in the microwave anechoic chamber, as seen in Fig. 6(a). Three pairs of standard horn antennas (working in 12-18 GHz, 18-26.5 GHz, and 26.5-40 GHz, respectively) were used in the measurement. The distance between the sample and antennas is over 10 m, satisfying the far-field condition at the highest considered frequency. In the process of measurement, the RCS of a metallic plate with the same dimension was first measured, and then we obtained the RCS of the sample and normalized it by the RCS of the metallic plate. It is seen in Fig. 6(c) that the metasurface without the RFSS layer can achieve the 10 dB RCS reduction between 14.8 and 22.4 GHz except for some frequency points where the monostatic RCS is reduced by about 8 dB. Its fractional bandwidth is calculated to be about 40.9%. After introducing the RFSS layer, the 10 dB RCS reduction is realized over a wide frequency band ranging from 13 to 31.5 GHz, and the fractional bandwidth reaches about 84%. In addition, it is found that there is still 8 dB RCS reduction level between 31.5 and 33 GHz. The above measured results agree well with the simulated ones shown in Fig. 2(a). From the above results, we can note that the employing RFSS layer not only enhances the RCS reduction level at the original operation band of the metasurface with only CSRR, but also expands the RCS reduction bandwidth in the higher frequency band. To further measure the incident angle dependence of our sample, an arch measurement system was adopted [39], as seen in Fig. 6(b). One horn antenna is set as transmitter and the other is used as the receiver, both of which are connected to the vector network analyzer. The radius of the arch is 3.5 m, which is obvious to be unsatisfied with the far-field condition. So we compared the experimental results of the sample under normal incidence using the above two measurement methods and found that the measured curve of the arch method is in a good agreement with that obtained in the microwave anechoic chamber. Therefore, we consider that the arch method can be an approximation to obtain the reflection performance of our planar sample under oblique incidence. As Fig. 6(d) shows, with the increase of the incident angle under both TE and TM polarizations, the −10 dB reflection bandwidth decreases a little, and most of the reflection levels are still about −8~-10 dB, except for the case of oblique incidence of 40° under TM polarizations. It may be due to the phase aberrations, which may cause the degradation of the backward scattering performance in the lower frequency band of 13-21.5 GHz. However, our metasurface still keeps the low scattering properties under oblique incidence.
Fig. 6 (a) Measurement setup in a microwave anechoic chamber and fabricated metasurface sample. (b) Arch measurement system. (c) Measured RCS reduction performance of the metasurface under normal incidence. (d) Measured Reflection of the metasurface for TE and TM polarizations under oblique incidence.
In order to better understand the performance of our designed low-RCS metasurface, we have listed bandwidth and thickness performances of some typical works classed by its working mechanism; Table 1. The traditional phase cancellation metasurfaces mainly focus on the optimization of reflection phase cells and their distributions to broaden the RCS reduction bandwidth [20,21,23,28], while our approach employs the hybrid mechanisms to present the wider or similar bandwidth level. And compared to the low-scattering surfaces proposed in [36] and [38] with hybrid mechanisms, our metasurface also has a wider 10 dB RCS reduction bandwidth. In [35], the absorption mechanism plays a major role in the RCS reduction, and the diffused energy is very small, which only reaches about 10-30% at different bands. It should be emphasized that our approach can further extend the RCS reduction bandwidth by optimizing the phase cells and their distribution. Here, our approach provides a feasible method to design the ultra-broadband or multi-spectra RCS-reduction metasurface.
Table 1. comparison of our work and previous researches
4. Conclusion
In conclusion, we have proposed a wideband low-scattering metasurface by using the hybrid physical mechanisms. The geometric phase cell and RFSS cell are employed to construct this metasurface, which can achieve wideband RCS reduction based on the combination of the phase cancellation and absorption mechanisms. Detailed physical analysis has been presented by studying the scattering patterns, electric field distribution and power loss density. The RCS reduction in the lower frequency band of 13-21.5 GHz is mainly due to the phase cancellation of the geometrical phase cells, while the RFSS plays the dominant role in the higher frequency band of 21.5-31.5 GHz by absorbing the incoming wave. Both the simulated and measured results reveal that our metasurface can effectively reduce the backward scattering energy by 10 dB from 13 to 31.5 GHz, which also has a good angular stability. The utilization of the RFSS in the traditional phase cancellation metasurface can sharply increase the fractional bandwidth of 10 dB RCS reduction from about 40.9% to 84%. Compared with the traditional single RCS-reduction mechanism, the use of hybrid mechanisms in our work can provide more freedom to achieve ultra-broadband or multi-spectra RCS reduction, which may find potential applications in some specific areas.
Funding
National Natural Science Foundation of China (61605213, 61775218).
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