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Abstract
Absorbers have high potential application values in the military field, such as electronic screening, radar cross-section reduction and invisible cloaking. However, most methods have the defects of narrow bandwidth, low absorptivity, complex three-dimensional structure and fixed polarizations. In this paper, we realize an ultra-broadband and full-polarization planar metamaterial absorber (PMA) with a three-layer composite structure, which exhibits multi-resonant and impedance matching properties by combining the ultra-light foams and indium tin oxide (ITO) films. The bottom two layers achieve a high-efficiency absorption rate at the low and medium spectrum, while the upper layer realizes a absorption property at a high frequency. Also, an equivalent circuit model is extracted to explain its operating mechanism. The experimental results show that our meta-absorber can achieve great absorber performance of better than 90% within 1-18 GHz for full-polarization incident waves, which is in great agreement with the numerical simulations. Moreover, our device is insensitive to oblique incidences and polarizations and possesses the physical characteristics of an ultralight, weighing 0.6 kg for a square meter, which is only 1/85.0-1/126.7 of the conventional absorbers under the same size. All these excellent performances determine that our research can be a good candidate for military stealth materials.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Electromagnetic (EM) stealth is one of the most important factors in modern science and technology. Compared with the fighter aircraft that could achieve stealth purpose by scattering EM wave to other parts by changing the shapes and structures, the use of absorber can achieve stealth effect in a more comprehensive way by fundamentally consuming EM wave. However, traditional absorbers utilizing natural materials such as wedge absorber [1–3] and ferrite [4,5], suffer from narrow bandwidth, fixed polarization, complex three-dimensional structure and huge weight due to the properties of the material itself and physical criteria restrictions. As shown in Fig. 1(a), most conventional absorbers can only operate within a very small frequency range (f0) and reflect them off when irradiated at lower frequencies (f1) and higher frequencies (f2). These defects greatly limit their practical application.
Fig. 1. Working principle of the ultra-wide and full-polarization meta-absorber. (a) Traditional narrow band absorber only operates in the vicinity of f0 frequency band. When EM waves at f1 and f2 frequency are incident on the absorber, they will be totally reflected. (b) The schematic of the proposed meta-absorber, whose working band covers from L to Ku bands. It’s a very light absorber made of three layers of ITO films separated by two layers of foams and weighs 0.6 kg for a square meter.
Recently, metamaterials [6–40] have been extensively studied for their remarkable performance. Metamaterials can realize special EM properties which do not exist in nature materials, such as negative refraction [8,9], invisibility cloaks [10–12], super lenses [13–16], surface plasmons [17,18] and perfect absorbers [19–35]. Perfect absorbers bring about new insight applications for metamaterial in stealth area. Various methods have been adopted to improve the performances of the absorbers, such as loading lumped elements [24–27] and introducing multi-resonant structures to expand the bandwidth [24,28–30], adopting the fractal patterns to achieve the miniaturization of the meta-absorbers [22,23], and using the complex three-dimensional structures to enhance the absorption efficiency [28,31,32]. However, most metamaterial absorbers are difficult to combine the advantages of high absorption efficiency, large bandwidth, full-polarization response, light weight as well as planar structures, which greatly limits their practical applications. The research progress of wave field manipulations based on artificial microstructure in recent years is reviewed [33], which provides beneficial guidance for researchers to realize miniaturization and integration of photoacoustic devices. The metasurfaces design method inspired by catenary electromagnetism can make the wideband absorber and flat antenna obtain the lightweight performance and excellent operation performance [34,35].
In this paper, we theoretically and experimentally proved a high efficiency (above 90%), full-polarization and ultra-wideband (from L to Ku bands) PMA, as shown Fig. 1(b), consisting of a three-resonant planar metamaterial structure. Due to the material properties of the foams and ITO, the whole structure is ultra-light with a density of 20 kg/m3, weighing 0.6 kg for a square meter, which is only 1/85.0-1/126.7 of the conventional absorbers under the same size [25,27]. Our study provides a new exploration method for the development of ultra-light, high-efficiency broadband full-polarization absorbers.
2. Concept and unit cell design
The unit cell of proposed PMA is composed of three layers of ITO, which separated by three layers of foams as shown in Fig. 2(a). In order to realize the absorption at low and moderate frequency, the bottom two layers are polystyrene foams with the same thickness of h1, dielectric constant of 1.0 and a tangent loss angel of 0.018 respectively. The bottom layer ITO is a square patch with the length l1 and the sheet resistance R1. The middle layer is an ITO square ring with a width of l2 and a resistance value of R2. On the top layer of the unit cell is a foam layer with thickness h2 and four-square ITO patches with the same size. Each square ITO has a length l3 and resistance value of R3. The back of the unit cell is covered by copper sheet with the conductivity 5.8 × 107 S/m and a thickness of 0.036 mm, which is designed to prevent the transmission of EM waves. Here, the structural parameters (l1, l2, l3, r1, h1, h2, h3) mainly determine the resonant frequencies and thus the working bandwidth, and the resistance values (R1, R2, R3) of the ITO layers mainly determine the absorption efficiency. Therefore, we can tune the corresponding parameters to obtain an ultra-wide band and high-efficiency absorber. The proposed high efficiency and ultra-broadband PMA is designed, analyzed and optimized by full-wave EM simulation with CST Microwave Studio. The periodic boundary conditions are applied to simulate the infinite periodic cells. By carefully optimizing, all of the parameters in the absorber structure are finally obtained as l1 = 49.7 mm, l2 = 16.5 mm, l3 = 18 mm, r1 = 10 mm, h1 = h2 = 12.2 mm, h3 = 5.6 mm, R1 = 230 Ω/sq, R2 = 220 Ω/sq, R3 = 300 Ω/sq and the period of the unit cell is p = 50 mm. The simulation result is shown in Fig. 2(c). The black solid line represents the reflection. It can be seen that the reflectivity is less than -10 dB from 1 to 18 GHz, which achieves broadband and high-efficiency absorbing performance. There are three obvious depressions at 1.6 GHz, 7.3 GHz and 15.7 GHz, which are caused by the magnetic resonances of the three-layer structures.
Fig. 2. Design of the meta-atom for the ultra-broad and full-polarization absorber. (a) The structure and parameters of the PMA. (b) The equivalent circuit model of the absorber. (c) The comparison of the S11 curve obtained by CST simulation and ADS circuit simulation. (d) The curve of the normalized real and imaginary parts of impedance obtained by deriving S parameters.
To further understand the absorption mechanism, an equivalent circuit model of the unit cell is shown in Fig. 2(b). In our meta-atom, the patch structures in three layers are equivalent to the inductance, while the gap or ring between patches can be considered as capacitances. The equivalent resistance is introduced based on the ITO structures in our meta-absorber, which can dissipate the incident waves with high efficiency. Each layer represents a series RLC circuit. Thus, three parallel series circuits constitute the equivalent circuit of the proposed PMA. It is necessary to mention that R1, L1 and C1 represent the equivalent circuit of the bottom layer, while R2, L2, C2 and R3, L3, C3 denote the other two layers. Since the dielectric constant of foam is the same as that of the air, the foam of each layer can be considered as the air impedance Z0. The bottom copper sheet can be seen as a short circuit. With a similar method as that in Refs. [41–43], we extracted that L1, L2, L3 as 21.33 nH, 15.24 nH, 9.57 nH, C1, C2, C3 as 453.5 fF, 31.36 fF, 10.71 fF and R1, R2, R3 as 235.81 Ω, 380.25 Ω, 478.33Ω respectively by using the Advanced Design Studio (ADS). With these L and C values, the resonance frequencies are calculated to be f1=1/[2π(LC)1/2] = 1.62 GHz, f2 = 7.28 GHz, f3 = 15.72 GHz respectively. Furthermore, the reflection of the circuit model simulated and compared with that obtained by the EM model, as shown in Fig. 2(c). Noting that the results are consistent at three resonant points, which verifies the feasibility of the proposed PMA from the perspective of the equivalent circuit.
The absorptivity of the absorber is defined as [21]
A(ω), R(ω) and T(ω) represent absorption rate, reflectivity and transmission rate, respectively. In order to maximize the absorptivity A(ω) we need to minimize the reflectivity R(ω) and transmission rate T(ω). Since our proposed PMA has a metal plate at the bottom that can prevent the transmission of the incident EM waves, the transmission rate is equal to zero. Thus, the formula for calculating the absorption rate can be simplified as $A(\omega )\textrm{ = }1 - R(\omega )= 1 - {|{{S_{11}}} |^2}$. We need to make the reflectivity as low as possible, and the reflectivity is determined by the equivalent impedance Zeff as $R(\omega )= \frac{{{Z_{eff}} - {\eta _0}}}{{{Z_{eff}}\textrm{ + }{\eta _0}}}$. Therefore, the absorption rate can be derived as From Eq. (2), it can be obtained that if the absorption rate is close to 1, the real part of the equivalent impedance Re(Zeff) of the absorber needs to be close to the air impedance η0 while the imaginary part Im(Zeff) is close to zero. We can obtain the impedance of the proposed PMA by using ${Z_{eff}} = \sqrt {\frac{{{{({1 + {S_{11}}} )}^2} - {S_{21}}^2}}{{{{({1 - {S_{11}}} )}^2} - {S_{21}}^2}}} $ [44], and the results are shown in Fig. 2(d). The black and red solid lines represent the real and imaginary parts of the equivalent impedance, respectively It can be seen from the curves that the real part of the normalized equivalent impedance fluctuates around 1 from 1 to 18 GHz, which indicates that the equivalent impedance of the PMA is close to that of the air (Z0 = 377 Ω) and the imaginary part is near zero, which indicate that our PMA can achieve broadband and highly efficient absorption.In order to figure out the working mechanism of the proposed absorber, we simulate the absorber performance for different layers. In the inset of Fig. 3(a), it is a single layer of foam with a square ITO patch attached to the surface. It can be seen from the simulation curve that the absorber can only achieve the performance of S11 less than -10 dB from 2.9 to 7.4 GHz with a strong resonance point at 5.2 GHz. We also simulate the single-layer circuit model in ADS with inductance and capacitance are 32.61 nH, 28.72 fF, respectively, and its results are consistent with that of the EM simulation. For the purpose of expanding the absorption bandwidth, we superimpose a foam layer with a square ring of ITO on the surface. The simulated reflection of ADS with L1 = 43.37 nH, L2 = 24.58 nH and C1 = 120.67 fF, C2 = 18.32 fF is shown in Fig. 3(b). The bandwidth is broadened from 2.9-7.4 GHz to 1.3-9.0 GHz with S11 less than -10 dB. Due to the addition of the second layer of ITO and foam, two resonance points have been generated at 2.2 GHz and 7.5 GHz. Our purpose is to design an absorber with S11 less than -10 dB in the range of 1-18 GHz, so we add a foam layer with four identical ITO square patches on the surface based on the previous structure, as shown in Fig. 3(c). At this form, a new resonance point appears at 15.7 GHz, which can indicate that the newly added top layer can achieve high frequency absorption. Similarly, we also compared the EM simulation results with circuit results and found that they are in good agreements with each other. On this basis, we achieved broadband high-efficiency absorption by superimposing three ITO layers with different shapes and different resistance values. The surface current distributions are shown in Figs. 3(d)-(f) at f1 = 1.6 GHz, f2 = 7.3 GHz and f3 = 15.7 GHz. The perfect absorbing effect at low frequency is mainly caused by the coupling between the bottom layer of square ITO and the metal sheet. As shown in Fig. 3(d), the parallel currents on surfaces are reversed. It can be seen as a magnetic dipole that excites a magnetic resonance. Figure 3(e) shows that a strong reverse current is formed between the bottom and the middle layer of ITO to achieve mid-frequency absorption. Therefore, owing to the presence of a magnetic response current loop between the layers, the magnetic field along the y-direction is restrained by the current loop and dissipated by ohmic loss, which will improve the absorption efficiency of the absorber. It is concluded that the ITO coupling between the middle and the top layer generates a ring current, which leads to the efficient absorption at high frequency range as shown in Fig. 3(f). It should be mentioned that the absorption from low frequency to high frequency is controlled by ITO from the lower layer to the upper layer, separately. This provides a way for us to adjust and optimize the parameters of the absorber rapidly and precisely.
Fig. 3. The responses of the EM model and the circuit model for different layers. Comparison of curves obtained by EM simulation and ADS circuit simulation of one (a), two (b) and three (c) layers. The surface current distributions of the proposed PMA at (d) 1.6 GHz, (e) 7.3 GHz, (f) 15.7 GHz, respectively.
We achieve broadband absorbing effect by combining the three-layer ITO structure. From the surface current distributions, it can be concluded that the bottom layer of ITO mainly affects the absorption performance in the low frequency band, while the second layer mainly works at the middle frequency band and the four small ITO patches on the top layer controls the absorption at the high frequency band. Therefore, we adjust and optimize the size and resistance of each ITO layer to achieve our expected goals. When l1 increases, the resonance point of the first absorption band will move to lower frequencies as shown in Fig. 4(a). The absorption rate will be enhanced with the increase of R1, as shown in Fig. 4(b). The influence of the size and resistance of the second ITO square ring layer on the resonance point and absorption rate of the mid-frequency band is the same as the impact of the first ITO layer on the low-frequency band, which is verified in Figs. 4(c) and 4(d). On the top layer of the ITO, there is a blue shift of resonance point with the increase of l3 while the absorption is still enhanced as R3 increased as shown in Figs. 4(e) and (f). Although the increase of R1, R2 and R3 will increase the absorption efficiency in low, medium and high frequency bands respectively. If the resistance value is increased blindly, they will have a certain impact on the absorption efficiency of each other's frequency bands. In summary, we fine-tuned and optimized the size and resistance of each ITO layer and finally realized an ultra-wideband, ultra-efficient absorber with an absorption rate over 90% at 1-18 GHz. The paraments are chosen as l1 = 49.7 mm, l2 = 16.5 mm, l3 = 18 mm, R1 = 230 Ω/sq, R2 = 220 Ω/sq, R3 = 300 Ω/sq.
Fig. 4. The influence of parameters on absorbing efficiency and working frequency band. S11 at the low frequency band varying against (a) the size l1 and (b) the resistance R1 of the bottom square patch layer. The influence of the (c) width l2 and (d) resistance R2 of the middle square ring layer on S11 in the mid-frequency band. The reflection coefficient varies with the (e) side length l3 and (f) resistance value R3 of the top four ITO patches in the high frequency range. S11 varying against the thickness of (g) the bottom layer h1, (h) the middle layer h2 and (i) the top layer h3.
The thickness of the absorber is a quite important factor affecting the absorbing efficiency and working bandwidth. As shown in Figs. 4(g)-(i), the influence of the thickness of each layer on the absorbing is shown. It can be seen from Fig. 4(g), when h1 increases, the minimum operating frequency decreases, resulting in an increase of bandwidth, and absorption at low frequency is realized. As shown in Fig. 4(h), when h2 increases, the resonant frequency points will move to the low frequency and the absorption efficiency of the middle frequency band will decrease slightly. With the increase of h3, the absorbing effect of high frequency band changes obviously, and the absorbing efficiency decreases with the decrease of resonant frequency, as shown in Fig. 4(i). Considering the specific influence of the thickness of each layer, we optimized and adjusted the unit in the simulation, so as to achieve 1-18 GHz broadband efficient absorption while keeping the overall thickness as thin as possible. The final thickness parameters are h1 = h2 = 12.2 mm, h3 = 5.6 mm.” The total thickness of the absorber is 1/10 of the maximum working wavelength, approaching the theoretical Rozanov limit, which provides the lower bound of the thickness of any physical absorber [45].
To verify the characteristics of the proposed absorber, we fabricated a 6 × 6 units sample (with the size of 300 mm × 300 mm × 30 mm), as shown in Fig. 5(a). For the foam board, we used a heated foam cutting machine to make two foam boards with a size of 300 mm × 300 mm × 12.2 mm and one with the size of 300 mm × 300 mm × 5.6 mm. For the ITO layer, firstly, we coated the ITO on the surface of Polyethylene Terephthalate (PET) by magnetron sputtering technology, and then etched the designed square patch and ring pattern by photolithography technology. Finally, we glue the ITO to the corresponding foam layer to make the complete absorber. In the experiment, we connected two linear polarization standard gain broadband horn antennas to an AV3672B Vector Network Analyzer as the transmitter and receiver of EM waves. In order to eliminate the influence of environmental interference, we used the time domain gate function in the vector network analyzer.
Fig. 5. Experimental setup and the measured results. (a) The photography of far field test and the fabricated sample. (b) Comparison of absorption rate between test and simulation results. The far-field scattering pattern of the sample and the metal plate tested at (c) 2 GHz, (d) 8 GHz and (e) 16 GHz (representing low frequency, intermediate frequency and high frequency respectively).
The testing result is shown in the Fig. 5(b), and the absorption rate of measured sample is in good agreements with the simulation result. The slight deviation in the low frequency range was attributed to the finite size of the sample and the inevitable fabrication errors. We also tested the far-field scatter patterns of the PMA and the metal sheet with the same size at three points of low frequency of 2 GHz, intermediate frequency of 8 GHz and high frequency of 16 GHz, as shown in Figs. 5(c)-(e). It is obvious that compared with the metal plate, the main lobe will drop by at least 10 dB with the proposed meta-absorber, which can illustrate that the absorber we proposed essentially absorbs the incident EM waves rather than scattering them in other directions.
We also study the absorbing performance of the absorber at large incident angles. In the simulation, we set up a periodic array based on the unit and excited the oblique incident electromagnetic wave. Then, the far-field patterns are simulated and compared with the patterns of metallic board with the same size. The results obtained are monostatic. For the TM mode shown in Fig. 6(a), when the incident angle gradually increases in the xoz plane, the low-frequency absorption efficiency will be reduced, but the overall reflection remains stable. It can be seen from Fig. 6(b) that under the TE polarization mode, when the incident angle increases from 0° to 45° in the yoz plane, the absorption rate remains stable (greater than 90%), until the incident angle increases to 60°, the absorptivity dropped to 80%. We analyze the reason for this phenomenon is that the change of equivalent impedance is different in these two modes, which can be expressed by following equations [46].
Fig. 6. Characteristics of the proposed meta-absorbers under different incident angles and polarizations. Under (a) TM and (b) TE polarization modes, the absorption rate varying against the incident angles. (c) The change trend of absorption rate and equivalent impedance with the increase of incident angle under different polarization modes. (d) The absorption rate at three polarization angles.
Depicted in Fig. 6(c), under TM mode, the equivalent impedance will slightly decrease with the increase of the incident angle and the absorption rate will remain stable (over 90%). However, the equivalent impedance increases obviously, resulting in the relative mismatch with the air impedance, which leads to the phenomenon that the absorption rate decreases with the increase of incident angle under TE mode, but it is still greater than 80%. Due to the mirror symmetric structure, the proposed absorber is insensitive to different polarization angles, as shown in Fig. 6(d), which increases its practical application. In general, the PMA is characterized by large angle incidence and polarization insensitivity.
3. Conclusion
In this paper, we have achieved an ultra-broadband and high-efficiency planar metamaterial absorber based on multilayer ITO structure, with the absorption mechanism analyzed through the equivalent circuit and impedance matching theories. We also made an experimental sample to test the performance of the PMA, the experimental and simulation results are in good agreements with each other. The absorber we proposed also has the characteristics of light weight, full-polarization and angle insensitivity, all these properties make the PMA a high application prospect in the military stealth field.
Funding
Key Projects of Aviation Foundation (201918037002); Natural Science Foundation of Shaanxi Province (2019JQ-013); National Natural Science Foundation of China (61701572, 61871394, 61901512).
Disclosures
The authors declare no conflicts of interest.
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