A novel ultrathin and broadband microwave

A novel ultrathin and broadband microwave metamaterial absorber Bei-Yin Wang, Shao-Bin Liu, Bo-Rui Bian, Zhi-Wen Mao, Xiao-Chun Liu, Ben Ma, and Lin Chen Citation: Journal of Applied Physics 116, 094504 (2014); doi: 10.1063/1.4894824 View online: http://dx.doi.org/10.1063/1.4894824 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Size-efficient metamaterial absorber at low frequencies: Design, fabrication, and characterization J. Appl. Phys. 117, 243105 (2015); 10.1063/1.4923053 Bandwidth-enhanced polarization-insensitive microwave metamaterial absorber and its equivalent circuit model J. Appl. Phys. 115, 104503 (2014); 10.1063/1.4868577 Integrating non-planar metamaterials with magnetic absorbing materials to yield ultra-broadband microwave hybrid absorbers Appl. Phys. Lett. 104, 022903 (2014); 10.1063/1.4862262 Novel triple-band polarization-insensitive wide-angle ultra-thin microwave metamaterial absorber J. Appl. Phys. 114, 194511 (2013); 10.1063/1.4832785 Microwave diode switchable metamaterial reflector/absorber Appl. Phys. Lett. 103, 031902 (2013); 10.1063/1.4813750

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JOURNAL OF APPLIED PHYSICS 116, 094504 (2014)

A novel ultrathin and broadband microwave metamaterial absorber Bei-Yin Wang,1,a) Shao-Bin Liu,1,2,a) Bo-Rui Bian,1 Zhi-Wen Mao,1 Xiao-Chun Liu,2 Ben Ma,1 and Lin Chen1

1 Key Laboratory of Radar Imaging and Microwave Photonics, Ministry of Education, College of Electronic and Information Engineering, Nanjing University of Aeronautics and Astronautics, PO Box 244, No. 29, Yudao St., Nanjing 210016, China 2 Key Laboratory in Aviation Science and Technology of High-performance Electromagnetic Window, Jinan, 250000, China

(Received 5 June 2014; accepted 26 August 2014; published online 5 September 2014) In this paper, the design, simulation, fabrication, and measurement of an ultrathin and broadband microwave metamaterial absorber (MMA) based on a double-layer structure are presented. Compared with the prior work, our structure is simple and polarization insensitive. The broadband MMA presents good absorption above 90% between 8:85 GHz and 14:17 GHz, with a full width at half maximum (FWHM) absorption bandwidth of 6:77 GHz and a relative FWHM absorption bandwidth of 57:3%. Moreover, the structure has a thickness of 1:60 mm (only k=20 at the lowest frequencies). The experimental results show excellent absorption rates which are in good correspondence with the simulated results. The broadband absorber is promising candidates as absorbing elements in scientific and technical applications because of its broadband absorption and C 2014 Author(s). All article content, except where otherwise noted, is polarization insensitive. V licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4894824] I. INTRODUCTION

Metamaterial absorber (MMA), which is a kind of electromagnetic absorber consisting of sub-wavelength metamaterial resonators and can exhibit near-perfect absorption characteristics, has been widely investigated in recently years.1 The nearly perfect absorption component is one of the fundamental building blocks in many potential applications, including antennas,2,3 radar imaging,4,5 thermal emission,6–10 and solar cells.11–15 Recently, the metamaterial is also fashioned to create perfect absorbers by manipulating the resonances in e and l.16–25 The periodic metal resonator structure provides an electric response to tune the electric permittivity of metamaterial eðxÞ, and the magnetic response is created by combining the resonator, and the ground plane tunes the magnetic permeability of metamaterial lðxÞ. Once the MMA satpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi isfies the condition of lðxÞ=eðxÞ ¼ ZðxÞ ¼ 1, the effective impedance of MMA will match the free space impedance Z0 and the reflection will be minimized. At the same time, if the MMA is high loss, which means the transmission is low, the nearly-unit absorption rate is obtained at certain frequency. However, the strong electric and magnetic resonance simultaneously will of course result in a narrow absorption bandwidth, which may impede its application in practice. Comparing with conventional materials, the thickness of metamaterial absorbers is much smaller. The design of metamaterial absorbers is more flexible and efficient. The bandwidth of a metamaterial absorber is one of the important aspects that may affect many applications. So far, most designs of metamaterial absorbers operate at a specific narrow frequency range. Dual-band26–28 and multi-band29,30 a)

Author to whom correspondence should be addressed. Electronic mail: [email protected]. Tel.: þ86-15996253890. Fax: þ86-2584892848.

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metamaterial absorbers have been demonstrated with distinct narrow absorption frequencies. More recently, broadband absorbers have also been investigated by utilizing the concept of multiple resonances31–33 and gradual impedance matching34,35 as well as the anti-reflection theory.36,37 In this paper, a microwave ultrathin and broadband metamaterial absorber with the special resonant structure has been designed and fabricated. The broadband MMA is composed of two dual-band sub-cells. The dual-band sub-cells presented us with a good way to realize broadband MMA. Numerical results show that the broadband MMA presents nearly perfect absorption above 90% with absorption ranging from 8:85 GHz to 14:17 GHz. In addition, the analysis of current and field distribution is performed to better understand the resonant mechanism. The experimental results show excellent absorption rates which agree well with the numerical simulation. II. UNIT CELL DESIGN

In this paper, we present a broadband MMA based on a dual-band MMA. The dual-band MMA consists of a metallic pentagon patch etched on lossy substrate realize insensitive polarization characteristics and two different resonant frequencies. The schematic of the dual-band MMA is presented in Fig. 1(a). The surface of the dual-band MMA is covered by a metallic pentagon patch and the bottom is a metallic film. All metals of the absorber are made by copper, which has a frequency independent conductivity r ¼ 5:8 107 S=m. epoxy glass cloth laminate (FR–4) performs as the dielectric spaces with the relative permittivity and loss tangent of 4:3 and 0:025, respectively. The geometric parameters are shown as follows: L ¼ 9:0 mm, A ¼ 7:0 mm, h ¼ 1:2 mm, and S ¼ 1:0 mm. The absorptive efficiency characterized is defined as A ¼ 1 TðxÞ RðxÞ ¼ 1 jS21 j2 jS11 j2 . A, jS11 j2 , and

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FIG. 1. (a) Perspective view of the dual-band MMA unit cell. (b) Simulated absorbance for the dual-band MMA. TE polarization (solid black) and TM polarization (red dashed). (c) Absorption with the size of S changed from 1:0 mm to 2:5 mm. The values of other parameters are unchanged.

jS21 j2 are the absorbance, reflectivity, and transmissivity, respectively. Because of the copper backplate, the S21 (transmission) of this system is zero. Thus, absorbance can be calculated using A ¼ 1 RðxÞ ¼ 1 jS11 j2 . Absorbance can be maximized by minimization of reflection from the top surface of the proposed structure. All the geometrical dimensions are chosen by using parametric variation so that the maximum absorptive efficiency is realized. A full wave electromagnetic (EM) simulation was performed to get the reflection parameter S11 based on the commercial program, CST MICROWAVE STUDIO 2010. The periodic boundary conditions were applied to the x and y directions and the absorbing boundary conditions were applied to the z direction. The plane wave is normally incident to the dual-band MMA along the z-axis with the electrical field y-polarized and magnetic field x-polarized. The simulation result of the absorption curve is depicted in Fig. 1(b). There are two absorption peaks at f2 ¼ 8:54 GHz and

f2 ¼ 11:29 GHz with absorption rates of 98:50% and 99:05%, respectively. It can be see that the dual-band MMA is polarization-insensitive. Fig. 1(c) shows the effect of parameter S on the absorption spectrum. In Fig. 1(c), when changing the parameter S from 1:0 mm to 2:5 mm, the two absorption peaks close to each other and finally become one peak. When the parameter S is equal to 2:0 mm, the two peaks form a continuous absorption spectrum between 8:5 GHz and 10:5 GHz with absorptions all above 90%. Next, we make some change on the structure of the dual-band MMA. Between the dielectric spaces and metallic backplate, we add an air-layer with thickness t. The schematic is presented in Fig. 2(a). The geometric parameters are as follows: L ¼ 9:0 mm, A ¼ 7:0 mm, h ¼ 0:4 mm, S ¼ 2:0 mm, and t ¼ 0:8 mm. Fig. 2 shows the absorption with the size of h changed from 0:1 mm to 0:4 mm. The absorption peaks shift to the low frequency with the parameter h increased. The dual-band MMA with air-layer is also a

FIG. 2. (a) Perspective view of the dual-band MMA adding air-layer. The geometric parameters are as follows: in millimeters: L ¼ 9:0 mm, A ¼ 7:0 mm, S ¼ 2:0 mm, h ¼ 0:4 mm, t ¼ 0:8 mm. (b) Absorption with the size of h changed from 0:1 mm to 0:4 mm.


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FIG. 3. Unit cell of the MMA: (a) the perfective view, (b) side view, (c) upper layer view, (d) lower layer view, and (e) photograph of fabricated MMA sample.

III. SIMULATION AND ANALYSIS

and P4 ) in the picture. We can see that the relative position of the four peaks is sensitive to the parameter S1 . When changing the parameter S1 from 1:6 mm mm to 2:2 mm, P1 and P2 come close to each other. P3 and P4 also have the same result. The absorption bandwidth becomes narrow with the increase in parameter S1 . Fig. 5(b) shows the absorptions with the parameter S2 changing from 2:1 mm to 2:7 mm. From the pictures, the relative position of the absorption peaks is also sensitive to the parameter S2 . But, the absorption bandwidth is essentially unchanged. Next, we discuss the effect of thickness h1 and h2 on the absorption curves, respectively. Fig. 6(a) shows the absorbance with the parameter h1 changing from 0:1 mm to 0:4 mm. From the picture, it is obviously seen that the absorption frequencies drift to the low frequency with the increase in h1 . The absorption frequencies at the high frequency are more sensitive to h1 . In Fig. 6(b), when the parameter h2 changes from 0:2 mm to 0:5 mm, the influence is more prominent at the low frequency. To compare the two pictures, we can see that the absorption frequencies at the low frequency are induced by the metal pentagon on the

As the simulated absorption spectrum for the broadband MA shown in Fig. 4, four absorption peaks of resonance are observed at f1 ¼ 9:14 GHz, f2 ¼ 10:46 GHz, f3 ¼ 11:86 GHz, and f4 ¼ 13:67 GHz, with nearly perfect absorption rates at these frequencies: 98:94%, 99:46%, 99:92%, and 99:59%, respectively. The four peaks are formed by the metallic pentagon patch etched on substrates, which to some extent form a continuous absorption spectrum between 8:85 GHz and 14:17 GHz with absorption all above 90%. The full width at half maximum (FWHM) absorption bandwidth is 6:77 GHz (8:43 15:2 GHz), with the relative FWHM absorption bandwidth of 57:3%. It indicates that the impedance of proposed MMA can be tuned to approximately match the free space in our interested frequency range. Here, we discuss the effect of parameters S1 and S2 on the absorption curves, respectively. First, in Fig. 5(a), we change the parameter S1 from 1:6 mm to 2:2 mm with other parameters fixed. There are four group peaks (P1 , P2 , P3 ,

FIG. 4. Simulated absorbance of the absorber with double-layer configuration. TE polarization (solid black) and TM polarization (red dashed).

broadband absorber. From the above analysis, the dual-band MMA with a metallic pentagon patch covered can easily form a broad-band polarization-insensitive metamaterial absorber. Compare the two structures above, we noticed that the structure of the two absorbers are similar, which can be easily stacked. With the geometric parameters changed, the two absorbers can form a continuous and wide absorption spectrum. Therefore, we designed a metamaterial absorber with double-layer structure. The schematic of the proposed absorber is presented in Fig. 3, which is composed of two dual-band sub-cells. The surface of each layer is covered by a metallic pentagon patch. The bottom of second layer is a metallic film. The layout of the unit cell structure is optimized and the geometric parameters are shown as follows: L ¼ 9:0 mm, A ¼ 7:0 mm, h1 ¼ 0:4 mm, h2 ¼ 0:4 mm, t ¼ 0:8 mm, S1 ¼ 2 mm, and S2 ¼ 2:5 mm. The complete MMA is the periodic extension of the unit cell in both x and y directions, as given in Fig. 3(e).


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FIG. 5. Simulated absorbance of the absorber with double-layer configuration. (a) The parameter S1 changes from 1:6 mm to 2:2 mm. The other parameters are fixed. (b) The parameter S2 changes from 2:1 mm to 2:7 mm. The other parameters are fixed.

FIG. 6. (a) Absorption with the size of h1 changed from 0:1 mm to 0:4 mm. The values of other parameters are unchanged. (b) Absorption with the size of h2 changed from 0:2 mm to 0:5 mm. The values of other parameters are unchanged.

surface of lower layer, and the absorption frequencies at the high frequency are induced by the metal pentagon on the surface of upper layer. Furthermore, we also discuss the effect of spacing t on the absorbance. Fig. 7 shows the absorption with the parameter t changing from 0:75 mm to 0:9 mm. The absorption frequencies drift to low frequency with the increase in t. Based on above analysis, the performance of absorption is sensitive to the geometry and material parameters. We could modulate the absorption performance by changing these parameters. To better understand the resonant mechanism and gain physical insight into the origin of the four absorption peaks, we have studied the surface current analysis and fielddistribution analysis, as shown in Figs. 8 and 9. The surface current and E-field distributions are obtained by CST. In these figures, the arrow indicates the direction of flow while color represents the intensity. From the view of absorber theory, the aim is to get the effective permittivity (e) and

FIG. 7. Absorption with the size of t changed from 0:75 mm to 0:90 mm. The values of other parameters are unchanged.


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FIG. 8. Simulated surface current distribution for the broadband MMA at resonant frequencies.

permeability (l) matched, so that the impedance of the absorber matches well to the free space. Both the electric response and magnetic response should be achieved. In Fig. 8, it can be clearly seen that counter-circulating current on the metallic pentagon patch is strongly associated with an electric response, which contributes to the effective e. At f1 ¼ 9:14 GHz and f2 ¼ 10:46 GHz, to compare the surface current distribution between each layer, as shown in Figs.

8(a) and 8(b), we can see that the current in the whole ground is anti-parallel to those exhibited in the lower layer, and the current loop is driven by the incident H-field, resulting in a strong magnetic resonance. As the electric response and magnetic response appear simultaneously at the resonant frequency f1 ¼ 9:14 GHz and f2 ¼ 10:46 GHz, the MMA can absorb the incident electric field and magnetic field. But the current in the upper layer is not anti-parallel to those


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FIG. 9. E-field distributions for the broadband MMA at resonant frequencies.

exhibited in the lower layer. The strong magnetic resonance does not exist. Therefore, the absorption frequencies f1 ¼ 9:14 GHz and f2 ¼ 10:46 GHz are induced by the electromagnetic resonance of the lower layer. In Figs. 9(a) and 9(b), the E-field is localized within the edge of the lower layer. Most of the power is lost due to the high confinement of energy. In the same way, at f3 ¼ 11:86 GHz and f4 ¼ 13:67 GHz, to compare the surface current distribution between each layer, as shown in Figs. 8(c) and 8(d), it is able to determine that the strong magnetic resonance can exist in each layer. So, the absorption frequencies f3 ¼ 11:86 GHz and f4 ¼ 13:67 GHz are induced by the electromagnetic resonance of the upper layer. The effect of the lower layer is to strengthen the electromagnetic resonance. In Figs. 9(c) and 9(d), the E-field is mainly distributed in the edge of the upper layer and the lower layer, which enables the electric energy to be mainly confined in those regions.

IV. EXPERIMENTS

To experimentally investigate the absorptive properties of the proposed microwave ultrathin and broadband MMA, we have fabricated a MMA sample, as shown in Figs. 3 and 10(a). The unit cells are fabricated using the print circuit board technique on the FR4 substrate, with total footprint of 270 270 mm2 and thickness of 1:60 mm as that used in numerical simulations. Fig. 10 illustrates the fabricated sample and the experimental simulation setup in which two pairs of horn antennas are connecting to the vector network analyzer (Agilent N5245A). The horn antennas have a voltage standing wave ration (VSWR) of