APPLIED PHYSICS LETTERS 93, 261115 共2008兲

Enhancing and tuning absorption properties of microwave absorbing materials using metamaterials Yanhong Zou, Leyong Jiang, Shuangchun Wen,a兲 Weixing Shu, Yongjun Qing, Zhixiang Tang, Hailu Luo, and Dianyuan Fan Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education, School of Computer and Communication, Hunan University, Changsha 410082, People’s Republic of China

共Received 30 July 2008; accepted 10 December 2008; published online 31 December 2008兲 We proposed and demonstrated a scheme to enhance and tune absorption properties of conventional microwave absorbing materials 共MAMs兲 by metamaterials 共MMs兲. By covering a MAM, say, carbonyl iron powder coating, with MMs composed of split ring resonators 共SRRs兲 and wires, we show both by experiments and by simulations that the maximum reflection loss 共RL兲 is increased significantly and the frequency region for absorption is shifted to lower frequency. The frequency region in which the maximum RL is less than ⫺10 dB shifts from 5–7 to 4.2–6.2 GHz for perpendicular polarization electromagnetic waves and to 4–9 GHz for parallel polarization waves. Simulation results reveal that the magnetic resonance obtained by SRRs and the electric resonance obtained by copper wires are the main factors in enhancing and tuning microwave absorption properties. © 2008 American Institute of Physics. 关DOI: 10.1063/1.3062854兴 Microwave absorbing material 共MAM兲 is a kind of functional material that can absorb electromagnetic wave effectively and convert electromagnetic energy into heat or make electromagnetic wave disappear by interference.1 MAMs have been widely applied to minimize electromagnetic reflection from large equipments such as airplanes, steamboats, tanks, and microwave darkrooms. However, whether traditional MAMs, such as carbon black and ferrite, or novel nanomaterials, they still have some disadvantages such as high density, narrow band, and low absorptivity.2 Therefore, the demands to develop more economical MAMs with low density, wide and adjustable frequency region for the microwave absorption, and large reflection loss 共RL兲 are ever increasing. Recently, metamaterials 共MMs兲, a kind of artificially structured materials composed of copper wires and split ring resonators 共SRRs兲, have attracted much attention on account of unique electromagnetic properties and their tunability. It is well known that the effective permittivity, caused by the electric resonance,3 and the effective permeability, caused by the magnetic resonance,4 of a MM can be tuned down to negative simultaneously in some frequency ranges, giving birth to the vibrant field of negative refraction.5–7 Absorption or loss is inevitable for a MM and is associated with the effective permittivity and permeability, so it can also be tuned simply by adjusting the shape, structure, dimension, or arrangement of the constitutive elements of MM.8–10 While people generally want to reduce absorption in MM, such as a subwavelength lens,5,11,12 there are other applications such as stealth technology or other types of cloaking devices where this is actually desired.13 In this letter, we propose a combined MAM, which is constructed by covering a single layer of MM on the surface of a conventional MAM, as shown in Fig. 1. The combined MAM has a higher absorptivity compared to the conventional MAM, and its absorption spectrum can be tunable in some frequency ranges. a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

0003-6951/2008/93共26兲/261115/3/$23.00

To demonstrate our scheme, we first fabricated a printedcircuit board with SRRs on one side and copper wires on the other according to Ref. 7 关see Fig. 1共b兲兴. The board material is FR-4 of a thickness of 0.35 mm, the SRRs and wire strips are 0.035-mm-thick copper. The unit cell, dimension, and arrangement of MM are shown in Fig. 1共a兲. We then chose carbonyl iron powder 共CIP兲 as the MAM and epoxy resin as agglomerant. After they have achieved appropriate viscosity by using acetone, we brushed them on an aluminum basal plate 共180⫻ 180⫻ 2 mm3兲 with a thickness of 2.5 mm, as shown in Fig. 1共c兲. Finally, the MM layer was covered on the surface with carbonyl iron powder coating 共CIPC兲 to produce a combined MAM, as shown in Fig. 1. We have measured the microwave absorption properties of the CIP, the MM layer, and the combined MAM, respectively, to identify the role of MM in enhancing and tuning the microwave absorption properties. The frequency dependence of the complex permittivity ␧ and magnetic permeability ␮ of the CIP, which were measured in the frequency range of 2–18 GHz using a transmission/reflection network analyzer 共Agilent 8720ET兲, is shown in Fig. 2共a兲 共black lines, left axis兲. The dielectric loss angular tangent value 共tan ␦␧ = ␧⬙ / ␧⬘, where ␧⬘ and ␧⬙ are the real and imaginary parts of ␧, respectively兲 and the magnetic loss angular tangent value 共tan ␦␮ = ␮⬙ / ␮⬘, where ␮⬘ and ␮⬙ are the real and imaginary

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FIG. 1. 共Color online兲 The proposed scheme for enhancing and tuning the microwave absorption property by covering a single MM layer 共b兲 on CIPC 共c兲 to produce a combined MAM. The left inset 共a兲 is the unit cell of MM, where c = 0.25 mm, w = 2.62 mm, d = 0.30 mm, and g = 0.46 mm.

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FIG. 2. 共Color online兲 Microwave absorption property of CIP. 共a兲 The relative complex permittivity ␧ and permeability ␮, dielectric loss angular tangent 共tan ␦␧兲, and the magnetic loss angular tangent 共tan ␦␮兲 vs frequency. 共b兲 The complex theoretical RL of CIP vs frequency for different thicknesses.

parts of ␮, respectively兲 are also shown 共blue lines, right axis兲. We see that the value of tan ␦␮ is larger than that of tan ␦␧ in the whole frequency range, suggesting that CIP is a magnetic loss MAM. For comparison, we have calculated the theoretical RL of CIPC under certain thickness according to ␧ and ␮ in Fig. 2共a兲. An aluminum plate with a thickness of 2 mm was put under CIPC, and the polarized incident wave, with electric field in the y direction and magnetic field in the x direction, is satisfied by applying perfect electric and magnetic boundary conditions, respectively. The results are shown in Fig. 2共b兲. We can see that as the thickness of CIPC increases, the matching frequency tends to shift to lower frequency, the RL at the matching frequency first increases after it reaches the maximum 共here, it is ⫺49 dB for the thickness of 1.5 mm兲, and it tends to become less. Therefore, theoretically, RL at low frequency region cannot be enhanced only by increasing the thickness of CIPC. At the same time, we also experimentally demonstrated the microwave absorption property of CIPC. The RL of CIPC with a thickness of 2.5 mm was measured in the frequency range of 2–18 GHz. Figure 3共a兲 shows the experimental and simulated RLs of CIPC. Both results agree very well over almost whole frequency range studied and show that the microwave absorption peaks at approximately 6 GHz. The simulated maximum RL is slightly larger than the experimental one probably due to the tolerance in the fabrication and assembly. The microwave absorption property of MM layer is indicated in Fig. 3共b兲. The electric field E and the magnetic field H are parallel to copper wires and SRR plane, respec0 RL (dB)

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FIG. 3. 共Color online兲 The experimental 共black and red lines兲 and simulated 共blue lines兲 RLs of CIPC 共a兲, MM layer 共b兲, and the combination of CIPC and MM 关共c兲 and 共d兲兴.

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Appl. Phys. Lett. 93, 261115 共2008兲

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FIG. 4. 共Color online兲 RL vs frequency for 共a兲 different MM structure, 共b兲 different length of copper wires, 共c兲 different width of copper wires, and 共d兲 different span between copper wires.

tively, and the wave vector k is parallel to the SRR axis. The simulated and experimental RLs of MM are all close to zero, illuminating that the MM layer does not absorb microwave in the frequency range of 2–18 GHz in this case. Now let us investigate the microwave absorption property of the combined MAM. Figure 3共c兲 shows the influence of MM layer on microwave absorption property of CIPC for incident waves with the electric field E normal to copper wires. The simulated RL of the combined MAM agrees well with the experimental one, but both are close to that of CIPC, suggesting that the MM layer does not increase the RL of CIPC significantly in this case. Figure 3共d兲 illustrates the results when the electric field E is parallel to copper wires. The overall qualitative agreement between experimental and simulated results is good; the remaining discrepancies are likely due to fabrication tolerances in the experiment. We see a notable increase in RL from ⫺14 to ⫺20 dB by using a MM layer. In addition, the frequency region in which the maximum RL is less than ⫺10 dB has been shifted to a significant lower frequency, from 5–7 to 4.2–6.2 GHz. The physical origin for the enhancement and tunability of microwave absorption property of the combined MAM can be understood from the structure and characteristics of MM. In Figs. 3共c兲 and 3共d兲, the magnetic fields H are both parallel to the SRRs plane, so the magnetic resonance cannot be obtained. However, the electric field E is parallel to the copper wires in Fig. 3共d兲, so the electric resonance is realized. Consequently, the RL of the combined MAM is much larger than that of CIPC. To further verify the reasonableness of this analysis, we have calculated the RLs for four combination cases: the CIPC only, the CIPC covered by a layer of SRRs, the CIPC covered by a layer of copper wires, and the CIPC covered by a layer of MM, with the results shown in Fig. 4共a兲. One can see that the effect of SRRs on the absorption is very weak because the magnetic resonance of SRRs cannot be realized. Therefore, the electric resonance obtained by copper wires is the main factor in increasing the RL for perpendicular polarization, i.e., the electric field E is parallel to copper wires. So here, we focused on the influence of copper wires on the microwave absorption property, as demonstrated in Figs. 4共b兲–4共d兲. Figure 4共b兲 shows that as the length of copper wires increases, the matching frequency tends to shift to lower frequency and the maximum RL be-

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comes less at the matching frequency. Figures 4共c兲 and 4共d兲 show that the influence of the width of copper wires and the span between two neighboring copper wires on RL are not significant. The underlying mechanism leading to resonant frequencies of the combined MAM can also be illuminated using the asymptotic models in Ref. 14 to some extent. From the above analysis, we speculate that the magnetic resonance caused by SRRs and the electric resonance obtained by copper wires will influence microwave absorption property together for different polarization of electromagnetic wave. Figure 5 illustrates the influence of polarization on microwave absorption property of the combined MAM. For parallel polarization the RL of the combination shows two absorption peaks at 4.8 and 7 GHz, respectively, and the frequency region in which the maximum RL is more than ⫺10 dB is 4–9 GHz, which is much better than that of CIPC. For perpendicular polarization, the RL of the combination has an almost duple absorption peak, and its corresponding frequency is shifted to a lower frequency compared to CIPC. This further demonstrates that we can use MMs to enhance and tune the microwave absorbing property of conventional

MAMs by adjusting the constitutive elements of MM. In conclusion, we have shown by experiments and simulations that the microwave absorption property of a conventional MAM can be enhanced and the corresponding frequency region can be tuned by using MM layers. Theoretical analysis reveals that the magnetic resonance obtained by SRRs and electric resonance obtained by copper wires are the main contributions to the enhanced absorption. Because the electromagnetic properties of MMs are engineerable, one can always design a proper MM to adjust the absorption properties of different kinds of conventional MAM. This work is supported by the National Nature Science Foundation of China 共Grant Nos. 10674045, 10804029, and 50802027兲. 1

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