Perfect metamaterial absorber with polarization and incident angleindependencies based on ring and cross-wire resonators for shieldingand a sensor application

Cumali Sabah a,n, Furkan Dincer b,d, Muharrem Karaaslan c,d, Emin Unal c,d,Oguzhan Akgol c,d, Ekrem Demirel e

a Middle East Technical University—Northern Cyprus Campus, Department of Electrical and Electronics Engineering, Kalkanli, Guzelyurt,TRNC/Mersin 10, Turkeyb Mustafa Kemal University, Department of Computer Engineering, Iskenderun, Hatay 31200, Turkeyc Mustafa Kemal University, Department of Electrical and Electronics Engineering, Iskenderun, Hatay 31200, Turkeyd Metamaterials and Photonics Research Group, Mustafa Kemal University, Iskenderun, Hatay 31200, Turkeye TUBITAK—UME, 41470 Gebze, Kocaeli, Turkey

a r t i c l e i n f o

Article history:Received 15 November 2013Received in revised form9 January 2014Accepted 14 February 2014Available online 25 February 2014

Keywords:MetamaterialAbsorberMicrowaveSensor

a b s t r a c t

We report the design, characterization and experimental verification of a perfect metamaterial absorber(MA) based on rings and cross wires (RCWs) configurations that operate in the microwave regime. Thesuggested MA provides perfect absorption with incident angle and polarization independencies whichcan be used for various shielding applications. Maximum absorption rate is 99:9% at 2:76 GHz forsimulation and 99:4% at 2:82 GHz for experiment, respectively. The experimental results of thefabricated prototype are in good agreement with the numerical simulations. We also present a numericalanalysis in order to explain physical interpretation of MA mechanism in detail. Moreover, a sensorapplication of the proposed MA is introduced to show additional feature of the model. As a result,proposed MA enables myriad potential applications in S band radar and medical technologies.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Metamaterials (MTMs) have unusual electromagnetic (EM)properties, such as artificial magnetism and negative refraction.They still draw interest of scientists due to practical importanceowing to varieties of the potential application areas [1–10]. Inaddition, such materials are manmade and can be artificiallyfabricated at the desired regimes of the EM spectrum from MHz[11], GHz [12], sub-THz [13], THz [14], sub-PHz [15], near-IR [16] tothe near optical frequency region [17]. Due to these exoticpotential behaviors in EM spectrum, many research groups havestudied the EM response of MTMs in order to understand theirfundamental features and investigated their variations to be usedin many applications such as super lens [18], sensing [19], cloaking[20], waveguide [21], filter [22], absorber [23] and so on [24–28].These types of studies can often be found in literature.

In this sense, the concept of metamaterial absorber (MA) hasachieved significant progress thanks to the development of the

MTM application technologies in absorber and absorber typedevices. In fact, there were several important attempts in theliterature to manufacture and use MA in microwave frequencies[29–38]. Bilotti et al. [29] worked on a resonant microwaveabsorber made of a proper array of split ring resonators (SRRs).Landy et al. [30] investigated a MA to achieve near unity absorp-tion. The proposed MA consists of two resonators coupled sepa-rately with the components of incident wave so as to hold allincident radiation within a single unit cell layer. Wang et al. [31]studied a resonant microwave absorber made of chiral MTM bothnumerically and experimentally. Zhu et al. [32] proposed apolarization insensitive microwave absorber study which includesdesign, fabrication and measurements. The proposed MA exhibitshigh microwave absorption with respect to different polarizedincident waves. In another study, [33] they also proposed amodulation of EM wave polarization by using tunable MTMreflector/absorber. In addition, Sun et al. [34] introduced anabsorber which consists of multilayer SRRs with destructiveinterference mechanism and an extremely broad frequency bandproperty. Lee and Lim [35] produced an enhanced bandwidthmicrowave absorber with a double resonant MTM. Huang et al.[36] introduced a MA with snowflake-shaped resonators. Lee et al.

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Optics Communications

http://dx.doi.org/10.1016/j.optcom.2014.02.0360030-4018 & 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Fax: þ90 392 661 2999.E-mail address: sabah@metu.edu.tr (C. Sabah).

Optics Communications 322 (2014) 137–142

[37] investigated ultra-thin hexagonal microwave MA for arbitrarypolarized EM wave. Also, Cheng and Yang [38] proposed a MAmade up of two resonators and a metal wire to realize strongabsorption in the microwave range. The data are tabulated andgiven in Table 1 to give a better visualization.

Unlike the others, in this study, first we took into account andevaluated MA studies in the literature. After that, we designed anew perfect MA that shows polarization and incident angleindependencies with perfect absorptivity in the microwave fre-quencies. The simulated and measured results are compared anddiscussed. In addition, advantages of the suggested MA model arecompared and evaluated in detail. The results show that the MAhas perfect absorption (99.9%) around frequency of 2.76 GHZ.Moreover, electric field and surface current distributions areinvestigated to verify the physical mechanism of the proposed MA.

2. Simulation and experimental studies

The suggested MA design consists of the rings and cross wires(RCWs)-shaped inclusions and a metallic layer. The resonator onthe top and background metallic layer on the bottom layersubstrate are assigned as copper sheet with electrical conductivityof 5:8� 107 S=m and thickness of 0.036 mm. FR-4 is selected as asubstrate between them due to its low cost fabrication and lowdielectric loss in the microwave region. FR-4 is a low-cost printedcircuit board material, manufactured from fiberglass clothembedded in epoxy resin. The “FR” in FR-4 stands for flameresistant. The thickness, loss tangent, and relative permittivity ofFR-4 are 1.6 mm, 0.02, and 4.2, correspondingly. The top resonator,bottom metallic plate and FR-4 substrate constitute the MA. Fig. 1shows the proposed MA structure. The parameters of dimensionsof the RCWs-shaped inclusion are presented in Fig. 1(a). Aftersimulations, the MA is manufactured as shown in Fig. 1(b). Thesample is fabricated with a dimension of 5�5 unit cells by usingconventional print circuit board technique. The overall size of oursample is 70�55 mm2.

The simulation of the periodic structure was performed with acommercial full-wave EM solver (CST Microwave Studio) based onfinite integration technique. The boundary conditions are periodicallyassigned with floquet port in the simulation. With the help of theperiodic boundary conditions and floquet port, simulation of theabsorber is performed. In addition, the measurement of S-parametersis achieved by using R&S ZVL6 Vector Network. Analyzer (VNA) andtwo horn antennasThe VNA supplies microwaves in the range of 1–6 GHz through two horn antennas. Transmitter horn antenna andreceiving horn antenna make small angle with the normal of thesurface of the structure. Fig. 2 shows some pictures from theexperiment. The measurements are realized in two steps. First, theplate microwave absorber was placed between the antennas toprevent cross coupling. Since the metal plate provides full reflection,a thick slab of copper was placed first for calibration purposes (Fig. 2(left-side)). As a second step, metamaterial absorber structure wasplaced to the same position of copper plate to measure the reflectionfrom metamaterial absorber structure (Fig. 2 (right-side). The mea-surement results of metamaterial absorber structure is calibrated withrespect to metallic plate. The schematic view of the experiment isalready shown in Fig. 2 [42]. The similar measurement setup can alsobe seen in Ref. 42 too.

3. Theoretical analysis and physical interpretation of theproposed MA mechanism

The absorption value with respect to frequency is defined asAðωÞ ¼ 1�RðωÞ�TðωÞ, where AðωÞ; R(ω) and T(ω) represent the

absorption, reflection and transmission of the system, respectively.Absorption AðωÞ value can be maximized by minimizing bothreflection RðωÞ ¼ jS11j2 and transmission TðωÞ ¼ jS21j2 at a certainfrequency range. However, absorption is generally defined asAðωÞ ¼ 1�RðωÞ due to the presence of the metallic ground plane,hence, the transmission has to be zero across the entire domain(TðωÞ-0). In other words, transmission value reduced down tozero to realize perfect MA [36,39].

It is well known that MTMs can be characterized by effectivemedium parameters. Reflectivity can be minimized (near-zero)when the effective permittivity εðωÞ and permeability μðωÞ havebeen simultaneously equal [35]. Therefore, it is possible to absorbthe penetrating wave by properly tuning εðωÞ and μðωÞ. A perfectMA can be achieved by varying these effective properties [30].The reflection and transmission values decay at a desired fre-quency range in an absorber due to the impedance matching andthe metallic background surface [40]. In the maximum absorptioncondition, the effective impedance of the overall system exactlymatches with the free space impedance Z(ω)¼Z0(ω) and thus thereflection decays [35,36].

Furthermore, we present a numerical analysis in order toexplain physical interpretation of MAs mechanism. Therefore, thesuggested model is analyzed with several material combinationsfor the FR4-dielectric (lossless and lossy) and for the metallic parts(copper and perfect electric conductor (PEC). Fig. 3 shows theresults of the simulation. It can be seen that lossy material playsleading role on absorption characteristic. This phenomenon can bemore understandable from a sensor application of the proposedmodel (which is given in MEP_L_fig10 Fig. 10). Lossy dielectricmaterial exactly absorbs the incident EM wave. Also, metallic partsaffect absorption characteristic and the difference between copperand PEC is caused by the presence of metallic loss [41].

4. Results and analysis

After designing the MA geometry, numerical calculations arerealized to obtain absorption properties of the system. To verify

Table 1Some MA studies in literature.

Landy et al. [30] 88% max. absorption at 11:5 GHz

Wang et al. [31] 98% max. absorption at 8 GHzZhu et al. [32] 97% max. absorption at 10:14 GHzZhu et al. [33] 499% max. absorption at 3:36 GHzSun et al. [34] 499% max. absorption at 15:2 GHzLee and Lim [35] 99% max. absorption at 9:8 GHzHuang et al. [36] 99:5% max. absorption at 11:28 GHzLee et al. [37] 499% max. absorption at 11:3 GHzCheng and Yang [38] 99:9% max. absorption at 10:4 GHz

Fig. 1. (a) Represents dimensions of the structure (a1¼2 mm, a2¼8 mm,a3¼7 mm, a4¼20 mm, a5¼1 mm, a6¼10 mm, a7¼3.5 mm). (b) picture of thefabricated sample.

C. Sabah et al. / Optics Communications 322 (2014) 137–142138

numerical and experimental studies, proposed MA is fabricatedand complex S-parameters are measured by VNA and two hornantennas, as mentioned before. Obtained numerical and experi-mental results are compared and evaluated to verify the charac-teristics of the MA. The measurement results show goodagreement with the simulation results. Proposed MA consists ofRCW-shaped inclusions as a resonator and a metal plate for zerotransmission with FR4 substrate between them. Results show thatthe proposed structure can be used for perfect absorption applica-tions. Numerical and experimental results are proved that theproposed model is a very good candidate for perfect MAs as shownin Figs. 4 and 5. Maximum absorption rate is observed approxi-mately 99.9% at 2.76 GHz for simulation and 99.4% at 2.82 GHz forexperiment, respectively. Note that the differences between theexperimental and simulation data are imputed by fabricationtolerances and dielectric dispersion of the substrate. The misalign-ment during the experiment may also be considered as a newsource of the error. The accuracy of the measurements can beexplained by the good agreement between the simulation andexperimental results.

Furthermore, fractional bandwidth (FBW) calculations of theresonance region are investigated to decide the qualification of theproposed MA. It is well known that the bandwidth of theresonance region is a crucially important factor in numerousapplications. Hence, the FBW which represents bandwidth of theMA is evaluated at maximum absorption frequency by dividing thehalf power bandwidth and the center frequency. In other words, itis obtained by the formula FBW ¼Δf =f 0, where Δf and f 0represent the half power bandwidth and the center frequency,respectively as shown in Figs. 4 and 5. In the proposed MA, theseparameters are obtained as f0¼2.76 GHz FBW � 4:42%, forsimulation and Δf ¼ 0:1 GHz, f 0 ¼ 2:82 GHz, FBW � 3:54%. Also,proposed model have an acceptable FBW according to themany studies in literature [23,42,43]. Moreover, the proposedstructure has approximately 120MHz bandwidth range referringto 4:42%-FBW which is quite enough for many applications.For example, if we want to use a patch antenna which has

approximately 3% FBW in an application, our structure wouldprovide enough margins to work with. These calculations showperformance quality of the suggested MA.

In order to monitor the absorbing performance of the proposedstructure, the effect of the distances between cross wires and rings(for a6 and a7 (Fig. 1)) are investigated for comparison. This isbeneficial for deeper understanding the operation mechanismof the absorber when they are combined. The results are shownin Fig. 6. For the studied spectrum, three different configurationshave one resonance. Note that, parameter sweep is performedduring the simulation to have as high as possible absorptionlevel for the selected parameter(s). As a first configuration (forred one a6¼ 10 mm; a7¼ 1:67 mm), the distance betweencross wires and rings are synchronized and closed. The structureprovides a resonance with A(ω)¼91.72% at the frequencyof 3.4 GHz. Besides, as a second configuration (for green onea6¼ 11:67 mm; a7¼ 3:5 mm), the distance between crosswires and rings are synchronized and opened. The structureoffers a resonance with A(ω)¼96.59% at the frequency of3.8 GHz. Moreover, for the third arrangement (for blue onea6¼ 11:67 mm; a7¼ 1:67 mm), the distance between cross wiresand rings are unsynchronized. The configuration shows a reso-nance with A(ω)¼95.64% at the frequency of 4.8 Ghz. This meansthat the combined structure can provide resonances at threedistinct frequencies. As a result, the structure is simulated accord-ing to some geometrical parameters (namely a6 and a7) and theeffect of the variation of the mentioned parameters appears as ashift in the resonant frequency and an alteration in the absorptionlevel (increase and/or decrease). If the other geometrical para-meters (a1, a2, a3, a4, and a5) are also altered, the similarobservation will be observed. Principally, the whole system canbe considered as a resonant LC circuit. When a geometricalparameter is changed, the capacitance and/or inductance valuesof the structure will be changed too. Thus the resonant frequencyand the absorption will be changed accordingly due to thealteration of the capacitance and/or inductance. Therefore, thespectral location of the resonant frequency and the absorptionlevel will be varied as a result of the variation of a1, a2, a3, a4, anda5 [44,45].

As the next investigation, we analyzed the effects of differentpolarization angles for the proposed MA. For this reason, theRCW-shaped inclusion is numerically rotated from 01 to 901 with151 steps as shown in Fig. 7. It can be seen that the suggested MAmodel provides very good absorption with some shift in theresonant frequency for all polarization angles due to the structuralsymmetry. In addition, when the polarization angle is changed, theMA provides dual band absorption between the frequency rangesof 2.5�3 GHz and 4.5�5 GHz. Shifts in the resonance frequenciesare still small with respect to the normal incidence case(Figs. 4 and 5). Also, the additional peaks increased or decreaseddepending on polarization and the incident angles (Figs. 7 and 8).

Fig. 2. A picture from measurement setup (left-side) and schematic view of the experiment (right-side) [42].

Fig. 3. Absorption values for several material combination.

C. Sabah et al. / Optics Communications 322 (2014) 137–142 139

The simulated absorption characteristics at various incidentangles for the transverse electric ðTEÞ and the transverse magneticðTMÞ polarized radiations are also obtained as shown in Fig. 8. It isseen that the peak absorptivity of 99.9% is obtained at normalincident for TE and TM cases. The simulated results prove that theproposed MA can be performed for a wide range of arbitrarypolarizations.

In order to view the behavior of the resonances, the electricfield and surface current distributions of the MA for resonancefrequency of 2.76 GHz are presented in Figs. 9 and 10, respectively.Especially, high concentration of electric field around upper andlower rings, cross wires and gaps between them verify absorberresonance mode which is created by the strong coupling effect ofthe electric response with electric field (Fig. 9). The electric field

supplies independent electric response as working like a dipole forthe applied polarization. This electric response (Fig. 9) excites freeelectrons as surface currents throughout the paths as shown inFig. 10. Therefore, magnetic dipole moment due to surface chargeinduces magnetic response and leads to resonance absorption.Whereas the circulating anti-parallel currents induce magneticresponse, parallel currents in the current distribution causeelectric response. These responses strongly couple with E and Hcomponents of the incident wave and produce strongly localizedEM field at the resonance frequency. It means both electric andmagnetic resonance occur at the resonance. Hence, impedancematching condition is provided (Z(ω)¼Z0(ω)) to confine theincident energy in the absorber that results in minimum reflectionand maximum absorption.

We calculated the real part of the impedance extracted fromthe calculated scattering parameter in order to show the workingmechanism and resonance behavior of the MA to the incoming

electromagnetic waves by zðωÞ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffið1þS11Þ2�S221=ð1�S11Þ2�S221

qas

shown in Fig. 11. Near perfect absorption is achieved at theabsorptive peaks of frequencies 2:76 GHz and 2.82 GHz for simu-lation and measurement, respectively. Effective impedance of theoverall system matched exactly to the free space impedance(ZðωÞ ¼ Z0ðωÞ). This condition provides nearly zero reflection [46].Note that the imaginary part of the impedance is zero at the

Fig. 5. Measured reflection and absorption and the FBW for the suggested microwave absorber.

Fig. 6. Simulation absorptions of the proposed MA for modified versions.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

Fig. 7. Simulated absorption characteristics at different polarization anglesfrom 01 to 901.

Fig. 4. Simulated reflection and absorption and the FBW for the suggested microwave absorber.

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resonant frequency which confirms the condition of impedancematching and provides full absorption [47].

5. Sensor application

The proposed MA structure can also be used for sensorapplications by inserting dielectric layer on the front face of the

absorber. That is why, in this section, the effect of a variation of theover-layer thickness on the reflection and absorption values isinvestigated. The over-layer thickness (all other parameters ofdesign and simulation are remained constant) is varied from 2 mmto 6 mm. The reflection and absorption results of the simulationsfor different thickness are shown in Fig. 12. Even though theabsorption value doesn't change much with the variation ofdielectric thickness (for 2 mm-over layer (98.49%) and for 6 mm-over layer (99.62%), the resonance frequency shifts to lowerfrequencies when the thickness of the over-layer is increased.The reason of this downward shift can be explained by thevariation of the capacitance of the overall structure. The incrementof the thickness of over-layer FR4 leads to increase capacitance,therefore resonance frequency slide downward with increasedover-layer thicknesses. Hence the proposed structure can also beused as a pressure sensor beside absorber applications. One of themost important properties of the proposed metamaterial absorberbased sensor is incident angle independency and its easily obtain-able frequency range. These are the superiority of the proposedsystem.

6. Summary and discussion

RCW-shaped resonator was proposed and its electromagneticfeatures are investigated as a perfect MA. Beside the numericanalysis, the MA structure was manufactured and reflection-transmission measurements are performed. The periodic dimen-sion which resembles the operation wavelength was alsoaddressed. The suggested MA provides perfect absorption andpolarization-incident angle independency in the microwave Sfrequency band while reflection is minimized. The experimentaland computational results show very good agreement. The pro-posed MA shows further flexibility and reliability in the design andenables myriad applications such as stealth and military technol-ogies. Especially, the feature of polarization-incident angle inde-pendency provides perfect absorption for different values. Manystudies in literature show only insensitive polarization and inci-dent angle for some angles and some of these have low insensitivepolarization and incident angle such as Ref. [35,38]. Also, it has amodel of flexible structure because of its variations [36].

7. Conclusion

We designed, simulated and measured a perfect MA. Obtainedresults are discussed and evaluated. Simulation results are in agood agreement with the measured results. Beside this RCWs canbe used as both angle and polarization independent absorber fordifferent shielding applications. By scaling the parameters, thedesign in microwave spectrum can be applied to terahertz fre-quency regime. In addition, the suggested model can be used in

Fig. 8. Simulation of angular dependence of the absorption properties for TE andTM incident cases.

Fig. 9. Electric field distribution at resonance frequency of 2.76 GHz.

Fig. 10. Surface current distribution at resonance frequency of 2.76 GHz.

Fig. 11. Normalized impedance curve of MA as a function of frequency.

Fig. 12. Reflection and absorption values for different thickness values of theover-layer.

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short wave communication applications. Furthermore, a sensorapplication of the proposed MA is presented to show additionalfeature of the model. Especially, this presented feature can also beapplied to pressure measurement applications. Additionally, theproposed model can also be used in by weather radar, surface shipradar, some communications satellites, and so on.

Acknowledgment

M.K. acknowledges the support of TUBITAK under the ProjectNumber of 113E290 and partial support of the Turkish Academy ofSciences.

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