An Ultrathin Compact Polarization-Sensitive Triple-band Microwave Metamaterial Absorber

Abstract

In this study, an ultra-compact metamaterial absorber (MMA) has been proposed for microwave applications comprising two modified square-shaped resonators printed on a dielectric substrate and terminated by a metallic plane. The proposed MMA exhibits perfect absorption at 3.36 GHz, 3.95 GHz and 10.48 GHz, covering S- and X-band applications. The absorber is ultra-compact (0.112 λ) in size and ultra-thin (0.018 λ) in thickness at the lowest resonating frequency. The normalized impedance, constitutive electromagnetic parameters, electric field and surface current distribution have been studied to understand the physical mechanism of the triple-band absorption. Furthermore, the absorber is analyzed with different polarization and incident angles for transverse electric waves. The proposed MMA has been experimentally demonstrated to verify the results obtained from simulations. Moreover, the effect of over-layer thickness is investigated to examine the sensing application of the absorber.

Introduction

Metamaterials (MMs)1 with unique electromagnetic (EM) properties have drawn considerable attention in several areas including superlens,2 sensor,3 cloaking,4 antenna5 and absorber applications.6 The first metamaterial absorber (MMA) was demonstrated in the microwave region by Landy et al. in 2008.6 Since then, several MMAs have been proposed to improve the compactness, thickness and absorption peaks of the structure from microwave to visible frequencies.7,8,9,10,11,12,13 Metamaterial absorbers are usually arranged in periodic arrays of resonators printed on a substrate with a ground plane at the bottom. The metallic resonator and dielectric substrate are excited by the incident electric and magnetic field, respectively.6 These structures have the ability to minimize the reflection by matching the impedance of the MMA to free space, and the transmission is eliminated by the ground plane to achieve perfect absorption.8

Till now, various metamaterial-based structures have been investigated, including split-ring structures,11 ring-shaped,14,15 square-shaped,10,16 cut-wire17 and spilt-Jerusalem cross-resonators18 to demonstrate their performance with dual-,11,14,17,19 triple-,15,17,20 multi-16,18 and broadband10 operations. To achieve multi-band absorption performance, several efforts have been carried out using orthogonal arrangements of different metallic resonators10,17 or multilayered metallic structures separated by dielectric substrates.16,20,21 Recently, a single-layer triple-band absorber was proposed utilizing three concentric closed circular ring resonators in a unit cell.15 However, these approaches may suffer from technical difficulties, like fabrication complexity, which significantly affect their utilization for practical applications. In addition, several MMAs with polarization-sensitive characteristics have been demonstrated and have drawn significant attention due to their applications in polarization detection and imaging.17,18

Despite previous studies on various MMAs, a few reports are available on the design of an ultra-thin and compact absorbers exhibiting perfect absorption and polarization-insensitive characteristics. Hence, here, we present a compact and ultra-thin MMA demonstrating the perfect absorption phenomenon for S- and X-band applications. The proposed novel MMA structure consists of two modified square-shaped resonators exhibiting three perfect absorption peaks, i.e. two (3.36 and 3.95 GHz) in the S-band and one (10.48 GHz) in the X-band with compact size of 10 × 10 mm2. Moreover, the proposed MMA is compact (0.112 λ) and ultra-thin (0.018 λ), where λ is the wavelength of the lowest resonating frequency. The normalized impedance, constitutive EM parameters, surface current and electric field (E-field) distribution of the absorber are analyzed to understand the physical insight of absorption mechanism. The proposed MMA is fabricated and experimentally demonstrated to verify the results obtained from computer simulations utilizing the finite element method (FEM) technique. The obtained results suggest the proposed MMA exhibits a higher absorption rate than that of the previously reported triple-band absorbers.9,15,20,21

Design of the Structure

The unit cell comprises a metallic resonator and a ground plane made of copper which is insulated by a FR4 substrate, as shown in Fig. 1a. The permittivity and loss tangent of the FR4 substrate are 4.4 and 0.02, respectively, and conductivity (σ) of copper is 5.8 × 107 S/m. The novel structure on the top layer of the MMA consists of two modified metallic square-shaped resonators to exhibit perfect absorption at all resonating frequencies. Each resonating element in the unit cell has its own absorptivity and resonant frequency;15,20,21 however, coupling between two metallic resonators of the proposed MMA creates an extra absorption peak utilizing the optimized geometrical structure and its dimensions.22 The optimized parameters of the MMA are a = 10, b = 6.75, c = 5.565, d = 2.92, e = 1.75, f = 0.94, g = 1.58, i = 0.4 and j = 0.3 mm. The periodic dimension is 10 mm with the thicknesses of 1.6 mm and 35 μm for the dielectric and metallic layers, respectively. The MMA is excited by incident electric and magnetic fields to adjust the effective permittivity (εeff) and the effective permeability (µeff), respectively. These simultaneous electric and magnetic fields of the incident wave result in perfect absorption at different resonating frequencies, as shown in Fig. 2. The proposed design is simulated using a high-frequency structural simulator (HFSS) with periodic boundary conditions (PBCs), as shown in Fig. 1b.

Fig. 1
figure1

Proposed metamaterial absorber (MMA). (a) Perspective view, (b) simulation model using periodic boundary conditions (PBC) in HFSS.

Fig. 2
figure2

Absorption (A), reflection (R) and transmission (T) response of the proposed MMA.

Results and Discussion

The absorption A(ω) is calculated by \( 1 - \left| {S_{11} |^{2} - } \right|S_{21} |^{2} \), where \( S_{11} \) and \( S_{21} \) are the reflection and transmission coefficients, respectively. However, the metallic ground plane makes the transmission zero; therefore, \( A\left( \omega \right) = 1 - |S_{11} |^{2} \). The reflection coefficient is \( R\left( \omega \right) = \left| {(Z - Z_{0} )/\left( {Z + Z_{0} } \right)} \right|^{2} \), where Z is the MMA impedance and Z0 is the free space impedance. The perfect absorption A \( \left( \omega \right) \) = 1 is achieved at Z ≈ Z0 = 377 Ω which is obtained by optimizing the geometrical structure and its dimensions.15

Figure 2 shows the absorption, reflection and transmission response of the proposed MMA. The resonating frequencies are 3.36 (f1), 3.95 (f2) and 10.48 GHz (f3) with peak absorptivities of 99.42, 99.10 and 99.90%, respectively. The modified shape of both the square resonators in proposed absorber not only miniaturizes the dimensions but also improves the absorbance level when compared to the previously reported triple-band absorbers.15,20,21 Additionally, the fractional bandwidth (FBW) of the proposed MMA, i.e. \( \Delta f/f_{0} \), is estimated, where \( \Delta f \) and \( f_{0} \) represent half power bandwidth and center frequency, respectively. The FBW of 2.9, 6.3 and 3.7% are obtained at resonating frequencies f1, f2 and f3, respectively.

The real and imaginary parts of the normalized impedance should be close to one and zero, respectively, to minimize the reflection. As shown in Fig. 3a, the normalized impedance (Z)9,18 of the proposed absorber is 0.88 + j0.08, 1.07 − j0.18 and 1.04 − j0.03 at 3.36 GHz, 3.95 GHz and 10.48 GHz, respectively. The equivalent circuit for the proposed absorber given in the inset of Fig. 3a is analogous than that of the previously reported studies.9,18,23,24 Figure 3b and c demonstrates the constitutive EM parameters (εeff and µeff), to understand the physical mechanism of the absorber. The values of the real and imaginary values of εeff and µeff should be almost same at all resonating frequencies as structure impedance equals the free space impedance, leading to zero reflection.11 Table I shows the real and imaginary values of the normalized impedance, and constitutive electromagnetic parameters are similar to the previously reported results to achieve the perfect absorption.11

Fig. 3
figure3

Frequency response of the retrieved parameters. (a) Real and imaginary normalized impedance (Z), (b) real values of εeff and μeff, (c) imaginary values of εeff and μeff. Inset in Fig. 3a shows the equivalent circuit of the proposed MMA.

Table I Comparison of normalized impedance and constitutive electromagnetic parameters of the proposed MMA

The proposed structure is demonstrated for different polarization (ϕ) and incident (θ) angles, with a step size of 15o, as illustrated in Fig. 4. The peaks in the S- and X-band are demonstrated at 3.36 GHz, 3.95 GHz and 10.48 GHz when ϕ = 0o. The peaks are gradually reduced as the polarization angle increases due to the asymmetrical nature of the structure (Fig. 4a).17 The polarization-sensitive nature of the structure demonstrates the practical applications in various fields including polarization detection and sensing.17,18 Further, it is observed that the absorber is insensitive to incident angles till 45o, and after that, absorption is reduced with increase in the angle, as illustrated in Fig. 4b.

Fig. 4
figure4

Absorption response for various (a) polarization angles (ϕ), (b) incident angles (θ).

Figure 5 shows that the E-field and surface current distribution have been analyzed to provide the physical insight of the absorption peaks. The E-field and surface current distribution on the top plane show that the first two peaks (3.36 and 3.95 GHz) are due to the outer modified square-shaped resonator, as shown in Figs. 5a and b. From Fig. 5c, it is noted that at 10.48 GHz, E-field and current distributions are maximum at the inner modified square-shaped resonator. Figure 5 demonstrates that the surface currents on the top and ground plane are anti-parallel and responsible for the magnetic coupling.19 However, excitation of both electric and magnetic fields simultaneously results in high absorption performance.

Fig. 5
figure5

Electric field and surface current distribution for the top plane and ground plane at (a) 3.36 GHz, (b) 3.95 GHz, (c) 10.48 GHz..

The proposed design is fabricated with an array of a 16 × 16 unit cell on a 1.6-mm-thick FR4 substrate, as illustrated in Fig. 6a. The reflection coefficient is measured using Keysight’s vector network analyzer (M9005A/M9374A) connected with two standard broadband horn antennas (1–18 GHz). Here, one antenna is utilized to transmit microwave signals, and another similar antenna is used to detect the reflected signal from the MMA. The schematic illustration of the measurement setup is shown in Fig. 6b. During free space measurements, the minimum angle (5°) has been used to avoid interference between incident and reflected waves. The test bench is calibrated to avoid the reflection from the nearby environment.18 Initially, the reflection coefficient of the metallic ground plane is measured to estimate the noise floor of the setup. Then the resultant reflection coefficient of MMA was obtained after subtracting the reflection loss of the ground plane, i.e. the floor noise, from the measured reflection coefficients of the fabricated sample.

Fig. 6
figure6

Fabricated structure and experimental setup. (a) Fabricated structure with its enlarged view, (b) experimental setup.

Figure 7a shows the reflection coefficients of −11.49 dB, −14.99 dB and −30.26 dB are obtained at 3.39 GHz, 4.00 GHz and 10.52 GHz, respectively, and the corresponding absorptivity values are 92.9, 96.83 and 99.9%, respectively (Fig. 7b). The reflection coefficient is measured with the change in polarization angle (0° to 45°) to verify the polarization dependence of the proposed MMA, as illustrated in Fig. 8a. In addition, the measured reflection coefficient response for different incident angles from 0° to 30° is observed in Fig. 8b. The measured results shown in Figs. 7 and 8 are analogous to simulated results with small deviation which can be attributed to the impurities in the material of the MMA prototype,16 fabrication imperfections,13,18,25 substrate nonuniformity26 and free space measurement tolerance.13,16,19

Fig. 7
figure7

Measured and simulated (a) reflection coefficient response, (b) absorption response.

Fig. 8
figure8

Measured reflection coefficient response for various (a) polarization angles, (b) incident angles.

The sensing performance of the MMA is demonstrated by adding an over-layer (OL) of FR4 dielectric material on top of the metallic structure. Figure 9a demonstrates the absorption response for different thickness varying from 0 mm to 0.15 mm. The increase in OL thickness increases the capacitance of the structure and causes the redshift in resonating frequency, as shown in Fig. 9a.27,28 Further, it is demonstrated that the increase in refractive index of the medium (1–1.4) also decreases the resonating frequency in Fig. 9b. These results suggest that the proposed MMA can be utilized for sensing applications by tuning the dielectric material properties, i.e. thickness and refractive index which in turn shift the resonating frequencies towards the lower side with apparent change in the absorption.

Fig. 9
figure9

Absorption response for different (a) over-layer thickness, (b) refractive index medium.

Table II compares the performance of the proposed MMA with the previously reported absorbers on the basis of absorptivity, FBW, unit cell size and dielectric thickness.9,14,15,19,20,21,25,26 The modified shape of both the square resonators in the proposed absorber not only miniaturizes the dimensions but also improves the absorbance level when compared to the previously reported triple-band absorbers. However, previously reported MMAs have achieved multi-band characteristics by incorporating multiple metallic resonators of different size14,15,20,25,26 and stacked metallic layers.20,21 The proposed single-layer MMA is compact (0.112 λ) and ultra-thin (0.018 λ), exhibiting absorption better than 99% at all resonating frequencies with the highest FBW of 6.3%. Additionally, the proposed MMA can be utilized for detection and sensing applications due to its polarization-sensitive nature.

Table II Comparison between previously reported absorbers and the proposed MMA

Conclusions

The absorption coefficients for the ultra-thin MMA at microwave frequencies have been demonstrated. The MMA shows three perfect absorption peaks at 3.36, 3.95 and 10.48 GHz. The structure is polarization-sensitive due to the asymmetric nature of the structure and angle-insensitive up to 45°. The normalized impedance, constitutive EM parameters, E-field and surface current distribution have been demonstrated to understand the absorption mechanism. The proposed absorber is experimentally demonstrated, and obtained results are in good accordance with the simulation results. The proposed compact (0.112 λ) and ultrathin (0.018 λ) polarization-sensitive absorber demonstrates perfect absorption at all resonating frequencies when compared to previously reported absorbers. The proposed MMA can be used for various practical applications such as polarization detection and sensing. Furthermore, the dependence of the over-layer thickness on absorption is examined to explore the sensing application of the MMA.

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Acknowledgments

PJ acknowledges financial support from the Ministry of Electronics and IT, Government of India, under the Visvesvaraya PhD. Scheme for Electronics and IT.

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Correspondence to Arun K. Singh.

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Jain, P., Singh, A.K., Pandey, J.K. et al. An Ultrathin Compact Polarization-Sensitive Triple-band Microwave Metamaterial Absorber. Journal of Elec Materi 50, 1506–1513 (2021). https://doi.org/10.1007/s11664-020-08680-z

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Keywords

  • Metamaterial absorber
  • ultrathin
  • triple-band
  • polarization sensitive
  • sensor
  • normalized impedance