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Abstract
A transparent low-profile polarization-insensitive metamaterial absorber with ultrawideband microwave absorption is presented. A fractional bandwidth of 125.2% (4.3–18.7 GHz, absorptance > 90%) is achieved using a simple patterned resistive metasurface. The thickness of the absorber is only ∼0.086 times the upper-cutoff wavelength. The experimental results agree with full-wave simulation results. A Cu-metal-mesh ground plane enhances the shielding efficiency and visible transparency. Radar cross-sections (RCS) are reduced across all reflection angles, over frequencies spanning the C, X, and Ku bands. With its visible-wavelength transparency, low profile, polarization insensitivity, excellent absorption, and wideband RCS reduction, the proposed absorber has wide applicability.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Microwave absorbers (MAs) are widely used in various fields ranging from military equipment to civil buildings and wireless communication facilities, owing to their effectiveness at dissipating microwave energy without generating any secondary pollution [1–3]. Most applications require broadband absorption, which is related to radar cross-section (RCS) reductions and electromagnetic compatibility [4,5]. As the classic microwave absorber, the Salisbury screen [6,7], suffers from narrow bandwidth, Jaumann absorbers [8,9] were developed by utilizing a multilayer structure to achieve broad-bandwidth absorption at the cost of increased thickness and weight. Recently, artificially structured metamaterial absorbers have been extensively studied because of their excellent designability [10–13]. The constitutive parameters of metamaterial absorbers can be engineered to achieve attractive qualities originating from the inherent resonances of their individual elements. There are two main approaches to obtain broadband absorption for metamaterial absorbers. The first involves multiple absorption peaks that are spectrally overlapped by the assembly of multilayered structures [14–17] or multiple resonators on a single layer [18,19]. The second approach is based on increasing the resistance of a periodic conductive pattern to gain the additional benefit of dissipating the energy by loading lumped resistors [20–21], painting resistive inks, [22,23] or using resistive films [24–29]. The first approach suffers from the extra burdens of increased thickness, great difficulty in processing, and high cost. In addition, it has been demonstrated that loading lumped resistors, the second approach, can cause a high-frequency parasitic effect and increase the processing difficulties, and resistive inks are usually all opaque. By contrast, the use of resistive films has the advantage of high accuracy, ease of fabrication, and low cost. In addition, transparent resistive sheets, such as indium tin oxide (ITO) films, have the unrivaled advantage of facilitating the realization of both visible observation and microwave absorption. This characteristic means that ITO films with periodic conductive patterns have potential for application in many fields, such as in stealth warship or aircraft window development, radio-frequency identification systems, and electronic toll-collection (ETC) systems [30–32].
In consequence of the promise offered by transparent resistive films for microwave absorption, in recent years, more and more effort has been devoted to developing optically transparent, broadband metamaterial absorbers. In 2014, Guo et al., developed a wideband transparent absorber consisting of an aluminum-grid structure that had an efficiency of greater than 90% for a frequency band spanning 5.8–12.2 GHz [33]. However, an operating-frequency band greater than 10 GHz is required for airborne applications and those related to surveillance radar signal absorption [29,34]. To this end, Guan et al., developed ITO as standing-up closed-ring resonators to span wide frequency ranges of absorption, 5.5–19.7 and 22.5–27.5 GHz, albeit only for a particular polarization [29]. Among the published transparent metamaterial absorbers, the highest relative absorption bandwidth (above 90%), 124.53%, was reported for interdigital capacitive structures. However, the design of this metasurface pattern is extremely complex, and there is a significant increase in the fabrication difficulty [26]. Furthermore, this absorber only generates two resonances, in common with the number of resonances reported for similar structures in most literature reports. Therefore, for practical applications, there remains a requirement for the development of simple patterned resistive elements, rather than complex elements and structures, for enhanced broadband absorption performance.
In this study, a new optically transparent, polarization-insensitive absorber with a wide absorption band, based on a simple patterned metasurface, is designed, fabricated, and characterized. The novelty of the proposed structure is three-fold. First, the use of a single-element composed of a simple combination of circles in the unit cell to generate three resonant modes and thus obtain ultra wide bandwidth (fractional bandwidth, FBW = 125.2%) absorption, has not been previously reported. Second, a more transparent and better-conducting metal mesh is used, in place of the conventional continuous ITO film as the ground plane, to improve both optical transmittance and shielding effectiveness. Third, the 10-dB RCS reduction bandwidth of the main lobe of the proposed transparent absorber spans a broad frequency range, from 4.3 to 17.9 GHz, covering the C, X, and Ku bands. In addition, our structure is shown to be insensitive to the polarization of the incident waves on account of the four-fold rotational symmetry of the low-profile metamaterial. The excellent performance of the absorber has been proved by simulation and experiment, and it promises to be a good candidate for numerous practical applications, for example, for EM shielding room observation windows, touch panel controls, radio-frequency identification systems, and transparent radio-frequency devices.
2. Design and simulation
Schematics of the patterned-metasurface transparent absorber and the structure of its unit cell are depicted in Fig. 1. The unit cell has a sandwich structure, being composed of three layers. The upper surface is comprised of a simple patterned resistive ITO resonator deposited on a flexible polyethylene terephthalate (PET) substrate, the ground plane is a copper micromesh with good conductivity that is adhered to a flexible PET substrate but separated from the ITO resonator layer by an air spacer; thus, air forms the middle layer in the sandwich structure. In order to ensure high optical transmittance for the proposed metamaterial absorber, all the materials used in the design have high visible-light transmission characteristics. The optically transparent ITO films used in the design are made to commercial standards. In this experiment, we utilize a resistive ITO film of 100 Ω/sq to form the patterned resistive metasurface. The use of a simple pattern allows the incorporation of lossy components into the resonant structure to efficiently dissipate the incident energy. The sheet resistance of the ITO film can be fine-tuned by precisely controlling the film thickness. The transparent absorbers reported in the literature mostly used ITO with low sheet resistances as the ground surface to completely reflect the incoming wave and ensure that there was no transmission of the microwaves. However, transmittance decreases obviously as the sheet resistance of the ITO films decreases. As shown in Fig. 2, the average transmittance of ITO films with sheet resistances of 6 Ω/sq and 100 Ω/sq and of a 0.2-Ω/sq Cu-metal-mesh are 74%, 79%, and 81%, respectively. (In each case, PET is used as the substrate.) The optical-microscopy image of a Cu-metal-mesh film with microgrooves of 6 µm in width and depth and a mesh line spacing of 175 µm is shown as an inset in Fig. 2. The Cu-metal mesh was fabricated through combining roll-to-roll imprinting and Cu electroplating [35]. This method allows a low sheet resistance, down to 0.2 Ω/sq, to be obtained, along with a high transmittance (81%). Herein, we use the more transparent and better-conducting metal mesh, replacing the conventional continuous ITO film ground plane, to improve both optical transmittance and shielding effectiveness.
Fig. 2. Optical transmittance of two types of ITO films and Cu-metal-mesh film. Inset: optical microscopy image of Cu-metal-mesh film
The structural parameters for the unit cell of the proposed absorber are defined in Fig. 3. The resistive ITO resonator pattern on the upper surface of a grounded substrate takes the form of a large circle with four small circles evenly distributed around its circumference, partially overlapping it. The supporting substrates of the ITO film and Cu-metal mesh are, in both cases, PET sheets designed with relative permittivity and dielectric loss tangent of 3 and 0.06, respectively, and thicknesses of 0.05 mm (upper layer) and 0.15 mm (lower layer). Full-wave numerical simulations of the transparent absorber were carried out by the finite integration technique using CST MWS software. Floquet ports with normal incident plane transverse electric (TE) waves and transverse magnetic (TM) waves for excitation were used. Periodic boundary conditions (PBCs) along the x and y directions were used to simulate the infinite periodic element. The geometry and size of the unit cell were optimized by numerical simulation to obtain good microwave-absorption performance. In addition, careful consideration was given to the physical conditions required for the fabrication of the structure. Thus the final optimized parameters, as defined in Fig. 3, were obtained as U = 7.4 mm, V = 3.3 mm, Y = 5.2 mm, P = 17.2 mm, tp1 = 0.05 mm, ta = 5.8 mm, tp2 = 0.15 mm, and RS1 = 100 Ω/sq; RS2 = 0.2 Ω/sq.
The absorptance of the proposed absorber under normal incidence can be defined as A(ω) = 1 – R(ω) – T(ω), where R(ω) = |S11|2 and T(ω) = |S21|2 are the reflectance and transmittance derived from the frequency-dependent complex S-parameter, respectively. The simulated spectral absorptance, reflectance, and transmisttance curves, obtained using a full-wave electromagnetic solver, are presented in Fig. 4(a). The proposed transparent absorber can obtain broadband microwave absorption with an efficiency above 90% over the frequency range of 4.3 to 18.7 GHz, with transmission levels close to zero apparent in Fig. 4(a). This low transmission value can be attributed to the low sheet resistance (0.2 Ω/sq) of the Cu-metal-mesh lower layer, which may be approximated as a reflector to avoid secondary scattering. Furthermore, three broad absorption bands, peaking at 5.5, 14, and 18.3 GHz, can be observed, and the peak absorptance in each case is 98%, 95%, and 99%, respectively.
Fig. 4. (a) Microwave-range characteristics of the proposed absorber under normal incident TE plane wave irradiation conditions. (b) Simulated effective impedance of the proposed absorber
A direct physical view of the absorption characteristics is obtained from the plot of the normalized input effective impedance Zeff(ω) of the proposed absorber [Fig. 4(b)], as calculated from the simulated S-parameters displayed in Fig. 4(a) via use of Eq. (1) [36]:
For insight into the physical mechanism of the broadband absorption performance, surface electric field E, surface magnetic field M, surface power loss density and surface-current I distributions of the proposed absorber under different frequencies were studied in Fig. 5. For the upper-layer patterned ITO metasurface, the electric field is mainly concentrated on the outer area or outward edge area of the simple pattern, as shown in Figs. 5(a)–5(c). In contrast, the magnetic field in 14 GHz concentrates on the edge area in the vertical direction of the simple pattern layer of the proposed absorber. The 5.5 GHz concentrates on the inner area of the edge in the vertical direction, and in 18.3 GHz it concentrates on the inner area of the simple pattern, as shown in Figs. 5(d)–5(f). The power loss density distributions as shown in Figs. 5(g)–5(i), which are similar to that of the electric field distributions, indicating that the power loss induced by the proposed absorber is attributed to the electric field coupling caused by resistance loss (ohmic dissipation). Meanwhile, as the observational frequency increases, the variation of the power loss distribution reveals that the power loss gradually increases and expands towards outer side of the ITO resistive pattern, correspondingly causing different absorption performances for different frequencies, as shown in Fig. 4(a).
Fig. 5. Electromagnetic responses of the proposed absorber under normally incident TE waves at 5.5 GHz, 14 GHz and 18.3 GHz. The distributions of (a–c) electric field (d–f) magnetic field (g–i) power loss density. Distributions of surface currents on (j–l) the ITO resistive metasurface and (m-o) the bottom ground layer.
Under the influence of the incident waves, electrons from the conductive ITO metasurface move with the external electric field, and a current is induced. By comparing the surface current distribution on the ITO resistive metasurface and the bottom ground layer at different frequency, it can be found that the intensity of surface current on the ITO resistive metasurface is stronger. This indicates that the absorption performance is mainly attributed to the resistive ITO film on the top layer, as shown in Figs. 5(j)–5(o). Furthermore, it can be observed that the direction of the current flow on bottom ground plane is opposite to that of the top ITO metasurface at 14 GHz and 18.3 GHz in the Figs. 5(g)–5(i). Such anti-parallel current flows imply the occurrence of magnetic resonances between top and bottom layer. However, the working mechanism for lower frequencies is quite different. The direction of the current flow between top and bottom layer is parallel at 5.5 GHz, that indicated electric resonances. Thus, the electric and magnetic excitations occur simultaneously, resulting in the strong, broadband absorption.
Furthermore, the surface current density is unevenly distributed on the ITO metasurface and it differs among the absorption peak frequencies, as shown in Figs. 5(j)–5(l). The power loss, PD, in the unit cell can be calculated as follows:
where J(x,y) is the surface current and RS is the effective resistance. Therefore, the enhanced surface current in the upper layer, combined with the ohmic loss from the ITO metasurface, make further contributions to the excellent absorption performance. Thus, the absorption bandwidth as well as the absorptivity of the absorber can be precisely controlled by the geometric and sheet resistance of the ITO metasurface.In order to further explore this mechanism, the effects of each part of the simple pattern on the absorption performance were analyzed. The simple pattern can be regarded as a combination of a large circle overlapped with four small circles evenly distributed around its circumference. Comparing Figs. 6(a) and 6(b), it is apparent that the addition of two small overlapping circles on opposite sides of the large circle results in higher and broader absorption, under TE polarization conditions. This absorption is almost the same as that for the entire low-profile pattern [Fig. 6(c)], whereas, little improvement in the absorption performance under TM polarization is seen in Fig. 6(b). On this basis, it is logical that by overlapping another two circles around the large circle to generate the full pattern for the proposed absorber, which has greater symmetry, the elimination of the polarization sensitivity, while maintaining the excellent absorption performance is achieved, as shown in Fig. 6(c). Furthermore, in Fig. 6(d) we demonstrate that the proposed transparent absorber has the advantage of polarization independence, thanks to its four-fold rotational symmetry.
Fig. 6. (a),(b),(c) Absorptance spectra for different elements of the proposed absorber pattern under normally incident plane-wave irradiation, for both TE and TM polarizations: (a) single large circle, (b) large circle plus two small overlapping circles, and (c) large circle plus four small overlapping circle (full pattern). (d) 3D plot of microwave absorptivity of the proposed absorber versus frequency and polarization angle (φ).
The primary origin of the broadband absorption is the high ohmic loss of the upper resistive film, since the design includes an air spacer, which has insignificant dielectric loss, as the dielectric layer. The quality factor (Q) can be used to quantify the loss capability of absorbers. This factor is defined in Eq. (3).
where f is the resonant frequency, Pr and Pa denote the stored and dissipated energies, respectively, and Δf represents the absorption bandwidth. The form of Eq. (3) suggests that the bandwidth is proportional to the dissipated energy, which means that a larger resistance may result in a wider absorption bandwidth. To further illustrate the effect of the upper resistive film on the absorption performance of the proposed structure, the absorption response of the absorber was studied for different sheet resistance values (RS1) of the upper layer. It is observed from Fig. 7 that as the upper-layer resistance increases, the first and third absorption peak intensities decrease while the second absorption peak intensity increases. At the same time, the first and the second absorption peaks shift toward each other but the position of the third absorption peak is essentially unchanged and thus the overall absorption bandwidth is reduced. Thus, it is apparent that there is an optimum sheet resistance value (RS1 = 100 Ω/sq) for the widest absorption bandwidth (above 90%). In this case, the three absorption peaks overlap and together forming a continuous broadband absorption of greater than 90%, from 4.3 to 18.7 GHz.Fig. 7. Simulated absorptance of the proposed structure for various upper-layer surface resistance (RS1) values.
In addition to polarization stability, in many practical applications it is necessary for absorbers to have good angular stability. In this regard, simulated absorptivity spectra under different oblique incidences of irradiation, for TE and TM modes, are illustrated in Fig. 8. Under TE mode irradiation, the absorption is maintained above 90% at incident angles from 0 to 15° while the absorption band structure remains nearly unchanged. When the incident angle exceeds 15°, however, the absorption slightly decreases. By comparison, for the TM mode, it can be observed that the absorptivity is higher at different incident angles, and the incident angle stability is better. In addition, for TM mode, it is clear that the absorption band slowly and gradually shifts to a higher frequency range [Fig. 8(b)]. Overall, the absorptivity of the metamaterial absorber is above 80% at incident angles from 0 to 45°, for both the TM and TE modes, across the working bandwidth, which spans the entire C-, X-, and Ku-bands. This result indicates that the proposed design features a reasonably stable angular performance.
Fig. 8. Simulated absorptivity as a function of incidence angle, θ, for (a) TE and (b) TM polarizations. The polarization orientation with respect to the surface in each case is shown schematically above the plot.
The RCS of the absorber with a cross-section of 293 × 293 mm (17 × 17unit cells) and that of a perfect electric conductor (PEC) plate of the same size were simulated using CST MWS. The bistatic RCS at 10 GHz as a function of reflection angle is shown in Figs. 9(a) and 9(b), respectively, for vertically and horizontally polarized normally incident waves. It is observed that the backscattered RCS of the transparent absorber is reduced with respect to that of the PEC plate, for every reflection angle. This proves that the absorption of the incident beam, rather than diffraction or scattering in other directions by the absorber, is responsible for the reflection reduction phenomenon in the case of this absorber. For better visualization, 3D plots of the RCS at 10 GHz for both the absorber and the above-mentioned PEC plate are given in Fig. 9(c). The monostatic RCS reduction capabilities of the proposed absorber are investigated for different frequencies and plotted in Fig. 9(d) for horizontally polarized normally incident waves. The magnitude of the main lobe of the RCS for the proposed absorber is reduced with respect to that of the PEC, and the 10-dB RCS reduction bandwidth spans a broad frequency range, from 4.3 to 17.9 GHz (FBW = 122.5%).
Fig. 9. Bistatic RCS of the proposed absorber as a function of reflection angle at 10 GHz for (a) vertically and (b) horizontally polarized incident waves, plotted with the same quantity for a PEC plate of the same size, for comparison. (c) 3D bistatic RCS plots for TE-polarized 10 GHz waves normally incident on the PEC plate (left) and the proposed absorber (right). (d) Monostatic RCS magnitude for the main lobe of the proposed absorber versus frequency, for horizontally polarized normally incident waves.
It is worth mentioning that the recently developed diffusion-like metasurfaces [37–39] have capabilities for backward RCS suppression that are similar to those of the absorber in reported in this article. However, the underlying functional mechanisms are quite different. Diffusion-like metasurfaces use the phase differences of the waves reflected by the different elements on the reflecting surface to average the incident energy into different directions, and hence they inherently enhance the magnitude of the side-lobes of the RCS. By contrast, the proposed absorber has the advantage that it allows broadband RCS suppression in all directions, as confirmed in Fig. 9.
3. Fabrication and characterization
To experimentally verify the absorption performance of the design, the proposed transparent absorber consisting of 17 × 17 unit cells (293 × 293 mm surface area) was fabricated, as shown in the inset of Fig. 10. The upper ITO film with a sheet resistance of 100 Ω/sq was deposited on a PET substrate by magnetron sputtering. Then, the resistive film was patterned by laser ablation of the ITO coating. In order to improve the optical transparency and shielding effect simultaneously, a Cu metal mesh structure was used in our structure, which also helps to broaden the application field. The bottom Cu metal mesh was fabricated by selective electroplating of Cu in roll-to-roll imprinted microgrooves on the PET substrate [35]. With microgroove widths of 6 µm and depths of 6 µm and a mesh line spacing of 175 µm, extremely low sheet resistance down to 0.2 Ω/sq and high transmittance of 81% have been achieved. Finally, the upper and lower layers were stretched and taped over a 5.5-mm thin square transparent frame, using a transparent double-sided adhesive, to form an air spacer. It was only possible to fabricate the polymethyl methacrylate (PMMA) frame with a thickness of 5.5 mm, hence the designed air spacer gap of 5.8-mm thickness was created by supplementing the PMMA depth with the transparent double-sided adhesive. The frame is a combination of an inner circular ring frame and an outer square frame, with four bridges running between them, both having minimum widths of 10 mm. This frame arrangement ensures not only that the upper and lower sheets are spread flat, without any bending, but also that the test is affected as little as possible by the presence of the PMMA frame.
Fig. 10. Measured optical transmittance spectrum for the fabricated sample. Inset: photograph of the fabricated absorber sample.
The use of the ground Cu metal mesh with a transmittance of 81% and a sheet resistance of 0.2 Ω/sq resulted in improvement in the electromagnetic shielding characteristics and optical transmittance of the absorber. The optical transmittance of the transparent absorber was measured by using a spectrophotometer (Beijing Purkinje General TU-1810). The results are presented in Fig. 11. The averaged transmittance of the transparent absorber in the visible band (400–760 nm) is approximately 65%.
Fig. 11. Schematics of setups for measurement of (a) transmittance of the proposed absorber under normal incidence and (b) reflectance of the proposed absorber under different incident angle θ. (c) Shielding effectiveness measurement results. (d) Comparison of measured and simulated absorptance for the transparent absorber. Measured absorption of the proposed metamaterial absorber for oblique incidence with (e) TE and (f) TM polarizations, with the polarization direction and the incident angle θ illustrated in the insets.
The transmittance coefficient (S21) and reflectance coefficient (S11) were measured using the free-space method and the arch method inside an anechoic chamber, respectively. Schematics of the measurements of the reflectance and transmittance of the proposed transparent absorber are depicted in Fig. 11. The shielding effectiveness (SE) can be calculated from the experimental S21 values, i.e., SE = T(ω) = |S21|2. Thus, calculated shielding efficiencies lower than −35 dB, in the range from 4 to 18 GHz, spanning the C, X, and Ku bands, were obtained, as shown in Fig. 11(c). This result means that the electromagnetic-wave transmission is very low and the sample absorber has a high shielding efficiency, which is consistent with the simulated transmission results displayed in Fig. 4(a).
A comparison of the measured and simulated absorptance of the transparent absorber is presented in Fig. 11(d). It can be seen the experimental and modeled results are in good agreement, as regards the overall trend, and similar to the simulation result, the measured response shows three resonance peaks, although its lower frequency edge is shifted to higher frequencies slightly. The surface resistance of the ITO film is uneven and values larger than the optimum may cause the spectral shift, as shown in Fig. 7. In addition, there are other factors that might possibly cause this deviation, such as a slight sagging of the unattached areas of the PET sheets, effects of the supporting frame structure that were neglected in the simulation, or low-frequency diffraction effects. Furthermore, the angular dependence of the measured absorptivity spectra for the transparent absorber under TE and TM modes are illustrated in Figs. 11(e) and 11(f), respectively, which are in good accordance with the simulations shown in Fig. 8 except some minor differences. The absorptivity of the proposed transparent absorber is above 80% at incident angles from 0 to 45° across the working bandwidth for both the TM and TE modes. The measured result indicates that the proposed design features a reasonably stable angular performance as predicted by the simulations.
4. Conclusion
In this article, an optically transparent broadband absorber based on a simple patterned resistive metasurface is designed on a Cu-metal-mesh ground plane. The structure offers microwave absorption at above 90% over a fractional bandwidth of 125.2% (4.3–18.7 GHz), and its thickness at the lowest cutoff frequency of 0.086λL. Compared with previously reported transparent absorbers, our design not only exhibits significant broadband absorption characteristics, but also shows excellent performance in terms of RCS reduction, as summarized in Table 1. The proposed transparent absorber has potential utilizations in practical applications, including the observation windows of stealth aircrafts and EM shielding rooms and in transparent radio-frequency identification systems, thanks to its low profile, polarization insensitivity, oblique incidence stability, visible-wavelength transparency, and wideband RCS reduction characteristics.
Table 1. Comparison with other transparent broadband absorbers
Funding
National Science Fund for Distinguished Young Scholars (51625201); National Key Research and Development Program of China (2016YFE0201600); National Natural Science Foundation of China (51702066, 51911530123, Key Project U1809210).
Disclosures
The authors declare no conflicts of interest.
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