Open Access
Add to BibSonomy
Abstract
In this work, an ultra-wideband microwave metasurface absorber with optically transparent and flexible properties is proposed. The metasurface is composed of a reflective backplane and a microwave absorption layer sandwiched between two dielectric substrates. The impedance matching curves of the microwave absorption layer are deduced based on the impedance matching theory, which is quite helpful and useful to improve the accuracy and efficiency of the broadband optimization design. Simulated results show that absorption higher than 90% can be achieved in the frequency band ranging from 5.8 GHz to 27.3 GHz, which covers the radar wavebands of C, X, Ku and K. The relative bandwidth reaches up to 130%, thus realizing ultra-wideband absorption while the thickness of the metasurface is only 0.12 times the upper-cutoff wavelength. For the TE (transverse electric) wave incidence, the metasurface maintains good performance when incident angle θ ≤ 50°, while for the TM (transverse magnetic) wave incidence, the absorption higher than 90% can be still achieved in a broad frequency band when θ ≤ 60°. It can be seen that using double-layer dielectric substrate in the metasurface not only greatly expands the microwave absorption bandwidth, but also improves the oblique incident properties. In addition, the metasurface is insensitive to polarization since its unit cell is symmetrical. Moreover, by rationally designing materials, the metasurface in this work is optically transparent and flexible, thus quite suitable for window radar stealth and equipment conformal stealth.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metamaterials and their two-dimensional equivalents (known as metasurfaces) are macroscopic composites of periodic or non-periodic structure, whose function is mainly determined by both the cellular architecture and the chemical composition [1]. Because metamaterials/metasurfaces can flexibly modulate electromagnetic waves, they have been widely used in many fields, such as polarization convertor, planar lens, meta-hologram and absorbers etc. [2–11].
In the field of electromagnetic absorbers, Landy et al. proposed a perfect metamaterial absorber with near unity absorbance in 2008. The structure consisted of two resonators that coupled separately to electric and magnetic fields so as to absorb all incident radiation within a single unit cell layer [11]. Since then, metamaterial/metasurface absorbers have been widely used in the frequency-domain field (from microwave to optical frequencies), and also in the time-domain field [12–15]. However, because the above absorption is achieved through resonance, the absorption bandwidth is usually very narrow. Numerous researches have been performed to expand the bandwidth for meeting the practical needs of broadband absorption in recent years.
One way is to create multiple resonances by placing resonant units with different sizes/shapes in a single layer or in different layers [16–25]. For the case of single layer, multiple resonances are usually discrete, and therefore the absorber is actually multi-band rather than wideband. The multilayer structure can realize resonances at multi-frequencies and the frequency overlapping leads to absorption over a wide waveband. However, the thickness of the multilayer structure is quite large since metallic units (e.g. Ag, Cu) are adopted in this structure. By using resistive units to replace the metallic units, the absorber is able to perform wideband absorption with thinner thickness [26–37]. The resistive units can be realized by loading lumped resistors or by using resistive materials (e.g. resistive ink and indium tin oxide (ITO)). Due to high cost and manufacturing complexity of the high frequency resistors, resistive materials are used more widely. In addition, metamaterials/metasurfaces combined with active components such as varactors and transistors can modulate absorption peak, polarization and working frequencies, thus broadening the absorption bandwidth [38–46]. These active metamaterial/metasurface absorbers show more complex manufacturing processes and higher cost than the passive ones.
Based on the above methods, the absorption bandwidth of metamaterial/metasurface absorbers has been greatly improved. However, a detailed theoretical guidance used for optimizing the broadband absorption design was absent in most literatures. Therefore, in this work, based on the impedance matching theory combined with the transmission line method and the equivalent circuit model, the impedance matching curves of a metasurface absorber using double-layer dielectric substrate is deduced in detail, which provides an accurate and efficient theoretical guidance for optimizing the broadband absorption. Then, a metasurface consisting of a reflective backplane and a microwave absorption layer sandwiched between two dielectric substrates is proposed, which can not only achieve ultra-wideband microwave absorption compared with most of the reported results, but also ensure the performance of large angle oblique incidence and polarization insensitivity. Finally, a sample is prepared and the ultra-wideband absorption performance is verified by experiments.
2. Theory and design
2.1 Theory
The impedance matching theory can be used to provide guidance for optimizing the broadband absorption design of metasurface. As shown in Fig. 1(a), the reflection-type metasurface in this work mainly includes dielectric substrate 2 (Sub2), microwave absorption layer (MAL) with periodic unit arrays, dielectric substrate 1 (Sub1) and reflective backplane. Based on the transmission line theory, the structure can be equivalent as a terminal short-circuit transmission line. Then, combined with the equivalent circuit theory, the equivalent circuit of the metasurface can be achieved (as seen in Fig. 1(b)), in which the resistive microwave absorption layer is equivalent as a RLC series circuit.
The impedance of the reflective backplane is Zsc = 0. According to the transmission line theory,
where Zc1, kc1 and d1 are the Sub1’s characteristic impedance, propagation constant and thickness, respectively. Z2 is the parallel equivalent impedance of Zsub1 and ZMAL (the impedance of MAL), and can be calculated asThe input impedance of the whole structure Zin can be expressed as
For the reflection-type metasurface, there is no transmission wave due to the reflective backplane. According to the impedance matching theory, when the input impedance of the whole structure Zin is equal to the air characteristic impedance Z0 (120π ≈ 377 Ω), there is no reflection wave. In this case, all the incident electromagnetic wave will enter into the metasurface and be absorbed. Therefore, in order to achieve broadband microwave absorption, the input impedance Zin should match the air characteristic impedance Z0 in the frequency band as wide as possible. For the metasurface shown in Fig. 1(a), according to equations (1) - (3), if the electromagnetic parameters and the thickness of the two dielectric substrates are fixed, the input impedance Zin is only related to the impedance of the microwave absorption layer ZMAL. That is, once the dielectric substrate is chosen, the broadband absorption is mainly determined by the microwave absorption layer. In other words, the broadband microwave absorption can be achieved by making the actual impedance of the microwave absorption layer (ZMAL_a) matches the ideal impedance of the microwave absorption layer (ZMAL_i) in a frequency band as wide as possible.
The ideal impedance of the MAL corresponds to the impedance of the microwave absorption layer when
At this circumstance, the ideal value of the parallel equivalent impedance Z2 can be obtained by
The real part Za and the imaginary part Zb of the ideal impedance of the MAL can be extracted by using the commercial digital software MATLAB as
The typical curves of ideal impedance of the microwave absorption layer are shown in Fig. 2, which presents that the ideal impedance of the microwave absorption layer is related to the thickness of the two dielectric substrates and their electromagnetic parameters. Setting that the thickness and electromagnetic parameters of the two substrates are the same, the curves of ZMAL_i as a function of the thickness of the dielectric substrate are shown in Fig. 2 (a). It can be seen that when the thickness increases, the curve moves to the lower frequency and becomes steeper. When d1 = d2 = 2 mm, the curves of ZMAL_i versus the real part of the relative dielectric constant are shown in Fig. 2 (b). As the real part of the relative dielectric constant increases, the curve moves towards the lower frequency and becomes steeper, and the overall value of the real part of the ideal impedance Za decreases.
The actual impedance of the microwave absorption layer can be deduced according to the simulation results. The reflection coefficient S11 can be calculated as
and then the actual input impedance of the whole structure is
Fig. 2. The ideal impedance of the MAL as a function of (a) thickness of the dielectric substrates and (b) the real part of relative dielectric constant of the dielectric substrates.
By combining Eq. (10) with Eq. (3), the actual parallel equivalent impedance Z2 can be calculated as
Then the real part Zre and the imaginary part Zim of the actual impedance of the MAL can be extracted by using MATLAB as
Since both the actual impedance of the microwave absorption layer ZMAL_a and the ideal impedance of the microwave absorption layer ZMAL_i are complex numbers, the matching of ZMAL_a with ZMAL_i means that their real part and imaginary part should be close to each other in a frequency band as wide as possible, as shown in Fig. 3. Based on the former analysis, in this case, the input impedance of the whole structure Zin will be approximately equal to the air characteristic impedance Z0 in a wide frequency band, and consequently ultra-wideband microwave absorption can be realized.
2.2 Structure and simulation
Figure 4(a) shows the overall three-dimensional schematic diagram of a cell for the proposed ultra-wideband metasurface absorber. The multilayered structure consists of the dielectric substrate 2 (Sub2), the microwave absorption layer (MAL), the dielectric substrate 1 (Sub1) and the reflective backplane. The PVC (polyvinyl chloride, its relative dielectric constant is εPVC = 2.4 (1-j0.06)) is optically transparent, flexible and moisture proof, and thus is chose as the two dielectric substrates. Both the MAL and the backplane consists of ITO (indium tin oxide)-coated-PET (polyethylene) film with the same thickness of d_PET = 0.175 mm and the PET relative dielectric constant is εPET = 3.0(1-j0.006).
Fig. 4. Schematic diagram of the ultra-wideband metasurface absorber. (a) Perspective view of one cell; (b) front view of the MAL for one cell; (c) equivalent circuit.
The equivalent circuit of this metasurface absorber is shown in Fig. 4(c), which is slightly different from Fig. 1(b) because the PET layer of the MAL is considered as a fraction of the transmission line. In this circumstance, according to the transmission line theory,
where Zp, kp and dp are the PET’s characteristic impedance, propagation constant and thickness, respectively.In order to realize optimal impedance matching and meet the requirement of polarization insensitivity, octagonal ring is selected as the unit cell of the MAL, as shown in Fig. 4(b). The equivalent inductance L and equivalent capacitance C can be adjusted by changing the geometrical sizes of the unit cell. The ITO surface resistance of the MAL is set as Rs and can be determined by optimal design. For the design convenience, the thickness of Sub1 and Sub2 are set as the same to reduce the number of optimization variables. Since the radar wavebands which we are interested in are the C, X, Ku and K bands, the thickness of each PVC dielectric substrate is fixed as d_sub1 = d_sub2 = 3 mm. When the ITO surface resistance of the backplane is set as Rplane = 5 Ω/sq, total reflection occurs. Based on the MAL impedance matching curves derived in Section 2.1, the optimal geometrical sizes labeled in Fig. 4(b) are a = 4 mm, l = 2.08 mm, w = 0.2 mm, and the ITO surface resistance of the MAL is Rs = 12.5 Ω/sq.
The simulated reflection coefficient versus frequency under the plane wave normal incidence is shown in Fig. 5(a). The reflection coefficient is less than -10 dB from 5.8 GHz to 27.3 GHz, realizing a total bandwidth of 21.5 GHz with a relative bandwidth of 130%. The corresponding impedance matching curves of the microwave absorption layer is shown in Fig. 5(b). It can be seen that the working frequency band (5.8 GHz to 27.3 GHz) is mainly determined by the matching points 1 and 2, and the three absorption peaks (7.5 GHz, 15.4 GHz and 24.5 GHz, as seen in Fig. 5(a)) correspond to the three matching points (point 3, 4 and 5) labeled in Fig. 5(b) respectively. It can be concluded that when the ideal impedance and the actual impedance of the MAL match well with each other in a wide frequency band, the ultra-wideband microwave absorption can be realized.
Fig. 5. (a) The simulated reflection coefficient of the metasurface absorber and (b) the MAL impedance matching curves under the plane wave normal incidence.
The microwave absorption is calculated by
where $R = {|{{S_{11}}} |^2}$ is the reflectance and $T = {|{{S_{21}}} |^2}$ is the transmission, S11 is the reflection coefficient and S21 is the transmission coefficient. The reflectance, transmission and absorption of the metasurface under the plane wave normal incidence are shown in Fig. 6(a). Since the ITO surface resistance of the backplane is Rplane = 5Ω/sq, the backplane realizes a total reflection, and therefore the metasurface transmission is 0. The absorption is higher than 90% in the frequency range from 5.8 GHz to 27.3 GHz, which covers four radar wavebands of C, X, Ku and K, and thus ultra-wide microwave absorption is achieved. The input impedance of the metasurface Zin normalized to the air characteristic impedance Z0 is presented in Fig. 6(b), and it shows that the real part of the normalized input impedance is close to 1 while the imaginary part approximates to 0 in the frequency band from 5.8 GHz to 27.3 GHz, which is consistent the with impedance matching theory.Fig. 6. (a) The reflectance, transmission and absorption and (b) the normalized input impedance of the metasurface under the plane wave normal incidence.
As shown in Fig. 6(a), there are three obvious resonant absorption peaks in the absorption curve, including 7.5 GHz, 15.4 GHz and 24.5 GHz. In order to further understand the absorption mechanism of this metasurface absorber, the distributions of surface current, surface loss density, surface electric field and volume loss density at these three resonant frequencies are studied in Fig. 7. As shown in Fig. 7(a-c), the free electrons in MAL’s ITO conductive film move under the incident wave, forming the induced surface current and the current direction is parallel to the direction of the external electric field. The free electrons in the ITO conductive film convert the incident wave energy into heat energy during the process of directional movement, resulting in the ohmic loss, as shown in Fig. 7(d-f). Moreover, the larger the induced current is, the stronger the ohmic loss is.
Fig. 7. Electromagnetic responses of the metasurface absorber at 7.5 GHz, 15.4 GHz and 24.5 GHz under the plane wave normal incidence. (a-c) Magnitude of the surface current; (d-f) surface loss density; (g-i) magnitude of the surface electric field; (j-l) volume loss density.
The positive charges move along the direction of the external electric field and gather at one side of the octagonal ring which is perpendicular to the direction of the external electric field, while the negative charges move against the direction of the external electric field and concentrate at the opposite side, so that the induced surface electric field forms between the adjacent unit cells, as shown in Fig. 7(g-i). Furthermore, under the action of the induced electric field, the electric dipoles in the two PVC substrates and MAL’s PET film would be arranged into the same direction, leading to the polarization of these dielectrics. During the process of rotation and friction of the electric dipoles, the microwave energy is converted into heat energy, resulting in the dielectric loss, as seen in Fig. 7(j-l). The dielectric loss is mainly from Sub2, while the dielectric loss in MAL’s PET film and Sub1 is relatively small. Therefore, the incident electromagnetic energy is finally dissipated by the ohmic loss (caused by the MAL) and the dielectric loss (caused by Sub2, Sub1 and MAL’s PET); and the ohmic loss (originated from resonances) dominates over the dielectric loss.
The absorption of the metasurface under oblique incidence is also simulated. The xoz plane (as shown in Fig. 4(a)) is chosen as the incident plane, and the simulated results under the TE wave and TM wave incidence with different incident angles are presented in Fig. 8(a) and 8(b), respectively. From Fig. 8(a), it can be found that with increasing the incident angle θ, the absorption frequency band moves towards higher frequency and the metasurface maintains good absorption performance when θ ≤ 50°. Figure 8(b) shows that as the incident angle θ increases, the absorption frequency band also shifts towards higher frequency and the absorption higher than 90% can still be achieved in a broad frequency band when θ ≤ 60°. Therefore, it can be concluded that adding a dielectric substrate on the microwave absorption layer can not only achieve ultra-wideband microwave absorption, but also improve the absorption performance under wide angle oblique incidence.
Fig. 8. The simulated absorption of the metasurface versus different incident angles under (a) the TE wave incidence and (b) the TM wave incidence.
Figure 9 shows the absorption of the metasurface under different polarization angle φ (as shown in Fig. 4(a)). Because the periodic unit is the octagonal ring with a symmetrical shape, the absorption almost does not change with the polarization angle φ, and therefore the metasurface is polarization insensitive.
3. Experiment and discussion
In order to confirm the simulated results, a metasurface sample with 200 mm × 200 mm area was fabricated. The key parameters including the thickness of the two dielectric substrates, the surface resistance of the MAL and the backplane, as well as the cell geometrical sizes are consistent with the optimal values achieved by simulation discussed above. The MAL octagonal ring arrays are fabricated through high-precision laser direct etching. The four layers (including the MAL, the two PVC substrates and the backplane) are assembled together by optically transparent glue. Figure 10(a) shows the optical photograph of the sample with enlarged details in the inset. Figure 10(b) shows the sample under the bending state, which proves its high flexible properties. Figure 10(c) shows the sample placed on a book, which demonstrates its good optical transparency.
Fig. 10. (a) Optical photograph of the metasurface sample and the inset shows the details of the cells; (b) the flexibility of the sample and (c) the optical transparency of the sample.
The reflection coefficient of the metasurface sample is measured by using the arch measurement method. The main measurement equipment consists of a vector network analyzer (Agilent N5224A) and two pairs of broadband horn antennas (with working frequency bands of 1-18 GHz and 18-26.5 GHz respectively). The curves of the reflection coefficient versus frequency are shown in Fig. 11, in which Fig. 11(a) shows the comparison curves of the reflection coefficient between the experimental and simulated results under the plane wave normal incidence when φ = 0°, and Fig. 11(b) is the comparison curves when φ = 90°.
Fig. 11. Comparisons of the reflection coefficient between the experimental and simulated results under the plane wave normal incidence when (a) φ = 0° and (b) φ = 90°. The experimental results are collected when the metasurface absorber is under the flat (rc = ∞) and curved (rc = 150 mm) states.
From Fig. 11, the experimental results and the simulated results agree well with each other for both φ = 0° and φ = 90° under the plane wave normal incidence, demonstrating that the metasurface indeed has ultra-wideband absorption properties. There are two differences between the experimental and simulated results (when the metasurface absorber is under the flat state): (i) the three resonant peaks shift towards higher frequency, resulting in the overall shift of the frequency band towards high frequency direction; (ii) the experimental value of reflection coefficient is higher than the simulated value, but it still remains below -10 dB and a good absorption can be achieved in the whole working frequency band. The reasons for the above discrepancies should be due to the following reasons: (i) firstly, the actual relative dielectric constant and thickness of the PVC substrates may be deviates from the values used in the simulation; (ii) secondly, since it is difficult to fabricate an ITO film with perfect uniform surface resistance on a large-area substrate, the actual ITO surface resistance for the MAL and the backplane is different from the values used in the simulation; (iii) moreover, the fabrication tolerance also contributes to the discrepancies. In conclusion, the experimental results agree well with the simulated results, which verify the validity of the simulation design and the feasibility of its ultra-wideband application.
The reflection coefficient of the metasurface absorber in curving state is also measured when the curvature radius is rc = 150 mm. The curved metasurface can still keep good performance in the whole working frequency band, as shown in Fig. 11. It is also found that the reflection coefficient is a little higher in some frequency bands and this can be explained as following: when the metasurface is bent, the plane wave normal incidence changes to oblique incidence for most areas of the sample. The equivalent circuit parameters change under this circumstance and therefore the impedance matching requirements are no longer satisfied.
The properties of typical reported wideband metasurface absorbers are summarized in Table 1. Owing to the impedance matching curves of the microwave absorption layer which is effective to optimize the absorber design, the matasurface absorber in this work shows larger total bandwidth and higher ratio of total bandwidth to thickness than those absorbers listed in Table 1, demonstrating the good performance of our absorber.
Table 1. Performances comparison between the metasurface absorber in this work and its counterparts
4. Conclusion
An ultra-wideband, optically transparent and flexible microwave metasurface absorber is achieved. The impedance matching curves of the microwave absorption layer is deduced for improving the broadband optimization design. Both the simulated and experimental results verify that the metasurface absorber in this work can realize ultra-wideband microwave absorption within a limited thickness. In addition, by rationally designing materials, the metasurface in this work has high optical transparency and good flexibility, which is promising in the applications of radar stealth fields.
Funding
National Natural Science Foundation of China (51977219).
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51977219).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. T.J. Cui, D.R. Smith, and R.P. Liu, “Metamaterials, theory, design and application,” NY, USA: Springer, 2 (2010).
2. A.V. Kildishev, A. Boltasseva, and V.M. Shalaev, “Planar photonics with metasurfaces,” Science 339(6125), 1232009 (2013). [CrossRef]
3. D.M. Lin, P.Y. Fan, E. Hasman, and M.L. Brongersma, “Dielectric gradient metasurface optical elements,” Science 345(6194), 298–302 (2014). [CrossRef]
4. G.X. Zheng, H. Muhlenbernd, M. Kenney, G.X. Li, T. Zentgraf, and S. Zhang, “Metasurface holograms reaching 80% efficiency,” Nat. Nanotechnol. 10(4), 308–312 (2015). [CrossRef]
5. A. Arbabi, Y. Horie, M. Bagheri, and A. Faraon, “Dielectric metasurfaces for complete control of phase and polarization with subwavelength spatial resolution and high transmission,” Nat. Nanotechnol. 10(11), 937–943 (2015). [CrossRef]
6. C.L. Holloway, E.F. Kuester, J.A. Gordon, J. O’Hara, J. Booth, and D.R. Smith, “An overview of the theory and applications of metasurfaces: the two-dimensional equivalents of metamaterials,” IEEE Ante. and Prop. Maga. 54(2), 10–35 (2012). [CrossRef]
7. T.J. Cui, M.Q. Qi, X. Wan, J. Zhao, and Q. Cheng, “Coding metamaterials, digital metamaterials and programmable metamaterials,” Light-Science and Applications 3(10), e218 (2014). [CrossRef]
8. M.S. Islam, J. Sultana, M. Biabanifard, Z. Vafapour, M.J. Nine, A. Dinovitser, C.M.B. Cordeiro, B.W.H. Ng, and D. Abbott, “Tunable localized surface plasmon graphene metasurface for multiband superabsorption and terahertz sensing,” Carbon 158, 559–567 (2020). [CrossRef]
9. M.R. Akram, G.W. Ding, K. Chen, Y.J. Feng, and W.R. Zhu, “Ultrathin single layer metasurfaces with ultra-wideband operation for both transmission and reflection,” Adv. Mate. 32(12), 1907308 (2020). [CrossRef]
10. E. Arbabi, A. Arbabi, S.M. Kamali, Y. Horie, M. Faraji-Dana, and A. Faraon, “MEMS-tunable dielectric metasurface lens,” Nature Comm.9(812), (2018). [CrossRef]
11. N.I. Landy, S. Sajuyigbe, J.J. Mock, D.R. Smith, and W.J. Padilla, “Perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef]
12. Y. Yao, R. Shankar, M .A. Kats, Y. Song, J. Kong, M. Loncar, and F. Capasso, “Electrically tunable metasurface perfect absorbers for ultrathin mid-infrared optical modulators,” Nano Lett. 14(11), 6526–6532 (2014). [CrossRef]
13. G.M. Akselrod, J.N. Huang, T.B. Hoang, P.T. Bowen, L. Su, D.R. Smith, and M.H. Mikkelsen, “Large-area metasurface perfect absorbers from visible to near-infrared,” Adv. Mate. 27(48), 8028–8034 (2015). [CrossRef]
14. X.Y. Liu, K.B. Fan, I.V. Shadrivov, and W.J. Padilla, “Experimental realization of a terahertz all-dielectric metasurface absorber,” Opt. Exp. 25(1), 191–201 (2017). [CrossRef]
15. Aman Nilotpal, S. Bhattacharyya, and P. Chakrabarti, “Frequency and time-domain analyses of multiple reflections and interference phenomena in a metamaterial absorber,” Journal of Optical Society of America B 37(3), 586–592 (2020). [CrossRef]
16. X.P. Shen, T.J. Cui, J.M. Zhao, H.F. Ma, W.X. Jiang, and H. Li, “Polarization-independent wide-angle triple-band metamaterial absorber,” Opt. Exp. 19(10), 9401 (2011). [CrossRef]
17. H. Li, L.H. Yuan, B. Zhou, X.P. Shen, Q. Cheng, and T.J. Cui, “Ultrathin multiband gigahertz metamaterial absorbers,” Jour. of Appl. Phys. 110(1), 014909 (2011). [CrossRef]
18. J.W. Park, P.V. Tuong, J.Y. Rhee, K.W. Kim, W.H. Jang, E.H. Choi, L.Y. Chen, and Y. Lee, “Multi-band metamaterial absorber based on the arrangement of donut-type resonators,” Opt. Exp. 21(8), 9691–9702 (2013). [CrossRef]
19. S. Bhattacharyya and K. V. Srivastava, “Triple band polarization-independent ultra-thin metamaterial absorber using ELC resonator,” J. Appl. Phys. 115(6), 064508 (2014). [CrossRef]
20. F. Ding, Y.X. Cui, X.C. Ge, Y. Jin, and S.L. He, “Ultra-broadband microwave metamaterial absorber,” Appl. Phys. Lett. 100(10), 103506 (2012). [CrossRef]
21. T. Jang, H. Youn, Y.J. Shin, and L.J. Guo, “Transparent and flexible polarization independent microwave broadband absorber,” ACS Photonics 1(3), 279−284 (2014). [CrossRef]
22. C.Y. Wang, J.G. Liang, T. Cai, H. P. Li, W. Y. Ji, Q. Zhang, and C. W. Zhang, “High-performance and ultra-broadband metamaterial absorber based on mixed absorption mechanisms,” IEEE Access 7, 57259–57266 (2019). [CrossRef]
23. J.W. Yu, Y. Cai, X.Q. Lin, and X. Wang, “Perforated multilayer ultrawideband absorber based on circuit analog absorber with optimal air spaces,” IEEE Ante. and Wire. Prop. Lett. 19(1), 34–38 (2020). [CrossRef]
24. X.J. Lu, Z.Y. Xiao, and M.M. Chen, “A broadband metamaterial absorber based on multilayer-stacked structure,” Mode. Phys. Lett. B 34(21), 2050216 (2020). [CrossRef]
25. L.H. He, L.W. Deng, Y.H. Li, H. Luo, J. He, S.X. Huang, and S.Q. Yan, “Design of a multilayer composite absorber working in the P-band by NiZn ferrite and cross-shaped metamaterial,” App. Phys. A: Materials Science and Processing 125(2), 130 (2019). [CrossRef]
26. S.J. Li, P.X. Wu, H.X. Xu, Y.L. Zhou, X.Y. Cao, J.F. Han, C. Zhang, H.H. Yang, and Z. Zhang, “Ultra-wideband and polarization-insensitive perfect absorber using multilayer metamaterials, lumped resistors, and strong coupling effects,” Nano. Res. Lett. 13(386), 7092 (2018). [CrossRef]
27. X. Begaud, A.C. Lepage, S. Varault, M. Soiron, and A. Barka, “Ultra-wideband and wide-angle microwave metamaterial absorber,” Materials 11(2045), 2045 (2018). [CrossRef]
28. T.T. Nguyen and S. Lim, “Angle-and polarization-insensitive broadband metamaterial absorber using resistive fan-shaped resonators,” Appl. Phys. Lett. 112, 021605 (2018). [CrossRef]
29. F. Costa, A. Monorchio, and G. Manara, “Analysis and design of ultra thin electromagnetic absorbers comprising resistively loaded high impedance surfaces,” IEEE Trans. on Ante. and Prop. 58(5), 1551 (2010). [CrossRef]
30. Z.H. Zhou, K. Chen, J.M. Zhao, P. Chen, T. Jiang, B. Zhu, Y.J. Feng, and Y. Li, “Metasurface salisbury screen: achieving ultra-wideband microwave absorption,” Opt. Exp. 25(24), 30241–30252 (2017). [CrossRef]
31. Q. Zhou, X.W. Yin, F. Ye, R. Mo, Z.M. Tang, X.M. Fan, L.F. Cheng, and L.T. Zhang, “Optically transparent and flexible broadband microwave metamaterial absorber with sandwich structure,” App. Phys. A: Mate. Sci. and Proc. 125(2), 131 (2019). [CrossRef]
32. H. Sheokand, S. Ghosh, G. Singh, M. Saikia, K.V. Srivastava, J. Ramkumar, and S.A. Ramakrishna, “Transparent broadband metamaterial absorber based on resistive films,” Jour. of Appl. Phys. 122(10), 105105 (2017). [CrossRef]
33. P.P. Min, Z.C. Song, L. Yang, B. Dai, and J.Q. Zhu, “Transparent ultrawideband absorber based on simple patterned resistive metasurface with three resonant modes,” Opt. Exp. 28(13), 19518–19530 (2020). [CrossRef]
34. S.F. Lai, Y.H. Wu, J.J. Wang, W. Wu, and W.H. Gu, “Optical-transparent flexible broadband absorbers based on the ITO-PET-ITO structure,” Opt. Mater. Express 8(6), 1585–1592 (2018). [CrossRef]
35. R.X. Deng, K. Zhang, M.L. Li, L.X. Song, and T. Zhang, “Targeted design, analysis and experimental characterization of flexible microwave absorber,” Materials and Design 162, 119–129 (2019). [CrossRef]
36. D.W. Hu, J. Cao, W. Li, C. Zhang, T.L. Wu, Q.F. Li, Z.H. Chen, Y.L. Wang, and J.G. Guan, “Optically transparent broadband microwave absorption metamaterial by standing-up closed-ring resonators,” Adv. Optical Mater. 5, 1700109 (2018). [CrossRef]
37. Y.Q. Zhang, H.X. Dong, N.L. Mou, L.L. Chen, R.H. Li, and L. Zhang, “High-performance broadband electromagnetic interference shielding optical window based on a metamaterial absorber,” Opt. Exp. 28(18), 26836–26849 (2020). [CrossRef]
38. H.Y. Li, F. Costa, Y. Wang, Q.S. Cao, and A. Monorchio, “A wideband multifunctional absorber/reflector with polarization-insensitive performance,” IEEE Trans. on Ante. and Prop. 68(6), 5033–5038 (2020). [CrossRef]
39. A.B. Li, Z.J. Luo, H. Wakatsuchi, S. Kim, and D.F. Sievenpiper, “Nonlinear, active, and tunable metasurfaces for advanced electromagnetics applications,” IEEE Access 5, 27439 (2017). [CrossRef]
40. J. Zhang, X.Z. Wei, I.D. Rukhlenko, H.T. Chen, and W.R. Zhu, “Electrically tunable metasurface with independent frequency and amplitude modulations,” ACS Photonics 7(1), 265−271 (2020). [CrossRef]
41. A. Li, S. Kim, Y. Luo, Y. Li, J. Long, and D.F. Sievenpiper, “High-power transistor-based tunable and switchable metasurface absorber,” IEEE Trans. on Microw. Theo. and Techn. 65(8), 2810–2818 (2017). [CrossRef]
42. Z.J Luo, J. Long, X. Chen, and D. Sievenpiper, “Electrically tunable metasurface absorber based on dissipating behavior of embedded varactors,” Appl. Phys. Lett. 109, 071107 (2016). [CrossRef]
43. F.W. Wang, K. Li, and Y.H. Ren, “Reconfigurable polarization rotation surfaces applied to the wideband antenna radar cross section reduction,” Int. J. RF Microw. Comput Aided Eng. 28, e21262 (2018). [CrossRef]
44. S. Kim, H. Wakatsuchi, J.J. Rushton, and D.F. Sievenpiper, “Switchable nonlinear metasurfaces for absorbing high power surface waves,” Appl. Phys. Lett. 108(4), 041903 (2016). [CrossRef]
45. J. Zhao, Q. Cheng, J. Chen, M. Q. Qi, W. X. Jiang, and T. J. Cui, “A tunable metamaterial absorber using varactor diodes,” New J. Phys. 15(4), 043049 (2013). [CrossRef]
46. J.L. Li, J.J. Jiang, Y. He, W.H. Xu, M. Chen, L. Miao, and S.W. Bie, “Design of a tunable low-frequency and broadband radar absorber based on active frequency selective surface,” IEEE Ante.Wireless Propag. Lett. 15, 774–777 (2016). [CrossRef]
