Open Access
Add to BibSonomy
Abstract
Absorbers have potential applications in the stealth field. However, limited bandwidth and low absorption rate persist in existing methods. Moreover, absorbers working in the low frequency range (1–4 GHz) with small size are much more difficult to realize. In this paper, we propose a novel absorption structure, which combines indium tin oxide film and metal resonator. The former realizes impedance matching with free space in a broad bandwidth at moderate frequency range while the latter shows the resonant property at low frequency. Based on this absorption structure, we design the zigzag-shaped structure to realize high-efficiency and ultra-broadband absorption. To demonstrate the feasibility of our method, we fabricate a sample and perform measurements. The measurement results show that our sample can achieve ultra-broadband absorption with high-efficiency of over 90% from 1 GHz to 18 GHz, which is in good agreement with simulation results. Our findings provide a valuable technique for broadband device design, which could bring about a wide range of applications in cloaking technology.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays, electronic countermeasures gradually become important fighting means in modern electronic warfare. Electromagnetic (EM) stealth is a major military technology for electronic countermeasures. Because absorbers can reduce the target radar cross-section effectively, they have become important in EM stealth technology. However, conventional absorbers, such as wedge absorber [1], ferrite [2], and natural composite absorber [3], still suffer from limited bandwidth, low absorption rate, electrically large size at low frequency, etc. These disadvantages limit its potential applications.
In recent years, metasurfaces have received great interest from researchers owing to its attractive EM properties [4,5], such as ultrathin lenses [6–13], planar holograms [14–16], and perfect absorber [17–19]. The EM wave absorption property gives metasurface a wide range of applications in the stealth field [20–24]. In 2008, Smith first proposed the concept of a perfect metamaterial absorber (PMA) and realized EM wave absorption [25]. Because the ideal absorption property of PMA is based on the resonance characteristics of the unit cell, the inherent narrow bandwidth performance limits its application. To realize broad bandwidth absorption, a variety of innovative technologies have been proposed, such as multilayer structure [26], single-layer multiresonant structure [27], lumped-element loading [28,29], and Salisbury screen [30,31]. With the development of materials science, many new materials have been envisioned by scientists. Indium tin oxide (ITO) can be used as an excellent material for absorbing EM waves efficiently [32]. Although the methods above can be used to enhance the absorption band, the bandwidth is still seriously limited, not to mention the low absorption rate and large size at the low frequency range.
In this paper, we propose a novel strategy to realize high-efficiency ultra-broadband absorption of above 90% from 1 GHz to 18 GHz using a zigzag-shaped structure. First, we propose a new structure by combining an ITO film and a metal resonator. The ITO film can realize impedance matching with the free space in broadband at moderate frequency, while the metal resonator can realize resonant absorption at low frequency. Then, we design a zigzag-shaped structure to realize high-efficiency ultra-broadband absorption, which is schematically shown in Fig. 1. Our finding paves the way for a high-efficiency ultra-broadband absorber that works at low frequency. It also has potential applications in the stealth field.
Fig. 1. Schematic of high-efficiency ultra-broadband zigzag-shape structure absorber. The frequencies of incident wave f1, f2 and f3 represent low frequency, moderate frequency and high frequency from 1 GHz to 18 GHz, respectively. The low, moderate, high frequency EM wave is mainly absorbed by lower-layer cutting metal wire, ITO film and upper-layer cutting metal wire, respectively.
2. Concept and unit cell design
We aim to design a microwave absorber structure to achieve high-efficiency absorption in the ultra-broadband range 1–18 GHz. Our design process is divided into three parts. In the first part, we proposed initial single layer structure composed of cutting metal wire and ITO film in order to realize absorption above 90% at low and moderate frequency. In the second part, we designed double layer structure to realize absorption at moderate and high frequency. At last, we combined single layer and double layer structure to realize ultra-broadband absorption by optimizing. First, in order to realize absorption at low frequency, we proposed a metal resonator structure, which is shown in Fig. 2(a). The substrate (blue part) stands on the metal sheet (gray part). It is made of FR-4 with relative permittivity of 4.3, a loss tangent of 0.025. On one side of the substrate is cutting metal wire (gray part). This structure for normal incidence can be modeled as a transmission line model, as shown in Fig. 2(b). The cutting metal wire can be equivalent to RLC series circuit. The gap is considered as the equivalent capacitance C, and the metal wire is thought to be a series of equivalent inductor L and resistor R. The metal sheet can be equivalent to a short-circuit at the end. The gap between cutting metal wire and metal sheet can be equivalent to characteristic impedance Z0. When EM wave is incident on the metal sheet directly and reflects totally, the cutting metal wire has no effect on it. Thus, the characteristic impedance can be equivalent to wave impedance of free space. As a result, the electrical length between RLC series circuit and short-circuit end is the same as the distance d between cutting metal wire and metal sheet. According to the equivalent element of cutting metal wire calculation formula [36], we can obtain
where $F(k, l) = \frac{{5k\textrm{h}}}{{\pi \lambda }}\ln (\csc \frac{{\pi l}}{{2k}})$. ${h_m}$ is thickness of metal (copper), 0.036 mm. $\sigma$ is conductivity of metal, 5.8×107 S/m. h is height of substrate. Thus, the resonant frequency is determined by the following relation: ${\omega _0} = \frac{1}{{\sqrt {LC} }}$. For the interest of realize high-efficiency absorption at about 3 GHz, according to formula (1), (2), and (3), we chose the parameter of the structure as following: h = 21 mm, l = 0.5 mm, d = 1 mm, p = 18 mm, q = 14 mm, t = 1 mm, k = 0.5 mm. R, L, and C are 17.10 $\Omega$, 0.166 nH, and 664.62 pF, respectively. We characterized the EM property of the unit cell using CST Microwave Studio. In the simulation, the electric field of the incident wave was set along the x-direction, the magnetic field was set along the y-direction, and the unit cell was arranged periodically. Figure 2(c) depicts the reflection coefficient rxx when the structure was shined normally along the z-axis. We can see that there is a reflection dip of |S11| at about 3.2 GHz, which is as we desire. In order to verify the reliability of transmission line model, we extracted the element parameters of the transmission line model. According to the simulation results of cutting metal wire structure, we obtained the parameters in terms of S11 by utilizing the AWR Design Environment. The calculated R, L, and C are 13.47 $\Omega$, 0.153 nH, and 638.28 pF, respectively. They are corresponding with the theoretical value. Then, we calculated the resonant frequency ${\omega _0}$ as 3.2 GHz, which agrees well with the resonant frequency of the simulation results. We also compared the S11 of the physical structure calculated by CST and that of transmission line model calculated by AWR, as shown in Fig. 2(c). The results were found to coincide well with each other. Thus, our results demonstrate the feasibility of the transmission line model. To determine the working mechanism of cutting metal wire structure, we study it furtherly. The influence of cutting size l of the metal wire at resonant frequency f1 is shown in Fig. 2(d). When there is no cutting, reflection curve has no dip along the whole spectrum. With increasing l, the depression of the reflection coefficient moves toward the higher frequency, but the dip remains stable. Increasing the cutting gap size l corresponds to an increase in the distance between parallel plates. Thus, with rising l, the equivalent capacitance C of the cutting metal wire decreased and the resonant frequency increased, according to formula (3). When there is no cutting, there is no equivalent capacitance C and no resonant frequency. The phenomenon above is corresponding with the theoretical prediction. Overall, we can acquire excellent absorption property at low frequency f1 by adjusting the parameters of the cutting metal wire. We also simulated the current distribution on the metal surface, which is shown in Fig. 2(e). We found that the current directions on the surface of the cutting metal wire and metal sheet were antiparallel, which demonstrates the magnetic resonance efficiently [33]. Therefore, owing to the existence of a magnetic response current loop between the cutting metal wire and metal sheet at the resonant frequency, the magnetic field along the y-direction was restrained by the current loop and dissipated by dielectric loss, while the electric field along the x-direction excited the current on the metal and was dissipated by ohmic loss. Thus, perfect absorption property could be realized.Fig. 2. Design of the initial unit cell. The geometrical parameters are listed as: h = 21 mm, l = 1 mm, d = 1 mm, p = 18 mm, q = 14 mm, t = 1 mm, a = 13.6 mm, k = 1 mm and the sheet resistance of ITO film (yellow part) Zf = 100 $\Omega /sq$. The substrate (blue part) is made of FR-4 with relative permittivity of 4.3, a loss tangent of 0.025. The metal (gray part) is made of copper with conductivity σ = 5.8×107 S/m and the thickness 0.036 mm. (a) Schematic of the proposed unit cell free view. (b) Equivalent transmission line model. (c) Comparison of S11 of physical structure and equivalent transmission line model. (d) Reflectivity varying against parameter of cutting metal wire l. (e) Current distributions of cutting metal wire. (f) Schematic of ITO film structure. Simulation results of (g) combination structure and (h) single ITO film.
Then in order to realize broadband absorption at moderate frequency, we proposed combination structure with ITO films and cutting metal wire. ITO film is a type of resistive film with high electrical conductivity and ohmic loss [34]. When the EM wave propagates along the surface of the film, current could be excited and dissipated by ohmic loss. ITO with gaps between films is attached to the other side of the substrate, as shown in Fig. 2(f). Figure 2(g) depicts the reflection coefficient rxx when the combination structure was shined normally along the z-axis. We can see that |S11| is below −10 dB from 2.2 GHz to 13.6 GHz. There are two reflection dips at low frequency f1 = 3.2 GHz and moderate frequency f2 = 10.4 GHz. The absorption rate of the absorber can be defined as $A(\omega )= 1 - R(\omega )- T(\omega )= 1 - {|{{\textrm{S}_{11}}} |^2} - {|{{\textrm{S}_{21}}} |^2}$, where A(ω), |S11|2, and |S21|2 are the absorbance, reflectivity, and transmissivity, respectively. As a result of the existence of a metal back sheet in our structure, the transmission (S21) is zero in theory.
We also examined the absorption property of the ITO film solely. We simulated the ITO film and reflection coefficient, as shown in Fig. 2(h). We found that |S11| is below −10 dB from 7.5 GHz to 14.2 GHz. As a result, we can realize reasonable impedance which agrees with the wave impedance of free space in the moderate frequency spectrum. The combination of wire resonance and ITO structure could lead to wider bandwidth.
To reduce the lower limit frequency of the absorber and further improve the bandwidth, we proposed a double-layer cutting metal wire and ITO film structure, which is shown in the inset of Fig. 3(a). The lower limit frequency of the absorption bandwidth above 90% fell to 1.2 GHz, which is much lower than that of a single-layer structure. Moreover, there was an additional reflection dip at high frequency f3 = 16.6 GHz, which does not occur in the single-layer structure. As a result, the upper limit frequency of the absorption bandwidth increased to 17.4 GHz. We also analyzed the role of cutting metal wire in this double-layer structure. We simulated the current distributions on the metal surface, which are shown in Figs. 3(b) and 3(c). From Fig. 3(b), we find that at f1 = 1.5 GHz, strong current distributions exist in the lower layer of the cutting metal wire and metal back sheet. The current directions are antiparallel, which is the same as that in the single-layer structure. In Fig. 4(c), the distributions are opposite. The current distributions in the upper layer of the cutting metal wire were strong and unidirectional at f3 = 16.6 GHz, which indicates strong electric resonance [25]. This is the main reason why the double-layer structure has better absorption property at higher frequency than the single-layer structure.
Fig. 3. (a) Simulation results of reflection coefficient of double-layer structure. Current distributions on metal surface of double-layer structure at (b) 1.5 GHz and (c) 16.6 GHz. (d) Simulation results of reflection coefficient of zigzag-shape structure. Absorption rate varying against (e) ITO film width a and (f) ITO film sheet resistance Zf.
Fig. 4. (a) The photography of fabricated sample. (b) Absorption of simulation and measurement results and (c) normalized impedance of finally optimized structure. The measurement far-field scattering patterns of our absorber and the mental sheet with the same size as our sample at (d) 1 GHz (e) 9 GHz (f) 18 GHz. The absorption rate varying against incident angle in (g) xoz plane and (h) yoz plane. (i) Average absorption among absorption band varying against incident angle. (j) The effective impedance varying against incident angle in xoz plane and yoz plane.
However, in the absorption band, |S11| is above −10 dB from 2.2 GHz to 6.3 GHz. By reviewing the reflectivity of our initial single-layer structure, we find that it has great impedance matching performance from 2.2 GHz to 6.3 GHz. For the single layer structure, the resonant frequency of cutting metal wire structure is 3.2 GHz. According to formula (2) and (3), the resonant frequency is related to the height of substrate. For the double layer structure, the height of substrate almost doubles. As a result, the resonant frequency f1 declines to 1.5 GHz. The absorption performance remains stable around resonant frequencies while deteriorates between resonant frequencies, which is the reason why the equivalent impedance from 2.2 GHz to 6.3 GHz is not matched well with the wave impedance of free space. Thus, we should carefully control each resonant frequency and the interval between them to reach a widest bandwidth.
In order to realize perfect absorption property across the whole spectrum, we proposed the zigzag-shaped structure by combining the double-layer structure and single-layer structure, as shown in the inset of Fig. 3(d). Then, we optimized the zigzag-shaped structure and the simulation results are shown in Fig. 3(d). The reflection coefficient is below −10 dB from 1.3 GHz to 18 GHz, which demonstrates that the effectiveness of the proposed method.
To obtain the optimum performance of the absorber, we studied some parameters which greatly influence the absorption property of our zigzag-shaped structure. Figure 3(e) illustrates the variations in absorption property with the width of ITO film a. As a increased from 13.4 mm to 13.8 mm, the lower limit frequency of the absorption band above 90% dropped steadily When a increased to 14 mm and there were no gaps between ITO films, the lower limit frequency sharply increased to 2.2 GHz. Then, we considered the effect of the sheet resistance of ITO films. Sheet resistance is a key index which determines the performance of ITO film. ITO is composed by the mixture of indium oxide and stannic oxide. Sheet resistance is determined by the proportion of indium oxide and stannic oxide and thickness of ITO. Figure 3(f) illustrates the variations in absorption property with the sheet resistance of ITO films Zf. When Zf increased from 50 $\Omega /sq$ to 150 $\Omega /sq$, the absorption of the zigzag-shaped structure increased smoothly, but the lower limit frequency of the absorption band slightly increased. When Zf increased to 200 $\Omega /sq$, the absorption property remained stable compared with 150 $\Omega /sq$. Thus, the sheet resistance is also an important parameter that greatly influences the impedance matching. In order to obtain good absorption property at lower frequency, we set Zf = 100 $\Omega /sq$ in our structure. Finally, we chose the following geometrical parameters for the optimized structure: h = 21 mm, l = 0.5 mm, k = 0.5 mm, d = 1 mm, p = 18 mm, q = 14mm, t = 1 mm, a = 13.8 mm, and sheet resistance of ITO film Zf = 100 $\Omega /sq$.
To demonstrate our design, we fabricated a sample consisting of a periodic array of optimized unit cells (15 × 20 cells with a total size of 420 × 360 mm2), its overall view is shown in Fig. 4(a). The fabrication process is carried out as following. Indium tin oxide (ITO) is coated on the surface of Polyethylene Terephthalate (PET) film by the technology of magnetron sputtering. Then, we use lithography technology to etch the predesigned ITO patterns on the surface of PET film. Similarly, the metal patterns on the surface of substrate are also etched by lithography technology. Next, ITO film is attached to the surface of the substrate. Two comb structures are erected on both sides of the metal sheet. The zigzag-shaped structures are embedded in the gaps of two comb structures so that they can stand on the surface of the metal sheet, which is shown in Fig. 4(a). The zigzag-shaped structure is also shown in the inset of Fig. 4(a). Then, we tested the fabricated sample. First, we measured the reflection coefficient of our absorber by comparing the scattering property of our sample with that of the metal sheet with the same size as the sample. In the experiments, the reflection coefficient was obtained by two horn antennas, i.e., a transmitting and receiving antenna, connected to an Agilent E8362C PNA vector analyzer to obtain electronic field information. Figure 4(b) shows the numerical and experimental reflective spectra against frequency for rxx. Our optimized structure realized high-efficiency broadband absorption above 90% from 1 GHz to 18 GHz. Good agreement was obtained between simulations and measurements, which indicates the feasibility of our method and design. The slight difference at low frequency was attributed to inevitable fabrication errors and imperfections of the incoming wavefronts generated by our microwave horn. To explore the physicics of high-performance absorption, we extracted the normalized impedance of our absorber from simulation results, shown in Fig. 4(c). The real part of the normalized impedance was close to 1 while the imaginary part was close to 0 in the absorption band, which proves that our absorber has great impedance matching property with free space. The slight fluctuation from 1 GHz to 6.7 GHz was caused by the magnetic resonance of the cutting metal wire; impedance matching is not very good in this band. Second, we measured the far-field scattering patterns of our sample and metal sheet at three representative frequencies of 1 GHz, 9 GHz, and 18 GHz respectively; the results are shown in Figs. 4(d)–4(f). We found that the main lobe of the absorber was more than 10 dB lower than that of metal sheet, while the side lobe stayed almost the same. As a result, the incident wave was mostly absorbed by our absorber instead of being scattered to other directions.
Finally, we considered the absorption property of our absorber, which is illustrated by oblique incident EM waves. The simulation results in Fig. 4(g) show that when the incident direction is in the xoz plane and the incident angle deviation from the normal direction of the array is ${\theta _E}$, the absorption property remains stable. When the incident angle increased to 60°, the absorption rate at low frequency decreased. As shown in Fig. 4(h), when the incident direction is in the yoz plane, the incident angle deviation from the normal direction of the array is ${\theta _H}$. With increasing incident angle, the absorption property gradually deteriorated. According to the absorption curves of oblique incidence, we also extracted the average absorption value from 1 GHz to 18 GHz, which is shown in Fig. 4(i). Therefore, the absorption property is insensitive to the incident angle of the TM wave, but sensitive to the TE wave. We considered the effective impedance of the TM case and TE case, which can be expressed as [35]
Figure 4(j) plots the curves of normalized effective impedance against varying incident angles. When the incident angle ${\theta _E}$ increased from 0° to 60°, ${\eta _{\textrm{e}ff - {\theta _E}}}$ decreased slightly from 1 to 0.5. The impedance matching was relatively stable, which is the main reason why absorption remains steady. When incident angle ${\theta _H}$ increased from 0° to 60°, ${\eta _{\textrm{e}ff - {\theta _H}}}$ increased much more significantly from 1 to 2 compared with ${\eta _{\textrm{e}ff - {\theta _E}}}$. As a result, the impedance matching deteriorated gradually with increasing incident angle in yoz plane, which is the reason for the decline in absorption. We also extracted the variations in effective impedance with incident angle of simulation results. First, we obtained the reflection coefficient S11 from the simulation results when incident angle changed from 0° to 60°, at intervals of 15°. Second, the effective impedance was determined by the following formula [36]We compared the performances of our work with the reported absorbers in term of bandwidth, thickness as well as angular stability in Table 1. We could find that the absorption bandwidth of our design is more outstanding than other work. What’s more, the absorption performance remains relatively stable for EM waves with large incident angles.
Table 1. The comparison between literatures and our work.
We also compared the absorption performance with the Razanov limit for non-magnetic absorbers to indicate the superiority of our work. K. N. Rozanov proposed that limitation of absorption bandwidth exists when the profile of a structure is constant [39]. As a result, our main target of this work is to get the widest absorption bandwidth when the thickness of structure is certain. K. N. Rozanov derived the absorption bandwidth limitation:
In our design, $\textrm{ln}{\rho _\textrm{0}}$ and $\mu$ are −0.5 and 1, respectively. Our design process includes three parts. In the first part, the initial single layer structure composed of cutting metal wire and ITO film realizes absorption above 90% from 2.2 GHz to 13.6 GHz. The height d is 21 mm. According to formula (7), we calculated that the upper limit frequency of absorption band is 13.7 GHz. Thus, the absorption band of our designed structure verges on Razanov limit but it does not pass the limit. In the second part, absorption band of double layer structure is from 6.3 GHz to 17.4 GHz. By calculation, we could find the bandwidth does not pass the Rozanov limit either. At last, we combined single layer and double layer structure to realize ultra-broadband absorption by optimizing.
3. Conclusion
In this paper, we proposed a novel structure by combining the ITO film and metal resonator. Based on this absorption structure, we designed a zigzag-shaped structure to realize ultra-broadband absorption with efficiency above 90% from 1 GHz to 18 GHz. The absorption performance was significantly enhanced compared with existing technology. In addition, a microwave sample was fabricated and the experimental results are in good agreement with simulation results, which demonstrates that our proposed structure is effective and can realize high-efficiency and ultra-broadband absorption as predicted. Finally, we analyzed absorber performance when illuminated by an oblique incident wave. The results show that the absorption property remains stable when the incident angle increased to 60° in xoz plane, but gradually deteriorates when the incident angle increased in yoz plane. Our proposed structure provides new insights on absorber design and will have potential applications in the stealth field.
Funding
National Natural Science Foundation of China (61701572, 61871394, 61901512); Natural Science Foundation of Shaanxi Province (2019JQ-013).
Disclosures
The authors declare no conflicts of interest.
References
1. M. Johansson, C. Holloway, and E. Kuester, “Effective electromagnetic properties of honeycomb composites, and hollow-pyramidal and alternating-wedge absorbers,” IEEE Trans. Antennas Propag. 53(2), 728–736 (2005). [CrossRef]
2. T. Deng, Y. Yu, Z. Shen, and Z. Chen, “Design of 3-D multilayer ferrite-loaded frequency-selective rasorbers with wide absorption bands,” IEEE Trans. Microwave Theory Tech. 67(1), 108–117 (2019). [CrossRef]
3. Z. Wang, Z. Zhang, X. Quan, and P. Cheng, “A perfect absorber design using a natural hyperbolic material for harvesting solar energy,” Sol. Energy 159, 329–336 (2018). [CrossRef]
4. M. Chen, M. Kim, A. M. Wong, and G. V Elefteriades, “Huygens’ metasurfaces from microwaves to optics,” Nanophotonics 7(6), 1207–1231 (2018). [CrossRef]
5. A. Epstetin and G. V Eleftheriades, “Huygens’ metasurfaces via the equivalence principle: design and applications,” J. Opt. Soc. Am. B 33(2), A31–A35 (2016). [CrossRef]
6. T. Cai, G. M. Wang, J. G. Liang, Y. Q. Zhuang, and T. J. Li, “High-performance transmissive meta-surface for C-/X-band lens antenna application,” IEEE Trans. Antennas Propag. 65(7), 3598–3606 (2017). [CrossRef]
7. L. Zhang, J. Ding, H. Zheng, S. An, H. Lin, B. Zheng, Q. Du, G. Yin, J. Michon, Y. Zhang, Z. Fang, M. Shalaginov, L. Deng, T. Gu, H. Zhang, and J. Hu, “Ultra-thin high-efficiency mid-infrared transmissive Huygens meta-optics,” Nat. Commun. 9(1), 1481 (2018). [CrossRef]
8. T. Cai, G. M. Wang, S. W. Tang, H. X. Xu, J. W. Duan, H. J. Guo, F. X. Guan, S. L. Sun, Q. He, and L. Zhou, “High-efficiency and full-space manipulation of electromagnetic wave fronts with metasurfaces,” Phys. Rev. Appl. 8(3), 034033 (2017). [CrossRef]
9. M. Jia, Z. Wang, H. Li, X. Wang, W. Luo, S. Sun, Y. Zhang, Q. He, and L. Zhou, “Efficient manipulations of circularly polarized terahertz waves with transmissive metasurfaces,” Light: Sci. Appl. 8(1), 16 (2019). [CrossRef]
10. T. Cai, S. W. Tang, G. M. Wang, H. X. Xu, S. L. Sun, Q. He, and L. Zhou, “High-performance bifunctional metasurfaces in transmission and reflection geometries,” Adv. Opt. Mater. 5(2), 1600506 (2017). [CrossRef]
11. H. P. Li, G. M. Wang, T. Cai, J. G. Liang, and X. J. Gao, “Phase-and-amplitude-control metasurfaces for antenna main-lobe and side-lobe manipulations,” IEEE Trans. Antennas Propag. 66(10), 5121–5129 (2018). [CrossRef]
12. Y. Y. Liang, H. Z. Liu, F. Q. Wang, H. Y. Meng, J. P. Guo, J. F. Li, and Z. C. Wei, “High-efficiency, near-diffraction limited, dielectric metasurface lenses based on crystalline titanium dioxide at visible wavelengths,” Nanomaterials 8(5), 288–293 (2018). [CrossRef]
13. H. P. Li, G. M. Wang, T. Cai, H. S. Hou, and W. L. Guo, “Wideband transparent beam-forming metasdevice with amplitude-and-phase-controlled metasurface,” Phys. Rev. Appl. 11(1), 014043 (2019). [CrossRef]
14. F. F. Qin, Z. Z. Liu, Z. Zhang, Q. Zhang, and J. J. Xiao, “Broadband full-color multichannel hologram with geometric metasurface,” Opt. Express 26(9), 11577 (2018). [CrossRef]
15. J. Deng, Z. Li, G. Zheng, J. Tao, Q. Dai, L. Deng, P. He, Q. Deng, and Q. Mao, “Depth perception based 3D holograms enabled with polarization-independent metasurfaces,” Opt. Express 26(9), 11843 (2018). [CrossRef]
16. L. L. Li, T. J. Cui, W. Ji, S. Liu, J. Ding, X. Wan, Y. B. Li, M. H. Jiang, C. W. Qiu, and S. Zhang, “Electromagnetic reprogrammable coding-metasurface holograms,” Nat. Commun. 8(1), 197 (2017). [CrossRef]
17. J. Schalch, G. Duan, X. Zhao, Z. Xin, and R. Averitt, “Terahertz metamaterial perfect absorber with continuously tunable air spacer layer,” Appl. Phys. Lett. 113(6), 061113 (2018). [CrossRef]
18. J. Suen, K. Fan, and W. Padilla, “A zero-rank, maximum nullity perfect electromagnetic wave absorber,” Adv. Opt. Mater. 7(8), 1801632 (2019). [CrossRef]
19. M. C. Tang, Y. N. Jiang, and X. P. Zeng, “Ultra-thin and broadband tunable metamaterial graphene absorber,” Opt. Express 26(2), 1681–1688 (2018). [CrossRef]
20. M. H. Luo, S. Shen, L. Zhou, S. L. Wu, Y. Zhou, and L. S. Chen, “Broadband wide-angle and polarization independent metamaterial absorber for the visible regime,” Opt. Express 25(14), 16715–16724 (2017). [CrossRef]
21. T. T. Nguyen and S. J. Lim, “Wide incidence angle-insensitive metamaterial absorber for both TE and TM polarization using eight-circular-sector,” Sci. Rep. 7(1), 3204 (2017). [CrossRef]
22. D. J. Lee, H. J. Jeong, and S. J. Lim, “Electronically switchable broadband metamaterial absorber,” Sci. Rep. 7(1), 4891 (2017). [CrossRef]
23. T. T. Nguyen and S. J. Lim, “Bandwidth-enhanced and wide angle-of-incidence metamaterial absorber using a hybrid unit cell,” Sci. Rep. 7(1), 14814 (2017). [CrossRef]
24. Y. Zhuang, G. Wang, Q. Zhang, and C. Zhou, “Low-Scattering Tri-Band Metasurface using Combination of Diffusion, Absorption and Cancellation,” IEEE Access 6, 17306–17312 (2018). [CrossRef]
25. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, “A perfect metamaterial absorber,” Phys. Rev. Lett. 100(20), 207402 (2008). [CrossRef]
26. F. Ding, Y. X. Cui, X. C. Ge, Y. Jin, and S. He, “Ultra-broadband microwave metamaterial absorber,” Appl. Phys. Lett. 100(10), 103506 (2012). [CrossRef]
27. N. T. Q. Hoa, T. S. Tuan, L. T. Hieu, and B. L. Giang, “Facile design of an ultra-thin broadband metamaterial absorber for C-band applications,” Sci. Rep. 9(1), 468 (2019). [CrossRef]
28. Y. J. Kim, J. S. Hwang, Y. J. Yoo, B. X. Khuyen, J. Y. Rhee, X. F. Chen, and Y. P. Lee, “Ultrathin microwave metamaterial absorber utilizing embedded resistors,” J. Phys. D: Appl. Phys. 50(40), 405110 (2017). [CrossRef]
29. D. Kundu, A. Mohan, and A. Chakrabarty, “Single-layer wideband microwave absorber using array of crossed dipoles,” IEEE Antennas Wirel. Propag. Lett. 15, 1589–1592 (2016). [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. Express 25(24), 30241 (2017). [CrossRef]
31. A. D. Khan and M. Amin, “Tunable Salisbury screen absorber using square lattice of plasmonic nanodisk,” Plasmonics 12(2), 257–262 (2017). [CrossRef]
32. J. Zhao, C. Zhang, Q. Cheng, J. Yang, and T. J. Cui, “An optically transparent metasurface for broadband microwave antireflection,” Appl. Phys. Lett. 112(7), 073504 (2018). [CrossRef]
33. Z. G. Dong and M. X. Xu, “Negative refraction with magnetic resonance in a metallic double-ring metamaterial,” Appl. Phys. Lett. 92(6), 064101 (2008). [CrossRef]
34. C. Granqvist and A. Hultaker, “Transparent and conducting ITO films: new developments and applications,” Thin Solid Films 411(1), 1–5 (2002). [CrossRef]
35. H. A. Wheeler, “Simple relations derived from a phased-array antenna made of an infinite current sheet,” IEEE Trans. Antennas Propag. 13(4), 506–514 (1965). [CrossRef]
36. D. R. Smith, D. C. Vier, T. Koschny, and C. M. Soukoulis, “Electromagnetic parameter retrieval from inhomogeneous metamaterials,” Phys. Rev. E 71(3), 036617 (2005). [CrossRef]
37. W. Jiang, L. L. Yan, H. Ma, Y. Fan, J. F. Wang, M. D. Feng, and S. B. Qu, “Electromagnetic wave absorption and compressive behavior of a three-dimensional metamaterial absorber based on 3D printed honeycomb,” Sci. Rep. 8(1), 4817 (2018). [CrossRef]
38. Y. J. Huang, J. Luo, M. B. Pu, Y. H. Guo, Z. Y. Zhao, X. L. Ma, X. Li, and X. G. Luo, “Catenary electromagnetic for ultra-broadband lightweight absorbers and large-scale flat antennas,” Adv. Sci. 6(7), 1801691 (2019). [CrossRef]
39. K. N. Rozanov, “Ultimate thickness to bandwidth ratio of radar absorbers,” IEEE Trans. Antennas Propag. 48(8), 1230–1234 (2000). [CrossRef]
