In this work, we proposed an ultra-wideband absorber using the dispersion characteristics of spoof surface plasmon polaritons (SSPP). The electromagnetic waves propagating on the SSPP structure have extremely slow group velocity and are confined in the sub-wavelength regions near the asymptotic frequency of the SSPP, and that electromagnetic energy can be absorbed by the high-lossy substrate and lumped resistors. The metallic strips of the SSPP structure are vertically bended and loaded with lumped resistors, and their lengths are linearly varied. This absorber performs excellent absorption ability, including ultra-wideband absorption, almost polarization insensitive performance, and high absorptivity under large incident angles for both transverse magnetic and transverse electronic waves. Based on the simulation analysis, an absorber sample was fabricated and measured, and the simulated and measured results are consistent with each other, validating the design method.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Over the past two decades, metamaterials have been proven that they can be used to manipulate electromagnetic (EM) waves [1–3]. For example, metamaterials can perform unique EM properties, such as anomalous refection and refraction, perfect lensing and invisibility cloaking [4–12]. Recently, absorbers based on metamaterials have become a research focus. In 2008, Landy et al. demonstrated a perfect metamaterial-based absorber which can achieve nearly-unity absorptivity in a narrow microwave band . Ever since then, many kinds of metamaterial-based absorbers have been proposed to extend their application abilities, such as wide operation band, high absorptivity, polarization insensitivity and so forth [14–17]. Basically, the working mechanism of these previously reported metamaterial-based absorbers can be described by effective medium theory. Metamaterial absorbers can be characterized by complex electric permittivity ɛ(ω)=ɛ1+jɛ2 and magnetic permeability μ(ω)=μ1+jμ2. When the impedance of metamaterial absorbers can match the free space impedance by adjusting the dimensions of electronic resonant component and magnetic resonant component, the incident EM waves will be absorbed, and the high absorption of metamaterial absorbers is mainly obtained due to the resonant loss [18–21]. However, for those traditional metamaterial absorbers, the absorptivity will deteriorate drastically when the incident angle increases, especially for the near-glancing incidence [18–26]. The reason of the absorptivity deterioration is that the resonance behaves intensely under the normal incidence and becomes weaker when the incident angle becomes larger.
Spoof surface plasmon polariton (SSPP) is a kind of surface wave which can mimic the high-confinement characteristics of surface plasmon polariton (SPP) at microwave frequencies [27–29]. To obtain SSPP, arrays of slits, holes, and blocks have been embellished on the metallic surface to support and propagate SSPP among those previously reported works [27–33]. SSPP structure has an asymptotic frequency. When the frequency approaches the asymptotic frequency, the group velocity of EM waves will become smaller and approach zero finally. The EM waves will be “stagnated” and confined tightly on the interface between metallic structure and substrate when its group velocity is extremely slow, and the high-lossy substrate, lumped resistors can absorb the EM energy. The first work using SSPP structure to design absorber was reported by Y. Q. Pang et al. . They used a periodically-arranged SSPP structure composed of corrugated metallic strips whose depth decreases linearly to obtain a broadband absorption. However, Pang’s work just dealt with the normally incident EM waves, but it opened up a new path to design absorber whose working mechanism is different with that of the previous absorbers. For the absorber under the oblique incidence, J. Yu et al. designed a bended SSPP-based absorber which can operate in a wide band, while maintaining a high angular stability . However, the bandwidth of the absorber can be further enhanced, and the absorber still can operate effectively when the incident angle increases up to higher degrees, even to the 80°-90°.
In this work, we proposed a SSPP-based absorber composed of two vertically placed metallic strip parts which are loaded with lumped resistors. This absorber can convert the incident EM waves with large incident angle to the high-confined SSPP mode, and the loaded resistors and high-lossy substrate can absorb the EM energy efficiently. This proposed absorber exhibits ultra-wideband absorption both under the normal and oblique incidence. Even when the incident angle increases up to 80°, the absorptivity still can be maintained above 80% in the frequency band of 7.29-15.79 GHz. Moreover, the presented absorber is almost polarization insensitive for both normal and oblique incidence. Based on the simulation results, we fabricated and measured a sample, and the measured results and simulated ones are consistent with each other. To the best of our knowledge, the absorber presented in this work is the first absorber which can achieve such high absorptivity under the large incident angle, while maintaining ultra-wide operation band. This proposed absorber will have great potential applications in stealth technology and mutual EM coupling reduction.
2. Exploration of the SSPP as an absorber
The unit cell of the SSPP structure in this work is schematically illustrated in Fig. 1(a). This unit cell has two layers: the top layer is a metallic strip, in which the periodicity and the gap width between two adjacent strips are marked as p and a, and the length of metallic strip is denoted as h. The bottom layer is substrate. The dielectric substrate is chosen as a commercial printed circuit board (PCB) with relative permittivity ɛr=4.4 and the loss tangent tanδ=0.034, and its thickness t is 1 mm, while the metal is chosen as copper with a thickness of 0.035 mm. At microwave frequencies, this structure can support the propagation of SSPP. Using the Eigen-mode solver of the commercial software CST Microwave Studio, we analyze the dispersion characteristics of this SSPP unit cell. First, we draw the dispersion relations of different modes and Ey component of near field distributions under a fixed h=8 mm, as displayed in Fig. 1(b). As presented, this SSPP structure can support two modes, corresponding to two dispersion relations. The simulated results show that the Ey-field is anti-symmetric for the first mode (odd mode), whereas the second mode (even mode) is symmetric which works in higher frequency. This is obvious that the odd mode is the fundamental mode of this proposed SSPP structure. The polarization of the incident EM waves is x-polarized (the electric field is polarized along the x direction), and the incident direction is along the z direction, which clearly show that the incident EM waves in free space is odd mode. Under the excitation of x-polarized EM waves, the proposed SSPP structure only support the odd mode, so we just need to analyze the fundamental mode of the SSPP structure for the incident EM waves.
Figure 2 illustrates the simulated dispersion relations of SSPP unit cell with respect to variation of metallic strip length h. We can observe that the dispersion curves deviate obviously from the lightline and finally approach the asymptotic frequencies, demonstrating that the EM waves can be tightly confined in the sub-wavelength region by the structured surface. Near the asymptotic frequencies, the EM waves propagate with extremely low group velocity. In this case, the SSPP waves will be dissipated when propagating along the interface between metals and substrate if the structure is lossy. To verify our analysis, we take a one-dimensional array along z direction with metallic strip length h=8 mm as an example, as shown in Fig. 3(a). In this case, the asymptotic frequency of SSPP structure is 10.5 GHz. The incident EM waves are polarized along the x direction and illuminate the SSPP structure from + z to -z direction. The S parameters are simulated and shown in Fig. 3(b). The absorptivity can be calculated by A=1-|S11|2-|S21|2, and its result is displayed in Fig. 3(b) as well. As expected, the absorptivity will be as high as above 60% in the frequency range of 10.4-11.1 GHz, which is near the asymptotic frequency. We also present the Ey component of near field distributions at 8 GHz, 10 GHz and 12 GHz as an example, as displayed in Fig. 3(c). It can be concluded that the SSPP structure can support the propagation of EM waves at 8 GHz and 10 GHz because these two frequencies are smaller than the asymptotic frequency, while the structure reflect the incident EM waves at 12 GHz for that this frequency is higher than asymptotic frequency.
Typically, most absorbers are reflected ones, so we add a metallic ground on the bottom of the one-dimensional array of SSPP structure, and the modified structure is shown in Fig. 4(a). In this case, the absorptivity can be determined as A=1-|S11|2 because there are no EM waves leaking out, and the absorptivity of this grounded SSPP structure and its comparison with that of SSPP structure without metallic ground are presented in Fig. 4(b). We can observe that the absorptivity of the grounded SSPP structure also can maintain at a high level near the asymptotic frequency. Moreover, its absorptivity even behaves better than that of the SSPP structure without the metallic ground, and the reason is that some unabsorbed EM waves are reflected by the metallic ground and are reabsorbed by the SSPP structure. From Fig. 2, we can observe that the asymptotic frequencies are different under different metallic strip length h. If the metallic strip length of one-dimensional array SSPP structure is consistent, then the absorption bandwidth will be very narrow. In order to broaden the bandwidth, we analyze a SSPP structure made of SSPP strips with linearly varied length from 3 mm to 8 mm, as shown in Fig. 4(c), and its absorption behavior is displayed in Fig. 4(d). As we can see, the SSPP structure has absorption ability when the frequency is larger than 10.2 GHz, which is near the asymptotic frequency corresponding to the SSPP strip with largest h value of 8 mm.
From above discussions, we can conclude that the dispersion characteristic is the defining factor to realize absorption. The SSPP structure can convert the incident EM waves to SSPP mode with limited low group velocity near the asymptotic frequency of SSPP structure, and these waves can be dissipated by the substrate with high loss. Furthermore, the linearly varied length of SSPP strips will contribute to broaden the bandwidth of absorber. In addition to the high-lossy substrate, the lumped resistors also can be used to absorb EM energy. The SSPP structure loaded with lumped resistors is illustrated in Fig. 4(e). As shown in Fig. 4(f), we simulate the absorptivity when the resistor values are set as 5, 20, 200, and 1000ohm, and it can be seen that the absorber has good absorptivity when the value of resistor varies from 20 to 200ohm. We can choose any values from 20-200ohm, and we choose the resistor with 200ohm in this paper. This SSPP structure can behave good absorption performance in the frequency range larger than 10 GHz, and it can even realize perfect absorption when the frequency is larger than 13 GHz.
3. Absorber design for large incident angle
The geometry of the proposed absorber is presented in Fig. 5(a). Considering the fabricated convenience, it is better to choose the number of plasmonic layers as 2 or 4, resulting to 90° or 45° angle between them respectively. In order to realize almost polarization insensitive absorptivity for both transverse magnetic (TM) and transverse electronic (TE) waves under the normal and oblique incidence, we choose the number of layers as 4, and the angle between them is 45°. Moreover, it is stressed that the metallic strips are vertically bended, which can maintain a high absorptivity for the oblique incident EM waves under large incident angles.
This absorber has two parts: the metallic ground and SSPP structure. The four plasmonic layers can be categorized as “PartI” and “PartII”, respectively, as shown in Fig. 5(b) and (c). For the “PartI”, the substrate is covered by four metallic structures, which are marked as M1, M2, M3 and M4. The top surface of the substrate is cover with M1 and M2, while the bottom surface is covered with M3 and M4. Note that M1 and M3 are symmetrical with each other, so are M2 and M4. “PartII” has the same formation with “PartI” except for two parameters of lds and ds. After proper optimization, the detailed geometry parameters are shown as follows: a=0.6 mm, p=0.9 mm, l=8 mm, dl=ds*sqr(2), ha=3 mm, hs=0.5 mm, ls=1 mm, s=0.4 mm, h=9.9 mm, ldl=5 mm, lds=2 mm, ds=10 mm, he=20 mm, t=0.8 mm, and Ri=200Ω. From Fig. 2, we can see that the asymptotic frequency is about 40 GHz when the metallic strip length is chosen as 2 mm. The asymptotic frequency is even higher than 40 GHz when ls=1 mm, thus we can predict that this absorber can operate in a wide frequency band, even at the frequency higher than 40 GHz. In order to get simplicity and brevity in the simulation process, we just simulate the proposed absorber in the frequency range of 2-40 GHz.
3.1 Absorption performance under the TM waves
The absorption performance of the proposed absorber mainly originates by the high loss of the substrate and the great loss of lumped resistors. For TM waves, because the electric field falls into the incident plane (which is determined by the incident direction and the normal vector of the sample), as shown in Fig. 6(a), and the metallic gratings are perpendicularly-bended ones, so the absorber even can maintain a high absorptivity with large incident angles. The simulated absorptivity results with different incident angles of θ are shown in Fig. 7. In the frequency range of 4.36-40 GHz, the absorptivity can be maintained above 90% when the incident angle varies from 0° (normal incidence) to 20°. As we mentioned above, the lower operating frequency of the straight grating structure shown in Fig. 5 is 10.2 GHz, while that of the perpendicularly bended one is 4.36 GHz. The reason is that the bended grating structure has longer route for inductive current, so the operating frequencies shift to the lower ones. When the incident angle tilts up to 40° and 60°, the absorptivity is above 90% in the frequency range of 6.33-40 GHz. When the incident angle tilts up to 70° and 75°, the absorptivity is larger than 80% in the frequency band of 6.45-40 GHz. Even when the incident angle reaches 80°, the absorptivity still can be maintained above 80% in the frequency band of 7.29-15.79 GHz.
Additionally, this proposed absorber is almost polarization insensitive. Because the absorber is a symmetric one and the angle between two adjacent plasmonic layers is 45°, so we just analyze the absorption performance under the polarization angle (φ) from 0° to 45°. Under the conditions with incident angles (θ) of 0°, 60° and 70°, the absorptivity properties with respect to different polarization angles (φ) are presented in Fig. 8. It clearly shows that the absorptivity curves nearly unchanged when the polarization angle (φ) varies from 0° to 45°.
3.2 Absorption performance under the TE waves
The proposed absorber has symmetric structure, so the absorption performances under the normal incidence for TM and TE waves are the same. For TE waves, the incident plane is perpendicular to the electric field, as shown in Fig. 6(b), so the absorption performance of TE waves under the oblique incidence will deteriorate compared with that of the TM waves. However, the absorber still has high absorptivity under the oblique incidence for TE waves, and the absorptivity curves with different incident angles of θ are displayed in Fig. 9. In the frequency range of 4.36-40 GHz, the absorptivity can be maintained above 90% when the incident angle increases from 0° to 40°. When the incident angle tilts up to 60° and 70°, the absorptivity curves vibrate sharply, indicating that the absorption performance deteriorate drastically. In the majority parts of the frequency range of 3.78-40 GHz, the absorptivity can be still maintained above 80%.
The absorptivity is still almost polarization insensitive, as shown in Fig. 10. When the polarization angle (φ) increases from 0° to 45° under the normal incidence, the absorptivity curves remain stable. In the case of θ=60°, the absorptivity can stay above 80% in the most parts of frequency range of 3.78-40 GHz under different polarization angles.
4. Scattering performance of the proposed absorber
In order to make a better understanding of the absorption performance of the proposed absorber, we simulate the scattering performance of an absorber sample which is composed of 13×13 unit cells, as shown in Fig. 11, and we also simulate a metallic plate with same area as a comparison. It is stressed that we just simulate in the frequency range of 5-20 GHz to economize the simulation time.
As discussed above, the absorption performance under the normal incidence for TE and TM waves are the same. Figure 12(a) displays the boresight bistatic radar cross section (RCS) of metallic plate and proposed absorber under normal incidence. It can be observed that the backward bistatic RCS of metallic plate can be significantly reduced by covering it with proposed absorber. As demonstrated in Fig. 12(a), the boresight RCS is suppressed by at least 8.5 dB from 5 GHz to 6.2 GHz and by more than 10 dB from 6.3 GHz to 20 GHz. The simulated 3-D scattering patterns of metallic plate and proposed absorber at 10 GHz and 14 GHz are illustrated in Fig. 12(b), and the boresight bistatic RCS of the proposed absorber can be dramatically reduced compared with that of the metallic plate.
For oblique incidence, the main beam of the scattering pattern of the metallic plate will point to the specular direction with respect to the incident direction, so we compare the specular bistatic RCS of metallic plate and proposed absorber. When the incident angle θ is chosen as 60°, the specular bistatic RCS for TE and TM waves of proposed absorber and their comparison with that of metallic plate are presented in Fig. 13(a) and Fig. 14(a), respectively. For TE waves, the specular RCS is suppressed by at least 8.4 dB from 9.1 GHz to 10.8 GHz and by more than 10 dB from 5 GHz to 9 GHz and from 10.9 GHz to 20 GHz. For TM waves, the specular RCS is suppressed by more than 10 dB from 7.3 GHz to 20 GHz. The simulated 3-D scattering patterns of metallic plate and proposed absorber at 10 GHz and 14 GHz for TE waves and TM waves are illustrated in Fig. 13(b) and Fig. 14(b), and the specular bistatic RCS of the proposed absorber can be dramatically reduced compared with that of the metallic plate. We note that there is another peak value in the reflecting direction for TE waves. However, this peak value still has 8.7 dB of RCS reduction compared with the peak value of same-sized metal. Moreover, the proposed absorber has relatively better absorption performance for TM waves compared with TE waves, according to the 3-D scattering patterns under oblique incidence.
5. Experimental validation
In order to validate the absorption performance of the proposed absorber, the experimental demonstration of this ultra-wideband absorber under large incident angle is performed. We use the super glue to fix the dielectric substrate on the metallic plate. The fabricated sample is shown in Fig. 15, and its size is 260×260m2, including 13×13 unit cells. As shown in Fig. 16(a), a pair of horn antennas is placed in front of the absorber under the normal incidence, the distance between the measured sample and antennas is 600 mm (10λ0, λ0 is the wavelength at 5 GHz), which satisfies the far-field condition of horn antennas to ensure plane waves are radiating to the measured absorber. The horn antenna is chosen as the commonly used 10180-SF horn antenna which can operate effectively in the frequency range of 1-18 GHz. These two antennas are connected with an Agilent vector network analyzer (VNA) to control the transmitting waves and analyze the receiving waves. First, we measured the reflect coefficient of the fabricated absorber. Second, we measured the reflect coefficient of a same-area metallic plate in the same measurement environment. Because the environment is unchanged when measuring the absorber and metallic plate, the propagation loss can be neglected as much as possible. Compared with the reflect coefficients of absorber and metallic plate, we can get the measured absorptivity. For oblique incidence, these two antennas move along a pathway symmetrically, and the shape of the pathway is an arc whose center is the middle of the measured absorber screen. The radius of this arc is also set as 600 mm (10λ0, λ0 is the wavelength at 5 GHz) to satisfy the far-field condition of horn antennas, as shown in Fig. 16(b). The operating band of these two horn antennas is 1-18 GHz, and the absorber has good absorption ability when the frequency is larger than 3 GHz based on simulated results, we just measured this absorber in the frequency band of 3-18 GHz. Moreover, it is a little difficult to define polarization angle (φ) precisely limited to our measured environment, so we simply measured the normal and oblique incidence with incident direction of (θ, φ) = (0°, 0°), (40°, 0°) and (70°, 0°) limited to our measured environment.
When the incident direction is set as (0°, 0°), the measured absorptivity curves under TM and TE waves are displayed in Fig. 17(a) and (b). When the incident direction is set as (40°, 0°), the measured absorptivity curves under TM and TE waves are displayed in Fig. 18(a) and (b), and the measured absorptivity curves under TM and TE waves with incident direction of (70°, 0°) are shown in Fig. 19(a) and (b), respectively. We see that the measured results for both TM and TE waves are perfectly consistent with the simulated ones. Note that the measured absorption spectra are slightly different with simulation, which is caused by the fabrication and measurement imperfection.
In summary, we utilize the dispersion relations of SSPP to design absorber. Near the asymptotic frequency of SSPP, the group velocity of SSPP is extremely slow and the SSPP will be confined within sub-wavelength regions, and then the EM waves can be dissipated by high-lossy substrate or lumped resistors. The proposed absorber is composed of two vertically placed metallic strip parts which are loaded with lumped resistors, and the length of metallic strips is linearly varied. This proposed absorber exhibits ultra-wideband absorption both under the normal and oblique incidence. Under the TM waves, this absorber even has strong absorption ability under the incident angle of 80°. The absorber also has high absorptivity with large incident angle under the TE waves. Moreover, this proposed absorber is almost polarization insensitive because it is symmetric. Based on the simulation analysis, an absorber sample was fabricated and measured. The measured results agree well with the simulated ones, validating the proposed design method. This proposed absorber is the first one which can realize high absorptivity under such large incident angle compared with those previously reported absorbers. This work has very promising applications in the RCS reduction and EM mutual coupling reduction.
Natural Science Basic Research Program of Shaanxi Province (2019JQ-103, 20200108, 2020022, 2020JM-350); National Postdoctoral Program for Innovative Talents (2019M653960, BX20180375); National Natural Science Foundation of China (61671464, 61701523, 61801508).
The authors declare no conflicts of interest.
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.
1. Y. Zhao, X. Y. Cao, J. Gao, X. Yao, T. Liu, W. Q. Li, and S. J. Li, “Broadband low-RCS metasurface and its application on antenna,” IEEE Trans. Antennas Propag. 64(7), 2954–2962 (2016). [CrossRef]
2. M. Mighani and G. Dadashzadeh, “Broadband RCS reduction using a novel double layer chessboard AM surface,” Electron. Lett. 52(14), 1253–1255 (2016). [CrossRef]
3. W. G. Chen, C. A. Balanis, and C. R. Birtcher, “Checkerboard EBG surfaces for wideband radar cross section reduction,” IEEE Trans. Antennas Propag. 63(6), 2636–2645 (2015). [CrossRef]
4. Y. Liu, K. Li, Y. T. Jia, Y. W. Hao, S. X. Gong, and Y. J. Guo, “Wideband RCS reduction of a slot array antenna using polarization conversion metasurfaces,” IEEE Trans. Antennas Propag. 64(1), 326–331 (2016). [CrossRef]
5. T. J. Cui, M. Q. Qi, X. Wan, J. Zhao, and Q. Cheng, “Coding metamaterials, digital metamaterials and programmable metamaterials,” Light: Sci. Appl. 3(10), e218 (2014). [CrossRef]
6. S. Liu, A. Noor, L. L. Du, L. Zhang, Q. Xu, K. Luan, T. Q. Wang, Z. Tian, W. X. Tang, J. G. Han, W. L. Zhang, X. Y. Zhou, Q. Cheng, and T. J. Cui, “Anomalous refraction and nondiffractive bessel-beam generation of terahertz waves through transmission-type coding metasurfaces,” ACS Photonics 3(10), 1968–1977 (2016). [CrossRef]
7. S. Liu, L. Zhang, Q. L. Yang, Q. Xu, Y. Yang, A. Noor, Q. Zhang, S. Lqbal, X. Wan, Z. Tian, W. X. Tang, Q. Cheng, J. G. Han, W. L. Zhang, and T. J. Cui, “Frequency-dependent dual-functional coding metasurfaces at terahertz frequencies,” Adv. Opt. Mater. 4(12), 1965–1973 (2016). [CrossRef]
8. S. Liu and T. J. Cui, “Flexible controls of scattering clouds using coding metasurfaces,” Sci. Rep. 6(1), 37545 (2016). [CrossRef]
9. Z. Y. Li, S. J. Li, B. W. Han, G. S. Huang, Z. X. Guo, and X. Y. Cao, “Quad-band transmissive metasurface with linear to dual-circular polarization conversion simultaneously,” Adv. Theory Simul. 4(8), 2100117 (2021). [CrossRef]
10. B. W. Han, J. Li, Z. Y. Li, G. S. Huang, J. H. Tian, and X. Y. Cao, “Asymmetric transmission for dual-circular polarized waves based on chiral metasurface,” Opt. Express 29(13), 19643–19654 (2021). [CrossRef]
11. H. H. Yang, T. Li, L. M. Xu, X. Y. Cao, L. R. Jidi, Z. X. Guo, P. Li, and J. Gao, “Low In-band-RCS antennas based on anisotropic metasurface using a novel integration method,” IEEE Trans. Antennas Propag. 69(3), 1239–1248 (2021). [CrossRef]
12. T. Li, H. H. Yang, Q. Li, L. R. Jidi, X. Y. Cao, and J. Gao, “Broadband low RCS and high gain microstrip antenna based on concentric ring-type metasurface,” IEEE Trans. Antennas Propag. 69(9), 5325–5334 (2021). [CrossRef]
13. 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]
14. S. Bhattacharyya, S. Ghosh, D. Chaurasiya, and K. V. Srivastava, “Wide-angle broadband microwave metamaterial absorber with octave bandwidth,” IET microwaves, antennas & propagation 9(11), 1160–1166 (2015). [CrossRef]
15. S. Bhattacharyya and K. V. Srivastava, “Triple band polarization-independent ultra-thin metamaterial absorber using electric field-driven LC resonator,” J. Appl. Phys. 115(6), 064508 (2014). [CrossRef]
16. H. Wakatsuchi, J. Paul, and C. Christopoulos, “Performance of customizable cut-wire-based metamaterial absorbers: absorbing mechanism and experimental demonstration,” IEEE Trans. Antennas Propag. 60(12), 5743–5752 (2012). [CrossRef]
17. Y. Z. Chen, Y. Nie, and R. Z. Gong, “A polarization-insensitive and omnidirectional broadband terahertz metamaterial absorber based on coplanar multisquares films,” Opt. Laser Technol. 48, 415–421 (2013). [CrossRef]
18. X. K. Kong, J. Y. Xu, J. J. Mo, and S. B. Liu, “Broadband and conformal metamaterial absorber,” Front. Optoelectron. 10(2), 124–131 (2017). [CrossRef]
19. Y. Z. Chen, H. L. Yang, Z. Z. Cheng, and B. X. Xiao, “A planar polarization-insensitive metamaterial absorber,” Photonics and Nanostructures-Fundamentals and Applications 9(1), 8–14 (2011). [CrossRef]
20. S. Ghosh, S. Bhattacharyya, and K. V. Srivastava, “bandwidth-enhancement of an ultrathin polarization incensitive metamaterial absorber,” Microw. Opt. Technol. Lett. 56(2), 350–355 (2014). [CrossRef]
21. B. Q. Lin, S. H. Zhao, X. Y. Da, Y. W. Fang, J. J. Ma, W. Li, and Z. H. Zhu, “Design of an ultra-compact metamaterial absorber,” Microw. Opt. Technol. Lett. 57(6), 1439–1441 (2015). [CrossRef]
22. Y. Z. Cheng and H. L. Yang, “Design, simulation, and measurement of metamaterial absorber,” Microw. Opt. Technol. Lett. 52(4), 877–880 (2010). [CrossRef]
23. J. P. Xu, J. Y. Wang, R. C. Yang, J. P. Tian, X. W. Chen, and W. M. Zhang, “Frequency-tunable metamaterial absorber with three bands,” Optik 172, 1057–1063 (2018). [CrossRef]
24. K. Qzden, O. M. Yucedag, and H. Kocer, “Metamaterial based broadband RF absorber at X-band,” International journal of electronics and communications (AEü) 70(8), 1062–1070 (2016). [CrossRef]
25. Y. Z. Cheng, H. L. Yang, Z. Z. Cheng, and N. Wu, “Perfect metamaterial absorber based on a split-ring-cross resonator,” Appl. Phys. A: Mater. Sci. Process. 102(1), 99–103 (2011). [CrossRef]
26. N. T. Q. Hoa, P. H. Lam, and P. D. Tung, “Wide-angle and polarization-independent broadband microwave metamaterial absorber,” Microw. Opt. Technol. Lett. 59(5), 1157–1161 (2017). [CrossRef]
27. W. X. Tang, H. C. Zhang, H. F. Ma, W. X. Jiang, and T. J. Cui, “Concept, theory, design, and applications of spoof surface plasmon polaritons at microwave frequencies,” Adv. Opt. Mater. 7(1), 1800421 (2019). [CrossRef]
28. P. H. He, H. C. Zhang, X. X. Gao, L. Y. Niu, W. X. Tang, J. Y. Lu, L. P. Zhang, and T. J. Cui, “A novel spoof surface plasmon polariton structure to reach ultra-strong field confinements,” Opto-Electron. Adv. 2(6), 19000101 (2019). [CrossRef]
29. H. F. Ma, X. Shen, Q. Chen, W. X. Jiang, and T. J. Cui, “Broadband and high-efficiency conversion from guided waves to spoof surface plasmon polaritons,” Laser Photonics Rev. 8(1), 146–151 (2014). [CrossRef]
30. H. Xiang, Y. Meng, Q. Zhang, F. F. Qin, J. J. Xiao, D. Han, and W. Wen, “Spoof surface plasmon polaritons on ultrathin metal strips with tapered grooves,” Opt. Commun. 356, 59–63 (2015). [CrossRef]
31. X. Gao, L. Zhou, Z. Liao, H. F. Ma, and T. J. Cui, “An ultra-wideband surface plasmonic filter in microwave frequency,” Appl. Phys. Lett. 104(19), 191603 (2014). [CrossRef]
32. D. W. Zhang, K. Zhang, Q. Wu, R. W. Dai, and X. J. Sha, “Broadband high-order mode of spoof surface plasmon polaritons supported by compact complementary structure with high efficiency,” Opt. Lett. 43(13), 3176–3179 (2018). [CrossRef]
33. H. C. Zhang, P. H. He, W. X. Tang, Y. Luo, and T. J. Cui, “Planar spoof SPP transmission lines: applications in microwave circuits,” IEEE microwave magazine 20(11), 73–91 (2019). [CrossRef]
34. Y. Q. Pang, J. F. Wang, H. Ma, M. D. Feng, Y. F. Li, Z. Xu, S. Xia, and S. B. Qu, “Spatial k-dispersion engineering of spoof surface plasmon polaritons for customized absorption,” Sci. Rep. 6(1), 29429 (2016). [CrossRef]
35. J. Yu, W. Jiang, and S. X. Gong, “Wideband angular stable absorber based on spoof surface plasmon polariton for RCS reduction,” IEEE antennas and wireless propagation letters 19(7), 1058–1062 (2020). [CrossRef]