Open Access
Add to BibSonomy
Abstract
This paper proposes a switchable broadband/narrowband absorber based on a hybrid metasurface comprising graphene and metal in the millimeter-wave regime. The designed absorber achieves broadband absorption when the surface resistivity of graphene Rs = 450 $\Omega/\Box$ and narrowband absorption when Rs = 1300 $\Omega/\Box$ and 2000 $\Omega/\Box$. The physical mechanism behind the graphene absorber is explored by analyzing the distributions of power loss, electric field, and surface current densities. An equivalent circuit model (ECM) based on transmission-line theory is derived to theoretically investigate the performance of the absorber, with ECM results in good agreement with simulation results. Furthermore, we fabricate a prototype and evaluate its reflectivity by applying various biasing voltages. The results obtained from the experiment are also consistent with those obtained from the simulation. When the external bias voltage is changed from +1.4 V to −3.2 V, the proposed absorber has an average reflectivity ranging from −5 dB to −33 dB. The proposed absorber has potential applications in radar cross-section (RCS) reduction, antenna design, electromagnetic interference (EMI) shielding, and EM camouflage techniques.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Electromagnetic (EM) absorbers have been widely used for microwave, millimeter-wave (mmW) and terahertz (THz) applications, including radar cross-section (RCS) reduction, EM compatibility and camouflage [1–3]. Conventional EM absorbers are usually made of lossy or non-reciprocal materials such as carbon and ferrite [4,5], which have inherent limitations of large thickness and high mass density, etc. Other techniques, such as the Salisbury screen [6] and the Jaumann absorber [7], have been proposed to address these issues. In fact, the Salisbury screen has a narrow bandwidth and the Jaumann absorber extends the bandwidth at the expense of increasing the total thickness. As opposed to the absorbers mentioned above, many modifications have been proposed, mainly focusing on thickness reduction [8], polarization-insensitivity [9], and bandwidth enhancement [10]. As for the frequency response, a variety of absorbers have been developed for single-band [11], multi-band [8,12], and broadband applications [13–15]. In some specific applications, tunable absorbers are more attractive due to their flexibility. Actively controlled elements, such as liquid crystal [16], PIN diodes [17], and vanadium dioxide (VO$_2$) [18,19], are adopted to realize reconfigurable characteristics. It should be emphasized that each type of tunable material or element has its unique property so they all face some obstacles or limitations. For example, the liquid crystal-based absorber has a narrow operating bandwidth. PIN diodes are usually used for relatively low frequencies. VO$_2$ exhibits a phase transition phenomenon under temperature control, but its performance is easy to be affected by operating temperature and is not suitable for high-power cases. Among tunable materials, graphene is a potential candidate due to its extraordinary performance in dynamic control and broadband response.
Graphene has the advantages of high electron mobility, optical transparency, and excellent mechanical property [20–24], etc. Alternatively, the conductivity of graphene can be dynamically controlled by electrostatic or chemical doping [24,25]. Recently, a large variety of graphene absorbers and multifunctional EM devices [25–29] from microwave to THz bands have been proposed and validated experimentally. By employing an electrostatic field bias, it is possible to alter the Fermi level of monolayer graphene, which causes a continuous change in the surface resistivity of graphene. The sandwiched graphene structure with ionic liquid is used in [25–31] to significantly enhance the control of surface resistivity. For example, Balci et al. propose a switchable radar-absorbing surface in the microwave regime [25]. Chen et al. propose a tunable perfect absorber based on graphene combined with metallic grating in the THz regime [26]. Song et al. demonstrate a graphene absorber that is both reconfigurable and broadband, which utilizes a resistive frequency selective surface (FSS) and a graphene capacitor [30]. In the latest work, some multifunctional devices are proposed with absorbing properties, which combine graphene and varactors to realize tunable amplitude and frequency, respectively [27–29,31]. However, most graphene-based absorbers are limited to a single function of narrowband, broadband, or multi-band absorption.
Recently, some attention has been attracted to reconfigurable multifunctional absorbers. Compared to traditional narrowband and broadband absorbers, switchable narrowband/broadband absorbers have the advantages of flexibility, cost-effectiveness, and improved bandwidth. Switchable broadband/narrowband bifunctional absorbers are proposed for applications in the increasingly complex EM environment. Some graphene-based absorbers with dual or multiple functionalities have been published. Chen et al. propose a switchable wideband-narrowband absorber based on graphene and VO$_2$ [32]. Zhuang et al. propose a switchable trifunctional absorber based on graphene for both broadband and narrowband operations in the THz regime [33]. Although these works show good performance, both of them have only performed numerical simulations without experimental verification.
In this article, a switchable broadband/narrowband absorber is proposed with the hybrid metasurface of graphene and metal structures. To the best of our knowledge, this is the first proposal and experimental validation of a switchable broadband/narrowband absorber based on large-area monolayer graphene in the mmW regime. When the surface resistivity of graphene $R_s$ = 1300 $\Omega/\Box$ and 2000 $\Omega/\Box$, the absorber achieves narrowband absorption at 27.6 GHz. When $R_s$ = 450 $\Omega/\Box$, the designed absorber achieves broadband absorption with a 10 dB bandwidth (with absorptivity $\ge$ 90%) of 18$\sim$37.3 GHz. The absorption amplitude can be finely adjusted by changing the bias voltage of graphene. In addition, we analyze the tunable physical mechanism of the proposed absorber using an equivalent circuit model (ECM) based on transmission-line theory, which shows good agreement with the full-wave simulation results. The ECM provides theoretical tools for the rapid design and analysis of the absorbers. Finally, a prototype is fabricated and measured, demonstrating that the operation state of the absorber can be tuned by applying different bias voltages. The proposed graphene absorber is designed to be switchable between narrowband and broadband absorptions and offers advantages in terms of wide bandwidth, reconfigurability, and adaptability. The switchable nature of the absorber allows for cost savings and a reduction in the number of absorbers needed for different applications. In addition, it provides improved performance and flexibility and can be adjusted to suit various scenarios.
2. Structure design and simulation
The graphene monolayer can be modeled as an infinitely thin conducting surface related to the Fermi energy level caused by chemical or electrostatic doping. The surface conductivity of graphene from the microwave to low terahertz range can be approximated from the well-known Kubo formula, which is dominated by intra-band contributions [34]:
The schematic of the proposed switchable graphene absorber is illustrated in Fig. 1(a). It consists of three parts from bottom to top, which are the metal Jerusalem cross FSS (JCFSS) layer on a grounded FR4 substrate, a foam substrate, and the graphene/ionic liquid/ITO structure (GIIS) layer [35]. The JCFSS is an array of crosses arranged in a pattern that filters EM waves in a specific frequency range [36]. Due to its resonant nature, the operating bandwidth of this FSS is typically narrow. The JCFSS is extensively applied in various fields, such as radar and communication systems, to eliminate unwanted frequencies and improve performance. The FR4 substrate is considered to be a dielectric layer with a relative permittivity of $\varepsilon _{r1}$ = 4.3, a loss tangent of tan $\delta _{1}$ = 0.035 and a thickness of $d_{1}$ = 0.1 mm. The bottom of the FR4 is a complete copper layer of 18 $\mu$m, which reflects all the incident waves. The foam substrate has a thickness of $d_{2}$ = 2 mm, and its relative permittivity and loss tangent are $\varepsilon _{r2}$ = 1.08 and tan $\delta _{2}$ = 0.005, respectively. The GIIS layer consists of a large area of monolayer graphene film and a high-resistivity (4500 $\Omega/\Box$) ITO film, both on a layer of polyethylene terephthalate (PET) substrate, which serves as two electrodes of GIIS. The Fermi level of graphene can be adjusted by employing an electrostatic field bias, resulting in a change of surface resistivity. The relative permittivity, loss tangent and thickness of PET substrate are $\varepsilon _{r3}$ = 3, tan $\delta _{3}$ = 0.02, and ${d}_{3}$ = 125 $\mu$m, respectively. A polyethylene (PE) film [30] containing the ionic liquid electrolyte [EMI][TFSA] [37] is sandwiched between the graphene and ITO film, forming a composite with relative permittivity of $\varepsilon _{r4}$ = 3, the electrical conductivity of $\sigma$ = 0.9 S/m and a thickness of ${d}_{4}$ = 12 $\mu$m. Figure 1(b) depicts one unit cell of the absorber and Fig. 1(c) shows the top view of a unit cell with the optimized geometric parameters described in the caption, where $P$, $l_1$, $l_2$, $w_1$, and $w_2$ represent the period, lengths, and widths, respectively.
Fig. 1. (a) Schematic illustration of the proposed switchable absorber. (b) Unit-cell of the hybrid structure with parameters of $d_1$ = 0.1 mm, and $d_2$ = 2 mm. (c) Top view of the metal unit cell with optimized parameters of $P$ = 2.7 mm, $l_1$ = 0.68 mm, $l_2$ = 1.5 mm, $w_1$ = 0.28 mm, and $w_2$ = 0.18 mm.
The structure is modeled and simulated using the full-wave EM software CST Microwave Studio, with Floquet boundary conditions in both the x- and y-directions, and with open (add space) conditions in the z-direction. The normal incident wave propagates along the negative z-axis, with the electric field component along the y-axis and the magnetic field component along the x-axis. In CST Microwave Studio, both the graphene monolayer and the ITO film are represented as Ohmic Sheets. The reflection coefficient ($S_{11}$) of the proposed graphene absorber can be obtained directly by full-wave simulation. Because the thickness of copper is significantly greater than the skin depth in the mmW range, the incident EM wave is predominantly reflected, resulting in a transmittance of $T = |S_{21} |^2 \approx 0$. Thus, the absorbance can be calculated using $A=1-R-T=1-|S_{11}|^{2}-|S_{21}|^{2} \approx 1-|S_{11}|^{2}$, where $R = |S_{11} |^2$ represents the reflectivity. As a result, the absorptance can be calculated simply by measuring the reflection coefficient ($S_{11}$). The computed reflection coefficient with different surface resistivity of graphene under normal incidence is shown in Fig. 2. For the 10 dB absorption condition, the proposed graphene absorber demonstrates narrowband absorption dips of $-$14 dB and $-$17.5 dB at 27.6 GHz, with corresponding surface resistivities of $R_s$ = 2000 $\Omega/\Box$ and 1300 $\Omega/\Box$, respectively. When $R_s$ = 450 $\Omega/\Box$, the designed absorber achieves broadband absorption dip of $-$37 dB with 10 dB bandwidth of 18$\sim$37.3 GHz. The fractional bandwidth (FBW) is calculated by the formula $FBW(\%) = 2\left (f_{upper}-f_{lower}\right )/\left (f_{upper}+f_{lower}\right )$, where $f_{upper}$ and $f_{lower}$ are the upper and lower frequencies, respectively, at which the reflection coefficient is less than $-$10 dB. For the broadband absorption state, the FBW of 10 dB ($FBW_{10dB}$) is calculated as 69.8%. Thus, the proposed structure can switch from a narrowband absorber to a broadband one. In addition, when $R_s$ = 125 $\Omega/\Box$, the reflectivity is around $-$5 dB in the frequency band of interest. The simulated results convincingly show that the proposed absorber can dynamically alter its reflected amplitude and bandwidth by changing the graphene resistivity in the frequency range of 15$\sim$40 GHz.
Fig. 2. Simulated reflection coefficients of the designed graphene absorber under various levels of graphene surface resistivity.
Furthermore, the reflection coefficients of the graphene absorber are calculated as the functions of incidence angle and frequency, as shown in Fig. 3. For the 10 dB absorption condition, the proposed graphene absorber demonstrates good switchable performance as the incidence angle varies from 0$^{\circ }$ to approximately 40$^{\circ }$, as depicted in Figs. 3(a), (c), and (d). The broadband absorption characteristic (with $R_s$ = 450 $\Omega/\Box$, as shown in Fig. 3(a)) undergoes a blue shift with increasing incidence angle, while the narrowband absorption characteristic is almost unchanged (with $R_s$ = 1300 $\Omega/\Box$, as shown in Fig. 3(c), and with $R_s$ = 2000 $\Omega/\Box$, as shown in Fig. 3(d)). It is indicated that the proposed absorber is not so sensitive to the oblique incidence angle.
Fig. 3. Calculated reflection coefficients with the surface resistivity $R_s$ of (a) 450 $\Omega/\Box$, (b) 760 $\Omega/\Box$, (c) 1300 $\Omega/\Box$, and (d) 2000 $\Omega/\Box$ as the functions of frequency and incidence angle.
To further study the proposed absorber, the impedance matching theory is introduced. The real and imaginary parts of the input impedance with varying surface resistivity of graphene are shown in Figs. 4(a) and (b). The input impedance ($Z_{in}$) is calculated as follow:
where $Z_0$ is the intrinsic impedance of free space. Figures 4(a) and (b) demonstrate that the input impedance of the absorber is influenced by the graphene surface resistivity. When $R_s$ = 760 $\Omega/\Box$, the real part of the input impedance is close to 377 $\Omega$ while the imaginary part approaches 0 at 27.6 GHz. In this case, the designed absorber has an input impedance that is equal to the intrinsic impedance of free space. Thus, the absorptance is relatively large within a narrow band. When $R_s$ = 450 $\Omega/\Box$, the real part of Z${}_{in}$ is close to 377 $\Omega$ in the frequency band of 18$\sim$37.3 GHz and the amplitude of the imaginary part is not so large, a broadband absorption state is provided.Fig. 4. (a) Real part and (b) imaginary part of the input impedance $Z_{in}$ under different surface resistivity of graphene.
3. Physical mechanism and equivalent circuit model (ECM)
To gain a deeper understanding of the physical mechanism, the power loss densities, electric field, and surface current density distributions in the cross-section of the absorber unit cell are simulated at the resonant frequency of 27.6 GHz with the surface resistivity $R_s$ of 125 $\Omega/\Box$, 450 $\Omega/\Box$, and 760 $\Omega/\Box$, as shown in Fig. 5. The power loss with $R_s$ = 125$\Omega/\Box$ is the smallest of the three cases because the graphene layer reflects a significant portion of the incident EM energy, as illustrated in Fig. 5(a). The power losses in the cases of $R_s$= 450 $\Omega/\Box$ and 760 $\Omega/\Box$ are mainly distributed on both the GIIS and JCFSS layers, especially on the JCFSS layer, as shown in Figs. 5(b) and (c). Similarly, it can be seen that there are strong electric fields on the GIIS and JCFSS layers, which indicates a resonance produced here, as shown in Figs. 5(e) and (f). In addition, we notice that currents also mainly distribute in the GIIS and JCFSS layers, as shown in Figs. 5(i) and (j). The currents on the top ITO and bottom graphene are in opposite directions. A similar current distribution can be noticed in the JCFSS layer. So near-perfect absorption will be achieved at the frequency of 27.6 GHz.
Fig. 5. Calculated (a)-(c) power loss densities, (d)-(f) electric field distributions, and (h)-(j) surface current distributions at the cross-section of the absorber unit cell at 27.6 GHz with the surface resistivity $R_s$ of 125 $\Omega/\Box$, 450 $\Omega/\Box$, and 760 $\Omega/\Box$, respectively.
To further get an insight into the physical mechanism, the ECM is applied to investigate the properties of the proposed absorber, as shown in Fig. 6. An RLC series circuit can represent the JCFSS. The impedance of the JCFSS unit cell can be expressed by:
where $R_m$, $L_m$, and $C_m$ denote the equivalent resistance, the distributed inductance, and the capacitance of the FSS, respectively. In the proposed model, the following approximate analytical expression can be utilized to compute the initial value of $L_m$ and $C_m$ [38]:Fig. 6. (a) Side view of the graphene-based absorber unit cell. (b) Equivalent circuit of the unit cell under normal incidence.
Figure 7 shows the calculated reflection coefficients with different graphene resistivity using two tools: the ECM and the CST full-wave simulator. The surface resistivity of graphene $R_s$ is selected as 125, 300, 450, 760, 1300, and 2000 $\Omega/\Box$. As shown in Fig. 7, the solid and dashed lines represent the CST and ECM results, respectively. The reflection coefficients obtained with the two tools are almost the same within the frequency range of 15 to 40 GHz. Therefore, the ECM can describe the EM behavior of the graphene absorber with good accuracy.
Fig. 7. Comparison of the reflection coefficients obtained with CST full-wave simulations and ECM calculations of the proposed graphene absorber under various values of surface resistivity.
4. Fabrication and measurements
To verify the proposed absorber experimentally, a prototype composed of GIIS and JCFSS is fabricated, as depicted in Fig. 8. The fabrication of the GIIS part is described in detail in our previous article [35]. The JCFSS is fabricated with the standard PCB technology which contains 48 $\times$ 48 unit cells printed on a 0.1 mm thick FR4 substrate, as shown in Fig. 8(e). Then the GIIS, the polymethacrylimide (PMI) foam, and the JCFSS parts are aligned and tightly pressed together to form the proposed absorber prototype using a 0.05 mm thick 3M tape, as shown in Fig. 8(c). The size of the entire absorber is 150 mm $\times$ 150 mm $\times$ 2.4 mm. In the measurement process, an Naval Research Laboratory (NRL) arch method [30,39] is utilized to assess the reflection properties of the prototype. The measurement system, shown in Fig. 8(a), is set up in an environment surrounded by absorbing material to minimize EM interference (EMI) errors. The system contains a vector network analyzer (VNA), a DC-regulated power supply, two horn antennas ($A_1$ and $A_2$), and the data acquisition and processing system with a personal computer (PC). The VNA is controlled by the PC using a LabVIEW host computer platform program to automatically acquire data and perform subsequent data processing. The two horn antennas are connected to the VNA through low-loss coaxial cables. One horn antenna transmits incident EM waves while the other horn antenna receives the corresponding reflected EM waves, with both the incidence and reflection angles of about 5$^{\circ }$. According to the NRL standard for far-field conditions, the distance between the horn antenna and the sample is 25 cm. To ensure optimal results, the antenna and the absorber prototype are set at the same height. The reflection measurement is calibrated before each measurement by placing a metal plate instead of the sample. The reflectance is obtained from the transmission coefficient (measured $S_{21}$ parameter) between the input and output ports of VNA. Here, we adopt the time-domain gating and background subtraction method to eliminate measurement errors well described in [39]. Figure 8(b). shows a photograph of the measurement setup, while Fig. 8(c) shows a top view of the prototype. Figure 8(d) shows a top view of the GIIS, where a thin metallic copper foil tape on the edge of the graphene film and ITO film serve as an electrode. Figure 8(e) and (f) show the top view and zoomed-in view of the JCFSS part, respectively. We employ the free space method described in [25] to obtain the relation between the surface resistivity of graphene and the applied external bias voltage, as shown in Fig. 9(a). It can be seen that the graphene resistivity can be changed from 2000 $\Omega/\Box$ to 125 $\Omega/\Box$ as the voltage shifts from +1.4 V to $-$3.2 V, respectively. The measured reflection coefficients under different bias voltages are shown in Fig. 9(b), showing an average reflectivity ranging from $-$5 dB to $-$33 dB. The CST full-wave simulations are also carried out and plotted for comparison. For the 10 dB absorption condition, the proposed graphene absorber exhibits narrowband absorption at the bias voltages of 1 V and 1.4 V at 27.6 GHz, with the absorption dips of $-$17 dB and $-$14 dB, respectively. Moreover, it achieves broadband absorption with the $FBW_{10dB}$ of 66.7% and the absorptivity of up to $-$33 dB at a bias voltage of $-$0.4 V. The measured results are in good agreement with the simulated ones. In Fig. 9(b), with a bias voltage of $-$0.4V, both absorption peaks have shifted towards high frequency. The shift may be caused by errors in the PMI foam thickness and incidence angle during testing, which could be due to manufacturing and compression deformation.
Fig. 8. (a) Schematic demonstration of the measurement system setup. Photographs of (b) measurement setup, (c) fabricated prototype, (d) GIIS, and (e), (f) JCFSS.
Fig. 9. (a) Measured surface resistivity of graphene with different bias voltages. (b) Comparison of measured values of reflection coefficient with full wave simulation.
Table 1 compares our proposed structure with the previously reported tunable absorbers. Most of the previous absorbers are with either broadband or narrowband absorption [16,26,29,30,40–42]. In addition, some absorbers have integrated PIN diodes or varactors [29,42] and their bias circuits are complicated. Although the absorbers in [32,43] can switch between the broadband and narrowband states, only numerical results are provided without experimental verification. In summary, the comparison indicates that the proposed work achieves a switchable graphene-based absorber between the broadband and narrowband responses, with good agreement obtained between the simulated and measured results. The absorber has a moderate thickness (0.21 $\lambda _0$) and provides 3% and 66.7% fractional bandwidths ($FBW_{10dB}$) for the narrowband and broadband absorptions, respectively. The center frequencies of both absorption modes are nearly identical, indicating excellent narrowband/broadband switching performance around the center frequency.
Table 1. Comparison with previously reported tunable absorbersa
5. Conclusion
In conclusion, we propose an absorber with a hybrid metasurface of graphene and metal structures, which can switch between broadband and narrowband modes in the mmW regime. The designed structure demonstrates broadband absorption with $R_s$ = 450 $\Omega/\Box$, while narrowband absorption is achieved with $R_s$ = 1300 $\Omega/\Box$ and 2000 $\Omega/\Box$. The tunable characteristics of the proposed absorber are analyzed using the full-wave simulation and the ECM methods, with the ECM results showing good agreement with the full-wave simulation. Furthermore, the physical mechanism is explored through the analysis of the distributions of power loss, electric field, and current densities of a unit cell. Finally, we fabricate the prototype and obtain the experimental results which also corresponded well with the simulated ones. When the external bias voltage is varied between +1.4 V and $-$3.2 V, the proposed absorber exhibits an average reflectivity ranging from $-$5 dB to $-$33 dB. The proposed absorber has potential applications in EM camouflage, antenna design, and RCS reduction techniques due to the electrically reconfigurable reflection characteristic.
Funding
National Natural Science Foundation of China (61831016, 62090011, 62188102).
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. F. Costa, A. Monorchio, and G. Manara, “Theory, design and perspectives of electromagnetic wave absorbers,” IEEE Electromagn. Compat. Mag. 5(2), 67–74 (2016). [CrossRef]
2. Y. Fan, J. Wang, X. Fu, Y. Li, Y. Pang, L. Zheng, M. Yan, J. Zhang, and S. Qu, “Recent developments of metamaterials/metasurfaces for RCS reduction,” EPJ Appl. Metamat. 6, 15 (2019). [CrossRef]
3. N. Lee, J.-S. Lim, I. Chang, H. M. Bae, J. Nam, and H. H. Cho, “Flexible Assembled Metamaterials for Infrared and Microwave Camouflage,” Adv. Opt. Mater. 10(11), 2200448 (2022). [CrossRef]
4. M. Li, S. Xiao, Y. Bai, and B. Wang, “An Ultrathin and Broadband Radar Absorber Using Resistive FSS,” IEEE Antennas Wirel. Propag. Lett. 11, 748–751 (2012). [CrossRef]
5. V. Sunny, P. Kurian, P. Mohanan, P. A. Joy, and M. R. Anantharaman, “A flexible microwave absorber based on nickel ferrite nanocomposite,” J. Alloys Compd. 489(1), 297–303 (2010). [CrossRef]
6. R. Fante and M. McCormack, “Reflection properties of the Salisbury screen,” IEEE Trans. Antennas Propag. 36(10), 1443–1454 (1988). [CrossRef]
7. L. Du Toit, “The design of Jauman absorbers,” IEEE Antennas Propag. Mag. 36(6), 17–25 (1994). [CrossRef]
8. N. Mishra, D. K. Choudhary, R. Chowdhury, K. Kumari, and R. K. Chaudhary, “An Investigation on Compact Ultra-Thin Triple Band Polarization Independent Metamaterial Absorber for Microwave Frequency Applications,” IEEE Access 5, 4370–4376 (2017). [CrossRef]
9. B. Zhao, C. Huang, J. Yang, J. Song, C. Guan, and X. Luo, “Broadband Polarization-Insensitive Tunable Absorber Using Active Frequency Selective Surface,” IEEE Antennas Wirel. Propag. Lett. 19(6), 982–986 (2020). [CrossRef]
10. F. Costa and A. Monorchio, “Electromagnetic Absorbers based on High-Impedance Surfaces: From ultra-narrowband to ultra-wideband absorption,” Adv. Electromagn. 1(3), 7–12 (2012). [CrossRef]
11. P. K. Singh, K. A. Korolev, M. N. Afsar, and S. Sonkusale, “Single and dual band 77/95/110 GHz metamaterial absorbers on flexible polyimide substrate,” Appl. Phys. Lett. 99(26), 264101 (2011). [CrossRef]
12. S. Ghosh and K. V. Srivastava, “Polarization-Insensitive Single-/Dual-Band Tunable Absorber With Independent Tuning in Wide Frequency Range,” IEEE Trans. Antennas Propag. 65(9), 4903–4908 (2017). [CrossRef]
13. H. Zhang, P. Zhou, H. Lu, Y. Xu, D. Liang, and L. Deng, “Resistance Selection of High Impedance Surface Absorbers for Perfect and Broadband Absorption,” IEEE Trans. Antennas Propag. 61(2), 976–979 (2013). [CrossRef]
14. 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]
15. Y. Tayde, M. Saikia, K. V. Srivastava, and S. A. Ramakrishna, “Polarization-Insensitive Broadband Multilayered Absorber Using Screen Printed Patterns of Resistive Ink,” IEEE Antennas Wirel. Propag. Lett. 17(12), 2489–2493 (2018). [CrossRef]
16. G. Deng, H. Hu, H. Mo, J. Xu, Z. Yin, H. Lu, M. Hu, J. Li, and J. Yang, “Tunable terahertz metamaterial wideband absorber with liquid crystal,” Opt. Mater. Express 11(12), 4026 (2021). [CrossRef]
17. J. Fang, J. Huang, Y. Gou, and Y. Shang, “Research on broadband tunable metamaterial absorber based on PIN diode,” Optik 200, 163171 (2020). [CrossRef]
18. G. Wu, G. Wu, X. Jiao, Y. Wang, Z. Zhao, Z. Zhao, Y. Wang, J. Liu, and J. Liu, “Ultra-wideband tunable metamaterial perfect absorber based on vanadium dioxide,” Opt. Express 29(2), 2703–2711 (2021). [CrossRef]
19. M. Zhang, M. Zhang, Z. Song, and Z. Song, “Switchable terahertz metamaterial absorber with broadband absorption and multiband absorption,” Opt. Express 29(14), 21551–21561 (2021). [CrossRef]
20. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, and A. A. Firsov, “Two-dimensional gas of massless Dirac fermions in graphene,” Nature 438(7065), 197–200 (2005). [CrossRef]
21. L. Ju, B. Geng, J. Horng, C. Girit, M. Martin, Z. Hao, H. A. Bechtel, X. Liang, A. Zettl, Y. R. Shen, and F. Wang, “Graphene plasmonics for tunable terahertz metamaterials,” Nat. Nanotechnol. 6(10), 630–634 (2011). [CrossRef]
22. K. S. Novoselov, V. I. Fal’ko, L. Colombo, P. R. Gellert, M. G. Schwab, and K. Kim, “A roadmap for graphene,” Nature 490(7419), 192–200 (2012). [CrossRef]
23. F. Schwierz, “Graphene transistors,” Nat. Nanotechnol. 5(7), 487–496 (2010). [CrossRef]
24. B. Sensale-Rodriguez, R. Yan, M. M. Kelly, T. Fang, K. Tahy, W. S. Hwang, D. Jena, L. Liu, and H. G. Xing, “Broadband graphene terahertz modulators enabled by intraband transitions,” Nat. Commun. 3(1), 780 (2012). [CrossRef]
25. O. Balci, E. O. Polat, N. Kakenov, and C. Kocabas, “Graphene-enabled electrically switchable radar-absorbing surfaces,” Nat. Commun. 6(1), 6628 (2015). [CrossRef]
26. X. Chen, Z. Tian, Y. Lu, Y. Xu, X. Zhang, C. Ouyang, J. Gu, J. Han, and W. Zhang, “Electrically Tunable Perfect Terahertz Absorber Based on a Graphene Salisbury Screen Hybrid Metasurface,” Adv. Opt. Mater. 8(3), 1900660 (2020). [CrossRef]
27. C. Huang, B. Zhao, J. Song, C. Guan, and X. Luo, “Active Transmission/Absorption Frequency Selective Surface With Dynamical Modulation of Amplitude,” IEEE Trans. Antennas Propag. 69(6), 3593–3598 (2021). [CrossRef]
28. C. Huang, J. Yang, C. Ji, L. Yuan, and X. Luo, “Graphene-Driven Metadevice for Active Microwave Camouflage with High-Efficiency Transmission Window,” Small Methods 5(2), 2000918 (2021). [CrossRef]
29. C. Huang, J. Song, C. Ji, J. Yang, and X. Luo, “Simultaneous Control of Absorbing Frequency and Amplitude Using Graphene Capacitor and Active Frequency-Selective Surface,” IEEE Trans. Antennas Propag. 69(3), 1793–1798 (2021). [CrossRef]
30. J. Song, C. Huang, J. Yang, X. Zhang, J. Peng, and X. Luo, “Broadband and Tunable Radar Absorber Based on Graphene Capacitor Integrated With Resistive Frequency-Selective Surface,” IEEE Trans. Antennas Propag. 68(3), 2446–2450 (2020). [CrossRef]
31. J. Zhang, X. Wei, I. D. Rukhlenko, H.-T. Chen, and W. Zhu, “Electrically Tunable Metasurface with Independent Frequency and Amplitude Modulations,” ACS Photonics 7(1), 265–271 (2020). [CrossRef]
32. W. Chen, C. Li, D. Wang, W. An, S. Gao, C. Zhang, and S. Guo, “Tunable wideband-narrowband switchable absorber based on vanadium dioxide and graphene,” Opt. Express 30(23), 41328–41339 (2022). [CrossRef]
33. L. Zhuang, W. Zhang, J. Liu, M. Chao, Q. Liu, B. Cheng, Y. Xu, and G. Song, “Switchable trifunctional terahertz absorber for both broadband and narrowband operations,” Opt. Express 30(25), 45848–45861 (2022). [CrossRef]
34. G. W. Hanson, “Dyadic Green’s functions and guided surface waves for a surface conductivity model of graphene,” J. Appl. Phys. 103(6), 064302 (2008). [CrossRef]
35. J. Feng, X. Chen, L.-S. Wu, and J.-F. Mao, “Broadband electrically tunable linear polarization converter based on a graphene metasurface,” Opt. Express 31(2), 1420–1431 (2023). [CrossRef]
36. R. Langley and A. Drinkwater, “Improved empirical model for the Jerusalem cross,” IEE Proc. H Microwaves, Opt. Antennas 129(1), 1 (1982). [CrossRef]
37. T.-T. Kim, H.-D. Kim, R. Zhao, S. S. Oh, T. Ha, D. S. Chung, Y. H. Lee, B. Min, and S. Zhang, “Electrically Tunable Slow Light Using Graphene Metamaterials,” ACS Photonics 5(5), 1800–1807 (2018). [CrossRef]
38. H. Li, Q. Cao, and Y. Wang, “A Novel 2-B Multifunctional Active Frequency Selective Surface for LTE-2.1 GHz,” IEEE Trans. Antennas Propag. 65(6), 3084–3092 (2017). [CrossRef]
39. W. Hofmann, C. Bornkessel, A. Schwind, and M. A. Hein, “Challenges of RF Absorber Characterization: Comparison Between RCS- and NRL-Arch-Methods,” in nternational Symposium on Electromagnetic Compatibility - EMC EUROPE (IEEE, 2019), pp. 370–375.
40. J. Zhang, Z. Liu, W. Lu, H. Chen, B. Wu, and Q. Liu, “A low profile tunable microwave absorber based on graphene sandwich structure and high impedance surface,” Int. J. RF Microw. Comput. Eng. 30(2), e22022 (2020). [CrossRef]
41. Q. Wang, Y. K. Liu, G. C. Wang, W. X. Jiang, and T. J. Cui, “Tunable triple-band millimeter-wave absorbing metasurface based on nematic liquid crystal,” AIP Adv. 12(1), 015127 (2022). [CrossRef]
42. Y. Li, H. Li, Y. Wang, Y. Wang, and Q. Cao, “A Novel Switchable Absorber/Linear Converter Based on Active Metasurface and its Application,” IEEE Trans. Antennas Propag. 68(11), 7688–7693 (2020). [CrossRef]
43. Z. Song, A. Chen, and J. Zhang, “Terahertz switching between broadband absorption and narrowband absorption,” Opt. Express 28(2), 2037 (2020). [CrossRef]
44. E. Park, W. Li, H. Jung, M. Lee, J.-H. Park, A. Shamim, and S. Lim, “Highly Scalable, Flexible, and Frequency Reconfigurable Millimeter-Wave Absorber by Screen Printing VO2 Switch Array onto Large Area Metasurfaces,” Adv. Mater. Technol., 2201451 (2023). [CrossRef]
