Reconfigurable design is an effective way to achieve multifunctional devices for system integration. Limited by the feeding network for multi-resonators, multimode absorbers with more than four modes are rarely reported. In this paper, a frequency-reconfigurable metamaterial absorber/reflector resonating at 3.05, 4.45 and 5.54 GHz is proposed. Based on a stereoscopic feeding network and a strategic arranged structure with loaded switching diodes, the proposed structure can realize the reconfigurable eight operating modes, including triple-band (111)/dual-band (110, 101, 011)/single-band (100, 010, 001) absorption and reflection (000) without re-optimizing and re-engineering the structure. The simulated results are confirmed by measuring a fabricated prototype. Our design provides a strategy to realize multifunction devices in microwave or even higher frequencies.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Metamaterial absorbers (MMAs), as a new branch of sub-wavelength artificial composite structures, have become one of the research hotspots because of the potential applications in attenuators, smart radars, stealth cloaks, sensors and thermal emitters etc [1–5]. Since Landy et al. proposed a microwave single-band perfect MMA consisting of the electric ring resonator, dielectric substrate and back cut-wire , various conventional MMAs with single-band [6,7], dual-band , multi-band [9–11] and broadband [12,13] absorption have been proposed by replacing the cut-wire with a continuous metal plate as bottom layer. Conventional MMAs feature the fixed frequency resonance once the samples are fabricated, which limits their applications. Recently, many efforts have been devoted to developing switchable/reconfigurable MMAs to overcome the shortcomings of the conventional MMAs. Switchable MMAs can selectively switch all or part of the absorption peaks by external excitation [12–25]. In terahertz regime, the frequency switchable MMAs were realized by incorporating tunable elements into the metaparticles such as photoconductive silicon , vanadium oxide (VO2) [15,16], germanium telluride (GeTe) , or graphene [18,19], whose conductivities can be tuned by thermal, optical or chemical potential excitation. Generally, the switchable MMAs in microwave regime are achieved by integrating semiconductor diodes, MEMS switches or injecting liquid metals/water in the structures [20–26]. For example, single-band switchable reflector/absorbers were realized by controlling the bias voltage of the diodes inserted in electric resonators [20,21]. By using grounding plate with the diodes and injecting liquid metal, broadband switchable absorbers were reported [22,23]. Based on planar biasing network, single and dual band switchable absorbers with embedded diodes were proposed [24,25]. However, limited by the design of biasing networks, there are rare reports on independently switchable MMAs with more than two frequency bands. In order to achieve reconfigurable multiband absorption, a new biasing network and an elaborately arranged structure are required to fulfill perfect matching, independent control as well as complete isolation between each other. Hence, it is a great challenge to realize switchable/reconfigurable multiband MMAs.
In this paper, we propose a reconfigurable MM absorber/reflector with eight operating modes for the first time. The unit structure consists of three sub-units embedded by three sets of switching diodes, respectively. Based on an elaborately designed feeding network, the absorption peaks resonating at 3.05, 4.45 and 5.54 GHz can be independently switched by controlling the three sets of switching diodes, resulting in eight absorption/reflection modes including triple- /dual- /single-band absorption or reflection mode. Compared with the previously reported switchable/reconfigurable absorbers, the designed structure provides a new strategy to realize multifunctional devices for system integration and is also expected to be used in electromagnetic stealth, biosensor, reflector and so on.
2. Structure design and experimental setup
The schematics and the fabricated prototype of the proposed reconfigurable absorber/reflector are illustrated in Fig. 1. As shown in Figs. 1(a) and 1(c), from top to bottom, the unit cell consists of five layers: top patterned metal resonators layer, middle double-sided coated copper FR4 dielectric substrate, metal backplane, one-sided coated copper FR4 dielectric layer and bottom feeding network layer. The top metal resonators are composed of three sub-units: a split square ring (SSR), a split cross (SC), and a split windmill (SW) structure, which are respectively inserted with three sets of identical diodes D1, D2 and D3, as shown in Fig. 1(b). The dark red bars in Fig. 1(c) illustrate the via holes connecting top resonators and bottom feeding network. A FR4 dielectric with one-sided coated copper is adopted as the bottom feeding plate to construct the stereoscopic biasing network. The metals on all layers are lossy copper with a thickness of 0.035 mm and a conductivity of . The dielectric constant and loss tangent of the FR4 dielectric substrate are 4.4 and 0.025, respectively. Figures 1(d) and 1(e) are the top and bottom layers of the fabricated 6 × 6 unit
cells prototype by printed circuit board technology. The optimized dimensions of the unit cell are given in the units of millimeters: a = 30, b = 7.3, d = 0.8, g = 1.7, R = 3.5, ϕ = 0.6, w1 = 1.4, w2 = 0.5, w3 = 1.6, t1 = 1.6 and t2 = 0.25, respectively. The overall size of the prototype is 210 mm × 210 mm and the total thickness is 1.955 mm, which is 1/28 and 1/50 of the minimum and maximum working wavelengths, respectively. As shown in Figs. 1(d) and 1(e), the resonator pattern with the surface feeding networks are etched on the top side of double-sided FR4 dielectric substrate. The BAP70-03 diodes are welded at the gaps of the SSR, SC and SW, respectively. The diodes D1 (in the SSR) are in series, whose the anodes and cathodes are respectively connected to the upper and down bias lines; the anodes of the diodes D2 (in the SC) and D3 (in the SW) are respectively connected to the bottom narrow feeding lines 1 and 2 through via nails (substituted by thin copper rods), and their cathodes are connected to the bottom wide feeding lines through via nails, where the bottom narrow feeding lines 1 and 2 are connected with the surface left bias line and right bias line through via nails, respectively. The thin via copper rods are isolated from the metal backplane to avoid short circuit by using thin insulated sleeves at the anode holes on the metal backplane side. In order to realize the independent switching of eight modes, the upper and lower bias lines (blue lines) provide bias voltage for diodes D1; the left bias line (red line) and the bottom wide feeding line provide bias voltage for diodes D2; the right bias line (green line) and the bottom wide feeding line provide bias voltage for diodes D3. Moreover, the HK1005R12J-T inductors with 120nh produced by TAIYO YUDEN are soldered at the gaps of the upper and lower bias lines to provide required isolation from the induced alternating currents. In this design, all diodes are the silicon PIN diode BAP70-03 produced by NXP , which is equivalent to a small resistance Ron of 1.5 Ω in series with an inductance Ld of 1.5 nH in on-state, or a large capacitance Coff of 0.1 PF in series with the inductance Ld in off-state. Figure 2 shows the equivalent circuit model of the structure, where Z0 is the impedance of free space and Zt1 is the characteristic impedance of the middle dielectric substrate (t1), respectively. When the EM wave is incident to the absorber surface, the sub-units SSR, SC and SW are equivalent to three parallel circuit units, which consist of an inductance Li and a resistance Ri (i = 1,2,3) in series with the diode equivalent impedance Zdiode, respectively. The input impedance Zin of the structure can be given by the following equations:28]: [λ/4]min = 13.5mm≥t1. Thus, the middle FR4 dielectric substrate (t1) can be equivalent to an inductance L and the resonant frequency can be determined by the following equation :Eq. (4) when the values of the equivalent circuit parameters are determined by the geometry structure [30,31]. However, for complicated structure, Eq. (4) is usually used to qualitatively explain the resonant frequency [5,32]. For the proposed structure shown in Fig. 1, when each sets of the diodes are in on-state under forward bias, according to the equivalent circuit model in Fig. 2, no resonance will occur in the corresponding sub-unit due to lack of equivalent capacitance in this case. However, when the diodes are in off-state under reverse bias, which provide an equivalent capacitance Coff, the corresponding sub-unit can generate a resonance at specific frequency. Therefore, the proposed structure can independently resonate at different frequencies, contributing different absorption modes. Thus, a frequency reconfigurable absorber with eight operating modes can be achieved through independently controlling the states of the three sets of diodes.
To demonstrate the performance of the proposed reconfigurable structure, we adopt the software CST Microwave Studio 2015 based on the finite integration technique (FIT) to model and simulate the structure, which will be verified by the experimental measurement later. In simulation, the unit cell boundary is set along the X-axis and the Y-axis direction shown in Fig. 1(a), and an open (add space) boundary is set along the Z-axis direction, in which the electric field and magnetic field of the incident EM wave are polarized along the Y-axis and the X-axis, respectively. In experiment, the transmitting and receiving horns are respectively connected to a vector network analyzer (Agilent N5222A) through a low loss coaxial cable, a DC stabilized power supply (SS2323) is applied to provide bias voltage for the switching diodes, the setup schematic of the measuring instrument is similar to Ref . It should be pointed out that the prototype needs to be measured under far-field condition , where is the distance between antennas and the prototype, is the diagonal length of horn antennas. According to the accurate standard [33,34], the experimental measurement requires a distance of more than 10 meters, which is difficult to be satisfied in a common anechoic chamber. Here we will measure the prototype by adopting an approximate NRL arch method which is generally used in practice . Here the measurement distance is about , which satisfies the NRL standard. The absorptivity of MMAs can be expressed as , where and are the reflectivity and transmissivity, respectively. Since a metal backplane in the structure is applied to block the EM wave, i.e. , the absorptivity can be reduced to .
3. Results and discussion
3.1. Reconfigurable eight operating modes
Based on the strategically designed resonators and the stereoscopic feeding network, the three sets of diodes D1 (in the SSR), D2 (in the SC), and D3 (in the SW) can be controlled independently. Therefore, the structure can operate in eight absorption/reflection modes according to different switching states of three sets of diodes, as shown in Table 1, where “1” and “0” represent the perfect absorption and reflection states, respectively. Figure 3 shows the simulated results (red solid lines) of eight absorption/reflection modes under the combination of the switching states of D1, D2 and D3. When all three sets of diodes are in off-state, thestructure exhibits the triple-band absorption mode (111) at 3.05, 4.45 and 5.54 GHz, as shown in Fig. 3(a). On the contrary, when all three sets of the diodes are working in on-state, all three absorption peaks are switched off and the structure is in the reflection state (000), as shown in Fig. 3(h). Following this way, when one set of the diodes are in on-state and the other two sets are in off-state, the structure exhibits the dual-band absorption modes (011, 101 or 110), as shown in Figs. 3(b), 3(c) and 3(d), respectively. Similarly, when two sets of diodes are in on-state and the other set of diodes are in off-state, the structure can only absorb EM waves at single frequency band, corresponding to the single-band absorption modes (001, 010 or 100), as shown in Figs. 3(e), 3(f) and 3(g), respectively. Therefore, by independently manipulating the states of the three sets of diodes, we can achieve arbitrary configuration of eight absorption/reflection modes (111, 011, 101, 110, 001, 010, 100, and 000) while keeping the high absorptivity of each peak. The measured results (blue marked lines) for each mode are also plotted in Fig. 3. Compared with the simulation results, the experimental results are in good agreement with the simulation ones except that the measured absorption peaks are a little shifted to lower frequencies, which might come from the errors of fabrication and measurement, such as dimension errors in the circuit printing process, the copper plate oxidation and the difference of the permittivity of dielectric substrate used in the measurement and simulation. The demonstrated results show that the proposed structure can achieve eight perfect absorption/reflection modes by the appropriate feeding network without re-optimizing and re-engineering the structure.
Furthermore, we retrieve the electromagnetic parameters through the parameter inversion method . Figure 4 illustrates the normalized impedance with respect to the free space impedance (Z0) by taking the modes of 111, 100, 010 and 001 as examples. It is clear to see that at each resonance frequency, the real parts of impedances are close to unit and the imaginary parts of the impedances are about zero. This implies that the impedance matching is satisfied at these resonance frequencies and then the proposed structure can achieve the perfect absorption at corresponding frequencies, which is consistent with the qualitatively explanation by Eqs. (1)–(4).
3.2. Reconfigurable absorption mechanism
Furthermore, in order to better illuminate the absorption mechanism of the reconfigurable absorber, we investigate the surface current and magnetic field distributions at the resonant frequencies 3.05, 4.45 and 5.54 GHz for the eight absorption/reflection modes, respectively, as shown in Figs. 5 and 6. It is observed from Figs. 5(a) and 6(a) for 111 mode that three pairs of strong anti-parallel currents appear between the SSR, SC, SW resonators and the metal backplane, and the magnetic field mainly focuses on the vicinity of the SSR, SC, SW resonators. It means that the three resonators respond respectively to the magnetic component of the incident EM wave at the corresponding frequency, which implies that there will be three absorption peaks. When the structure is operating in 011, 101, and 110 modes, the anti-parallel current and magnetic field distributions become very weak at the frequency position corresponding to ‘0’ state, while the current and magnetic field distributions at the other two resonance frequencies corresponding ‘1’ state are almost unaffected, so there are only two different magnetic responses in the 011, 101, and 110 modes, as shown in Figs. 5(b)–5(d) and 6(b)–(d) respectively. Similarly, when the structure is working in 001, 010 and 100 modes, only the anti-parallel current and magnetic field distributions at the resonance frequency corresponding to ‘1’ state are formed, so only one magnetic response occurs in corresponding resonator in 001, 010 and 100 modes, as shown in Figs. 5(e)–5(g) and 6(e)–6(g) respectively. Particularly, for the 000 mode, all the current and magnetic field distributions at the three frequencies 3.05, 4.45 and 5.54 GHz are very weak, as shown in Figs. 5(h) and 6(h), suggesting the three absorption peaks are switched off. Looking at Figs. 4 and 5, it can be inferred that the three absorption peaks at 3.05, 4.45 and 5.54 GHz are mainly excited by the magnetic response between the SSR, SC, SW and the metal backplane. Moreover, we noticethat the current and magnetic field distributions at each frequency are only focused on the corresponding sub-unit structures, and there is almost no crossover between them. Therefore, the coupling among the three sub-unit structures is very weak, and the absorption peaks in all states are independent. Looking at Figs. 4, 5 and 6, the proposed structure can be arbitrarily switched between three-/ dual-/ single-band absorption modes and the reflection mode by changing the states of the three sets of diodes embedded in the SSR, SC and SW sub-units, respectively.
3.3. Polarization-insensitive and wide-angle absorption
In practice, the incident EM waves are not always polarized in a fixed direction and are not completely perpendicular to the absorber surface, so it is important to appraise the absorption performance under different polarization and oblique incidence. Taking the three-band absorption (111 mode) as an example, we further discuss the absorption performance at different polarization angles and oblique incident angles of the EM waves. Since the designed structure has a quadruple rotation symmetry, which is coincident with the rotation around the centre of the patterns by 90 degrees, Fig. 7(a) only presents the simulated absorptivity at the polarization angles from 0° to 45° under normal incidence. It is clear in Fig. 7(a) that the frequency position and absorptivity of the three absorption peaks are almost unaffected when the polarization angle is varied from 0° to 45°. The simulated absorptivity for TE and TM waves with different oblique incident angles are respectively shown in Figs. 7(b) and 7(c). For both TE and TM waves, the absorptivity at the three resonant frequencies gradually decreases when the incident angle is increased from 0° to 60° for a polarization angle , but the lowest absorptivity can still remain above 78% and 78.5% for TE and TM waves, respectively. These results are also confirmed by measured ones, which are illustrated in Figs. 7(d)–7(f) corresponding to Figs. 7(a)–7(c), respectively.
We have proposed a reconfigurable MM absorber/reflector that can be arbitrarily switched between eight modes including three- /dual- /single-band absorption and the reflection. The unit structure is composed of three sub-units: SSR, SC and SW resonators embedded by three sets of silicon diode switches D1, D2 and D3 in the corresponding splits. When all three sets of diodes are switched off, the absorber can achieve perfect absorption at 3.05, 4.45 and 5.54 GHz. Through a skillfully designed stereoscopic feeding network, the three sets of switching diodes can be independently controlled. Thus, the proposed structure can achieve eight operating modes, including triple-band (111) /dual-band (110, 101, 011) /single-band (100, 010, 001) absorption and reflection (000) without re-engineering the structure. Moreover, the absorption mechanism was illuminated by the equivalent circuit model and the surface current distributions of the structure for eight working modes. Taking the triple-band absorption as an example, the absorption characteristics of the proposed structure under different polarization and incident angles were investigated. Furthermore, the simulated results were examined by measuring the fabricated prototype, the measurement results are in good agreement with the simulation ones. The elaborately designed structure with the new stereoscopic feeding network may provide a new way to realize multifunctional devices for system integration in microwave even higher frequencies. The proposed reconfigurable absorber/reflector is expected to be applied to single-band, dual-band and three-band electromagnetic stealth, biosensor, reflector and so on.
National Natural Science Foundation of China (NSFC) (61775126); Natural Science Foundation of Shanxi Province (201601D011038, 201801D121119).
Authors wish to thank Dr. Saptarshi Ghosh from Indian Institute of Technology for beneficial discussions.
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