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Abstract
Perfect absorbers are highly desired in many engineering and military applications, including radar cross section (RCS) reduction, cloaking devices, and sensor detectors. However, most types of present absorbers can only absorb space propagation waves, yet absorption for the surface wave (SW) has not been researched intensively. In reality, when the space wave illuminates on the metal under large oblique angles, surface waves can be excited on the interface between metal and dielectric and thus would increase the RCS and influence the stealth performance. Here, based on the wave vector and impedance matching theories, we propose a broadband absorber for the surface wave under spoof surface plasmon polariton (SSPP) mode. The former theory ensures that surface waves can enter the absorber efficiently, and the latter guarantees perfect absorption. The experimental results indicate that our absorber can achieve a broadband (9.4-18 GHz) performance with an absorption ratio better than 90%, which is in great agreement with the simulations. Therefore, our device can be applied in RCS reduction for the metal devices, antenna array decoupling and many other applications. Also, this work provides a unique methodology to design new types of broadband surface wave absorbers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Electromagnetic (EM) stealth technology has become a key point in modern military science and technology. Absorber has been demonstrated as an important method to realize the EM stealth. Traditional absorbers are usually constituted of natural material, such as wedge absorbers [1–2] and ferrite [3]. However, they are inconvenient for integration application, as their bulky forms, narrow bandwidth, and sensitivity to the working environment. Metamaterial and metasurface have the strong abilities to manipulate the wavefronts of transmitted and reflected EM wave [4–36]. They have become a field of hot research topic with remarkable achievements, including invisibility cloaks [4,5], super lenses [6,7], planar holograms [8,9] and perfect absorbers [15–27]. Since 2008, Metamaterial/metasurface absorbers have attracted strong attention [15], due to their outstanding performance. Recently, series of methods have been used to improve the performance for the metamaterial/metasurface absorbers. Papers [16–19] adopt multiple point resonance structures or load lumped elements to expand the bandwidth. Papers [20–22] introduce improved meta-atom to make the absorber have great polarization insensitive or incident angle insensitive characteristics. Papers [23–24] take complex three-dimensional structures, such as Huygens structure and zigzag-shaped structure, to improve absorption efficiency.
However, there are few studies on surface wave in current metamaterial/metasurface absorbers. Surface wave can be easily excited on the interface between metal and dielectric, when the metal equipment is illuminated under large oblique angles. As a result, surface wave absorption has many applications on RCS reduction, decoupling in array antennas, electromagnetic shielding and many other regions. Current absorbers cannot absorb surface wave as their electromagnetic properties are fundamentally different from those of space wave. Thus, there are two main difficulties in designing surface wave absorbers. First, surface wave is unstable and easy to be decoupled into space. How to reduce their scattering and make them enter the absorbers? Second, the impedance of surface wave is different from the impedance of space wave. How to match the impedance of the surface wave with that of the absorber to ensure a high efficiency absorption?
Here, we propose a universal strategy to realize broadband surface wave absorption by combining wave vector matching and impedance matching conditions. When the wave vector of surface wave is matched with the absorber, the surface wave will propagate into the absorber without scattering and reflection. And with the condition of impedance matching, the electromagnetic energy carried by surface wave will be absorbed perfectly. To demonstrate this conception, a realistic device composed of a metasurface absorber and a SSPP exciter is designed and fabricated, shown in Fig. (1). Simulation and measurement have indicated that both reflection and transmission coefficients of our device are below −10 dB in the range of 9.4-18 GHz, and scattering energy on other directions also maintain a low level. And the absorption ratio of the absorber can be up to 90%. Besides verifying the feasibility of the surface wave absorption mechanism, our work also provides a unique methodology to design new types of broadband surface wave devices.
Fig. 1. Schematics and working principles of the proposed absorber. (a) Proposed SSPP exciter, the surface wave under SSPP mode can perfectly propagate from the feed port to the terminal. (b) SSPP exciter loaded with the metasurface absorber, EM energy is dissipated in the absorber.
2. Design of the SSPP exciter
First, we discuss the design principle of the SSPP exciter. To realize absorption for surface wave, it is important to research the characteristics of surface wave. Usual surface wave bounded at the metal–dielectric interface does not satisfy the eigenmode in microwave region. This kind of surface wave can only spread a short distance alone the surface and can be easily decoupled. Thus, adopting usual surface wave is inconvenient to explore its performance. SPP is an eigen electromagnetic mode which is caused by the interaction between photons and electrons [37–47]. And the wave vector of SPP under TM and TE mode should yield the following two formulas on the interface, respectively.
Fig. 2. Schematics and performance of the proposed SSPP exciter. (a) Structure and parameters configuration of the exciter. (b) Reflection and transmission spectrum of SSPP exciter. (c) Dispersion curve of the SSPP on the exciter. (e), (h) Electric field and energy distribution of SSPP under TE mode at 11 GHz. (d), (g) Electric field and energy distribution of SSPP under TE mode at 14 GHz. (f), (i) Electric field and energy distribution of SSPP under TE mode at 17 GHz.
Table 1. parameter configuration of SSPP exciter (mm)
Then, we simulate the performance of the exciter. As shown in Fig. 2(b), the reflection coefficient is below −10 dB in the range of 9.4-18 GHz, and the transmission coefficient is over −1 dB at the same frequency band. The power flow and electric distribution shown in Figs. 2(d)–2(i) demonstrate the energy propagation is steady, and the dispersion curve of the exciter is derived in Fig. 2(c). These results indicate that our device can effectively excite SSPP in a wide band, which is proper to be selected as a carrier to research SW absorption.
3. Absorption mechanisms and meta-atom design
Next, we introduce the mechanism for surface wave absorption. There are two conditions obligatory to reach. The basic condition is wave vector matching which ensures the EM energy can transmit into the absorber without reflection. According to the microwave theory, the EM field can transmit among different systems, as long as the field mode in these systems are similar. Therefore, the wave vector which represents the field mode should be the same among different systems. The other necessary condition is impedance matching. When the resistance of the absorber is same as the SSPP, the energy can be perfectly absorbed by the devices. According to the above two principles, we propose a type of metasurface absorber, and the meta-atom is shown in Fig. 3(a). To realize wave vector matching, an “H” shaped structure which is similar to the exciter is adopted as the meta-atom pattern. Furthermore, the absorber is designed as a double-layered structure to enhance the absorption ability. The H-shaped structure is etched on the Polyethylene Terephthalate (PET) with the thickness of “h”. And the PET is glued on the F4B (dielectric constant of 2.65 and a tangent loss angel of 0.001, respectively) as the meta-atom of the absorber, shown in Fig. 3(a). The geometric parameters of meta-atom can affect the wave vector. As shown in Fig. 3(b), the dispersion performance will be changed against arm length “l” of the “H” shaped structure. According to this trend, we adjust these parameters to ensure the dispersion curve of the absorber coincide with the SSPP excited by the exciter in Fig. 2(c), and the condition of wave vector is finally realized. The final geometric parameters of the meta-atom are shown in Table 2.
Fig. 3. Design progress of SW absorber. (a) The meta-atom structure of the meta-structure of the surface absorber (b) Dispersion curve of the meta-atom. (c) The change trend of the surface wave reflection performance when the ITO square resistance of the absorber changes. (d) The change trend of the transmission performance when the ITO square resistance of the absorber changes. (e) The change trend of the transmission performance when the number of absorber layers changes. (f) The change trend of the absorption ratio when the number of absorber layers changes.
Table 2. Geometric parameters of meta-atom (mm)
Then, we adjust the equality resistance of the absorber to realize matching condition. The equality resistance is affected by the conductivity of meta-structures. Here, we select the indium tin oxide (ITO) as the material to construct the meta-structure, which is convenient to adjust its conductivity. Figures 3(c) and 3(d) represent the absorption performance changed with the sheet resistance of the ITO film. It is noticed that the sheet resistance of ITO has no affection to the reflection coefficients shown in Fig. 3(c), the |S11| always keeps beneath −10 dB in the range of 9.4-18 GHz, which means the reflection performance is mainly determined by wave vector matching. On the other hand, the sheet resistance has a dramatically affection to transmission performance. Figure 3(d) illustrates that the |S21| between two ports will be changed when the sheet resistance of ITO film is adjusted. In order to obtain the best absorption effect, we select a proper value of ITO sheet resistance to realize minimum transmission coefficient. The final value of sheet resistance is determined to be 100 Ω/square. Finally, we explore the most suitable number of absorber layers. We adopt a single-layered, a double-layered and a triple-layered absorber to demonstrate their absorption effect, respectively. Figures 3(e) and 3(f) illustrate the absorption performance changed with the number of absorber layers. As the number of absorber layers increases, its absorption effect is also enhanced. For the single-layered structure absorber, its absorption effect does not achieve the desired effect (90%) in a wide frequency range. Meanwhile, for the triple-layered structure absorber, although its absorption has some enhancement compared with double-layered structure absorber, the enhanced effect is not obvious. Therefore, both high efficiency absorption and engineering practice are considered, we adopt the double-layered structure as the absorber structure.
After series of optimization, we finally obtain an effective SW absorber working in a wideband. Figure 4 illustrates the simulation results of the absorber. It is observed that the transmission coefficient of the absorber is suppressed under −10 dB in the range of 9.4-18 GHz, shown in Fig. 4(b). And the reflection coefficient is also beneath −10 dB, shown in Fig. 4(a). The above simulations demonstrate that most part of EM energy has been absorbed in a wideband. As indicated in Figs. 4(c) and 4(d), the electric field and the power flow of SW is clear and steady on the SSPP exciter. But when the SW transmits into the absorber, the field pattern is broken and the amplitude of electric field attenuated rapidly, which illustrates the dissipation of SW energy. Besides, we also consider that some energy may scatter from the juncture between the absorber and exciter, due to the imperfect matching. As shown in Figs. 4(e) and 4(f), two waveguide ports are positioned at the top and bottom surface of the absorber, respectively. Then the scatter characteristic can be acquired by calculated the transmission coefficient among these feed ports. It is noticed that the scattering level of surface wave is beneath −30 dB among 9.4-18 GHz on upper and lower profile, which indicates that the energy of surface wave is absorbed by the absorber instead of being radiated to other directions. Figures 4(g) and 4(h) illustrate the electromagnetic energy of absorber and SSPP exciter in the xoz plane, respectively. The scattering energy of absorber sustain a low level compared with the exciter.
Fig. 4. Simulated results of SW absorber. (a), (b) Simulated reflection and transmission performance of the SW absorber. (c), (d) Electric field and energy distribution on the absorber at 11 GHz, 14GHz, 17GHz, respectively. (e) Transmission coefficients between top surface of the absorber and input port. (f) Transmission coefficient between bottom surface and input port. (g), (h) Electromagnetic energy distribution on the xoz plane.
In order to verify the characteristics of the proposed absorber, we fabricated a realistic SSPP exciter and a surface wave absorber, shown in Fig. 5(e). Here, we observed that part of the energy will be scattered from the original slow-wave structure after adding absorber on the exciter, as shown in Fig. 4(c). And this little part energy is not absorbed by the absorber. If it does not transmit to the receive port, the results will be inaccurate. Therefore, we add some extra slow-wave structures around the large CPW port to make this part energy can transmit to the receive port. Figures 5(a) and 5(b) illustrate the reflection and transmission performance of designed SSPP exciter. The measured results are matched well with the simulated one, the reflection coefficient is below −10 dB in the range of 9.4-18 GHz, and the transmission coefficient sustain a high level in low frequency range. Here, we should mention that as the surface wave transmits alone the substrate, the dissipation effect of the F4B board will affect the working performance, and the measured and simulated results have some differences at high frequency range. Figures 5(c) and 5(d) illustrate the reflection/transmission coefficients by adding the absorber on the SSPP exciter. The measured results have a good agree with the simulated one. Both the reflection and transmission coefficients are below −10 dB in range of 9.4-18 GHz. By comparing the transmission coefficients of SSPP exciter and SW absorber, the absorption ratio of the surface wave absorber can be calculated, shown in Fig. 5(f). It is noted that our device can effectively absorb SSPP surface wave in the range of 9.4-18 GHz with an absorption ratio better than 90%, and the simulation results are in good agreement with the experimental test results. The series of experiments demonstrate that our device can absorb the surface wave under SSPP mode with high efficiency.
Fig. 5. Measured and simulated results of SSPP exciter and SW absorber. (a), (b), Measured and simulated reflection and transmission performance of the SSPP exciter. (c), (d) Measured and simulated reflection and transmission coefficients of SW absorber. (e) Photograph of the fabricated SSPP exciter and SW absorber. (f) Measured and simulated absorption ratio of the SW absorber.
4. Conclusion
In summary, based on wave vector matching and impedance matching theories, we design a broadband and high-efficiency surface wave absorber. By adjusting the geometrical parameters of the meta-atom, the wave vector matching between the absorber and the surface wave can be realized to guarantee the high efficiency for surface wave energy entering the absorber. Meanwhile, the absorption ratio is enhanced by properly choosing the resistance of ITO to satisfy the impedance matching condition. Simulated and experimental results indicate that the absorber can effectively absorb SSPP surface wave in the range of 9.4-18 GHz with an absorption ratio better than 90%, while the energy scattering in other directions is also maintained at a low level. Besides verifying the feasibility of the surface wave absorption mechanism, our work also lays a theoretical and technical foundation for the next step in the development of surface wave devices.
Funding
Key Projects of Aviation Foundation (KPAF) (201918037002); Natural Science Foundation of Shaanxi Province (2019JQ-013); National Natural Science Foundation of China (61701572, 61871394, 61901512).
Disclosures
The authors declare no conflicts of interest.
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