Open Access
Add to BibSonomy
Abstract
An electromagnetic absorber is realized based on the plasma metamaterial and lumped resistors. For a TE wave, the tunable absorption spectra can be obtained, and the properties of absorption can be improved by exciting the different plasma resonance structures. The proposed absorber can work in S, L and C bands, which is covered at 1.6115-4.0798 GHz (absorption rate is larger than 0.9), and its relative bandwidth is 86.7%. However, for a TM wave, the reflection coefficient is near to 1. The proposed electromagnetic absorber not only can realize the ultra-broadband absorption for the TE wave, but also can act as a reflector for the TM wave. A polarization splitter can be realized by such an absorber.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the development of science and technology, the electromagnetic metamaterial has gradually become a research central issue in the industry. Based on some novel physical properties of electromagnetic metamaterial, it can be used to realize some special microwave devices [1]. An important application of electromagnetic metamaterial is the generation of electromagnetic absorbers. The electromagnetic absorbers [2,3] are functional materials that absorb and consume the energies of the incident electromagnetic waves and exhibit a periodic arrangement in space. As a kind of electromagnetic metamaterials, they have two main characteristics: (1) The reflection of the incident wave is small, and the most of incident wave can enter the internal of the absorption layers; (2) The energy of incident wave can be lost [4], and the electromagnetic absorber is an energy-consuming device which can consume the electromagnetic energy when the incident wave propagates through the absorber. Based on those features, in recent years, many new types of microwave absorbing materials have emerged and also are widely used, such as the stealth absorbing materials [5], the magnetic absorbing materials [6], the absorbing materials for antenna applications [7], Carbon-based absorbers [8], etc. .
In 2008, American scholar Landy and associates [9,10] firstly designed an electromagnetic resonance absorber based on the electromagnetic metamaterial, and proposed a polarization-insensitive electromagnetic absorber in the THz frequency regime. The simulated absorption rate is 95%, and the experimentally measured absorption rate at a frequency of 1.145 THz is 77%. In 2011, Shen et al [11] proposed a polarization-independent wide-angle triple-band metamaterial absorber, which has three absorption peaks at frequencies 4.06GHz, 6.73GHz and 9.22GHz with absorptivity 99%, 93%, and 95%, respectively. In 2013, Kearney et al [12] present a terahertz metamaterial absorber with an embedded resistive layer whose maximum absorption rate is 94%.
With the development of electromagnetic absorbers, how to design and implement the broadband electromagnetic absorbers with tunable absorption bandwidths [13] is a research hotspot. At present, the main technologies for broadening the absorption bandwidths of the electromagnetic absorbers include loading the lumped elements [14,15], the multi-layer stacking structures [16], and the multi-cell structures [17], and so on. Compared with the method of loading the lumped elements, the design technologies based on the multi-stacked structures and multi-cell structures are relatively complicated. Therefore, loading the lumped resistances to realize the ultra-broadband absorbers is a better choice. On the other hand, to obtain the tunable absorption, the plasma metamaterial which includes the solid plasma can be utilized to realize the electromagnetic absorbers since the amazing features can be observed in the solid plasma. When the solid plasma is not excited, it can exhibit the dielectric properties. However, when the solid plasma is excited, it has similar metallic conductivity characteristics [18–20]. Therefore, the solid plasma and other dielectrics can form the plasma metamaterials. As a branch of metamaterials, the plasma metamaterial becomes a new research focus. In this paper, the resonant structures of the proposed absorber are realized by the plasma metamaterial. If the excited areas and states of solid plasma resonators can be manipulated dynamically, the adjustable absorption spectrum can be obtained. When the regions of solid plasma resonators are properly excited, for TE wave, the absorption regime of the proposed absorber can effectively cover the entire S band, which also can be extended to the L and C bands. However, in the absorption frequency region of TE wave, the reflectivity of TM wave also is every high, which almost is close to 1. Obviously, the proposed electromagnetic absorber also can be acted as a splitter.
2. Structure and design
In Fig. 1, the spatial structure diagrams of the unit cell for the proposed absorber are given. As shown in Fig. 1(c), the proposed electromagnetic absorber consists of three layers. The bottom layer is a copper plate, whose electric conductivity σ = 5.8 × 107 s/m. The middle layer is a dielectric substrate (εr = 3.2, tangent delta = 0.003). The top layer is distributed with the plasma resonators. It can be seen from Figs. 1(a)-(b), in the top layer of absorber, there are three arrow-shaped plasma resonance units, which are named resonance units 1, 2 and 3, respectively. To each resonance unit, a resistance element is loaded in the middle of two arrow-shaped plasma resonators. The resistance element R1 = 190 Ω is loaded in the middle of the resonance unit 1, and the resistance element R2 = 600Ω is loaded in the middle of the resonance units 2 and 3, respectively. The thickness of the dielectric substrate is h = 13.6mm. Both the top solid plasma layer and the bottom metal copper plate have a thickness of w = 0.1 mm. The resonance unit 1 is composed of two trapezoidal and rectangular plasma resonance structures. The height of this trapezoidal resonator is a = 17.4 mm. The lengths of the lower and upper bases are k = 2.4 mm and k1 = 1.2 mm, respectively. The large rectangular resonance structure has a length of b = 9.6 mm and a width of c = 2.4 mm. Similarly, the small rectangular resonance structure has a length of b1 = 4.8 mm and a width of c = 2.4 mm. On the other hand, the structures of resonance units 2 and 3 are exactly same. The height of such a trapezoidal structure is a1 = 7.25 mm, and the lengths of the lower and upper bases are j = 1 mm and j1 = 0.5 mm, respectively. The four rectangular resonance structures also are same. The parameters of rectangular unit are i = 2 mm and c1 = 1 mm, respectively. The detail information of other parameters can be seen in Table 1. As we know [18–20], the relative permittivity of solid plasma can be described by Drude model, which can be written as . The plasma frequency is , and the collision frequency is .The incident electromagnetic wave is along -z direction. In this paper, TE wave is considered that the electric field is parallel to y axis and the magnetic field is parallel to x axis. TM wave is assumed that the electric field is parallel to x axis and the magnetic field is parallel to y axis. Obviously, the absorption can be expressed as , where is reflectivity, and is transmittance.
Fig. 1 Structure schematic views of the unit cell for the proposed absorber: (a) the front view of unit cell; (b) the schematic of resonance cell; (c) the side view of unit cell
Table 1. The parameters of the proposed absorber
3.Results and Discussions
In Fig. 2, the absorption spectra of TE wave and the reflection spectrum of TM wave are plotted. In Fig. 2(a), only one case is considered, which is that only the resonance unit 1 is excited and the resistance elements R1 and R2 do not exist. We can see from Fig. 2(a) that there is only one absorption peak at 4.873 GHz with absorption rate of 88.58%. Figure 2(b) shows the result of one case which is that when only the resonance unit 1 is excited and the resistance element R1 is loaded. It can be seen from Fig. 2(b) that there are three absorption peaks at 1.9085 GHz, 3.861 GHz and 4.642 GHz with absorption rate of 92.76%, 99.99% and 97.75%, respectively. The obtained two absorption regions (absorption rate is larger than 0.9) are covered 1.7-4.062 GHz and 4.5-4.761 GHz, respectively. In Fig. 2(c), the result of one case is given which is all of the plasma resonance units are excited, and the resistance elements R1 and R2 are loaded. As shown in Fig. 2(c), there are two absorption peaks at 1.903 GHz and 3.9215 GHz with absorption rate of 95.55% and 99.42%, respectively. The reflectivity is below −10 dB cover 1.6115-4.0798 GHz, and the absorption rate is above 90% whose the relative bandwidth is 86.7%. Compared Fig. 2(c) with Figs. 2(a)-(b), in such a case, the proposed absorber has a larger absorption bandwidth, whose absorption rate is above 90%. We also can see from Fig. 2(d) that, for TM wave, the reflectivity of such an absorber is larger than 90% which is covered from 0 to 4.885 GHz. Thus, for TM wave, the proposed absorber can act as a reflector. Obviously, for TE wave, when all of the solid plasma resonators are excited, the absorption bandwidth (the absorption rate is above 90%) is 2.4683 GHz, and the ultra-broadband absorption can be achieved. For TM wave, the bandwidth of reflection runs from 0 to 4.885 GHz, whose reflectivity is larger than 90%. As mentioned above, the ultra-broadband absorption can be realized by loading the lumped resistance elements, and the absorption bandwidth also can be broadened by exciting the small plasma resonators beside the resonance unit 1. In a word, a polarization splitter also can be realized by the proposed absorber.
Fig. 2 Absorption and reflection spectra of the proposed absorber: (a) the resonance unit 1 is excited but the resistance elements do not exist; (b) the resonance unit 1 is excited and the resistance element R1 is loaded; (c) The absorption spectrum of TE wave when all of the resonators are excited and the resistance elements R1 and R2 are loaded; (d) The reflection spectrum of TM wave when all of the resonators are excited and the resistance elements R1 and R2 are loaded.
In order to explore the physical mechanism of the proposed absorber, the electric field distributions of the absorber surface and the surface current distributions at two different resonant frequencies 1.903 GHz and 3.9215 GHz are displayed in Fig. 3. As we know, when the electromagnetic wave is incident to two wires, the different kinds of surface current distributions can be obtained. If the two wires maintain an anti-parallel current, the magnetic resonance can be formed. In such a case, it also can be looked as the positive and negative charges are concentrated at both ends of the two wires, respectively. Similarly, if the two wires have a parallel current, the electrical resonance can be obtained. Such a case can be equivalent to the same polarity of charges are concentrated at both ends of two wires. Thus, we can use the surface electric field and surface current distributions of the resonator and the underlying metal plate to explain the physical mechanism of the proposed absorber, respectively. We can observe from Figs. 3(a)-(b) that the electric fields are mainly concentrated in the middle of the resonance unit 1 (marked by the symbol “○”) at 1.903 GHz and 3.9215 GHz, respectively. In those cases, the positive charges can be equally placed at the the middle of resonant unit 1 (marked by the symbol “○”). Figures 3(c)-(d) show the current distributions of the bottom copper plate at the frequencies of 1.903 GHz and 3.9215 GHz, respectively. The directions of the black arrows indicate the directions of currents flow. We can see from Fig. 3(c)-(d) that the surface currents in the bottom copper plate are mainly along the y axis, and such phenomena are equivalent to the negative charges placed there (marked by the symbol “○”). When the incident electromagnetic wave propagates through the proposed absorber, the upper surface and the bottom metal surface of the absorber can be equivalent to a electric dipole, which can be equivalent to a situation which is the reverse of the flow of the currents flowing on the upper and lower surfaces. Obviously, the physical mechanism of such an absorber is the magnetic resonance, which leads to consume the energy of incident electromagnetic wave.
Fig. 3 The electric field distributions of the absorber surface and the surface current distributions at different frequencies: (a) the electric field distribution of the absorber surface at f = 1.903 GHz; (b) the electric field distribution of the absorber surface at f = 3.9215 GHz; (c) the surface current distribution of the bottom copper plate at f = 1.903 GHz; (d) the surface current distribution of the bottom copper plate at f = 3.9215 GHz.
In order to further understand the loss mechanism of such an absorber, the power loss density distributions are plotted in Fig. 4. It can be seen from Figs. 4(a)-(b) that the energies of the incident electromagnetic wave at two different resonant frequencies 1.903 GHz and 3.9215 GHz are mainly consumed by the ohmic losses of the lumped resistances, which are loaded on the resonance units. We also can see from Figs. 4(c)-(d) that only a small part of the energies are lost by the dielectric substrate since the loss tangent of the dielectric substrate is small (tanδ = 0.003). Obviously, for the proposed absorber, the effective absorption of incident electromagnetic wave mainly depends on the magnetic resonances between the upper and lower layers of dielectric substrate, which leads to the ohmic losses of the loaded lumped resistances. Therefore, we can realize such an ultra-broadband absorber by loading the lumped resistances.
Fig. 4 (a) The power loss density of the surface of absorber when f = 1.903 GHz;(b) the power loss density of the surface of absorber surface when f = 3.9215 GHz; (c) the power loss density of the inside dielectric substrate when f = 1.903 GHz; (d) the power loss density of the inside dielectric substrate when f = 3.9215 GHz
Figure 5 shows the relationships between structural parameters a and q of the proposed absorber and absorption and reflection spectra of TE and TM waves, respectively. As shown in Fig. 5(a), with the increasing value of a (the length of the trapezoidal structures of the resonance unit 1), the absorption bandwidth of the absorber gradually increases but the absorption rate gradually decreases. One also can see from Fig. 5(c) that the curves of reflection are hardly changed with the increasing value of a. According to the results in Figs. 5(a)-(c), we can know that, for TE wave, a is an important parameter to manipulate the absorption bandwidth of such an absorber. However, changing the value of a is almost no effect on the tuning of reflection for TM wave. In Fig. 5(b), the relationships between the parameter q (the definition of q can be seen in Fig. 1) and the absorption spectra are given. From Fig. 5(b), we can discover that both the bandwidth of absorption and absorption rate will gradually increase when the value of q is increased. However, the different trends can be found in Fig. 5(d). As shown in Fig. 5(d), the reflection spectra of TM wave almost are never changed with varying the parameter q. The reflection coefficients of TM wave are larger than 0.9 in the frequency region 0-0.486 GHz, when the value of q is increased from −15°to 15°. Obviously, q is a significant parameter to tailor the absorption bandwidth of the proposed absorber. As mentioned above, if the excited regions of the plasma resonance units can be artificially controlled, the tunable absorption spectrum will be achieved. It means that the frequency regime of absorption for TE wave can be enlarged by optimizing the corresponding parameters, and the performance of such an absorber also can be improved. However, for TM wave, the present absorber can work as a reflector, and the better reflection characteristics can be observed in such a devices. Changing the parameters a and q will not effect on the reflection coefficient of TM wave. Thus, the polarization splitter can be realized by the proposed absorber.
Fig. 5 The relationships between structural parameters a, q and absorption and reflection spectra of TE and TM waves: (a) the absorption spectra of TE wave for a = 14,15,16 mm; (b) the absorption spectra of TE wave for q = −15°, 0°, 15°; (c) the reflection spectra of TM wave for a = 14,15,16 mm; (d) the reflection spectra of TM wave for q = −15°, 0°, 15°.
In Fig. 6, the relationships between parameters R1 and R2 of the proposed absorber and absorption and reflection spectra of TE and TM waves are plotted, respectively. As shown in Fig. 6(a), with the increasing value of R1, the frequency range of absorption (the absorption rate is above 90%) will increase first and then decrease. The largest bandwidth of absorption can be found at the case of R1 = 190 Ω, which is located at 1.6115-4.0798 GHz. If we continue to increase the value of R1, the bandwidth of absorption will gradually decrease, which covers 1.70-4.09 GHz. It can be seen from Fig. 6(b) that varying R2 has little effect on the performance of the proposed absorber. If R2 = 400, 500, 600, 700 Ω, the frequency ranges of absorption are located at 1.65-4.0798 GHz, 1.65-4.0798 GHz, 1.6115-4.0798 GHz, 1.62-4.0798 GHz, respectively. Obviously, the bandwidth of absorption can be slightly widened with the increasing value of R2. In Figs. 6(c)-(d), the reflection spectra with different R1 and R2 are given. We can observe from Fig. 6(c) that the reflection spectra of TM wave are hardly changed with varying the value of R1. The reflection spectra of TM wave are located at 0-4.885 GHz which are larger than 0.9, when R1 = 140, 190, 240 Ω. The similar trends of reflection spectra also can be seen in Fig. 6(d). As mentioned above, we can conclude that tuning the values of R1 and R2 can tailor the region of absorption spectrum of TE wave but has little effect on the reflection spectrum of TM wave.
Fig. 6 The relationships between parameters R1, R2 and absorption and reflection spectra of TE and TM waves: (a) the absorption spectra of TE wave for R1 = 140, 190, 240 Ω; (b) the absorption spectra of TE wave for R2 = 400, 500, 600, 700 Ω; (c) the reflection spectra of TM wave for R1 = 140,190,240 Ω; (d) the reflection spectra of TM wave for R2 = 400, 500, 600, 700 Ω.
In Fig. 7, the relationships between parameters h and w of the proposed absorber and absorption and reflection spectra of TE and TM waves are plotted, respectively. As shown in Fig. 7(a), with the increasing value of h (the thickness of the dielectric substrate), the frequency range of absorption (the absorption rate is above 90%) will almost never changed. If h = 14.1 mm, the frequency range of absorption is located at 1.5545-4.0269 GHz, and the absorption rate is above 90% (the relative bandwidth is 88.5%). It also can be seen from Fig. 7(b) that varying w (the thickness of the solid plasma resonators) has little effect on the performance of the proposed absorber. As shown in Fig. 7(c), the reflection spectra with the different h are given. If h = 13.1, 13.6, 14.1 mm, the frequency ranges of reflection are located at 0-4.8431GHz, 0-4.8874GHz, 0-4.906GHz, respectively. It is obvious that the reflection range of TM wave will increase slightly with the increasing value of h. The trends of reflection spectra in Fig. 7(c)-(d) are similar. As mentioned above, we can conclude that tuning the values of h can tailor the region of absorption spectrum of TE wave and the reflection spectrum of TM wave. However, changing the values of w has little effect on the region of absorption spectrum of TE wave and the reflection spectrum of TM wave.
Fig. 7 The relationships between parameters h, w and absorption and reflection spectra of TE and TM waves: (a) the absorption spectra of TE wave for h = 13.1, 13.6, 14.1 mm; (b) the absorption spectra of TE wave for w = 0.05, 0.1, 0.15mm; (c) the reflection spectra of TM wave for h = 13.1, 13.6, 14.1 mm; (d) the reflection spectra of TM wave for w = 0.05, 0.1, 0.15mm.
4. Conclusion
In summary, an ultra-broadband electromagnetic absorber is realized based on the lumped resistors and plasma metamaterial, whose relative bandwidth of absorption is 86.7% (absorption rate is larger than 0.9). The calculated results show that the largest frequency range of absorption of TE wave is covered 1.6115-4.0798 GHz (absorption rate is larger than 90%), and the tunable absorption spectrum of TE wave also can be achieved by exciting the different solid plasma resonance units. According to the results of analyzing the electric fields, surface current distributions and power loss densities of the different layers at the different frequencies, we can know that the energy of incident TE wave can be consumed in the proposed absorber due to the magnetic resonances between top and bottom layers of absorber. It means that the impedance of such an absorber is matched with the impedance of the free space. However, for TM wave, the reflectivity is close to 1, whose frequency region is covered 0-4.885 GHz. Therefore, the proposed absorber not only can realize the ultra-broadband absorption of TE wave, but also can work as a reflector for TM at some time. In the other word, a polarization splitter can be realized by such an absorber in the frequency range of 1.6115-4.0798 GHz.
Funding
Chinese Jiangsu Planned Projects for Postdoctoral Research Funds and Project Funded by China Postdoctoral Science Foundation; Special Grade of China Postdoctoral Science Foundation (1501016A, 2015M581790 and 2016T90455).
References and links
1. K. B. K. Teo, E. Minoux, L. Hudanski, F. Peauger, J. P. Schnell, L. Gangloff, P. Legagneux, D. Dieumegard, G. A. Amaratunga, and W. I. Milne, “Microwave devices: carbon nanotubes as cold cathodes,” Nature 437(7061), 968 (2005). [CrossRef] [PubMed]
2. D. Micheli, R. Pastore, C. Apollo, M. Marchetti, G. Gradoni, and V. M. Primiani, “Broadband electromagnetic absorbers using carbon nanostructure-based composites,” IEEE Trans. Microw. Theory Tech. 59(10), 2633–2646 (2011). [CrossRef]
3. Q. Cheng, T. J. Cui, W. X. Jiang, and B. G. Cai, “An onmidirectional electromagnetic absorber made of metamaterials,” New J. Phys. 12(6), 063006 (2010). [CrossRef]
4. D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite medium with simultaneously negative permeability and permittivity,” Phys. Rev. Lett. 84(18), 4184–4187 (2000). [CrossRef] [PubMed]
5. K. Iwaszczuk, A. C. Strikwerda, K. Fan, X. Zhang, R. D. Averitt, and P. U. Jepsen, “Flexible metamaterial absorbers for stealth applications at terahertz frequencies,” Opt. Express 20(1), 635–643 (2012). [CrossRef] [PubMed]
6. Y. Zhao, X. Y. Cao, J. Gao, and W. Q. Li, “Broadband radar absorbing material based on orthogonal arrangement of CSRR etched artificial magnetic conductor,” Microw. Opt. Technol. Lett. 56(1), 158–161 (2014). [CrossRef]
7. J. Shao, G. Fang, J. Fan, Y. C. Ji, and H. J. Yin, “TEM horn antenna loaded with absorbing material for GPR applications,” IEEE Antennas Wirel. Propag. Lett. 13(5), 523–527 (2014). [CrossRef]
8. J. R. Liu, M. Itoh, T. Horikawa, K. Machida, S. Sugimoto, and T. Maeda, “Gigahertz range electromagnetic wave absorbers made of amorphous-carbon-based magnetic nanocomposites,” J. Appl. Phys. 98(5), 054305 (2005). [CrossRef]
9. 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] [PubMed]
10. N. I. Landy, C. M. Bingham, T. Tyler, N. Jokerst, D. R. Smith, and W. J. Padilla, “Design, theory, and measurement of a polarization insensitive absorber for terahertz imaging,” Phys. Rev. B 79(12), 125104 (2009). [CrossRef]
11. X. Shen, T. J. Cui, J. Zhao, H. F. Ma, W. X. Jiang, and H. Li, “Polarization-independent wide-angle triple-band metamaterial absorber,” Opt. Express 19(10), 9401–9407 (2011). [CrossRef] [PubMed]
12. B. Kearney, F. Alves, D. Grbovic, and G. Karunasiri, “Terahertz metamaterial absorbers with an embedded resistive layer,” Opt. Mater. Express 3(8), 1020–1025 (2013). [CrossRef]
13. B. Zhu, Y. Feng, J. Zhao, C. Huang, Z. Wang, and T. Jiang, “Polarization modulation by tunable electromagnetic metamaterial reflector/absorber,” Opt. Express 18(22), 23196–23203 (2010). [CrossRef] [PubMed]
14. W. Yuan and Y. Cheng, “Low-frequency and broadband metamaterial absorber based on lumped elements: design, characterization and experiment,” Appl. Phys., A Mater. Sci. Process. 117(4), 1915–1921 (2014). [CrossRef]
15. C. Zhang, Q. Zhang, and M. Hu, “Frequency Selective Surface absorber loaded with lumped-element,” in International Symposium on Electromagnetic Compatibility. IEEE: 539–542, (2008).
16. S. J. Li, X. Y. Cao, J. Gao, T. Liu, Y. J. Zheng, and Z. Zhang, “Analysis and Design of Three-Layer Perfect Metamaterial- Inspired Absorber Based on Double Split-Serration-Rings Structure,” IEEE Trans. Antenn. Propag. 63(11), 5155–5160 (2015). [CrossRef]
17. Y. Wen, W. Ma, J. Bailey, G. Matmon, X. Yu, and G. Aeppli, “Planar broadband and high absorption metamaterial using single nested resonator at terahertz frequencies,” Opt. Lett. 39(6), 1589–1592 (2014). [CrossRef] [PubMed]
18. Y. K. Pozhela, R. B. Tolutis, and Z. K. Yankauskas, “Gaussian helicon-wave excitation in a magnetized solid-state plasma,” Radiophys. Quantum Electron. 27(6), 552–558 (1984). [CrossRef]
19. A. A. Vedenov, “Solid state plasma,” Sov. Phys. Usp. 7(6), 809–822 (1965). [CrossRef]
20. V. A. Manasson, L. S. Sadovnik, A. Moussessian, and D. B. Rutledge, “Millimeter-wave diffraction by a photo-induced plasma grating,” Microwave Theory and Techniques, IEEE Transactions on 43(9), 2288–2290 (1995). [CrossRef]
