A novel tunable frequency selective surface absorber with dual-DOF for broadband applications Peng Kong, XiaoWei Yu, ZhengYang Liu, Kai Zhou, Yun He, Ling Miao, and JianJun Jiang* School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China * [email protected]
Abstract: A novel tunable frequency selective surface (FSS) with dualdegrees of freedom (DOF) is presented, and firstly applied to broadband absorber. Based on a simple prototype unit cell resonator, an approach for achieving multi-resonances is studied. A unit cell pattern with gradient edges is discussed, and variable resistor and variable capacitor are introduced to fully utilize its characteristic of multi-resonances. Bias line is designed to provide bias voltage respectively for two variable devices and provide two operational DOF for FSS. Simulation and measurement results both show that the tunable FSS absorber with dual-DOF has wideband absorption with the reflectivity below −10 dB in 1−5 GHz and with a total thickness of about 10 mm. ©2014 Optical Society of America OCIS codes: (350.4010) Microwaves; (300.1030) Absorption; (260.5740) Resonance; (160.3918) Metamaterials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
A. Tennant and B. Chambers, “Adaptive radar absorbing structure with PIN diode controlled active frequency selective surface,” Smart Mater. Struct. 13(1), 122–125 (2004). J. C. Liu, C. Y. Liu, C. P. Kuei, C. Y. Wu, and Y. S. Hong, “Design and analysis of broadband microwave absorber utilizing FSS screen constructed with circular fractal configurations,” Microw. Opt. Technol. Lett. 48(3), 449–453 (2006). L. K. Sun, H. F. Cheng, Y. J. Zhou, and J. Wang, “Broadband metamaterial absorber based on coupling resistive frequency selective surface,” Opt. Express 20(4), 4675–4680 (2012). W. Ma, Y. Z. Wen, and X. M. Yu, “Broadband metamaterial absorber at mid-infrared using multiplexed cross resonators,” Opt. Express 21(25), 30724–30730 (2013). G. H. Yang, X. X. Liu, Y. L. Lv, J. H. Fu, Q. Wu, and X. M. Gu, “Broadband polarization-insensitive absorber based on gradient structure metamaterial,” J. Appl. Phys. 115(17), 17E523 (2014). Q. Chen, J. J. Jiang, X. X. Xu, L. Zhang, L. Miao, and S. W. Bie, “A thin and broadband tunable radar absorber using active frequency selective surface,” in 2012 IEEE Antennas And Propagation Society International Symposium (IEEE, 2012). M. Yoo and S. Lim, “Polarization-independent and ultrawideband metamaterial absorber using a hexagonal artificial impedance surface and a resistor-capacitor layer,” IEEE Trans. Antenn. Propag. 62(5), 2652–2658 (2014). B. X. Wang, L. L. Wang, G. Z. Wang, W. Q. Huang, X. F. Li, and X. Zhai, “Theoretical investigation of broadband and wide-angle terahertz metamaterial absorber,” IEEE Photon. Technol. Lett. 26(2), 111–114 (2014). Y. Z. Wen, W. Ma, J. Bailey, G. Matmon, X. M. Yu, and G. Aeppli, “Planar broadband and high absorption metamaterial using single nested resonator at terahertz frequencies,” Opt. Lett. 39(6), 1589–1592 (2014). B. X. Wang, L. L. Wang, G. Z. Wang, W. Q. Huang, X. F. Li, and X. Zhai, “Metamaterial-based lowconductivity alloy perfect absorber,” J. Lightwave Technol. 32(12), 2293–2298 (2014). W. C. Li, X. J. Qiao, Y. Luo, F. X. Qin, and H. X. Peng, “Magnetic medium broadband metamaterial absorber based on the coupling resonance mechanism,” Appl. Phys., A Mater. Sci. Process. 115(1), 229–234 (2014). X. X. Xu, J. J. Jiang, S. W. Bie, Q. Chen, C. K. Zhang, and L. Miao, “Optimal design of electromagnetic absorbers using visualization method for wideband potential applications,” J. Electromagn. Waves Appl. 26(8– 9), 1215–1225 (2012). M. Li, S. Xiao, Y. Y. Bai, and B. Z. Wang, “An ultrathin and broadband radar absorber using resistive FSS,” IEEE Antennas Wirel. Propag. Lett. 11, 748–751 (2012).
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30217
14. W. R. Xu and S. Sonkusale, “Microwave diode switchable metamaterial reflector/absorber,” Appl. Phys. Lett. 103(3), 031902 (2013). 15. B. Zhu, Y. J. Feng, J. M. Zhao, C. Huang, and T. A. Jiang, “Switchable metamaterial reflector/absorber for different polarized electromagnetic waves,” Appl. Phys. Lett. 97(5), 051906 (2010). 16. C. Mias, “Varactor tunable frequency selective absorber,” Electron. Lett. 39(14), 1060–1062 (2003). 17. P. S. Taylor, E. A. Parker, and J. C. Batchelor, “An active annular ring frequency selective surface,” IEEE Trans. Antenn. Propag. 59(9), 3265–3271 (2011). 18. B. Sanz-Izquierdo, E. A. Parker, and J. C. Batchelor, “Dual-band tunable screen using complementary split ring resonators,” IEEE Trans. Antenn. Propag. 58(11), 3761–3765 (2010). 19. G. M. Coutts, R. R. Mansour, and S. K. Chaudhuri, “Microelectromechanical systems tunable frequencyselective surfaces and electromagnetic-bandgap structures on rigid-flex substrates,” IEEE Trans. Microw. Theory Tech. 56(7), 1737–1746 (2008). 20. L. Boccia, I. Russo, G. Amendola, and G. Di Massa, “Tunable frequency-selective surfaces for beam-steering applications,” Electron. Lett. 45(24), 1213–1214 (2009). 21. F. Costa, A. Monorchio, and G. P. Vastante, “Tunable high-impedance surface with a reduced number of varactors,” IEEE Antennas Wirel. Propag. Lett. 10, 11–13 (2011). 22. A. Fallahi, A. Yahaghi, H. Benedickter, H. Abiri, M. Shahabadi, and C. Hafner, “Thin wideband radar absorbers,” IEEE Trans. Antenn. Propag. 58(12), 4051–4058 (2010).
1. Introduction Microwave absorber is widely applied to reduce the radar cross section (RCS) of fighter plane and warship. In the fields of metamaterial and frequency selective surface (FSS), there have been numerous reports concerning wideband absorber in recent years [1–10]. The resonances in the absorber have been used to explain the broadband properties [2–5,9–13]. From this standpoint, more resonances are required for achieving wideband performance. It is an effective approach to expanding bandwidth whether the absorber is tunable or not [3,4,6]. Smart steal system (SSS) is able to tune its working band to absorb electromagnetic (EM) wave at desired frequency, and finally achieves an equivalent broadband absorption. Tunable absorber is required for this smart application, while the inactive absorbers failed to meet this demand. For tunable absorber, insert of active devices is always an important work and is able to provide the degrees of freedom (DOF) for tuning [1,6,14–16]. In the fields of FSS and metamaterial, including the ones not used for absorption, the previous work indicated that variable resistor [1,6,17,18] and variable capacitor [16,18–21] are both benefit to tunable performance. The variable devices are able to achieve impedance matching surface in certain frequency band. For tunable FSSs with variable capacitors, when the variable capacitors change, the resonance frequency is able to be changed, and they all have the best fit capacitance for the strongest resonance [16,21]. It indicates that the impedance matching of FSSs loses when the variable capacitors are away from the best fit capacitance. It means that it is not good enough for broadband performances with only variable capacitors. In order to maintain the impedance matching at the time of capacitance change, other devices such as variable resistors and variable inductors should be introduced for parametric compensation. However, there have been few studies about the effect of using two or three of them in FSS or metamaterial or other electromagnetic surfaces. And there have been few studies highlighting a broadband absorber applied in 1−5 GHz, both in the fields of tunable and un-tunable absorber. In this letter, a novel tunable FSS absorber with dual-DOF for 1−5 GHz applications is reported. An approach for achieving multi-resonances is studied based on a simple prototype unit cell resonator. A pattern with gradient edges is then discussed, and based on which the bias line is designed to provide bias voltage respectively for variable resistor and variable capacitor. Simulation and measurement results both show that multi-resonances exist in the structure and the effects of two variable devices are benefit to broadband absorption. 2. Optimization of the prototype FSS absorber RLC circuit is one of the simplest resonators, and is chosen as the prototype unit cell pattern of FSS absorber. The three-dimensional sketch of the absorber is shown in Fig. 1. The absorber consists of substrate layer, isolation layer and ground plane. Depends on the manufacturing process, the material of substrate layer is chosen as FR4 with a permittivity of
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30218
4.4, a loss tangent of 0.02, and a thickness of 0.7 mm (h1). And honeycomb support plane is used as isolation layer between metallic pattern and conductor floor with a thickness of 9.5mm (h2) for its high strength and low-density. These two layers are constant throughout the paper, and the primary design target is focused on the pattern of FSS unit cell. The unit cell pattern of FSS is modeled in High Frequency Structure Simulator (HFSS). And Finite Element Method (FEM) is used to simulate the electromagnetic properties of FSS structure. The period of FSS unit cell is 36 mm. The periodical boundary condition (masterslave boundary) is adopted in the side faces. Floquet port is assigned on the top boundary along Z direction, and bottom side is grounded by the metallic plate. The incident wave is modeled as normal incident wave with electric field polarized along X axis. The metallic patch of pattern is model by perfect electric conductor (PEC) boundary. And the materials of substrate layer, isolation layer and ground plane are selected as FR4, air and copper in materials library. The simulation model is shown as the inset of Fig. 1.
Fig. 1. Three-dimensional sketch of the absorber.
The schematic structure of the prototype unit cell pattern before optimization is shown in Fig. 2(a). The prototype unit cell pattern is constructed with metallic patches and a single value resistor named R. The metallic patches and the gap between them contribute inductors and capacitors for unit cell. Its distribution of electric field on the FSS pattern at 3.24 GHz is shown in Fig. 2(b). And Fig. 2(c) shows its reflectivity as function of frequency. As shown in Fig. 2(b), the stronger electric field distributes along circuit lines, between the gap of patches and around resistor. It indicates that the resistor, the gap of patches and circuit line are combined in a simple RLC resonator and contribute a strong resonance at 3.24 GHz. It has an absorption peak but a narrow absorptive bandwidth, as shown in Fig. 2(c). It also means that the shape of the unit cell pattern has a great influence on the absorptive properties.
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30219
Fig. 2. Prototype unit cell pattern before optimization. (a) Schematic structure of unit cell pattern. (b) Distribution of electric field on the FSS pattern at 3.24 GHz. (c) Simulation reflectivity as function of frequency. Ws = 4 mm, Ls = 20 mm, Wl = 2 mm, Gs = 2 mm, R = 1000 Ω.
As the shape of metallic patch transforms, its inductance and capacitance change regularly, and will generate new resonant status. It is a possible way to construct the pattern of multi-resonances by adjusting its shape. Genetic Algorithm (GA) has been proved to be useful for optimizing the EM performances of FSS because of its superiority of solving multi-variables optimization problems [22]. To achieve wider absorptive bandwidth, an optimization is performed based on FEM. Four dimension parameters and resistance of the single value resistor are optimized by using GA, including Ls, Ws, Gs, Wl and R. So the shape of pattern will be changed during optimization to search those patterns of wider absorptive bandwidth. Both the magnitude and bandwidth of the reflectivity are considered for evaluating absorptive performance. In considering magnitude, the different values above or below the threshold are treated differently, in order to incline to the value nearby the threshold rather than the lower value. The fitness function is established as follows: Fitness = α
η Pn + μ Pm n +β N N
(1)
where N is the number of points in reflectivity curve, n and m are the number of points below and above the desired threshold, P are the magnitude of reflectivity, α and β are the weighting factors of bandwidth and average reflectivity, η and μ are the weighting factors of sum reflectivity below and above the desired threshold. Such a treatment is helpful to achieve a not so strong but wider absorptive bandwidth, more than some strong but narrow absorption peaks. Optimization goals set as the absorptive bandwidth with the reflectivity below −10 dB in the frequency range of 1−5 GHz. As shown in Fig. 3, optimized unit cell obviously has wider absorptive bandwidth. The main absorption peak is at 3.3 GHz, and there is a secondary absorption peak at about 4.5 GHz. The distributions of electric field shown in Fig. 3(c) and 3(d) indicate that the
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30220
resonances at the different absorption peak occur in different areas of the unit cell pattern. And two resonances all occur nearby the edge.
Fig. 3. Prototype unit cell pattern after optimization. (a) Schematic structure of unit cell pattern. (b) Simulation reflectivity as function of frequency. (c) Distribution of electric field on the FSS pattern at 3.3 GHz. (d) Distribution of electric field on the FSS pattern at 4.5 GHz. Ws = 13.07 mm, Ls = 35 mm, Wl = 7.67 mm, Gs = 2.5 mm, R = 140 Ω.
All parameters of individuals in GA optimization are extracted and processed to plot a contour map of fitness values. Ws and Wl are set as X- and Y-axes. The individual with maximum fitness value is kept and others are nullified when they have the same combination of Ws and Wl. As shown in Fig. 4, it could be noticed that almost all optimized patterns, which have higher fitness values, incline to have gradational edges, such as the edges of optimal solution. It means that achieving multi-resonances and wider bandwidth is probable by designing the pattern with gradational edges.
Fig. 4. Contour map of fitness values as function of Ws and Wl.
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30221
3. Broadband tunable absorber with dual-DOF Based on the above discussions, a unit cell pattern with gradational edges is developed, shown as Fig. 5. To fully utilize the pattern’s characteristic of multi-resonances, a variable resistor R-variable and a variable capacitor C-variable are introduced into unit cell. R-variable is between two patches, and C-variable is at the gap. They are able to provide two operational DOF for FSS. Two variable devices and inductors introduced by the patch construct a tunable RLC resonator. Simulation reflectivity in different values of R-variable and C-variable is shown in Fig. 5(b). Comparing orange and blue curves in Fig. 5(b), which have the same value of Cvariable, some absorption peaks disappear and the others appear as the value of R-variable changes. And comparing blue and green curves, which have the same value of R-variable, the reflectivity varies and the absorption peaks move along frequency direction as the value of C-variable changes. It demonstrates that the variable devices are both benefit for achieving wider operational bandwidth. Envelope curve, shown as the black curve in Fig. 5(b), is the synthesis result for the smaller values of other original curves, and it is processed to represent the comprehensive absorptive performance for all different tunable states. It indicates that an equivalent broadband absorption is achieved as the values of devices change. Figure 5(c)-5(e) indicates that there are strong electric fields distributing in different areas at different resonant status. This is similar with that of optimal prototype unit cell pattern shown in Fig. 3(c)-3(d), and further indicates more resonant status. It demonstrates that multi-resonances exist in this pattern with gradational edges.
Fig. 5. Unit cell pattern of the broadband absorber with gradational edges. (a) Schematic structure of unit cell pattern. (b) Simulation reflectivity of FSS with different resistance and capacitance. (c)−(e) Distribution of electric field on the FSS pattern at 1.2 GHz, 2.6 GHz and 4.0 GHz.
In samples fabrication, a PIN diode (BAP50-03, NXP) is used as the variable resistor and a varactor (BB131, NXP) is used as the variable capacitor. The former device works in forward bias, and the latter device works in reverse bias. Bias line is required for biasing those devices welded on the unit patterns. Taking too many liberties with design of bias line will broke the original resonator. It is found that cutting the patch by using extremely narrow split is able to afford bias lines respectively for two kinds of devices without impeding the original resonances, because the narrow split’s big capacitance allows resonant currents to flow
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30222
through it in microwave frequency range. We design bias line for the broadband tunable absorber with dual-DOF by using this approach. Fabricated 180 mm × 180 mm sample of the broadband tunable absorber with dual-DOF and distribution of its bias lines are shown in Fig. 6. The white lines shown in Fig. 6(b) are the extremely narrow splits, at the both side of which are the bias lines for two devices respectively. The bias lines with same polarity are connected together at the sides of sample. So all the same devices are in parallel and biased at the same voltage.
Fig. 6. Fabricated sample of the broadband tunable absorber with dual-DOF and distribution of its bias lines. (a) Photo of 180 mm × 180 mm sample. (b) Magnified picture of the unit cell pattern.
The microwave reflectivity of the sample is tested by arch test method based on two broadband double-ridged horn antennas and vector network analyzer (Agilent 8720ES). The experimental results and comparison between simulation and measurement are plotted in Fig. 7, in which the performances of tunable absorber in different bias voltages and their envelope are shown. The PIN diodes are biased forward at the first voltage, and the varactors are biased reversely at the second voltage. The PIN diodes work as resistors with a high resistance in microwave frequency band at small forward bias voltage, and with a low resistance at large forward voltage. The varactors have a high capacitance at small reverse bias voltage, and have a low capacitance at large reverse voltage. The comparisons between measurements and simulations are shown in Fig. 7(a)-7(d). In the same states of measurements and simulations, the trends of reflectivity changing as the function of frequency are similar with each other. When bias voltage changes, the tunable FSS has different absorption peaks, just as the simulation predicted. It also should be noticed that there are some differences between measurements and simulations. These differences mean that the addition of narrow splits slightly affects the performance. Still, the main design goals, tunability and broadband absorption performance of FSS, are maintained. Shown as the black curve in Fig. 7(e), the envelope of all reflectivity shows that there are obvious multiple absorption peaks in operational band, similarly with the simulation results expected in Fig. 5(b). Shown as the other curves in Fig. 7(e), the tunable effects of two devices are both obvious and also consistent with the simulation results shown in Fig. 5(b). It demonstrates that PIN diodes and varactors both contribute to shifting resonant frequency. It proves that the multiple resonant modes exist on this unit pattern with gradational edges and are able to be excited respectively in different electric parameters, thus generate broadband absorptive properties.
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30223
Fig. 7. Measurement reflectivity of the broadband tunable absorber with dual-DOF. (a)−(d) Comparison between simulation and measurement in different working states. (e) Measurement reflectivity.
The measurement results show that the broadband tunable absorber has wideband absorption with the reflectivity below −10 dB from 1.3 GHz to 4.7 GHz. The total thickness is about λ/23 of the lower limit frequency and λ/10 of the center frequency. In normal operating conditions, the 180 mm × 180 mm sample only needs below 0.5 W input. 4. Conclusion In this paper, based on a simple prototype unit cell resonator, the resonances on the FSS pattern is studied. It is found that there are multi-resonances exist on the pattern which has gradational edges. By designing the pattern with gradational edges and introducing variable resistor and variable capacitor, the tunable FSS absorber with multi-resonances and wider bandwidth is achieved. Based on this broadband FSS unit cell, the bias lines of two kinds of variable devices are designed with proper use of extremely narrow splits. Finally, a novel tunable FSS broadband absorber with dual-DOF is accomplished. It has wideband absorption with the reflectivity below −10 dB in 1−5 GHz and with a total thickness of about 10 mm. In the future, after dealing with the polarization- and angle- insensitive problems, the tunable FSS broadband absorber with dual-DOF will have broad prospects in the application of smart steal system. Acknowledgments This work was supported in part by the National Natural Science Foundation of China under Grant Nos. 61172003.
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30224
Abstract: A novel tunable frequency selective surface (FSS) with dualdegrees of freedom (DOF) is presented, and firstly applied to broadband absorber. Based on a simple prototype unit cell resonator, an approach for achieving multi-resonances is studied. A unit cell pattern with gradient edges is discussed, and variable resistor and variable capacitor are introduced to fully utilize its characteristic of multi-resonances. Bias line is designed to provide bias voltage respectively for two variable devices and provide two operational DOF for FSS. Simulation and measurement results both show that the tunable FSS absorber with dual-DOF has wideband absorption with the reflectivity below −10 dB in 1−5 GHz and with a total thickness of about 10 mm. ©2014 Optical Society of America OCIS codes: (350.4010) Microwaves; (300.1030) Absorption; (260.5740) Resonance; (160.3918) Metamaterials.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
A. Tennant and B. Chambers, “Adaptive radar absorbing structure with PIN diode controlled active frequency selective surface,” Smart Mater. Struct. 13(1), 122–125 (2004). J. C. Liu, C. Y. Liu, C. P. Kuei, C. Y. Wu, and Y. S. Hong, “Design and analysis of broadband microwave absorber utilizing FSS screen constructed with circular fractal configurations,” Microw. Opt. Technol. Lett. 48(3), 449–453 (2006). L. K. Sun, H. F. Cheng, Y. J. Zhou, and J. Wang, “Broadband metamaterial absorber based on coupling resistive frequency selective surface,” Opt. Express 20(4), 4675–4680 (2012). W. Ma, Y. Z. Wen, and X. M. Yu, “Broadband metamaterial absorber at mid-infrared using multiplexed cross resonators,” Opt. Express 21(25), 30724–30730 (2013). G. H. Yang, X. X. Liu, Y. L. Lv, J. H. Fu, Q. Wu, and X. M. Gu, “Broadband polarization-insensitive absorber based on gradient structure metamaterial,” J. Appl. Phys. 115(17), 17E523 (2014). Q. Chen, J. J. Jiang, X. X. Xu, L. Zhang, L. Miao, and S. W. Bie, “A thin and broadband tunable radar absorber using active frequency selective surface,” in 2012 IEEE Antennas And Propagation Society International Symposium (IEEE, 2012). M. Yoo and S. Lim, “Polarization-independent and ultrawideband metamaterial absorber using a hexagonal artificial impedance surface and a resistor-capacitor layer,” IEEE Trans. Antenn. Propag. 62(5), 2652–2658 (2014). B. X. Wang, L. L. Wang, G. Z. Wang, W. Q. Huang, X. F. Li, and X. Zhai, “Theoretical investigation of broadband and wide-angle terahertz metamaterial absorber,” IEEE Photon. Technol. Lett. 26(2), 111–114 (2014). Y. Z. Wen, W. Ma, J. Bailey, G. Matmon, X. M. Yu, and G. Aeppli, “Planar broadband and high absorption metamaterial using single nested resonator at terahertz frequencies,” Opt. Lett. 39(6), 1589–1592 (2014). B. X. Wang, L. L. Wang, G. Z. Wang, W. Q. Huang, X. F. Li, and X. Zhai, “Metamaterial-based lowconductivity alloy perfect absorber,” J. Lightwave Technol. 32(12), 2293–2298 (2014). W. C. Li, X. J. Qiao, Y. Luo, F. X. Qin, and H. X. Peng, “Magnetic medium broadband metamaterial absorber based on the coupling resonance mechanism,” Appl. Phys., A Mater. Sci. Process. 115(1), 229–234 (2014). X. X. Xu, J. J. Jiang, S. W. Bie, Q. Chen, C. K. Zhang, and L. Miao, “Optimal design of electromagnetic absorbers using visualization method for wideband potential applications,” J. Electromagn. Waves Appl. 26(8– 9), 1215–1225 (2012). M. Li, S. Xiao, Y. Y. Bai, and B. Z. Wang, “An ultrathin and broadband radar absorber using resistive FSS,” IEEE Antennas Wirel. Propag. Lett. 11, 748–751 (2012).
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30217
14. W. R. Xu and S. Sonkusale, “Microwave diode switchable metamaterial reflector/absorber,” Appl. Phys. Lett. 103(3), 031902 (2013). 15. B. Zhu, Y. J. Feng, J. M. Zhao, C. Huang, and T. A. Jiang, “Switchable metamaterial reflector/absorber for different polarized electromagnetic waves,” Appl. Phys. Lett. 97(5), 051906 (2010). 16. C. Mias, “Varactor tunable frequency selective absorber,” Electron. Lett. 39(14), 1060–1062 (2003). 17. P. S. Taylor, E. A. Parker, and J. C. Batchelor, “An active annular ring frequency selective surface,” IEEE Trans. Antenn. Propag. 59(9), 3265–3271 (2011). 18. B. Sanz-Izquierdo, E. A. Parker, and J. C. Batchelor, “Dual-band tunable screen using complementary split ring resonators,” IEEE Trans. Antenn. Propag. 58(11), 3761–3765 (2010). 19. G. M. Coutts, R. R. Mansour, and S. K. Chaudhuri, “Microelectromechanical systems tunable frequencyselective surfaces and electromagnetic-bandgap structures on rigid-flex substrates,” IEEE Trans. Microw. Theory Tech. 56(7), 1737–1746 (2008). 20. L. Boccia, I. Russo, G. Amendola, and G. Di Massa, “Tunable frequency-selective surfaces for beam-steering applications,” Electron. Lett. 45(24), 1213–1214 (2009). 21. F. Costa, A. Monorchio, and G. P. Vastante, “Tunable high-impedance surface with a reduced number of varactors,” IEEE Antennas Wirel. Propag. Lett. 10, 11–13 (2011). 22. A. Fallahi, A. Yahaghi, H. Benedickter, H. Abiri, M. Shahabadi, and C. Hafner, “Thin wideband radar absorbers,” IEEE Trans. Antenn. Propag. 58(12), 4051–4058 (2010).
1. Introduction Microwave absorber is widely applied to reduce the radar cross section (RCS) of fighter plane and warship. In the fields of metamaterial and frequency selective surface (FSS), there have been numerous reports concerning wideband absorber in recent years [1–10]. The resonances in the absorber have been used to explain the broadband properties [2–5,9–13]. From this standpoint, more resonances are required for achieving wideband performance. It is an effective approach to expanding bandwidth whether the absorber is tunable or not [3,4,6]. Smart steal system (SSS) is able to tune its working band to absorb electromagnetic (EM) wave at desired frequency, and finally achieves an equivalent broadband absorption. Tunable absorber is required for this smart application, while the inactive absorbers failed to meet this demand. For tunable absorber, insert of active devices is always an important work and is able to provide the degrees of freedom (DOF) for tuning [1,6,14–16]. In the fields of FSS and metamaterial, including the ones not used for absorption, the previous work indicated that variable resistor [1,6,17,18] and variable capacitor [16,18–21] are both benefit to tunable performance. The variable devices are able to achieve impedance matching surface in certain frequency band. For tunable FSSs with variable capacitors, when the variable capacitors change, the resonance frequency is able to be changed, and they all have the best fit capacitance for the strongest resonance [16,21]. It indicates that the impedance matching of FSSs loses when the variable capacitors are away from the best fit capacitance. It means that it is not good enough for broadband performances with only variable capacitors. In order to maintain the impedance matching at the time of capacitance change, other devices such as variable resistors and variable inductors should be introduced for parametric compensation. However, there have been few studies about the effect of using two or three of them in FSS or metamaterial or other electromagnetic surfaces. And there have been few studies highlighting a broadband absorber applied in 1−5 GHz, both in the fields of tunable and un-tunable absorber. In this letter, a novel tunable FSS absorber with dual-DOF for 1−5 GHz applications is reported. An approach for achieving multi-resonances is studied based on a simple prototype unit cell resonator. A pattern with gradient edges is then discussed, and based on which the bias line is designed to provide bias voltage respectively for variable resistor and variable capacitor. Simulation and measurement results both show that multi-resonances exist in the structure and the effects of two variable devices are benefit to broadband absorption. 2. Optimization of the prototype FSS absorber RLC circuit is one of the simplest resonators, and is chosen as the prototype unit cell pattern of FSS absorber. The three-dimensional sketch of the absorber is shown in Fig. 1. The absorber consists of substrate layer, isolation layer and ground plane. Depends on the manufacturing process, the material of substrate layer is chosen as FR4 with a permittivity of
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30218
4.4, a loss tangent of 0.02, and a thickness of 0.7 mm (h1). And honeycomb support plane is used as isolation layer between metallic pattern and conductor floor with a thickness of 9.5mm (h2) for its high strength and low-density. These two layers are constant throughout the paper, and the primary design target is focused on the pattern of FSS unit cell. The unit cell pattern of FSS is modeled in High Frequency Structure Simulator (HFSS). And Finite Element Method (FEM) is used to simulate the electromagnetic properties of FSS structure. The period of FSS unit cell is 36 mm. The periodical boundary condition (masterslave boundary) is adopted in the side faces. Floquet port is assigned on the top boundary along Z direction, and bottom side is grounded by the metallic plate. The incident wave is modeled as normal incident wave with electric field polarized along X axis. The metallic patch of pattern is model by perfect electric conductor (PEC) boundary. And the materials of substrate layer, isolation layer and ground plane are selected as FR4, air and copper in materials library. The simulation model is shown as the inset of Fig. 1.
Fig. 1. Three-dimensional sketch of the absorber.
The schematic structure of the prototype unit cell pattern before optimization is shown in Fig. 2(a). The prototype unit cell pattern is constructed with metallic patches and a single value resistor named R. The metallic patches and the gap between them contribute inductors and capacitors for unit cell. Its distribution of electric field on the FSS pattern at 3.24 GHz is shown in Fig. 2(b). And Fig. 2(c) shows its reflectivity as function of frequency. As shown in Fig. 2(b), the stronger electric field distributes along circuit lines, between the gap of patches and around resistor. It indicates that the resistor, the gap of patches and circuit line are combined in a simple RLC resonator and contribute a strong resonance at 3.24 GHz. It has an absorption peak but a narrow absorptive bandwidth, as shown in Fig. 2(c). It also means that the shape of the unit cell pattern has a great influence on the absorptive properties.
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30219
Fig. 2. Prototype unit cell pattern before optimization. (a) Schematic structure of unit cell pattern. (b) Distribution of electric field on the FSS pattern at 3.24 GHz. (c) Simulation reflectivity as function of frequency. Ws = 4 mm, Ls = 20 mm, Wl = 2 mm, Gs = 2 mm, R = 1000 Ω.
As the shape of metallic patch transforms, its inductance and capacitance change regularly, and will generate new resonant status. It is a possible way to construct the pattern of multi-resonances by adjusting its shape. Genetic Algorithm (GA) has been proved to be useful for optimizing the EM performances of FSS because of its superiority of solving multi-variables optimization problems [22]. To achieve wider absorptive bandwidth, an optimization is performed based on FEM. Four dimension parameters and resistance of the single value resistor are optimized by using GA, including Ls, Ws, Gs, Wl and R. So the shape of pattern will be changed during optimization to search those patterns of wider absorptive bandwidth. Both the magnitude and bandwidth of the reflectivity are considered for evaluating absorptive performance. In considering magnitude, the different values above or below the threshold are treated differently, in order to incline to the value nearby the threshold rather than the lower value. The fitness function is established as follows: Fitness = α
η Pn + μ Pm n +β N N
(1)
where N is the number of points in reflectivity curve, n and m are the number of points below and above the desired threshold, P are the magnitude of reflectivity, α and β are the weighting factors of bandwidth and average reflectivity, η and μ are the weighting factors of sum reflectivity below and above the desired threshold. Such a treatment is helpful to achieve a not so strong but wider absorptive bandwidth, more than some strong but narrow absorption peaks. Optimization goals set as the absorptive bandwidth with the reflectivity below −10 dB in the frequency range of 1−5 GHz. As shown in Fig. 3, optimized unit cell obviously has wider absorptive bandwidth. The main absorption peak is at 3.3 GHz, and there is a secondary absorption peak at about 4.5 GHz. The distributions of electric field shown in Fig. 3(c) and 3(d) indicate that the
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30220
resonances at the different absorption peak occur in different areas of the unit cell pattern. And two resonances all occur nearby the edge.
Fig. 3. Prototype unit cell pattern after optimization. (a) Schematic structure of unit cell pattern. (b) Simulation reflectivity as function of frequency. (c) Distribution of electric field on the FSS pattern at 3.3 GHz. (d) Distribution of electric field on the FSS pattern at 4.5 GHz. Ws = 13.07 mm, Ls = 35 mm, Wl = 7.67 mm, Gs = 2.5 mm, R = 140 Ω.
All parameters of individuals in GA optimization are extracted and processed to plot a contour map of fitness values. Ws and Wl are set as X- and Y-axes. The individual with maximum fitness value is kept and others are nullified when they have the same combination of Ws and Wl. As shown in Fig. 4, it could be noticed that almost all optimized patterns, which have higher fitness values, incline to have gradational edges, such as the edges of optimal solution. It means that achieving multi-resonances and wider bandwidth is probable by designing the pattern with gradational edges.
Fig. 4. Contour map of fitness values as function of Ws and Wl.
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30221
3. Broadband tunable absorber with dual-DOF Based on the above discussions, a unit cell pattern with gradational edges is developed, shown as Fig. 5. To fully utilize the pattern’s characteristic of multi-resonances, a variable resistor R-variable and a variable capacitor C-variable are introduced into unit cell. R-variable is between two patches, and C-variable is at the gap. They are able to provide two operational DOF for FSS. Two variable devices and inductors introduced by the patch construct a tunable RLC resonator. Simulation reflectivity in different values of R-variable and C-variable is shown in Fig. 5(b). Comparing orange and blue curves in Fig. 5(b), which have the same value of Cvariable, some absorption peaks disappear and the others appear as the value of R-variable changes. And comparing blue and green curves, which have the same value of R-variable, the reflectivity varies and the absorption peaks move along frequency direction as the value of C-variable changes. It demonstrates that the variable devices are both benefit for achieving wider operational bandwidth. Envelope curve, shown as the black curve in Fig. 5(b), is the synthesis result for the smaller values of other original curves, and it is processed to represent the comprehensive absorptive performance for all different tunable states. It indicates that an equivalent broadband absorption is achieved as the values of devices change. Figure 5(c)-5(e) indicates that there are strong electric fields distributing in different areas at different resonant status. This is similar with that of optimal prototype unit cell pattern shown in Fig. 3(c)-3(d), and further indicates more resonant status. It demonstrates that multi-resonances exist in this pattern with gradational edges.
Fig. 5. Unit cell pattern of the broadband absorber with gradational edges. (a) Schematic structure of unit cell pattern. (b) Simulation reflectivity of FSS with different resistance and capacitance. (c)−(e) Distribution of electric field on the FSS pattern at 1.2 GHz, 2.6 GHz and 4.0 GHz.
In samples fabrication, a PIN diode (BAP50-03, NXP) is used as the variable resistor and a varactor (BB131, NXP) is used as the variable capacitor. The former device works in forward bias, and the latter device works in reverse bias. Bias line is required for biasing those devices welded on the unit patterns. Taking too many liberties with design of bias line will broke the original resonator. It is found that cutting the patch by using extremely narrow split is able to afford bias lines respectively for two kinds of devices without impeding the original resonances, because the narrow split’s big capacitance allows resonant currents to flow
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30222
through it in microwave frequency range. We design bias line for the broadband tunable absorber with dual-DOF by using this approach. Fabricated 180 mm × 180 mm sample of the broadband tunable absorber with dual-DOF and distribution of its bias lines are shown in Fig. 6. The white lines shown in Fig. 6(b) are the extremely narrow splits, at the both side of which are the bias lines for two devices respectively. The bias lines with same polarity are connected together at the sides of sample. So all the same devices are in parallel and biased at the same voltage.
Fig. 6. Fabricated sample of the broadband tunable absorber with dual-DOF and distribution of its bias lines. (a) Photo of 180 mm × 180 mm sample. (b) Magnified picture of the unit cell pattern.
The microwave reflectivity of the sample is tested by arch test method based on two broadband double-ridged horn antennas and vector network analyzer (Agilent 8720ES). The experimental results and comparison between simulation and measurement are plotted in Fig. 7, in which the performances of tunable absorber in different bias voltages and their envelope are shown. The PIN diodes are biased forward at the first voltage, and the varactors are biased reversely at the second voltage. The PIN diodes work as resistors with a high resistance in microwave frequency band at small forward bias voltage, and with a low resistance at large forward voltage. The varactors have a high capacitance at small reverse bias voltage, and have a low capacitance at large reverse voltage. The comparisons between measurements and simulations are shown in Fig. 7(a)-7(d). In the same states of measurements and simulations, the trends of reflectivity changing as the function of frequency are similar with each other. When bias voltage changes, the tunable FSS has different absorption peaks, just as the simulation predicted. It also should be noticed that there are some differences between measurements and simulations. These differences mean that the addition of narrow splits slightly affects the performance. Still, the main design goals, tunability and broadband absorption performance of FSS, are maintained. Shown as the black curve in Fig. 7(e), the envelope of all reflectivity shows that there are obvious multiple absorption peaks in operational band, similarly with the simulation results expected in Fig. 5(b). Shown as the other curves in Fig. 7(e), the tunable effects of two devices are both obvious and also consistent with the simulation results shown in Fig. 5(b). It demonstrates that PIN diodes and varactors both contribute to shifting resonant frequency. It proves that the multiple resonant modes exist on this unit pattern with gradational edges and are able to be excited respectively in different electric parameters, thus generate broadband absorptive properties.
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30223
Fig. 7. Measurement reflectivity of the broadband tunable absorber with dual-DOF. (a)−(d) Comparison between simulation and measurement in different working states. (e) Measurement reflectivity.
The measurement results show that the broadband tunable absorber has wideband absorption with the reflectivity below −10 dB from 1.3 GHz to 4.7 GHz. The total thickness is about λ/23 of the lower limit frequency and λ/10 of the center frequency. In normal operating conditions, the 180 mm × 180 mm sample only needs below 0.5 W input. 4. Conclusion In this paper, based on a simple prototype unit cell resonator, the resonances on the FSS pattern is studied. It is found that there are multi-resonances exist on the pattern which has gradational edges. By designing the pattern with gradational edges and introducing variable resistor and variable capacitor, the tunable FSS absorber with multi-resonances and wider bandwidth is achieved. Based on this broadband FSS unit cell, the bias lines of two kinds of variable devices are designed with proper use of extremely narrow splits. Finally, a novel tunable FSS broadband absorber with dual-DOF is accomplished. It has wideband absorption with the reflectivity below −10 dB in 1−5 GHz and with a total thickness of about 10 mm. In the future, after dealing with the polarization- and angle- insensitive problems, the tunable FSS broadband absorber with dual-DOF will have broad prospects in the application of smart steal system. Acknowledgments This work was supported in part by the National Natural Science Foundation of China under Grant Nos. 61172003.
#220690 - $15.00 USD Received 11 Aug 2014; revised 5 Nov 2014; accepted 18 Nov 2014; published 25 Nov 2014 (C) 2014 OSA 1 December 2014 | Vol. 22, No. 24 | DOI:10.1364/OE.22.030217 | OPTICS EXPRESS 30224
