Applied Physics Express 7, 112204 (2014) http://dx.doi.org/10.7567/APEX.7.112204
Active metasurface for controlling reflection and absorption properties Minyeong Yoo and Sungjoon Lim* School of Electrical and Electronics Engineering, Chung-Ang University, Seoul 156-756, Republic of Korea E-mail:
[email protected] Received August 12, 2014; accepted October 1, 2014; published online October 31, 2014 A novel active metasurface is proposed in this paper. The proposed metasurface can be electronically switched between the reflection and the absorption modes by a tunable component, such as a PIN diode. When the PIN diode is in the on-state, the proposed metasurface exhibits nearperfect absorptivity at 10 GHz. On the other hand, when the PIN diode is in the off-state, the proposed metasurface exhibits near-perfect reflectivity over a wide bandwidth at 10 GHz. A unit cell is designed to include a DC bias network, which yields reliable performance. © 2014 The Japan Society of Applied Physics
ince Russian physicist Veselago first performed theoretical studies of metamaterials in 1968, metamaterials have received considerable attention because of their extraordinary characteristics.1) In addition, their permittivity and permeability can be manipulated by employing artificial conductive patterns. For instance, a metasurface can be implemented using periodic metallic patterns on a planar dielectric substrate. Many applications of metamaterials have been studied, such as invisibility cloak technology,2) antennas,3) sensors,4) superlenses,5) reflectors,6) and perfect absorbers.7–16) In general, the bandwidth of a metasurface is very narrow given that it exhibits a resonant structure. For example, a metamaterial absorber can absorb electromagnetic (EM) energy only at a resonant frequency. Although narrowband operation is useful for sensor detection, broadband operation is preferred for most absorber applications for radar-crosssection (RCS) reduction or suppression of electromagnetic interference.9–11) The frequency limitation can be overcome by the loading of tuning components on the metasurface. Therefore, the resonant frequency of active metasurfaces can be continuously changed and can cover a wide spectrum. In addition, although conventional metasurfaces are only capable of having a single type of effect on an incident EM wave, active metasurfaces can serve multiple functions by switching of their electrical properties. Multifunctional active metasurfaces can be applied in diverse EM environments. In recent years, several active metasurfaces have been reported with various tuning mechanisms, such as diodes,12–15) microelectromechanical systems (MEMS),17) and lasers.18) Their resonant frequencies,12) power levels,13) polarizations,14) and modes15) can be varied electronically or optically. In this paper, we propose an active metasurface that allows its reflectivity and absorptivity to be controlled at 10 GHz. The proposed metasurface can be switched between the reflection and absorption modes by changing the bias voltage. In this work, PIN diodes are employed as tuning components because of their fast electronic tuning capability and simple fabrication. The capability of switching between reflection and absorption modes has been previously demonstrated using a frequency-tunable metasurface.15) In this work, we propose a metasurface that can be switched between reflection and absorption modes where broadband reflection is achieved by pushing the resonant frequency of the reflection mode to a level that is much higher than that achieved by tuning the frequency only slightly. It should be noted that a bias network is required to apply the bias voltage to the PIN diodes; without
S
this bias network, the performance of the active metasurface will not be reliable. In this work, the active metasurface is implemented using a stable bias network. We find that the simulation and measurement results are in good agreement. In general, metasurfaces exhibit frequency-dependent permittivities ¾(½) and permeabilities ®(½) under effectivemedium approximations.16) When an EM wave is normally incident on a metasurface with relative permittivity (¾M) and relative permeability (®M), the intrinsic impedance (ZM) of the metasurface is determined by the relation rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi M ð!Þ0 ZM ¼ ; ð1Þ "M ð!Þ"0 where ¾0 and ®0 are the relative permittivity and relative permeability of free space, respectively. If ¾M and ®M are manipulated such that they are identical, the impedance of the metasurface can be matched to the impedance of air (Z0 = 377 ³), which results in no reflection from the metasurface, as suggested by the following governing equation: ð!Þ ¼
ZM ð!Þ Z0 : ZM ð!Þ þ Z0
ð2Þ
In addition, when the transmitted EM energy is completely dissipated from the conductive and dielectric losses, the metasurface serves as an absorber. On the other hand, when the impedance of the metasurface does not match with that of air, the incident EM waves are reflected. From Eq. (1), we find that the metasurface can be made to operate as a perfect reflector by manipulating ¾M and ®M. Therefore, a metasurface with controllable reflection and absorption can be designed through the electronic control of ZM. Figure 1(a) shows the switchable unit cell of the proposed active metasurface. It consists of a periodic array of a splitring resonator (SRR) on an FR-4 epoxy substrate. The bottom layer is covered with a conductor to prevent backward transmission. To electrically switch the impedance, a single PIN diode is placed on the central wire of the SRR, which is the critical point for controlling the current flow. Therefore, the electric and magnetic responses can be switched by changing the bias voltages of the PIN diode, which results in dramatic changes in the impedance. When the PIN diode is forward-biased, current flows through the PIN diode so that the unit cell works in the absorption mode. In the absorption mode, electric resonance is generated when the incident electric field is polarized along the axis parallel to the central wire of the SRR. In addition, magnetic resonance is generated by the flow of the anti-
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© 2014 The Japan Society of Applied Physics
Appl. Phys. Express 7, 112204 (2014)
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Reflection Coefficients [dB]
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Fig. 2. Simulated reflection coefficients for the on- and off-states of the PIN diode.
(b) Fig. 1. (a) Top and bottom views of the proposed switchable unit cell and (b) 3D configuration of the unit cell for full-wave simulation: a = 1.4, b = 0.7, c = 0.8, d = 0.8, e = 0.8, f = 0.4, g = 3.4, h = 3.4, i = 0.45, j = 0.2, k = 8.5, l = 8.5, and m = 2.5 (in units of mm).
parallel surface current through the central wire of the SRR at the top and bottom layers. When either zero bias or reverse bias is applied to the PIN diode, current cannot flow through the PIN diode, and the impedance changes dramatically. Consequently, the unit cell is switched to the reflection mode. To electronically switch the bias state of the PIN diode, it is necessary to apply a DC voltage with a suitable bias network to obtain reliable performance. The bias network consists of an inductor, a via hole, and a partition gap for each unit cell, as illustrated in Fig. 1(a). The inductor works as an RF choke to prevent the leakage of the RF signal through the bias line. In general, high-frequency inductors have relatively small inductance values. Therefore, two inductors are used in series to achieve a sufficiently high inductance for the RF choking function. A metallic wire is used to connect the unit cell with the DC power supply. When the metallic wire is placed on the top plane, facing the incident EM wave, the presence of the wire may cause scattering, which would result in inadequate performance. Therefore, the metallic wire is placed on the bottom plane, which is electrically connected to the top plane through the via hole (which has a 0.2 mm radius), as shown in Fig. 1(a). The partition gap is located in the middle of the bottom plane to prevent an electrical DC short between the forward and zero bias lines. Given that the partition gap can result in some transmitted waves, as it acts as a radiation slot, its orientation is parallel to the polarization of the incident electric field. Therefore, the effect of the partition gap is minimized. Figure 1(b) shows the three-dimensional (3D) configuration of the unit cell for full-wave analysis. The full-wave analysis is performed using a commercial finite element method (FEM)-based EM simulator, ANSYS HFSS. Master/ slave boundary conditions are applied to realize infinite arrays of the proposed metasurface. The EM wave is normally
incident on the proposed metasurface, and its electric field is polarized along the x-axis, which is parallel to the central wire of the SRR. Each unit cell has a bias network and a PIN diode with the same orientation, as illustrated in Fig. 1(b). The PIN diode for the EM simulation is a series of inductance (LS) and resistance (RS) in the on-state and a series of inductance (LS) and capacitance (CT) in the off-state. The simulated reflection coefficients of the proposed metasurface are plotted in Fig. 2. The proposed metasurface works as an absorber when the PIN diode is in the on-state. The metasurface has a reflection coefficient of ¹25 dB at 10.972 GHz, which results in near-perfect absorptivity. On the other hand, the proposed metasurface has a reflection coefficient of ¹0.213 dB at 10.972 GHz when the PIN diode is in the offstate. Therefore, near-perfect absorptivity and reflectivity are respectively achieved for the on- and off-states of the PIN diode. Because of the junction capacitance of the PIN diode in the off-state, resonance is unavoidable in the reflection mode. However, the resonant frequency in the reflection mode occurs at 20.948 GHz, which is much higher than 10.972 GHz. Therefore, a broadband reflection mode is achieved in this work. The electric and magnetic responses of the proposed metasurface can be understood by examining the electric-field distributions and surface current densities (see the online supplementary data available at http://stacks.iop.org/APEX/7/ 112204/mmedia). A prototype sample, shown in Fig. 3, is fabricated to experimentally demonstrate the switching capability of the proposed metasurface. The fabricated prototype has 10 © 10 unit cells on a 2.5 mm FR-4 substrate with a dielectric constant of 3.9 and a loss tangent of 0.016. An M/A-COM MA4PBL027 beam-lead PIN diode is provided as a switching component. A Murata LQW04AN5N1C00 chip inductor was chosen as the RF choke inductor. Its inductance is 5.1 nH, and its self-resonance frequency (SRF) is greater than 10 GHz. Because two inductors are used in series, a 10-nH inductance is achieved for the RF choke. Each lumped component is attached to the conductive patterns of the top layer using surface-mount technology (SMT) with leads, as shown in the inset of Fig. 3(a). In addition, a photoimageable solder resist (PSR) ink is printed on the bottom layer to prevent an electrical DC short between the forward-bias and zero-bias lines, and a metallic wire is soldered to the bottom layer to connect the prototype to a DC power supply, as shown in Fig. 3(b). The bias network has the effect of maintaining the performance of the proposed metasurface, regardless of the length of the wire.
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Appl. Phys. Express 7, 112204 (2014)
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Relative Permittivity / Permeability
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(a) Fig. 3. Fabricated prototype sample of the proposed active metasurface: (a) top layer and (b) bottom layer.
Relative Permittivity / Permeability
The absorptivity can be calculated from the reflection coefficient R (½) and the transmission coefficient T(½). The absorptivity is given by A(½) = 1 ¹ R (½) ¹ T (½). To measure the reflection and transmission coefficients, the bistatic RCS measurement setup11) and waveguide method19) are used. Using a retrieval method20) and Eq. (1), the relative permittivity and permeability can be calculated from the measured S-parameters for the on- and off-states. Since it is inaccurate to extract the complex value of the impedance from a free-space measurement, we use the waveguide method21) to extract the complex impedance of the fabricated metasurface. Figure 4 shows the relative permittivity and permeability of the proposed metasurface for the on- and off-states. In the absorption mode (the on-state), it is observed in Fig. 4(a) that the real parts of the relative permittivity and permeability are the same. Therefore, we can expect the impedance of the proposed metasurface to be matched with that of air, and the proposed metasurface works as a perfect absorber. On the other hand, Fig. 4(b) shows that the real parts of the relative permittivity and permeability of the proposed metasurface are different in the reflection mode (the off-state). Therefore, high reflectivity is expected, and the proposed metasurface functions as a perfect reflector. Figure 5(a) shows the simulated and measured reflection and transmission coefficients under normal incidence from 9 to 13 GHz. When a bias voltage of 1.3 V is applied to the PIN diode, the active metasurface serves as an absorber. In the absorption mode, the measured reflection coefficient and transmission coefficient are ¹22.41 and ¹40.92 dB, respectively, at 10.972 GHz, and the 10-dB bandwidth is 6.09%. The absorptivity is calculated to be 99.4%. When the PIN diode turns off under zero bias, the active metasurface works as a reflector. In the reflection mode, the measured reflection coefficient and transmission coefficient are ¹0.281 and ¹30.63 dB, respectively, at 10.972 GHz. The calculated reflectivity for the reflection mode is 93.8%. The simulation and measurement results are in good agreement with the reflection coefficients. The simulated and measured transmission coefficients are slightly different as a result of the finite size of the fabricated sample. An infinite periodic structure is assumed in the fullwave simulation, whereas only some of the unit cells are used in the measurement. Nevertheless, the measured transmission coefficients are sufficiently small and can be ignored. The performance of the proposed metasurface was investigated at oblique incident angles and for various polarizations of the EM wave. Figures 6(a) and 6(b) show the simulated and measured absorptivities for different incident
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(b) Fig. 4. Extracted EM characteristics from the measurement results of the fabricated metasurface; the measured real and imaginary parts of the relative permittivity and permeability for (a) the on-state and (b) the off-state.
Fig. 5. Simulated and measured reflection and transmission coefficients of the active metasurface in the on- and off-modes at normal incidence from 9 to 13 GHz.
angles of the EM waves, respectively. Although the resonant frequency in the absorption mode decreases from 10.972 to 10.4 GHz in the simulation and to 10.791 GHz in the measurement as the incident angle increases, the absorptivity is maintained at a value higher than 90% at the resonant frequency. In the reflection mode, the proposed metasurface has low absorptivity, which represents high reflectivity, for oblique incident angles. However, the absorptivity starts decreasing from an incident angle of 60°, while the absorptivity is almost constant from 0 to 50°. Figures 6(c) and 6(d) show the simulated and measured absorptivities for different polarizations under normal incidence, respectively. In order to change the polarization of the incident wave, the horn antennas are turned from 0 to 90° in steps of 15°. In the absorption mode, the simulated absorptivity is almost 90% at the resonant frequency from 0 to 45° in spite of the resonant frequency changing from 10.972 to 11.1 GHz, as shown in Fig. 6(c). Similarly, the measured
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© 2014 The Japan Society of Applied Physics
Appl. Phys. Express 7, 112204 (2014)
M. Yoo and S. Lim
(a)
(b)
(c)
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Fig. 6. (a) Simulated and (b) measured absorptivity of the active metasurface at various incident angles and (c) simulated and (d) measured absorptivity of the active metasurface at various polarizations for the on- and off-states.
absorptivity is almost constant and higher than 96.8% at 10.972 GHz from 0 to 45° in the absorption mode, as shown in Fig. 6(d). However, both the simulated and measured absorptivity drastically decrease from 60°. In addition, both the simulated and measured absorptivity in the reflection mode are less than 13.7% at 10.972 GHz for all polarization angles. In summary, we have proposed an active metasurface that can be switched between reflection and absorption modes. Mode switching is achieved by incorporating PIN diodes and a bias network with an SRR. When 10.972 GHz EM radiation is normally incident, the proposed metasurface shows an absorptivity of 99.4% when the PIN diode is in the on-state and a reflectivity of 93.8% when the PIN diode is in the off-state. When the incident angle is increased from 0 to 50°, an absorptivity of 94.2% and a reflectivity of 86.7% are still achieved for the absorption and reflection modes, respectively, at 10.972 GHz. Under normal incidence, the absorptivity in the absorption mode is higher than 96.8% at 10.972 GHz when the polarization angle ranges from 0 to 45°. Under normal incidence, the absorptivity in the reflection mode is lower than 13.7% at 10.972 GHz when the polarization angle ranges from 0 to 90°. The simulation and measurement results are in good agreement because of the stable bias network. The reflective array and radar absorber are widely used in radar battlefield applications. Therefore, the proposed active metasurface can be used in diverse radar environments. For instance, the metasurface would operate in the absorption mode when a radar signal is received from a foe. Otherwise, the metasurface would be switched to the reflection mode when a friendly radar signal is received. Acknowledgment This research was partly supported by the MSIP (Ministry of Science, ICT & Future Planning), Korea, under the ITRC (Information Technology Research Center) support program (NIPA-2014-H0301-
14-1015) supervised by the NIPA (National ICT Industry Promotion Agency) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0022562).
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