Nonlinear, Active, and Tunable Metasurfaces for Advanced Electromagnetics Applications

SPECIAL SECTION ON TUNABLE DEVICES FOR MODERN COMMUNICATIONS: MATERIALS, INTEGRATION, MODELING, AND APPLICATIONS

Received October 5, 2017, accepted November 13, 2017, date of publication November 23, 2017, date of current version December 22, 2017.

Digital Object Identifier 10.1109/ACCESS.2017.2776291

INVITED PAPER

AOBO LI 1, (Student Member, IEEE), ZHANGJIE LUO2, HIROKI WAKATSUCHI3, (Member, IEEE), SANGHOON KIM4, (Member, IEEE), AND DANIEL F. SIEVENPIPER1, (Fellow, IEEE) 1Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, CA 92093, USA 2Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621999, China 3Department of Electrical and Mechanical Engineering, Nagoya Institute of Technology, Aichi 466-8555, Japan 4JMA Wireless, Liverpool, NY13088, USA Corresponding author: Sanghoon Kim ([email protected]) This work was supported in part by ONR under Grant N00014-11-1-0460 and Grant N00014-15-1-2062, in part by NSF under Grant 1306055, in part by JSPS through Grants-in-Aid for Young Scientist (A) under Grant 15H05527, and in part by MIC through Strategic Information and Communications Research and Development Promotion Programme under Grant 165106001.

ABSTRACT We demonstrate a series of nonlinear, active, and tunable metasurfaces for a variety of electromagnetic applications. The metasurfaces have achieved a range of exotic properties by populating nonlinear or active circuit components on a periodically patterned metallic surface. The circuit components such as diodes, varactors, transistors, and other devices can be controlled manually, actively, or self-adaptively. This allows nonlinear metasurfaces to have active tuning, power-dependent behavior, self-focusing, reconfigurable surface topology, or frequency self-tuning capabilities. The power-dependent metasurfaces can be applied to active RF absorbers that only absorb high-power surface waves to prevent malfunction or damages to sensitive devices. The rectifier-based waveform-dependent metasurface absorber can be specifically designed to absorb either high power pulsed waves or continuous waves. The transistor- based surface wave metasurface absorber provides another degree of freedom in that it can be manually switched to tune the absorber, or it can be tuned using computer controlled feedback. The self-focusing effect has been demonstrated for the first time at RF frequencies to automatically collimate high-power surface waves. The reconfigurable and self-tuning metamaterial surfaces can be implemented to support a broadband reconfigurable antenna system or to adapt to a wide range of incoming frequencies. In this paper, the concepts of nonlinear and active tunable metasurfaces are discussed, including results of full-wave simulation analysis, EM/circuit co-simulation, and experimental results in waveguides, using a near-field scanner, as well as far-field measurements in an anechoic chamber.

INDEX TERMS Electromagnetic metamaterials, metasurface, high impedance surface, nonlinear, active metamaterials, tunable circuits.

I. INTRODUCTION other kinds of interaction. These novel electromagnetic func- Nonlinear or active tunable metasurfaces consist of an artifi- tionalities provided by artificial impedance surfaces can be cial impedance surface that includes nonlinear circuit compo- applicable to surface waveguides [1], [2] by defining high nents. The artificial impedance surfaces are constructed using impedance regions, anisotropic impedance surfaces [3], [4] planar periodic metallic unit cells that are sub-wavelength in by defining an impedance tensor matrix [5], holographic scale. These unit cells are designed to have the particular sur- surfaces [6]–[8] by modulating the surface impedance to face impedance by modeling the two-dimensional patterned radiate leaky waves as low-profile surface antennas [9]–[14] conductive array on a grounded dielectric substrate. The sur- by assigning non-uniform impedances. They can also be face impedance can be designed to control surface waves to used as high impedance surfaces [15] by forming conductive support propagation, to scatter or absorb them, or to provide paths through the vias between the patches and ground plane.

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These remarkable applications are achieved by engineering passive structures, however, their applications are restricted by the inherit limitations such as the tradeoff between bandwidth and surface thickness, as well as the effects of losses. By adding re-configurability and tunability to metasurfaces, these limitations can potentially be mitigated. There are several approaches to achieve the desired tunability, ranging from applying tunable elements [16], [17] on the surface to introducing nonlinearities with active circuit components such as diodes, varactors, transistors, etc. Recent develop- ments in the tunable and reconfigurable metasurfaces with the active electronic circuits have extended to the unique electromagnetic functionalities and properties that are not achievable with conventional passive approaches. In this paper, we present recent advances in nonlinear and active tunable metasurfaces as well as their unique applications that are achievable when nonlinearity is introduced. By using simple diodes, a new type of power dependent absorber [18] is created which responds to the strength of the incident waves. These nonlinear surfaces absorb high power signals to avoid potentially damaging effects, while ignoring low power signals such as from communication systems. By combing rectifiers with metasurfaces, a waveform-dependent absorber [19] is demonstrated, which can convert propagating surface microwaves into static charges that are dissipated in resistors either in parallel with shunt capacitors or in series with inductors, yielding the first waveform-selective absorbing metasurface. When transistors are used [20], the absorbing metasurface can have manual tunability and switching ability. Combined with active feedback, this can yield a sharper response to changes in FIGURE 1. Adapted with permission from [18]. Overview of the power dependent switchable metamaterial absorber. (a) Mechanism of the incident power. By using varactor diodes that provide tun- metasurface with two distinct states defined by a threshold power. Insets ability which is controlled by an electrical bias, the self- figures: the OFF-and ON-states of dispersion diagrams. (b) Geometry of focusing [21] and self-tunable metamaterial surfaces [22] are the unit cell p = 17, t = 3 : 175, g = 2, a = 15, b = 4 : 49, c = 2 : 762, d = 0 : 864, and r = 1 : 0 (all in mm).(c) A photograph of the fabricated introduced, which react to the incoming powers or frequen- sample. (d) An EM simulation model with nine cascaded unit cells in a cies. The self-tuning concept is implemented by using a fre- row with periodic boundary conditions to simulate a 2-D array. quency detecting circuit which controls the surface properties through varactor tuning by using a feedback circuit. Finally, defined as open or short circuit depending on the incoming we will examine a reconfigurable ground plane by deploy- power level. A network of resistors is interconnected between ing RF MOSEFETs as switches to support broadband and the patches to provide absorption. At low powers, the diodes reconfigurable antenna and communication systems while are off, and the metasurface appears as an ordinary frequency maintaining artificial magnetic conductor characteristics. selective surface. The diodes turn on when illuminated by high power, and the metasurface is electrically transformed II. POWER DEPENDENT SWITCHABLE ABSORBERS to a resistor-loaded high impedance surface, which can be Power dependent metamaterial absorbers are able to switch highly lossy near its resonance frequency. The electrical tran- between two states depending on the magnitude of the incom- sition between these two states can be verified by the disper- ing waves. For example, by placing these surfaces on the sion diagram of the insets shown in Fig. 1 (a). By switching outside of a conductive enclosure, they can mitigate potential from OFF to ON states when illuminated with high power threats which would otherwise cause malfunction or dam- waves, the band edge in the dispersion diagram is shifted age to electronic devices when illuminated by high power to the intended operating frequency of 2.4 GHz. A unit microwaves. In contrast, the metasurface does not interact cell geometry and photo of an actual fabricated sample are with low power signals such as for communication systems. shown in Fig. 1 (b) and (c). Fig. 1 (d) shows an EM/circuit The mechanism of the switchable metasurface is illustrated co-simulation of a row of the nine unit cells with periodic in Fig. 1 (a). By attaching paired diodes in opposite direc- boundary conditions. In the circuit simulation, SPICE data tions at the location of vertical conductive vias that connect of the packaged diode model, Avago HSMS-286c, was used, patterned top patches to a ground plane, the diodes can be including all parasitic values.

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behave as slot antennas. In order to demonstrate suppression of this leakage field into the enclosure, we use the method shown in Fig. 2 (e). High power waves at 2.4 GHz are launched from a horn antenna located on the wall of the anechoic chamber. A metal box covered by the metasurface is located 2.5 m away from the source antenna, and a coax- ial probe is placed beneath a small 9mm gap between the metasurfaces. The measured leakage field inside the box is shown in Fig. 2 (f) versus the impinging field magnitude. The switchable metasurface is compared to the conventional magnetic radar absorbing material (MAGRAM) and a bare metal surface. For both of the linear surfaces, the MAGRAM and the bare aluminum surface, the leakage field magnitude is a linear function of the incident field strength. However, the nonlinear metasurface shows a nonlinear response that exhibits a saturation effect, where the leakage field is limited to 4 mW/cm2, regardless of the external source power.

III. WAVEFRORM-DEPENDENT ABSORBING METASURFACES This section introduces metasurfaces that enable us to sense different waves even at the same frequency in response to their waveforms or pulse widths [19], [23], [24]. The waveform dependence is achieved using a periodic unit cell composed of a conducting patch, dielectric substrate, and ground plane together with several circuit elements (see Figs. 3 (a) and (b)). Four Schottky diodes are arranged as a diode bridge, and thus convert the incoming waveform (i.e., sin(2πft) where f and t respectively denote frequency and time) to a rectified waveform (i.e., |sin(2πft)|) that con- FIGURE 2. Simulation and measurement data of absorptivity of the switchable metasurface absorber. (a) Transition of absorptivity under the tains an infinite set of harmonics. However, most of the low (10 mW) and high (32 W) power inputs. Dashed and solid lines energy is at zero frequency. Therefore, the energy of a short represent simulation and measurement results, respectively. (b) Absorption at 2.4 GHz versus the different incoming power levels. pulse is stored in a capacitor to be dissipated with a neighbor- Near-field measurement results of the surface field profiles under the ing resistor, resulting in a strong level of absorption. If a long low (c) and high (d) power waves. (e) A photo of the measurement setup for the leakage field experiment in an anechoic chamber. (f) The leakage pulse or continuous wave (CW) illuminates the metasurface, fields under different circumstances. however, the capacitor is fully charged up so that induced electric charges are not effectively rectified and the absorbing Simulation and measurement results of absorption, performance is limited. This metasurface was numerically near-field scans, and leakage field into a metal enclo- tested inside a waveguide with a slightly modified geometry sure are shown in Fig. 2. Absorption is calculated as (see Figs. 3 (c) and (d)). As plotted by the black curve of A(ω) = 1– T(ω) –R (ω). (Here, A(ω): absorption, T(ω): Fig. 4 (a), the absorptance was gradually decreased from transmission, and R(ω): reflection with frequency depen- 0.8 to 0.3 by increasing the pulse width from 50 ns to 10 µs, dence (ω).) Note that reflection is negligible in all cases. although the frequency remained unchanged at 4.2 GHz. In Fig. 2 (a), absorption plots exhibit different properties The surface can be designed to preferentially absorb long depending on the incoming power level, and the correspond- pulses by replacing a pair of a capacitor and resistor with an ing transition of the absorption versus the input power levels inductor connected to a resistor in series (Fig. 3 (e)). In this is verified at the desired frequency at 2.4 GHz. This transition case, electric charges are not allowed to enter the diode bridge can also be seen in the surface field measurement as shown during an initial time period due to the presence of the electro- in Fig. 2 (c) and (d). At low power, the launched surface motive force of the inductor. This force, however, is gradually waves at the edge of the metasurface can propagate across mitigated by the zero-frequency component so that more the surface away from the source with small attenuation as electric charges reach the resistor. Such a simulation result is shown in Fig. 2 (c). At high power, surface waves are strongly shown by the black curve of Fig. 4 (b). This demonstrates that attenuated as shown in Fig. 2 (d). Illuminating a conductive the absorptance was improved from 0.3 to 0.8 by increasing enclosure with high power signals results in surface waves the pulse width. which travel over the outside of the enclosure, and may leak One of the important points of these structures is that inside through small gaps or openings, which effectively the design of waveform-dependent absorption is readily

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FIGURE 4. Simulation results of (a) capacitor-based and (b) inductor-based waveform-dependent metasurfaces. In this simulation, frequency was fixed at 4.2 GHz, while the pulse width was varied from 50 ns to 10 µs. The black curve of the capacitor-based metasurface set capacitance and resistance to 1 nF and 10 k, respectively, while that of the inductor-based metasurface used 100 µH inductors and 5.5 resistors. For the dashed curves (or for the dotted curves), the capacitance and inductance were increased (or decreased) by a factor of 10 so that the black curves were entirely shifted to a large (or small) pulse width.

FIGURE 5. Transistor-loaded metasurface in the ON and OFF states for surface wave absorption. (a) Transistors loaded at the via, Topology A. FIGURE 3. Adapted with permission from [19] (a, b) Waveform-dependent (b) Transistors loaded at the gap, Topology B. metasurface containing capacitor-based circuits. The energy of a short pulse is stored at capacitors and dissipated with resistors before a next pulse comes in. (c) Geometry modified for simulation. The design with a finite bandwidth [25], angular dependence [26], appli- parameters were given as cx = 7.6, cy = 1.7, dx = 1.3, dy = 0.5, g = 1.0, cation to wireless communications [24], field visualization m = 2.4, p = 18.0, rx = 1.0, and ry = 2.0 (all in mm). The diodes and substrate were respectively modeled as Schottky diodes (Avago, method in simulation [27], and control of other scattering HSMS-2863/2864) and Rogers3003 (1.5 mm thick). (d) The metasurface parameters [28]. was deployed at the bottom of a waveguide (22 mm tall and 18 mm wide). (e) Waveform-dependent metasurface containing inductor-based circuits. IV. TRANSISTOR BASED METASURFACE ABSORBER We have also studied the first transistor-based tunable and modified by changing the circuit parameters. This is because switchable surface wave absorbing metasurface. The surface the performance is determined by the time constants of the can be manually controlled to ON or OFF states, or to any circuit elements used, i.e., capacitor, inductor, and resistor. absorption state as needed by simply controlling the bias to For example, increasing (or decreasing) the capacitance and the gates of the transistors. It provides surface wave sup- inductance leads to entirely shifting the absorptance curve pression to protect electronic devices from leakage of high to a large (or small) pulse width, as represented by the power signals or surface currents through discontinuities in dashed (or dotted) curves of Figs. 4 (a) and (b). In addi- the enclosing structure. Two topologies are introduced in this tion, combining both of the capacitor-based and inductor- work and their working principles are indicated in Fig. 5, based circuits leads to more complex waveform-dependent which are both designed based on periodic patches with a via response, such as absorption for an intermediate pulse width. in the center. For topology A, the drain and source terminals This is demonstrated in our previous studies that also show of a pair of transistors are placed in shunt with the absorbing experimental validation [19], [24], performance for a pulse resistors at the gaps of the unit cells. When the gates are

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negatively biased, the drain-to-source channel appears as an open circuit, and surface waves at the desired frequency will be absorbed by the resistors through each cycle resonating in the cavity. However, when the transistors are positively biased, the drain-to-source channel will look like a short circuit, thus shorting the gap resistors and transforming the surface from periodic patches into a continuous conductive sheet that can support TM wave propagation with minimal absorption, shown in Fig. 5 (a). Topology B adopts the same metallic electromagnetic band gap structures but with the drain-to-source channel of the transistors placed at the via of the patches. When the gates of the transistors are negatively biased, then the vias are isolated from the patches which make the vias invisible to the surface, thus supporting surface waves at the desired frequency. On the other hand, when the gates are positively biased, the vias are connected to the surface and changed to the absorbing state, demonstrated in Fig. 5 (b). Additionally, for both of the topologies, when transistors are biased at different DC voltage levels between ON and OFF states, the metasurfaces can be manually controlled to give different absorption levels, since the resistance of drain-to-source channel is changed to yield different absorption performance. The two topologies are simulated using the EM/Circuit co- simulation technique, and they are measured in waveguide WR-430. The results are shown in Fig. 6. The switching property of the two topologies is clearly apparent at low input power (10 mW) in Fig. 6 (a) and (b). At higher input power levels, the absorption rate of both topologies drops due to saturation of the drain-to-source channel of the transistors, which could be resolved by replacing them with higher power transistors. Compared to the diode-based absorber, the use of ﬁeld-effect transistors (FET), gives an additional degree of freedom because absorption is controlled through the gate voltage, rather than the diode turn-on voltage, making this surface switchable and tunable at both low and high microwave power levels. Furthermore, the absorption rate can be easily tuned by adjusting the bias voltage of the transistors shown in Fig. 6 (c). A larger panel of the topology B is fabricated with a slit in the middle, mimicking a discontinuity in the surface. It is measured in the anechoic chamber to study its performance in a realistic scenario, shown in Fig. 7. In the ON and OFF states, FIGURE 6. Waveguide measurement results (a) Absorption rate for ON the radiation pattern of the surface is measured and plotted and OFF states of topology A with different levels of input power. in Fig. 8 (a) at the center frequency 2.15GHz. Large attenu- (b) Absorption rate for ON and OFF states of topology B with different ation is observed between the ON and OFF states, especially levels of input power. (c) Measured tunable absorption rate of topology B with different gate biasing. at grazing angles, demonstrating about 18dB difference. The tunability of the surface is also demonstrated in Fig. 8 (b), showing that when the DC bias is changed from −1 V to 0 V, of this FET-based surface is that one can program the gate different gain levels are acheived. Compared to the diode- biasing voltage in response to different levels of incoming based switchable metasurface, the FET-based metasurface power. In other words, a particular power density level can be can be maintained in a particular tuning state with no static preset as a threshold to turn the surface ON, OFF, or to any power draw, since the FETs are voltage-controlled. On the state in between, such as to provide a sharper response. This other hand, the FET’s drain-to-source channel is symmetric, setup is shown in Fig. 9 (a), where the leakage power through which means the active absorbers we are proposing can be the slit is detected with a power meter, and the information tuned without affecting their linearity. One other advantage is fed to a controlling computer in real time. When the level

VOLUME 5, 2017 27443 A. Li et al.: Nonlinear, Active, and Tunable Metasurfaces for Advanced Electromagnetics Applications

FIGURE 8. (a) Relative amplitude for the input power leaking through the slit in the ON and OFF states in the normal incidence mode. (b) Tunable absorption rate with different DC bias for grazing incidence.

FIGURE 7. (a) Measurement setup for surface wave absorption in the where ω is angular frequency, C is sheet capacitance, and L is anechoic chamber. (b) Position of the surface for normal incidence (left) sheet inductance. Based on this, varactor diodes are employed and grazing incidence (right), with larger view of the surface. to adjust the capacitance. They are controlled by reverse DC bias voltages, providing the possibility of power-controlled is greater than the preset threshold value, the DC source is characteristics. controlled by the computer instantly to turn the transistors on, The array configuration is illustrated in Fig. 10, which can switching the surface to the absorbing state. From Fig. 9 (b), be viewed as modified mushroom structures embedded with 18 dB of attenuation is achieved when input power exceeds varactors on the upper substrate and the EM power sensing the threshold, which yields a very sharp response. circuits on the lower substrate. For every cell, the five vertical V. NONLINEAR POWER-DEPENDENT IMPEDANCE metal vias are the key elements that connect the varactors and SURFACE AND APPLICATIONS the circuits, and support EM currents at the same time. Each A new nonlinear impedance surface is proposed that presents unit cell responds independently to its local power level. The surface waves with a tunable surface impedance depending sensing circuits transform the EM power level information on power level of the incident waves [29]. The control of the into corresponding DC voltage signals, and feed them back surface impedance can offer more flexibility in manipulating to the varactors. EM waves, thus giving rise to a new range of applications. The power-dependent surface impedance property is stud- According to the effective medium model of the high- ied experimentally. The configuration and a photograph of impedance surface, the surface impedance of an array of sub- the measurement setup are shown in Fig. 11. The VNA is wavelength mushroom cells is partly determined by its sheet set to be working in ‘‘external test set mode’’ [29], with its capacitance, as indicated effectively by the impedance of a four ports arranged as a source port (S), reference port (R), parallel resonant circuit test ports for reflection (A), and transmission (B), respectively. The incident signal is measured in the R port after jωL the power amplifier, so the mismatch or drift of the ampli- Z = (1) 1 − ω2LC fier is canceled. The prototype is placed at the bottom of a

27444 VOLUME 5, 2017 A. Li et al.: Nonlinear, Active, and Tunable Metasurfaces for Advanced Electromagnetics Applications

FIGURE 11. The power-dependent measurement. (a) Setup configuration. (b) Photograph.

FIGURE 9. (a) Experimental setup for sharp response absorbing ability measurement. (b) Sharp response of absorption with different power From the phase of S21, surface impedance of the prototype is level thresholds. achieved from r φ Zs = Z 1 − ( )2 (3) 0 kl s f 2 k = k 1 − c (4) 0 f

where Z0 is the impedance of free space, ϕ is the phase, l is the length of the array, k0 is the wave number in free space, and fc is the cutoff frequency of the waveguide. The results are shown in Fig. 12. As is clearly observed, at frequencies from about 1.64 to 1.67 GHz, the surface impedance is tuned by the input power as it is swept from 16 to 27 dBm. Typically, the largest impedance range was j385 to j710 at 1.66 GHz. One of the intriguing applications of the nonlinear impedance surface is the self-focusing effect. The effect

FIGURE 10. Power-dependent impedance surface, not to scale. It consists is a nonlinear phenomenon typically studied in the optical of a lattice of modified slotted mushroom-like cells embedded with community, which relies on media with beam-magnitude- varactor diodes and sensing circuits. dependent refractive index. When a high-power beam is injected into such a medium, such as a nonlinear crystal, it enhances the local index and automatically produces a high- WR430 waveguide. Voltage transmission, or S , of the 21 index channel, which, in turn, guides the beam with its energy waveguide is measured by conﬁned within the region. In contrast, low-magnitude beams B have little impact on the index, and thereby tend to spread S = (2) 21 R normally in the same medium.

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vertically polarized waves of the waveguide. The simulation starts with a homogeneous low-impedance surface, and the impedances of the cells are then updated in accordance with the resulting E-field magnitude distribution. With the new impedance surface, the next time-step simulation is carried out. The process repeats five times until a stable E-field distribution is achieved. Fig. 14 shows the E-field distributions at different time steps, indicating a gradually focused beam profile, which means a high-impedance channel is formed. In essence, the high-impedance waveguide is automatically induced by the beam interacting with the nonlinear unit cells.

FIGURE 12. Measured power-dependent surface impedance performance of the prototype sample.

The effect has drawn intensive investigations for decades, delivering many optical applications such as self-trapping waveguiding, beam-splitting, optical interconnects, and so on [30]. It is not until recently that the development of the nonlinear impedance surface allows the nonlinear self-focusing behavior to occur at microwave frequencies for the ﬁrst time. Because the surface impedance is directly related to the effective refractive index for surface waves, it is the RF analog of FIGURE 14. Adapted with permission from Ref [21]. E-field distributions the optical self-focusing that shares the same principle. in the discrete-time simulations, representing the forming process of the self-focusing effect. Result of each step: (a)first, (b) second, (c) third, (d) fourth, and (e) fifth.

The effect has also been characterized using the measurement configuration shown in Fig. 15. A large prototype panel including almost 380 cells was fabricated. Amplified RF signals generated by the VNA were radiated by a standard WR430 waveguide adaptor placed next to the left side of the board. The E-fields above the surface were sensed by the vertical probe and then fed back into the VNA through a 10-dB protecting attenuator. Normalized E-field magnitude distributions on the board with varying emitted power levels are presented in Fig. 16. If the power sensors are turned off, the prototype board degenerates into a homogeneous linear surface with constant FIGURE 13. Adapted with permission from [21]. Simulation model for the self-focusing effect of surface waves. low impedance. Therefore, the scattered E-field distribution plotted in Fig. 16 (a) was obtained regardless of the power In an effort to predict the beam forming process of levels. When the circuits were turned on, with power less self-focusing on the nonlinear impedance surface, a series than 31 dBm, the natural diffraction of the surface wave was of discrete-time simulations are performed with Ansys measured, as shown in Fig. 16 (b). The result is nearly the HFSS 15.0. The model is shown in Fig.13, which represents same as in Fig. 14 (a), as expected. As the power reached a nonlinear surface attached with a rectangular waveguide 34 dBm, the wave was observed to be guided within a straight on the left side. The surface consists of hundreds of cells, channel with very little leakage, as shown in Fig. 16 (c). each of which is set to be an ‘‘impedance boundary’’ with According to the simulations, it can be claimed that the a reactance depending on a local electric field (E-field). channel is a high-impedance area that is automatically main- When the wave port is excited, transverse-magnetic-mode tained by the interaction between the beam and the nonlinear (TM-mode) surface waves are coupled on the surface by the surface. The E-field magnitude at the edge of the beam profile

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observed, in which the energy converges to a very narrow region in which it is dissipated in a round hot spot, as shown in Fig.16 (d). The extreme phenomenon can be attributed to the loss of the prototype that is enhanced with the power. In addition, from the viewpoint of a band gap structure, the band moves downwards to the frequency of interest as the power is increased, and at that point the wave can no longer propagate. This is not predicted in the simulations because the bandgap effect is not taken into account in HFSS using the impedance boundary condition. The self-focusing effect represents the balance between the nonlinearity of a metasurface and natural diffraction of a beam. Besides the injected power level, loss of the metasurface can also affect this balance. The power-dependent nonlinear surface presented above is a lossy medium, in which the dissipating effect increases with increasing power. As a consequence, the propagation of the beam is observed to be cut off in Fig. 16 (d). In [31], the key role of the varactors in the electrically controlled dissipation of the metasurface is analyzed in detail.

FIGURE 15. Measurement setup of the self-focusing effect. (a) Configuration (adapted with permission from [21]). (b) Photograph.

FIGURE 17. Adapted with permission from [31]. Simulated dissipating behaviors of the lattice of the mushroom-like cells. (a) Simulation model. (b) and (c) are results with the resistance and capacitance connected in series across the slot: (b) resistance is zero; (b) resistance is 1.2. (d) Results with varactor SPICE models connected across the slot. FIGURE 16. Adapted with permission from [21]. Measured E-field distributions. (a) Diffracting beam when the nonlinearity of the surface is turned off. (b)–(d) Nonlinear propagations of the surface waves with The tunable dissipating behavior of the varactors is varying power levels: (b) normal scattering when incident power is less characterized numerically, as plotted in Fig. 17 (a). than 31 dBm, (c) self-focusing propagation when the power reaches 34 dBm, and (d) extreme focusing effect when the power is In co-simulations, a lattice of the unit cells is built at the 37dBm or larger. bottom of a TEM waveguide, and each of the embedded varactors is approximately modeled as a resistor and measures 9.5 dB higher than the outside, which agrees capacitor in series. The dissipation of surface waves by well with the simulated result. When the power is the lattice as a function of the capacitance are compared increased beyond 37dBm, a sharply focused proﬁle is in Fig. 17 (b) and (c). It can be seen that when the resistance

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By delicately adjusting the parameters of the circuits, results shown in Fig.18 (b) are obtained, in which the surface exhibits a sharp dissipation response to the incoming power level. The dissipating behavior and tuning role of the varactors in the impedance surface can be used for microwave suppression applications. For example, if their bias polarity is reversed, the surface impedance would decrease with increasing power, leading to devices such as saturable absorbers. Furthermore, a high-power surface wave beam propagating on such a surface would induce a low-index proﬁle surrounded by high-index region, causing its energy to be more strongly diffracted. This could provide an attractive alternative to mitigate the destructive effects of high power microwaves.

FIGURE 19. An illustrator of the EM/circuit co-simulation scheme. The sensing part provides rectified voltages to feed the varactors that give capacitance to tune the metasurface.

VI. SELF-TUNING ABSORBERS

FIGURE 18. Measured dissipating behaviors of the prototype. (a) With DC One of the fundamental limitations of sub-wavelength struc- tuning signals. (b) In accordance with the incident power (adapted with tures is their narrow bandwidth due to their electromagnetic permission from [31]). resonance properties. Additionally, when the metasurface is electrically thin, its absorbing performance and bandwidth are inversely related to the thickness [33], [34]. Thus, it is is zero, the absorption is small except for several ripples, not possible to make a broadband absorber with an arbitrarily which can be explained by the dissipation of the lossy sub- thin substrate. There are various approaches to achieve broad- strate at the resonances of the lattice. In contrast, a small resis- band absorbing properties such as multilayer, multi-resonant tance causes significant dissipation within a frequency band structures, but none have exceeded the fundamental thickness that moves with the changing capacitance. Fig. 17 (d) shows limitation. Here, we suggest using the novel approach of the result when SPICE models of the varactors are taken self-tunability [22], to create the broadband metasurfaces into account. With various DC bias voltages, the curves look absorber that responds to the incoming frequency and tunes almost identical with Fig. 17 (c). If the series resistance of its resonance frequency to match that of the incident wave. the varactors is set to be zero, a similar result as in Fig. 17 (b) To achieve this performance, the metasurface is divided into is obtained, indicating the significant effect of the resistance sensing and responding parts. The sensing part is a cir- in the tunable dissipation characteristics of the lattice. The cuit which detects the incoming frequency and provides a dissipating effect of the array on plane waves is analyzed frequency-dependent DC bias. The responding part is con- in [32], where a similar tunable performance is also found. structed of the high impedance surface with varactor diodes The dissipating behavior of a prototype tuned by DC sig- populated on the surface. The rectified DC bias is fed into the nals is measured [31], and the results are shown in Fig. 18 (a). network of varactors using a feedback circuit, and the meta- At the frequencies of interest, they match well with the sim- surface automatically tunes its resonance frequency based on ulated performance. Based on this, power-dependent proper- the provided DC bias, which depends on the input frequency. ties are further examined with EM power sensors included. The EM/circuit co-simulation is shown in Fig. 19. We use a

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periodic metasurface model with varactors, as well as a set of sensing circuits including the RF-DC rectiﬁer, op-amps, and limiter to provide the frequency-dependent control voltages. The simulation result of the self-tuning absorber is shown in Fig. 20, demonstrating the broad tunable bandwidth that is much wider than the instantaneous bandwidth at each individual tuning state.

FIGURE 20. Absorptivity simulation data of the self-tuning metasurface. The thick red line represents the self-tuning absorption. The thin solid lines represent instantaneous tuning results by assigning each DC bias.

VII. RECONFIGURABLE GROUND PLANE Modern communication systems are rapidly emerging for the requirements of high speed access, large data capacity, and diverse usage formats. One key technology for broadband communication systems is reconfigurable and broadband antennas. One of the key challenges for their development is that the reconfigurable elements require an adjustment of the FIGURE 21. Overview of the diagram and mechanism of the local reflection properties from the ground plane to operate reconfigurable impedance ground plane. (a) An illustrator of the over a broad frequency range, because the antennas occupy a reconfigurable metasurface with deployed RF switches. (b) Examples of fixed distance from the ground plane. A tunable ground plane the configured surface topology to 3 × 3 and 1 × 1. (c) The reflection phase curves with multi-octave tunabilities. would allow the reflection phase to be controlled to match the requirements of the antenna at any given tuning state. The proposed tunable ground plane [35] is constructed using a high impedance surface with RF MOSFETs populated at the gaps between the metal patches as shown in Fig. 21 (a). The FETs are used as RF switches to reconfigure the surface topology by connecting or disconnecting nearby periodic patches. When the switch is turned on, the internal resistance between the drain and source of the FETs will dominate. Similarly, the capacitance between patches and parasitics of the FETs will dominate when the switch is turned off. For example, 3×3 and 1×1 configured surfaces are shown in Fig. 21 (b). To configure the surface for lower FIGURE 22. Simulation results with ideal RF switches of the frequency bands, inner switches are closed to connect adja- reconfigurable impedance ground plane with the reflection phases of cent patches, and outer ones are opened to disconnect that N × N surface topology. group of patches from their neighbors. Similarly, to configure for a higher frequency band, all the switches are opened, and scaling the lateral dimensions by opening and closing the reconfigured patches are at their minimum size. In other switches. By switching between these two different sur- words, the impedance ground is reconfigured by simply face topologies, the ground plane can change its resonance

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frequency, and the reflection phase can be adjusted to match described a type of reconfigurable ground plane to support the requirements of a tunable antenna. Fig. 21 (c) shows the broadband and reconfigurable antenna systems. These tech- reflection phases of the lower and higher tuning states as a nologies can be expected to be used to dramatically increase function of the frequency for two different surface topologies. the capabilities of metamaterials for applications such as The resonance frequency is defined as the point where the active absorbers, surface waveguides, or low profile antennas. reflection phase crosses through 0◦, and the bandwidth is Unlike classical and conventional electromagnetic sur- defined the frequency range over which the reflection phase faces, the nonlinear, active and tunable metasurfaces pro- lies between −π/2 and π/2. Transforming the surface is posed in this paper provide new ways of manipulating simply a matter of reconfiguring the ground plane between the interaction between electromagnetic waves and mate- these two states for the lower and higher bands. In addition, rials, enabling unique properties like broad bandwidth, any number of states are achievable by manipulating the power or waveform dependency and reconfigurability. Based configuration of the switches, so that the surface can have on the techniques introduced here, more flexible metasurface multi-octave tunability. designs for applications in absorbers, conformal antennas, The proposed impedance ground plane is numerically ana- leaky wave structures and control of surface waves may be lyzed using a co-simulation method including the transistor achieved in the future. switches to reconfigure the surface topology. For the simulation, N × N unit cells are constructed with periodic boundary ACKNOWLEDGMENT conditions, and lumped ports are assigned at the position of H. Wakatsuchi, Z. Luo, and S. Kim were with the Depart- the gaps between the patterned patches. The geometry of the ment of Electrical and Computer Engineering, University unit cell has a period of 3.2 mm, a gap distance of 0.81 mm of California at San Diego, La Jolla, CA, USA. (Aobo Li, on a dielectric substrate, RO5880 (εr = 2.2) with a thickness Zhangjie Luo, Hiroki Wakatsuchi, and Sanghoon Kim con- of 2.87 mm. N-channel RF MOSFETs are controlled by the tributed equally to this work.) biasing network to provide open and short channels between the patches. The metallic lattice of patches are arranged into REFERENCES × × groups of 2 2 through 11 11 or simply as individual dis- [1] R. Quarfoth and D. Sievenpiper, ‘‘Artificial tensor impedance sur- connected cells, for the 1 × 1 case. Reflection phases, 6 S11, face waveguides,’’ IEEE Trans. Antennas Propag., vol. 61, no. 7, are compared in each different topology. In the 1 × 1 case, pp. 3597–3606, Jul. 2013. all of the patches are disconnected, so the structure behaves [2] R. G. Quarfoth and D. F. Sievenpiper, ‘‘Nonscattering waveguides based on tensor impedance surfaces,’’ IEEE Trans. Antennas Propag., vol. 63, as an ordinary high impedance surface with a resonant fre- no. 4, pp. 1746–1755, Apr. 2015. quency of 11.86 GHz defined by the single unit cell geometry. [3] J. Lee and D. F. Sievenpiper, ‘‘Patterning technique for generating arbitrary By increasing the number of connected channels to form anisotropic impedance surfaces,’’ IEEE Trans. Antennas Propag., vol. 64, no. 11, pp. 4725–4732, Nov. 2016. N × N cells, the capacitance between patches increases with [4] R. Quarfoth and D. Sievenpiper, ‘‘Broadband unit-cell design for highly the expanded effective patch size, so the resonance frequen- anisotropic impedance surfaces,’’ IEEE Trans. Antennas Propag., vol. 62, cies is reduced as shown in Fig. 22. Each reflection phase of no. 8, pp. 4143–4152, Aug. 2014. [5] B. H. Fong, J. S. Colburn, J. J. Ottusch, J. L. Visher, and D. F. Sievenpiper, the configured surfaces slightly overlaps with the neighboring ‘‘Scalar and tensor holographic artificial impedance surfaces,’’ curve to provide a continuous range from 2 to 17 GHz where IEEE Trans. Antennas Propag., vol. 58, no. 10, pp. 3212–3221, the structure maintains high surface impedance, to facilitate Oct. 2010. [6] L. Matekovits, ‘‘Analytically expressed dispersion diagram of unit cells efficient conformal antennas. for a novel type of holographic surface,’’ IEEE Antennas Wireless Propag. Lett., vol. 9, pp. 1251–1254, 2010. VIII. CONCLUSIONS AND FUTURE WORKS [7] M. Li, S.-Q. Xiao, and D. F. Sievenpiper, ‘‘Polarization-insensitive holo- This paper provides a review of recent advances in nonlinear graphic surfaces with broadside radiation,’’ IEEE Trans. Antennas Propag., vol. 64, no. 12, pp. 5272–5280, Dec. 2016. and active metamaterial surfaces as well as a wide range [8] S. Pandi, C. A. Balanis, and C. R. Birtcher, ‘‘Design of scalar impedance of related applications. In each case the surface impedance holographic metasurfaces for antenna beam formation with desired polar- is tuned by active electronics such as diodes, transistors, ization,’’ IEEE Trans. Antennas Propag., vol. 63, no. 7, pp. 3016–3024, Jul. 2015. varactors, etc. The nonlinear tuning mechanism and applica- [9] A. T. Pereda et al., ‘‘Dual circularly polarized broadside beam metasurface tions are classified into groups based on their functionalities antenna,’’ IEEE Trans. Antennas Propag., vol. 64, no. 7, pp. 2944–2953, and capabilities. By applying diodes to the surface, power Jul. 2016. [10] G. Minatti, F. Caminita, M. Casaletti, and S. Maci, ‘‘Spiral leaky-wave dependent surface wave absorbers can be built. Using a full- antennas based on modulated surface impedance,’’ IEEE Trans. Antennas wave rectifier, a new type of waveform-dependent surface Propag., vol. 59, no. 12, pp. 4436–4444, Dec. 2011. wave absorber is achieved. Transistor based absorbing meta- [11] G. Minatti, S. Maci, P. D. Vita, A. Freni, and M. Sabbadini, ‘‘A circularly- surfaces provide manual gate control, which can make the polarized isoflux antenna based on anisotropic metasurface,’’ IEEE Trans. Antennas Propag., vol. 60, no. 11, pp. 4998–5009, Nov. 2012. surface programmable, or allow the use of feedback to cre- [12] G. Minatti et al., ‘‘Modulated metasurface antennas for space: Synthesis, ate a sharp absorption response. We have also demonstrated analysis and realizations,’’ IEEE Trans. Antennas Propag., vol. 63, no. 4, active self-tunable metasurfaces for self-focusing of surface pp. 1288–1300, Apr. 2015. [13] M. Faenzi et al., ‘‘Realization and measurement of broadside beam mod- waves and self-tuning for expanding the frequency bandwidth ulated metasurface antennas,’’ IEEE Antennas Wireless Propag. Lett., beyond the limits of passive absorbers. In addition, we have vol. 15, pp. 610–613, 2016.

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[14] A. Vallecchi, J. R. De Luis, F. Capolino, and F. De Flaviis, ‘‘Low AOBO LI (S’16) received the B.S. degree from profile fully planar folded dipole antenna on a high impedance sur- Shanghai Jiao Tong University, Shanghai, China, face,’’ IEEE Trans. Antennas Propag., vol. 60, no. 1, pp. 51–62, in 2010, and the dual M.S. degree in electrical Jan. 2012. engineering and telecommunications from Shang- [15] D. Sievenpiper, L. Zhang, R. F. J. Broas, N. G. Alexopolous, and hai Jiao Tong University and the Georgia Insti- E. Yablonovitch, ‘‘High-impedance electromagnetic surfaces with a for- tute of Technology, respectively, in 2013. From bidden frequency band,’’ IEEE Trans. Microw. Theory Techn., vol. 47, 2013 to 2014, he was an Application Engineer with no. 11, pp. 2059–2074, Nov. 1999. National Instruments. He is currently pursuing the [16] D. F. Sievenpiper, J. H. Schaffner, H. J. Song, R. Y. Loo, and G. Tangonan, Ph.D. degree with the University of California ‘‘Two-dimensional beam steering using an electrically tunable impedance surface,’’ IEEE Trans. Antennas Propag., vol. 51, no. 10, pp. 2713–2722, at San Diego, La Jolla, CA, USA. His research Oct. 2003. focuses on advanced electromagnetics applications, including active meta- [17] D. Shrekenhamer, W.-C. Chen, and W. J. Padilla, ‘‘Liquid crystal tunable surface microwave absorbers, metasurface microwave sources, wireless metamaterial absorber,’’ Phys. Rev. Lett., vol. 110, no. 17, p. 177403, power transmission, and phased arrays systems. 2013. [18] S. Kim, H. Wakatsuchi, J. J. Rushton, and D. F. Sievenpiper, ‘‘Switchable nonlinear metasurfaces for absorbing high power surface waves,’’ Appl. Phys. Lett., vol. 108, no. 4, p. 041903, 2016. [19] H. Wakatsuchi, S. Kim, J. J. Rushton, and D. F. Sievenpiper, ‘‘Waveform-dependent absorbing metasurfaces,’’ Phys. Rev. Lett., vol. 111, no. 24, p. 245501, 2013. [20] A. Li, S. Kim, Y. Luo, Y. Li, J. Long, and D. F. Sievenpiper, ‘‘High-power transistor-based tunable and switchable metasurface absorber,’’ IEEE Trans. Microw. Theory Techn., vol. 65, no. 8, pp. 2810–2818, Aug. 2017. ZHANGJIE LUO received the B.S. degree in elec- [21] Z. Luo, X. Chen, J. Long, R. Quarfoth, and D. Sievenpiper, ‘‘Self-focusing tronic and information engineering and the Ph.D. of electromagnetic surface waves on a nonlinear impedance surface,’’ Appl. degree in radio physics from Sichuan University, Phys. Lett., vol. 106, no. 21, p. 211102, 2015. Chengdu, China, in 2008 and 2015, respectively. [22] D. Sievenpiper, S. Kim, J. Long, and J. Lee, ‘‘Advances in nonlinear, active, From 2012 to 2014, he was with the Applied and anisotropic artificial impedance surfaces,’’ in Proc. 46th Eur. Microw. Electromagnetics Group, University of California Conf. (EuMC), London, U.K., Oct. 2016, pp. 799–802. at San Diego, La Jolla, CA, USA, as a Visit- [23] G. V. Eleftheriades, ‘‘Electronics: Protecting the weak from the strong,’’ ing Ph.D. Student. He is currently a Research Nature, vol. 505, no. 7484, pp. 490–491, 2014. Staff with the Institute of Electronic Engineer- [24] H. Wakatsuchi, D. Anzai, J. J. Rushton, F. Gao, S. Kim, and ing, China Academy of Engineering Physics, D. F. Sievenpiper, ‘‘Waveform selectivity at the same frequency,’’ Sci. Mianyang, China. His research interests include microwave metasurfaces Rep., vol. 5, Apr. 2015, Art. no. 9639. and millimeter-wave technologies. [25] H. Wakatsuchi et al., ‘‘Experimental demonstration of nonlinear waveform-dependent metasurface absorber with pulsed signals,’’ Electron. Lett., vol. 49, no. 24, pp. 1530–1531, 2013. [26] H. Wakatsuchi, F. Gao, S. Yagitani, and D. F. Sievenpiper, ‘‘Responses of waveform-selective absorbing metasurfaces to oblique waves at the same frequency,’’ Sci. Rep., vol. 6, Aug. 2016, Art. no. 31371. [27] H. Wakatsuchi, D. Anzai, and C. Smartt, ‘‘Visualization of field distributions of waveform-selective metasurface,’’ IEEE Antennas Wireless Propag. Lett., vol. 15, pp. 690–693, 2016. [28] H. Wakatsuchi and D. F. Sievenpiper, ‘‘Waveform-selective scattering HIROKI WAKATSUCHI (M’14) received the control through circuit-based metasurfaces at the same frequency,’’ pre- B.Eng. and M.Eng. degrees from Aoyama Gakuin sented at the IEEE Antennas Propag. Symp., San Diego, CA, USA, University, Kanagawa, Japan, in 2006 and 2008, Jul. 2017. respectively, and the Ph.D. degree from the [29] Z. Luo, X. Chen, J. Long, R. Quarfoth, and D. Sievenpiper, ‘‘Nonlinear power-dependent impedance surface,’’ IEEE Trans. Antennas Propag., University of Nottingham, Nottingham, U.K., vol. 63, no. 4, pp. 1736–1745, Apr. 2015. in 2011, all in electrical and electronic engineer- [30] Z. Chen, M. Segev, and D. N. Christodoulides, ‘‘Optical spatial solitons: ing. He was with Electromagnetic Compatibility Historical overview and recent advances,’’ Rep. Prog. Phys., vol. 75, no. 8, (EMC) Group, Applied Electromagnetic Research p. 086401, 2012. Center, National Institute of Information and Com- [31] Z. Luo, J. Long, X. Chen, and D. Sievenpiper, ‘‘Electrically munications Technology, Japan, from 2005 to tunable metasurface absorber based on dissipating behavior of 2008. In 2008, he joined the Department of Electrical and Electronic Engi- embedded varactors,’’ Appl. Phys. Lett., vol. 109, no. 7, p. 071107, neering, George Green Institute for Electromagnetics Research, Univer- 2016. sity of Nottingham, U.K. From 2011 to 2013, he was with the Applied [32] Z. Luo, L. Zhao, C. Xue, and D. Sievenpiper, ‘‘An electrically tun- Electromagnetics Research Group, Department of Electrical and Computer able absorbing metasurface for surface waves and plane waves,’’ in Engineering, University of California at San Diego, as a Research Scholar. Proc. Asia–Pacific Microw. Conf. (APMC), New Delhi, India, Dec. 2016, Since 2013, he has been with the Nagoya Institute of Technology, Japan, pp. 1–4. as an Assistant Professor, and where he is currently an Associate Professor. [33] K. N. Rozanov, ‘‘Ultimate thickness to bandwidth ratio of radar He received many distinctions, including the York EMC Services Ltd., Best absorbers,’’ IEEE Trans. Antennas Propag., vol. 48, no. 8, pp. 1230–1234, Paper Prize at the Festival of Radio Science URSI, the second prize of the Aug. 2000. [34] J. L. Wallace, ‘‘Broadband magnetic microwave absorbers: Fundamen- AP-RASC’10 Student Paper Competition at AP-RASC’10 2010 Asia-Pacific tal limitations,’’ IEEE Trans. Magn., vol. 29, no. 6, pp. 4209–4214, Radio Science Conference in 2010, the Young Researcher Award from Nov. 1993. the Funai Foundation for Information Technology, the Telecommunications [35] S. Kim and D. F. Sievenpiper, ‘‘Reconfigurable impedance ground plane Technology Award from the Telecommunications Advancement Foundation, for broadband antenna systems,’’ presented at the IEEE Antennas Propag. and the Ando Scholar Award. His current research interests include metama- Symp., San Diego, CA, USA, Jul. 2017. terial research and its applications, especially with focus on absorbers.

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SANGHOON KIM (S’12–M’17) received the DANIEL F. SIEVENPIPER (M’94–SM’04–F’09) M.S. and B.S. degrees in physics from the Konkuk received the B.S. and Ph.D. degrees in electrical University, Seoul, South Korea, in 2008 and 2005, engineering from the University of California at respectively, and the Ph.D. degree from the Uni- Los Angeles, Los Angeles, USA, in 1994 and versity of California at San Diego in 2017. For 1999, respectively. Ph.D., his research area was focused to invent He was the Director of the Applied Electromag- novel nonlinear active metamaterial surfaces to netics Laboratory, HRL Laboratories, Malibu, CA, handle high power surface waves. He is currently where his research included artiﬁcial impedance an antenna engineer with JMA Wireless, Syracuse, surfaces, conformal antennas, tunable and wear- NY, USA. His research focuses to build phased able antennas, and beam steering methods. He is array antennas for base stations. currently a Professor with the University of California at San Diego, where his research focuses on antennas and electromagnetic structures. He has over 70 issued patents and over 120 technical publications. In 2008, he received the URSI Issac Koga Gold Medal. Since 2010, he has served as an Associate Editor for the IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS.

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