Dynamic Control of Ultrathin Electromagnetic Absorber Using Active High Impedance Metasurfaces

In recent years, active metasurfaces have induced more interests, which provides great freedom of wave manipulation and gives rise to many novel phenomena. High impedance metasurfaces are a kind of artificial structures characterized by the in-phase reflection at the resonant frequency. It works as a magnetic mirror and can be applied in subwavelength cavity, low profile reflector antenna, etc. When introduce in tunable components, it possesses versatile functionalities and broader application. In this paper, we proposed an active high impedance metasurface which exhibits highly efficient absorption to the electromagnetic incidence. As an array of diodes are loaded between the gap of periodic gratings and applied with positive bias, the absorption frequency can be dynamically controlled by the level of bias voltage. At the resonant frequency, the reflection from the metasurface can be less than 1%, implying that most of the incidence is absorbed by the magnetic resonance of high impedance metasurface. When the bias voltage is tuned from 0 to +1.33 V (the forward voltage), the resonant peak rises from 8.5 to 9.1 GHz, providing a flexible control of electromagnetic wave absorption in X band.

High impedance surface (HIS) [22,23]is a kind of sandwich structures comprised of periodical metallic patterns, dielectric spacer and metallic ground plane. They behave as a highly efficient magnetic conductor (PMC) to the incident wave at resonant frequency with high surface impedance, giving zero phase shift to the reflected wave [24,25]. HIS is widely investigated and found to be an electromagnetic band gap (EBG) material which suppresses the propagation of surface waves. These properties can be utilized for low-profile antenna, and back lobe suppression, etc [26,27]. Meanwhile, it is worth noting that the local field is enhanced by several orders around the resonant frequency. HIS can also be used as an electromagnetic absorber when appropriate loss is introduced. It works with small loss factor and electrically thin thickness, while gives high absorption efficiency [28][29][30]. In 2019, Liu et al. introduced PIN diode into a HIS, making a reconfigurable metasurface with multiple applications [31].
In this paper, we have designed and measured for the first time the highly efficient absorption effect of an active high-impedance metasurface. we fabricated a HIS which consists of a metallic grating and a ground plane, with a dielectric substrate sandwiched between them. Then a diode and a passive resistor are loaded on each gap of the grating. Highly efficient electromagnetic absorption can be achieved by dedicated design of the resistance of passive resistor; and the absorption frequency can be dynamically changed with the tunable characteristics of the diode. It is observed with finite-difference time-domain (FDTD) simulations that, the absorption efficiency is 99% at 8.5 GHz without the DC bias; while applied with positive bias voltage, the series resistance of the diode decreases, resulting in a higher resonant frequency of HIS; and at the forward voltage +1.33 V of the diode, the absorption frequency increases to 9.7 GHz. Reflection from the sample is measured with microwave experiments. The tuning range is from 8.5 to 9.1 GHz with a minimum reflection of −30 dB observed during the measurements, which agrees well with the theoretical predictions. This work can make good contributions to instrument protection, electromagnetic wave shielding and other areas.

STRUCTURE AND SIMULATIONS
The proposed active HIS is a three-layered periodic structure as schematically shown in Figure 1. A metallic grating is on the top surface with a period of px 12.2 mm and a gap of g 3 mm, so that the width of the grating is w 9.2 mm. Then a diode and a passive resistor are loaded in series across each gap and periodically arrayed along y direction with an interval of py 12 mm. We adopted Aluminum Gallium Arsenide (AlGaAs) flip-chip PIN diodes (MA4AGP907) with a small junction capacitance of 0.025 pF, and the resistance of the passive resistor is R 100 Ω. On the bottom layer is a metallic ground plane. The thickness of the dielectric layer is h 0.78 mm, and that of each metallic layer is t 0.017 mm, giving rise to a total thickness of 0.814 mm of the sample. The dielectric constant of the substrate is ε r 2.65.
According to the equivalent circuit model theory [23], a HIS can be modeled as a parallel RLC resonant circuit. The gap of grating behaves as a capacitor and the resonant loop forming among the top and the bottom layer provides an equivalent inductance, giving rise to a constant resonant frequency determined by the structural geometry. When the gap is loaded with a diode and a passive resistor, it turns into an active HIS with equivalent circuit model as shown in Figure 2. The diode forms a parallel RC circuit with the junction capacitance C D and a variable resistance R D , in series with the passive resistor R, integrated into the HIS circuit. The variable  In the simulation, periodical boundary conditions are imposed along x axis and y axis. The incident wave propagates along z − direction and is ensured with no reflection at z + boundary by UPML boundary condition. The excitation in time domain is a Gauss pulse. The diode is simulated using passive components with C D 0.025 pF, and R D 4.2 Ω (on-state) or R D ∞ Ω (offstate). z component of Poynting vector is probed at 5 mm above the sample by setting a probe plane. Calculated reflection spectrum is shown in Figure 3. For on-state, highly efficient absorption happens at 8.5 GHz with a reflection coefficient of −26.5 dB, while the electromagnetic incidence is mostly reflected back with a reflection coefficient of −1.8 dB at 9.7 GHz. For offstate, the absorption peak is blue shifted to 9.7 GHz with a low reflection level of −11 dB, while the reflection rises to −3 dB at 8.5 GHz. At this point, the dynamic control of resonant absorption peak, that is the switch between absorption and reflection state at 8.5 GHz and 9.7 GHz, can be achieved by using the on-state and off-state of the diode.

EXPERIMENTS AND DISCUSSIONS
For verification, a HIS sample is fabricated with printed circuit board (Taconic TLX-8) with a lateral size of 180 × 180 mm, as shown in Figure 4. 224 chip diodes and an equal number of chip resistors are surface mounted with gold silk bonding technology on the top layer of the sample. By alternatively applying positive and negative DC voltage on adjacent gratings, the diode is biased with tuneable voltage level. The reflection is measured in a microwave anechoic chamber, with the sample placed on a foam and two standard gain horn antennas placed in front of it. The main lobe of horn antennas is perpendicularly incident on the sample surface, as schematically shown in Figure 4B. The horn emitter and the horn receiver are connected to an Agilent PNA52224 vector network analyzer to measure the S parameter S 21 . At first, a metal plate with the same size as the sample is measured to record the calibration field intensity. When the sample is under test, we get the reflected field intensity. Then the reflection coefficient is derived from the difference between these data.   We measured the reflection spectra for 0 V and +1.33 V bias voltage, corresponding to the off-state and on-state of diode respectively. Figure 5 shows the normalized reflection coefficient for both states from measurements. It can be observed that, when the bias is not loaded (0 V), the resonant absorption peak appears at 8.5 GHz with a reflection minimum of −24 dB. While applied with +1.33 V bias voltage, the absorption peak shifts to a higher frequency of 9.1 GHz, which agrees well with the theoretical calculation.
In addition to the on-state and off-state, the resistance of the diode will continuously vary during the increase of positive bias voltage from 0 V to +1.33 V. The reflection of the sample is measured at several intermediate levels of bias voltage as shown in Figure 6. It is proved that the resonant absorption peak can be dynamically tuned between 8.5 and 9.1 GHz, by adjusting the bias voltage. The reflection of the sample is low than −18 dB during the whole range. When +1 V bias voltage is applied, the reflection coefficient drops below −30 dB, implying a wonderful absorption of incidence.

CONCLUSION
In conclusion, we have designed an active HIS to achieve dynamic control of the electromagnetic absorption. The impedance matching between the HIS and the incident wave is met by introducing a passive series resistor on the top surface of the HIS. By adjusting the positive bias voltage of the diode, the resistance of the diode can be tuned in a range to achieve dynamic control of the highly efficient electromagnetic absorption. Experimental samples are fabricated and measured to verify this phenomenon for the first time. Both theoretical simulations and experimental measurements demonstrate that the active HIS can achieve electromagnetic absorption with an absorption efficiency greater than 99% at resonant frequencies.
By changing the diode from off-state to on-state, a dynamic switch between the electromagnetic absorption and reflection is observed at 8.5 and 9.1 GHz in experiments. Furthermore, continuous changing of the absorption peak between 8.5 and 9.1 GHz can be achieved by gradually varying the positive bias voltage of the diode. This work has great potential in electromagnetic wave stealth, shielding, and electromagnetic compatibility.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

AUTHOR CONTRIBUTIONS
ZW conceived the idea and supervised the whole study. HL and WX conducted the numerical calculations and performed the experiments. YC derived the theory, carried out the analysis and drafted the manuscript. All authors contributed to the review of manuscript.