Intelligent Metasurfaces with Continuously Tunable Local Surface Impedance for Multiple Reconfigurable Functions

Electromagnetic metasurfaces can be characterized as intelligent if they are able to perform multiple tunable functions, with the desired response being controlled by a computer influencing the individual electromagnetic properties of each metasurface inclusion. In this paper, we present an example of an intelligent metasurface that operates in the reflection mode in the microwave frequency range. We numerically show that, without changing the main body of the metasurface, we can achieve tunable perfect absorption and tunable anomalous reflection. The tunability features can be implemented using mixed-signal integrated circuits (ICs), which can independently vary both the resistance and reactance, offering complete local control over the complex surface impedance. The ICs are embedded in the unit cells by connecting two metal patches over a thin grounded substrate and the reflection property of the intelligent metasurface can be readily controlled by a computer. Our intelligent metasurface can have a significant influence on future space-time modulated metasurfaces and a multitude of applications, such as beam steering, energy harvesting, and communications.

Figure 1

Schematic of the unit cell for the proposed intelligent metasurface. We select the basic topology of a high-impedance surface and find the suitable dimensions for operation in the target frequency range (around 5 GHz) using the analytical formulas [34]. The dimensions are $d_x=d_y/2=9.12$ mm, $t=1.016$ mm, $w=8.12$ mm, and $g=1$ mm. A tunable IC chip is incorporated to provide a variable complex impedance and locally modify the surface impedance of the metasurface in a continuous way.

Figure 2

The required $R$ and $C$ values of the electronic elements (series topology; see the inset in Fig. 1) for achieving perfect absorption at different incidence angles for the two orthogonal polarizations (see Fig. 1). The operation frequency is 5 GHz.

Figure 3

Independent control over the reflection amplitude in decibels (a) and phase in degree (b) by tuning the load resistance $R$ and capacitance $C$. The white and black curves show example contours of constant amplitude and constant phase respectively.

Figure 4

(a) Schematic view of the supercell for anomalous reflection in the $x$-$z$ plane. Different colors of the lumped elements represent different capacitance settings. (b) Port naming convention and correspondence with reflected diffraction orders for normal incidence. (c) Anomalous reflection angles for different numbers of cells exploiting the first and second diffraction orders (assuming normal incidence).

Figure 5

Look-up table: Reflection phase for a uniform metasurface as a function of the capacitance of the tunable IC. The operating frequency is 5 GHz and the incident wave impinges at normal incidence. The corresponding reflection amplitude is shown in the inset.

Figure 6

Anomalous reflection with an $N=8$ supercell promoting the first diffraction order. (a),(b) Capacitance values for the meta-atoms before optimization (linear phase prescription) and after optimization. The corresponding phase profiles are shown in the insets. (c) Port naming convention and the three reflection scenarios: $1\to 3$, $2\to 2$, $3\to 1$. (d),(e),(f) Scattered electric field for the three reflection scenarios. Planar wavefronts tilted to the desired direction are observed.

Figure 7

Anomalous reflection with an $N=17$ supercell promoting the second diffraction order. (a),(b) Capacitance values for the meta-atoms before optimization (linear phase prescription) and after optimization. The corresponding phase profiles are shown in the insets. (c) Scattered electric field when illuminating from port 1: perfect anomalous reflection toward ${50.7}^{\circ }$ (port 5).

Figure 8

Anomalous reflection in the $y$-$z$ plane (TM polarization) with a $N=5$ supercell along the $y$ axis promoting the first diffraction order. (a) Schematic of unit-cell stacking along the $y$ axis with $N=3$. (b) Capacitance values for the meta-atoms after optimization; the corresponding phase profile is shown in the inset along with the linear phase profile prescription (dashed line). (c) Scattered magnetic field when excited from port 1: perfect anomalous reflection toward ${41.1}^{\circ }$ (port 3).