Programmable metasurfaces have received a great deal of attention due to their ability to dynamically manipulate electromagnetic (EM) waves. Despite the rapid growth, most of the existing metasurfaces require manual control to switch among different functionalities, which poses severe limitations on practical applications. Here, we put forth an intelligent metasurface that has self-adaptive EM functionality switching in broadband without human participation. It is equipped with polarization discrimination antennas (PDAs) and feedback components to automatically adjust functionalities for the different incident polarization information. The PDA module can first perceive the polarization of incident EM waves, e.g., linear or circular polarization, and then provide the feedback signal to the controlling platform for switching the EM functionality. As exemplary demonstrations, a series of functionalities in the 9–22 GHz band has been realized, including beam scanning for $x$-polarization, specular reflection for $y$-polarization, diffuse scattering for left-handed circular polarization (LCP), and vortex beam generation for right-handed circular polarization (RCP) waves. Experiments verify the good self-adaptive reaction capability of the intelligent metasurface and are in good agreement with the designs. Our strategy provides an avenue toward future unmanned devices that are consistent with the ambient environment.

1. INTRODUCTION

During the past few decades, metamaterials have drawn much attention from the scientific community due to their remarkable electromagnetic (EM) properties that are not available in nature [1–3]. As forms of planar metamaterials, metasurfaces eliminate the bulk of metamaterials and lower the loss due to their negligible electrical thickness. Thus, metasurfaces have facilitated many intriguing functional devices such as lens antennas [4,5], polarization converters [6–8], vortex beam generators [9], and chromatic aberration-free metadevices [10]. Nevertheless, the above-mentioned works are passive, leading to fixed functionalities and characteristics.

To explore a new perspective of metasurfaces, programmable metasurfaces have been proposed and exploited, which can offer a flexible platform for implementing diverse functions [11–13]. Various control mechanisms have been explored to design the programmable metasurfaces, including the PIN diodes [14], varactors [15], photodiodes [16], and micro-electromechanical systems (MEMS) [17], and attained dynamic modulations of EM responses via external stimulus. In early research, most of the realized dynamic metasurfaces focused on the reconfiguration of a single freedom (amplitude, phase, polarization, etc.) under special polarization [18–21]. For example, the PIN diodes have been applied on metasurfaces to achieve beam scanning by changing the phase distributions [22,23]. The latest efforts started to be devoted to the design of programmable metasurfaces with multifunctional EM properties, which can dynamically regulate multiple degrees of freedom of EM waves. For instance, active dual-polarized metasurfaces loaded with varactors or PIN diodes can dynamically reshape the far-field scatterings for two orthogonal linear-polarization channels [24–26]. A series of metasurfaces has been proposed to achieve the surface wave directional radiation with customizable radiation intensity and switchable radiation pattern in two circular polarization channels [27]. Based on this, the number of channels can be further extended to four or even six channels [28,29]. In addition, various types of multifunctional programmable metasurfaces have also been reported, including scattering and radiation modes, full-space modes, and multi-polarized modes [30–33]. However, the EM functionality switching of the above-mentioned tunable metasurfaces must rely on the control of human beings.

In order to achieve intelligent manipulation of EM waves, metasurfaces should be capable of perceiving automatically external information and then adjusting adaptively its operating states to a changing environment. With this strategy, many intelligent metasurfaces with excellent performance have been proposed [34–37]. Very recently, an intelligent metasurface architecture integrated with sensing and feedback components has been presented, which can modulate adaptively the reflected patterns under different microwave incidences [38]. Furthermore, the combined use of deep-learning algorithms and detectors allows intelligent metasurfaces to be applied to more complex usage environments [39–41]. Nevertheless, most of the aforementioned intelligent metasurfaces either implement under special polarization or operate on narrow bandwidth.

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To overcome the above limitations, here we propose a broadband intelligent metasurface with the capability of self-adaptive EM manipulation according to different states of polarization (SOPs) of the incident wave. It consists of sensing and feedback components to constitute a closed-loop system without the need for manual instructions. The sensing component uses polarization discrimination antennas (PDAs), which can recognize two orthogonal linear polarizations ($x$-LP and $y$-LP) and two orthogonal circular polarization waves (CP, i.e., LCP and RCP that are left- and right-handed CP). The overall process of our intelligent metasurface system is as follows: first, the PDAs sense the polarization of the incident EM wave; then, an operation instruction from the control system according to the polarization information is acquired; finally, dynamic switching of its EM functionality is made. As a proof of the concept, we experimentally realize an intelligent metasurface that automatically switches its EM functionality among beam scanning, specular reflection, diffuse scattering, and the vortex beam. Our approach provides a solid platform for intelligent and cognitive wideband metadevices.

2. THEORETICAL ANALYSIS AND META-ATOM DESIGN

A. Principle of the Intelligent Metasurface

Figure 1 illustrates the schematic of the proposed intelligent metasurface, which consists of the sensing module, the microcontroller unit (MCU), the control system, and the executing device. The sensing module has two channels (linear and circular PDA channels) to perceive the polarization of the illuminating wave and transform the polarization signal into the feedback voltage. Then, the MCU is used for information processing, which can select the coding sequence of the EM function corresponding to the output voltage. Many coding sequences need to be pre-stored in the MCU platform for diversified EM properties. The control system is mainly used to control the orientation of each meta-atom individually. The executing device adopts a broadband programmable metasurface with fully polarized characteristics, and a micromotor is assembled behind each meta-atom to realize dynamic reconfigurability. Our intelligent metasurface can self-adaptively implement four functions under different SOPs of the incident EM wave, including beam scanning, specular reflection, diffuse scattering, and the vortex beam. In brief, all the above-mentioned EM functions can be self-adaptively adjusted throughout the entire process.

Figure 1.Schematic configuration of the proposed intelligent metasurface.

In order to obtain the intelligent metasurface controlled by the polarizations of the illuminating wave, the key step is to design tunable meta-atoms that operate in full-polarization waves. Since CP waves are the eigenstates in analyzing the phase responses of metasurfaces, then arbitrary polarization waves can be decomposed into two orthogonal CP waves, that is, LCP and RCP waves. As a result, a metadevice capable of working under dual-CP waves is thus able to simultaneously work under an arbitrary polarization state. However, according to Pancharatnam–Berry (PB) phase theory, the functionalities under LCP and RCP waves are essentially locked together due to the opposite phase profile flipping as the spin state of incident waves. Limited by this, PB phase by rotating meta-atoms cannot achieve polarization-insensitive devices. Then, the core of the question naturally becomes how to achieve reprogrammable metasurfaces that completely decouple optical functions for different photon spins in a wide band. For a reflection meta-atom under the Cartesian coordinate system, the CP reflections with linear base after a rotation angle $\alpha $ can be written as (Appendix A) $${r}_{LL}=\frac{1}{2}[({r}_{xx}-{r}_{yy})-j({r}_{xy}+{r}_{yx})]{e}^{-2\alpha j},$$$${r}_{LR}=\frac{1}{2}[({r}_{xx}+{r}_{yy})+j({r}_{xy}-{r}_{yx})],$$$${r}_{RL}=\frac{1}{2}[({r}_{xx}+{r}_{yy})-j({r}_{xy}-{r}_{yx})],$$$${r}_{RR}=\frac{1}{2}[({r}_{xx}-{r}_{yy})+j({r}_{xy}+{r}_{yx})]{e}^{2\alpha j}.$$

Here, ${r}_{mn}=|{r}_{mn}|\xb7{\mathit{e}}^{j{\phi}_{mn}}$($m$, $n=x$, $y$, $L$, and $R$), where the first subscript indicates the polarization state of the reflective wave and the second subscript indicates the incident polarization. From the equation we can clearly see that the phases ${\phi}_{LL}$ and ${\phi}_{RR}$ can be arbitrarily and independently manipulated through co-polarized or cross-polarized reflection coefficients.

The co-LP scheme indicates that complete distinct phases at two CP states can be achieved when $|{r}_{xx}|=|{r}_{yy}|=1$ and ${\phi}_{xx}-{\phi}_{yy}=\pm \pi $ [42–44]. Therefore, the phases that are imparted to the LCP and RCP incident waves now are ${\phi}_{LL}={\phi}_{xx}-2\alpha $ and ${\phi}_{RR}={\phi}_{xx}+2\alpha $. Theoretically, if we simultaneously impart phase patterns ${\phi}_{LL}={\phi}_{RR}$, the metasurface can operate at any state of polarization in Poincaré spheres. The underlying reason is that any incident polarization state can be described by the superposition of two orthogonal CP states. In this case, the rotation angle $\alpha $ and the required co-polarization phase ${\phi}_{xx}$ and ${\phi}_{yy}$ can be obtained by $${\phi}_{xx}={\phi}_{LL}={\phi}_{RR},$$$${\phi}_{yy}={\phi}_{xx}\mp \pi ,$$$$\alpha =0.$$

On the other hand, the cross-LP scheme is adopted to decouple phases ${\phi}_{LL}$ and ${\phi}_{RR}$ for full-polarization modes [45]. Assuming a reciprocal system without mirror symmetry with the condition $|{r}_{xy}|=|{r}_{yx}|=1$ and ${\phi}_{xy}={\phi}_{yx}$, we immediately obtain ${\phi}_{LL}={\phi}_{xy}-2\alpha -\pi /2$ and ${\phi}_{RR}={\phi}_{xy}+2\alpha +\pi /2$. Similarly, we impart phase patterns ${\phi}_{LL}={\phi}_{RR}$ to achieve full-polarization devices. Then the required cross-polarization phase ${\phi}_{xy}$ and rotation angle $\alpha $ can be derived as $${\phi}_{xy}={\phi}_{yx}={\phi}_{LL}={\phi}_{RR},$$$$\alpha =-\frac{\pi}{4}.$$

It is noted that the condition of the co-LP scheme also applies to the cross-LP scheme when the original 0°-orientated counterpart rotates $\pm 45\xb0$. Consider a reflection meta-atom exhibiting mirror symmetry; then its EM characteristics can be described by diagonal Jones matrix $\mathbf{R}=\left[\begin{array}{cc}{r}_{xx}& 0\\ 0& {r}_{yy}\end{array}\right]$. If rotated with an in-plane orientation angle $\theta $, the meta-atom will exhibit a reflection Jones matrix $\mathbf{R}(\theta )$, which can be extracted from Eq. (A2) in Appendix A and written as $$\mathbf{R}(\theta )=\left[\begin{array}{cc}{r}_{xx}(\theta )& {r}_{xy}(\theta )\\ {r}_{yx}(\theta )& {r}_{yy}(\theta )\end{array}\right]=\left[\begin{array}{cc}{r}_{xx}\text{\hspace{0.17em}}{c\mathrm{os}}^{2}\theta +{r}_{yy}\text{\hspace{0.17em}}{\mathrm{sin}}^{2}\theta & \frac{{r}_{xx}-{r}_{yy}}{2}\text{\hspace{0.17em}}\mathrm{sin}\text{\hspace{0.17em}}2\theta \\ \frac{{r}_{xx}-{r}_{yy}}{2}\text{\hspace{0.17em}}\mathrm{sin}\text{\hspace{0.17em}}2\theta & {r}_{xx}\text{\hspace{0.17em}}{\mathrm{sin}}^{2}\theta +{r}_{yy}\text{\hspace{0.17em}}{\mathrm{cos}}^{2}\theta \end{array}\right].$$

According to the condition of the co-LP scheme $|{r}_{xx}|=|{r}_{yy}|=1$ and ${\phi}_{xx}-{\phi}_{yy}=\pm \pi $, the reflection matrix $\mathbf{R}(\theta )$ can be simplified as $\mathbf{R}(\theta )=\left[\begin{array}{cc}\mathrm{cos}\text{\hspace{0.17em}}2\theta & \mathrm{sin}\text{\hspace{0.17em}}2\theta \\ \mathrm{sin}\text{\hspace{0.17em}}2\theta & -\mathrm{cos}\text{\hspace{0.17em}}2\theta \end{array}\right]{e}^{j{\phi}_{xx}}$. It is implied that the reflection coefficients met the condition ($|{r}_{xy}|=|{r}_{yx}|=1$ and ${\phi}_{xy}={\phi}_{yx}$) of the cross-LP scheme when the orientation angle of the co-LP scheme element is $\theta =\pm 45\xb0$.

Based on the above-mentioned analysis, we design a basic building block for broadband polarization insensitivity, as shown in Figs. 2(a) and 2(b). Moreover, the micromotor technique is used to achieve dynamic modulation of EM functions for intelligent metasurfaces. The meta-atom is composed of a metallic layer with a mirror C-shaped structure supported by a circular substrate to facilitate rotation. The FR4 (${\epsilon}_{r}=4.2$, $\mathrm{tan}\text{\hspace{0.17em}}\delta =0.015$) is selected as the substrate with thickness ${h}_{1}=1\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$, and the metallic structure is determined as an aluminum thin film with a thickness of 0.036 mm. A copper plate is used as the ground plane with ${h}_{2}=3\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$ below the substrate to ensure a high reflection efficiency. The formed thick air layer between them further improves the meta-atom bandwidth.

Figure 2.The proposed meta-atom and its symmetric operation mode. (a) Top view and (b) perspective view. In this work, the following parameters are: $w=0.8$, $R=3.6$, $b=1.5$, $r=1.2$, $P=9$ (all in millimeters), and $\alpha =21\xb0$, $\beta =160\xb0$. (c) and (d) are the surface current distributions under $x$- and $y$-polarized wave illumination, respectively.

Notably, the orientation angle of the meta-atom is set as $\alpha =0\xb0$ to validate the bi-co-polarized reflection property. Since the meta-atom is mirror symmetric with respect to the $x$ and $y$ axes, the reflection coefficients for the cross-polarized mode are all suppressed to zero, while the co-polarized reflection coefficients are entirely enhanced to 100%. Figures 2(c) and 2(d) show the simulated surface currents under the illumination of $x$- and $y$-polarized waves. The simulations were done in the microwave band using commercial software CST Microwave Studio. It is clear that the connecting rod of the mirror C-shaped pattern resonates when the $y$-polarized wave impinges on the meta-atom, whereas the lateral arcs resonate when the element is impinged by the $x$-polarized wave. In other words, ${\phi}_{xx}$ and ${\phi}_{yy}$ can be carefully regulated by the connecting rod and the arcs to engineer a broadband high-efficiency co-LP system.

Given the criterion of the co-LP scheme from Eq. (2), we can see that the key step to realize the polarization-insensitive property is to achieve a meta-atom with $|{r}_{xx}|=|{r}_{yy}|=1$ and ${\phi}_{xx}-{\phi}_{yy}=\pm \pi $ in broadband. Figure 3(a) illustrates the reflection coefficients for two orthogonal LP waves at normal incidence. Within a wide frequency band 9–22 GHz, we do find that $|{r}_{xx}|=|{r}_{yy}|\approx 1$ (with realistic losses considered), and the condition ${\phi}_{xx}-{\phi}_{yy}=180\xb0$ is approximately satisfied. Multi-resonances are excited by such a structure, leading to the broadband property [46]. The thickness of the substrate and air layer will affect the working bandwidth. Based on the above analysis, we can acquire the conditions of the cross-LP scheme through a $\pm 45\xb0$ rotating operation to the meta-atom. To validate this criterion, we depict the simulated amplitude and phase of cross-polarization [${r}_{xy}(45\xb0)$ and ${r}_{yx}(45\xb0)$] after the meta-atom was rotated 45° in Fig. 3(b). As a comparison, we draw the theoretical results of Eq. (4) in the figure, which is calculated by ${r}_{xy}(45\xb0)={r}_{yx}(45\xb0)=({r}_{xx}-{r}_{yy})/2$. It is apparent that good agreements can be observed between the simulation results and the theoretical predictions so that the condition $|{r}_{xy}|=|{r}_{yx}|=1$ and ${\phi}_{xy}={\phi}_{yx}$ can be fulfilled within a broadband. Results demonstrate that the criterion of the co-LP scheme is the basis for achieving full polarization characteristics.

Figure 3.The reflection characteristics of the meta-atom. Simulation reflection coefficients versus frequency for orientation angle (a) $\alpha =0\xb0$ and (b) $\alpha =45\xb0$ shined by LP plane wave excitations. Amplitude and phase of the reflected coefficient (c) ${r}_{xx}$ and (d) ${r}_{yy}$ as functions of parameter $\alpha $ at 14 GHz. (e), (f) Amplitude and phase difference of the ${r}_{xx}$ and ${r}_{yy}$ as a function of $\alpha $ and frequency.

We quantitatively studied the influence of angle $\alpha $ on the reflection when the meta-atom was illuminated by LP waves. Figure 3(c) shows the simulated amplitude and phase of ${r}_{xx}$ versus angle $\alpha $ at 14 GHz. We see that the reflection amplitude $|{r}_{xx}|$ decreases from almost one to zero when rotation angle $\alpha $ rotates from 0° to 45° while keeping other parameters constant. The reflection phase ${\phi}_{xx}$ is fixed at 180°, and it will be fixed at 0° when the angle $\alpha $ changes from 45° to 90°. The variation of the amplitude and phase follows ${r}_{xx}(\alpha )=-\mathrm{cos}\text{\hspace{0.17em}}2\alpha $, which agrees well with the theoretical calculation. It implies that the maximum of amplitude can be achieved when $\alpha $ is set to 0° or 90°, with an abrupt change of 180° in the phase response. On the other hand, ${r}_{yy}$ satisfies the relationship ${r}_{yy}(\alpha )=\mathrm{cos}\text{\hspace{0.17em}}2\alpha $ [see Fig. 3(d)]. As shown, ${r}_{yy}$ has the same amplitude distribution as ${r}_{xx}$ while having a 180° phase shift.

To further observe the wideband characteristic of the meta-atom, the amplitude and phase difference of ${r}_{xx}$ and ${r}_{yy}$ with different $\alpha $ from 8 to 24 GHz are shown in Figs. 3(e) and 3(f). The results show that the amplitude of ${r}_{xx}$ and ${r}_{yy}$ varying against frequency has good parallelism properties across the whole band from 9 to 22 GHz. In addition, the phase difference remains constant in two separated ranges of 0° to 45° and 45° to 90° in this band, which is approximately fixed at $\pm 180\xb0$. Therefore, we can see that both ${r}_{xx}$ and ${r}_{yy}$ have nearly 100% reflection efficiency when $\alpha $ is 0° or 90° with an abrupt phase change of 180°. Such characteristics can be well applied to a 1-bit coding metasurface. Meanwhile, the two orientation states of the meta-atom satisfy the conditions of the co-LP scheme ($|{r}_{xx}|=|{r}_{yy}|=1$ and ${\phi}_{xx}-{\phi}_{yy}=\pm \pi $), which implies it can be applied to polarization-insensitive devices in broadband. To achieve tunable meta-atoms, each element is connected to a micromotor by a shaft, and the orientation can be tunable with 0° or 90°.

3. DESIGN OF A FULL-POLARIZATION RECONFIGURABLE METASURFACE

Based on the analysis described above, we can design an electrically controlled reprogrammable metasurface for full polarization, whose meta-atom can be freely tunable with 0° or 90° orientation. We define two working states of the meta-atom as binary codes “0” and “1.” Here, the “0” element is characterized by a reflection phase of 0° whereas the “1” element is characterized by 180°. We denote the exhibited meta-atoms as “0” and “1” for different SOPs as illustrated in Fig. 4(a). It is noteworthy that, despite a 180° phase difference in ${\phi}_{xx}$ and ${\phi}_{yy}$, they share the same encoding state due to the adoption of the binary encoding method.

Figure 4.Full-polarization metasurface for double beam deflection. (a) Configured 1-bit coding elements working independently for phase control of linear and circular polarizations. (b) Required phase distribution. (c) The theoretical and simulated 2D normalized scattering patterns under the $x$-polarized wave in the $yoz$ plane at 14 GHz.

In addition, an example is given to demonstrate that the reprogrammable metasurface can operate under any polarization incidence. We design a metasurface consisting of $40\times 40$ meta-atoms with the coding sequence of “00001111…” along the $y$ direction, and the binary phase diagram is displayed in Fig. 4(b). The phase coding sequence of “00001111…” means that the metasurface is encoded by the period of 00001111 in a line-controlled manner. The anomalous reflection angle of the beams can be calculated by the generalized Snell’s reflection law [47], $${\theta}_{r}=\pm \mathrm{arcsin}(\lambda /\mathrm{\Gamma}),$$where $\lambda $ is the free-space wavelength, and $\mathrm{\Gamma}$ is the periodicity of the coding sequence. In this scheme, the deviation angle of the beams apart from the $z$-axis is $\pm 17.3\xb0$, with the working frequency set as 14 GHz. Figure 4(c) shows the simulated normalized scattering pattern under the $x$-polarized wave in the $yoz$ plane at 14 GHz, and the deflection angle is $\pm 17\xb0$. Under the normal incidence of plane waves, the scattering pattern from the metasurface can be calculated as $${E}_{\mathrm{total}}(\theta ,\phi )=\sum _{m=1}^{M}\sum _{n=1}^{N}{e}^{-j{\phi}^{mn}-jkP\text{\hspace{0.17em}}\mathrm{sin}\text{\hspace{0.17em}}\theta [(m-1/2)\mathrm{cos}\text{\hspace{0.17em}}\phi +(n-1/2)\mathrm{sin}\text{\hspace{0.17em}}\phi ]},$$where $\theta $ and $\phi $ are the elevation and azimuth angles of an arbitrary direction, respectively, and ${\phi}^{mn}$ denotes the reflected phase from the ($m$, $n$) particle. $k$ is the wavenumber in free space, and $P$ is the periodicity of each element. In order to give a detailed comparison, we also plot in Fig. 4(c) the calculated scattering pattern, where good agreements are observed between the simulated result and the theoretical prediction.

As mentioned above, the rotation angles $\alpha =0\xb0$ and 90° play a key role as they guarantee the identically spin-locked phase for CP waves (${\phi}_{LL}={\phi}_{RR}$). Such a feature allows the designed metasurface to operate at any polarization state in Poincaré spheres. To verify the full polarization properties, the simulated 2D scattering patterns for five input polarization states are exhibited in Fig. 5, including $x$-LP, $y$-LP, LCP, RCP, and elliptical polarization (EP) states. All results are almost identical, and the main beams are split into two symmetrical pencil beams, which are in good consistency with theory [see Fig. 4(c)]. To further illustrate the working bandwidth of the full-polarization reprogrammable metasurface, 3D radiation patterns for different polarization states at various frequencies are shown in Fig. 10 (Appendix B), where the highly directive dual beams are achieved between 9 and 21 GHz. It indicates that the reprogrammable metasurface can operate at arbitrary polarization states in broadband.

Figure 5.Poincaré sphere representation of some exemplary states from A to E, corresponding to RCP, EP, $x$-LP, LCP, and $y$-LP, respectively. The simulated far-field radiation patterns represent the co-polarized beam under the normal incidence of the full-polarization metasurface with the SOP from (a) to (e).

4. RESULTS AND DISCUSSION OF THE INTELLIGENT METASURFACE

A. Design of the Sensing Module

To realize the self-adaptive response to the polarization changes of incidence, two polarization discrimination antennas (PDAs) are meticulously designed to comprise the sensor for perceiving the incident polarization. The PDAs consist of two parts: the linearly PDA (LPDA) is applied to detect the $x$- and $y$-polarized incident waves, and the circular PDA (CPDA) is to LCP and RCP waves [48,49]. As Figs. 6(a) and 6(b) show, the LPDA integrates a patch array antenna and the metal ground with four interconnected slots. Four PIN diodes are employed to connect surrounding and central metal. The area of the antenna is $40\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}\times 40\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$ (see Fig. 11 in Appendix C). A standard horn antenna was used to emit $x$- or $y$-polarized EM waves, and the antenna was excited by a vector network analyzer and power amplifier with 35 dBm radiation power. The distance between the emitting antenna and the LPDA was set as 1 m. The LPDA was connected directly to a digital multimeter to measure the DC voltage level, and the measured output DC voltages are presented in Fig. 6(c). Within the frequency range of 13.5–14.5 GHz, the output DC voltages are divided into positive and negative values, respectively, for $x$- and $y$-polarized incident waves, and their absolute values are greater than 5 mV. However, the absolute values of measured output DC voltages are less than 5 mV when the LPDA is excited by a standard CP horn antenna. We set the threshold of voltage of $\pm 5\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mV}$ as effective values, which indicates that the LPDA is applicable for discriminating $x$- and $y$-polarized waves. The proposed CPDA integrates a patch array antenna and four PIN diodes loaded on the slot ring, as illustrated in Figs. 6(d) and 6(e). More details about the working mechanism of the CPDA are provided in Fig. 12 (Appendix D). Figure 6(f) shows the measured output DC voltages of the CPDA. The results of the CPDA confirm the orthogonal CP detection ability.

Figure 6.Sensing module for the incident polarization detection. Prototype and measured output DC voltages to (a)–(c) LPDA and (d)–(f) CPDA, respectively.

B. Intelligent Metasurface for Different EM Functions

Based on tunable meta-atoms with full polarization characteristics and the designed PDAs, we propose an intelligent metasurface that can switch EM functions automatically for different polarized incidences. First, the PDAs perceive the polarization of the incident EM wave and then convert the polarization information into the different feedback voltages. Second, the MCU selects the preset required coding pattern based on the feedback voltage. Finally, the control system independently regulates the orientation states of each meta-atom according to the coding pattern of functions. As schematically shown in Fig. 7, we can generate four functions—beam scanning for $x$-LP, specular reflection for $y$-LP, diffuse scattering for LCP, and vortex beams for RCP incident waves. The reflective beams for different polarizations are generated by a metasurface consisting of $40\times 40$ elements with a total size of $360\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}\times 360\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$.

Figure 7.Intelligent metasurfaces achieve different EM functions for different polarized incidences. The phase distribution of function and its simulated results for (a) beam scanning under $x$-polarization, (b) specular reflection under $y$-polarization, (c) diffuse scattering under LCP, and (d) vortex beams under the RCP incident wave. The columns i and ii are phase distributions and simulated 3D scattering patterns. The column iii contains 2D scattering patterns and the phase patterns of the vortex beam. The column iv shows the simulated normalized $E$-field patterns on the $yoz$ plane and the monostatic RCS reductions from 8 to 24 GHz.

For beam scanning, four coding sequences of the metasurface and their EM responses under the $x$-polarized normally incident wave are shown in Fig. 7(a). The phase coding sequences are “0000011111…,” “00001111…,” “000111…,” and “0011…” along the $y$ direction [Fig. 7(a-i)]. These sequences enable a twin beam to be asymptotically close to the $z$-axis in the $yoz$ plane, as demonstrated by the simulated 3D far-field scattering patterns of the metasurface at 14 GHz in Fig. 7(a-ii). The deviation angle of the twin beams can be calculated by Eq. (5), which results in $\pm 13.8\xb0$, $\pm 17.3\xb0$, $\pm 23.4\xb0$, and $\pm 36.5\xb0$ for the four cases. In all cases, deviation angles of the simulated 2D far-field radiation patterns are consistent with the theoretical predictions [see Fig. 7(a-iii)]. The simulated E-field pattern from 8 to 24 GHz is also illustrated in Fig. 7(a-iv) to analyze the operating bandwidth. The angles of the maximum scattering direction are in good coincidence with the theoretical results (blue asterisks) from 9 to 22 GHz. Next, for $y$-polarized incidence, the EM function is switched into specular reflection, and the coding pattern and the EM responses are shown in Fig. 7(b). The simulated far-field scattering patterns indicate that the metasurface can serve as a reflector.

Supposing the LCP wave illuminates the intelligent metasurface, the EM function is switched into diffuse scattering. The coding particles are randomly distributed on the surface, and the corresponding coding phase pattern is given in Fig. 7(c-i). Figure 7(c-ii) depicts the simulated 3D scattering pattern at 19 GHz. We observe that the incident wave is diffused in numerous directions uniformly. For comparison, we plot the 2D RCS pattern at 19 GHz, and the monostatic RCS reductions of the metasurface and the reference metallic plate from 8 to 24 GHz, as drawn in Figs. 7(c-iii) and 7(c-iv). All results indicate that the metasurface can be used to reduce RCS in broadband.

To generate vortex beams, the intelligent metasurface is illuminated by a normally incident RCP plane wave. The coding pattern of the vortex beam with orbital angular momentum (OAM) mode of $l=\pm 1$ is shown in Fig. 7(d-i). Details can be found in Appendix E. In Fig. 7(d-ii), the simulated 3D scattering pattern reveals that the main beam exhibited a ring-shaped intensity profile with a hollow center, which coincides with the characteristic profile of vortex beams. In addition, the phase profiles of the generated beams are illustrated in Fig. 7(d-iii), where OAM modes of $\pm 1$ are observed for the deflected beams in the $yoz$ plane. More detailed information of the simulated 3D radiation patterns at different frequencies is shown in Fig. 13 (Appendix E), where vortex beams are achieved between 9 and 21 GHz. The simulated scattering pattern from 8 to 24 GHz is shown in Fig. 7(d-iv), where the blue asterisks represent the peak angles at different frequencies obtained by theory. As the working frequency increases from 9 to 22 GHz, the elevation angles of the reflective beams change from 27.5° to 11°.

5. EXPERIMENTAL MEASUREMENT AND DISCUSSION

To experimentally demonstrate the self-adaptive EM manipulation capability of the proposed intelligent metasurface, we fabricated a sample based on printed circuit board (PCB) technology, as shown in Fig. 8(a). The sample contained $20\times 20$ meta-atoms, and its dimension was $180\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}\times 180\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$. The size of the fabricated sample is reduced to half of the model in the simulations. We used 3D printing technology to design a frame placed beneath the copper plate so that the thickness of the air layer is exactly 3 mm. The photograph of the individual meta-atom loaded with the micromotor is shown in the inset of Fig. 8(a), where the metallic ground plane is removed for a clear view of the structure and the micromotor. The DC micromotor and limit structure were adopted in the prototype to rotate each meta-atom with 0° or 90°. The response time and power consumption of the prototype were 0.3 s and 44 W, respectively. Then, all of the micromotors were linked to the I/O ports of the control system. Since each micromotor is individually and dynamically regulated, the metasurface can be reprogrammed to enable different functions for various SOPs.

Figure 8.The experimental setup in an anechoic chamber. (a) Photograph of the fabricated sample. The inset of the picture shows a single meta-atom whose ground plane is removed for a clear view. (b) Photograph of the measurement setup in the microwave anechoic chamber.

In the experiments, the intelligent metasurface, PDAs, and emitting source are fixed on a rotatable table in a microwave chamber, as shown in Fig. 8(b). The metasurface is surrounded by absorbers to reduce the impact of the control system on the EM environment. The distance between the metasurface and the emitting source was about 2.2 m to ensure a quasi-plane wave illumination. Here, the PDAs are placed between the metasurface and the source, and the distance between the PDAs and the source was 1 m. By automatically rotating the table, the radiation from the metasurface was recorded by a vector network analyzer through a receiver.

We first demonstrate the beam scanning performance when the intelligent metasurface is illuminated by an $x$-polarized wave. Figure 9(a) plots the measured 2D scattering pattern of the metasurface with different phase coding sequences at 14 GHz, the two cases corresponding to “00001111…” and “0011….” The result shows that the main beams are deflected to the angles around $\pm 17\xb0$ and $\pm 36\xb0$, which is in good agreement with the simulated one displayed in the figure in line. To quantitatively estimate the operating bandwidth, we measured the frequency-dependent 2D far-field scattering pattern in case 1, where the blue asterisks represent the deflection angle obtained by simulations, as illustrated in Fig. 9(b). It verifies that the proposed metasurface can work well in a certain frequency band from 10 to 22 GHz. By changing the polarization of the incident wave into $y$, the intelligent metasurface automatically switches its EM function to reflection. Figure 9(c) presents the measured and simulated 2D scattering pattern at 14 GHz. The pattern shows the radiation in a pencil beam.

Figure 9.Experiments with the fabricated intelligent metasurface. Measured and simulated 2D normalized scattering patterns in the $yoz$ plane for (a) $x$-LP, (c) $y$-LP, (f) RCP incidence at 14 GHz, and (d) LCP incidence at 19 GHz. (b) The normalized 2D pattern within 8–24 GHz in the $yoz$ plane for $x$-LP incidence. (e) Comparison of the measured and simulated RCS reductions for LCP incidence.

Similarly, under LCP wave excitation, the EM function is switched into diffuse scattering. Figure 9(d) compares the measured and simulated 2D RCS patterns in the $xoz$ plane at 19 GHz, which is obtained by normalizing the reflection to a same-sized metallic plate under normal incidence. It can be seen that the experimental result is in good agreement with the simulation. Our sample can suppress the beam intensity by about 15 dB, achieving scattering reduction in all directions. Moreover, Fig. 9(e) plots the RCS reductions in the experiment and simulation, and the $-10\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{dB}$ RCS reduction covers the frequencies 17.5–21 GHz. Finally, when the incident is an RCP wave, the intelligent metasurface can generate vortex beams by automatically reconfiguring the meta-atoms. The performance of vortex beams was measured by far-field and near-field experiments. The measured 2D scattering pattern on $yoz$ plane at 14 GHz is shown in Fig. 9(f), and the energy null is located in the directions of $\pm 36\xb0$. The measured result is in good agreement with the simulated one, demonstrating the capability of generating well-defined vortex beams. The results of the near-field experiment can be found in Fig. 14 (Appendix E).

Finally, the performances of the proposed design are compared with that of some representative intelligent metasurfaces, as summarized in Table 1. It is clear that the proposed intelligent metasurface can work in orthogonal linear and circular polarization channels, and has a much wider frequency band. The advantages enable our design to be applied in broadband and multi-polarized intelligent systems.

Comparison of Performances with Reference Papers

Designs

Detection Signal

Control Amplitude and Phase

Control Method

Polarization

Frequency

Ref. [34]

Direction

Phase

Micromotor

CP

3.75–4.6 GHz

Ref. [35]

Intensity

Amplitude

PIN diode

LP

5–10 GHz

Ref. [38]

Azimuth

Phase

PIN diode

LP

9 GHz

Ref. [37]

Velocity

Phase

Varactor

LP/CP

2.78–6.32 GHz

Ref. [50]

LP/Frequency

Amplitude

Varactor

LP

3.8–6.8 GHz

Ref. [36]

Location

Phase

Varactor

LP

5.6 GHz

Proposed

Polarization

Phase

Micromotor

LP/CP

9–22 GHz

6. CONCLUSION

In summary, we have demonstrated an intelligent metasurface operating in broadband with high self-adaptability, which can achieve EM manipulation in response to different SOPs of the incident wave without human control. It is mainly composed of the sensing module, the MCU, the control system, and the executing device, and it establishes a complete sensing and feedback system. The sensing module is accomplished by two PDAs, which directly convert the polarization information of the incident EM wave to DC voltage signals. The polarization information encompasses orthogonal linear and circular polarizations, specifically $x$-LP, $y$-LP, LCP, and RCP. The MCU is used for information processing, and the control system switches the EM functions of the executing device. A reprogrammable metasurface with full polarization characteristics is adopted as the executing device. The intelligent metasurface can switch four EM functions for different polarizations (including beam scanning, reflection, diffuse scattering, and vortex beams) by rotating the meta-atoms that are controlled by the electrically driven micromotors. Experimental results verified that the realized intelligent metasurface can adjust its operation state automatically, according to the perceived polarization of the incoming wave. In comparison to the smart metasurface we previously proposed, the new intelligent metasurface is the first to combine polarization information for self-adaptive EM manipulation in broadband. We believe that this work will further promote the development of intelligent and cognitive metasurfaces, which may find potential application in intelligent materials and systems.

APPENDIX A: THEORY FOR CP REFLECTION COEFFICIENTS

Without the loss of generality, we start from the relationship between the incident and reflected electric field under the $xoy$ coordinate, which can be expressed as $$\left[\begin{array}{c}{E}_{x}^{r}\\ {E}_{y}^{r}\end{array}\right]=\mathbf{R}\left[\begin{array}{c}{E}_{x}^{i}\\ {E}_{y}^{i}\end{array}\right]=\left[\begin{array}{cc}{r}_{xx}& {r}_{xy}\\ {r}_{yx}& {r}_{yy}\end{array}\right]\left[\begin{array}{c}{E}_{x}^{i}\\ {E}_{y}^{i}\end{array}\right],$$in which ${E}_{x}^{r}$ and ${E}_{y}^{r}$ represent the $x$- and $y$-polarized components of the reflected fields, ${E}_{x}^{i}$ and ${E}_{y}^{i}$ represent the $x$- and $y$-polarized components of the incident fields, and $\mathbf{R}$ is the reflection matrix of an anisotropic meta-atom.

If the meta-atom is rotated by an angle $\alpha $ along the $z$-axis, the effect can be described using Jones calculus, $$\mathbf{R}(\alpha )={\mathbf{M}}^{-1}(\alpha )\mathbf{RM}(\alpha )\phantom{\rule{0ex}{0ex}}=\left[\begin{array}{cc}\mathrm{cos}(\alpha )& -\mathrm{sin}(\alpha )\\ \mathrm{sin}(\alpha )& \mathrm{cos}(\alpha )\end{array}\right]\left[\begin{array}{cc}{r}_{xx}& {r}_{xy}\\ {r}_{yx}& {r}_{yy}\end{array}\right]\left[\begin{array}{cc}\mathrm{cos}(\alpha )& \mathrm{sin}(\alpha )\\ -\mathrm{sin}(\alpha )& \mathrm{cos}(\alpha )\end{array}\right],$$where $\mathbf{M}(\alpha )$ describes the rotation matrix. The CP components can be expressed with the linear polarization components by $$\left[\begin{array}{c}{E}_{L}^{i}\\ {E}_{R}^{i}\end{array}\right]=\mathbf{C}\left[\begin{array}{c}{E}_{x}^{i}\\ {E}_{y}^{i}\end{array}\right]=\frac{1}{\sqrt{2}}\left[\begin{array}{cc}1& j\\ 1& -j\end{array}\right]\left[\begin{array}{c}{E}_{x}^{i}\\ {E}_{y}^{i}\end{array}\right],\phantom{\rule{0ex}{0ex}}\left[\begin{array}{c}{E}_{L}^{r}\\ {E}_{R}^{r}\end{array}\right]={\mathbf{C}}^{\prime}\left[\begin{array}{c}{E}_{x}^{r}\\ {E}_{y}^{r}\end{array}\right]=\frac{1}{\sqrt{2}}\left[\begin{array}{cc}1& -j\\ 1& j\end{array}\right]\left[\begin{array}{c}{E}_{x}^{r}\\ {E}_{y}^{r}\end{array}\right],$$in which ${E}_{L}^{i}$ and ${E}_{R}^{i}$ (${E}_{L}^{r}$ and ${E}_{R}^{r}$) represent the LCP and RCP components of the incident (reflected) fields. Due to the opposite directions of propagation between the incident and reflected waves, the rotation directions of the LCP (and RCP) waves are also opposite. Then combining the above equations, there is $$\left[\begin{array}{c}{E}_{L}^{r}\\ {E}_{R}^{r}\end{array}\right]={\mathbf{C}}^{\prime}{\mathbf{M}}^{-1}(\alpha )\mathbf{RM}(\alpha ){\mathbf{C}}^{-\mathrm{l}}\left[\begin{array}{c}{E}_{L}^{i}\\ {E}_{R}^{i}\end{array}\right]=\left[\begin{array}{cc}{r}_{LL}& {r}_{LR}\\ {r}_{RL}& {r}_{RR}\end{array}\right]\left[\begin{array}{c}{E}_{L}^{i}\\ {E}_{R}^{i}\end{array}\right],$$where the four CP reflection coefficients with a linear base can be described as follows: $${r}_{LL}=\frac{1}{2}[({r}_{xx}-{r}_{yy})-j({r}_{xy}+{r}_{yx})]{e}^{-2\alpha j},$$$${r}_{LR}=\frac{1}{2}[({r}_{xx}+{r}_{yy})+j({r}_{xy}-{r}_{yx})],$$$${r}_{RL}=\frac{1}{2}[({r}_{xx}+{r}_{yy})-j({r}_{xy}-{r}_{yx})],$$$${r}_{RR}=\frac{1}{2}[({r}_{xx}-{r}_{yy})+j({r}_{xy}+{r}_{yx})]{e}^{2\alpha j}.$$

APPENDIX B: ADDITIONAL 3D RADIATION PATTERNS OF THE FULL-POLARIZATION METASURFACE

Further insight into the working band of the proposed full-polarization reconfigurable metasurface is provided by 3D radiation patterns under different input polarization states at different frequencies. The simulated 3D radiation patterns for five input polarization states at specifically 9 GHz, 15 GHz, and 21 GHz are shown in Fig. 10. As illustrated, the metasurface achieves satisfying performances in producing a highly directive dual beam for incident waves with $x$-LP, $y$-LP, LCP, RCP, and EP over a wideband range.

Figure 10.Three-dimensional radiation patterns at the different frequencies of 9 GHz, 15 GHz, and 21 GHz for (a) $x$-LP, (b) $y$-LP, (c) LCP, (d) RCP, and (e) EP incident waves.

APPENDIX C: DESIGN OF THE LINEAR POLARIZATION DISCRIMINATION ANTENNA

Figures 11(a) and 11(b) show the basic structure of the proposed LPDA, having a total size of $40\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}\times 40\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$. Nine square patches with microstrip feed lines are periodically arranged on the substrate [48]. Four staggered slot lines with PIN diode (SMS7621-040LF) are printed on the other side of the substrate. The metallic structure is etched on the FR4 (${\epsilon}_{r}=4.2$, $\mathrm{tan}\delta =0.015$) substrate with a thickness of 0.8 mm.

Figure 11.Basic structure of the LPDA. (a) Top view of the antenna, (b) bottom view of the antenna. The optimized parameters are ${a}_{1}=4.9$, ${b}_{1}=5.3$, ${l}_{1}=40$, and ${w}_{1}=25.5$, all in the unit of millimeters.

Suppose an $x$-polarized wave illuminates the LPDA; then the signal received by the patch array antenna is the horizontal component. The working state of the left or right PIN diode is “ON,” the rest is “OFF,” and the output voltage is positive. Similarly, the output voltage is a negative value, which means the received signal is the $y$-polarized wave. Therefore, we can discriminate $x$- and $y$-polarized waves by the positive and negative values of the output voltage.

APPENDIX D: DESIGN OF THE CIRCULAR POLARIZATION DISCRIMINATION ANTENNA

The geometry of the proposed CPDA is shown in Figs. 12(a) and 12(b), having a total size of $55\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}\times 55\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{mm}$. The CPDA consists of twelve patch elements and a double-balanced multiplier [49]. A slot-ring and four PIN diodes (SMS7621-040LF) used in the RF multiplier also act as an antenna and amplitude detector, respectively. The equivalent circuits of PIN diodes are exhibited in Fig. 12(c), in which the values of $R$, $L$, and $C$ are 12.78 Ω, 0.45 nH, and 0.053 pF, respectively. Furthermore, slot lines which are parts of feeding circuits also act as slot antennas. The commercial dielectric material FR4 with a thickness of 0.8 mm was adopted in the design. A $\pm \pi /4$ phase shift can be realized by bending the slot line parallel to the $x$ (or $y$) axis inward (or outward).

Figure 12.The basic structure of the CPDA. (a) Top view of the antenna, (b) bottom view of the antenna. (c) Equivalent circuit models of the PIN diodes in the OFF and ON states. (d) Simulated ${S}_{11}$ of the CPDA. (e) Phase difference of the two parts. The optimized parameters are ${a}_{2}=4.8$, ${b}_{2}=9.1$, $n=13.7$, $m=16.35$, ${l}_{2}=55$, ${w}_{2}=19.7$, $r=5.44$, all in the unit of millimeters, and $\beta =28.2\xb0$.

The received RF signals from the array antenna are processed at the RF multiplier. From the polarity of the multiplier output voltage, the polarization of the received signal can be detected. The output voltage is either positive or negative, which corresponds to RCP or LCP. The simulated S-parameters of the Subarrays A and B (Parts A and B) are shown in Fig. 12(d), which are below $-10\text{\hspace{0.17em}\hspace{0.17em}}\mathrm{dB}$ in the frequency range of 13.58–14.38 GHz. The insets provide the radiation pattern and gain of the array antenna at 14 GHz. The simulated results for the phase difference of the two subarrays are shown in Fig. 12(e), which are designed to be 90° to detect CP waves.

APPENDIX E: ADDITIONAL RESULTS FOR THE VORTEX BEAM GENERATOR

In order to obtain two symmetric vortex beams in the $yoz$ plane, a periodic coding pattern of “00001111…” along the $y$ direction should be encoded onto the metasurface. The phase for an OAM beam of order $l$ in Cartesian coordinates can be written as $$\phi (x,y)=l\text{\hspace{0.17em}}\mathrm{arctan}\left(\frac{y}{x}\right)+k\text{\hspace{0.17em}}\mathrm{sin}\text{\hspace{0.17em}}{\theta}_{r}y.$$

Here, $l$ represents the mode number of the generated vortex beam, while $k$ depicts the wave number in free space. The ${\theta}_{r}$ is the deflection angle of the periodic coding pattern metasurface, which can be calculated by Eq. (5) in the main text. Then, discretize the phase profile into a 1-bit coding sequence by employing the discreteness criteria of $${\phi}_{1-\mathrm{bit}}(x,y)=\{\begin{array}{cc}0\xb0,& 0\xb0\le \phi (x,y)<180\xb0\\ 180\xb0,& 180\xb0\le \phi (x,y)<360\xb0\end{array}.$$

Figure 13 presents the numerically simulated 3D far-field radiation patterns of the intelligent metasurface from 9 to 21 GHz in steps of 4 GHz under normal RCP incidence. We can clearly see that all those 3D far-field radiation patterns at different frequencies show a typical doughnut-shaped intensity with a singularity in the center, indicating that the radiation energy in the normal direction is very low, which is completely identical to the far-field distribution of the ideal vortex beam. Obviously, the vortex beam generator can maintain stable performances with the desired anomalous angle and OAM modes in a broad frequency range.

Figure 13.Scattering patterns of the designed vortex beam generator at (a) 9 GHz, (b) 13 GHz, (c) 17 GHz, and (d) 21 GHz under RCP incidence, respectively. The left panels show the amplitude of the scattering patterns while the right panels show the phase patterns of each vortex beam.

Figure 14.Measured results of the near-field intensity distribution, phase distribution, and mode spectra of OAM beams with mode (a) $l=1$ and (b) $l=-1$.