The programmable metasurface, as a promising functional platform, has attracted widespread attention owing to its real-time electromagnetic (EM) manipulation and easy deployment features [1–7]. Typically, the programmable metasurface is constructed by combining the active metasurface incorporated with PIN diodes, varactors, liquid crystals, or other tunable components with the digital programmable devices, such as field programmable gate array (FPGA) [8–13]. By changing dynamically the digital control sequences, the amplitude, phase, polarization, and other characteristics of EM waves can be manipulated in real time on programmable metasurfaces. With this capability, programmable metasurfaces have been adopted to realize various advanced devices and systems for communication, imaging, and radar [14–23]. For example, a reconfigurable intelligent surface (RIS) that can actively regulate the channel environment has been developed based on programmable metasurfaces, which shows great potential in sixth-generation (6G) wireless communications [24–27]. However, most previous programmable metasurfaces can only provide tunable functionalities for the certain single polarization, and their elements are controlled in whole or column way, leaving much potential unexploited.

Recently, some works focused on this problem have been demonstrated [28–32]. In Ref. [28] a polarization-controlled dual-programmable metasurface was proposed and realized for independent manipulations of orthogonally polarized EM waves in real time. However, the designed metasurface element (meta-element) only offers a 1-bit phase resolution and cannot be controlled independently. In Ref. [29] an active anisotropic coding metasurface consisting of 16×16 meta-elements was implemented to achieve the 1-bit and 2-bit phase modulations for two independent linear-polarization channels. But limited by the control circuit, the meta-elements in a column (or row) share an identical voltage, and thus the metasurface just operates in one-dimensional (1D) EM manipulation. More recently, a dual linearly polarized programmable metasurface composed of 12×12 independently controllable meta-elements was realized [32], but the 2-bit working bandwidth is narrow due to the used PIN diodes, and the array scale is small.

Here, we design and realize an anisotropic programmable metasurface with individually controllable meta-elements for independent and real-time manipulations of dual-polarized EM waves in the two-dimensional (2D) direction. Importantly, the meta-element can be programmed to achieve the 2-bit phase modulation in a wide frequency band, which enhances the performance of the programmable metasurface. Based on this anisotropic programmable metasurface, two different functions of real-time beam scanning and vortex wave generation are demonstrated numerically and experimentally for orthogonally polarized microwave incidences. Compared to previous work, this anisotropic programmable metasurface can realize more advanced functions and can achieve better performance in terms of programmability, bandwidth, and phase resolution.

A schematic of the wideband 2-bit anisotropic programmable metasurface is illustrated in Fig. 1, in which the orthogonally polarized phase responses of the meta-element can be manipulated independently in real time. To control each anisotropic meta-element independently, we design a scalable multi-channel control circuit that can provide (2×m×n)-way biases for (m×n) anisotropic meta-elements. We also design meticulously a pixel-level bias network for routing bias signals for all meta-elements independently. The next paragraph provides some detailed discussions on this bias network design. This design ensures abundant anisotropic meta-elements to be addressed individually within x- and y-polarized channels separately, which provides a high degree of capability in the efficient modulation of dual-polarized EM waves. Owing to its distinctive feature of dual-polarized wave independent control, wavefront shaping of the metasurface can be anisotropic, enabling different EM functionalities in the x- and y-polarized channels, such as pencil-type beam scanning and dynamic vortex beam generation, respectively. The anisotropic programmable metasurface offers two decoupled polarization channels for real-time EM control in 2D direction, which facilitates an increase in manipulation efficiency and functional diversity and could push programmable metasurfaces one step closer towards more complicated applications.

To control all meta-elements independently, especially for dual-polarization working mode, we further implement a pixel-level bias network, as shown schematically in Fig. 1. The bias network contains three different kinds of bias lines I, II, and III (indicated by green, pink, and white lines) as well as necessary metallic vias. Bias lines I and II are located on the top of bias layer, which are connected to two groups of varactors along the x and y directions respectively, through the reserved metallic vias (see meta-element design for more details). Bias lines III are located on the back of bias layer, which are designed and wired to connect bias lines I or II for powering each meta-element separately. For example, when m=n=6, the voltage interface 1 consisting of 18 bias lines III is used to independently control three columns of meta-elements on the right half of the anisotropic programmable metasurface under x-polarized incidence, and the voltage interface 2 is used to control these meta-elements independently under y-polarized incidence. Similarly, the meta-elements in the left three columns can also be controlled independently using this biasing method. We note that by carefully routing the bias line III and using more voltage interfaces, it is able to independently control more meta-elements (see sample fabrication for more details).

To realize the high-performance anisotropic programmable metasurface, we first design a dynamic anisotropic meta-element with large phase variations in a wide band, as shown in Fig. 2(a). It consists of two dual-metal layer structures and an F4B dielectric sandwiched in the middle. To achieve decoupling control of orthogonally polarized EM waves, a special top-layer metal pattern is designed elaborately, comprising a central square copper patch and four symmetrically distributed rectangular copper patches. To further realize the phase tuning, four varactors are embedded into the reserved gaps for changing the equivalent capacitances between the rectangular patches and the central square patch. The two groups of varactors in the x and y directions are used to tune x- and y-polarized responses, respectively. Therefore, by altering the capacitance values in the x and y directions separately, the meta-element reflections for two orthogonal polarizations can be modulated independently. The bottom layer of dielectric is fully copper-plated for improving the reflectivity of EM waves. To supply two independent voltages for two groups of varactors, two feeding lines along the x and y directions, respectively, are designed, which are patched onto the front of the bias layer [Fig. 2(a)]. It is worth noting that we have carefully selected and adopted the “MAVR-000120-14110P” varactor as a tuning component for achieving the large phase difference and wideband tunability, due to its high capacitance change varying from 1.15 to 0.14 pF.

Next, we investigate numerically and analyze the reflection performance of the designed anisotropic meta-element under dual-polarized incidences. To achieve large phase change and broadband characteristics, several important parameters are finally optimized as p=20.00mm, h=1.50mm, l1=14.20mm, l2=2.00mm, l3=14.75mm, and d=0.127mm. In the simulations, the integrated varactor was modeled as an equivalent series circuit with a resistance R=0.3Ω, an inductance L=0.4nH, and a variable capacitance C varying from 1.15 to 0.14 pF. Through changing the capacitances Cx and Cy in the x and y directions, respectively, we observe the simulated reflection amplitudes and phases of the anisotropic meta-element, and the results are shown in Figs. 2(b) and 2(c). It is obvious that the resonant frequencies of the meta-element shift from 3.7 to 5.2 GHz as the capacitances change from 1.15 to 0.14 pF, exhibiting broadband tuning feature. In addition, the amplitude of reflection coefficient is greater than −2.4dB at each resonant frequency. The meta-element can achieve a reflected phase difference larger than 250° in a wide band from 4.0 to 5.0 GHz. The dynamic reflected amplitudes and phases can be achieved for both x- and y-polarized incidences. Since the varactors in the x and y directions can be controlled independently, decoupling manipulation can be realized for the two polarizations.

Moreover, we further investigate the cross-polarization response of the meta-element, and the simulated reflection responses are shown in Fig. 2(d). We observe that for x-polarized incidences, changing the capacitances Cy in the y direction has almost no effect on the resonance responses, which indicates that the realized meta-element has high cross-polarization isolation. In our current design, we aim to realize a 2-bit phase modulation, and thus a ∼270° phase difference is enough. Because the realized meta-element is a single resonant structure, it will suffer the deficiency in phase tuning range [33]. To achieve a full 360° phase difference, an effective method is to design a meta-element with two resonances. In general, multiple resonances will lead to more reflection losses.

We note that in addition to the meta-element design, another key challenge is how to achieve independent control of a large number of meta-elements, which is also an advance from the existing global and 1D control manners. To achieve this goal, a scalable and multi-channel control circuit is designed. It is composed of an ESP32 microcontroller unit (MCU) and a driving circuit. For the driving circuit, analog multiplexers are utilized to supply control voltages, and each of them, equipped with three logic input pins, can supply to at most eight distinct voltage values. The MCU is interfaced with the analog multiplexers through bus switchers, and control voltages to the meta-elements are provided via I/O expanders. Considering the scalability and flexibility of the driving circuit, the bus switcher and the I/O expanders are implemented on printed circuit boards (PCBs) separately, which can provide 64 independent outputs. In this case, we can provide enough bias channels to control the metasurface by increasing the number of PCBs. As a demonstration, a prototype of the anisotropic programmable metasurface composed of 21×20 meta-elements was fabricated, as shown in Fig. 2(e). Because each PCB of the I/O expander provides biases for 60 meta-elements across three columns of the metasurface, 14 PCBs (7×2=14) are adopted for the independent control of 420 dual-polarized meta-elements through the voltage interfaces. The measured reflection phases of the sample under different reverse biases are shown in Fig. 2(f). It is clear that when the reversed bias is changed from 0 to 10 V, the metasurface sample can achieve a phase difference exceeding 250° in a wide band ranging 4.1–5.0 GHz. Therefore, the realized anisotropic programmable metasurface can be used to realize the 2-bit phase coding. From the simulation amplitude curves in Fig. 2(b), we see that the reflection loss is greater at low frequencies. Therefore, we also measured the reflection amplitudes of the metasurface sample at several low frequencies, as shown in Fig. 2(g). It is clear that the measured minimum reflection amplitude is greater than −2.62dB at 4.1 GHz.

To demonstrate the anisotropic characteristic of the 2-bit programmable metasurface, beam scanning and dynamic vortex beam generation are realized for x- and y-polarized incidences, respectively. In function design, we first choose four different capacitances to realize the 2-bit coding elements of “00”, “01”, “10”, and “11” with a phase difference of 90° between adjacent elements. Since the reflection phase of the meta-element varies with frequencies, the four capacitances for 2-bit coding are different at several operating frequencies. For example, the four capacitances of 0.914, 0.604, 0.475, and 0.171 pF are selected to realize the 2-bit phase coding elements at 4.5 GHz. For beam scanning, we design four different coding patterns for achieving the different pointing angles under x-polarized wave incidence, as shown in Fig. 3. Figures 3(a)–3(d) display the coding patterns and the corresponding simulated three-dimensional (3D) beams at 4.5 GHz. We see clearly that the beam scanning angles are pointed at −10°, −20°, −30°, and −40°, respectively, under these four 2-bit coding patterns. Moreover, to verify the broadband capability of the metasurface, the beam scanning performance at 4.0 and 5.0 GHz is also simulated, and the results are illustrated in Figs. 3(e) and 3(f), respectively. The metasurface can also realize good beam scanning at 4.0 and 5.0 GHz, showing a broadband beam scanning feature.

Different from the beam scanning, vortex beam generations are realized based on the anisotropic programmable metasurface under y-polarized wave incidence. In the simulation, the feed antenna was located 150 mm away from the metasurface. The scanning surface was an 800mm×800mm plane, positioned 500 mm from the metasurface. Figure 4 presents the simulation results of the generated vortex beams carrying orbital angular momentum (OAM) with different modes of l=±1 at 4.0, 4.5, and 5.0 GHz. At the three frequencies, the electric-field amplitude distributions for l=±1 modes are in doughnut-shaped patterns, featuring a toroidal region and null at its center. In the phase distribution of the +1st order OAM electric field, a 360° phase shift is observed in a counterclockwise direction. Conversely, for the −1st order OAM, the phase shift of 360° is evident moving in a clockwise direction. Both the amplitude and phase distributions prove that the metasurface can generate good vortex waves of different orders in the wideband frequency range.

As verifications, the far-field and near-field experiments were carried out in a standard microwave anechoic chamber to measure the real-time beam scanning and vortex beam generation, respectively. Figure 5(a) shows the far-field measuring environment, in which the anisotropic programmable metasurface sample and the feeding antenna were placed on a supporting board fixed on an automatic antenna turntable. The receiving antenna is an ultra-wideband dual-polarized horn antenna. The measured far-field reflected beams with different deflection angles of the sample under x-polarized incidence at 4.5 GHz are shown in Fig. 5(b). We observe that the anisotropic programmable metasurface shows good beam scanning characteristics, and the beam deflections are ranging from −10° to −40° by changing the coding patterns, which agrees well with the simulated results. For near-field OAM beam measurement, the y-polarized feeding antenna was fixed in front of the metasurface sample and the distance was 150 mm; the probe scanning plane was 500 mm away from the sample, and the size of the scanning plane was 640mm×640mm, as shown in Fig. 5(c). During the measurement, the waveguide acts as a scanning probe to record the amplitude and phase of the reflected electric field at 4.5 GHz, and the measured results are shown in Fig. 5(d). It is obvious that there are ring-shaped amplitude distributions and two reverse spiral-like phase distributions on the monitoring plane, which shows the vortex beams carrying OAM with modes of l=1 and −1, respectively.

To summarize, we have demonstrated an anisotropic programmable metasurface that can manipulate independently dual-polarized EM waves in real time. Moreover, based on the designed scalable multi-channel control circuit, each meta-element can be controlled independently to realize 2-bit phase modulation. We fabricated an anisotropic programmable metasurface prototype with 21×20 meta-elements. The measured results verify that the metasurface prototype can provide phase differences exceeding 250° in a wide band ranging from 4.1 to 5.0 GHz. Based on this anisotropic programmable metasurface, the two different functions of real-time beam scanning and vortex beam generation are realized for two different linearly polarized waves, respectively. The numerical results agree well with the experimental ones, indicating the dual-channel anisotropic programmability of the metasurface, which could enrich the applications of metasurfaces.