Metasurfaces, as 2D planar structures of electromagnetic (EM) metamaterials, possess the unique ability to independently or collectively control the polarization, amplitude, and phase of EM waves solely through discontinuities in the cross-sectional field values [1]. In general, metasurfaces can be categorized into two main types, i.e., transmission and reflection, which offer advantages of low profile and easy integration and have been widely applied to realize many fascinating physical phenomena, such as metasurface holograms [2,3], abnormal reflection and refraction [4,5], and novel manipulation of polarization states [6–9]. In recent years, there has been growing focus on metasurfaces designed to operate in the full space. Unlike those limited to reflection or transmission modes, full-space metasurfaces offer enhanced spatial EM manipulation capabilities, expanding their potential applications, such as omnidirectional Janus metasurfaces [10–12] and full-space multifunctional metasurfaces for frequency or polarization multiplexing [13–16]. However, the majority of proposed full-space metasurfaces are passive, and their functionalities cannot be changed once they are manufactured.

In 2014, digital coding and programmable metasurfaces were proposed to provide an effective method for real-time manipulation of EM waves, bringing a link between the physical world and digital world [17], which has been widely used in antennas [18,19], imaging systems [20–22], wireless communication systems [23,24], and reconfigurable intelligent surfaces (RISs) [25–27]. To realize a full-space programmable digital coding metasurface, a type of metasurface that can switch between transmission and reflection modes has been proposed [28,29], but it cannot simultaneously manipulate the wavefronts of reflected and transmitted waves. In order to manipulate reflected and transmitted waves independently at the same time, the metasurfaces were divided into two regions by interleaved arrangement of the meta-atoms, that is, part of the meta-atoms work in the reflection mode, while the other part works in the transmission mode [30,31]. However, due to the interleaved arrangement of the meta-atoms, only half of metasurface aperture can be used for the manipulation of reflected and transmitted waves, respectively. In 2021, a full-space programmable metasurface that can simultaneously and independently manipulate the wavefronts of x-polarized waves in the reflection space and y-polarized waves in the transmission space was proposed [32].

Compared with the linearly polarized (LP) EM waves mentioned above, circularly polarized (CP) waves have better transmission performance and can overcome the spatial multipath effect, which plays an important role in the optical region and microwave frequency band. The Pancharatnam–Berry (PB) structures with geometric phase response are usually used to manipulate the CP waves [33]; however, the geometric phase responses of left-handed polarized (LCP) and right-handed circularly polarized (RCP) waves are opposite and inherently coupled with each other. By introducing the propagation phase and combining the geometric phase, the phase responses of LCP and RCP waves can be completely decoupled, thus achieving independent control of LCP and RCP waves [34,35]. However, since geometric phase control requires unit rotation and propagation phase control requires unit size adjustment, this type of metasurface makes it difficult to implement independent reconfigurable or programmable control of LCP and RCP waves. In Ref. [36], a reconfigurable spin-locked metasurface was proposed by mechanically rotating the meta-atom to realize retroreflection over a wide continuous angle range; however, the control of LCP and RCP waves cannot be decoupled. On the other hand, the chirality-assisted meta-atoms were proposed to realize spin-decoupled phase control [37,38], which enables the possibility of independent reconfigurable control of LCP and RCP waves in reflection spaces [39]. In order to realize independent control of CP waves in full space simultaneously, the metasurfaces based on receiver–transmitter integrated unit structures were proposed, which have been widely used in antennas [40,41], chip-integrated CP detectors [42,43], and full-space energy distributor [44,45] meta-devices design. However, it is still a big challenge to implement independent reconfigurable control of reflected and transmitted CP waves simultaneously based on a single metasurface.

Here, we propose a full-space programmable metasurface that can independently control the reflected and transmitted CP waves in real time. The polarization states of reflected and transmitted waves can be arbitrarily customized by precise design of meta-atoms, while the reflection and transmission phases can be independently controlled in real time by adjusting the bias voltages of PIN diodes loaded on meta-atoms, thus realizing the real-time independent control of transmitted and reflected waves. As an example, the full-space programmable CP metasurface that can achieve copolarized reflection for RCP incident waves and cross-polarized transmission for LCP incident waves was designed, fabricated, and measured. The results show that the reflected and transmitted waves can be controlled to real-time scan in a large angle range through independently adjusting the reflection and transmission phase coding patterns of a metasurface. In addition, a space-multiplexing wireless communication scheme was established based on the proposed full-space programmable CP metasurface, which successfully delivered two independent images to the receivers in reflection and transmission spaces, respectively. This work provides a feasible solution for real-time manipulation of full-space EM waves based on programmable metasurfaces, which is promising to be applied in future new generation antennas, radars, wireless communications, and so on.

Figure 1 shows a schematic diagram of the proposed full-space programmable CP metasurface, which can achieve independent manipulation of CP waves in the transmission and reflection spaces simultaneously. In this design, the metasurface can realize copolarized reflection for RCP incident waves and cross-polarized transmission for LCP incident waves. In addition, the wavefronts of the reflected and transmitted waves can be further independently tailored by reprogramming the reflection and transmission phase coding patterns of metasurface, respectively.

The meta-atom of metasurface is a receiver–transmitter integrated structure, as illustrated in Fig. 2(a), which consists of six metallic layers separated by four dielectric substrate layers of F4B (gray layers) with a relative permittivity εr=3.5 and a loss tangent tanδ=0.001. Four dielectric substrate layers are glued together through Rogers 4350F layers (pink layers), the relative permittivity is εr=3.52, and the loss tangent is tanδ=0.004. The thicknesses of the dielectric substrate layers and the Rogers layers are as follows: h1=3mm, h2=0.1mm, h3=0.254mm, h4=0.2mm, and h5=1.5mm. Top and bottom layers are soldered with PIN diodes (M/A-COM MADP-000907-14020x), which are equivalent to a series of a resistor R=7.8Ω and an inductor L=30pH in the ON state and a series of a capacitor C=28fF and an inductor L=30pH in the OFF state. The specific views of the six metallic layers are shown in Figs. 2(b)–2(g). Layer 1 takes the form of an arc-shaped patch, serving as the receiver patch, with inner radius r1=2.12mm, outer radius r2=5.2mm, and an opening angle of α=40°. Additionally, a gap with a width of g=1mm is etched on the arc-shaped patch for soldering the reflected PIN diode (PIN 1). The position of the gap is indicated in polar coordinates, specifically at β=35°. Layer 2 is a ground plane with two circular holes etched at the positions of metalized vias 1 and 2, where the diameters of two circles are 1.2 and 0.7 mm, respectively. Vias 1 and 2 pass through these two circles, while the other two metalized vias, vias 3 and 4, are directly connected to the ground plane, where the diameter of via 1 is 0.6 mm, and vias 2, 3, and 4 are all 0.35 mm. The azimuth angles corresponding to vias 1 and 2 are θ1=−116° and θ2=−64°, respectively. The arc-shaped receiver patch is connected to metallic layer 2 through metalized vias 3 and 4. It is worth mentioning that via 3 and via 4 play a crucial role in the performance of the meta-atom. On the one hand, they connect the negative terminal of PIN 1, the positive terminal of PIN 2, and the negative terminal of PIN 3 to the common DC ground (layer 2). On the other hand, the design of the two vias (via 3 and via 4) can keep the stability of the transmission or reflection amplitude during the phase modulation. Via 3 is located on the upper part of the receiver patch (90°<θ3<180°), while via 4 is positioned on the lower part of the receiver patch (180°<θ4<270°). In practice, via 4 has a significant impact on the reflected amplitude, whereas via 3 is primarily to keep the stability of the transmitted or reflected amplitude during the phase modulation. By optimizing the trade-off between transmission/reflection amplitude and stability, θ3 and θ4 were ultimately optimized to 135° and 210°, respectively. The schematic diagrams of bias lines 1 and 2 are depicted in layers 3 and 4, where the fan-shaped branch structures (r3=3mm and θ5=50°) are designed at the vias to prevent induced currents from flowing into the bias lines. Layer 5 is another metallic ground plane, which can effectively block the signal radiation of the bias lines. Layer 6 adopts a square ring-shaped transmitter patch structure, which is directly connected to the arc-shaped receiver patch through metalized via 1. Two PIN diodes (PIN 2 and PIN 3) are soldered onto the square ring metal patch, with the polarity indicated in Fig. 2(g). It is worth mentioning that the bias lines 1 and 2 are connected to the arc-shaped receiver patch and the square ring metal patch through vias 2 and 5, respectively, to provide bias voltages for PIN diodes loaded on the receiver patch and transmitter patch. The period of the meta-atom is p=14mm, and the other parameters shown in the meta-atom are w1=4mm, w2=3.05mm, w3=6.1mm, and w4=1.5mm.

To further explore the potential physical mechanism of independent modulation of reflected and transmitted waves, we studied the surface current of the meta-atom under different CP normal incidence with phase delays of 0°, 90°, 180°, and 270°, as illustrated in Fig. 3. The receiver patch effectively concentrates surface currents on the right side of the arc-shaped patch when RCP waves are incident on the meta-atom, as depicted in Fig. 3(a). In this case, the surface currents are unable to reach via 1 located on the left side of the receiver patch, preventing current from being transmitted to the transmitter patch. As a result, the current intensity on the transmitter patch is low, as shown in Fig. 3(b). This causes total reflection of the RCP waves, with PIN 2 and PIN 3 having minimal impact on the reflected wave. Conversely, when LCP waves are incident on the meta-atom, surface currents concentrate on the left side of the receiver patch, as depicted in Fig. 3(c). Therefore, the surface currents can flow through via 1 toward the transmitter patch and radiate to the transmission space in the form of EM waves due to the match between the transmitter patch and the receiver patch, as shown in Fig. 3(d). It is important to note that the transmitted waves are minimally affected by PIN 1.

More importantly, when RCP waves are incident, the surface current on the receiver patch is concentrated near PIN 1. When the working state of PIN 1 changes, the induced current on the receiver patch can be reconfigured, as illustrated in Figs. 4(a) and 4(b), thus achieving the phase control of the reflected waves. Conversely, when the LCP waves are incident, the induced current flows toward the transmitter patch. The surface currents on the transmitter patch can be reversed by changing the working states of PIN 2 and PIN 3, as depicted in Figs. 4(c) and 4(d), implying that 180° phase shift of transmitted waves can be realized.

Figures 5(a)–5(d) show the simulated copolarized reflection amplitudes and phases (R_rr) of the meta-atom under the RCP incident waves when PIN 1, PIN 2, and PIN 3 are in different working states. The results show that the reflection amplitudes are always greater than 0.85 from 9.5 to 10.8 GHz whether PIN 1 is ON or OFF, as shown in Fig. 5(a), while the reflection phase shifts 180°, as shown in Fig. 5(b). Switching of the working states of PIN 2 and PIN 3 does not affect the reflection amplitude and phase of the RCP waves, as shown in Figs. 5(c) and 5(d). Figures 5(e)–5(h) show the simulated cross-polarization amplitudes and phases (Tlr) of the meta-atom under the LCP incident waves when PIN 1, PIN 2, and PIN 3 are in different working states. The results show that the transmission amplitudes are greater than 0.8 from 9.75 to 10.15 GHz when PIN 2/PIN 3 are ON/OFF and OFF/ON, as shown in Fig. 5(e), while the transmission phase shifts 180°, as shown in Fig. 5(f). Similarly, the switching of working state of PIN 1 also does not affect the transmission amplitude and phase responses of LCP incident waves, as shown in Figs. 5(g) and 5(h). Therefore, the proposed meta-atom can achieve completely independent real-time phase control for RCP copolarized reflected waves and LCP cross-polarized transmitted waves. In addition, it is worth mentioning that the polarization states of reflected and transmitted waves can be freely customized by simply mirroring the receiver patch and transmitter patch, respectively.

It should be noted that the transmission bandwidth is lower than the reflection bandwidth. This is because modulation of the reflection phase is achieved through the geometric phase structure, which inherently has a relatively wide operating bandwidth. In contrast, the transmitted waves are emitted by the rectangular patch, and, according to the antenna theory, the radiation bandwidth of a patch antenna is relatively narrow. In the future, magneto-electric dipole structures could be considered to expand the transmission bandwidth.

A full-space programmable metasurface consisting of 14×14 meta-atoms was designed to validate its robust capability in controlling reflected and transmitted EM waves. A standard rectangular waveguide is employed as quasispherical waves to feed the metasurface, which is placed 100 mm away from the metasurface. The LP waves with a quasispherical wavefront emitted by the waveguide can be decomposed into RCP and LCP waves with equal amplitude, which can be independently controlled by the metasurface in the reflection and transmission spaces, respectively. For simplicity, the reflection phases with 180° difference are encoded as binary digital codes 0 and 1, respectively, which are corresponding to the OFF and ON states of PIN 1, named as reflection code (R code). Similarly, the transmission phases with 180° difference are also encoded as binary digital codes 0 and 1, respectively, which are corresponding to the ON/OFF and OFF/ON states of PIN 2/PIN 3, respectively, named as transmission code (T code). Therefore, the reflected RCP waves under RCP incidence and transmitted RCP waves under LCP incidence can be independently controlled by changing the coding patterns of R code and T code in real time.

Based on the theory of coding metasurfaces, the phase distribution to reflect or transmit a single beam can be calculated [40]. Then, by applying the principle of field superposition, the required phase distributions required to generate multiple beams can be obtained: φmeta(m,n)=arg(∑b=1Tab(m,n)exp(jφb(m,n))),(1)in which the subscript b represents the number of individual beams, and φb represents the phase distribution required to generate the bth beam. Finally, by discretizing φmeta into 1 bit, the final coding pattern can be obtained.

In the first case, we set three sets of coding patterns to generate single beams in reflection and transmission spaces, as shown in Figs. 6(a)–6(c), where the reflection codes 0 and 1 are represented by white squares and black squares, respectively, while transmission codes are represented by yellow squares and blue squares, respectively. The corresponding 3D far-field radiation patterns are presented in Figs. 6(d)–6(f). To clarify the radiation directions of the reflected and transmitted beams, we define the θ range for the reflection space as −90° to 90°, and the θ range for the transmission space as 90° to 270°, which is the angle with respect to the +z axis, as indicated in Fig. 6(d). The reflected/transmitted RCP beams are arbitrarily designed and directed to θr=10°/θt=190°, θr=−20°/θt=180°, and θr=−30°/θt=210°, respectively, verifying the real-time independent programmable capability of the proposed metasurface in the reflection and transmission spaces. Except for the single beam, the metasurface can also generate dual beams in reflection or transmission spaces. In the second case, we have configured two sets of coding patterns for generating one/two and two/one beams in the reflection and transmission spaces, respectively, as shown in Figs. 6(g) and 6(h). The corresponding simulated 3D far-field radiation patterns are shown in Figs. 6(j) and 6(k). All the results show that the metasurface has good performance in arbitrary beamforming under the incidence of a quasispherical incident wave.

In addition, when the metasurface is illuminated by a plane wave, two symmetrical beams will be generated in reflection and transmission spaces due to the 1 bit coding. In the third case, the metasurface is illuminated by an LP plane wave, and its reflection and transmission coding patterns are configured as shown in Fig. 6(i), which are set to 01100110011001 and 00111110000111, respectively. The corresponding simulated 3D far-field radiation patterns are shown in Fig. 6(l). The result shows that the RCP component is reflected and generates two RCP beams directing to θr1=−32° and θr2=32°, while the LCP component is transmitted and generates two RCP beams directing to θt1=165° and θt2=195°, which agree with the theoretical expectations.

Figures 7(a)–7(c) show the measured and simulated far-field radiation patterns with single beam in reflection and transmission spaces in the yoz plane at 10 GHz, corresponding to the 3D far-field radiation patterns demonstrated in Figs. 6(d)–6(f). It can be seen that the radiation directions of the reflected beam and transmitted beam can be precisely and independently controlled by the reflection and transmission coding patterns in real time, and the measurements agree well with the simulations. Figure 7(d) shows the simulated and measured reflection and transmission far-field radiation patterns in the yoz plane at 10 GHz under different CP plane wave incidence, corresponding to the 3D far-field radiation pattern demonstrated in Fig. 6(l). The upper part of the polar plot displays the RCP reflected far-field radiation patterns, and the lower part of the polar plot displays the RCP transmitted far-field radiation patterns. The measured results also show a good agreement with the simulations, with two reflected beams pointing to θr1=−32° and θr2=32° and two transmitted beams pointing to θt1=165° and θt2=195° at 10 GHz. In addition, the beam scanning performance of a metasurface with 10° intervals was also measured in reflection and transmission spaces. Figure 7(e) shows the measured scanning far-field radiation patterns of a reflected RCP beam in the yoz plane at 10 GHz under LP quasi-spherical wave incidence when the transmission code is uniform and programmed to 1. To reconfigure the reflection coding pattern by controlling the bias voltage applied to PIN 1, the reflected beam can be controlled to continuously scan from −50° to 50° and maintain good performance within the scanning range. Figure 7(f) illustrates the measured scanning far-field radiation patterns of a transmitted beam in the yoz plane at 10 GHz when the reflection code is uniform and programmed to 1. The results also show that the transmitted beam can continuously scan within a 100° angle range (from 130° to 230°) and maintains good performance within the scanning range. Meanwhile, the measured results also show that the metasurface can operate well within the frequency bands of 9.6–10.4 GHz in reflection mode and 9.6–10 GHz in transmission mode (see Section 6.B for more details).

In order to show the potential application of the proposed metasurface, a space-multiplexing wireless communication scheme is established and experimentally demonstrated, as shown in Fig. 8. The metasurface serves as a full-space RIS, which can independently manipulate EM waves in reflection and transmission spaces, respectively. Therefore, two different modulated signals can be simultaneously controlled by the metasurface and independently sent to the target receivers in reflection and transmission spaces, such as two images here (Image 1 and Image 2), where Images 1 and 2 are delivered by reflection channel (RCP reflection under RCP incidence) and transmission channel (RCP transmission under LCP incidence), respectively. In this scheme, the incoming signals (Image 1 and Image 2) are carried by a plane wave; when they illuminate on the metasurface, Image 1 will be delivered to two receivers in reflection space through reflection channels, and Image 2 will be delivered to two receivers in transmission spaces through transmission channels, respectively. It should be noted that two channels in the reflection and transmission spaces are symmetrical with the normal direction of the metasurface due to the limitation of 1 bit design, which can be solved by developing high bit phase.

Without loss of generality, we set the directions of the reflection beams to offset the +z direction by ±15° with θr1=−15° and θr2=15° and the directions of the transmission beams to offset −z direction by ±32° with θt1=148° and θt2=212°, as shown in Fig. 9(a). In this situation, the corresponding reflection encoding sequence is 00111110000111, and the transmission encoding sequence is 01100110011001. The results indicate good performance of two reflection and two transmission channels accurately directing to the preset directions. Since the two reflected beams carry the same image information, we randomly selected the beams with θr1=−15° in reflection space and θt2=212° in transmission space for the wireless communication measurements.

During the wireless communication experiment, the quadrature phase shift keying (QPSK) modulation is used to modulate and demodulate the transmitted images. The experimental results are shown in Fig. 9(b). When the metasurface is working normally, that is, the bias voltages are applied to the PIN diodes loaded on metasurface, forming the reflection and transmission coding sequences of 00111110000111 and 01100110011001 at 10 GHz, respectively, two images can be successfully delivered to the receivers in the reflection and transmission spaces. However, when the metasurface is not working, that is, all the bias voltages applied to the metasurface are turned off, the incident EM waves cannot be reshaped and directed to the receivers in the preset directions. As a result, the EM energy received by the receiving horns is very low, leading to chaotic constellation diagrams and received signals; therefore, the images cannot be delivered, as shown in Fig. 9(c). In addition, it is worth mentioning that, since the proposed full-space metasurface is programmable, we can also reprogram the reflection and transmission coding sequences independently to adjust the channel directions as desired.

We proposed a full-space programmable CP metasurface that can independently manipulate the wavefronts of reflected and transmitted CP waves in real time. The meta-atoms of a metasurface are transmitter–receiver integrated structures loaded with PIN diodes. By controlling the working state of PIN diodes with bias voltages, the reflection and transmission phases of the metasurface can be controlled independently, thereby achieving the independent manipulations of the reflected and transmitted waves. As a practical application, a space-multiplexing wireless communication scheme was demonstrated based on the proposed full-space programmable CP metasurface for delivering two independent images to the target receivers located at preset locations in reflection and transmission spaces, respectively. The proposed full-space programmable CP metasurface effectively improves the capacity of a metasurface to control EM waves in different spatial regions in real-time and is promising to be applied in wireless communication systems, radar, holographic imaging, and other fields.

The metasurface consists of 14×14 meta-atoms (196mm×196mm) with a total size of 246.4mm×224mm (including the via area connected to the bias layer), as provided in Fig. 10(a). To realize the independent feeding of PIN 1 and PIN 2/PIN 3, we have carefully designed the bias networks in layers 3 and 4 of the metasurface, as shown in Figs. 10(b) and 10(c), where the bias lines in layers 3 and 4 are used to control PIN 1 and PIN 2/PIN 3, respectively. Each bias line is then fed separately using an FPGA to realize independent control of each meta-atom.

The top view (receiver patches) and bottom view (transmitter patches) of the fabricated metasurface are shown in Figs. 11(a) and 11(b). For measurement of the far-field radiation pattern, the metasurface is placed on a rotating platform in one side of a microwave anechoic chamber, and a standard X-band rectangular waveguide antenna is placed 100 mm away from the metasurface to serve as a point feeding source, generating incident quasi-spherical waves, as shown in Fig. 11(c). The measured far-field radiation pattern is shown in Figs. 11(d)–11(o). It is worth mentioning that, for plane wave incidence, the distance between the transmitting horn and the metasurface is set to 2.5 m. In another side of the microwave anechoic chamber, there is a standard CP horn antenna for receiving the far-field signal, which is 10 m way from the metasurface and is not displayed here.

The workflow diagram of one of the wireless communication channels is provided in Fig. 12(a). In the transmitting part, the image information to be transmitted is modulated by the universal software radio peripheral (USRP, transmitting at 1.5 GHz). The modulated signal is then upconverted to the working frequency of metasurface (10 GHz) through Mixer 1. Next, the EM waves carrying the information are transmitted to the metasurface using a feeding horn antenna. Finally, the metasurface directs EM waves to the receiving horn by beamforming. In the receiving part, the information received by the receiving horn antenna is amplified by a low-noise amplifier (LNA). Next, the signal will be downconverted to the working frequency band of the USRP by using Mixer 2. In order to isolate the transmitting and receiving signals from each other, the receiving frequency of the USRP is set to 1.6 GHz. Finally, the USRP demodulates the signal to recover the original image. The mentioned experimental equipment is marked one by one in Fig. 12(b). In the communication experiment, the metasurface serves as a full-space RIS; thus, the communication rate depends on the rate of the USRP (1 Mbps in this communication experiment). It should be noted that, for the measurement of each channel, two signal sources and two CP horn antennas are required. If we measure space-multiplexing wireless communications in reflection and transmission channels simultaneously, four signal sources and four CP horns will be required. Due to the limitation of experimental equipment, the wireless communications for the reflection channel and transmission channel are measured separately.