Metamaterials are artificially engineered electromagnetic (EM) materials achieved by structuring subwavelength metallic and/or dielectric composites (e.g., “meta-atoms”) with pre-designed EM responses[1–4]. The concept of artificial media was initially proposed to construct the so-called left-handed metamaterials with simultaneously negative permittivity and permeability for the purpose of achieving negative reflection[5–7] and super imaging[8–11]. Later, transformation optics was developed, and significant efforts have been devoted to the design of metamaterial-based devices for engineering EM spaces[12–16]. Metamaterials have shown unique prospects in the manipulation of EM waves that leads to a series of new types of metadevices[17–21]. However, many metamaterials realized so far have complex structure, large sizes, and relatively high losses, which hinder their practical use.
Metasurfaces[22–24], which are quasi-two-dimensional (2D) metamaterials consisting of planar arrays of subwavelength meta-atoms, have received increasing attention in recent years. Metasurfaces offer new opportunities for controlling the amplitude[25–28], phase[29–36], and polarization[37–39] of EM waves by fully exploiting the abrupt phase changes at the interface via engineering the interactions between EM waves and meta-atoms. Their operation principle is completely different from that of bulk metamaterials whose functionalities critically rely on wave propagation inside a sufficiently thick (bulk) medium. Owing to the planar configuration and deep subwavelength thickness, metasurfaces have a series of advantages over bulky metamaterials, such as low loss, ease of fabrication, and compatibility with the existing fabrication techniques[40–45].
While metasurfaces have been extensively studied and applied in many areas in microwave[46–48], terahertz (THz)[49–51], and optical devices and systems[51–53], a common restriction in passive metasurfaces is that each functionality typically requires a distinct design, making each metasurface suitable only for a specific task, significantly limiting its applications[54,55]. It is becoming increasingly interesting to make use of reconfigurable techniques for achieving tunable functionalities via incorporating active materials/elements into traditional passive metasurfaces. Under external stimuli, those active materials/elements could change their electrical properties and, in turn, affect the EM functionalities of metasurfaces, resulting in a dynamical tunability in metasurfaces. A great variety of active materials/elements are available for designing tunable metasurfaces, such as liquid crystal (LC)[56–58], 2D materials (i.e., graphene and )[59–65], phase change materials (PCMs)[66–69], active lumped elements[70–75], ferroelectric materials[76–78], water[79–81], and semiconductors[82,83]. Metasurfaces have different responses and can be classified into different tuning mechanisms, such as electrical[84,85], thermal, optical[87,88], and mechanical controls[89,90]. For example, indium tin oxide (ITO) can be stimulated electrically or optically, and graphene can be tuned by electrical or chemical doping. Among various tuning techniques, electrical tuning is of particular interest, since it enables a route for fine integration of metasurfaces with electronic devices/systems and potentially offers much faster switching speeds and more precise control than those in other tuning techniques.
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Until now, many good review articles have been published to summarize the developments of tunable metasurfaces[24,91–93]. However, those review articles only focused on some types of tunable metasurfaces, such as electrically reconfigurable microwave metasurfaces with active lumped elements. In this article, we will briefly review the key achievements in the fast developing field of electrically reconfigurable metasurfaces. Particularly, this review focuses on electrical tuning mechanisms and practical applications of active metasurfaces at microwave frequencies. We hope such a review can offer a brief guidance for new researchers in this research field. The rest of this article is organized as follows. In Section 2, we introduce reconfigurable metasurfaces based on LCs. Section 3 focuses on the latest progress of graphene-based metasurfaces and related metadevices. In Section 4, we discuss reconfigurable metasurfaces based on active lumped elements, which can actively control EM waves in a uniform manner. Section 5 describes reconfigurable metasurfaces based on other active materials, such as PCMs and ferroelectric materials. In Section 6, we summarize the recent efforts of programmable metasurfaces, that is, tunable and reconfigurable inhomogeneous metasurfaces with constitutional meta-atoms locally tuned by external stimuli. Section 7 reviews the development of time-modulated metasurfaces and Section 8 concludes the review, giving some perspectives on future developments and challenges in the field.
2. Reconfigurable Metasurfaces Based on Liquid Crystals
LCs are mesophase materials that flow like liquids, but the structural elements of which are packed in a crystalline-like manner. LCs can exist in different phases, such as smectics, nematics, and isotropics, featuring different properties and different order degrees. One of the most common phases of LCs is the nematic phase, where the rod-shaped LC molecules are orientationally ordered, but lack the long-range transnational order. Therefore, nematic LCs are inherently anisotropic, exhibiting different dielectric constants along and perpendicular to the long axis of the molecules. Importantly, external electric and magnetic fields can control the orientation of the LC molecules and thereby change the dielectric constant and refractive index[96,97]. This makes LCs, and nematics in particular, one of the most popular active materials achieving various functional devices, such as filters, phase shifters, delay lines, and antenna arrays[56–58].
By incorporating LCs into metasurfaces, one can create electrically tunable microwave devices through the application of an external bias voltage [Fig. 1(a)]. Various types of resonators are used to measure the dielectric properties of LCs at microwave frequencies, including patch resonators[104,105] and inductive coupled ring resonators. Apart from the characterization of LC materials, there are also works focused on the design of LC-based tunable resonators from the perspectives of applications. Yaghmaee et al. presented a tunable electric-LC (ELC) resonant metasurface, where voltage-controlled tunability is achieved by using LCs running through the central gaps, as shown in Fig. 1(b). The simulation results demonstrated a continuous frequency tuning range of 6% at around 4.5 GHz, opening up the opportunity for the design of frequency selective surfaces (FSSs) using LCs. In another experimentally validated example, a tunable subwavelength Fabry–Perot resonator consisting of metallic capacitive meshes filled with 130-µm-thick E3 nematic LC was proposed. It demonstrated a 2% frequency shift at the Ka band, as shown in Fig. 1(c). The ultrathin nematic LC layer significantly reduces the bias voltage to about 10 V and the response time to the subsecond range, showing more perspectives from the practical point of view. More recently, Wang et al. presented an example of a microwave tunable metasurface absorber based on a nematic LC, where triple absorbing frequency bands can be dynamically adjusted by changing the bias voltage.
Figure 1.(a) Working principle of voltage-induced nematic LC[
Another significant category of an LC-based metasurface is electrically tunable filters, including band-stop filters and bandpass filters. In nearly all cases, the original working frequencies of LC-based filters are determined by pattered metallic structures, and the tunability of frequency can be achieved by changing the dielectric constant of the LC. Based on this mechanism, a tunable FSS, which generates an electronically tunable bandpass filter response at 110–170 GHz, was proposed. A 3% frequency shift in the passband is achieved when a 10 V bias voltage is applied to a 130-µm-thick LC layer. The metallic capacitive meshes provide a distinct frequency dispersion of the transmittance and also deliver the bias voltage across the nematic LC layer. The nematic LC can be employed in fishnet metamaterial at lower frequencies around 9 GHz. Although the necessary bias voltage increases to 60 V, it provides a noticeable 340 MHz shift of the passband as well as a remarkable 30° phase shift of the transmitted wave. Another example of a microwave continuously tunable split ring resonator (SRR) bandpass filter based on nematic LCs is presented in Fig. 1(d), showing a larger passband frequency tuning range of 750 MHz with low tuning voltage. A more efficient tuning method at the X band has been achieved by employing short wire-pair metamaterials infiltrated by a nematic LC [see Fig. 1(e)]. Applying bias voltage from 0 to 100 V, the LC molecules experience a nearly 90° reorientation, resulting in a continuous and reversible frequency shift from 9.91 GHz down to 9.55 GHz in the magnetic resonance of the metamaterial. Besides, Ding et al. reported a new design of filtering a tunable LC phase shifter based on spoof surface plasmon polaritons, showing self-embedded fixed filtering performance and continuously tunable phase shifts.
Besides the considered examples, other types of microwave LC-based devices have been reported, such as gradient index lens, digital metamaterial, and beam-steering antenna[113,114]. For instance, Zhao et al. proposed an LC-based digital metasurface, where encoding is realized by biasing LCs to shift the phase of the incident wave. This digital metamaterial can realize beam steering or radar cross-section (RCS) reduction by setting properly the code patterns, which provides a very feasible solution for further applications.
3. Reconfigurable Metasurfaces Based on Graphene
Graphene, a 2D material with unprecedented properties, has become a widely used active material since its discovery in 2004. Compared to traditional materials such as silicon and GaAs, graphene demonstrates superior physical properties in its mono-atomic thickness, optical transparency, large thermal conductivity, and strong mechanical ductility[116,117]. Moreover, the Fermi energy of graphene can be shifted by bias voltage due to the low carrier density of states near the Dirac point, which allows the use of graphene for manipulating EM waves over a broad spectral range.
According to Kubo’s formula, the surface conductivity of graphene is given by , where is the electron charge, and are the Boltzmann constant and the reduced Planck constant, and and are the scattering rate and temperature. Then, graphene surface impedance can be calculated using . Although the resistance term () and reactance term () of the surface impedance in the visible and infrared regions can be tuned on a large scale, especially for the microwave region, these tunabilities are quite different. Increasing the graphene Fermi energy from zero to 0.5 eV leads to the dramatic decrease of without a noticeable change of at microwave frequencies, which means that the reactance term of the surface impedance can be ignored. Therefore, graphene behaves more like a tunable resistive film, so that large phase difference and uniform amplitude of the reflection response cannot be achieved simultaneously only by changing . Moreover, the experimental realization of large-area, high-quality, and periodically patterned graphene is still challenging due to the immature technique to synthesize, transfer, and control the graphene at a reasonable scale for the long wavelength region.
Until now, graphene has been studied in different types of tunable metasurfaces from THz to visible frequencies by taking advantage of its tunable sheet impedance[120–122], while only a few works have focused on the microwave region. Since graphene is highly resistive for microwaves, its straightforward application is the design of EM absorbers. In 2014, experimental realization of a transparent graphene-based tunable Salisbury screen and Jaumann absorbers operating at millimeter wavelengths was presented by Wu et al. [Fig. 2(a)], following the theoretical exploration on a graphene-based Salisbury screen operating at a THz frequency. The monolayered graphene is synthesized on the wafer utilizing the chemical vapor deposition method. After coating a layer of polymethylmethacrylate (PMMA) and etching the wafer, the graphene-PMMA film was transferred onto a quartz substrate. Furthermore, multilayers of graphene were also fabricated by stacking the graphene-PMMA film on the top of monolayer graphene and repeating the coat-and-etch step. In this way, the surface impedance can be reduced to 377 Ω for designing the Salisbury screen. This graphene-based Salisbury screen absorbs 95% of the incident EM waves at a fundamental frequency near 29.6 GHz. Moreover, in order to enhance its bandwidth, a few graphene/quartz slices were also stacked layer by layer to realize the Jaumann absorber, which demonstrates effective absorption from 125 to 165 GHz.
Figure 2.(a) Multiple transfer-etch processing for a two-layer device[
Recently, it was demonstrated that a sandwich graphene structure provides a more efficient way to electrically control graphene over the traditional dielectric structure. The sandwich graphene structure is made of ionic liquid sandwiched between two graphene layers. The thickness of the sandwich graphene structure is usually about 200 µm, which is negligible compared to the microwave wavelengths. When a bias voltage as low as 3.5 V is applied, it induces a homogeneous electric field between the top and the bottom graphene layers. The ions inside the liquid tend to accumulate on the anode/cathode graphene, creating regions of high ionic concentrations in the vicinity of the graphene layers. Thus, the graphene layers on both sides are doped, and their surface impedance can vary significantly when a small bias voltage as low as 3.5 V is applied. Zhang et al. experimentally demonstrated the operation of an electrically tunable, broadband coherent perfect absorption at microwave frequencies by harnessing the coherent absorption features of a graphene-electrolyte-graphene sandwich structure, as shown in Fig. 2(d). A reasonably good agreement between the experimental and simulated results confirms that the microwave coherent absorptivity of the sandwich graphene structure can be tuned dynamically from nearly 50% to 100% by applying a bias voltage. The real electrically tunable Salisbury screen was proposed by Balci et al. in 2015. By replacing the traditional resistive film with a sandwich graphene structure, this graphene Salisbury screen shows its electrically tunable reflection from to at a single frequency of 10.5 GHz [see Figs. 2(c) and 2(e)]. Most recently, a low profile dynamically tunable microwave absorber was experimentally demonstrated [Fig. 2(g)]. By combining the sandwiched graphene structure with a metallic high impedance surface, tunable reflection was achieved from to at an operating center frequency of 11.2 GHz. Compared to previous works, the entire thickness of this absorber was only 2.8 mm, nearly one tenth of the working wavelength, which promotes the actual applications of graphene-based metasurfaces at microwave frequencies. Moreover, amplitude modulation also can be realized in transmission metasurfaces. Zhang et al. experimentally demonstrated dynamic control of microwaves in the classical metasurface analog of electromagnetically induced transparency (EIT) using graphene. As shown in Fig. 2(h), the EIT effect is resonated by coupling between static metallic resonators, and the electrically tunable graphene is implemented onto those that convert the static coupled resonators into active ones. By applying bias voltage to graphene layers, the amplitude value of the EIT peak can be modulated from 0.4 to more than 0.6, which represents an effective approach for sensitive sensors and communication systems.
In order to reduce the thickness and expand the working bandwidth, a large amount of research on graphene metasurfaces has been conducted in the past few years[61–63]. These designs employ metal or dielectric resonators with patterned graphene to realize the tuning function, such as reconfigurable bandwidth and switchable amplitude. Figure 2(f) shows a multilayer absorber configuration, which contains five layers of patterned graphene patches separated by 50-nm-thick films. Simulated results demonstrate that the magnitude of the reflection coefficient varies from nearly 0 to 0.8 for different applied bias voltages at around 10 GHz.
Xu et al. theoretically designed a tunable metasurface absorber by inserting cross-shaped graphene into an FSS structure. Both narrowband and broadband tunable absorption can be achieved with the same structure by tuning the conductivity of graphene. It should be noted that the bias voltage applied to electrostatically dope the graphene is quite large, which is impractical for experiment. Furthermore, the unit cells of the patterned graphene in many theoretical works are separated from each other, which makes those designs unavailable for electrical tuning. The method of fabricating patterned graphene and the effects of interconnecting wires for the implementation of bias voltage have not been considered, which makes it difficult to realize a tunable performance in practice.
In 2017, Yi et al. experimentally demonstrated a mechanically reconfigurable absorber with a square-patterned graphene on a polyethylene terephthalate (PET) substrate. Although this absorber lacks dynamic tunability, it exhibits tunable resonant frequency from 12.5 to 13.3 GHz and from 12.3 to 13.5 GHz by utilizing graphene with different or stacking the graphene/PET layers. Chen et al. proposed a microwave metasurface absorber with reconfigurable bandwidth using a multilayer graphene-based FSS. Different working bandwidths can be obtained by using multilayered graphene with different sheet resistances, which is realized by simply changing its growth temperature. Later on, based on a similar method, they also investigated microwave beam reconfiguration based on graphene ribbons. Although an excellent agreement in these works has been achieved between the measurement and simulation results, none of their samples exhibit a truly electrically reconfigurable performance that has been predicted by theoretical analyses. Rencently, Zhang et al. experimentally proposed an optically transparent absorbing metasurface with tunable absorption bandwidth. As shown Fig. 2(i), such an absorbing metasurface is made of patterned graphene layers on a PET substrate backed by an ITO film. By controlling the bias voltage applied to graphene layers, the absorbing band can be dynamically and continuously shifted among the single band (13 GHz), dual band (7 and 17.6 GHz), and wide band (7–18 GHz). Moreover, another scheme of tunable metasurfaces based on a combination of graphene with varactor-loaded active resonators is shown in Fig. 2(j). Such an electrically dual-tunable metasurface is driven by two independent bias voltages that enable continuous frequency tuning from 3.4 to 4.55 GHz and resonant reflection from to simultaneously. This work enriches the multifunctional microwave metasurface and creates great possibilities for graphene-based devices in EM stealth technology.
4. Reconfigurable Metasurfaces Based on Active Lumped Elements
A p-i-n (PIN) diode is a typical switchable lumped element, which has two states, ‘ON’ and ‘OFF’. Under zero or reverse bias, the PIN diode acts as a capacitance, which blocks the electric signals (this is the ‘OFF’ state). Under the forward bias, a typical PIN diode behaves as a low resistance, making it a good conductor for radio frequency (RF) signals (this is ‘ON’ state). The PIN-based metasurfaces can realize real-time control. The incorporation of PIN diodes enables the metasurfaces with more versatile functions than conventional passive metasurfaces, particularly in the gigahertz (GHz) band.
In 2012, Zhang et al. proposed a tunable circuit analog absorber, as shown in Fig. 3(a). By employing PIN diodes to control the RF signal path, different effective thicknesses can be realized, which result in absorbance frequency switching between 0.85–1.88 GHz and 2.66–5.23 GHz. Xu et al. demonstrated a tunable metamaterial absorber, the reflection responses of which can be adjusted over the entire 2–18 GHz range by changing the bias voltage of the active PIN diode array loaded between the combination shaped metal film.
Figure 3.Reconfigurable metasurfaces with active lumped elements. (a) Design of switchable microwave absorber[
Recently, switchable perfect absorbers and reflectors using a PIN diode integrated active FSS (AFSS) have attracted enormous interest[70,71,137]. For example, Zhao et al. proposed a metasurface that can be electronically switched between the reflection and absorption modes by a PIN diode. Such an AFSS absorbs almost all incident power at 3.5 GHz for the ‘ON’ state, while showing total reflection for the ‘OFF’ state. Furthermore, an equivalent circuit model was developed to characterize the AFSS’s switchable EM performance. Similarly, transmissive or absorptive AFSSs have also been reported using switchable PIN diodes. Very recently, Tao et al. proposed an active metasurface to achieve reconfigurable wave transmissions and reflections with cross-polarization conversions [Fig. 3(b)]. Li et al. demonstrated a multi-functional reconfigurable polarization conversion metasurface for controlling both the propagation and polarization of EM waves, as shown in Fig. 3(c). By controlling the biasing, it can switch its function between the reflection-type converter and the transmission-type converter. Most recently, Song et al. reported dynamical switching between nearly complete reflection, transmission, and absorption in a PIN-diode-based active metasurface.
Varactor, a different electrically sensitive material, is another excellent candidate to help pixel reconfigurable metasurfaces in microwave regimes, thanks to its variable capacitance with more flexibility than PIN diodes. Metasurface meta-atoms are integrated with one varactor diode each to manipulate the EM response. Varactor diodes can be adjusted in a continuous way. In the simplest scenario, all varactors are controlled by the same voltage, which effectively gives frequency tunability to the functionality the metasurface is designed for[72,73,140–145].
The most widely investigated functionality is tunable perfect absorption, where, by regulating the reverse bias voltage on the varactor diode, the absorption frequency of the designed unit can be controlled continuously[72,140–143]. In 2015, Zhu et al. proposed tunable microwave metamaterial absorbers using varactor-loaded split loops and experimentally demonstrated its flexible frequency tunability of the microwave reflection in the range of 5–6 GHz. When the varactors are loaded on the splits, this means that a capacitor is connected in parallel, which increases the total capacitance in the circuit and lowers the resonant frequency significantly. For instance, in 2016, Luo and coworkers experimentally demonstrated an electrically tunable metasurface absorber in the GHz regime based on dissipating behavior of embedded varactors [Fig. 3(d)]. Owing to the varactor diodes and biasing circuits embedded in the lattice structure, the absorption rate can be tuned over a broad range by varying the direct current biasing voltage. In addition, employing varactors on metasurfaces enables tunable frequency and phase properties.
In one case, the unit cell of the metasurface is a multiple resonance structure with two resonance poles and one resonance zero, capable of providing a 360° reflection phase variation, active tuning within a finite frequency band, and linear reflection phase tuning [see Fig. 3(e)]. Zhu et al. demonstrated that the EM reflections for orthogonally polarized incident waves can be tuned independently by adjusting the bias voltages on the corresponding diodes. Owing to this feature, the reflected EM waves can be electrically controlled to a linear polarization with a continuously tunable azimuth angle from 0° to 90° at the resonant frequency or an elliptical polarization with tunable azimuth angle of the major axis when off the resonant frequency.
5. Reconfigurable Metasurfaces Based on Other Active Materials
PCMs are solid materials that are characterized by a unique combination of properties. The most distinctive property of PCMs is that electronic excitation or laser pulses can rapidly switch them from a disordered-amorphous state to an ordered-crystalline state, possessing significantly different optical and EM properties[147–149]. As one type of nonvolatile PCMs, chalcogenide PCMs are a category of amorphous alloys, including germanium (Ge), antimony (Sb), and tellurium (Te), which provide an adaptable structure for tunable and reconfigurable metasurfaces[150–152]. PCMs have been widely introduced into the tunable configurations for the realization of nanoantennas, modulators, and gradient metasurfaces. In 2020, Mou and coworkers experimentally demonstrated a broadband tunable metamaterial absorber. The proposed metadevice, which is made of a PCM (GST), exhibits high absorptivity () within a broad wavelength band (480–1020 nm).
The tuning of the dielectric constant can also be realized at microwave frequencies using ferroelectric materials. Ferroelectric materials are crystal structures that lack a center of symmetry, resulting in a spontaneous electric polarization that can be reversed by an external electric field[153,154]. Most common ferroelectric materials used to design tunable metasurfaces are yttrium iron garnet (YIG) and barium strontium titanate (BST). A tunable metamaterial at microwave frequencies is demonstrated by introducing YIG rods into a periodic array of SRRs. When applying a magnetic field bias, the permeability of YIG rods can be tuned from negative to positive, causing continuous and reversible frequency shifts in response of the metamaterial. Similarly, a tunable magnetic metamaterial based on BST thin film capacitor load with metallic split rings was proposed for enabling a 140 MHz tunable slab with center frequency of 1.75 GHz through a voltage bias. In addition, tunable metamaterial resonators based on BST films have also been validated experimentally.
6. Programmable Metasurfaces
We have summarized the progress in the design of global reconfigurable metasurfaces, whose meta-atoms are tuned by external stimuli in a uniform manner. On the other hand, great concerns with pixel reconfigurable metasurfaces have risen from researchers in the fields of materials and electronics. On account of the possibilities to independently control each meta-atom by the tunable element embedded, one can assemble these meta-atoms to form a pixel reconfigurable metasurface with amplitude or phase profiles, that is, a programmable metasurface. The key point is to combine proper active stimuli with each unit cell. Obviously, some external controllers, like an LED for adjusting light intensity or a heater for controlling temperature, are not suitable for pixel metasurfaces due to the relatively large size as compared to the unit cell. It seems more suitable to construct programmable metasurfaces with meta-atoms incorporating voltage-driven elements such as varactor or PIN diodes, so that their EM characteristics can be dramatically tuned through varying the applied voltage. By accurately determining the voltages applied to different meta-atoms, the metadevices can realize the desired wave-manipulation dynamic functions. To some extent, a more exciting progress has been made using such a scheme compared with global reconfigurable metasurfaces.
Programmable metasurfaces can be enabled by incorporating PIN diodes in the structure design. When the locally applied voltage is different for each unit cell of the metasurface, it enables us to achieve a range of tunable applications, such as tunable reflection (steering)[156–158], multi-beam generation, beam diffusion[158,159], beam focusing, and holography.
Recently, Cui et al. presented the concept of programmable coding metasurfaces to dynamically manipulate incident radiation in the microwave regime[156,157,160]. Controlling the bias voltage applied across the PIN diode loaded on the top of a meta-atom can yield two different reflection phases, i.e., 0 and to mimic “0” and “1” states, respectively. Therefore, by reprogramming the bias voltages applied to such meta-atoms using field-programmable gate arrays (FPGAs) directly, one can control the phase distribution encoded on the entire metadevice. Real-time controllable digital beam steering[156,157] and dynamically switched holographic images were realized [Fig. 4(a)]. In 2020, Bai et al. presented experimetally a transmissive-type programmable metasurface with high efficiency. They further extended the operation bands of programmable metasurfaces from microwave to millimeter-wave frequencies. Although these digital reconfigurable metasurfaces can realize various functions, their beam manipulation capability is very limited due to the use of only 1 bit digital coding. Compared with them, the beam manipulation capability of a 2 bit digital metasurface with two PIN diodes loaded on meta-atoms is obviously enhanced.
Figure 4.Programmable metasurfaces. (a) EM programmable coding metasurface holograms[
In 2017, Huang et al. proposed a design of a 2 bit digitally controlled coding metasurface that can achieve beam deflection, multi-beam, and beam diffusion. The dynamical switching of these different scattering patterns was completed by a programmable electric source. Moreover, a few recent works showed the possibility of loading switchable PIN diodes on a Pancharatnam–Berry (PB) metasurface and a water-based metasurface. A tunable PB metasurface with frequency reconfigurability was designed. By controlling the external voltages applied to the diodes, the operation band with a 180° phase difference between orthogonal reflection coefficients can be dynamically controlled. As such, the resulting PB metasurface composed of these orderly rotated meta-atoms exhibits a broadband photonic spin Hall effect with nearly 100% conversion efficiency in the ON state and switching to dual well-separated bands in the OFF state. As for reconfigurable water-based metasurface integrated with PIN diodes, the wavefront reflected by the metasurface can be modulated by both the degree of salinity and the diode pattern. With these two manipulating methods, the metasurface can not only control the amplitude of the scattered beams but also the beam deflection angles, which promises a more flexible and economical way to manipulate the wavefront [Fig. 4(b)]. The PIN-based metasurfaces face the challenges of narrow band width, lower efficiency than passive metasurface, and complex processing technology in industrial, scientific, and medical (ISM) applications, which is also the reason why there are few relevant applications of electrically reconfigurable microwave metasurfaces since they were proposed for so long.
Moving one step towards more elaborate functionality, the locally applied continuous tuning voltage is allowed to be different for each unit cell of the metasurface by loading varactors. In 2003, Sievenpiper et al. proposed an electrically steerable reflector based on a resonant textured surface loaded with varactor diodes in the microwave regime [Fig. 4(c)]. By programming the reflective phase gradient on the device, the authors demonstrated 2D beam steering over a range of for both polarizations. Inspired by this work, many other metasurfaces were proposed to achieve diversified functionalities in microwaves[165–167], such as tunable frequency and phase properties[168,169], beam deflection[166,167], beam scanning, beam focusing[166,170,171], holography, and predesigned scattering field generation[172,173]. Xu et al. showed that a tunable gradient metasurface exhibits single-mode high-efficiency operation within a wide frequency band, while its passive counterpart only works at a single frequency but exhibits deteriorated performance at other frequencies. Second, the metasurface can be dynamically switched from a specular reflector to a surface-wave convertor.
Ratni et al. experimentally demonstrated an active metasurface for reconfigurable reflectors, which can produce anomalous reflection properties within a broad frequency range and scan the direction of the reflected beam within an angular range. In addition, this kind of reconfigurable planar metasurface is of frequency agility and can realize beam focusing. The active metadevices mentioned above are all working in reflection geometry, which are relatively easy to realize but are sometimes unfavorable for certain applications. To realize tunable or reconfigurable metadevices in the transmission mode, one needs to precisely control both phase and amplitude of the locally transmitted wave through each meta-atom, which is quite challenging.
So far, reconfigurable pixel metasurfaces in the transmission mode have been available in some areas, such as vortex beam generation and dynamical focusing. In 2017, Chen et al. experimentally demonstrated a design of writing a letter by dynamically changing the focal point with a tunable microwave Huygens’ metasurface [Fig. 4(d)]. Such an active metadevice was constructed by a 2D array of composite meta-atoms exhibiting both electric and magnetic responses; inside each meta-atom a voltage-controlled varactor was embedded. Again, by varying the voltages applied across these varactors, the distribution of transmission phase on the metasurface can be precisely and dynamically controlled, yielding a dynamical manipulation on the field pattern generated at the transmission side. Recently, an active metasurface for three orders of vortex beams (, , ) generation was designed. The transmission phase of each metasurface unit cell can be fully controlled by the capacitance of the varactor diodes loaded on the units. Different modes of vortex beams can be separately generated by simply changing the bias voltage applied to the varactor diodes on the elements, which has potential for super resolution imaging [Fig. 4(e)].
The combination of multiple tunable elements (e.g., PIN diodes or varactors) provides the possibility to simultaneously achieve multiple EM functionalities and real-time reconfigurability in a single design[175,176]. For example, Wu et al. presented and experimentally characterized a microwave active absorber that has dual ability of simultaneous but dividable modulation on absorbing frequency and intensity. The study of Huang et al. reports a reconfigurable metasurface for multifunctional control of EM waves. By controlling tunable elements, the proposed metasurface can dynamically change its local phase distribution to generate predetermined EM responses. Here, the metasurface can generate beam-splitting performance that can be used to reduce backward scattering waves, and its scattering reduction frequency is tunable. In addition, such a metasurface can also achieve dynamical beam deflection and polarization transformation through reconfigurable design of the phase distribution. Furthermore, active amplifiers provide another option for continuously tunable pixel reconfigurable metasurfaces. A spatial-energy digital-coding metasurface with active amplifiers is proposed to realize arbitrary editing of the energy of spatial propagating waves in the microwave frequency range. Based on the proposed metasurface, the spatial energy of a linearly polarized propagating wave can be amplified and edited into arbitrary magnitudes by controlling the voltage.
7. Time-Modulated Metasurfaces
Meanwhile, time-modulated metasurfaces have attracted great interest. In previous works, the properties of active metasurfaces could be switched to different states. However, in different states, their physical properties remained stationary. As a kind of material that can dynamically manipulate physical properties, time-modulated metasurfaces have begun attracting more and more attention in recent years[178–181]. This is because time-modulated metasurfaces show unique characteristics that time-invariant metasurfaces do not have and allow the exploration of new dimensions of longitudinal wave propagation.
In 2018, Dai et al. designed and experimentally characterized a reflective time-domain digital coding metasurface. The magnitudes and phases of harmonic components can be accurately and separately engineered [Fig. 5(a)]. Figure 5(b) demonstrates the combination of a nonlinear convolution theorem and a time-domain digital coding metasurface, which is able to achieve flexible and continuous harmonic wavefront control. Bui et al. presented a nonreciprocal time-varying metasurface shown in Fig. 5(c). Through two examples of subwavelength waveguides operating at 13 and 13.5 MHz, it is proven that the reconfigurable metasurface can realize an active switching control of field-localized waveguides. An anisotropic time-domain digital coding metasurface was also designed and fabricated [Fig. 5(d)]. It not only realizes linear and nonlinear polarization synthesis, but also can realize programmable control in real-time. Most recently, Wang et al. proposed and experimentally verified a time-modulated transparent nonlinear active metasurface loaded with varactors. By applying a bias voltage to the time-modulated sequence, frequency mixing can be realized while suppressing unnecessary higher-order harmonics [Fig. 5(e)]. The above applications show that it is meaningful to use the time dimension by using the dynamic switching of the coding sequence. Time-modulated metasurfaces are expected to have important applications in new wireless communications, radar systems, and other fields.
Figure 5.Time-modulated metasurfaces. (a) Reflective time-domain digital coding metasurface with independent control of the harmonic amplitude and phase[
8. Conclusion and Outlook
In summary, we have briefly reviewed the recent progress of electrically reconfigurable microwave metasurfaces. Compared with passive microwave metasurfaces, the flexibility of electrically reconfigurable metasurfaces in EM wave manipulation has been significantly enhanced, and the functionality richness of reconfigurable metadevices has also been greatly increased. At present, researchers proposed a variety of electrically reconfigurable microwave metasurfaces with different tuning factors, such as reconfigurable metasurfaces based on LC, graphene, active lumped elements, PCMs, and ferroelectric materials, in addition to programmable metasurfaces and time-modulated metasurfaces. Compared with microwave metasurfaces controlled by other kinds of stimuli, such as light, temperature, mechanics, and power, the use of additional electrical stimuli to control the EM response characteristics of microwave metasurfaces to achieve reconfigurable function not only reduces the complexity and cost of the system to some extent, but also improves the practicability and operability. In addition, it is easy to achieve precise control with metasurface applications. The electrically reconfigurable metasurfaces introduced in this review are in the microwave band. Up to now, there is less work to extend them to THz and other bands. For example, the PIN diode switches can only work at a relatively low frequency (). The traditional microwave electronic device design method cannot be easily extended to the frequency range of THz. Expanding the working frequency band of electrically reconfigurable metasurfaces so that it can play its unique role in each frequency band is also the direction of researchers.
It is worth noting that reconfigurable intelligent surfaces (RISs) have recently begun attracting more and more attention of researchers. The RIS is a passive array composed of a large number of metasurface units. Different components can independently reflect the incident signal by controlling its amplitude and/or phase. It can reconfigure wireless channels by reflecting the incident EM waves towards desired directions with high array gains[187–189] so as to flexibly regulate the EM wave in the wireless environment with low power consumption and cost and greatly improve the received signal quality. RISs have been envisioned as a promising candidate for future 6G wireless communications. Compared to passive RISs, active RISs integrating amplifiers have the ability of reflecting signals actively, which can further improve the channel capacity. We believe that in the near future, intellectualization is a development trend of electrically reconfigurable microwave metasurfaces.
In a word, we believe that the future progress of electrically reconfigurable metasurfaces and metadevices in the microwave regime will receive more and more attention. The improvement of the richness of new microwave devices will provide enough confidence to meet the challenges of the next generation of microwave technologies.
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