Multifunctional Metasurfaces Based on the “Merging” Concept and Anisotropic Single-Structure Meta-Atoms

Metasurfaces offer great opportunities to control electromagnetic (EM) waves, attracting intensive attention in science and engineering communities. Recently, many efforts were devoted to multifunctional metasurfaces integrating different functionalities into single flat devices. In this article, we present a concise review on the development of multifunctional metasurfaces, focusing on the design strategies proposed and functional devices realized. We first briefly review the early efforts on designing such systems, which simply combine multiple meta-structures with distinct functionalities to form multifunctional devices. To overcome the low-efficiency and functionality cross-talking issues, a new strategy was proposed, in which the meta-atoms are carefully designed single structures exhibiting polarization-controlled transmission/reflection amplitude/phase responses. Based on this new scheme, various types of multifunctional devices were realized in different frequency domains, which exhibit diversified functionalities (e.g., focusing, deflection, surface wave conversion, multi-beam emissions, etc.), for both pure-reflection and pure-transmission geometries or even in the full EM space. We conclude this review by presenting our perspectives on this fast-developing new sub-field, hoping to stimulate new research outputs that are useful in future applications.


Introduction
Facing the increasing demands on data-storage capacity and information processing speed in modern science and technology, electromagnetic (EM) integration plays a more and more important role, which has intrigued intensive attention with remarkable applications. An ultimate goal pursued by scientists and engineers along this development is to make miniaturized devices as small as possible, yet equipped with powerful functionalities as many as possible. However, available efforts based on conventional materials suffer from the issues of device thickness, low efficiency, and restricted functionalities, caused by the fact that natural materials only exhibit electric responses with small variation range of permittivity ε and, thus, they only have limited capabilities on manipulating EM waves.

Multifunctional Meta-Devices Based on the "Merging" Concept
A simple scheme developed in early years utilized the so-called "merged" meta-structures to design multifunctional metasurfaces. In such a scheme, people first design individual metasurfaces exhibiting their own functions and then construct a multifunctional device simply through merging the two structures together. Below we present several examples to illustrate how the scheme works. Figure 1a presents an optical bifunctional metasurface that can realize a hologram image or a vortex beam, depending on the helicity of excitation light [74]. To achieve their end, the authors Appl. Sci. 2018, 8,555 3 of 18 first design two individual metasurfaces (both utilizing the metal-bar structure as basic meta-atoms) which can realize one of the needed functionalities when they are shined by incident light taking circular polarizations (CP) with different helicities (see Figure 1a). The desired phase profiles on two metasurfaces are created by the Pancharatnam-Berry (PB) principle [54,88,89] through rotating the metallic bars at different positions by appropriate angles. Since the two metasurfaces exhibit identical periodic structures and there are enough open spaces between metallic bars, the authors then merge two metasurfaces together to obtain the final design in which all metallic bars do not touch with each other. Such a device was finally fabricated out and experimentally characterized, showing nice bifunctional performances (Figure 1a). However, the working efficiency of the device is quite low, which is found to be around 9% [74].  Figure 1a presents an optical bifunctional metasurface that can realize a hologram image or a vortex beam, depending on the helicity of excitation light [74]. To achieve their end, the authors first design two individual metasurfaces (both utilizing the metal-bar structure as basic meta-atoms) which can realize one of the needed functionalities when they are shined by incident light taking circular polarizations (CP) with different helicities (see Figure 1a). The desired phase profiles on two metasurfaces are created by the Pancharatnam-Berry (PB) principle [54,88,89] through rotating the metallic bars at different positions by appropriate angles. Since the two metasurfaces exhibit identical periodic structures and there are enough open spaces between metallic bars, the authors then merge two metasurfaces together to obtain the final design in which all metallic bars do not touch with each other. Such a device was finally fabricated out and experimentally characterized, showing nice bifunctional performances (Figure 1a). However, the working efficiency of the device is quite low, which is found to be around 9% [74]. Reproduced from Ref. [74] with permission. (b) A metasurface that can reconstruct different images to different polarization channels. Reproduced from [64] with permission. (c) A metasurface that can generate multiple hologram images as shined by circularly polarized light with different helicity. Reproduced from [63] with permission. (d) A metasurface that can generate optical vortices with distinct topological charges at different longitudinal focal planes. Reproduced from [58] with permission. LCP, left CP; RCP, right CP; OAM, orbital angular momentum; LHCP, left-handed circularly polarized ; RHCP, right-handed circularly polarized. Such a "merging" concept has been straightforwardly applied to realize many other multifunctional meta-devices [58,[61][62][63][64][65]. Figure 1c presents a reflective bifunctional meta-device which can form a holographic image of a 'flower' or a 'bee' as the helicity of incident light changes from left CP (LCP) to right CP (RCP) [63]. In Figure 1b, a new type of meta-hologram device is introduced, which can exhibit various hologram patterns depending on the helicity the incident light [64]. Figure 1d shows a meta-device which can control the polarization state (i.e., spin angular momentum) and the position of the focal planes by manipulating the helicity of the incident light. [58].
In reviewing these meta-devices based on the "merging" concept, we find that the proposed design strategy is physically transparent and easy to implement. However, to make the "merging" process work, the adopted meta-atoms must be very simple structures (say, metal bar) to avoid metallic overlapping. Unfortunately, these meta-atoms typically do not satisfy the 100%-efficiency criterion established for PB metasurfaces [54] and, thus, one type of meta-atoms can generate background noise in addition to the desired functionalities. As a result, such meta-devices typically  Such a "merging" concept has been straightforwardly applied to realize many other multifunctional meta-devices [58,[61][62][63][64][65]. Figure 1c presents a reflective bifunctional meta-device which can form a holographic image of a 'flower' or a 'bee' as the helicity of incident light changes from left CP (LCP) to right CP (RCP) [63]. In Figure 1b, a new type of meta-hologram device is introduced, which can exhibit various hologram patterns depending on the helicity the incident light [64]. Figure 1d shows a meta-device which can control the polarization state (i.e., spin angular momentum) and the position of the focal planes by manipulating the helicity of the incident light. [58].
In reviewing these meta-devices based on the "merging" concept, we find that the proposed design strategy is physically transparent and easy to implement. However, to make the "merging" process work, the adopted meta-atoms must be very simple structures (say, metal bar) to avoid metallic overlapping. Unfortunately, these meta-atoms typically do not satisfy the 100%-efficiency criterion established for PB metasurfaces [54] and, thus, one type of meta-atoms can generate background noise in addition to the desired functionalities. As a result, such meta-devices typically suffer from issues of low operating efficiencies and functionality cross-talking, except [63] where the issue was partially solved by seeking a high-efficiency PB meta-atom in the reflection geometry.

Multifunctional Metasurfaces Based on Anisotropic Single-Structure Meta-Atoms for Two Polarizations
Having understood the key issues in the "merging" concept, people propose a new strategy to design multifunctional metasurfaces, i.e., using single structures as the basic meta-atoms to design the final device instead of merging two separately determined ones. In what follows, we first discuss the basic concept, and then introduce several meta-devices realized with such a strategy, classified into four sub-categories as detailed below.
Suppose that the adopted meta-atoms possess mirror symmetries, we can then describe the EM response for a meta-atom located at a position (x, y) on a metasurface by two diagonal Jones's matrices: and: where r xx , r yy , t xx , t yy denote the reflection/transmission coefficients of the meat-atom (periodically repeated to form a periodic metasurface), respectively. In order to make the devices exhibit working efficiencies as high as possible, ideally one requires the designed meta-atoms to be either perfectly reflective: or perfectly transparent: Once such conditions are satisfied meaning that all meta-atoms are of highest working efficiencies, people then adjust the geometrical parameters of each meta-atom to make the whole metasurface exhibit desired polarization-dependent phase profiles (i.e., ϕ xx (x, y) and ϕ yy (x, y)) to achieve certain wave-manipulation functionalities. A typical example is schematically shown in Figure 2, for which the wanted bi-functionalities are focusing and beam bending, respectively. To achieve this end, the final phase distributions on the metasurface should be: with F being the focal length and k 0 being the free-space wave-vector, and: with ξ being the phase gradient responsible for the anomalous reflection angle and C 1 being an a constant. We note that a conversion from propagating wave (PW) to surface wave (SW) can happen if ξ > k 0 . We emphasize that the high-efficiency conditions Equations (1) and (2) can only be approximately satisfied since losses inevitably exist, especially at high frequencies. Moreover, the achieved functionalities are not limited to the two described above. If different wave-manipulation functionalities are desired, one may easily replace Equations (5) and (6) by the phase distributions related to those functionalities.

Multifunctional Metasurfaces Exhibiting Similar Functionalities
Historically, multifunctional metasurfaces designed by single-structure meta-atoms were first realized in reflection geometry, because high-efficiency reflective meta-atoms are much easier to find than their transmissive counterparts. A commonly-used reflective meta-atom is the so-called metalinsulator-metal (MIM) structure, which consists of a metallic planar resonator and a continuous metal sheet separated by a dielectric spacer (see Figure 3a). Such an MIM structure was widely used to design high-efficiency reflective metasurfaces at frequencies ranging from microwave to visible [22,23,44,72,79,91], simply because its metallic ground plane can help reflect all incoming waves back (thus, Equation (1) can be easily satisfied) while the phase of reflected wave can undergo a continuous −180° to 180° variation as frequency passes through a resonance inside the "meta-atom". Such a resonance is usually called "magnetic resonance" [14] or "gap-plasmon resonance" [22,72], and its properties are carefully analyzed in [91,92]. Therefore, tuning the geometrical parameters of such a meta-atom can efficiently modify its two reflection phases ( and ) (see Figure 3b), which then offers enough freedoms to design multifunctional meta-devices. Figure 3d depicts a bifunctional meta-device working at telecom wavelengths, which can excite surface plasmon polaritons (SPPs) to flow along two orthogonal directions when the device is shined by external light with polarizations along x or y direction [72]. The fabricated device consists of 3 × 3 MIM meta-atoms, with geometrical parameters carefully adjusted such that both ( , ) and ( , ) satisfy Equation (6) with . Experimental characterizations reveal that the fabricated device can indeed exhibit the desired bi-functionalities (see Figure 3a). Based the same concept, other bi-functional meta-devices were realized based on MIM meta-atoms with patches replaced by other type of planar resonators, to achieve the polarization-dependent anomalous reflection (beam splitting) (Figure 3c) [79] and polarization-controlled dual-image hologram ( Figure 3e) [59]. Two common features of these meta-devices are worth mentioning: the realized functionalities are identical or similar, and the working efficiencies are not very high, partially caused by the metallic losses at optical frequencies.

Multifunctional Metasurfaces Exhibiting Similar Functionalities
Historically, multifunctional metasurfaces designed by single-structure meta-atoms were first realized in reflection geometry, because high-efficiency reflective meta-atoms are much easier to find than their transmissive counterparts. A commonly-used reflective meta-atom is the so-called metal-insulator-metal (MIM) structure, which consists of a metallic planar resonator and a continuous metal sheet separated by a dielectric spacer (see Figure 3a). Such an MIM structure was widely used to design high-efficiency reflective metasurfaces at frequencies ranging from microwave to visible [22,23,44,72,79,91], simply because its metallic ground plane can help reflect all incoming waves back (thus, Equation (1) can be easily satisfied) while the phase of reflected wave can undergo a continuous −180 • to 180 • variation as frequency passes through a resonance inside the "meta-atom". Such a resonance is usually called "magnetic resonance" [14] or "gap-plasmon resonance" [22,72], and its properties are carefully analyzed in [91,92]. Therefore, tuning the geometrical parameters of such a meta-atom can efficiently modify its two reflection phases (ϕ xx and ϕ yy ) (see Figure 3b), which then offers enough freedoms to design multifunctional meta-devices. Figure 3d depicts a bifunctional meta-device working at telecom wavelengths, which can excite surface plasmon polaritons (SPPs) to flow along two orthogonal directions when the device is shined by external light with polarizations along x or y direction [72]. The fabricated device consists of 3 × 3 MIM meta-atoms, with geometrical parameters carefully adjusted such that both ϕ xx (x, y) and ϕ yy (x, y) satisfy Equation (6) with ξ > k 0 . Experimental characterizations reveal that the fabricated device can indeed exhibit the desired bi-functionalities (see Figure 3a). Based the same concept, other bi-functional meta-devices were realized based on MIM meta-atoms with patches replaced by other type of planar resonators, to achieve the polarization-dependent anomalous reflection (beam splitting) (Figure 3c) [79] and polarization-controlled dual-image hologram (Figure 3e) [59]. Two common features of these meta-devices are worth mentioning: the realized functionalities are identical or similar, and the working efficiencies are not very high, partially caused by the metallic losses at optical frequencies.  [93] with permission. (c) A polarization beam splitter made by bifunctional gradient metasurface constructed with MIM metaatoms consisting of metal-cross planar resonators and metal ground plane (yellow) separated by a dielectric spacer (blue). Reproduced from [79] with permission. (d) Experimental characterizations on a unidirectional polarization-controlled SPP (red) coupler at telecom wavelength. Reproduced from [72] with permission. (e) A bi-functional meta-hologram device consisting of metal pattern and metal ground plane (yellow) separated by a dielectric spacer (blue) that can generate different hologram image depending on the incident linear polarization. Reproduced from [59] with permission.

Multifunctional Reflective Metasurfaces Combining Distinct Functionalities
The proposed design strategy is so general that it can also be used to realize multifunctional metasurfaces exhibiting distinct functionalities. For example, the first reflective multifunctional metadevice possessing two distinct functionalities (focusing and PW-to-SW conversion) was demonstrated in [93] in the microwave regime (see Figure 4a,b). Taking meta-atoms the same as shown in Figure 3a, the authors successfully designed a reflective bi-functional metasurface which exhibits two phase distributions satisfying Equations (5) and (6). Shining a y-polarized microwave at frequency 9.3 GHz normally onto the sample, the authors measured the Re[ ⃗ ] distributions on two orthogonal planes. Results displayed in Figure 4c indicate clearly that the incoming plane wave has been focused to a focal point. To demonstrate the PW-to-SW conversion effect for the ⃗ ||x polarization, the authors purposely fabricated a mushroom structure supporting eigen spoof SPP [30,32] and then put it at the right side of the metasurface to guide out the driven SW (see inset to Figure 4d). Shining the metasurface by an x-polarized microwave (9.3 GHz), the authors used a nearfield scanning technique to map out the Re[ ⃗ ] field pattern (see Figure 4d). The measured field pattern represents a very well-defined spoof SPP with = 206.5 m , in good agreement with the theoretically-calculated value ( = 209.4 m ). For both functionalities, the experimentallyestimated working efficiencies are higher than 90%, which is not surprising since metallic losses are (c) A polarization beam splitter made by bifunctional gradient metasurface constructed with MIM meta-atoms consisting of metal-cross planar resonators and metal ground plane (yellow) separated by a dielectric spacer (blue). Reproduced from [79] with permission. (d) Experimental characterizations on a unidirectional polarization-controlled SPP (red) coupler at telecom wavelength. Reproduced from [72] with permission. (e) A bi-functional meta-hologram device consisting of metal pattern and metal ground plane (yellow) separated by a dielectric spacer (blue) that can generate different hologram image depending on the incident linear polarization. Reproduced from [59] with permission.

Multifunctional Reflective Metasurfaces Combining Distinct Functionalities
The proposed design strategy is so general that it can also be used to realize multifunctional metasurfaces exhibiting distinct functionalities. For example, the first reflective multifunctional meta-device possessing two distinct functionalities (focusing and PW-to-SW conversion) was demonstrated in [93] in the microwave regime (see Figure 4a,b). Taking meta-atoms the same as shown in Figure 3a, the authors successfully designed a reflective bi-functional metasurface which exhibits two phase distributions satisfying Equations (5) and (6). Shining a y-polarized microwave at frequency 9.3 GHz normally onto the sample, the authors measured the Re[ → E ] distributions on two orthogonal planes. Results displayed in Figure 4c indicate clearly that the incoming plane wave has been focused to a focal point. To demonstrate the PW-to-SW conversion effect for the → E x polarization, the authors purposely fabricated a mushroom structure supporting eigen spoof SPP [30,32] and then put it at the right side of the metasurface to guide out the driven SW (see inset to Figure 4d). Shining the metasurface by an x-polarized microwave (9.3 GHz), the authors used a near-field scanning technique to map out the Re[ → E z ] field pattern (see Figure 4d). The measured field pattern represents a very well-defined spoof SPP with k spp = 206.5 m −1 , in good agreement with the theoretically-calculated value (k spp = 209.4 m −1 ). For both functionalities, the experimentally-estimated working efficiencies are higher than 90%, which is not surprising since metallic losses are negligible at microwave frequencies. Such a concept was successfully extended to realize the same type of bifunctional meta-device, but working at optical wavelengths (Figure 4e) [94], which is more close to realistic integration-optics applications. Appl. Sci. 2018, 8  In designing these devises, people noticed that an arbitrary anisotropic meta-atom might exhibit  ] pattern on the xy-plane using a monopole antenna placed vertically and 8 mm above the metasurface and the mushroom structure, when the metasurface is illuminated by a normally incident x-polarized plane wave. Reproduced from [93] with permission. (e) The working principle of similar type of bi-functional metasurface operating for visible light realized in [94]. The top panel shows the schematic of the unit cell consisting of an Ag nanobrick (grey) on top of a spacer (green) and Ag substrate. Reproduced from [94] with permission. SPPs, surface plasmon polaritons.
In designing these devises, people noticed that an arbitrary anisotropic meta-atom might exhibit undesired polarization cross-talking (i.e., varying one parameter of the meta-atom can influence its EM responses for both polarizations). Such an effect hinders the fast designs of meta-devices with complex functionalities, which typically require 2D parameter searching to realize the desired phase profiles. To overcome such cross-talking and to enlarge the working bandwidth, Xu et al. proposed a new type of anisotropic meta-atom [95] which consists of multilayers of planar resonators coupled with a metallic ground plane (Figure 5a). A crucial improvement is to add a wire loop to surround each planar resonator (see Figure 5a), which can significantly degrade the polarization cross-talking due to the screening effect [95].

Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 17
Measured radiation patterns (Figure 5e,f) demonstrated clearly the bi-functionalities possessed by the fabricated device. Another device realized in [95] can deflect beam anomalously and achieve small-angle multi-beam emissions for two polarizations. With such a unique meta-atom structure at hand, people can realize other multifunctional meta-devices exhibiting powerful and complex wavemanipulation functionalities.

Multifunctional Transmissive Metasurfaces Combining Distinct Functionalities
Compared to reflective devices, high-efficiency multifunctional meta-devices in transmission Based on such meta-atoms, the authors successfully realized two bifunctional metasurfaces combining complex wave-control functionalities [95]. One device is shown in Figure 5 which can achieve the functionalities of focusing (Figure 5c) and large-angle multi-beam emissions (Figure 5d) under external excitations with two polarizations. While the phase distribution responsible for the focusing functionality is easy to obtain (Equation (5)), that for another one needs to be carefully determined (see Figure 5b). The structural parameters of all meta-atoms can be quickly determined from two phase profiles, thanks to the low polarization cross talking property of the meta-atoms. Measured radiation patterns (Figure 5e,f) demonstrated clearly the bi-functionalities possessed by the fabricated device. Another device realized in [95] can deflect beam anomalously and achieve small-angle multi-beam emissions for two polarizations. With such a unique meta-atom structure at hand, people can realize other multifunctional meta-devices exhibiting powerful and complex wave-manipulation functionalities.

Multifunctional Transmissive Metasurfaces Combining Distinct Functionalities
Compared to reflective devices, high-efficiency multifunctional meta-devices in transmission geometry are much more difficult to realize, simply because more channels (transmission and reflection ones) exist in this geometry and thus Equation (2) is difficult to meet. People utilized the concept of Huygens' surface to construct transmission-mode metasurfaces with high efficiencies, but such devices usually require complex 3D non-flat meta-atoms with both electric and magnetic resonators incorporated [24,26]. Due to the complexities in meta-atom design, multifunctional meta-devices realized based on such a scheme are rarely seen, except a transmissive functionality-tunable device recently realized with controllable active elements incorporated [96].
Meanwhile, meta-atoms in multilayer geometry (with deep-subwavelength total thicknesses) are found as alternative candidates to construct high-efficiency transmission-mode metasurfaces. Many functional meta-devices are fabricated using such kind of meta-atoms [27,49], generating interesting effects such as high-efficiency SPP excitations [31] and photonic spin-Hall effect [54]. Straightforwardly, there appear recent efforts on using such meta-atoms to construct transmissive multifunctional meta-devices with very high efficiencies.
Cai et al. proposed to use a four-layer structure, with each layer containing a metallic mesh coupled with a metal patch [93], as the basic meta-atom to construct transmissive bifunctional meta-devices (Figure 6a). Each single layer exhibits perfect EM transmission at a particular frequency due to the interaction between the patch resonator and the opaque mesh. Mutual interactions between different layers can significantly enlarge the transparency window and the transmission-phase variation range (see Figure 6b). With such a high-performance structure, the authors designed and fabricated a transmissive metasurface (see Figure 6a for sample picture) with meta-atoms carefully adjusted to yield transmission phases (ϕ xx and ϕ yy ) satisfying Equations (5) and (6). Experimental results shown in Figure 6e,f indicate that the device can focus y-polarized PW to a point and can refract x-polarized PW anomalously, illustrating the desired bi-functionality. The achieved working efficiency is 72%, much higher than those of other transmissive metasurfaces [25][26][27]49].
range (see Figure 6b). With such a high-performance structure, the authors designed and fabricated a transmissive metasurface (see Figure 6a for sample picture) with meta-atoms carefully adjusted to yield transmission phases ( xx and yy) satisfying Equations (5) and (6). Experimental results shown in Figure 6e,f indicate that the device can focus y-polarized PW to a point and can refract x-polarized PW anomalously, illustrating the desired bi-functionality. The achieved working efficiency is 72%, much higher than those of other transmissive metasurfaces [25][26][27]49].

Multifunctional Metasurfaces for Full-Space Manipulation of EM Waves
The multifunctional metasurfaces introduced in the previous sections, whether working in reflection or transmission geometries, leave half of the EM space completely unexplored. It is highly desired to expand the wave-manipulation capabilities of metasurfaces to the full EM space, offering the metasurfaces independently controlled functionalities at their two different sides. In this section, we introduce recent efforts to design metasurfaces that can manipulate the wave fronts of EM waves in the full EM space with very high efficiencies [97].
The key step in this new strategy is to design a collection of meta-atoms which are perfectly transparent or reflective for incident waves polarized along two orthogonal directions. As shown in the inset to Figure 7a, the meta-atom consists of four metallic layers separated by three dielectric spacers. A crucial difference of present meta-atom with a usual transmissive one (Figure 6a) is that here the x-orientated metallic stripes of the bottom two layers are continuous ones, which can ensure a high reflection for x-polarized waves. Meanwhile, the reflection-phase ϕ r xx (x) for this polarization can vary from −180 • to 180 • (Figure 7b) as frequency changes. For the y-polarization, however, the coupling between different layers creates a wideband transparent window with a controllable transmission phase, which can again cover the whole 360 • range (Figure 7c).
Based on such unique meta-atoms, the authors successfully design several bi-functional meta-devices combining similar or distinct functionalities to manipulate EM waves at different sides of the metasurfaces. For instance, the authors fabricated a bifunctional metasurface, which can anomalously reflect x-polarized incident wave to the reflection side ( Figure 7d) and focusing the y-polarized incident wave to a focal point at the transmission side ( Figure 7e). As shown in Figure 7f,g, microwave experiments perfectly demonstrated the desired bi-functionalities of the fabricated meta-device. Both functionalities exhibit very high efficiencies (in the range of 85-91%) and the total thickness of the device is only~λ/8 [97].
Such a strategy has also been extended to design bi-functional PB metasurfaces that can manipulate the wave-fronts of circularly-polarized waves in the full space [89]. The key step is again to find a high-performance PB meta-atom exhibiting unique helicity-dependent transmission/reflection responses. Assisted by an elegant Jones' matrix analysis, Cai et al. were able to find a particular PB meta-atom (see Figure 8a) that can (nearly) perfectly reflect RCP light to RCP light at the reflection side (Figure 8b), and (nearly) transmit LCP light to RCP light at the transmission side (Figure 8c). Such unique polarization-conversion properties make the designed meta-atom ideal candidate to construct bifunctional PB meta-devices for controlling CP lights in the full space [54]. One can simply rotate the meta-atoms with appropriate angles to realize the desired phase distributions to achieve certain functionalities. The authors used such a meta-atom to fabricate a bifunctional meta-lens, and experimentally demonstrated that it behaves as a transmission-mode focusing lens for LCP wave (Figure 8d,f), but changes to a reflection-mode diverging lens for RCP wave (Figure 8e,g). Other bi-functional meta-devices can also be realized. However, one should note that the bi-functional meta-devices realized by such a scheme can only exhibit the same type of wave-manipulation functionalities for two polarizations, since the two phase distributions are intrinsically linked with each other.

Conclusions and Discussion
In summary, we presented a concise review on the development of multifunctional metasurfaces, focusing on the physical mechanisms and practical meta-devices realizations. The design strategies include a simple "merging" concept and more sophisticated ones relying on single anisotropic meta-atoms, and the realized meta-devices can achieve two or more wave-manipulation functionalities at frequencies ranging from microwave to the visible. In reviewing the development of this field, we find that the meta-atom design is of particular importance, since any new type of high-performance multifunctional meta-atom can surely stimulate a series of meta-devices with diversified functionalities. We already see many interesting new designs in the field (for example, the polarization-dependent full-space meta-atom) which help people realize useful functional devices with more flexibilities in controlling EM waves. However, we also noticed that this field is far from mature, since many new ideas are only realized in the microwave regime, leaving plenty of

Conclusions and Discussion
In summary, we presented a concise review on the development of multifunctional metasurfaces, focusing on the physical mechanisms and practical meta-devices realizations. The design strategies include a simple "merging" concept and more sophisticated ones relying on single anisotropic meta-atoms, and the realized meta-devices can achieve two or more wave-manipulation functionalities at frequencies ranging from microwave to the visible. In reviewing the development of this field, we find that the meta-atom design is of particular importance, since any new type of high-performance multifunctional meta-atom can surely stimulate a series of meta-devices with diversified functionalities. We already see many interesting new designs in the field (for example, the polarization-dependent full-space meta-atom) which help people realize useful functional devices with more flexibilities in controlling EM waves. However, we also noticed that this field is far from mature, since many new ideas are only realized in the microwave regime, leaving plenty of rooms for researchers working at more challenging frequency domains. We hope that this review can serve as a useful guide to help researchers quickly jump into this field, eventually pushing these conceptual laboratory-level protocol devices to integration-optics platforms.
Before concluding this review, we would like to discuss more on the differences in the design of meta-atoms for microwave, Infrared Radiation (IR), or optical frequencies. Given the different behavior of materials in these frequency ranges, the geometry of the meta-atoms needed to achieve a chosen functionality are necessarily different. For example, whereas in the microwave regime metals behave as perfect electric conductors, the metallic losses can be very significant at IR and optical frequencies. As a result, while meta-atoms with multilayer metallic structures are widely used in designing high-efficiency transmissive multifunctional metasurfaces in low-frequency domains, these meta-atom structures are difficult to be used directly at high frequencies because of the fabrication challenges and the material losses. Instead, simple metallic structures (say, metal bars) are currently used in designing transmission-mode multifunctional meta-devices at high frequencies, although the realized meta-devices suffer from low-efficiency issues. Very recently, all-dielectric metasurfaces began to appear, which seems to be a very promising route to overcome such issue for transmissive multifunctional meta-devices at high frequencies. Along with the fast development in nanofabrication technologies, we expect that more excellent works can appear in this field, generating meta-devices which can eventually be used in practice.