## 1 Introduction
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]. 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  
Figure 1: Multifunctional devices designed with merged structures. (**a**) Design strategy, sample picture, and experimental characterizations of a multifunctional metasurface than can generate holographic images or a vortex beam depending on the helicity of incident circularly polarized light. 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. 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:  
\[\mathbf{R}=\left( \begin{array}{cc}r_{xx}(x,y)&0\\ 0&r_{yy}(x,y)\end{array} \right) \tag{1}\]  
and:  
\[\mathbf{T}=\left( \begin{array}{cc}t_{xx}(x,y)&0\\ 0&t_{yy}(x,y)\end{array} \right) \tag{2}\]  
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.