## 4 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  
Figure 8: **(a)** Topology of a meta-atom consisting of three metallic layers (yellow) separated by two diecltric spacers (gray). **(b,c)** Amplitude/phase (Blue circle/red star) responses of the meta-atom rotated with certain angles when illuminated by normally incident (**b**) RCP and **(c)** LCP waves. Schematics of a bifunctional metasurface which behaves as a transmissive focusing lens **(d)** and a reflective diverging lens **(e)** under excitations of LCP and RCP waves, respectively. **(f)** Measured \(|E_{x}|^{2}\) distributions on both _xoz_ and _yoz_ planes at the transmission side when the meta-device is illuminated by a normally incident LCP wave. **(g)** Measured Re(\(E_{x}\)) distributions on the _xoz_ plane at the reflection side of the metasurface under excitation of a normally-incident RCP wave. Reproduced from [89] with permission. 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. This work is supported by the National Natural Science Foundation of China (grant no. 11604167, no. 11474057, no. 11404063, no. 11734007, no. 11674068, and no. 61501499), National Basic Research Program of China (grant no. 2017 YFA0303504), K. C. Wong Magna Fund in Ningbo University and the Shanghai Science and Technology Committee (grant no. 16 ZR1445200 and no. 16 JC1403100).