## 1 Introduction
### 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 \(\varphi_{xx}^{r}(\mathrm{x})\) for this polarization can vary from \(-180^{\circ}\) to \(180^{\circ}\) (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 \(360^{\circ}\) 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 \(\sim\)\(\lambda/8\)[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, [PERSON] 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. Figure 7: (**a**) Schematics of a 4-layer meta-atom composed by four metallic layers separated by three spacers.