## 1 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 \(\varepsilon\) and, thus, they only have limited capabilities on manipulating EM waves. Metamaterials (MTMs) [1; 2], consisting of deep-subwavelength-sized EM microstructures (e.g., meta-atoms) arranged in periodic or non-periodic orders, have drawn much attention recently. Through tailoring the microstructures of meta-atoms, MTMs can, in principle, exhibit arbitrary values of permittivity \(\varepsilon\) and permeability \(\mu\), which offers MTMs extraordinarily strong capabilities to control EM waves. Many fascinating wave-manipulation effects have been demonstrated based on MTMs, such as negative refraction [3; 4], super-resolution imaging [5; 6], cloaking [7; 8; 9], polarization-control [10; 11; 12; 13], perfect light absorption [14], and transparency [15; 16], and unusual wave-control effects realized by zero-index MTMs [17; 18]. Attempts have also been made to achieve multifunctional EM devices based on MTMs. However, the realized devices typically exhibit bulky sizes and low efficiencies, since MTMs are three-dimensional (3D) materials composed by resonant metallic structures which can easily absorb EM waves. Moreover, such 3D devices require complex fabrication processes, adding more disadvantages for EM integration [19; 20]. Metasurfaces, ultrathin MTM layers constructed by planar meta-atoms of pre-determined EM responses arranged in specific two-dimensional (2D) orders, can largely overcome the difficulties faced by MTMs. Tuning the EM responses of meta-atoms to realize certain transmission/reflection phase distributions on the metasurfaces, one can use these ultra-thin devices to efficiently reshape the wave-fronts of incident EM beams based on Huygens' principle, achieving unusual effects, including anomalous beam bending based on generalized Snell's law [21; 22; 23; 24; 25; 26; 27; 28; 29], propagating wave to surface waves conversion [30; 31; 32], polarization-control [33; 34; 35; 36; 37; 38; 39], focusing [40; 41; 42], holograms [43; 44; 45], flat-lens imaging [46; 47; 48; 49; 50], tunable devices [51; 52; 53], and photonic spin-Hall effect [54; 55; 56], etc. Typically, these devices are flat, much thinner than the wavelength, and exhibit much higher efficiencies than their bulky MTM counterparts, all being highly favorable for integration-optics applications. These attractive properties make metasurfaces the best candidates to construct multifunctional EM devices. Indeed, many efforts have recently been devoted to designing multifunctional optical devices based on metasurfaces [57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98;  
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.