introducing abrupt phase changes via an ultrathin sheet of subwavelength resonators (usually known as meta-atoms). The first such metasurface was proposed by Yu and Capasso [2] Afterward, significant number of novel metasurfaces have been proposed in microwave, [3] terahertz, [4] near-infrared, [5] and visible ranges. [6] These metasurfaces are utilized for realizing many applications such as focusing with subwavelength planar lenses, [7] optical cloaking, [8,9] generating nondiffracting beams, [10] manipulating phase profiles having orbital angular momentum (OAM), [11] photon spin Hall effect, [12] and performing computation with the coding metasurfaces. [13,14] Although metasurfaces have proved their unique freedom in wavefront manipulation, most metasurfaces generally suffer from low efficiencies. This is especially critical for transmissive metasurfaces where both the magnetic and electric resonances should be precisely controlled. For example, the efficiency of the first ever demonstrated metasurface was only 5%. [2] A few transmissive metasurfaces have since been introduced with higher efficiencies based on the Huygens' principle [15,16] or meta-transmit arrays. [17,18] However, the operation of these metasurfaces is severely restricted, e.g., may require specific polarization, added complexity in manufacturing, or demand complex, costly computations for each distinct phase.Recently, it has been demonstrated that metasurfaces can be realized based on the photon spin Hall effect. [19][20][21][22] Spin Hall effect is a phenomenon where moving electrons with opposite spins can be transversely separated. Spin Hall effect can be intrinsic based on spin-orbit coupling of electrons [23] or extrinsic due to the spin-dependent scatterings by impurities. [24] Both of these could be utilized to achieve extraordinary wave propagation. It is known that the experimentally observed intrinsic phenomena are usually very weak. [23] On the contrary, extrinsic spin Hall is observed in a special class of metasurfaces, [12] which utilizes subwavelength scatterers to achieve full phase control in the range of 0-2 π. The transmission/ reflection phase control of a cross-polarized wave is achievable through the rotation of the designed scatterers, which in fact make it more promising than the other types of meta-atoms that require phase dispersions to achieve phase control. The maximum achievable efficiency of such metasurfaces was theoretically predicted to be 25% in a single layer structure since only electric responses can be realized in such structures. [25,26] This limitation was overcome by designing reflective metasurfaces with ground planes, [27][28][29] where both the electric and the Metasurfaces offer unprecedented freedom to manipulate electromagnetic waves at deeply subwavelength scales. However, realizing a highly efficient metasurface, yet simple enough to conceptualize, design, and fabricate, is a challenging task. In this paper, a novel approach is proposed for designing meta-atoms which can achieve full phase cont...

Metasurfaces have been extensively studied for generating electromagnetic waves carrying orbital angular momentum (OAM). In particular, programmable metasurfaces enable real‐time switching between multiple OAM modes in a digital manner. However, the current programmable metasurfaces are mostly based on reflective mode, which suffer from low efficiency as well as serious feed blockage. In this paper, a transmissive programmable metasurface is presented for the highly efficient generation of multimode convergent OAM beams. The proposed transmissive metasurface is composed of electronically reconfigurable units with 1‐bit phase resolution (0/π), which are obtained by integrating two PIN diodes in the radiating layer for current direction modulation. Through the antisymmetry configuration of the two PIN diodes, nearly uniform transmission magnitudes but inversed phase states in a wide band can be obtained. The simulation results show that the proposed reconfigurable unit can achieve good 1‐bit phase tuning, with minimum insertion loss of 0.2 dB and 2 dB transmission bandwidth of more than 10%. Through the dynamic modulation of the quantized code distributions on the metasurface, programmable multimode OAM beams can thus be constructed. Both simulated and measured results verify the effectiveness of the proposed design.

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Recently, vortex beam carrying orbital angular momentum (OAM) for radio communications has attracted much attention for its potential of transmitting multiple signals simultaneously at the same frequency, which can be used to increase the channel capacity. However, most of the methods for getting multi-mode OAM radio beams remain up to now mainly at simulation level, since their implementations are of complicated structure and very high cost. This paper provides an effective design of generating five-mode OAM radio beams by using the Rotman lens-fed antenna array. The Rotman lens is a viable beamforming approach instead of electronically scanned arrays for its low cost and the ease of implementation. The lens-fed array employs a two-layer structure for size reduction, and the lens body and the antenna array are segregated by a common ground plane to eliminate spurious radiation and thus improve the performance of the OAM beams. The measured results coincide with the simulated ones, which verified the effectiveness of the proposed design for generating multi-mode OAM beams.

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