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this technology, the spatial resolution and imaging efficiency of the metasurface holograms are significantly improved, compared with the conventional geometrical optical structures suffering high-noise and low-reliability properties. Accordingly, some new achievements such as the polarization-independent, multi-color, non-linear, and high-efficiency holograms have been implemented with much more convenience under the metasurface-based technologies. [10][11][12][13][14][15][16] Digital coding metasurfaces have attracted much attention in recent years, which build up a bridge for the physical world and the digital information world. [17,18] The phase or amplitude response of the digital units are always encoded as N-bit with adjustment of some meta-unit parameters. For example, for a coding unit of transmission-type metaunit, the transmission phases of 0° and 180° refer to be digital bits "0" and "1", which bring great innovation for traditional designs. We can directly realize the transmission, reception, and processing of information in the microwave fields without considering the base-band digital modulation and processing, which greatly simplifies the design of the conventional transmitters and receivers. In addition, new wireless communication systems and advanced imaging systems have also been developed using this technology with good performance. [19][20][21] On this basis, adding active elements such as PIN diodes, varactors, and micro-electro-mechanical systems (MEMS) into the digital meta-unit, [22][23][24][25] the programmable metasurfaces can be realized through a field programmable gate array (FPGA). Electromagnetic reprogrammable coding metasurface holograms have been completed by using 1-bit reflection-type metasurface. [26] However, in the actual implementations of the conventional metasurface holography and the programmable digital-coding metasurface holography, the position of the feeding source is always placed far from the metasurfaces, which greatly increases the profile of metasurfaces and emerges unwanted blockage under the reflection-type designs. [27] Nowadays, the folded reflectarray, as an improved form of Cassegrain reflectarray, has also received much attention. [28][29][30] Although this structure reduces the profile and overcomes the shielding effect of the reflection-type metasurfaces, it brings some challenges in designing the feed and the rotation of polarization. [28][29][30] The introduction of Fabry-Perot (FP) cavity will pave a way to solve these problems. Compared with the folded reflectarray, Metasurface holography has been well studied due to its features that are superior to the traditional holographic technologies. However, high profiles of the reflection-and transmission-type configurations in the metasurface holography restrict its applications. On the other hand, digital coding metasurfaces have received great attention recently because they build a link between the information theory and surface electromagnetic fields, which also bring new viewpoint and convenience fo...

The electromagnetic (EM) controls of spatial nonreciprocal performance have gained much attention due to the extreme attraction of the unusual physical phenomenon. Especially, the emergence of metasurfaces, has enormously promoted the design of manipulating the spatial wave and field to achieve various EM functions. Herein is proposed a method to realize the spatial nonreciprocal manipulation for the EM polarizations using an active metasurface which elaborately integrates the elements of power amplifiers. The proposed nonreciprocal metasurface can have the distinct transmission appearance of EM polarizations. When the spatial wave impinges on metasurface with vertical polarization from one side, the transmitted wave has the same polarization; however, when the spatial wave with vertical polarization is incident from the opposite side, the polarization of the transmitted wave will be converted to horizontal. This novel performance is realized by using the meta‐atoms composed of three cascaded components: receiving component, nonreciprocal guiding system, and transmitting component. A prototype of the active metasurface is fabricated and measured, and good agreement is observed between the simulation and measurement results.

In the past decades, metasurfaces have opened up a promising venue for manipulating lights and electromagnetic (EM) waves. In the field of nonlinearity, second-harmonic generation (SHG) is a research focus due to its diverse applications. There have been many researches for realizing SHG in optical regime using nonlinear characteristics of optical materials, but its efficiency is low. In microwave frequencies, SHGs are basically studied in the guided-wave systems. Here, high-efficiency SHGs of spatial waves are presented in the microwave frequency using nonlinear metasurface loaded with active chips at the subwavelength scale. The nonlinear meta-atom is composed of receiving antenna, transmitting antenna, and active circuit of frequency multiplier, which can realize strongly nonlinear response and link the EM signals from the receiving to transmitting antennas. Correspondingly, to achieve the function of spatial-wave frequency multiplication, the working frequency of the transmitting antenna in the meta-atom should be twice as that of the receiving antenna, and hence the active chip is well matched to obtain the signal transforming with high efficiency. Good performance of the spatial-wave frequency multiplication is demonstrated in the proof-of-concept experiments with the best transform efficiency of 85.11% under normal incidence, validating the proposed method.

Due to the strong ability of recognizing electromagnetic (EM) environment and adaptively control of EM waves, the intelligent metasurfaces have received great attention recently. However, the intelligent metasurface with frequency recognition for adaptive manipulation of the EM waves has not been studied. Here, we propose a frequency-recognition intelligent metasurface to precisely control the spatial EM waves under the agile frequencies with the help of a real-time radio-frequency sensor and an adaptive feedback control system. An active meta-atom is presented to reach 2 bit phase coding and 1 bit amplitude coding capacities to control the amplitude and phase independently. Experimental results demonstrate that the metasurface can recognize different frequency of the incoming wave with very high resolution, and can adaptively realize the self-defined multiple frequency agilities to manipulate the reflected EM waves without any human participation. As example, the intelligent metasurface with frequency recognition can adaptively operate wave absorption at 5.36 GHz, reflection to normal direction at 5.38 GHz, deflection to −30° at 5.40 GHz, random diffusion at 5.42 GHz, and deflection to +33° at 5.44 GHz by detecting the incoming frequency at the resolution of 0.02 GHz.