Metasurfaces have provided unprecedented freedom for manipulating electromagnetic waves. In metasurface design, massive meta-atoms have to be optimized to produce the desired phase profiles, which is time-consuming and sometimes prohibitive. In this paper, we propose a fast accurate inverse method of designing functional metasurfaces based on transfer learning, which can generate metasurface patterns monolithically from input phase profiles for specific functions. A transfer learning network based on GoogLeNet-Inception-V3 can predict the phases of 2^8×8 meta-atoms with an accuracy of around 90%. This method is validated via functional metasurface design using the trained network. Metasurface patterns are generated monolithically for achieving two typical functionals, 2D focusing and abnormal reflection. Both simulation and experiment verify the high design accuracy. This method provides an inverse design paradigm for fast functional metasurface design, and can be readily used to establish a meta-atom library with full phase span.

Lossless coding metasurfaces and metamaterial absorbers have been widely used for radar cross section (RCS) reduction and stealth applications, which merely depend on redirecting electromagnetic wave energy into various oblique angles or absorbing electromagnetic energy, respectively. Here, an absorptive coding metasurface capable of both the flexible manipulation of backward scattering and further wideband bistatic RCS reduction is proposed. The original idea is carried out by utilizing absorptive elements, such as metamaterial absorbers, to establish a coding metasurface. We establish an analytical connection between an arbitrary absorptive coding metasurface arrangement of both the amplitude and phase and its far-field pattern. Then, as an example, an absorptive coding metasurface is demonstrated as a nonperiodic metamaterial absorber, which indicates an expected better performance of RCS reduction than the traditional lossless coding metasurface and periodic metamaterial-absorber. Both theoretical analysis and full-wave simulation results show good accordance with the experiment.

A two-dimensional (2D) coding phase gradient metasurface (CPGM) is proposed for radar cross section (RCS) reduction in this work. The 2D phase gradient super cell is employed to serve as the coding element. The primary pattern of the coding element will be modulated by the designed 2D phase gradient. Thus, a more flexible method of scattering manipulation will be achieved by both the 2D phase gradient and coding sequences. The specific scattering patterns of the 2D CPGM under the modulation of phase gradient and coding sequence were analyzed. A controllable backward diffusion scattering coding phase gradient metasurface was realized based on Pancharatnam–Berry phase by modulating both the phase gradient and coding sequence. Both simulated and measured results demonstrate its excellent performance on RCS reduction.

The modern design of advanced functional materials or surfaces has increasingly undergone miniaturization and integration, so as to implement multiple functions using the same aperture. Metasurfaces, as an emerging kind of artificial functional surface, have provided unprecedented freedom in manipulating electomagnetic (EM) waves upon two-dimensional surfaces. It is desirable that the aperture of metasurfaces can be multiplexed with a number of functions for EM waves. Based on previous research, an artificial neural network-forward design can achieve an inverse design. In this paper, we propose an inverse design method of multiplexing metasurface apertures. A deep learning network (DLN) is trained to serve as a forward model for a genetic algorithm (GA), that is, a deep-leaning-forward genetic algorithm (DLF-GA). With the DLF-GA model, the phase of meta-atoms can be predicted for orthogonally linearly polarized waves simultaneously. The DLN was trained by a data set consisting of 70 000 samples with an accuracy of 95% on the test data. To demonstrate the competency of this method, we demonstrate the design of a multiplexed metasurface that can achieve focusing and diffuse scattering for polarization. The metasurface, which consists of 24 × 24 meta-atoms, can be generated monolithically with the input phase profile. A prototype was fabricated and measured. The predicted, simulated, and measured results are well consistent and underscore the validity of this inverse design method. This method provides an efficient and accurate method for the fast design of multiplexed metasurfaces and will find applications in microwave engineering such as in satellite communications.

Vortex beams with orbital angular momentum (OAM) can remarkably increase the channel capacity without signal process, thus attract more and more attention of researchers, and various novel methods of generating vortex beams blow out. In this work, a circular-polarized vortex beam reflector antenna was proposed via integrating vortex beam generated metasurface and beam focusing metasurface. The antenna consists of the metasurface reflector and a feed helical antenna. The phase distribution of the metasurface combines the OAM phase distribution and the beam focusing required phase distribution which is used to generate vortex beam from a point source. The measured results agree well with the simulated results, and both demonstrate that the reflector antenna can generate a vortex beam with OAM from 12 GHz to 18 GHz, and the realized gain is 22.4 dB at the center frequency 15 GHz.

In this paper, a transparent absorption-diffusion-integrated metamaterial (ADMM) based on standing-up lattice structure is proposed which takes full advantage of electromagnetic absorption and destructive interference simultaneously for the suppression of broadband backward scattering within a wide angular domain, especially for the lower-frequency scattering. The proposed ADMM is constituted by two kinds of rhombic and squared ITO lattices arranged in a pseudorandom distribution and then backed with ITO film. Calculation, simulation, and experimental measurement show that the proposed ADMM can achieve low scattering with normalized reflection less than 0.1 in the frequency band of 6.1-21.0GHz. In addition, owing to the standing-up lattice structure, the averaged optical transmittance of our ADMM reaches the optimal value of around 82.1% in the visible wavelength range of 380-780nm, promising an excellent optical transparency. The proposed comprehensive scheme provides an effective way to achieve broadband scattering suppression and high compatibility with optical transparency, enabling a wide range of applications in the window glass of stealth armament, electromagnetic compatibility facility and photovoltaic solar device.