Independent wavefront manipulation of broadband spin-decoupled metasurfaces based on equivalent circuit model

Various functional meta-devices have exhibited powerful manipulation of orbital angular momentum spectrums, but they are still faced with the issue of inherent spin-correlated characteristic and lack of effective method to design metasurface. In this work, a broadband spin-decoupled metasurface is presented to realize independent regulation of dual circularly polarized waves. Based on the transmission line theory, an equivalent circuit model of meta-atom is proposed to establish the relationship between reflection coefficients and structural parameters. By utilizing the model, a group of meta-atoms has been designed with a step phase difference of 45 degrees, which can form a 3-bit coding metasurface. The proposed meta-atoms have a broad frequency band covering 11.56 - 18.17 GHz (44.5%). Moreover, the bifunctional reflection-type metasurfaces are constructed for producing the vortex waves along the +z axis, the off-axis pencil beams, and their synthetic beams. Based on the Fourier transform analysis, the mode purity and the electric field distribution of the generated beams are discussed in detail. The experimental measurements have good agreement with the simulation results, thus revealing the effectiveness of the proposed methodology to design metasurface.