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Vortex beams (VBs) carrying orbital angular moment (OAM) modes have been proven to be promising resources for increasing communication capacity. Although considerable attention has been paid on metasurface-based VB generators due to the unprecedented advantages of metasurface, most applications are usually limited at a single band with a fixed OAM mode. In this work, an emerging dual-band reflection-type coding metasurface is proposed to mitigate these issues by newly engineered meta-atoms, which could achieve independent 2-bit phase modulations at two frequency bands. The proposed coding metasurface could efficiently realize and fully control dual-band VBs carrying frequency selective OAM modes under the linearly polarized incidence. As the first illustrative example, a dual-band VB generator with normal beam direction is fabricated and characterized at two widely used communication bands (Ku and Ka bands). Moreover, by encoding proper coding sequences, versatile beams carrying frequency selective OAM modes can be achieved. Therefore, by adding a gradient phase sequence to the first VB generator, the second one is designed to steer the generated beams to a preset direction, which could enable diverse scenarios. The measurement results of both VB generators agree very well with the numerical ones, validating the full control capability of the proposed approach.

Metasurface has drawn much attention due to its unprecedented wave‐front manipulation abilities with an ultrathin flat profile. However, the metasurface as a diffractive device usually suffers from chromatic aberrations, which greatly hinders the design freedom at different wavelengths. In this work, it is demonstrated that this limitation can be overcome by a multifunctional metasurface with completely independent phase modulations at three arbitrarily wavelengths. Specifically, a novel single‐layer tri‐spectral meta‐atom composed of three alternatively arranged slot and metallic resonators is proposed to operate at three distinct wavelengths, where 2π geometric phase modulations under the circularly polarized incidence can be achieved independently by rotating the corresponding resonators. As proof of concept demonstrations, a tri‐wavelength vortex beam generator and a meta‐hologram are designed to verify the proposed method. First, a vortex beam generator with arbitrary topological charge numbers at three wavelengths is designed and verified through theoretical calculation and full‐wave simulation. Moreover, a meta‐hologram generated by the computer‐generated holography is designed to display three frequency selective holographic images on the same image plane. The tri‐wavelength meta‐hologram is validated through theoretical calculation, full‐wave simulation, and experiment. The experimental results agree very well with the numerical ones, demonstrating the attractive capabilities of multifunctionalities at three wavelengths.

Meta‐devices have attracted great interest due to the unprecedented capabilities of manipulating wavefronts. Complex‐amplitude hologram can provide high‐quality images that can be free of ghost images and undesired diffraction orders. However, conventional meta‐holograms usually operate at a single band with phase‐only modulation. Here, a reflective 2‐bit meta‐hologram is proposed to operate with independent complex‐amplitude modulations at two frequency bands. The high‐efficiency meta‐atom is composed of a top perforated metallic layer, on which two C‐shape split ring resonators (CSRRs) are located in the centers of a circular hole and an annular slot. By tuning the sizes of the two CSRRs, dual‐band 2‐bit phase modulations can be individually achieved, while the amplitude profile can be continuously tailored at each band by rotating the corresponding CSRR without affecting the phase responses. Based on this emerging meta‐atom, a dual‐band bifocal metalens is demonstrated numerically and a bispectral meta‐hologram is validated both numerically and experimentally at two widely used communication bands. The proposed method features all desirable advantages of the coding metasurfaces with extra degrees of freedom by providing independent frequency control and amplitude modulation, which can provide great opportunities in multifunctional applications with enhanced performance and boosted information capacity.