Frequency-Multiplexed Complex-Amplitude Meta-Devices Based on Bispectral 2-Bit Coding Meta-Atoms

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.