The research field of metamaterials has become one of the most important areas in the past two decades due to the meta materials' unconventional electromagnetic properties that do not readily exists in natural materials. A large number of inno vative applications based on metamaterials have been proposed and experimentally verified, such as negative refraction [1] and cloaking. [2] Unfortunately, the widespread applications of metamaterials have been severely limited due to the fabrication challenges and material losses.Metasurfaces [3,4] are 2D arrangement of subwavelength metaatoms (unit cells) engineered to manipulate the properties (e.g., phase, amplitude, and polarization) of the electromagnetic (EM) waves, which fea ture easy fabrication and reduced absorp tion loss compared to 3D meta materials. Typically, metasurfaces are constructed of metal [5,6] or dielectric [7,8] metaatoms with delicately designed geometries and orientations to locally modulate the pro perties of the scattered EM wave. Due to the unpre cedented abilities to manipulate the EM wavefronts, metasurfaces have attracted enormous interests in different parts of the frequency spectra, and enabled a lot of novel planar metadevices, such as ultrathin skin cloak, [9] metaholograms, [10] polarization con verters, [11] spin Hall effects, [12,13] metalenses, [14,15] and vortex beam generators. [16][17][18][19] In addition, the concept of meta surface has also been adopted in many other research areas, such as the elastic meta surfaces [20,21] and acoustic metasurfaces. [22,23] Although metasurfaces have been widely studied since their emergence, most metasurface devices are designed to operate at a single wavelength, and their function alters or the performance deteriorates as the wavelength is varied due to the dispersion nature. The wavelengthdependent behavior of the metasurfaces is one of the critical limitations in existing metasurface devices. Therefore, a number of investigations have been conducted to circumvent this limitation at two or more distinct wavelengths recently. [24][25][26][27][28][29][30][31][32][33] For instance, several multiwavelength devices are realized by grouping different sized resonators into a building block. [24,28,29,32] Another means for realizing multiwavelength metasurfaces is to interleave several functional sections com posed of particular resonators operating at one wavelength of the incident light. [27,31] However, the metasurface devices have Metasurfaces are planar structures that can offer unprecedented freedoms to manipulate electromagnetic wavefronts at deep-subwavelength scale. The wavelength-dependent behavior of the metasurface could severely reduce the design freedom. Besides, realizing high-efficiency metasurfaces with a simple design procedure and easy fabrication is of great interest. Here, a novel approach to design highly efficient meta-atoms that can achieve full 2π phase coverage at two wavelengths independently in the transmission mode is proposed. More specifically, a bilayer meta-atom is d...

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.

The Airy beam has attracted considerable interests due to its remarkable non‐diffracting, self‐bending, and self‐healing properties. An auto‐focusing Airy (AFA) beam can be achieved by constructing two counter‐propagation Airy beams, which is a novel method to generate focusing effect. Traditionally, because of the complexity of the generation mechanism, it is hard to realize the Airy beam with complex amplitude modulation. The reported ones usually require bulky optical systems and typically work at one single wavelength. Here, a general scheme is proposed to launch dual‐band Airy and AFA beams based on the single‐layer geometric metasurface, consisting of a newly designed meta‐atom that can realize independent complex amplitude modulations at two arbitrary terahertz wavelengths. Based on the proposed meta‐atom, the aforementioned attractive properties of the Airy beam are demonstrated at two terahertz wavelengths. Besides, the dual‐band AFA beams are illustrated to have the practically important adjustability of the focal length. Furthermore, owing to the self‐healing property, the proposed focusing scheme (i.e., AFA beam) shows a much better capability of circumventing the PEC obstacles compared to the conventional metalens, which could have potential applications in various important scenarios such as biomedical imaging/therapy and nondestructive evaluation with compact size and multiwavelength functionalities.

The wavelength-dependent behavior of the metasurface is one of the severe disadvantages in the metasurface-based applications, which greatly limits its applications. In this work, a novel reflective building block (unit cell) has been proposed to work at two arbitrary terahertz frequency bands with independent phase control at each band. The geometric phase building block can reflect the incident circularly polarized wave and convert it to the wave with opposite helicity and high conversion efficiencies at both bands. As proof of concept demonstrations, a dual-band cylindrical meta-lens and two vortex beam generators operating at 0.45 and 0.7 THz are investigated to verify the performance of the proposed building block. The simulated results of the metalens agree very well with the theoretical calculations, which are consistent with the design goals. Moreover, the designed vortex metasurfaces reveal a convenient and low-cost way to realize the vortex beam carrying different/same orbital angular momentum modes at two bands.INDEX TERMS Dual-band, independent phase control, high efficiency, reflection.

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