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...
