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Frequency‐Dependent Dual‐Functional Coding Metasurfaces at Terahertz Frequencies

In view of the aforementioned shortcomings for 3D metamaterials, it would be extremely beneficial to reduce the thickness to that of a surface with only one or a few layers of metallic structures. This aim has led to the development of 2D equivalence of metamaterials, referred to metasurfaces,[18–26] which can manipulate the wavefronts of EM waves through the phase discontinuities introduced at a subwavelength scale on the surface of substrates, thus taking much less physical space and offering less lossy structures than 3D metamaterials. Due to easy fabrications of planar structures using standard photolithography process, a number of metasurface designs have been implemented at the terahertz and optical frequencies including the orbit angular momentum generations,[19,20] absorbers,[21,22] holography,[23,24] polarizer,[25] and asymmetric transmission,[26] etc. Recently, inspired by the digital circuit that only possesses two distinct states “on” and “off”, the concept of coding metamaterials was proposed to realize various manipulations to EM waves such as anomalous beam reflections and random diffusions by distributing coding particles with opposite reflection phases on a surface with predesigned coding sequences.[27] As the retrieval method[28] originally used to characterize the 3D metamaterial was made on the assumption of the periodic boundary condition, it does not apply for coding metasurfaces because of its subwavelength thickness in the propagation direction. Instead, it is straightforward to characterize coding metasurfaces, in analogy to the discrete digital circuits, by phase responses of coding particles “0” and “1”, possessing opposite reflection phases, respectively. Lately, coding metamaterials have been extended to terahertz frequency to realize anomalous reflections and broadband diffusions using Minkovski fractal coding particles[29] and ring resonators.[30] Interestingly, as the coding particle is designed as anisotropic, a single encoded metasurface could exhibit dual functionalities under orthogonal polarizations.[31] In this work, a dual-band 1-bit coding metasurface is developed in terahertz frequencies to work independently with completely diverse functions at two different frequencies, as conceptually illustrated in Figure 1a,b. Under the normal illumination of terahertz waves at the higher frequency (see Figure 1a), the metasurface shows the alternative coding sequence as “0101...” that can redirect the normal incidence to two oblique directions having equal angle with respect to the z-axis in the upper half space; while at the lower frequency (see Figure 1b), the coding pattern becomes the chess-board distribution that splits the normal incidence to four identically Frequency-Dependent Dual-Functional Coding Metasurfaces at Terahertz Frequencies