The terahertz region is a special region of the electromagnetic spectrum that incorporates the advantages of both microwaves and infrared light waves. In the past decade, metamaterials with effective medium parameters or gradient phases have been studied to control terahertz waves and realize functional devices. Here, we present a new approach to manipulate terahertz waves by using coding metasurfaces that are composed of digital coding elements. We propose a general coding unit based on a Minkowski closed-loop particle that is capable of generating 1-bit coding (with two phase states of 0 and 1806), 2-bit coding (with four phase states of 0, 906, 1806, and 2706), and multi-bit coding elements in the terahertz frequencies by using different geometric scales. We show that multi-bit coding metasurfaces have strong abilities to control terahertz waves by designing-specific coding sequences. As an application, we demonstrate a new scattering strategy of terahertz waves-broadband and wide-angle diffusion-using a 2-bit coding metasurface with a special coding design and verify it by both numerical simulations and experiments. The presented method opens a new route to reducing the scattering of terahertz waves.
Loss in photonic devices is intentionally minimized, as it limits the efficiency and potential functionalities. Here, we report a loss-assisted non-Hermitian electromagnetic metasurface operating at an exceptional point (EP), showing extraordinary angular asymmetry at EP condition. Spatially engineered losses are essential for the exotic scattering response of this metasurface. As a proof of concept, a functional EP metasurface, composed of a judiciously tailored tri-meta-atom supercell, is shown to exhibit unidirectional retro-reflection: totally suppressed reflection when light illuminates from the left, but highly efficient reflection from the right. Our results open promising possibilities for developing new mechanisms and designing functional photonic devices for wave manipulation and fuse exceptionalpoint physics with flatland optics.
We demonstrate the coexistence of two tunable symmetric and asymmetric resonances in a metamaterial composed of asymmetrical split-rings (ASRs) patterned on a dielectric layer numerically and experimentally. The full-wave simulation and measurement results demonstrate that the metamaterial reveals a symmetric cross-polarization transmission band with a ripple-free peak and asymmetric co-polarization transmission bands characterized by trapped-mode resonances. Both symmetric and asymmetric resonances can be easily tuned via the incident angle of electromagnetic waves. The resonant excitation and coupling of the electric and magnetic dipole moments contribute to the conversion of two orthogonal linear polarizations. The ASR metamaterial shows a directionally asymmetric transmission for both linearly and circularly polarized waves for large angles of incidence. The proposed ASR metamaterial is of importance to develop novel metamaterial-based devices.
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