Broadband diffusion of terahertz waves by multi-bit coding metasurfaces

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 180°), 2-bit coding (with four phase states of 0, 90°, 180°, and 270°), 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. A team in China has demonstrated a new strategy for controlling terahertz waves by using ‘coding’ metasurfaces to attain broadband diffusion. Metamaterials have previously been used to control terahertz waves and develop functional devices. Now, Tie Jun Cui and co-workers have developed metasurfaces composed of one-, two- and three-bit digital coding elements based on Minkowski loops. They demonstrated their coding surfaces by showing that metasurfaces with appropriately designed coding sequences can be used to strongly manipulate terahertz waves. In particular, they realized broadband, wide-angle diffusion using a two-bit coding metasurface with a special design and obtained good agreement between the measured results and numerical simulations. The proposed method offers a new way to control scattering of terahertz waves and can be implemented using conventional lithography.