Novel tunable graphene-encoded metasurfaces on an uneven substrate for beam-steering in far-field at the terahertz frequencies.

In this paper, we present a novel tunable graphene coding metasurface structure using a circular graphene patch on an uneven substrate. By changing the Fermi level of graphene or the thickness of the substrate, we can achieve obvious phase variation. Firstly, we put forward two construction methods of 1-bit coding metasurface based on this mechanism. The first method is to change the thickness of the substrate when the Fermi levels of the two-unit cells are the same, so that the two-unit cells exhibit different digital states of '0' and '1'. Furthermore, we change the working frequency band in real-time by switching the Fermi level from 0.05 eV to 0.85 eV. The second method is to change the Fermi level of graphene on the two-unit cells when the physical structure is fixed, so that the two-unit cells exhibit different digital states of '0' and '1'. In this case, we can achieve the regulation of the direction and number of far-field reflected waves in the frequency range of 2.65 THz ∼ 2.85THz. Then, to obtain a single beam of reflected waves deviating from the normal direction, we create a 2-bit method in combination with two 1-bit construction methods. At 1.9 THz, the four-cell structures have a phase difference of approximately 90° and the same reflection coefficient. We also set several coding modes to analyse the control of the reflected wave on the 2-bit coding metasurface. Finally, we realized the real-time regulation of the reflected wave in eight directions from 0° to 360° by controlling the Fermi level of the graphene. Therefore, this article proposes a potentially effective approach to the design of functional devices for beam splitting and beam deflection.