Graphene based tunable metasurface for terahertz scattering manipulation

Recently, the terahertz waves have attracted increasing attention due to the growing practical applications in astronomy, communication, imaging, spectroscopy, etc. While the metasurfaces, with extraordinary ability to control the electromagnetic waves, have been increasingly employed to tailor their interaction with terahertz waves and offer fascinating capabilities unavailable from natural materials. However, there are more and more requirements for the dynamical tune of the responses to electromagnetic components for the practical applications such as the terahertz stealth in variable environment. As such, considerable attention to terahertz frequencies has been focused on the tunable metasurfaces. Graphene has been proved to be a good candidate to meet the requirements for tunable electromagnetic properties, especially at the terahertz frequencies. In this paper, we design a tunable terahertz metasurface and achieve dynamically manipulating the scattering of terahertz waves. The metasurface is constructed by embedding double graphene layers with voltage control into the polyimide substrate of the diffuse scattering metasurface, which consists of the random array of rectangular metal patches, polyimide substrate, and metal ground. By adjusting the bias voltage on the double graphene layers, the terahertz scattering distribution can be controlled. At zero bias, the conductivity of graphene approaches to zero, and the random phase distribution is formed over the metasurface so that the reflected terahertz waves are dispersed into the upper half space with much lower intensity from various directions. With the bias voltage increasing, the conductivity of graphene increases, then the changeable range of the phase over the metasurface can be changed from 2 pi to pi/4. As a result, the random phase distribution of the metasurface is gradually destroyed and increasingly transformed into a uniform phase distribution, resulting in the scattering characteristic changes from the approximate diffuse reflection to the specular reflection. The expected performance of proposed metasurface is demonstrated through the full-wave simulation. The corresponding results show that the terahertz scattering pattern of the metasurface is gradually varied from diffuse scattering to specular reflection by dynamically increasing the Fermi level of graphene through increasing the bias voltage. Moreover, the performance of the proposed metasurface is insensitive to the polarization of the incident wave. All of these indicate that the proposed metasurface can continuously control the scattering characteristics of terahertz wave. Thus, the proposed metasurface can be well integrated into the changing environment, and may offer potential stealth applications at terahertz frequencies. Moreover, as we employ complete graphene layers as the controlling elements instead of structured graphene layers in other metamaterial designs, the proposed metasurface may provide an example of relating the theory to possible experimental realization in tunable graphene metasurfaces.