Tunable dual-band terahertz metalens based on stacked graphene metasurfaces

In this paper, we propose and theoretically investigate a tunable stacked graphene metasurface, which can independently manipulate electromagnetic field at different terahertz frequency. By tuning the Fermi levels of graphene ribbons, the designed two-layers graphene ribbons can realize a required phase shift of ∼2π for manipulating the terahertz wavefronts owing to the excitation of plasmon resonances at each layer of graphene ribbons. A reflective double-frequencies focusing metalens has been designed working at 3.5 THz and 7.0 THz. For the designed lens with focal lengths of 300μm, the focus depth of 3.5 THz and 7.0 THz electromagnetic wave (full width at half maximum along the Z direction) is 174. 7μm and84μm, respectively. In addition, the resolution of focal points (full width at half maximum along the X direction) are close to the half-wavelength in the focusing plane, which are 48. 2μm and 21.5μm respectively, demonstrating that the proposed metalens have superior performance. Furthermore, the focal length and offset distance of focal point of the lens can be tuned actively by changing gate voltages. The present graphene metasurface paves the way to engineering various potential applications for tunable, multiband, multifunctional metasurfaces.