Terahertz focusing of multiple wavelengths by graphene metasurfaces

Metasurfaces can achieve nearly arbitrary wavefront control based on manipulation of the wave phase profile. We propose a metasurface based on double graphene cut-wire resonators which can cover the complete 2p phase region with high reflection efficiency. This full phase coverage is essential for efficient wavefront manipulation, without reflecting energy into unwanted channels. A mirror capable of focusing multiple wavelengths is demonstrated numerically based on the proposed structure. The mirror can effectively focus terahertz (THz) waves from 1.2 to 1.9 THz to the same focal point by changing the Fermi level of each graphene resonator separately. The presented metasurface could provide a powerful platform for controlling THz waves, including focusing, beam steering, beam shaping, and holography