▎ 摘　　要

The realization of electrically tunable beam focusing using a properly designed conductivity pattern along a strip on a background single graphene flake with operation in the terahertz regime is proposed and numerically investigated. The strip is illuminated with a guided surface plasmon polaritons (SPP) plane wave and the physical origin of the design procedure is evaluated from the phase of effective mode index of propagating SPP wave on graphene. Upon tuning a gate voltage between the graphene sheet and the substrate, the focus tuning is achieved. Finite-difference time-domain numerical technique is employed to explore the propagation characteristic of SPP wave and the performance parameters of the lens include the focal length, full-width half-maximum, and focusing efficiency. Such a one atom thick planar lens with the capability of electrical focus tuning besides the compatibility with current planar optoelectronic systems can find valuable potential applications in the field of transformational plasmon optics. (C) 2014 AIP Publishing LLC.