▎ 摘　　要

A graphene-based graded index (GRIN) planar lens with the capability of being tuned by the external magnetostatic bias field is proposed and numerically investigated. Based on the effective mode index of propagating transverse magnetic (TM) surface plasmon polaritons (SPP) on the graphene sheet, a design procedure to achieve a beam-focusing phenomenon is introduced. For this purpose, the required conductivity pattern along a strip on a background graphene layer is obtained by applying a gate voltage between the graphene sheet and a properly designed ground plane. Using the finite-difference time-domain (FDTD) numerical technique, the simulations are conducted to explore the propagation characteristics of SPP waves and the performance parameters of the lens, which include the focal length (FL), the full width half maximum (FWHM) and the focusing efficiency. Valuable potential applications can be envisioned for the active modulation of a terahertz light beam via the magnetic, in addition to the electric tunability of a one-atom-thick graphene sheet, such as optical data processing, imaging, and so on.