▎ 摘　　要

We theoretically study the topological transition of dispersion types and propose a tunable planar lens based on graphene hyperbolic metamaterials (HMMs). By tuning the chemical potential (mu(c)) of graphene, the dispersion relation of the HMM is topologically switchable between ellipse (mu(c)<0.6 eV) and hyperbola (mu(c)>0.6 eV) where positive and negative refractions occur respectively. Especially, for mu(c)>0.6 eV, a Gaussian light beam is negatively refracted twice and focuses at a far-field point finally, acting well as a planar lens. Furthermore, its focal length l can be sensitively tuned by controlling mu(c), and Delta l reaches 260 mu m (from 528 to 268 mu m) while mu(c) varies with only 0.05 eV (from 0.65 to 0.7 eV). The physical reason is attributed to the different anisotropy degrees of EFCs for different mu(c). Such a compact, high-speed, and sensitively tunable planar lens holds great promise in photonic integration, photonic imaging, and directional coupling applications. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement