Magnetically Controlled Nanofocusing of a Graphene Plasmonic Lens

In this paper, we propose a method to tailor the nanofocusing of plasmons on graphene plasmonic lens, which is composed of graphene and circular dielectric gratings of magneto-optical material beneath it. With an external magnetic field parallel to graphene surface, the magneto-optical effect of substrate leads to the difference in modal indices of graphene plasmons, which also introduces an additional relative phase difference between these two plasmons during excitation and propagation. Together, these two effects enable us to tailor the position of focal points through external magnetic field, which has been described by an analytical approach based on phase matching and verified by numerical simulations. With an operation wavelength of 8500 nm and an external magnetic field from B = ?1 T to B = 1 T, a shift distance over one and a half times of plasmons wavelength for focal points or donut-shaped field profiles can be obtained under linearly or circularly polarized light. The proposed scheme has potentials in diverse applications, such as the tunable nanofocusing and particle manipulation.