A fork-shaped plasmonic device with polarization-controllable optical confinement

Abstract

It is shown that fork-shaped plasmonic gratings can display a hybrid mode that features both plasmonic mode (TMmode) and dielectric mode (TE-mode) characteristics with wide range of tunable group velocities. A dielectric gap is introduced in the middle of metallic grating and it is found that this gap plays an important role in controlling the TE-TM mode coupling. By controlling the polarization angle we can switch from plasmonic mode to dielectric mode. Thus, a new scheme for manipulating the optical confinement by using a polarizer is realized. (see Figure) We can combine the plasmonic mode and dielectric mode to reduce the intrinsic loss of Plasmon-polariton due to the free-carrier absorption in the conducting material with the same degree of confinement. The fork structure provides an easier way to control the group velocity in a wide range. The dispersion relations were calculated by using Rigorous Coupled Wave Analysis. We obtain tunable group velocities ranging from 0.2 c to almost zero (i.e. achieving localized Surface Plasmon-polariton) and from 0.05 c to 0.3 c by varying the pillar and dielectric (made of Si 3 N 4 ) thicknesses respectively. This fork structure is expected to have applications in surface plasmon polariton (SPP) mixed with guided-mode based optical devices, such as optical buffering, hybrid waveguides, splitters and lasers and especially for applications requiring slow light propagation.