Analysis of coupling in the semi-cylindrical surface plasmonic couplers

In this paper, we have investigated the performance of a nano-optical directional coupler based on gap plasmon waveguides. The coupler consists of two waveguides having a localized coupled plasmon propagating between two semi-cylindrical surfaces. It is clear that the wave number and correspondingly light confinement in the waveguides are the most effective parameters in coupling strength and coupling length. Some expected and unexpected dependencies of the coupling length on the structure parameters are shown. Simulation results of the coupler obtained by the compact-2D finite-difference time-domain (FDTD) method comply with those derived by an analytic method with the aid of the finite-element frequency-domain (FEFD) software package of COMSOL. The considered structures, because of their small coupling length and dimensions are appropriate for use in optical integrated circuits.