Mode Properties of Flat-top Silver Nano-ridge Surface Plasmon Waveguides

We investigate surface plasmon modes supported by flat-top silver nano-ridges. We calculate the mode electromagnetic field distribution, the dispersion curve, the travel range, and the figure-of-merit of the nano-ridge mode. We find that the nano-ridge surface plasmon modes are quasi-TEM modes with longitudinal field components three orders of magnitude smaller than the transverse field components. The quasi-TEM nature of mode profiles reveals that the propagation of free electron oscillations on the top of the nano-ridge contributes mainly to the tightly confined ridge mode. We also find that as the width of the nano-ridge decreases, the ridge mode becomes more tightly confined on the ridge top. As the width of the nano-ridge increases, the nano-ridge mode approaches two decoupled right-angle wedge plasmon modes.