  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.
