  A simple yet accurate model for the transmission of surface plasmon
polaritons (SPPs) in a finite metal-insulator-metal (MIM) waveguide to the
sides of the apertures is proposed and demonstrated to be more accurate than
available models. It is as simple as using a magnetic current density across
the plane of the aperture whose value is defined by SPPs with any number of
modes in the waveguide through the equivalence principle. Then, the generated
SPPs on the both sides of the aperture are extracted from the convolution
integral of the equivalent current density and Green's function. As a result,
the model provides the transmission coefficients of SPPs in the MIM to the side
walls of the aperture accurately and efficiently; not only for symmetric MIMs
with a single isolating layer but also for non-symmetric ones with
multi-layered insulating materials. The results are in very good agreement with
those obtained by the FDTD method and better than the other approximations
available in literature for a wide range of aperture widths.
