Role of local surface plasmon modes in the extraordinary optical transmission of sub-wavelength apertures

The extraordinary optical transmission through single sub-wavelength apertures is investigated in this paper. SPP modes on the upper and lower in-plane surfaces and Local Surface Plasmon (LSP) modes propagating along the vertical hole wall are clearly distinguished. We have numerically studied the procedure of wave propagating through a single sub-wavelength hole with a 3D finite-difference time-domain (FDTD) method and traced the wave vectors during the enhanced transmission using full wave analysis. It shows clear evidence that the LSP modes propagating along the hole wall is the origin of the extraordinary optical transmission of single sub-wavelength apertures, as a strong support to the previous experimental results. The role of these modes is identified by comparison of transmission properties of the sandwich-like structures made of different metallic materials. Transmission at inclined incidence for p-polarization and s-polarization are also calculated and the results can be properly explained with our view.