Propagation properties of surface plasmon along metal nano-aperture and nano-wire

Surface plasmon propagation properties for nano-sized metal aperture and nano-wire structures are studied based on the waveguide theory. It is verified that there are only TM0 and HE1 mode existing in both structures. For nano-wire structure, however, there is no cut-off radii for both TM0 and HE1 mode. On the contrast, there is a cut-off radius for nano-sized silver aperture structure, and even more, there are several characteristic radius coefficients a/lambda, -0.32, 0.73. For each section, the EM wave may takes on different features, e.g., totally-bounded features along the aperture interface for a/lambda > 0.73, or oscillation inside core or strong attenuation along z direction for 0.32 < a/lambda < 0.73). HE1 mode of metal aperture doesn't show cutoff radius as well, but the wave-number along z direction monotonically increase with the radius coefficient a/lambda in contrast with existence of a characteristic radius coefficient a/lambda of 0.41 for nano-wire.