  We studied experimentally and theoretically the optical transmission through
asymmetrical holes of a metal film, which is constructed by introducing small
protuberances to the sides of individual square holes. Due to the symmetry
breaking of hole shape, an interesting transmission feature appears: both the
Ag-glass (1, 0) and Ag-glass (1, 1) peaks split distinctly. Detailed studies
indicate that the peak splitting is actually associated with the two
asymmetrical waveguide surface-plasmon (WSP) modes confined on the surface of
opposite hole walls. The finding demonstrates the crucial role of WSP modes and
enriches our understanding of the phenomenon.
