  We investigate a short (~1.5{\mu}m) partially-corrugated tapered waveguide
structure for mode coupling from a silicon micro-slab to a plasmonic nano-gap
waveguide at the optical communication frequency. More than 80% transmission
efficiency is reported numerically for the first time. The result indicates
that the corrugated waveguide structure should not only be helpful for
realizing full on-chip silicon plasmonic devices but also a good choice for
mode coupling enhancement from dielectric waveguides to plasmonic waveguides.
Meanwhile, we point out that the coupling mechanism reported here is different
from that achieved by exciting surface plasmon polaritions (SPPs) at metal
surfaces reported in [17] and [18].
