  The finite-difference time-domain method was employed to simulate light
propagation in tapered near-field fiber probes with small metal aperture. By
conducting large-volume simulations, including tapered metal-cladding waveguide
and connected optical fiber waveguide, we illustrated the coupling between
these guiding modes as well as the electric field distribution in the vicinity
of the aperture. The high collection efficiency of a double-tapered probe was
reproduced and was ascribed to the shortening of the cutoff region and the
efficient coupling to the guiding mode of the optical fiber. The dependence of
the efficiency on the tapered structure parameters was also examined.
