  Excitation of localized surface plasmons (LSPs) of metal nanoparticles (NPs)
residing on a flat metal film has attracted great attentions recently due to
the enhanced electromagnetic (EM) fields found to be higher than the case of
NPs on a dielectric substrate. In the present work, it is shown that even much
higher enhancement of EM fields is obtained by exciting the LSPs through
extended surface plasmons (ESPs) generated at the metallic film surface using
the Kretschmann-Raether configuration. We show that the largest EM field
enhancement and the highest surface-enhanced fluorescence intensity are
obtained when the incidence angle is the ESP resonance angle of the underlying
metal film. The finite-difference time-domain simulations indicate that
excitation of LSPs using ESPs can generate 1-3 orders higher EM field intensity
than direct excitation of the LSPs using incidence from free space. The
ultrahigh enhancement is attributed to the strong confinement of the ESP waves
in the vertical direction. The drastically intensified EM fields are
significant for highly-sensitive refractive index sensing, surface-enhanced
spectroscopies, and enhancing the efficiency of optoelectronic devices.
