  As an interesting surface plasmon phenomenon discovered several years ago,
electromagnetic field redistribution in nanoparticle dimer on film system
provides a novel thought to enhance the light power on a plain film which could
been widely used in surface enhanced Raman scattering (SERS), solar cells,
photo-catalysis, etc. Homodimers on film are mainly investigated in past years,
while the properties of heterodimers on film are still unclear. In this work,
size difference induced electromagnetic field redistribution in Ag nanoparticle
dimer on Au film system is investigated first. The results obtained from finite
element method indicate that the smaller nanoparticle has much greater ability
to focus light energy on Au film, which even reached more than 5 time compared
to the larger one. Further researches indicate that this energy focusing
ability has a strong relationship to the wavelength and diameter ration in
dimer. Similar focusing phenomenon is found in the system of thick wire-smaller
particle on film. Later, the SERS spectra collected in the small
nanoparticle-large nanowire system provide an experimental evidence for this
theoretic predication. Our results strengthen the understanding of surface
plasmon on plane film and have potential application prospects in the surface
plasmon related fields.
