  Optical properties of coupled metallic particles (MNPs) have been widely
reported due to their unique characteristics such as peak shift/splitting of
the coupling spectra and electromagnetic enhancement at sub-wavelength scale,
etc. In a previous work, we have investigated the coupling spectra of two
coupled MNPs with parallel polarized excitation. In this study, we investigate
the vertical polarization case in detail. Different from the parallel one, the
vertical one has its unique properties: (a) three coupling coefficients; (b)
positive coupling terms in the coupling equations; (c) blue-shifts of the peaks
with the increasing coupling strength for identical MNPs spectra, including
scattering, absorption, and photoluminescence. Comparison with published
experimental results shows the validity of this model. This work provides a
deeper understanding on the optical properties of coupled MNPs and is
beneficial to relevant applications.
