  We performed Cathodoluminescence (CL) spectroscopy and imaging in a high
resolution scanning electron microscope to locally and selectively excite and
investigate the plasmonic property of a multibranched gold nanostar on silicon
substrate. This method allows us to map the local density of optical states
from the nanostar with a spatial resolution down to a few nanometers. We
resolve both in spatial and spectral domain, different plasmon modes associated
with the nanostar. Finite- difference time-domain (FDTD) numerical simulations
are performed to support the experimental observations. We investigate the
effect of substrate on the plasmonic property of these complex shaped
nanostars. The powerful CL-FDTD combination helps us to understand the effect
of the substrate on plasmonic response of branched nanoparticles.
