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

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... e-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.