Local electron beam excitation and substrate effect on the plasmonic response of single gold nanostars.

We performed cathodoluminescence (CL) spectroscopy and imaging in a high-resolution scanning electron microscope to locally and selectively excite and investigate the plasmonic properties of a multi-branched gold nanostar on a 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 the 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 the substrate on the plasmonic properties of these complex-shaped nanostars. The powerful CL-FDTD combination helps us to understand the effect of the substrate on the plasmonic response of branched nanoparticles.