  Cathodoluminescence (CL) imaging spectroscopy is an important technique to
understand resonant behavior of optical nanoantennas. We report high-resolution
CL spectroscopy of triangular gold nanoantennas designed with near-vacuum
effective index and very small metal-substrate interface. This design helped in
addressing issues related to background luminescence and shifting of dipole
modes beyond visible spectrum. Spatial and spectral investigations of various
plasmonic modes are reported. Out-of-plane dipole modes excited with vertically
illuminated electron beam showed high-contrast tip illumination in panchromatic
imaging. By tilting the nanostructures during fabrication, in-plane dipole
modes of antennas were excited. Finite-difference time-domain simulations for
electron and optical excitations of different modes showed excellent agreement
with experimental results. Our approach of efficiently exciting antenna modes
by using low index substrates is confirmed both with experiments and numerical
simulations. This should provide further insights into better understanding of
optical antennas for various applications.
