  In this paper, we present a systematic investigation of symmetry-breaking in
the plasmonic modes of triangular gold nanoprisms. Their geometrical C3
symmetry is one of the simplest possible that allows degeneracy in the
particle's mode spectrum. It is reduced to the non-degenerate symmetries Cv or
E by positioning additional, smaller gold nanoprisms in close proximity, either
in a lateral or a vertical configuration. Corresponding to the lower symmetry
of the system, its eigenmodes also feature lower symmetries (Cv), or preserve
only the identity (E) as symmetry. We discuss how breaking the symmetry of the
plasmonic system not only breaks the degeneracy of some lower order modes, but
also how it alters the damping and eigenenergies of the observed Fano-type
resonances.
