  We report that rhomb-shaped metal nanoantenna arrays support multiple
plasmonic resonances, making them favorable bio-sensing substrates. Besides the
two localized plasmonic dipole modes associated with the two principle axes of
the rhombi, the sample supports an additional grating-induced surface plasmon
polariton resonance. The plasmonic properties of all modes are carefully
studied by far-field measurements together with numerical and analytical
calculations. The sample is then applied to surface-enhanced Raman scattering
measurements. It is shown to be highly efficient since two plasmonic resonances
of the structure were simultaneously tuned to coincide with the excitation and
the emission wave- length in the SERS experiment. The analysis is completed by
measuring the impact of the polarization angle on the SERS signal.
