  Regular arrays of metal nanoparticles on metal films have tuneable optical
resonances that can be applied for surface enhanced Raman scattering or
biosensing. With the aim of developing more surface selective geometries we
investigate regular gold nanoparticle arrays on 25nm thick gold films, which
allow to excite asymmetric surface plasmon modes featuring a much better field
confinement compared to the symmetric modes used in conventional surface
plasmon resonance setups. By optical extinction spectroscopy we identify the
plasmonic modes sustained by our structures. Furthermore, the role of thermal
treatment of the metal structures is investigated, revealing the role of
modifications in the crystalline structure of gold on the optical properties.
