When perforated metal films are sufficiently thin, in addition to exciting surface plasmon-polariton
(SPP) modes by conventional two dimensional grating scattering, there is also the possibility of
coupling to the localised modes associated with the holes. Here, experimental transmission spectra are
obtained from focused ion beam fabricated hole arrays exhibiting localized modes in the visible
frequency region. We employ both analytical and numerical (finite element) modeling to understand
the fundamental properties of the localized mode. Finally, the sensitivity of the optical response to
changes in refractive index is explored, and its potential for sensing applications is discussed.