  We present a theoretical analysis of the effects of short range surface
plasmon polariton excitation on sub-wavelength bridges in metal gratings. We
show that localized resonances in thin metal bridges placed within the slit of
a free-standing silver grating dramatically modify transmission spectra and
boost absorption regardless of the periodicity of the grating. Additionally,
the interference of multiple localized resonances makes it possible to tailor
the absorption properties of ultrathin gratings, regardless of the apertures'
geometrical size. This tunable, narrow-band, enhanced-absorption mechanism
triggered by resonant, short range surface plasmon polaritons may also enhance
nonlinear optical processes like harmonic generation, in view of the large
third-order susceptibility of metals.
