  The specular reflectivity of lamellar gratings of gold with grooves 0.5
microns wide separated by a distance of 3.5 microns was measured on the 2000
cm$^{-1}$ - 7000 cm$^{-1}$ spectral range for p-polarized light. For the first
time, experimental evidence of the excitation of electromagnetic surface shape
resonances for optical frequencies is given. In these resonances the electric
field is highly localized inside the grooves and is almost zero in all other
regions. For grooves of depth equal to 0.6 microns, we have analyzed one of
these modes whose wavelength (3.3 microns) is much greater than the lateral
dimension of the grooves.
