Surface electromagnetic modes of lamellar metallic gratings made of gold are analyzed both theoretically and experimentally in the 0.2-1.0 eV spectral range for p-polarized light. For deep enough grooves, we show how waveguide resonances that show no dispersion with parallel momemtum appear in the photonic band structure of these surface plasmons. In these resonances the electric field is highly localized inside the grooves and is almost zero in all other regions. We also illustrate the existence of hybrid modes, combination of standing waves with propagating surface plasmon polaritons. Experimental evidence of the excitation of both kind of localized surface plasmons is given for lamellar gratings of period 3.5 m with grooves 0.5 m wide and 0.6 m deep.
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