Using a silver grating of variable groove depth overcoated by a thin film of copper phthalocyanine, we measured an increase of the Raman efficiency of 104as compared to the flat case (copper phthalocyanine on a flat silver surface). This effect is due to the resonant excitation of delocalized surface plasmons. The value of 104corresponds to an optimum groove depth of the grating which acts as a high gain directive antenna when surface plasmons resonance is achieved at the Stokes frequency. ©1995 American Institute of Physics.