It is theoretically shown that nanometric silver lamellar gratings present very strong visible light absorption inside the grooves, leading to electric field enhancement of several orders of magnitude. It is due to the excitation of quasistatic surface plasmon polaritons with particular small penetration depth in the metal. This may explain the abnormal optical absorption observed a long time ago on almost flat Ag films. Surface enhanced Raman scattering in rough metallic films could also be due to the excitation of such quasistatic plasmon polaritons in grain boundaries or notches of the films.

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PACS. 71.30.+h Metal insulator transitions and other electronic transitions - 71.38.+i Polarons and electron-phonon interactions - 71.10.-w Theories and models of many electron systems,