Resonant response of a bare metallic grating to S-polarized light

This review is motivated by two oft-noted observations concerning surface enhanced Raman Scattering (SERS): a) the SERS excitation spectrum is not well-correlated to the reflectivity (or extinction) spectrum; b) SERS is often a short-ranged, first-layer effect due preponderantly to molecules adsorbed at interstitial sites. The question addressed herein is: can classical electromagnetic causes explain these facts? The answer we provide is affirmative as concerns (molecules adsorbed at) highly conducting, one-dimensional, lamellar gratings (grooves with rectangular cross sections) exposed to S-polarized light (electric field vector perpendicular to the plane of incidence and parallel to the rulings). Our analysis thus complements earlier discussions of this question, that were concerned with molecules adsorbed on metallic gratings or randomly rough metal surfaces exposed to P-polarized light (magnetic field vector perpendicular to the plane of incidence and parallel to the rulings). We find that the grooves of the lamellar gratings studied act as cavity resonators; the cavity field is strongly enhanced in the neighborhood of the resonance wavelength, but this has no significant effect on the field outside the cavity (in both the near and far zones). This findings is illustrated with the example of a silver grating with a period of 0.38 μm and grooves 2.5 μm deep and 0.35 μm wide, which is shown to produce a (modulus squared) electric field enhancement within the grooves of about 440 near the resonance wavelength of about 0.7 μm, at the same time that the far-field response (i.e. reflectivity) of the grating is very nearly that of a flat silver mirror throughout the visible.