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.PACS numbers: 71.36.+c, 73.20.Mf, 78.66.Bz Metallic gratings can exhibit absorption anomalies [1]. One of these anomalies which is particularly remarkable is observed for p-polarized light only, and is due to surface plasmon polariton (SPP) excitations. SPP excitation induces a minimum on the specular reflectance spectra which is indicative of the amount of energy flowing parallel to the surface. In a first order approximation, the spectral position of the minimum does not depend on the shape or the amplitude of the grooves but depends only on the dielectric constant and period of the grating. One interesting problem which has been raised long time ago and which is still of interest today is the near field dependence of these modes on the grating shape [2]. Linked to this problem is the possible existence of modes localized in grooves of prominent shape and their relation with non-linear optical effects observed in certain rough metal surfaces [3][4][5][6][7][8].At the beginning of the century Rayleigh pointed out that flat rigid surfaces with cylindrical holes can present acoustic resonances for well defined depths of the holes [9]. Rayleigh showed that under these resonant conditions the acoustic energy is concentrated in the holes and he suggested that similar effects could occur with light. More recently Rendell and Scalapino [10] suggested the possible existence of localized plasmons in order to explain light emission in metal-oxide-metal structures. These plasmon modes are qualitatively different from propagative SPPs on a flat surface excited by attenuated total reflection or by a gentle surface corrugation. Despite the conceptual and practical interest of these surface shape resonances and well documented theoretical predictions, till now no experimental evidence of these electromagnetic resonances has been reported for optical frequencies.In this letter we show that for lamellar gratings with deep rectangular cross sections, localized waveguide resonances which are equivalent to the acoustic resonances described by Rayleigh, can be excited in the channels when the impinging light has an electric field component perpendicular to the grooves direction. The experiments here presented also illustrate the existence of hybrid modes, combination of standing waves localized in the grooves with propagating SPPs.Measured samples consist in periodic arrays of metallic grooves of nominal width ...

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.

No abstract