In this letter we show how transmission metallic gratings with very narrow and deep enough slits can exhibit transmission resonances for wavelengths larger than the period of the grating. By using a transfer matrix formalism and a quasi-analytical model based on a modal expansion, we show that there are two possible ways of transferring light from the upper surface to the lower one: by the excitation of coupled surface plasmon polaritons on both surfaces of the metallic grating or by the coupling of incident plane waves with waveguide resonances located in the slits. Both mechanisms can lead to almost perfect transmittance for those particular resonances.PACS numbers: 78.66. Bz, 73.20.Mf, 42.79.Dj, 71.36.+c Absorption anomalies in metallic gratings have attracted much attention since their discovery by Wood [1] in 1902. One of these anomalies is only observed for p-polarized light (H parallel to the grating grooves) and appears as a minimum on the specular reflectance. Now it is well known [2] that this anomaly stems from the excitation of surface plasmon polaritons (SPPs) by the incident electromagnetic radiation. The dependence of these SPP modes on the grating shape and the possible existence of modes localized in the grooves have been studied for a number of years and are still of interest [2][3][4][5][6][7][8]. On the other hand, the activity of the last decade in the field of photonic crystals [9] has originated a renewed interest in the properties of SPPs, as they can be viewed as surface electromagnetic modes propagating in 1D periodic dielectric media [10].Besides, experimental evidence of the excitation of optical waveguide modes inside the narrow grooves of zeroorder reflection gratings has been recently given [11]. Both from fundamental and practical points of view it would be very interesting to analyze the transmission properties of these waveguide modes. Also very recently some experiments carried out in arrays of submicrometre cylindrical holes in metallic films have shown an extraordinary optical transmission at wavelengths up to ten times larger than the diameter of the holes [12]. The similarities between this last structure and metal gratings suggest the possibility of equivalent resonant effects in transmission metal gratings of very narrow slits.In this letter we test theoretically this possibility by analyzing the response of transmission metallic gratings to p-polarized electromagnetic radiation. We will show how, for very narrow slits, the coupling of the incident light with surface electromagnetic modes of the grating can lead to almost perfect transmission resonances appearing at wavelengths larger than the period of the grating and hence much larger than the lateral dimensions of the slits.Inset of Fig. 1 shows a schematic view of the structures under study with the definition of the different parameters: the period of the grating (d), the width (a) and height (h) of the slits. The substrate is characterized by a dielectric constant, ǫ. Advances in material technology have allowed ...

In this Letter we show that a single rectangular hole exhibits transmission resonances that appear near the cutoff wavelength of the hole waveguide. For light polarized with the electric field pointing along the short axis, it is shown that the normalized-to-area transmittance at resonance is proportional to the ratio between the long and short sides, and to the dielectric constant inside the hole. Importantly, this resonant transmission process is accompanied by a huge enhancement of the electric field at both entrance and exit interfaces of the hole. These findings open the possibility of using rectangular holes for spectroscopic purposes or for exploring non-linear effects.

We develop a theory to study apertureless scanning near-field optical microscopy which takes into account retardation, higher multipoles of the tip, and the multiple scattering between the tip and the surface. We focus on metallic systems and discuss the implication of the formation of tip-induced surface plasmon modes in the tip-surface system. We discuss the effects associated with the shift in energy of those modes as a function of the tip-surface distance. Both the local field and the scattering cross section are enhanced when the tip approaches the surface, but there is no general correspondence between the two enhancements.

We derive a general expression for the signal in scanning near-field optical microscopy. This expression, based on the reciprocity theorem of electromagnetism, is an overlapping integral between the local electric field and a function that characterizes the tip. In particular, it includes the dependence on wavelength, illumination conditions, and polarization. To illustrate the potentiality of this theory, we discuss the polarization behavior and the spectral response of the apertureless setup.