Surface plasmons are waves that propagate along the surface of a conductor. By altering the structure of a metal's surface, the properties of surface plasmons--in particular their interaction with light--can be tailored, which offers the potential for developing new types of photonic device. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics.

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The emerging field of plasmonics is based on exploiting the coupling between light and collective electronic excitations within conducting materials known as surface plasmons. Because the so-called surface plasmon polariton (SPP) modes that arise from this coupling are not constrained by the optical diffraction limit, it is hoped that they could enable the construction of ultracompact optical components 1,2 . But in order that such potential can be realized, it is vital that the relatively poor light-SPP coupling be improved. This is made worse by the fact that the incident light that is conventionally used to launch SPPs in a metal film 3-6 is a significant source of noise, unless directed away from a region of interest, which then decreases the signal and increases the system's size. Back-side illumination of subwavelength apertures in optically thick metal films 7-13 eliminates this problem but does not ensure a unique propagation direction for the SPP. We propose a novel back-side slit-illumination method that incorporates a periodic array of grooves carved into the front side of a thick metal film. Bragg reflection enhances the propagation of SPPs away from the array, enabling them to be unidirectionally launched from, and focused to, a localized point.A picture of the proposed surface plasmon polariton (SPP) launcher is shown in Fig. 1. A periodic array of one-dimensional indentations is fabricated at the (output) metal surface close and parallel to the illuminated slit. The design of this device is based on two facts. The first one is that the reflection of SPPs by a periodic array of indentations presents maxima at the low-l edges of the plasmonic bandgaps [14][15][16] . For subwavelength indentations, the spectral locations of these edges can be obtained by folding the dispersion relation of SPPs for a flat metal surface into the first Brillouin zone, satisfying the following expression:where P is the period of the array, k p holds for the in-plane plasmon wavevector and m is the band index. Remarkably, although the reflectance maxima depend on the groove geometry (width and depth) and the number of grooves, their spectral locations do not. The second fact is that the phase picked up by the SPP on reflection is just mπ, precisely at the condition given by

We present a new theoretical and numerical framework for the study of the optical properties of micrometric and nanometric three-dimensional structures of arbitrary shape. We show that the field distribution induced inside and outside such a structure by different external monochromatic sources can be obtained from a unique generalized field propagator expressed in direct space. An application of the method to the confinement of optical fields due to the scattering of subwavelength objects is presented. 02.30.Tb, 02.60.Nm, 61.16.Ch Over the past few years, considerable efforts have been devoted to the understanding of the response properties of micrometric and nanometric structures isolated in gas phase [1] or integrated on a surface [2]. Recent continuous progress in scanning near-field optical microscopy (SNOM) [3][4][5][6] has strongly enhanced our insight into the field distributions in the vicinity of such subwavelength structures. From a theoretical point of view, dealing with low-symmetry, three-dimensional (3D) systems renders the intricate problem related to the electromagnetic boundary conditions at the material interfaces intractable. Therefore, most of the numerical approaches based on the matching of these boundary conditions encounter difficulties when applied to geometrically complex mesoscopic and subwavelength systems made of realistic materials. This fact was demonstrated by the theoretical challenge raised by the development of SNOM.In order to circumvent this obstacle inherent to the matching of electromagnetic boundary conditions, we present in this Letter a new theoretical framework for a large class of problems dealing with 3D objects of arbitrary shape and dielectric functions. More precisely, we show that the entire field distribution induced by different sources, inside and outside a 3D structure, can be derived from a unique generalized field propagator expressed in direct space. This approach is based on Green's dyadic technique. Although the power of this technique has long been recognized, its use was impeded by the difficult construction of Green's dyadics associated with complex geometries. In this Letter, we point out that electromagnetic scattering theory gains much efficiency by adopting certain procedures from quantum scattering theory. Particularly, the introduction of a dyadic Dyson's equation enables the straightforward construction of Green's dyadics associated with arbitrarily complex geometries.Let us consider a nonmagnetic scattering system described by a dielectric tensor´͑r, v͒ ´r ͑v͒ 1´s͑r, v͒, embedded in an infinite homogeneous reference mediuḿ r ͑v͒. This scattering system must not necessarily be a single region, but can be formed by disconnected parts embedded in the reference system, with the tensor´s͑r, v͒ vanishing outside of the scattering system.When an incident field E 0 ͑r͒ (a monochromatic field with the usual exp͓2ivt͔ time dependence is assumed throughout this Letter, but the method is able to handle arbitrary incident waves) impinges on that system...