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

Materials showing electromagnetic properties that are not attainable in naturally occurring media, the so called metamaterials, have been lately, and still are, among the most active fields in optical and materials physics and engineering. Among those properties, one of the most attractive is the sub-diffraction resolving capability predicted for media having index of refraction of -1. Here we propose a fully 3D, isotropic metamaterial with strong electric and magnetic responses in the optical regime, based on spherical metallo-dielectric core-shell nanospheres. The magnetic response stems from the lowest, magnetic-dipole resonance of the dielectric shell with high refractive index, and can be tuned to coincide with the plasmon resonance of the metal core, responsible for the electric response. Since the response does not originate from coupling between structures, no particular periodic arrangement needs to be imposed. Moreover, due to the geometry of the constituents, the metamaterial is intrinsically isotropic and polarization independent. It could be realized with current fabrication techniques with materials such as Silver (core) and Silicon or Germanium (shell). For these particular realistic designs, the metamaterials present negative index in the range within 1.2 − 1.55 µm.

In this work, the scattering of surface plasmons by a finite periodic array of one-dimensional grooves is theoretically analyzed by means of a modal expansion technique. We have found that the geometrical parameters of the array can be properly tuned to achieve optimal performance of the structure either as a Bragg reflector or as a converter of surface plasmons into light. In this last case, the emitted light is collimated within a few degrees cone. Importantly, we also show that a small number of indentations in the array are sufficient to fully achieve its functional capabilities. PACS numbers: 73.20.Mf, 41.20.Jb Surface plasmons (SPs) are well known for their capabilities for concentrating light in sub-wavelength volumes and guiding light through the surface of a metal [1]. This has recently raised the prospect of SP-based photonic circuits, with length scales much smaller than those currently achieved [2,3,4]. In order to reach this goal, it is mandatory to know the scattering properties of SPs by simple surface profiles, as pointed out by recent experimental studies [2,5,6,7,8,9] From the theoretical side, this is still an open problem, even after a few seminal works based on the Rayleigh expansion [10,11,12] and the Green's function dyadic [13,14]. Both these methods require the evaluation of sophisticated scattering functions from which physical insight is not easily inferred.In this work we present an alternative formalism for calculating the scattering of SPs by indentations perforated on a thick metal film. For that purpose, we have extended to real metals the modal expansion technique previously developed within the perfect conductor approximation (PCA) [15,16], therefore incorporating surface plasmon polaritons into the model. This approach enables us to describe the scattering properties of an arbitrary set of indentations without any restriction over their position or shape. Besides, our method does not require any adjustable parameter, inasmuch as the wavelength-dependent dielectric function of the metal ε(λ) is previously known. In addition to this, it provides a very compact representation of the electromagnetic (EM) fields and simple expressions for the scattering magnitudes, which permits us to extract underlying physical mechanisms much more easily.Within our theoretical framework, the way of launching SPs onto a set of indentations resembles the back-side illumination employed in some experimental works [5]. We consider a single slit flanked by a set of N indentations placed in the output surface of an infinite metallic film of thickness h (see Fig.1). Eventually, the distance between the slit and indentations will be taken to be infinity. In this way, the slit merely plays the role of a theorist's SP-launcher. More precisely speaking, SPs are taken into account by applying surface impedance boundary conditions (SIBC) [17] to the metal/dielectric interface along the film surface.In this work, we consider the simplest case of 1D subwavelength indentations (grooves). Additionally, we impos...

Single metallic nanorods acting as half-wave antennas in the optical range exhibit an asymmetric, multi-resonant scattering spectrum that strongly depends on both their length and dielectric properties. Here we show that such spectral features can be easily understood in terms of Fano-like interference between adjacent plasmon resonances. On the basis of analytical and numerical results for different geometries, we demonstrate that Fano resonances may appear for such single-particle nanoantennas provided that interacting resonances overlap in both spatial and frequency domains. 12 Appendix A. Calculation techniques 12 Appendix B. Details of the fitting of scattering efficiency to Fano-like lineshape 13 References 15