Marcelo Lester

Coupling of evanescent s-polarized waves to the far field by waveguide modes in metallic arrays

In this paper we show that enhanced optical transmission through a 1D periodic slit array comprising real metallic cylinders is possible for s-polarization when the array is near a dielectric interface. We investigate the behaviour of this structure under s-polarized illumination, in which case surface plasmons are not excited. Numerical results show that the transmitted intensity appears as a periodic function of the slit depth, for propagating as well as for evanescent incidence, suggesting that this behaviour is related to the excitation of waveguide modes in the slits. In particular, the coupling of evanescent to propagating electromagnetic waves is investigated. It is shown that s-polarized evanescent waves generated at the interface can be transformed into propagating waves if the optical width of the slits allows propagation of the first waveguide mode. As the interface approaches the array, the transmitted intensity increases for evanescent incidence.

An optical nanoantenna made of plasmonic chain resonators

We propose a novel structure that behaves like an optical antenna and converts evanescent waves into propagating waves. The system comprises metallic subwavelength cylinders distributed in a dual-period array. It is illuminated by an evanescent wave generated by total internal reflection in a close interface. For particular wavelengths, the system exhibits resonances and the inhomogeneous wave is converted into propagating waves that radiate to the far field. This effect can be controlled by varying the geometrical parameters of the structure, such as the period and the inclination angle. Therefore, the transmitted intensity can be sent to a predesigned direction. This structure could be used in highly sensitive detection devices, among other applications.

Blaze produced by a dual-period array of subwavelength cylinders

We propose an alternative way of enhancing the intensity diffracted by a grating in a given direction using a dual-period array. Each period comprises several identical cylinders with diameters much smaller than the incident wavelength (subwavelength cylinders), whose axes are aligned in a plane which is tilted with respect to the periodicity direction. We present results for metallic and dielectric cylinders, and show that in both cases this structure behaves like a blazed grating in the sense of its capability to enhance the intensity in a pre-designed direction. This blazed-like behavior is found for both incident polarization modes. If we consider the quick evolution of manufacturing techniques of nanogratings, such structures constitute a realizable alternative not only for the microwave and millimeter wave regions but also for optical devices.

Internal symmetries in conical diffraction from metallic gratings

Abstract Previous investigations in conical diffraction have shown that for a metallic grating illuminated under conditions of resonant excitation of surface plasmons, the maximum efficiency of polarization conversion is obtained when the plane of incidence forms an angle φ = 45% with the main section of the grating. This fact has been understood as a consequence of the evident symmetry imposed by the incident wave and it has not been questioned up to now. Here we show that, apart from this highly symmetrical configuration, the polarization conversion efficiency has local maxima also in other cases which do not exhibit such an evident symmetry. We demonstrate that the occurrence of both the local maxima for the new configurations and the maximum corresponding to φ 45% are a consequence of internal symmetries imposed by the propagation direction of the resonantly excited surface plasmons.

Reflection of electromagnetic waves from index-matched surfaces

The connection between the eigenmodes supported by a flat interface between two permeable, index-matched media and the singular behavior of its reflectivity at grazing incidence has been investigated. We show that these eigenmodes can be coupled to non-grazing incident waves through a corrugation and that the coupling is correlated with strong peaks in the curves of reflectivity versus angle of incidence. The shape and position of these peaks is studied for different incident polarizations and height of corrugation. These behaviours are discussed in terms of the evolution with corrugation of the complex poles of the analytical continuation of the reflectivity.

Scattering of electromagnetic waves at the corrugated interface between index-matched media

Abstract We consider the scattering of s- and p-polarized electromagnetic waves incident onto the periodically corrugated boundary between index-matched media. First we find the eigenmodes supported by this kind of interface by means of a perturbative solution to the corresponding homogeneous problem and show that these eigenmodes are intimately related to the occurrence of strong peaks in the curves of reflectivity versus angle of incidence. Second, we use the Rayleigh hypothesis to derive integral equations for the amplitudes of the reflected and transmitted waves and give a solution to these equations in the form of series of powers of the surface profile. This perturbative treatment is useful to follow the strong changes that a weal corrugation introduces in the reflectivity of index-matched interfaces.

Study of resonant modes of a periodic metallic array near a dielectric interface : evanescent-to-propagating coupling via surface plasmon excitation

The structural modes of a 2D periodic array of metallic cylinders near a dielectric interface are studied, for large and small wavelength-to-period ratios. We show that there are two clearly distinguished kinds of resonances: waveguide modes of the gap between the array and the interface, and eigenmode excitations (plasmons). While waveguide modes are present for both polarizations, surface plasmon polaritons (SPPs) only occur for p polarization, and, under particular conditions, this mechanism can produce an efficient coupling (about 60%) between the incident evanescent wave and the eigenmodes supported by the structure, which produces an enhanced transmitted propagating order. The response of the structure is analysed in detail by varying its relevant parameters, paying particular attention to the interplay between SPPs and Rayleigh anomalies, and its effect on the grating response.

Control of the diffracted response of wire arrays with double periods

The possibility of controlling the diffracted response of a periodic structure is investigated by using dual-period arrays, i.e., periodic arrays with a compound unit cell. We consider wire gratings in which each period comprises several cylinders with circular cross sections and all the cylinder axes are contained in the same plane. It is shown that this kind of structure permits one to control the diffracted response, regardless of the cylinder material and the incident polarization. Our numerical results suggest that the effect produced by wire gratings with dual-period characteristics is basically a geometric effect, and it can be present for other shapes of individual scatterers within each subarray.

Enhanced transmission via evanescent-to-propagating conversion in metallic nanoslits: role of Rayleigh anomalies

We analyze the enhanced transmission phenomenon in subwavelength slit structures near a dielectric interface. In particular, we investigate the influence of Rayleigh anomalies in the spectral position as well as in the bandwidth of Fabry?Perot resonances excited on such structures. We consider the cases of propagating and evanescent incidence, i.e., when the metallic structure is illuminated from the dielectric medium side with an incidence angle larger than the critical angle. We show that Rayleigh anomalies strongly interact with Fabry?Perot resonances, and make them deviate from the spectral positions predicted by the infinitely thin slit model. To get physical insight into this problem, we develop a simplified electromagnetic model and show that there is a close correspondence between the transmitted response of the structure and the behavior of certain function that depends on the geometrical and the illumination parameters. Our results suggest that Rayleigh anomalies strongly modify the electromagnetic response of the structure due to the existence of surface waves that modify the coupling condition between the fields inside and outside the slits. Besides, we show that even in absence of Fabry?Perot resonances, it is possible to produce enhanced transmission by taking advantage of the pseudoperiodicity condition of the fields.

Detection of eccentricity in silver nanotubes by means of induced optical forces and torques

In previous works (Abraham et al 2011 Plasmonics 6 435; Abraham Ekeroth and Lester 2012 Plasmonics 7 579; Abraham Ekeroth and Lester 2013 Plasmonics 8 1417; Abraham Ekeroth R M and Lester M 2015 Plasmonics 10 989–98), we have conducted an exhaustive study about optical properties of metallic realistic two-dimensional (2D) nanotubes, using an experimental-interpolated dielectric function (Palik 1985 Handbook of Optical Constants of Solids (Toronto: Academic Press)). In the case of non-homogeneous metallic shells, we suggested (in a theoretical form) a procedure to detect the non-uniformity of shells in parallel, disperse and randomly oriented long nanotubes (2D system). This detection is based exclusively on the plasmonic properties of the response (Abraham Ekeroth and Lester 2012 Plasmonics 7 579). Here we consider exact calculations of forces and torques, exerted by light on these kinds of nanostructures, illustrating the mechanical effects of plasmonic excitations with one example of silver shell under p-polarized incidence. This study continues with the methodology implemented in the previous paper (Abraham Ekeroth R M and Lester M 2015 Plasmonics 10 989–98), for homogeneous nanotubes. The features of the electromagnetic interaction in these structures, from the point of view of mechanical magnitudes, make it possible to conceive new possible interesting applications. Particularly, we point out some results regarding detection of eccentricity in nanotubes in vacuum (when Brownian movement is not taken into account). We interpret the optical response of the realistic shells in the framework of plasmon hybridization model (PHM), which is deduced from a quasi-static approximation. Our integral formalism provides for retardation effects and possible errors is only due to its numerical implementation.