A. Polemi

Dispersion Characteristics of a Metamaterial-Based Parallel-Plate Ridge Gap Waveguide Realized by Bed of Nails

The newly introduced parallel-plate ridge gap waveguide consists of a metal ridge in a metamaterial surface, covered by a metallic plate at a small height above it. The gap waveguide is simple to manufacture, especially at millimeter and sub-millimeter wave frequencies. The metamaterial surface is designed to provide a frequency band where normal global parallel-plate modes are in cutoff, thereby allowing a confined gap wave to propagate along the ridge. This paper presents an approximate analytical solution for this confined quasi-TEM dominant mode of the ridge gap waveguide, when the metamaterial surface is an artificial magnetic conductor in the form of a bed of nails. The modal solution is found by dividing the field problem in three regions, the central region above the ridge and the two surrounding side regions above the nails. The fields within the side regions are expressed in terms of two evanescent TE and TM modes obtained by treating the bed of nails as an isotropic impedance surface, and the field in the central ridge region is expanded as a fundamental TEM parallel-plate mode with unknown longitudinal propagation constant. The field solutions are linked together by equalizing longitudinal propagation constants and imposing point-continuity of fields across the region interfaces, resulting in a transcendental dispersion equation. This is solved and presented in a dispersion diagram, showing good agreement with a numerical solution using a general electromagnetic solver. Both the lower and upper cutoff frequencies of the normal global parallel-plate modes are predicted, as well as the quasi-TEM nature of the gap mode between these frequencies, and the evanescent fields in the two side regions decay very rapidly away from the ridge.

Incremental theory of diffraction: a new-improved formulation

In this paper, a general systematic procedure is presented for defining incremental field contributions. They may provide effective tools to describe a wide class of scattering and diffraction phenomena at any aspect, within a unitary, self-consistent framework. This procedure is based on a generalization of the incremental theory of diffraction (ITD) localization process for uniform cylindrical, local canonical problems with elementary source illumination and arbitrary observation aspects. In particular, it is shown that the spectral integral formulation of the exact solution for the local canonical problem may also be represented as a spatial integral convolution along the longitudinal coordinates of the cylindrical configuration. Its integrand is then directly used to define the relevant incremental field contribution. For the sake of convenience, but without loss of generality, this procedure is illustrated for the case of local wedge configurations. Also, a specific suitable asymptotic analysis is developed to derive new closed form high-frequency expressions from the spectral integral formulation. These expressions for the incremental field contributions explicitly satisfy reciprocity and are applicable at any incidence and observation aspect. This generalization of the ITD localization process together with its more accurate asymptotic analysis provides a definite improvement of the method.

High-frequency Green's function for a semi-infinite array of electric dipoles on a grounded slab .I. formulation

A uniform, high-frequency solution is presented for the electromagnetic field radiated at finite distance by a semi-infinite array of elementary electric dipoles placed on an infinite grounded dielectric slab. This solution is useful for the efficient analysis of printed arrays. The field is represented in terms of a series encompassing propagating and evanescent truncated Floquet waves together with their corresponding diffracted rays, which arise from the edge of the array. The high-frequency formulation also includes surface and leaky wave contributions excited at the array edge. The diffracted waves contain discontinuities which compensate the disappearance of surface, leaky and truncated Floquet waves at their pertinent shadow boundaries.

Efficient disc on pillar substrates for surface enhanced Raman spectroscopy

In this work, geometrical optimizations of Ag disc on pillar (DOP) hybrid plasmonic nanostructures were conducted and allowed us to achieve reproducible average enhancement factors of 1 × 10(9) and greater.

A Mesh-Adapted Closed-Form Regular Kernel for 3D Singular Integral Equations

The Greens functions employed in the method of moments (MoM) diverge when observation and source points coincide; this is at the origin of the difficulties in computing the MoM matrix entries, and in handling the near-field interactions in fast Fourier transform (FFT)-based fast methods and other sampling-based methods. In this paper, we show that this singularity can be avoided, and a modified regular Greens function can be used instead. This latter is obtained from the spectral representation of the usual Greens function via windowing of its spectrum; the width of the spectral window depends on the size of the mesh employed for discretizing the problem, so that the proposed regular Greens function is a mesh-adapted regular kernel. We address a general 3D problem; we relate the MoM reaction integrals to the 2D Fourier spectrum of the Greens function, that allows to discuss the necessary spectral bandwidth for the windowed Greens function. We employ a tapered window, and present a closed-form expression for the spatial Greens function. Numerical results are presented for 3D antenna and scattering problems discretized with Rao-Wilton-Glisson (RWG) functions, and for uniform and nonuniform meshing. They show that the proposed method yields accurate solutions also for the antenna input impedance. The meaning of the regularized Greens function is also discussed and put in perspective.

On the Polarization Properties of a Dielectric Leaky Wave Antenna

In this letter, the polarization properties of composite planar dielectric structures fed by point sources are investigated. With an appropriate choice of substrate heights and dielectric constants, the structure is a leaky wave antenna (LWA), based on a (Fabry-Peacuterot)-like effect, which enhances the directivity of isotropic sources (e.g., dipoles or slots). These antennas have been deeply investigated in the past, especially from the antenna gain point of view. Nevertheless, the aspect concerning with the polarization has not been well explored yet. In our analysis, we show that this high-gain antenna is very well polarized when the excitation is provided by a perfectly polarized feeding source. This concept is important in the design of overlapped apertures in multifeed aperture systems

Distance dependent quenching effect in nanoparticle dimers

In this paper, we investigate the emission characteristics of a molecule placed in the gap of a nanoparticle dimer configuration. The emission process is described in terms of a local field enhancement factor and the overall quantum yield of the system. The molecule is represented as a dipolar source, with fixed length and fed by a constant current. We first describe the coupled dimer-molecule system and compare these results to a single sphere-molecule system. Next, the effect of dimer size is investigated by changing the radius of the nanoparticles. We find that when the radius increases, a saturation effect occurs that trends towards the case of a radiating dipole between two flat interfaces, which we refer to as a parallel plate waveguide geometry. An analytical solution for the parallel plate waveguide geometry is presented and compared to the results for the spherical dimer configuration. We use this approximation as a reference solution, and also, it provides useful guidelines to understand the physical mechanism behind the energy transfer between the molecule and the dimer. We find that the emission intensity undergoes a quenching effect only when the inter-nanoparticle gap distance of the dimer is very small, meaning that strong coupling prevails over energy engaged in the heating process unless the molecule is extremely close to the metal surface.

High-frequency Green's function for a semi-infinite array of electric dipoles on an infinite grounded stratified dielectric slab: part II-spatial domain parameterization

This second part of a three-paper sequence deals with the spatial domain parametrization and physical interpretation of the relevant asymptotic high-frequency Greens function for a semi-infinite phased array of parallel dipoles on an infinite stratified grounded dielectric slab. This array Greens function (AGF) has been previously derived using a spectral domain formulation; the relevant asymptotic solution contains contributions associated with Floquet waves (FWs), and corresponding surface, leaky and diffracted waves excited at the array edge. Both the truncated-FW series and the series of corresponding diffracted field contributions exhibit excellent convergence properties. In the present paper, through application of the Poisson summation, the AGF for a plane-stratified grounded dielectric slab is developed in terms of space domain FW-dependent Kirchhoff radiation integrals which are synthesized by superposition of periodicity-modulated phased line sources oriented parallel to the edge. The asymptotic evaluation of each Kirchhoff radiation integral leads to a grouping of various asymptotic terms, which provide physically appealing interpretations of a variety of wave processes, encompassing slab-modulated propagating (radiating) and evanescent (nonradiating) FWs, slab-guided surface waves (SWs) or leaky waves (LWs), and their edge-coupled phenomenologies. The present space domain parametrization leads to the same asymptotics as that from the spectral domain parametrization, but allows a clear description of the spatial wave interaction processes.

A leaky-wave groove antenna at optical frequency

In the framework of nanoantennas functioning at optical frequencies, we present here a new kind of leaky-wave antenna realized as a groove in an aluminum superstrate, supported by a silver substrate. The antenna works in the optical frequency range where the silver acts as a dielectric with equivalent refractive index between zero and one. Under these conditions, the dominant mode launched in the structure exhibits a phase velocity larger than the speed of light in free-space, thus producing a leaky-wave radiation in free-space. We propose a simplified analytical form of the dispersion characteristic of the fundamental mode supported by the structure, which allows for identification of the radiative leaky-wave condition. We also propose approximate formulas for calculating the antenna gain and loss efficiency. The results obtained through these formulas are successfully compared with full-wave simulations. The final parametric study shows how the radiation characteristic is affected by the variation of ge...

High-frequency Green's function for a semi-infinite array of electric dipoles on an infinite grounded stratified dielectric slab: part III

The particular effects of phase matching in interactions between different wave species and interaction mechanisms pertaining to the asymptotic Greens function for a semi-infinite phased array of parallel dipoles on an infinite grounded stratified dielectric slab are addressed. The two previous parts have lead to a grouping of certain asymptotic terms, which provides physically appealing interpretations of interacting wave processes that involve slab-modulated propagating (radiating) Floquet waves (FW), slab-guided leaky waves (LW), evanescent (nonradiating) FWs and surface waves (SW), together with their associated conical edge-coupled wave constituents. Special attention is now given to the role, in these wave interaction functions,of the difference between the propagating wave numbers (i.e., the degree of phase matching) that characterizes any pair of propagating FWs and LWs, and evanescent FWs and SWs. Cutoff transitions of FWs from the evanescent to the propagating regime are described also in terms of a second wave interaction function, structured similarly to the first. In connection with applications, the interaction between SWs and evanescent FWs, as well as FW transitions from evanescent to propagating, are discussed from the perspective of scan blindness in actual arrays. Various numerical studies highlight the role of phase matching in different problem scenarios.