We present a full experimental characterization of isotropic artificial optical magnetism at visible frequencies produced by “raspberrylike” plasmonic nanoclusters synthesized by a bottom-up ...approach. Experimental measurements of extinction and light scattering associated with numerical simulations enable one to extract absolute values of the magnetic scattering cross section across the whole visible spectral range. The strength of the resonant magnetic response of a single magnetic nanocluster is illustrated as the effective magnetic permeability of a homogeneous sphere of identical volume.
Superdirective antennas developed over the last century have received renewed interest in recent years from the development of metamaterials. These arrays of electromagnetic resonators (or ...meta-atoms) carrying short wavelength electro- and/or magneto-inductive waves support current distributions with very high spatial frequency as required by the classical conditions for superdirectivity. As meta-atoms can have both electric and magnetic dipole characteristics (and hence radiation properties), developing antennas exploiting these distributions can challenge conventional intuitions regarding the optimal configurations required. In this work we are reporting the development of a genuinely superdirective array using split ring resonators (SRRs). We provide a comprehensive analytical model characterizing the radiation from SRR dimers in which excitation of only one split ring leads to superdirective radiation via mutually coupled modes. Our model exploits simple circuit descriptions of coupled resonant circuits, combined with standard radiation formulae for curvilinear current distributions. Using this simple model we are able to map directivity against possible SRR locations and orientations in two dimensions and identify the unique optimal configuration which meets the requirements for superdirective emission. We validate the theoretical findings by comparison to both full wave simulations and experiments showing that our SRR dimer achieves endfire directivity very close to the maximum theoretical value.
Closed-form representations of the physical optics (PO) field scattered in the far zone by plane penetrable dielectric angular sectors of arbitrary opening angle featuring a conical-section boundary ...are derived in terms of incomplete cylindrical functions (ICFs). The proposed expressions, possibly in combination with PO formulas for the scattering from polygonal plates, allow one to evaluate the scattering from flat dielectric plates with both convex and concave curved edges in a very efficient manner and constitute a useful tool to improve the accuracy of the geometrical characterization of a class of natural scatterers. In order to check the correctness and computational effectiveness of the proposed scattered-field solutions, comparisons with results obtained by accurate completely numerical PO calculations are provided. Simulation data by the method of moments are also presented to assess the applicability of the PO approximation to the considered sample scattering geometries.
The design of a dual-polarized microstrip series-fed linear traveling-wave array is described in this paper. The array is composed of two identical subarrays formed by cascading an equal number of ...four-port aperture-coupled cross-patch elements and terminated on a two-port radiating matched load. By properly exciting the array, dual linear or circular polarization can be accomplished and by virtue of the symmetric arrangement of the antenna cross-polarization is annihilated. A straightforward design strategy is proposed for the synthesis of a desired current distribution along the array antenna. Some proof-of-concept linear arrays are developed and the corresponding numerical results, obtained through a full-wave approach based on the method of moments (MoM), are provided. As an independent validation, supplementary analyses by the finite integration technique (FIT) are also reported. Very close agreement is found between prescribed and synthesized array performance, which qualifies the proposed design approach as an accurate and effective tool for the synthesis of series-fed aperture-coupled patch arrays.
Reconfigurable bistate metasurfaces composed of interwoven spiral arrays with embedded pin diodes are proposed for single and dual polarisation operation. The switching capability is enabled by pin ...diodes that change the array response between transmission and reflection modes at the specified frequencies. The spiral conductors forming the metasurface also supply the dc bias for controlling pin diodes, thus avoiding the need of additional bias circuitry that can cause parasitic interference and affect the metasurface response. The simulation results show that proposed active metasurfaces exhibit good isolation between transmission and reflection states, while retaining excellent angular and polarisation stability with the large fractional bandwidth (FBW) inherent to the original passive arrays.
We present a fully printable effective negative refractive index (NRI) metamaterial responsive to arbitrarily linearly polarized incident waves. The proposed metamaterial is composed of a periodic ...array of tightly coupled Jerusalem cross conductor pairs printed on the opposite sides of a dielectric substrate. Each pair supports both symmetric and antisymmetric resonance modes, whose superposition can lead to an effective NRI of the composite medium. A thorough characterization of the transmission properties of such metamaterial is performed, and conclusive evidences of the medium exhibiting effective NRI properties and impedance matching to free space are presented for a range of the structure parameters.
The properties and characteristics of a recently proposed anisotropic metamaterial based upon layered arrays of tightly coupled pairs of “dogbone” shaped stripe conductors have been explored in ...detail. It has been found that a metamaterial composed of such stacked layers exhibits artificial magnetism and may support backward wave propagation. The equivalent network models of the constitutive conductor pairs arranged in the periodic array have been devised and applied to the identification of the specific types of resonances, and to the analysis of their contribution into the effective dielectric and magnetic properties of the artificial medium. The proposed “dogbone” configuration of conductor pairs has the advantage of being entirely realizable and assemblable in planar technology. It also appears more prospective than simple cut-wire or metal-plate pairs because the additional geometrical parameters provide an efficient control of separation between the electric and magnetic resonances that, in turn, makes it possible to obtain a fairly broadband left-handed behaviour of the structure at low frequencies.
A fully planar antenna design incorporating a high impedance surface (HIS) is presented. The HIS is composed by a periodic array of subwavelength dogbone-shaped conductors printed on top of a thin ...dielectric substrate and backed by a metallic ground plane. First, the characteristics of a dipole over PEC or PMC layers, a dielectric slab, and the HIS are compared and studied in detail, highlighting the advantages provided by the use of the HIS. Then, the design of a low profile folded dipole antenna working at 5.5 GHz on top of the HIS is described. The surface provides close to 6% antenna impedance bandwidth and increased gain up to 7 dBi, while shielding the lower half space from radiation. The antenna structure comprises three metal layers without any vias between them, and its overall thickness is 0.059λ 0 . The dipole is fed by a balanced twin lead line through a balun transformer integrated in the same antenna layer. A prototype has been built and measurements confirming simulation results are provided.
The three-dimensional plane wave scattering from anisotropic impedance half-planes having their principal anisotropy axes arbitrarily oriented with respect to the diffracting edge is analyzed. Two ...classes of exactly solvable half-plane configurations are identified and the corresponding rigorous analytical expressions for the total scattered field are derived in terms of a generalization of the Maliuzhinets special function. Then, explicit uniform asymptotic field expressions are provided along with samples of numerical results proving their validity.
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