Metamaterials最新文献

A miniature printed antenna exploiting magnetic photonic crystal (MPC) modes is presented. The MPC mode is typically found in volumetric anisotropic materials and is realized here using a set of coupled printed transmission lines. The printed element is formed by a pair of unit cells, cascaded to realize a circularly periodic structure. To excite the nonreciprocal MPC mode, the coupled microstrip lines are printed on a uniform substrate with magnetic material inserts. A printed and slot (cavity-backed) version of the MPC-based element is designed and constructed. It is found that the cavity-backed version is better suited for reducing platform interactions, thus, avoiding detuning when installed on smaller platforms. Tunable magnetic biasing is also explored to improve functionality. The paper concludes by providing the in situ performance of the cavity-backed and recessed MPC antennas.

Generalized electromagnetic boundary conditions are derived for interfaces containing surface layers of electric and magnetic polarization. Next, the singularities in the fields and polarization densities at the inner surfaces of spherical and circular cylindrical cloaks illuminated by sources inside the cavities of the cloaks are determined from a plane-wave analysis of fields incident on a half space of material characterized by a relative permittivity–permeability dyadic equal to that of the inner material surfaces of the cloaks. Lastly, these singular fields and polarization densities are shown to satisfy the generalized boundary conditions. For sources inside the cloak cavities, it is found that the inner surfaces of spherical cloaks, unlike the inner surfaces of cylindrical cloaks, behave as “DB boundaries” at which $\hat{\text{n}}\cdot \text{D}$ and $\hat{\text{n}}\cdot \text{B}$ are zero.

In this contribution, we discuss the possibility of applying the scattering-cancellation-based plasmonic cloaking technique to reduce the scattering from a sensing device, lowering its disturbance on sensing without affecting its ability to measure and “sense” the external world. The plasmonic cloak, while allowing the external fields to penetrate into the cover, may be able to suppress the scattering without necessarily shielding its interiors from the impinging light, which may be of great use if we wish to “sense” the external world, or receive an external signal, from inside the cloak. Our results may provide a unique sensing system that may receive and transmit signals, while its presence may not be perceived by the surrounding. This may be of great importance in a wide range of biological, optical, physical and engineering applications.

The surface plasmons (SPs) guided by the films of noble metals have been modelled using the measured optical constants of Ag, Au and Cu. It is shown that Drude permittivity with the slowly varying plasma ω_p(λ) and collision ν(λ) frequencies, retrieved from the experimental data, provides a comprehensive description of the main features of the guided SP modes and the SP resonances. The effects of dispersion and losses on the SP properties have been separated by scaling ν(λ) in the numerical analysis of the full-wave dispersion equation. Using Drude permittivity with the scaled ν(λ), it is demonstrated that in the absence of SP resonances, the ω_p(λ) dispersion but not damping primarily determine the maximum attainable slow wave factor of SP. A detailed qualitative interpretation of the SP characteristics in the metallic films with the measured permittivity shed the light on the phenomenological mechanisms underlying the SP properties in the films of noble metals on dielectric substrates.

An exact solution of the boundary electrodynamic problem for a thin layer of uniaxial absorbing metamaterial characterized by the effective permittivity (permeability) tensor with near-zero values of the axial component is investigated. The parameters of optical anisotropy, absorption, and geometry of the problem are determined when the layer exhibits plane electromagnetic wave reflection and transmission that are selective with respect to one or several incident angle ranges. The numerical analysis shows that even small deviations from ideal conditions (losses, non-zero values of the axial component, nonorthogonality of the optical axis to the layer boundaries) can lead to remarkable changes of the metamaterial properties. The low level of losses, required for applications of the material (spatial filtering of the radiation, etc.), seems to be unattainable nowadays for passive metamaterials, so the realization of the concept can be concerned with active metamaterials.

A flexible birdcage-type resonant RF detector for magnetic resonance imaging is described. The circuit consists of a polygonal ring of magnetically coupled L–C resonators, a periodic structure supporting backward magnetoinductive waves. The elements are mechanically linked to allow relative rotation, and the pivot point is optimised to hold the nearest-neighbour coupling coefficient invariant to small changes in the angle of an undistorted joint. Simple theory based on a parallel wire approximation to rectangular inductors is developed to allow the variation of the coupling coefficient with angle and radius to be estimated, and hence determine the location of the pivot. The optimised pivot is shown to reduce resonance splitting in octagonal rings. However, second-neighbour interactions degrade performance. The theory is verified experimentally using printed circuit board elements coupled by flexible hinges, and the invariance of the nearest-neighbour coupling coefficient is confirmed. Octagonal ring resonators are constructed for operation at 63.8 MHz frequency and the mode spectra of regular and distorted rings are measured. Magnetic resonance imaging properties are investigated using ¹H MRI of simple objects in a 1.5 T field. Images are obtained from undistorted and distorted resonators and the effect of distortion on SNR is quantified.

M. Kerker et al. [J. Opt. Soc. Am. 73 (1983) 765–767] established that under certain optical conditions and in the far-field regime, one isolated dipole-like particle may not scatter light in the forward or in the backward direction. New applications within the nanoscience community require knowledge of the scattering behaviour in the neighbourhood of structures in the near-field region. In this work we analyse the conditions derived by Kerker et al., as a function of the distance to the particle, from the near field to the far-field zone to discover the regions of applicability.

The novel folded ring resonator (FRR) is uniaxial bi-anisotropic and possesses a strong resonant magnetic bandgap. Within the bandgap at 4.75 GHz, the cubic FRR is significantly subwavelength because it is $\lambda$/13 along any coordinate axis and $\lambda$/7 along the body diagonal. The bulk behavior of a cubic lattice of the novel structure demonstrates backward mode behavior centered near the frequency 4.5 GHz with a bandwidth of 12.2% as derived from the dispersion curve. Parametric sweeps of the critical dimensions are presented showing the tunability of the structure. The field nature of the backward mode is explained in terms of the realized simultaneous electric and magnetic dipole reponses. For finite thickness layers of the novel FRR, the transmitted phase is shown to indicate the presence of a backward mode in agreement with the bulk response. The novel structure also functions as a linear to circular polarizer for a single layer. Due to the asymmetry of the structure, asymmetric transmission occurs within the backward mode in the form of circular cross-polarization dichroism and direction dependent transmission. Although the backward mode occurs for any incident angle it is shown to be anisotropic in nature. A backward mode bandwidth as wide as 23.3% is achieved along the body diagonal of the cubic unit cell, which is the principal axis.