Trapped mode resonances in metalo-dielectric structures with electric asymmetry materials

This paper investigates the possibility of exciting high quality trapped resonant modes on frequency selective surfaces consisting of identical sub-wavelength metallic inclusions (symmetrically split rings) with no structural asymmetry but exhibitting electrical asymmetry. The electrical symmetry is broken by using different dielectric substrates. The existence of such anti-symmetric trapped mode on geometrical symmetric structure is demonstrated through numerical simulation. Numerical results suggest that the high quality factor observed for these resonant modes is achieved via weak coupling between the "trapped modes" and free space which enables the excitation of these modes

Opencage radio frequency coil for magnetic resonance imaging

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Wood’s anomalies for arrays of dielectric scatterers

The Rayleigh Wood anomalies refer to an unexpected repartition of the electromagnetic energy between the several interference orders of the light emerging from a grating. Since Hessel and Oliner (Appl. Opt. 4, 1275-1297 (1965)), several studies have been dedicated to this problem, focusing mainly on the case of metallic gratings. In this paper, we derive explicit expressions of the reflection coefficients in the case of dielectric gratings using a perturbative approach. This is done in a multimodal description of the field combined with the use of the admittance matrix, analog to the so-called electromagnetic impedance. Comparisons with direct numerical calculations show a good agreement with our analytical prediction

Optimized microwave illusion device

This work was funded by the Engineering and Physical Sciences Research Council (EPSRC), UK under a Programme Grant (EP/I034548/1) ‘The Quest for Ultimate Electromagnetics using Spatial Transformations (QUEST

Electromagnetic coupling in a planar periodic configuration of resonators

We are studying arrays composed of a periodic arrangement of sub-wavelength resonators. An analytical model is developed inside an array of 4 by 4 multi-gap split ring resonators. To describe the frequency splitting of the single fundamental resonance, we propose a simple model based on the approximation of each resonator as an electrical dipole and a magnetic dipole that are driven by the same complex amplitude. We show that the relative strength of the two dipoles strongly depends on cell symmetry. With this approximation, the dispersion relation can be obtained for an infinite size array. A simple matrix diagonalization provides a powerful way to deduce the resonant frequencies for finite size array. These results are comforted by numerical simulations. Finally, an experimental demonstration of a tunable antenna based on this study is presented

Volume coil for MRI based on metasurface

International audienceConventional birdcages are radiofrequency coils for magnetic resonance imaging (MRI) that consists of closed, right-hand or left-hand transmission lines with sub-wavelength periodicity. Analyzing these coils in terms of metasurface leads to engineering their properties as will and achieve original applications. To that end, we control the intrinsic impedance and the phase shift of each unit cell. As a demonstration, we use this concept to break the periodicity of the structure in order to provide a wide aperture allowing an easy access to the region of interest. We show that this coil, that we have called opencage, achieves good isolation between two driving ports and high homogeneity of the magnetic field. The results of measurements realized with phantom and in-vivo in MRI are presented. These results have confirmed the good performances predicted by our theoretical approach and the numerical calculations. This opencage coil could be used for many applications such as pre-clinical imaging of small animals or human head clinical imaging