70 research outputs found
Near-field Image Transfer by Magneto-Inductive Arrays: a Modal Perspective
A simple model of near-field pixel-to-pixel image transfer using
magneto-inductive arrays is presented. The response of N-dimensional
rectangular arrays is first found as an excitation of eigenmodes. This
analytical method involves approximating the effect of sources and detectors,
and replaces the problem of solving large numbers of simultaneous equations
with that of evaluating a sum. Expressions are given for the modal expansion
coefficients, and in the low-loss case it is shown that the coefficient values
depend only on the difference in reciprocal frequency space of the operating
frequency from the resonant frequency of each mode. Analytic expressions are
then derived for quasi-optical quantities such as the spatial frequency
response, point-spread function and resolving power, and their implications for
imaging fidelity and resolution are examined for arrays of different dimension.
The results show clearly that there can be no useful image transfer for in-band
excitation. Out-of-band excitation allows image transfer. Provided the array is
larger than the expected image by at least the size of the point spread
function, the effect of the array boundaries may be ignored and imaging is
determined purely by the properties of the medium. However, there is a tradeoff
between fidelity and throughput, and good imaging performance using thick slabs
depends on careful choice of the operating frequency. The approximate analytic
method is verified by comparison of exact numerical solution
Superdirectivity from arrays of strongly coupled meta-atoms
This is the final version of the article. Available from AIP Publishing via the DOI in this record.We explore the possibility of achieving superdirectivity in metamaterial-inspired endfire antenna arrays relying on the good services of magnetoinductive waves. These are short-wavelength slow waves propagating by virtue of coupling between resonant meta-atoms. Magnetoinductive waves are capable of providing a rapidly varying current distribution on the scale of the free space wavelength. Using dimers and trimers of magnetically coupled split ring resonators with only one element driven by an external source, we introduce an analytical condition for realising superdirective current distributions. Although those current distributions have been known theoretically for a good 60 years, this is the first time that a recipe is given to realise them in practice. Our key parameters are the size of the array, the resonant frequency and quality factor of the elements, and their coupling constant. We compare our analytical results for coupled magnetic dipoles with numerical results from CST simulations for meta-atoms of various shapes. The calculated bandwidth of 5 MHz for a dimer operating at 150 MHz indicates that, contrary to popular belief, superdirective antennas exist not only in theory but may have practical applications.Financial support by the John Fell Fund (University of Oxford) and by the EPSRC UK (SYMETA, EP/N010493/1) is gratefully acknowledged
A theory of metamaterials based on periodically loaded transmission lines: Interaction between magnetoinductive and electromagnetic waves
Published versio
Nonlinear Localization in Metamaterials
Metamaterials, i.e., artificially structured ("synthetic") media comprising
weakly coupled discrete elements, exhibit extraordinary properties and they
hold a great promise for novel applications including super-resolution imaging,
cloaking, hyperlensing, and optical transformation. Nonlinearity adds a new
degree of freedom for metamaterial design that allows for tuneability and
multistability, properties that may offer altogether new functionalities and
electromagnetic characteristics. The combination of discreteness and
nonlinearity may lead to intrinsic localization of the type of discrete
breather in metallic, SQUID-based, and symmetric metamaterials. We
review recent results demonstrating the generic appearance of breather
excitations in these systems resulting from power-balance between intrinsic
losses and input power, either by proper initialization or by purely dynamical
procedures. Breather properties peculiar to each particular system are
identified and discussed. Recent progress in the fabrication of low-loss,
active and superconducting metamaterials, makes the experimental observation of
breathers in principle possible with the proposed dynamical procedures.Comment: 19 pages, 14 figures, Invited (Review) Chapte
A theory of metamaterials based on periodically loaded transmission lines: Interaction between magnetoinductive and electromagnetic waves
Magnetoelastic nonlinear metamaterials
We introduce the concept of magnetoelastic metamaterials with electromagnetic
properties depending on elastic deformation. We predict a strong nonlinear and
bistable response of such metamaterials caused by their structural reshaping in
response to the applied electromagnetic field. In addition, we demonstrate
experimentally the feasibility of the predicted effect.Comment: 4 pages, 5 figure
Rotational resonance of magnetoinductive waves: Basic concept and application to nuclear magnetic resonance
Published versio
Wireless power transfer in the presence of a conducting interface: Analytical solution
A comprehensive model of wireless power transfer (WPT) between a pair of inductively coupled loop resonators in the vicinity of a conducting slab is developed by combining electromagnetic (EM) theory with an equivalent circuit representation of the system. The model incorporates all EM interactions occurring between the coils both directly and through the slab. The design of an optimal WPT regime for two coils in the presence of a conducting slab with variable conductivity is demonstrated by exploiting the model to predict the coil detuning produced by the slab. It is further shown that the model also constitutes an effective tool to estimate the losses in the conducting material solely in terms of the parameters of the generalised equivalent circuit and to analyse the power flow in a WPT system. The validity of the analytical results is proved by comparison with rigorous full-wave simulations. The proposed analytical method can have particular relevance in the field of wireless charging of electric vehicles and WPT for supplying implanted medical devices and body sensor networks, where it can be instrumental for the design and optimisation of the system to be compliant with existing RF exposure safety levels
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