Manuel J. Freire

Experimental demonstration of a μ=−1 metamaterial lens for magnetic resonance imaging

In this work a μ=−1 metamaterial (MM) lens for magnetic resonance imaging (MRI) is demonstrated. MRI uses surface coils to detect the radio frequency (rf) energy absorbed and emitted by the nuclear spins in the imaged object. The proposed MM lens manipulates the rf field detected by these surface coils so that the coil sensitivity and spatial localization are substantially improved. Beyond this specific application, we feel that the reported results are the experimental confirmation of a new concept for the manipulation of rf field in MRI, which paves the way to many other interesting applications.

Planar magnetoinductive lens for three-dimensional subwavelength imaging

A planar near-field magnetoinductive lens operating in the microwave range is presented. The proposed device consists of two parallel planar arrays of metallic broadside coupled sBCd split-ring resonatorssSRRsd, or BC-SRRs. The power coming from a point-like source located in front of the lens is focused into a receiver located in free space beyond the lens. This focus is clearly separated from the back side of the lens, and has a size that is an order of magnitude smaller than the free space wavelength of the incoming field. The imaging properties of the device relies mainly on the excitation of magnetoinductive surface waves on the BC-SRR arrays. By simply scaling the BC-SRRs’ size, as well as the arrays periodicity, the operation frequency of the device can be tuned in a wide frequency range. Thus, the proposed design is potentially useful for many applications ranging from megahertz to terahertz.

Three-dimensional superresolution in metamaterial slab lenses: Experiment and theory

This paper presents a theoretical and experimental study on the viability of obtaining two- and threedimensional superresolution i.e., resolution overcoming the diffraction limit for all directions in space by means of metamaterial slab lenses. Although the source field cannot be actually reproduced at the back side of the lens with superresolution in all space directions, the matching capabilities of metamaterial slabs does make possible the detection of images with three-dimensional superresolution. This imaging takes place because of the coupling between the evanescent space harmonic components of the field generated at both the source and the detector.

Planar magnetoinductive wave transducers: Theory and applications

Transduction of magnetoinductive waves (MIWs) in planar technology is demonstrated. A transducer consisting of a one-dimensional periodic array of metallic split squared ring resonators (SSRR), placed between a pair of microstrip lines on a planar substrate has been fabricated and measured. The microstrip lines are inductively coupled to the SSRRs located at the ends of the periodic array and excite MIWs that propagate along the array. The theoretical model for the dispersion of MIWs is used to predict the dispersion relation and the delay time in the device. The delay time was measured and a good agreement was found with the theoretical predictions. The transmission coefficient of the device was also measured. The theoretical and experimental results suggest that the proposed configuration can find application in the design of delay lines and other microwave devices. In fact, the behavior of the proposed transducer is similar to that of the conventional ferrite magnetostatic-wave transducer. However, fer...

An excitation theory for bound modes, leaky modes, and residual‐wave currents on stripline structures

The nature of the current on a general multilayered printed-circuit stripline structure excited by a delta-gap source is investigated. The current is obtained through the construction of a semianalytical three-dimensional (3-D) Greens function, which accounts for the presence of the infinite conducting strip and the layered background structure. The 3-D Greens function is obtained by Fourier transforming the delta-gap source in the longitudinal (z) direction, which effectively resolves the 3-D problem of a delta-gap source into a superposition of 2-D problems, each of which is infinite in the z direction. The analysis allows for a convenient decomposition of the strip current into a sum of constituent parts. In particular, the strip current is first resolved into a set of bound-mode current waves and a continuous-spectrum current. The continuous-spectrum current is then represented as a set of physical leaky-mode currents in addition to a set of “residual-wave” currents, which arise from the steepest-descent integration paths. An asymptotic analysis reveals that the residual-wave currents decay algebraically as z−3/2. Far away from the source, the residual-wave currents dominate the continuous-spectrum strip current. Results are shown for a specific type of stripline structure, but the analysis and conclusions are valid for arbitrary multilayer stripline structures.

On the applications of μr=-1 metamaterial lenses for magnetic resonance imaging

In this work some possible applications of negative permeability magnetic metamaterial lenses for magnetic resonance imaging (MRI) are analyzed. It is shown that using magnetic metamaterials lenses it is possible to manipulate the spatial distribution of the radio-frequency (RF) field used in MR systems and, under some circumstances, improve the sensitivity of surface coils. Furthermore a collimation of the RF field, phenomenon that may find application in parallel imaging, is presented. MR images of real tissues are shown in order to prove the suitability of the theoretical analysis for practical applications.

Electroinductive waves in chains of complementary metamaterial elements

Electronductive waves supported by chains of resonators drilled on a metallic plate are presented. Propagation of energy comes as a consequence of the electric coupling between these resonators. Therefore, these waves are termed as electroinductive waves. They can be interpreted as the dual counterpart of the so-called magnetoinductive waves, which are due to the mutual inductances along chains of resonators. In order to show their existence, some electromagnetic simulations and experiments have been carried out, using as resonators the complementary particle of the split ring resonator. The reported result opens the way to a high variety of applications in one- and two-dimensional devices, such as transducers, delay lines, bends, power dividers, couplers, antennas, lenses, etc.

Evolution of leaky modes on printed-circuit lines

The frequency evolution of dominant (quasi-TEM) and higher order modes on an open printed-circuit structure such as a microstrip is examined. Three different mode types are considered, including bound modes (BMs), leaky modes that leak into the surface wave of the background structure, and leaky modes that also leak into space. One of the fundamental goals is to establish the conditions under which one type of mode can transition into another type as the frequency changes. One important conclusion is that the dominant BM can never transition into a leaky mode for a microstrip structure with an isotropic substrate, but such a transition is possible for an anisotropic substrate, observed originally by Tsuji et al. and Shigesawa et al. However, higher order BMs can directly transition into leaky modes, as shown by Oliner and Michalski and Zheng. On other structures such as coplanar strips, where the bound dominant mode exhibits odd symmetry, a transition from a bound dominant mode to a leaky mode is possible, as shown by Shigesawa et al. and Tsjui et al. In addition to examining the mathematical transitions that are possible, the physical continuation of modes is also investigated, by examining the frequency evolution of the currents excited by a practical source. It is concluded that there may be physical continuity between modes, even if there is no mathematical continuity.

Near-field imaging in the megahertz range by strongly coupled magnetoinductive surfaces: Experiment and ab initio analysis

In this work, the previously reported near-field imaging by two strongly coupled arrays of planar magnetic resonators is further studied. Experiments are performed to clarify the physical mechanisms underlying such an effect. The specific aim of these experiments is to clarify both the role played by magnetoinductive surface waves (MISWs) and the presence in the device of evanescent Fourier harmonics amplification. In addition to the experimental work, an ab initio theoretical analysis is developed to obtain a first approximation of the above effects. This model assumes that MISWs play the same role as plasmon-polaritons in negative refractive slabs, thus producing amplification of evanescent Fourier harmonics in the device. It also predicts that imaging occurs close to the resonators’ resonant frequency, between the passbands for the two MISW branches that can be excited in the lens. Both predictions from the theoretical model are in qualitative agreement with the experimental results. Quantitative agree...

Nonlinear split-ring metamaterial slabs for magnetic resonance imaging

This work analyzes the ability of split-ring metamaterial slabs with zero/high permeability to reject/confine the radiofrequency magnetic field in magnetic resonance imaging systems. Split-ring slabs are designed and fabricated to work in a 1.5 T system. Nonlinear elements consisting of pairs of crossed diodes are inserted in the split-rings, so that the slab permeability can be switched between a value close to unity when interacting with the strong field of the transmitting coil, and zero or high values when interacting with the weak field produced by protons in tissue. Experiments are shown where these slabs locally increase the signal-to-noise-ratio.