Anomalous Reflection From Hyperbolic Media

Despite the apparent simplicity, the problem of refraction of electromagnetic waves at the planar interface between two media has an incredibly rich spectrum of unusual phenomena. An example is the paradox that occurs when an electromagnetic wave is incident on the interface between a hyperbolic medium and an isotropic dielectric. At certain orientations of the optical axis of the hyperbolic medium relative to the interface, the reflected and transmitted waves are completely absent. In this paper, we formulate the aforementioned paradox and present its resolution by introduction of infinitesimal losses in a hyperbolic medium. We show that the reflected wave exists, but became extremely decaying as the loss parameter tends to zero. As a consequence, all the energy scattered into the reflected channel is absorbed at the interface. We support our reasoning with analytical calculations, numerical simulations, and an experiment with self-complementary metasurfaces in the microwave region. In addition to the great fundamental interest, this paradox resolution discovers a plethora of applications for the reflectors, refractors, absorbers, lenses, antennas, camouflage and holography applications.Comment: 16 pages, 4 figure

Extending a Birdcage Coil for Magnetic Resonance Imaging of a Human Head with an Artificial Magnetic Shield

In magnetic resonance imaging, a birdcage coil is the most commonly used volumetric resonator creating a highly homogeneous radiofrequency magnetic field in a conductive sample. An artificial magnetic radiofrequency shield was recently shown to improve the magnetic field amplitude per unit power (transmit efficiency) of a preclinical birdcage coil by reducing the intrinsic losses in the coil and increasing power absorbed by the sample. In this paper, we propose a new application of an artificial shield in clinical MRI. Thanks to the proposed artificial shield a birdcage coil for human brain imaging operating at 300 MHz (Larmor frequency of protons at static fields of 7 T) can be expanded to increase free space. As a result, the coil becomes more comfortable for the patient and keeping comparable transmit efficiency. The same extended coil with a conventional copper shield would have at least 10% lower efficiency. The proposed artificial shield is implemented as an annular-ring cavity-backed slot in a copper cylinder that tightly surrounds the birdcage. To demonstrate the effect, radiofrequency magnetic field and specific absorption rate distributions were compared numerically and experimentally for the initial and extended coils with different shields.Peer reviewe

A practical realization of an artificial magnetic shield for preclinical birdcage RF coils

In the most of magnetic resonance imaging (MRI) systems, a conventional radiofrequency (RF) electric shield is typically placed around an RF volume coil to avoid the interaction with the other components of the system. Disadvantageously metal shields reduce the transmit efficiency of the RF coil as well as its receive sensitivity due to out-ofphase reflection of electromagnetic waves. In contrast, an ideal magnetic shield having high surface impedance provides in-phase reflection, which can be promising for improvement of RF coil's performance. In this work, we propose an artificial magnetic shield based on a cylindrical miniaturized corrugated structure to improve characteristics of a small-animal birdcage coil at 7T. The coil was simulated in the presence of the metal and ideal magnetic shield as well as the proposed structure. The results demonstrate enhancement of the coil's performance in the presence of the proposed shield, which is comparable with an ideal one.Peer reviewe

Microwave analogy of Förster resonance energy transfer and effect of finite antenna length

Abstract The near-field interaction between quantum emitters, governed by Förster resonance energy transfer (FRET), plays a pivotal role in nanoscale energy transfer mechanisms. However, FRET measurements in the optical regime are challenging as they require nanoscale control of the position and orientation of the emitters. To overcome these challenges, microwave measurements were proposed for enhanced spatial resolution and precise orientation control. However, unlike in optical systems for which the dipole can be taken to be infinitesimal in size, the finite size of microwave antennas can affect energy transfer measurements, especially at short distances. This highlights the necessity to consider the finite antenna length to obtain accurate results. In this study, we advance the understanding of dipole–dipole energy transfer in the microwave regime by developing an analytical model that explicitly considers finite antennas. Unlike previous works, our model calculates the mutual impedance of finite-length thin-wire dipole antennas without assuming a uniform current distribution. We validate our analytical model through experiments investigating energy transfer between antennas placed adjacent to a perfect electric conductor mirror. This allows us to provide clear guidelines for designing microwave experiments, distinguishing conditions where finite-size effects can be neglected and where they must be taken into account. Our study not only contributes to the fundamental physics of energy transfer but also opens avenues for microwave antenna impedance-based measurements to complement optical FRET experiments and quantitatively explore dipole–dipole energy transfer in a wider range of conditions

Experimental evidence of Förster energy transfer enhancement in the near field through engineered metamaterial surface waves

International audiencePlasmonics has been demonstrated to provide fine tuning of the emission properties of single quantum sources (brightness, polarization, directivity, spectrum, lifetime…). However, significantly less is known about the role of surface plasmons in mediating subwavelength Förster resonant energy transfer (FRET) when a second emitter is introduced. Here, we report microwave experiments showing that excitation of surface waves on a dedicated metasurface can strongly mediate FRET in the near-field regime. This work paves the way for metasurfaces engineered to control dipole-dipole energy transfer with applications in lighting sources, photovoltaics, quantum information processing and biophysics

Experimental evidence of Förster energy transfer enhancement in the near field through engineered metamaterial surface waves

Abstract Plasmonics has been demonstrated to provide fine tuning of the emission properties of single quantum sources (brightness, polarization, directivity, spectrum, lifetime…). However, significantly less is known about the role of surface plasmons in mediating subwavelength Förster resonant energy transfer (FRET) when a second emitter is introduced. Here, we report microwave experiments showing that excitation of surface waves on a dedicated metasurface can strongly mediate FRET in the near-field regime. This work paves the way for metasurfaces engineered to control dipole-dipole energy transfer with applications in lighting sources, photovoltaics, quantum information processing and biophysics