R. Joffe

Novel microwave near-field sensors for material characterization, biology, and nanotechnology

The wide range of interesting electromagnetic behavior of contemporary materials requires that experimentalists working in this field master many diverse measurement techniques and have a broad understanding of condensed matter physics and biophysics. Measurement of the electromagnetic response of materials at microwave frequencies is important for both fundamental and practical reasons. In this paper, we propose a novel near-field microwave sensor with application to material characterization, biology, and nanotechnology. The sensor is based on a subwavelength ferrite-disk resonator with magnetic-dipolar-mode (MDM) oscillations. Strong energy concentration and unique topological structures of the near fields originated from the MDM resonators allow effective measuring material parameters in microwaves, both for ordinary structures and objects with chiral properties.

Microwave magnetoelectric fields: An analytical study of topological characteristics

Abstract The near fields originated from a small quasi-two-dimensional ferrite disk with magnetic-dipolar-mode (MDM) oscillations are the fields with broken dual (electric-magnetic) symmetry. Numerical studies show that such fields – called the magnetoelectric (ME) fields – are distinguished by the power-flow vortices and helicity parameters (E.O. Kamenetskii, R. Joffe, R. Shavit, Phys. Rev. E 87 (2013) 023201). These numerical studies can well explain recent experimental results with MDM ferrite disks. In the present paper, we obtain analytically topological characteristics of the ME-field modes. For this purpose, we used a method of successive approximations. In the second approximation we take into account the influence of the edge regions of an open ferrite disk, which are excluded in the first-approximation solving of the magnetostatic (MS) spectral problem. Based on the analytical method, we obtain a “pure” structure of the electric and magnetic fields outside the MDM ferrite disk. The analytical studies can display some fundamental features that are non-observable in the numerical results. While in numerical investigations, one cannot separate the ME fields from the external electromagnetic (EM) radiation, the present theoretical analysis allows clearly distinguish the eigen topological structure of the ME fields. Importantly, this ME-field structure gives evidence for certain phenomena that can be related to the Tellegen and bianisotropic coupling effects. We discuss the question whether the MDM ferrite disk can exhibit properties of the cross magnetoelectric polarizabilities.

Planar Ka band antenna for satellite communication based on metamaterial technology

In this paper, we present a multilayer flat lens antenna with effective zero index. It is made of a multilayer and periodic structure of printed elements to enhance a single patch element gain from 6 dBi to 19 dBi with radiation efficiency close to 85%. This high radiation efficiency enables to reduce the antenna aperture size. In the literature, the concept of a flat lens with zero index fed symmetrically by a balanced dipole operating at X band is described]. In our study the feeding dipole was replaced by a patch backed by a ground plane and optimized at Ka band. An array of 2×2 patch elements with a flat lens on top was designed and optimized for radiation efficiency, gain and radiation pattern. In the presentation a comparison between measured and computed results will be shown.

Manipulation of the Radiation Characteristics of a Patch Antenna by Small Ferrite Disks Inserted in Its Cavity Domain

In this paper, it is shown how the radiation characteristics of a patch antenna can be manipulated by a small number of normally magnetized ferrite disks inserted in the resonant region of the patch. It is shown that a one- and dual-band circular polarized microstrip antenna can be obtained by taking advantage of the interaction of the antenna cavity field with the magnetized ferrite disks. The scattering and radiation parameters of the antenna are investigated. The dependence of the axial ratio and the return loss of the antenna on the position and the number of ferrite disks underneath the patch are analyzed. Experimental and simulation results are in good agreement.

Improved Back-Projection Cortical Potential Imaging by Multi-resolution Optimization Technique

Electroencephalogram (EEG) has evolved to be a well-established tool for imaging brain activity. This progress is mainly due to the development of high-resolution (HR) EEG methods. One class of HR-EEG is the cortical potential imaging (CPI), which aims to estimate the potential distribution on the cortical surface, which is much more informative than EEG. Even though these methods exhibit good performance, most of them have inherent inaccuracies that originate from their operating principles that constrain the solution or require a complex calculation process. The back-projection CPI (BP-CPI) method is relatively new and has the advantage of being constraint-free and computation inexpensive. The method has shown relatively good accuracy, which is necessary to become a clinical tool. However, better performance must be achieved. In the present study, two improvements are proposed. Both are embedded as adjacent stages to the BP-CPI and are based on the multi-resolution optimization approach (MR-CPI). A series of Monte-Carlo simulations were performed to examine the characteristics of the proposed improvements. Additional tests were done, including different EEG noise levels and variation in electrode-numbers. The results showed highly accurate cortical potential estimations, with a reduction in estimation error by a factor of 3.75 relative to the simple BP-CPI estimation error. We also validated these results with true EEG data. Analyzing these EEGs, we have demonstrated the MR-CPI competence to correctly localize cortical activations in a real environment. The MR-CPI methods were shown to be reliable for estimating cortical potentials, enabling researchers to obtain fast and robust high-resolution EEGs.

Azimuthally unidirectional transport of energy in magnetoelectric fields: topological Lenz’s effect

Abstract Magnetic-dipolar modes (MDMs) in a quasi-2D ferrite disc are microwave energy-eigenstate oscillations with topologically distinct structures of rotating fields and unidirectional power-flow circulations. At the first glance, this might seem to violate the law of conservation of an angular momentum, since the microwave structure with an embedded ferrite sample is mechanically fixed. However, an angular momentum is seen to be conserved if topological properties of electromagnetic fields in the entire microwave structure are taken into account. In this paper, we show that due to the topological action of the azimuthally unidirectional transport of energy in a MDM-resonance ferrite sample there exists the opposite topological reaction on a metal screen placed near this sample. We call this effect topological Lenz’s effect. The topological Lenz’s law is applied to opposite topological charges: one in a ferrite sample and another on a metal screen. The MDM-originated near fields – the magnetoelectric (ME) fields – induce helical surface electric currents and effective charges on a metal. The fields formed by these currents and charges will oppose their cause.

Multiresonance Measurement Method for Microwave Microscopy

Nondestructive tests in microwave frequencies for dielectric parameters evaluation with high spatial resolution have a great significance in many fields of material science. In this paper, we present a new method for the dielectric parameter characterization based on the displacement of the multiresonance spectrum of the magnetostatic (MS) oscillations in a thin-film ferrite disk due to loading by a dielectric sample. The effect is manifested by shifting and widening of the MS spectrum that is dependent on the relative permittivity of the sample. The correlation between the MS spectrum characteristics is used in a postprocessing analysis to enhance the separability of noisy data and thereby increase the measurement accuracy. The experimental and numerical results have shown the ability to accurately evaluate the dielectric parameters of the load sample under consideration. The simplicity of the measurement in a wide range of operating frequencies along with nondestructive sensing with a subwavelength resolution of 100

Induced torsion effects in microwave structures with magnetoelectric fields

\mu \text{m}

MAGNETIC-DIPOLAR-MODE OSCILLATIONS FOR NEAR- AND FAR-FIELD MANIPULATION OF MICROWAVE RADIATION

complies with the current requirements of scanning microwave microscopy.

Corrections to "Manipulation of the radiation characteristics of a patch antenna by small ferrite disks inserted in its cavity domain"

At ferromagnetic-resonance frequencies, in small ferrite samples so-called magnetic-dipolar-mode oscillations can be excited. The near fields in vacuum, originated from a ferrite disk with these oscillations, are unidirectional rotating fields. When a thin metal wire is placed above such disk, unique torsion electric currents are induced. We analyze analytically these torsion effects. The torsion currents create the fields with unique topology. The shown results could be useful for development of microwave microscopy of chiral biological structures.