Elena Semouchkina

FDTD study of resonance Processes in metamaterials

The finite-difference time-domain simulations in the frequency domain are used to study resonance phenomena in left-handed metamaterials consisting of the arrays of split-ring resonators (SRRs) and metal rods. It is demonstrated that, at frequencies corresponding to the band of enhanced transmission of the metamaterial, the half-wavelength resonances occur in both the SRRs and rods. The observed resonances in rods make questionable the applicability of the plasma concept to the analysis of the metamaterial performance. We also show that overlapping of electric or magnetic fields at resonance causes coupling between resonators and assembling them in three-dimensional groups, which rearrange in dependence on frequency inside the transmission band. As the result, the resonance phenomena in the metamaterial proceed essentially nonuniform, although the size of the metamaterial units is less than the wavelength. We suggest that coupling between resonators is capable of providing the electromagnetic response, similar to that observed at the backward-wave propagation in double-negative media. The latter is demonstrated on the example of the all-dielectric metamaterial composed from an array of dielectric resonators.

An infrared invisibility cloak composed of glass

We propose to implement a nonmetallic low-loss cloak for the infrared range from identical chalcogenide glass resonators. Based on transformation optics for cylindrical objects, our approach does not require metamaterial response to be homogeneous and accounts for the discrete nature of elementary responses governed by resonator shape, illumination angle, and inter-resonator coupling. Air fractions are employed to obtain the desired distribution of the cloak effective parameters. The effect of cloaking is verified by full-wave simulations of the true multiresonator structure. The feasibility of cloak fabrication is demonstrated by prototyping glass grating structures with the dimensions characteristic for the cloak resonators.

Design optimization and implementation of bandpass filters with normally fed microstrip resonators loaded by high-permittivity dielectric

Different approaches for designing bandpass filters with transmission zeroes are investigated by using the transmission-line theory and the finite-difference time-domain method. The filters are composed of capacitively coupled uniform impedance microstrip resonators, stepped-impedance microstrip resonators, and tapped feed lines. Different design modifications are discussed and, as a result, a design with double-coupled resonators is proposed. Based on this structure, a miniaturized filter capacitively loaded by high-permittivity dielectric inclusions and fabricated by using low-temperature co-fired ceramic (LTCC) technology is presented. The measured and simulated S-parameter spectra of the LTCC-filter are in good agreement.

Broadband, Miniaturized Stacked-Patch Antennas for L-Band Operation Based on Magneto-Dielectric Substrates

Design of stacked-patch antennas using magneto-dielectric substrates was investigated. In particular, special types of substrates with identical relative permittivity and permeability were considered. The optimal design parameters were determined using a genetic algorithm. It will be shown that by employing these matched magneto-dielectric substrates, a significant miniaturization of up to 60% can be achieved while providing a large operating bandwidth (20%). Several design examples were considered and their performance was evaluated using a full-wave analysis technique based on the method of moments. The tradeoff between antenna gain and degree of miniaturization for a fixed bandwidth was also investigated. A fabrication methodology for obtaining the required matched magneto-dielectric substrate materials is also proposed.

Mie scattering as a cascade of Fano resonances

We reveal that the resonant Mie scattering by high-index dielectric nanoparticles can be presented through cascades of Fano resonances. We employ the exact solution of Maxwells equations and demonstrate that the Lorenz-Mie coefficients of the Mie problem can be expressed generically as infinite series of Fano functions as they describe interference between the background radiation originated from an incident wave and narrow-spectrum Mie scattering modes that lead to Fano resonances.

High Q calcium titanate cylindrical dielectric resonators for magnetic resonance microimaging.

At high magnetic fields radiation losses, wavelength effects, self-resonance, and the high resistance of typical components all contribute to increased losses in conventional RF coil designs. High permittivity ceramic dielectric resonators create strong uniform magnetic fields in a compact structure at high frequencies and can potentially solve some of the challenges of high field coil design. In this study an NMR probe was constructed for operation at 600 MHz (14.1T) using an inductively fed CaTiO(3) (relative permittivity of 156) cylindrical hollow bore dielectric resonator. The design has an unmatched Q value greater than 2000, and the electric field is largely confined to the dielectric itself, with near zero values in the hollow bore which accommodates the sample. Experimental and simulation mapping of the RF field show good agreement, with the ceramic resonator giving a pulse width approximately 25% less than a loop gap resonator of similar inner dimensions. High resolution images, with voxel dimensions less than 50 microm(3), have been acquired from fixed zebrafish samples, showing excellent delineation of several fine structures.

Implementation of Low Scattering Microwave Cloaking by All-Dielectric Metamaterials

We present a cylindrical microwave cloak, which provides low total scattering cross width (TSCW) for cloaked objects larger than the wavelength. The cloak is composed of identical dielectric resonators, while different air fractions are employed to realize the spatial dispersion of the cloak medium effective permeability required by the transformation optics relations. The cloak performance is investigated by simulating the true multi-resonator cloak structure and by experiments. The results of simulations and measurements are in good agreement and confirm the existence of the cloaking band, in which the TSCW of the cloaked target decreases below that of the bare target.

New approaches for designing microstrip filters utilizing mixed dielectrics

A strategy is developed for designing capacitively loaded microstrip filters on low-temperature co-fired ceramic (LTCC) substrates with inclusions or superstrate layers of higher permittivity dielectrics. Finite-difference time-domain simulations of the field distribution at resonant frequencies are used to determine the optimal locations and size of capacitive loads. It is demonstrated that strategic capacitive load placement enables altering the center and attenuation pole frequencies, the shape and width of the passband, and input impedance of the filter by modification of selected resonant modes. Capacitive loading with higher permittivity dielectrics is shown to be very efficient in decreasing dimensions of microstrip filters with low-permittivity substrates. The designs of novel compact resonators and filters have been developed and the prototypes fabricated by using LTCC technology. The results of prototype measurements agree with the simulation results, which validates the proposed approach

Design optimization of microstrip square-ring band-pass filter with quasi-elliptic function

Resonant processes in dual-mode microstrip square-ring band-pass filter fed through L-shaped coupling arms have been studied using the Finite Difference Time Domain (FDTD) simulations. The performed field analysis has revealed that coupling arms act as additional half-wavelength resonators coupled with the ring through both electric and magnetic fields. New filter designs with horseshoe resonators instead of coupling arms have been developed, which provide either pure magnetic or electric coupling at the resonant frequency. The obtained results demonstrate the possibility to essentially improve filter characteristics. Simulations results have been verified by the measurements of fabricated prototypes.

Electromagnetic response of the split-ring resonator placed inside a waveguide

The EM response of split ring resonators, conventionally employed to design the left-handed metamaterials, was studied by modeling the behavior of one ring located in a standard waveguide. It was shown that, besides the expected half and full-wavelength resonances, an additional resonance is excited in the SRR at an intermediate frequency due to coupling between the SRR and the waveguide. This resonance supports longitudinal mode in the waveguide and magnetic dipole-type fields near the SRR. While the response of the SRR at the first resonance can be described by negative permeability, the full-wavelength resonance features negative permittivity and the intermediate resonance exhibits more complex behaviors of the constitutive parameters. The properties of this resonance could be used to design novel metamaterials.