Kaan Guven

Investigation of magnetic resonances for different split-ring resonator parameters and designs

We investigate the magnetic resonance of split-ring resonators (SRR) experimentally and numerically. The dependence of the geometrical parameters on the magnetic resonance frequency of SRR is studied. We further investigate the effect of lumped capacitors integrated to the SRR on the magnetic resonance frequency for tunable SRR designs. Different resonator structures are shown to exhibit magnetic resonances at various frequencies depending on the number of rings and splits used in the resonators.

Experimental observation of true left-handed transmission peaks in metamaterials

We report true left-handed (LH) behavior in a composite metamaterial consisting of a periodically arranged split ring resonator (SRR) and wire structures. We demonstrate the magnetic resonance of the SRR structure by comparing the transmission spectra of SRRs with those of closed SRRs. We have confirmed experimentally that the effective plasma frequency of the LH material composed of SRRs and wires is lower than the plasma frequency of the wires. A well-defined LH transmission band with a peak value of -1.2 dB (-0.3 dB/cm) was obtained. The experimental results agree extremely well with the theoretical calculations.

Observation of negative refraction and negative phase velocity in left-handed metamaterials

We report the transmission characteristics of a two-dimensional (2D) composite metamaterial (CMM) structure in free space. At the frequencies where left-handed transmission takes place, we experimentally confirmed that the CMM structure has effective negative refractive index. Phase shift between consecutive numbers of layers of CMM is measured and phase velocity is shown to be negative at the relevant frequency range. Refractive index values obtained from the refraction experiments and the phase measurements are in good agreement.

Experimental observation of left-handed transmission in a bilayer metamaterial under normal-to-plane propagation

We demonstrate experimentally the double-negative (epsilon < 0, micro < 0) transmission band of a one-dimensional metamaterial structure under normal-to-plane propagation in the microwave regime. The structure consists of stacked bilayers of metal cutwire and wire pairs, which are separated by a thin dielectric layer. The existence of the negative index of refraction is inferred from the transmission and phase spectra obtained by using multilayer metamaterial samples. Another metamaterial structure incorporating non-magnetic (electrically shorted) cutwire pairs does not exhibit the corresponding transmission band, which supports the true left-handed behavior of the metamaterial.

Effect of disorder on magnetic resonance band gap of split-ring resonator structures

We investigated the influence of periodicity, misalignment, and disorder on the magnetic resonance gap of split-ring resonators (SRRs) which are essential components of left handed-metamaterials (LHMs). The resonance of a single SRR which is induced by the split is experimentally demonstrated by comparing transmission spectra of SRR and closed ring resonator. Misaligning the SRR boards do not affect the magnetic resonance gap, while destroying the periodicity results in a narrower band gap. The disorder in SRR layers cause narrower left-handed pass band and decrease the transmission level of composite metamaterials (CMMs), which may significantly affect the performance of these LHMs.

Metamaterials with negative permeability and negative refractive index: experiments and simulations

We report the transmission characteristics of split-ring resonator and left-handed metamaterials (LHM) in the microwave frequency regime. A left-handed transmission band is observed at the frequencies where both dielectric permittivity and magnetic permeability are negative. The reflection characteristics of ordered and disordered LHMs are studied. The two-dimensional LHM structure is verified to have a negative refractive index. We employed three different methods to observe negative refraction: the beam shift method, refraction through wedge-shaped negative-index metamaterial, and phase-shift experiments.

Electromagnetic cloaking with canonical spiral inclusions

We report an electromagnetic cloaking structure that is composed of identical canonical spiral particles. By using the Clausius-Mosotti formula, the electric and magnetic polarizabilities of a single spiral particle are related to the relative permittivity and permeability of the sparse distribution of particles. The permittivity and permeability of the distribution are, in turn, defined according to the coordinate transformation, which leads to the cloaking effect. Spirals are optimized to exhibit equal permittivity and permeability response so that the cloak consisting of these spirals will work for both transverse electric (TE) and transverse magnetic (TM) polarizations. Measurement of the cloaking device surrounding a metal cylinder inside a parallel waveguide was performed. The steady-state propagation of an electromagnetic wave was reconstructed from the amplitude and phase data, which demonstrates that the field largely restores to a free-space propagation pattern after the cloak.

Modeling of Spirals with Equal Dielectric, Magnetic, and Chiral Susceptibilities

Abstract In this article, we study spiral particles with optimized design parameters, which can make possible the realization of media with equal dielectric, magnetic, and chiral susceptibilities. Two different spiral structures are investigated: the canonical spiral (which consists of a split loop with two straight-wire sections, orthogonal to the loop plane and connected to the edges of the gap) and the true helix (which is obtained by bending a wire with a constant pitch angle). The transmission and reflection coefficients of arrays of spiral particles are obtained under plane wave excitation by numerical simulation. The properties of slabs formed by periodic chiral and racemic arrays of spirals are investigated. Good agreement is found between the presented results and the previously reported theoretical and experimental studies.

Bilayer metamaterial: analysis of left-handed transmission and retrieval of effective medium parameters

We report an experimental and numerical analysis of a planar metamaterial designed for normal-to-plane propagation, and operating at microwave frequencies. The metamaterial consists of cutwire and wire patterns, which are arranged periodically on both sides of a dielectric layer, in the form of a bilayer. The left-handed transmission band of the metamaterial is demonstrated experimentally. The effective index of refraction retrieved from the S parameters is found to be negative within this transmission band. An independent negative refraction experiment supports the existence of the negative index of refraction for the metamaterial.

Decoupling of Multifrequency Dipole Antenna Arrays for Microwave Imaging Applications

The mutual coupling between elements of a multifrequency dipole antenna array is experimentally investigated by -parameter measurements and planar near-field scanning of the radiated field. A multifrequency array with six dipoles is analyzed. In order to reduce the coupling between dipoles, a planar metasurface is placed atop the array acting as superstrate. The mutual coupling of the antenna elements in the absence and presence of the superstrate is presented comparatively. Between 3 and 20 dB mutual coupling reduction is achieved when the superstrate is used. By scanning the field radiated by the antennas and far-field measurements of the radiation pattern, it is observed that the superstrate confines the radiated power, increases the boresight radiation, and reduces the endfire radiation.