Liisi Jylhä

Backward-wave regime and negative refraction in chiral composites

Possibilities to realize a negative refraction in chiral composites in dual-phase mixtures of chiral and dipole particles are studied. It is shown that because of a strong resonant interaction between chiral particles (helices) and dipoles, there is a stop band in the frequency area where the backward-wave regime is expected. The negative refraction can occur near the resonant frequency of chiral particles. Resonant chiral composites may offer a root to realization of negative-refraction effect and superlenses in the optical region.

Transmission-Line Networks Cloaking Objects From Electromagnetic Fields

We consider a novel method of cloaking objects from the surrounding electromagnetic fields in the microwave region. The method is based on transmission-line networks that simulate the wave propagation in the medium surrounding the cloaked object. The electromagnetic fields from the surrounding medium are coupled into the transmission-line network that guides the waves through the cloak thus leaving the cloaked object undetected. The cloaked object can be an array or interconnected mesh of small inclusions that fit inside the transmission-line network.

Modeling of isotropic backward-wave materials composed of resonant spheres

A possible realization of isotropic artificial backward-wave materials is theoretically analyzed. An improved mixing rule for the effective permittivity of a composite material consisting of two sets of resonant dielectric spheres in a homogeneous background is presented. The equations are validated using the Mie theory and numerical simulations. The effect of a statistical distribution of sphere sizes on the increase of losses in the operating frequency band is discussed and some examples are shown.

Equation for the effective permittivity of particle-filled composites for material design applications

A new simple equation for the effective permittivity of particle-filled composites is presented. The equation does not involve any free parameters in addition to the volume fraction of inclusions, the semi-axes of ellipsoidal inclusions and the permittivities of materials. Alternatively, instead of semi-axes, the percolation threshold can be used as a parameter. It is derived in a similar manner as the Maxwell Garnett mixing equation, but by including an enhanced background permittivity effect when the volume filling ratio of inclusions increase. The result is a mixing equation which has the Maxwell Garnett mixing equation as a low volume filling ratio limit, but which approaches the Bruggeman mixing equation as the volume fraction of inclusions increases. The mixing equation is compared with classical mixing formulae, and to numerical simulations and measurements from the literature.

Numerical modeling of disordered mixture using pseudorandom simulations

The electrostatic effective permittivity of samples of three-dimensional random material consisting of equisized spheres is analyzed numerically. The electric field inside a cubical computation domain is calculated by using finite-element method and field calculation software Opera in a supercomputer. The spheres occupy random positions in the cubic computation cell. As the effective permittivity is analyzed numerically, the finite calculation domain makes the structure infinite and periodic. This kind of structure is called pseudorandom material. This study suggests that a relatively small computational domain (around five times the inclusion sphere radius) could be used when modeling random mixture, if the same samples are analyzed using three orthogonal field orientations. The effective permittivity as a function of the volume fraction of inclusions can be described with generalized mixing formula containing a parameter, which is fitted to numerical results.

Microstructure-based numerical modeling method for effective permittivity of ceramic/polymer composites

Effective permittivity was modeled and measured for composites that consist of up to 35vol% of titanium dioxide powder dispersed in a continuous epoxy matrix. The study demonstrates a method that enables fast and accurate numerical modeling of the effective permittivity values of ceramic/polymer composites. The model requires electrostatic Monte Carlo simulations, where randomly oriented homogeneous prism-shaped inclusions occupy random positions in the background phase. The computation cost of solving the electrostatic problem by a finite-element code is decreased by the use of an averaging method where the same simulated sample is solved three times with orthogonal field directions. This helps to minimize the artificial anisotropy that results from the pseudorandomness inherent in the limited computational domains. All the required parameters for numerical simulations are calculated from the lattice structure of titanium dioxide. The results show a very good agreement between the measured and numericall...

TUNABILITY OF GRANULAR FERROELECTRIC DIELECTRIC COMPOSITES

Electrical tunability of a composite consisting of small ferroelectric spheres randomly dispersed into a dielectric background is studied. A new method to calculate the effective permittivity of such a nonlinear composite is introduced. The method is based on the Bruggeman effective medium theory and a specific model for the nonlinear permittivity of the ferrite. The resulting tunability (defined as a measure of the change in the permittivity due to the bias field) is a continuous function of the volume fraction of the ferroelectric material phase in the composite. As an example,SrTiO 3 is studied with two different nontunable background materials.

High-order resonant modes of a metasolenoid

The higher-order resonant modes of a new artificial magnetic particle — metasolenoid — are characterized and the dependency of the resonant frequencies on the dimensions of the metasolenod are studied both analytically and numerically. The study is relevant to the design of new artificial magnetic materials, slow-wave structures, and filters.

Approximations and full numerical simulations for the conductivity of three dimensional checkerboard geometries

This paper focuses on the problem of macroscopic effective permittivity (permeability, conductivity) of a regular three-dimensional two-phase checkerboard structure. There is no exact solution for the Laplace equation in this geometry. Various classical mixing rules and matrix methods exist in the literature to estimate the effective permittivity of mixtures and they are often used beyond their area of applicability. We compare the results from these methods with our full numerical solution of the problem for various contrasts between the phases. The limitations of the approximate models are pointed out

Microwave Transmission-Line Lens Matched with Free Space

In order to mitigate the problem of unwanted reflections from a homogeneous dielectric lens, a lens based on periodic networks of transmission lines is proposed. Since the propagation constant and the impedance of the network can be varied to a large extent independently from each other, it is possible to realize a lens with a high index of refraction while having almost perfect impedance matching with the surrounding medium.