1. 
 Engström, Christian, et al.
(författare)

A comparison of two numerical methods for homogenization of Maxwell's equations
 2004

Rapport (övrigt vetenskapligt)abstract
 When the wavelength is much larger than the typical scale of the microstructure in a material, it is possible to deﬁne eﬀective or homogenized material coeﬃcients. The classical way of determination of the homogenized coeffi cients consists of solving an elliptic problem in a unit cell. This method and the FloquetBloch method, where an eigenvalue problem is solved, are numerically compared with respect to accuracy and contrast sensitivity. The FloquetBloch method is shown to be a good alternative to the classical homogenization method, when the contrast is modest.


2. 
 Gustafsson, Mats, et al.
(författare)

Optical theorem and forward scattering sum rule for periodic structures
 2011

Rapport (övrigt vetenskapligt)abstract
 Based on energy conservation, an optical theorem is constructed for a slab having an arbitrary periodic microstructure in a plane. A sum rule for low pass structures is derived using analytic properties of Herglotz functions based on causality and passivity. The sum rule relates the extinction cross section to the static polarizability per unit cell, and quantifies the interaction between the slab and electromagnetic fields possible over all wavelengths. The results are illustrated with several numerical and experimental examples.


3. 
 Gustafsson, Mats, et al.
(författare)

Physical bounds and sum rules for highimpedance surfaces
 2010

Rapport (övrigt vetenskapligt)abstract
 Highimpedance surfaces are artificial surfaces synthesized from periodic structures. The high impedance is useful as it does not short circuit electric currents and reflects electric fields without phase shift. Here, a sum rule is presented that relates frequency intervals having high impedance with the thickness of the structure. The sum rule is used to derive physical bounds on the bandwidth for highimpedance surfaces. Numerical examples are used to illustrate the result, and show that the physical bounds are tight.


4. 
 Gustafsson, Mats, et al.
(författare)

Sum rules and physical bounds on passive metamaterials
 2010

Rapport (övrigt vetenskapligt)abstract
 Frequency dependence of the permittivity and permeability is inevitable in metamaterial applications such as cloaking and perfect lenses. In this paper, Herglotz functions are used as a tool to construct sum rules from which we derive physical bounds suited for metamaterial applications, where the material parameters are often designed to be negative or near zero in the frequency band of interest. Several sum rules are presented that relate the temporal dispersion of the material parameters with the difference between the static and instantaneous parameter values, which are used to give upper bounds on the bandwidth of the application. This substantially advances the understanding of the behavior of metamaterials with extraordinary material parameters, and reveals a beautiful connection between properties in the design band (finite frequencies) and the low and highfrequency limit.


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6. 
 Karlsson, Anders, et al.
(författare)

Frequency Selective Structures with Stochastic Deviations
 2003

Rapport (övrigt vetenskapligt)abstract
 This paper deals with the performance of frequency selective structures with defects. A frequency selective structure is in this case a periodic pattern of apertures in a conducting plate. The plate can be of arbitrary thickness. The defects can be by deviations in the placing of the apertures, in the material parameters, or in the shape of the apertures. First the perturbation to the far ﬁeld pattern from a deviation in one aperture is analyzed. Then a statistical analysis is performed for a frequency selective structure where the apertures have a stochastic variation.


7. 
 Kristensson, Gerhard, et al.
(författare)

Homogenization of corrugated interfaces in electromagnetics
 2004

Rapport (övrigt vetenskapligt)abstract
 A surface with periodic corrugations of suffciently small periodicity is shown to be electromagnetically equivalent to an inhomogeneous transition region (slab). Explicit expressions for the inhomogeneous transition region are found for onedimensional corrugations and for twodimensional corrugations a local elliptic problem has to be solved in order to ﬁnd the equivalent electromagnetic properties. The homogenized surface can be characterized by its surface impedance dyadic or its reﬂection dyadic. A few numerical examples illustrate the theory.


8. 
 Larsson, Christer, et al.
(författare)

Waveguide measurements of the permittivity and permeability at temperatures up to 1000 C
 2010

Rapport (övrigt vetenskapligt)abstract
 This paper describes a method to measure the permittivity and the permeability at temperatures from room temperature up to 1000 C using a single rectangular waveguide. The hardware design of the setup that can handle these temperatures and the procedure that is required to correct for the thermal expansion is developed. This includes the sample displacement, the displacement of the calibration reference planes, the thermal expansion of the waveguide and the gap between the sample and the waveguide wall. Measurements on Macor\textregistered and NiZn Ferrite samples are performed in order to evaluate the performance of the setup and the procedure that is used to determine the permittivity and permeability from the measured Sparameters.


9. 
 Ramprecht, Jörgen, et al.
(författare)

Biased magnetic materials in RAM applications
 2007

Rapport (övrigt vetenskapligt)abstract
 The magnetization of a ferro or ferrimagnetic material has been modeled with the LandauLifshitzGilbert (LLG) equation. In this model demagnetization effects are included. By applying a linearized small signal model of the LLG equation, it was found that the material can be described by an effective permeability and with the aid of a static external biasing field, the material can be switched between a Lorentzlike material and a material that exhibits a magnetic conductivity. Furthermore, the reflection coefficient for normally impinging waves on a PEC covered with a ferro/ferrimagnetic material, biased in the normal direction, is calculated. When the material is switched into the resonance mode, we found that there will be two distinct resonance frequencies in the reflection coefficient, one associated with the precession frequency of the magnetization and one associated with the thickness of the layer. The former of these resonance frequencies can be controlled by the bias field and for a bias field strength close to the saturation magnetization, where the material starts to exhibit a magnetic conductivity, one can achieve low reflection (around 20 dB) for a quite large bandwidth (more than two decades).


10. 
 Ramprecht, Jörgen, et al.
(författare)

Magnetic losses in composite materials
 2008

Rapport (övrigt vetenskapligt)abstract
 We discuss some of the problems involved in homogenization of a composite material built from ferromagnetic inclusions in a nonmagnetic background material. The small signal permeability for a ferromagnetic spherical particle is combined with a homogenization formula to give an effective permeability for the composite material. The composite material inherits the gyrotropic structure and resonant behavior of the single particle. The resonance frequency of the composite material is found to be independent of the volume fraction, unlike dielectric composite materials. The magnetic losses are described by a magnetic conductivity which can be made independent of frequency and proportional to the volume fraction by choosing a certain bias. Finally, some concerns regarding particles of small size, i.e., nanoparticles, are treated and the possibility of exciting exchange modes are discussed. These exchange modes may be an interesting way to increase losses in composite materials.

