Browsing by Author "Wallen, Henrik"

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  • Backscattering enhancement, vanishing extinction, and morphological effects of active scatterers
    (2023-01-01) Sihvola, Ari; Wallen, Henrik; Yla-Oijala, Pasi; Parveen, Rashda
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Using analytical Lorenz-Mie scattering formalism and numerical methods, we analyze the response of active particles to electromagnetic waves. The particles are composed of homogeneous, non-magnetic, and dielectrically isotropic medium. Spherical scatterers and sharp and rounded cubes are treated. The absorption cross section of active particles is negative, thus showing gain in their electromagnetic response. Since the scattering cross section is always positive, their extinction can be either positive, negative, or zero.We construct a five-class categorization of active and passive dielectric particles.We point out the enhanced backscattering phenomenon that active scatterers display and also discuss extinction paradox and optical theorem. Finally, using COMSOL Multiphysics and an in-house method-of-moments code, the effects of the non-sphericity of active scatterers on their electromagnetic response are illustrated.
  • Complex electromagnetic responses from simple geometries
    (2011) Kettunen, Henrik
     Sähkötekniikan korkeakoulu | Doctoral dissertation (article-based)
    The electromagnetic properties of a material arise from its intrinsic microstructure, which may often be very complex. However, materials are usually characterized more simply using macroscopic material parameters, electric permittivity and magnetic permeability. This thesis considers the principles of material modeling from the electromagnetics point of view. The analysis is mostly based on electrostatics. The aim of the thesis is to enhance the understanding of the interaction between matter and the electromagnetic fields, and further, the relation between matter and geometry. The contents of the thesis can be divided into three parts. The first part discusses the concepts of polarization and polarizability and considers the electric reponses of particles with different geometries. Polarizabilities of a three-dimensional hemisphere and a two-dimensional half-disk are solved. The second part studies negative material parameters. The emphasis lies on negative permittivity. Interfaces between permittivities of opposite signs are found supporting surface plasmons, or electrostatic resonances. The occurrence of these resonances is especially studied for a hemisphere and a half-disk. Moreover, it is showed that sharp edges with negative permittivity may support unphysically singular field modes, which in numerical simulations can result in non-convergent solutions. The most efficient way to overcome this problem in computational modeling is to slightly round all sharp corners. The third part focuses on homogenization of composite media. Effective material parameters modeling the response of a thin dielectric composite slab are retrieved. Computational homogenization techniques and their limitations are studied. The results indicate that for a successful homogenization, the unit cells of the slab must remain very small compared with the wavelength. Also, the boundary layers of the slab show higher effective permittivity than the corresponding bulk medium.
  • Finite element method for planar antenna analysis
    (2000) Wallen, Henrik
     Helsinki University of Technology | Master's thesis
  • Integral Operator-Based Characteristic Mode Theory for Conducting, Material, and Lossy Structures
    (2023) Yla-Oijala, Pasi; Kuosmanen, Matti; Wallen, Henrik
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Integral operator-based theory of characteristic modes (CMs) for conducting, material, and lossy structures is reviewed. CMs are defined as solutions of a generalized eigenvalue equation (GEE). These GEEs are presented for various surface and volume integral operators and material structures. Interpretation of the characteristic eigenvalues in terms of electromagnetic power is studied based on the Mie expansion and integral operator formalism. Orthogonality and diagonalizing properties of the CMs are summarized. Challenges related to dielectric-magnetic bodies, lossy structures and spurious modes are discussed, as well as differences between the surface and volume operator approaches.
  • Polarizability of conducting sphere-doublets using series of images.
    (2004) Wallen, Henrik; Sihvola, Ari
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The classical electrostatic problem of two nonintersecting conducting spheres in a uniform incident electric field is considered. Starting from the basic Kelvin’s image principle, the two spheres are replaced with equivalent series of image sources, from which the polarizability is calculated. Explicit expressions for the axial and transversal components of the polarizability dyadic are found by solving the recurrence equations. Efficient numerical evaluation of the different series is also discussed.
  • Polarizability of conducting sphere-doublets using series of images.
    (2004) Wallen, Henrik; Sihvola, Ari
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The classical electrostatic problem of two nonintersecting conducting spheres in a uniform incident electric field is considered. Starting from the basic Kelvin’s image principle, the two spheres are replaced with equivalent series of image sources, from which the polarizability is calculated. Explicit expressions for the axial and transversal components of the polarizability dyadic are found by solving the recurrence equations. Efficient numerical evaluation of the different series is also discussed.
  • Properties of Hybridized Modes in Core-Shell Scatterers
    (2018-09-24) Sihvola, Ari; Tzarouchis, Dimitrios C.; Yla-Oijala, Pasi; Wallen, Henrik
     A4 Artikkeli konferenssijulkaisussa
    The character of resonances in plasmonic core-shell structures is analyzed. In particular, the focus is on the behavior of the weaker (antibonding) resonance. Using the residue expansion of the polarizability, it is shown that to maximize the effect of the antibonding resonance, the ratio of the core radius to the whole sphere radius should be 0.596. The polarizability-based results are compared with full Mie scattering calculations showing fair agreement up to size parameters x=1/3.