Abdelilah Mejdoubi

Dielectric response of perforated two-dimensional lossy heterostructures: A finite-element approach

Finite-element simulations of the effective complex permittivity of perforated two-dimensional (2D) lossy heterostructures are reported. The method is computationally inexpensive and is suited for simulations where the tacit assumptions are the following: (1) the composite behaves like a homogeneous medium with an effective (relative) permittivity e=e′−je″ and (2) the porous medium is characterized by a perforated 2D object having arbitrary shape, e.g., split rings, honeycomb lattice, and Sierpinski carpet. These shape functionals have many applications to the scattering of wave and are also important for describing effective properties of particle dispersions. Our calculations provide insights into a variety of tuning parameters influencing e including the surface fraction and perimeter of inclusion, the permittivity contrast between the inclusion and the matrix, and the shape of the holes. For a 2D composite structure containing a deterministic fractal inclusion we explicitly demonstrate that the e′ and...

Numerical calculations of the intrinsic electrostatic resonances of artificial dielectric heterostructures

In order to study the intrinsic electrostatic resonances (ERs) of artificial dielectric heterostructures, we develop an efficient effective-medium-based method for modeling the effective permittivity, with careful attention paid to several key factors controlling ERs. Our method relies on finite element modeling and is applicable to inclusions with complex boundaries, e.g., fractal inclusion. A series of isolated and square arrays of several types of negative-permittivity media is considered. The inclusion shapes investigated can be considered as cross sections of infinite three-dimensional objects, where the properties and characteristics are invariant along the perpendicular cross-sectional plane. The continuum model used in this work is accurate only if the homogeneous representation of the composite structure makes sense, i.e., quasistatic approximation. It is found, among the conclusions of the article, that the effective permittivity of the composite (lossless) structures versus surface fraction cur...

Finite-difference time-domain simulation of heterostructures with inclusion of arbitrarily complex geometry

Currently, there is a great interest in tailoring the polarization properties of composite materials with the goal of controlling the dielectric behavior. This paper reports finite-difference time-domain (FDTD) modeling of the dielectric behavior of two-dimensional (2D) lossless two-phase heterostructures. More specifically, we present extensive results of 2D FDTD computations on the quasistatic effective permittivity of a single inclusion, with arbitrarily complex geometry (regular polygons and fractals), embedded in a plane. The uniaxial perfectly matched layer-absorbing boundary condition is found adequate for truncating the boundary of the 2D space because it leads to only very small backreflections. The effectiveness of the method is demonstrated by the variety of geometries modeled, i.e., regular polygons and fractals, and permittivity contrast ratios which allows us to distinguish between effects of surface fraction and effects of morphology. Our calculations show that geometrical effects can give ...

Intrinsic electrostatic resonances of heterostructures with negative permittivity from finite-element calculations: Application to core-shell inclusions

Herein, we report finite-element calculations of the effective (relative) permittivity of composite materials consisting of inclusions and inclusion arrays with a core-shell structure embedded in a surrounding host. The material making up the core of the two-dimensional structures, or cross sections of infinite three-dimensional objects (parallel, infinitely long, and identical cylinders) where the properties and characteristics are invariant along the perpendicular cross sectional plane, is assumed to have a negative real part of the permittivity, while the coating material (annular shell) is considered to be lossless. While strictly valid only in a dc situation, our analysis can be extended to treat electric fields that oscillate with time, provided that the wavelengths and attenuation lengths associated with the fields are much larger than the microstructure dimension in order that the homogeneous (effective-medium) representation of the composite structure makes sense. While one may identify features of the electrostatic resonance (ER) which are common to core-shell structures characterized by permittivities with real parts of opposite signs, it appears that the predicted ER positions are sensitive to the shell thickness and can be tuned through varying this geometric parameter. For example, we observe that the ER is broadened and shifted as the loss and the shell thickness are increased, respectively. We also argue that such core shell may also be valuable in controlling ER characteristics via polarization in an external electric field. In addition, by considering calculations of the electric field distribution, we find that the ER results in very strong and local-field enhancements into small parts of the shell perimeter. Our findings open up possibilities for the development of hybrid structures that could exploit the ER features for a particular application.

A compact model of precessional spin-transfer switching for MTJ with a perpendicular polarizer

Magnetic Tunnel Junction (MTJ) devices are CMOS compatible with high stability, high reliability and non-volatility. A macro-model of MTJ with precessional switching is presented in this paper. This model is based on Spin-Transfer Torque (STT) writing approach. The current-induced magnetic switching and excitations was studied in structures comprising a perpendicularly magnetized polarizing layer (PL), an in-plane magnetized free layer (FL), and an in-plane magnetized analyzing layer (AL).

Controllable effective complex permittivity of functionally graded composite materials: A numerical investigation

A ubiquitous issue in dielectric heterostructures is to understand the relation between unconventional materials and their effective polarization properties (complex permittivity, polarizability, factor of depolarization). In this context, graded composite materials (GCMs), in which the constituent material properties can vary continuously in space, provide an interesting playground. We report effective permittivity calculations of two-phase GCM, using finite-element (FE) calculations, to understand the effects of shape, size, and intrinsic permittivity of the different components of the material. Our analysis shows that purposely introduced gradients in the permittivity of inclusion can be used to tune the effective permittivity of the GCM. Our FE calculations quantitatively test recent predictions of the effective permittivity of GCM having general power-law gradient inclusions based on the recently developed Wei-Poon-Shin theory [Phys. Lett. A 336, 264 (2005)]. The agreement between the FE data and the...

Controlling intrinsic electrostatic resonances of negative permittivity artificial multilayers

We report a numerical study of the electrostatic resonances (ERs) in arrays of elliptical particles with a core-shell structure embedded in a surrounding host. The core medium is supposed to have a complex permittivity with a negative real part, while the shell and the host have real and positive permittivity. These simulations are valid within the quasistatic approximation, when that all length scales must be much smaller compared to both the wavelength of the wave in the medium and the skin depth. The ER features can be tuned by properly selecting the core and shell material parameters, i.e., by tuning the shell thickness, the core loss, and the aspect ratio of the elliptical particle. In addition, very large enhancements of the local field of the order of a few hundreds can be achieved at the resonance. Since the enhanced fields are localized at the perimeter of the core-shell structures, they can serve as a local probe of the dielectric environment in small parts of the particle perimeter. Apart from ...

SPICE modelling of precessional spin-transfer switching in MRAM cells with a perpendicular polarizer

The development of hybrid Magnetic-CMOS circuits such as MRAM (Magnetic RAM) and Magnetic logic circuit requires efficient simulation models to describe the magnetic devices electrical behavior. A compact-model of Magnetic Tunnel Junctions (MTJ) is presented in this paper. This device is the most commonly used magnetic components in CMOS circuits. This model is based on Spin-Transfer Torque (STT) with a perpendicular polarizer writing approach. The current-induced magnetic switching and excitations was studied in structures comprising a perpendicularly magnetized polarizing layer (PL), an in-plane magnetized free layer (FL), and an in-plane magnetized analyzing layer (AL).