Shabnam Ghadarghadr

An optical reflectarray nanoantenna: the concept and design.

This paper presents the concept and design of a reflectarray nanoantenna at optical frequencies whose elements are nano-sized concentric spherical particles with the core made of ordinary dielectrics and the shell made of a plasmonic material. Modeling approaches based on finite difference time domain (FDTD) numerical method and dipole-modes scattering theory are used to characterize and tune the reflectarray design. A 6x6 elements reflectarray nanoantenna operating at wavelength 357.1nm with narrow beamwidth is presented, and its scanned radiation characteristics for 15 degrees and 30 degrees are demonstrated.

Plasmonic array nanoantennas on layered substrates: modeling and radiation characteristics

In this paper, we theoretically characterize the performance of array of plasmonic core-shell nano-radiators located over layered substrates. Engineered substrates are investigated to manipulate the radiation performance of nanoantennas. A rigorous analytical approach for the problem in hand is developed by applying Greens function analysis of dipoles located above layered materials. It is illustrated that around the electric scattering resonances of the subwavelength spherical particles, each particle can be viewed as an induced electric dipole which is related to the total electric field upon that particle by a polarizability factor. Utilizing this, we can effectively study the physical performance of such structures. The accuracy of our theoretical model is validated through using a full-wave finite difference time domain (FDTD) numerical technique. It is established that by novel arraying of nano-particl and tailoring their multilayer substrates, one can successfully engineer the radiation patterns and beam angles. Several optical nanoantennas designed on layered substrates are explored. Using the FDTD the effect of finite size substrate is also explored.

Dispersion Diagram Characteristics of Periodic Array of Dielectric and Magnetic Materials Based Spheres

In this paper, the characteristics of electromagnetic (EM) waves supported by three dimensional (3D) periodic arrays of dielectric and magneto-dielectric spheres are theoretically investigated. The sphere particles have the potential to offer electric and magnetic dipole modes, where their novel arrangements engineer desired metamaterial performance. A full wave spherical modal analysis is applied to express the electromagnetic fields in terms of the electric and magnetic dipole modes and the higher order terms. Imposing the boundary conditions, determine the required equations for obtaining kd-betad dispersion characteristics. A metamaterial constructed from unit-cells of two different spheres is created, where one set of spheres develops electric modes, and the other set establishes magnetic modes. It is demonstrated that such a composite of high dielectric spheres provides double negative (DNG) metamaterial in a narrow frequency band spectrum where kd,betad<1. We also investigate the dispersion diagram for a 3D array of one-set of highly coupled dielectric spheres. Here, the couplings between the electric and magnetic dipoles of the spheres generate hybrid modes, resulting in a backward wave medium for kd > 1. The developments of DNG and backward wave metamaterials utilizing 3D array of magneto-dielectric spheres located inside space, and dielectric spheres embedded in a negative mu host, are also addressed. The use of magnetic materials allows accomplishing wider dispersion characteristic bandwidths and tunable feasibility.

Negative Permeability-Based Electrically Small Antennas

The goal of this letter is to present the behavior of mu-negative (MNG) metamaterial-based electrically small antennas. The Greens function analysis is applied to characterize the performance of a hemispherical negative permeability (MNG) resonator excited by a slot aperture. The method of moment (MoM) is used to obtain the current distribution over the source excitation. It is illustrated how a resonator composed of negative permeability medium can successfully establish a small antenna element. For small-size structure , the approximated-form Greens function demonstrates the relation between the resonant frequencies and the material parameters. The obtained results are integrated into the design of a MNG slab radiator. The radiation performance of a slab resonator is detailed using a finite difference time domain (FDTD) full wave analysis. The obtained observations may provide road maps for the future design of metamaterial-based subwavelength antennas.

Coupled Dielectric Nanoparticles Manipulating Metamaterials Optical Characteristics

In this paper, we investigate the concept and theory of all-dielectric metapatterned structures that manipulate electric and magnetic optical characteristics. A 3-D array of dielectric particles is designed, where the spheres operate in their magnetic modes and their couplings offer electric modes. An analytical solution for the problem of plane wave scattering by 3-D array of dielectric nanospheres is presented. FW multipole expansion method is applied to express the optical fields in terms of the electric and magnetic dipole modes and the higher order moments. By enforcing the boundary conditions at the surface of each sphere, with the use of the translational addition theorem for vector spherical wave functions, required equations to determine the scattering coefficients are obtained. Novel materials features in optics are demonstrated. Electric and magnetic scattering coefficient resonances around the same frequency band are obtained. It is highlighted how a metapatterned structure constructed from dielectric nanosphere unit cells can provide electric and magnetic modes resulting in backward wave phenomenon. A comprehensive circuit model based on the RLC (resistor, inductor, and capacitor) realization is presented to successfully analyze the scattering performance of a dielectric nanosphere. To better understand the physics of an array of spheres, circuit models for the interactions, and couplings between spheres are also accomplished. The engineered dispersion diagram for a 3-D array of identical highly coupled nanospheres is scrutinized, verifying that the high couplings between spheres can offer the backward wave characteristics.

Characterization of metamaterial-based electrically small antennas

The purpose of this work is to theoretically investigate the behavior of an electrically small antenna enclosed in a metamaterial sphere. We consider the use of magneto-dielectric and negative permittivity materials for constructing the small antenna. The Greens function for the evaluation of the input impedance is derived and the method of moment with Galerkins procedure is used to determine the probe current from which the input impedance of the resonator is calculated. A physical insight is provided, and the effect of metamaterials for bandwidth enhancement is addressed.

Electrically small antennas embedded in metamaterials: Closed-form analysis and physical insight

The main goal of this work is to understand the behavior of electrically small antennas embedded in metamaterials. To do so, the Greens function for small-size antenna located in small-size metamaterial sphere is approximated, then by applying the method of moment with Galerkins procedure the probe current and input impedance as a function of sphere radius, antenna length, and material properties are derived. The results clearly demonstrate that in most of the designs considering the first dominant mode is not sufficient and one needs to consider higher order terms in Greens function expansion to provide an accurate analysis for antenna performance. A physical insight is highlighted.

Array of Plasmonic Nanoparticles Enabling Light Coupling and Guiding in Solar Systems: A Theoretical Analysis

This paper demonstrates a novel approach for enhancing energy coupling into thin-layered solar cells enabled by depositing non-periodic arrays of plasmonic nanoparticles. Theoretical models are developed to characterize the structure and determine novel physical performance.