Alexander B. Yakovlev

Analytical modeling of conformal mantle cloaks for cylindrical objects using sub-wavelength printed and slotted arrays

Following the idea of “cloaking by a surface” [A. Alu, Phys. Rev. B 80, 245115 (2009); P. Y. Chen and A. Alu, Phys. Rev. B 84, 205110 (2011)], we present a rigorous analytical model applicable to mantle cloaking of cylindrical objects using 1D and 2D sub-wavelength conformal frequency selective surface (FSS) elements. The model is based on Lorenz-Mie scattering theory which utilizes the two-sided impedance boundary conditions at the interface of the sub-wavelength elements. The FSS arrays considered in this work are composed of 1D horizontal and vertical metallic strips and 2D printed (patches, Jerusalem crosses, and cross dipoles) and slotted structures (meshes, slot-Jerusalem crosses, and slot-cross dipoles). It is shown that the analytical grid-impedance expressions derived for the planar arrays of sub-wavelength elements may be successfully used to model and tailor the surface reactance of cylindrical conformal mantle cloaks. By properly tailoring the surface reactance of the cloak, the total scattering from the cylinder can be significantly reduced, thus rendering the object invisible over the range of frequencies of interest (i.e., at microwaves and far-infrared). The results obtained using our analytical model for mantle cloaks are validated against full-wave numerical simulations.

Rectangular waveguide with dielectric-filled corrugations supporting backward waves

A new application for corrugated waveguides as left-handed (LH) meta-material guided-wave structures is investigated. The waveguide is operated below the cutoff of the dominant mode, where the waveguide has an inherent shunt inductance. The dielectric-filled corrugations are used to provide a series capacitance, which, along with the shunt inductance, create the necessary environment to support backward waves. A simple equivalent-circuit model is constructed, and proves quite accurate in determining the dispersion, as well as the scattering characteristics of the structure. Experimental verification of the occurrence of backward waves in the corrugated waveguide is presented. Very good agreement between the results obtained using the equivalent-circuit model and the full-wave finite-difference time-domain solution is achieved. The effect of the various design parameters on the LH propagation bandwidth is investigated. The advantages and possible applications of the structure are discussed.

Nanostructured graphene metasurface for tunable terahertz cloaking

We propose and analyze a graphene-based cloaking metasurface aimed at achieving widely tunable scattering cancelation in the terahertz (THz) spectrum. This ‘one-atom-thick’ mantle cloak is realized by means of a patterned metasurface comprised of a periodic array of graphene patches, whose surface impedance can be modeled with a simple yet accurate analytical expression. By adjusting the geometry and Fermi energy of graphene nanopatches, the metasurface reactance may be tuned from inductive to capacitive, as a function of the relative kinetic inductance and the geometric patch capacitance, enabling the possibility of effectively cloaking both dielectric and conducting objects at THz frequencies with the same metasurface. We envision applications for low-observable nanostructures and efficient THz sensing, routing and detection.

Effects of Spatial Dispersion on Reflection From Mushroom-Type Artificial Impedance Surfaces

In a spatially dispersive medium, the electric dipole moment of an inclusion cannot be related to the macroscopic electric field through a local relation. Several recent works have emphasized the role of spatial dispersion in wire media, and demonstrated that arrays of parallel metallic wires may behave very differently from a uniaxial local material with negative permittivity. Here, we investigate the effect of spatial dispersion on reflection properties of the mushroom structure introduced by Sievenpiper, based on local and nonlocal homogenization methods. The objective of this paper is to clarify the role of spatial dispersion in the mushroom structure and demonstrate that, under some conditions, it is suppressed. The metamaterial substrate, or metasurface is modeled as a wire medium covered with an impedance surface. Surprisingly, it is found that, in such a configuration, the effects of spatial dispersion may be nearly suppressed when the slab is electrically thin, and that the wire medium can be modeled very accurately using a local model.

Theory and implementation of dielectric resonator antenna excited by a waveguide slot

Excitation of dielectric resonator antennas (DRAs) by waveguide slots is proposed as an alternative to traditionally used excitation mechanisms in order to enhance the frequency bandwidth of slotted waveguide radiators and to control the power coupled to the DRA. The analysis is based on the numerical solution of coupled integral equations discretized using the method of moments (MoM). The dielectric resonator (DR) is modeled as a body-of-revolution based on the integral equation formulation for the equivalent electric and magnetic surface current densities. The analysis of an infinite or a semi-infinite waveguide containing longitudinal or transverse narrow slots uses the appropriate dyadic Greens function resulting in closed-form analytical expressions of the MoM matrix. The scattering parameters for a slotted waveguide loaded with a dielectric resonator antenna disk are calculated and compared with finite-difference time-domain results. Bandwidth enhancement is achieved by the proper selection for the antenna parameters.

Characterization of Surface-Wave and Leaky-Wave Propagation on Wire-Medium Slabs and Mushroom Structures Based on Local and Nonlocal Homogenization Models

In this paper, a nonlocal homogenization model is proposed for the analysis of the spectrum of natural modes on sub-wavelength mushroom-type high-impedance surfaces composed of a capacitive grid connected to a grounded wire-medium (WM) slab. Modal characteristics of mushroom structures are studied in conjunction with the surface-wave and leaky-wave propagation on WM slabs based on local and nonlocal homogenization models, showing the importance of spatial dispersion (SD) in WM. It is shown that mushroom structures support proper real (bound) forward and backward modes, whose dispersion determines the stopband properties of the mushroom structure, and proper (exponentially decaying from the surface) and improper (exponentially growing from the surface) complex leaky-wave modes related to the backward and forward radiation, respectively. Results obtained by different homogenization models are compared leading to important conclusions. Specifically, an interesting observation concerns the mushroom structures with short vias, wherein the SD of the WM slab is significantly reduced, and the results of local and nonlocal homogenization models are in excellent agreement.

Excitation of discrete and continuous spectrum for a surface conductivity model of graphene

Excitation of the discrete (surface-wave/plasmon propagation mode) and continuous (radiation modes) spectrum by a point current source in the vicinity of graphene is examined. The graphene is represented by an infinitesimally thin, local, and isotropic two-sided conductivity surface. The dynamic electric field due to the point source is obtained by complex-plane analysis of Sommerfeld integrals, and is decomposed into physically relevant contributions. Frequencies considered are in the GHz through mid-THz range. As expected, the TM discrete surface wave (surface plasmon) can dominate the response along the graphene layer, although this depends on the source and observation point location and frequency. In particular, the TM discrete mode can provide the strongest contribution to the total electric field in the upper GHz and low THz range, where the surface conductivity is dominated by its imaginary part and the graphene acts as a reactive (inductive) sheet. V C 2011 American Institute of Physics. [doi:10.1063/1.3662883]

Global modeling of spatially distributed microwave and millimeter-wave systems

Microwave and millimeter-wave systems have generally been developed from a circuit perspective with the effect of the electromagnetic (EM) environment modeled using lumped elements or N-port scattering parameters. The recent development of the local reference node concept coupled with steady-state and transient analyses using state variables allows the incorporation of unrestrained EM modeling of microwave structures in a circuit simulator. A strategy implementing global modeling of electrically large microwave systems using the circuit abstraction is presented. This is applied to the modeling of a quasi-optical power-combining amplifier.

Circuit modeling of the transmissivity of stacked two-dimensional metallic meshes.

This paper presents a simple analytical circuit-like model to study the transmission of electromagnetic waves through stacked two-dimensional (2-D) conducting meshes. When possible the application of this methodology is very convenient since it provides a straightforward rationale to understand the physical mechanisms behind measured and computed transmission spectra of complex geometries. Also, the disposal of closed-form expressions for the circuit parameters makes the computation effort required by this approach almost negligible. The model is tested by proper comparison with previously obtained numerical and experimental results. The experimental results are explained in terms of the behavior of a finite number of strongly coupled Fabry-Pérot resonators. The number of transmission peaks within a transmission band is equal to the number of resonators. The approximate resonance frequencies of the first and last transmission peaks are obtained from the analysis of an infinite structure of periodically stacked resonators, along with the analytical expressions for the lower and upper limits of the pass-band based on the circuit model.

Reduction of Mutual Coupling Between Neighboring Strip Dipole Antennas Using Confocal Elliptical Metasurface Cloaks

In this paper, we propose a novel approach to reduce the mutual coupling between two closely spaced strip dipole antennas with the elliptical metasurfaces formed by conformal and confocal printed arrays of subwavelength periodic elements. We show that by covering each strip with the metasurface cloak, the antennas become invisible to each other and their radiation patterns are restored as if they were isolated. The electromagnetic scattering analysis pertained to the case of antennas with frequencies far from each other is shown to be as a good approximation of a 2-D metallic strip scattering cancellation problem solved by expressing the incident and scattered fields in terms of radial and angular Mathieu functions, with the use of sheet impedance boundary conditions at the metasurface. The results obtained by our analytical method are confirmed by full-wave numerical simulations.