Yashwanth R. Padooru

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.

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.

Line-source excitation of realistic conformal metasurface cloaks

Following our recently introduced analytical tools to model and design conformal mantle cloaks based on metasurfaces [Padooru et al., J. Appl. Phys. 112, 034907 (2012)], we investigate their performance and physical properties when excited by an electric line source placed in their close proximity. We consider metasurfaces formed by 2-D arrays of slotted (meshes and Jerusalem cross slots) and printed (patches and Jerusalem crosses) sub-wavelength elements. The electromagnetic scattering analysis is carried out using a rigorous analytical model, which utilizes the two-sided impedance boundary conditions at the interface of the sub-wavelength elements. It is shown that the homogenized grid-impedance expressions, originally derived for planar arrays of sub-wavelength elements and plane-wave excitation, may be successfully used to model and tailor the surface reactance of cylindrical conformal mantle cloaks illuminated by near-field sources. Our closed-form analytical results are in good agreement with full-w...

A Generalized Additional Boundary Condition for Mushroom-Type and Bed-of-Nails-Type Wire Media

An additional boundary condition (ABC) for mushroom or bed-of-nails metamaterials is generalized for thin 3-D or 2-D material patches or ground planes. It is shown that the usual ABC necessary for the homogenization of these wire-medium metamaterials fails for thin imperfect conductors, and a generalization is presented based on charge conservation. The new ABC leads to results that are in good agreement with full-wave simulations.

New Absorbing Boundary Conditions and Analytical Model for Multilayered Mushroom-Type Metamaterials: Applications to Wideband Absorbers

An analytical model is presented for the analysis of multilayer wire media loaded with 2-D arrays of thin material terminations, characterized in general by a complex surface conductivity. This includes the cases of resistive, thin metal, or graphene patches and impedance ground planes. The model is based on the nonlocal homogenization of the wire media with additional boundary conditions (ABCs) at the connection of thin (resistive) material. Based on charge conservation, new ABCs are derived for the interface of two uniaxial wire mediums with thin imperfect conductors at the junction. To illustrate the application of the analytical model and to validate the new ABCs, we characterize the reflection properties of multilayer absorbing structures. It is shown that in such configurations the presence of vias results in the enhancement of the absorption bandwidth and an improvement in the absorptivity performance for increasing angles of an obliquely incident TM-polarized plane wave. The results obtained using the analytical model are validated against full-wave numerical simulations.

Nonlocal homogenization model for the analysis of absorbing properties of mushroom structures with graphene patches at microwaves

Graphene as a one-atom-thick planar sheet of carbon atoms bonded in a hexagonal structure [1] is characterized by unique electrical, thermal, and mechanical properties, including high electrical conductivity and high electron mobility at room temperature, optical transparency, and breaking strength much greater than steel, among others. All these exotic features make graphene attractive in various applications. In particular, one of the electromagnetic applications of graphene, which is related to the subject of this paper, concerns its use in electrically-thin absorbing structures at microwaves. In [2], it has been shown that graphene patches can be used in high-impedance surfaces acting as tunable absorbing structures. Recent developments in the homogenization methods for the analysis of electromagnetic properties of metamaterial substrates (e.g., wire medium (WM) and mushroom-type surfaces) [3], [4] have opened up a possibility for modeling metamaterials with patterned screens made of graphene. Also, recent advancements in the chemical vapor deposition techniques make possible to fabricate large-scale graphene ribbons and patches [5].

Dual capacitive-inductive nature of graphene metasurface: Transmission characteristics at low-terahertz frequencies

We report on the dual nature (capacitive and inductive) of the surface impedance of periodic graphene patches at low-terahertz frequencies. The transmission spectra of a graphene-dielectric stack shows that patterned graphene exhibits both the low-frequency (capacitive) passband of metal patch arrays and the higher-frequency (inductive) passband of metal aperture arrays in a single tunable configuration. The analysis is carried out using a transfer matrix approach with two-sided impedance boundary conditions, and the results are verified using full-wave numerical simulations.

Mantle cloaking using sub-wavelength conformal metallic meshes and patches

In this paper, we present for the first time a rigorous analytical model of mantle cloaking for cylindrical objects using 2-D sub-wavelength conformal square metallic meshes and patches. The model is based on Lorentz-Mie scattering theory which applies the two-sided impedance boundary condition at the interface of the sub-wavelength elements. It is shown that the analytical grid-impedance expressions derived for the planar FSS grids may be successfully used to model and tailor the surface reactance of cylindrical conformal mantle cloaks. The results obtained using our analytical model for mantle cloaks are validated against full-wave numerical simulations.

Air gap tuning of patch antenna resonance

An adjustable air gap was proposed in 1984 to tune the resonant frequency of patch antennas. This method was investigated theoretically and experimentally for the case of coaxially fed patch only. The main disadvantage of applying this method to tune the frequency of coaxially fed patches is that the center conductor has to be de-soldered and re-soldered every time the width of the air gap is changed. In this paper, we investigate the air gap tuning of a rectangular patch resonance frequency. The patch is fed using aperture coupling or by a stripline. In both cases, changing the air gap width requires no de-soldering and re-soldering. It is found that for the same ratio of air gap width to substrate thickness, the resonant frequency is changed by similar percentage for all these feeding methods.

Multilayered Wire Media: Generalized Additional Boundary Conditions and Applications

The history of homogenization methods describing the interaction of electromagnetic waves with materials/matter (formed by a large number of periodic metal-lattices/atoms) goes a long way back (see, for a detailed historical review, the books by [1] and [2]). Typically, these methods are applied when the size of the material inclusions is small compared to the wavelength of the incident wave. In such cases, the microscopic fluctuations are averaged out to obtain smooth and slowly varying macroscopic quantities that can be used to characterize the long range variations of the electromagnetic waves [3].