Chandra S. R. Kaipa

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.

Generalized additional boundary conditions for wire media

We generalize additional boundary conditions (ABCs) for wire media by including arbitrary wire junctions with impedance loading. Special attention is given to the conditions at the interface of two uniaxial wire media with metallic patches at the junction. The derived ABCs are validated against full-wave numerical 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.

Mushroom-Type High-Impedance Surface With Loaded Vias: Homogenization Model and Ultra-Thin Design

In this letter, we study the reflection properties and natural modes (surface waves and leaky waves) of the mushroom-type surfaces with impedance loadings (as lumped loads) at the connection of the vias to the ground plane. The analysis is carried out using the nonlocal homogenization model for the mushroom structure with a generalized additional boundary condition for loaded vias. It is observed that the reflection characteristics obtained with the homogenization model strongly depend on the type of the load (inductive or capacitive) and are in a very good agreement with the full-wave simulation results. The proposed concept of lumped loads enables the design of an ultrathin mushroom-type surface with high-impedance resonance characteristics (zero reflection phase) for oblique incidence at low frequencies with a broad stopband for surface waves.

Transmission through stacked 2D periodic distributions of square conducting patches

In this paper, we study the transmissivity of electromagnetic waves through stacked two-dimensional printed periodic arrays of square conducting patches. An analytical circuit-like model is used for the analysis. The model accounts for the details of the transmission spectrum provided that the period of the unit cell of each patterned layer is well below the wavelength in the dielectric slabs separating the printed surfaces. In particular, we analyze the low-pass band and rejection band behavior of the multilayer structure, and the results are validated by comparison with a computationally intensive finite element commercial electromagnetic solver. The limiting case of an infinite periodic structure is analytically solved and the corresponding band structure is used to explain the passband/stopband behavior of finite structures. In addition, we study in depth the elementary unit cell consisting of a single dielectric slab coated by two metal patch arrays, and its resonance behavior is explained in terms of Fabry-Perot resonances when the electrical thickness of the slab is large enough. In such case, the concept of equivalent thickness of the equivalent ideal Fabry-Perot resonator is introduced. For electrically thinner slabs it is also shown that the analytical model is still valid, and its corresponding first transmission peak is explained in terms of a lumped-circuit LC resonance.In this paper, we study the transmissivity of electromagnetic waves through stacked two-dimensional printed periodic arrays of square conducting patches. An analytical circuit-like model is used for the analysis. The model accounts for the details of the transmission spectrum provided that the period of the unit cell of each patterned layer is well below the wavelength in the dielectric slabs separating the printed surfaces. In particular, we analyze the low-pass band and rejection band behavior of the multilayer structure, and the results are validated by comparison with a computationally intensive finite element commercial electromagnetic solver. The limiting case of an infinite periodic structure is analytically solved and the corresponding band structure is used to explain the passband/stopband behavior of finite structures. In addition, we study in depth the elementary unit cell consisting of a single dielectric slab coated by two metal patch arrays, and its resonance behavior is explained in terms o...

Characterization of negative refraction with multilayered mushroom-type metamaterials at microwaves

In this paper, it is shown that bulk metamaterials formed by multilayered mushroom-type structures enable broadband negative refraction. The metamaterial configurations are modeled using homogenization methods developed for a uniaxial wire medium loaded with periodic metallic elements (for example, patch arrays). It is shown that the phase of the transmission coefficient decreases with the increasing incidence angle, resulting in the negative spatial shift of the transmitted wave. The homogenization model results are obtained with the uniform plane-wave incidence, and the full-wave results are generated with a Gaussian beam excitation, showing a strong negative refraction in a significant frequency band. Having in mind a possible experimental verification of our findings, we investigate the effect of introducing air gaps in between the metamaterial layers, showing that even in such simple configuration the negative refraction phenomenon is quite robust.

Mushroom-type structures with the wires connected through diodes: Theory and applications

In this paper, we establish a general formalism to quantify the interaction of electromagnetic waves with mushroom-type structures (high impedance surface and bi-layer) with diodes inserted along the direction of the wires. The analysis is carried out using the nonlocal homogenization model for the mushroom structure with the generalized additional boundary conditions at the connection of the wires to diodes. We calculate numerically the magnitude and phase of the reflected/transmitted fields in the presence of an ideal and realistic PIN diodes. It is observed that the reflection/transmission characteristics of the mushroom-type structures can be controlled by tuning the working states of the integrated PIN diodes. We realize a structure with a multi-diode switch to minimize the undesired transmission for a particular incident angle. In addition, a dual-band subwavelength imaging lens is designed based on the resonant amplification of evanescent waves, wherein the operating frequency can be tuned by chang...

Broadband negative refraction at microwaves with a multilayered mushroom-type metamaterial

In this paper, a multilayered mushroom-type structure is proposed as a bulk metamaterial, which creates a broadband negative refraction. The metamaterial is modeled using homogenization methods as a multilayered structure formed by a uniaxial wire medium loaded with periodic metallic elements (for example, patch arrays). It is shown that the phase of transmission coefficient decreases with the increasing incidence angle, resulting in the negative spatial shift of the transmitted wave. The homogenization model results are obtained with the uniform plane-wave incidence, and the full-wave CST results are generated with a Gaussian beam excitation, showing a strong negative refraction in a wide frequency band.

Partial focusing by a bulk metamaterial formed by a periodically loaded wire medium with impedance insertions

In this paper, a uniaxial wire medium periodically loaded with metallic patches and lumped impedance insertions is proposed for partial focusing of electromagnetic radiation due to a magnetic line source. The analysis is based on the nonlocal homogenization model for a bi-layer mushroom structure with generalized additional boundary conditions for loaded vias, and it is extended to a multilayered configuration with the transfer matrix approach. The proposed structure exhibits a high transmission and is nearly insensitive to the losses. The analytical results are validated against full-wave numerical simulations.

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.