Scott M. Rudolph

Generation of Propagating Bessel Beams Using Leaky-Wave Modes: Experimental Validation

We present the experimental generation of Bessel beams using a leaky radial waveguide. The radial waveguide consists of a capacitive sheet over a ground plane. The capacitive sheet is composed of patch elements printed on both sides of a dielectric substrate. The radial waveguide is coaxially fed and supports an azimuthally invariant leaky-wave mode whose normal electric-field component is a truncated, zeroth-order Bessel function. Two prototypes are presented with the same propagation constant and lateral extent, but different attenuation constants. 2D electric field measurements and their respective Fourier transforms validate the operation of the prototypes as Bessel-beam launchers at two frequency bands. Cleaner patterns are achieved by the prototype with lower attenuation constant. The dual-band capability and associated frequency dependent resolution can be useful in near-field planar focusing systems. The proposed structure can be used for generating arbitrary zeroth-order propagating Bessel beams at microwave and millimeter-wave frequencies.

Volumetric negative-refractive-index medium exhibiting broadband negative permeability

In this article, a broadband, volumetric, negative-refractive-index metamaterial is presented and its operation is explained. The structure achieves a negative permeability bandwidth of 60% using a periodic cage of backward-wave transmission lines. The permeability is negative over a broad bandwidth due to contradirectional coupling between a backward wave guided by a transmission line and a forward, free-space wave. Negative permittivity is realized using an array of inductively loaded wires. Propagation within the infinite negative-refractive-index (NRI) medium as well as transmission through finite slabs of the NRI medium is presented. The results demonstrate that the proposed NRI medium exhibits a refractive index equal to −1 at 2.45 GHz and is well matched to free space throughout the NRI bandwidth.

A Broadband Three-Dimensionally Isotropic Negative-Refractive-Index Medium

When designing negative-refractive-index (NRI) metamaterials, scientists have struggled to overcome three primary obstacles: polarization dependence, narrow bandwidth, and fabrication challenges. Here, we address these issues with the design of a three-dimensional, fully isotropic, broadband NRI medium that can be produced on a large scale. The simulated structure exhibits a NRI bandwidth of 24.1%, more than twice that achievable using a typical split-ring resonator/wire medium. A four-hundred-unit-cell slab measuring 15 cm×15 cm×6 cm is fabricated and used as a flat NRI lens. The lens produces a super- resolved focus independent of the type of source or its polarization.

Super-Resolution Focusing Using Volumetric, Broadband NRI Media

We describe the design, fabrication and testing of broadband negative permeability and broadband negative-refractive-index (NRI) metamaterials. The NRI metamaterial is designed to address two major limitations of current NRI media: high loss and narrow bandwidth of operation. The proposed NRI design has a backward-wave bandwidth of 44.3%. This large bandwidth allows the structure to operate away from resonant frequencies, where the loss is significantly lower. The stopband characteristics of a lambda0/3 slab of the negative permeability medium were measured. In addition, the focusing characteristics of the NRI medium were tested and resolution beyond the diffraction limit was experimentally observed. The slab measured in this paper achieved a resolution enhancement of 2.0 over a bandwidth of 0.33%.

Design and Free-Space Measurements of Broadband, Low-Loss Negative-Permeability and Negative-Index Media

We present the design and measurement of broadband, volumetric negative-permeability and negative-refractive-index (NRI) media. Both of these media are fabricated using standard printed-circuit-board techniques and operate at X-band frequencies. The S-parameters of four-cell slabs of the negative-permeability and NRI media are measured, and the material parameters of the NRI lens are extracted. The four-cell-thick (λ0/3) NRI lens exhibits a backward-wave bandwidth of 41.2% and a total loss of 0.67 dB at the operating frequency (where μr ≈ -1). Super-resolved focusing in free space is also demonstrated, and spatial frequencies beyond the free-space wavenumber are recovered over a bandwidth of 7.4%. A focus with a half-power beamwidth of 0.27λ0 is achieved at 10.435 GHz.

Broadband Switching Nonlinear Metamaterial

In this letter, we present a nonlinear metamaterial capable of transitioning between a broadband reflective and broadband nonreflective state depending on the incident power level. Below the activation voltage, broadband reflectivity is created by forbidding propagation through the metamaterial as a result of the effective permittivity and permeability having different signs. Above the activation voltage, reflection is suppressed by matching the effective permittivity and permeability. Full-wave simulations are performed to analyze the electromagnetic characteristics of the structure, with experimental results presented at S-band frequencies in a WR-284 waveguide. These measurements demonstrate a 10 dB reduction in reflected amplitude over an 18% bandwidth, a 3-dB reduction over a greater than 30% bandwidth, and a maximum difference in reflection of nearly 42 dB.

A broadband three-dimensional isotropic NRI medium

In this paper, a three-dimensional, fully-isotropic, broadband negative-refractive-index medium is proposed. The structure has a negative-index bandwidth of 24% and a simulated loss of 0.063dB/cell at the operating frequency of 1.5GHz (where n ≈ −1). The fabrication process for this metamaterial, which utilizes stereolithography, is described. This process allows for mass production of the metamaterials constitutive elements, making the construction of large slabs possible.

The Design of Broadband, Volumetric NRI Media Using Multiconductor Transmission-Line Analysis

Multiconductor transmission-line (MTL) analysis is used to model a broadband, volumetric negative-refractive-index (NRI) medium. Equations for the two-dimensional dispersion characteristics and the Bloch impedance are derived using a simplified periodic MTL analysis that provides insight into the operation of the NRI structure, while still accounting for spatial dispersion. Equations for the resonant points of the NRI medium are also derived, including the magnetic and electric plasma frequencies and the low-frequency backward-wave cutoff. The nature of each resonant point is discussed as well. Finally, this paper presents the modeling of finite structures with rigorous MTL analysis. Using this method, the reflection and transmission coefficients for normal incidence on a four-cell slab of the NRI medium are calculated and compared to full-wave simulation.

Broadband, low-loss negative-permeability and negative-index media for free-space applications

We present broadband negative permeability and negative-refractive-index (NRI) media that operate in free space. Both of these media operate at X-band frequencies and are fabricated using standard printed-circuit-board techniques. The S-parameters of four-cell slabs of the negative permeability and NRI media are measured using a quasi-optical Gaussian beam measurement system. Using these data, the material parameters of the NRI lens are calculated. The four-cell-thick (λ0/3) lens exhibits a backward-wave bandwidth of 41.2% and a total loss of 0.67dB at the operating frequency. Super-resolved focusing in free space is also demonstrated over a bandwidth of 8.4%. A focus with a half-power beamwidth of 0.27λ0 is achieved at 10.435GHz.

Arbitrary Transformation of Antenna Radiation Using a Cylindrical Impedance Metasurface

In this letter, we show that a cylindrical, single-layer impedance metasurface enclosing a radiating source is capable of reshaping the electromagnetic field pattern at a predetermined radius outside of the metasurface. We first present the design procedure of a cylindrical impedance metasurface and then proceed to an example demonstrating the achievable pattern accuracy. Simulation results validate the design methodology and show that complete amplitude control can be achieved in the azimuthal direction for a vertically polarized source.