Justin G. Pollock

Below-Cutoff Propagation in Metamaterial-Lined Circular Waveguides

This paper investigates the propagation characteristics of circular waveguides whose interior surface is coated with a thin metamaterial liner possessing dispersive, negative, and near-zero permittivity. A field analysis of this system produces the dispersion of complex modes, and reveals in detail intriguing phenomena such as backward-wave propagation below the unlined waveguides fundamental-mode cutoff, resonant tunneling of power, field collimation, and miniaturization. It is shown how the waveguide geometry and metamaterial parameters may be selected to engineer the lined waveguides spectral response. Theoretical dispersion and transmission results are closely validated by full-wave simulations.

Anisotropic metamaterial optical fibers.

Internal physical structure can drastically modify the properties of waveguides: photonic crystal fibers are able to confine light inside a hollow air core by Bragg scattering from a periodic array of holes, while metamaterial loaded waveguides for microwaves can support propagation at frequencies well below cutoff. Anisotropic metamaterials assembled into cylindrically symmetric geometries constitute light-guiding structures that support new kinds of exotic modes. A microtube of anodized nanoporous alumina, with nanopores radially emanating from the inner wall to the outer surface, is a manifestation of such an anisotropic metamaterial optical fiber. The nanopores, when filled with a plasmonic metal such as silver or gold, greatly increase the electromagnetic anisotropy. The modal solutions in such anisotropic circular waveguides can be uncommon Bessel functions with imaginary orders.

Miniaturized Circular-Waveguide Probe Antennas Using Metamaterial Liners

This work presents the radiation performance of open-ended circular-waveguide probe antennas that have been miniaturized by the introduction of thin metamaterial liners. The liners introduce an HE11 mode well below the natural cutoff frequency, which provides substantial gain improvements over a similarly sized waveguide probe. A new feeding arrangement employing a shielded-loop source embedded inside the miniaturized waveguide is developed to efficiently excite the HE11 mode and avoid the excitation of other modes across the frequency-reduced band while maintaining the antennas compactness. A metamaterial-lined circular-waveguide probe antenna operating over 42% below its natural cutoff frequency is designed to provide a radiation efficiency of up to 28.8%. A simple, printed-circuit implementation of the metamaterial liner based on inductively loaded wires is proposed and its dispersion features are discussed.

Realization of ∈-negative-near-zero metamaterial liners for circular waveguides

This paper investigates the realization of epsilon-negative and near-zero (ENNZ) metamaterial liners for below-cutoff propagation in circular waveguides using an inductively loaded thin-wire grid composed of radially and azimuthally directed wires. Full-wave parametric simulations show how parameters such as periodicity in ρ, φ, and z, along with the degree of inductive loading can alter the dispersion of the structures supported modes.

Effective-Medium Properties of Cylindrical Transmission-Line Metamaterials

This letter presents a cylindrical negative-refractive-index transmission-line (NRI-TL) metamaterial composed of radially oriented NRI-TL layers. This arrangement introduces radially inhomogeneous and anisotropic effective permeability and permittivity, whose profiles may be extracted from full-wave simulations using a discrete multilayer approximation. It is shown that a cylindrical NRI-TL metamaterial of single-unit-cell thickness may be described as a homogeneous, anisotropic effective medium exhibiting resonances akin to those observed in isotropic cylindrical structures, which may be used to enhance the radiated power of nearby antennas. An analytical formulation describing the near fields, radiation patterns, and power-ratio enhancement displays very good agreement with full-wave simulations.

A class of circular waveguiding structures containing cylindrically anisotropic metamaterials: Applications from radio frequency/microwave to optical frequencies

This paper investigates a class of circular waveguiding structures containing anisotropicmetamaterials and explores their potential benefits in applications from RF to optical frequencies. The introduction of anisotropy in these waveguides is shown to provide substantial control of the dispersion and field distributions of several supported modes. For exotic material parameters such as permittivity and permeability that are typically associated with metamaterials, intriguing propagation phenomena such as backward-wave behavior, frequency-reduced modes, monomodal propagation, and field confinement are observed and provide enabling functionalities for a wide range of RF/microwave and optical applications.

Experimental Verification of Below-Cutoff Propagation in Miniaturized Circular Waveguides Using Anisotropic ENNZ Metamaterial Liners

This paper presents experimental verification of below-cutoff transmission through miniaturized waveguides whose interior is coated with a thin anisotropic metamaterial liner possessing epsilon-negative and near-zero properties. These liners are realized using a simple printed-circuit implementation based on inductively loaded wires, and introduce an HE11 mode well below the natural cutoff frequency. The inclusion of the liner is shown to substantially improve the transmission between two embedded shielded-loop sources. A homogenization scheme is developed to characterize the liners anisotropic effective-medium parameters, which is shown to accurately describe a set of frequency-reduced cutoffs. The fabrication of the lined waveguide is discussed, and the experimental and simulated transmission results are shown to be in agreement.

Miniaturized circular waveguide probe antennas using ENNZ metamaterial liners

This paper investigates the miniaturization of circular open-ended waveguide (OEWG) probe antennas by lining their interior with thin epsilon-negative-and-near-zero (ENNZ) metamaterial liners. Comparisons are drawn between the radiation characteristics of the miniaturized and conventional-sized OEWG probes, focusing on their return loss, far-field patterns, and radiation efficiency.

Anisotropic Metamaterial optical fibers: Bessel modes with imaginary orders and nanoporous alumina microtubes

A nanoporous alumina microtube with radially emanating nanopores or plasmonic nanocylinders creates a structural anisotropy and makes possible anisotropic optical fibers with mode structure and properties unattainable hitherto in step-index or photonic crystal fibers.

Analysis of propagation in metamaterial-lined circular waveguides

This paper studies the propagation characteristics of circular waveguides whose interior surface is coated with a thin metamaterial liner possessing dispersive, negative, and near-zero permittivity. Treating the waveguide as inhomogeneously filled, a field analysis of this system produces the dispersion of complex modes, and reveals intriguing phenomena such as below-cutoff propagation and field collimation.