Ashwin K. Iyer

Planar negative refractive index media using periodically L-C loaded transmission lines

Recent demonstrations of negative refraction utilize three-dimensional collections of discrete periodic scatterers to synthesize artificial dielectrics with simultaneously negative permittivity and permeability. In this paper, we propose an alternate perspective on the design and function of such materials that exploits the well-known L-C distributed network representation of homogeneous dielectrics. In the conventional low-pass topology, the quantities L and C represent a positive equivalent permeability and permittivity, respectively. However, in the dual configuration, in which the positions of L and C are simply interchanged, these equivalent material parameters assume simultaneously negative values. Two-dimensional periodic versions of these dual networks are used to demonstrate negative refraction and focusing; phenomena that are manifestations of the fact that such media support a propagating fundamental backward harmonic. We hereby present the characteristics of these artificial transmission-line media and propose a suitable means of implementing them in planar form. We then present circuit and full-wave field simulations illustrating negative refraction and focusing, and the first experimental verification of focusing using such an implementation.

Negative refractive index metamaterials supporting 2-D waves

Recent demonstrations of negative refraction utilize three-dimensional collections of discrete periodic scatterers to synthesize artificial dielectrics with simultaneously negative permittivity and permeability. In this paper, it is shown that planar, two-dimensional L-C transmission line networks in a high pass configuration can demonstrate negative refraction as a consequence of the fact that such media support propagating backward waves. Simulations illustrating negative refraction and focusing at 2 GHz are subsequently presented.

Transmission line models for negative refractive index media and associated implementations without excess resonators

Recently, three-dimensional composite periodic media comprising split-ring resonators (SRR) and thin wires have been shown to exhibit a negative refractive index in the frequency range around the SRR resonance. In this letter, we propose transmission line models for studying and interpreting the electromagnetic propagation behavior of such materials. Based on these equivalent transmission line models, we show that by periodically loading a network of transmission lines with series capacitors and shunt inductors, a negative refractive index medium can be synthesized without excess resonators, thus leading to wideband behavior. These proposed media have tailorable properties over a broad frequency range. Moreover, they are completely planar, frequency scalable, more compact, and easier to implement for RF/microwave circuit applications than their SRR/wire counterparts.

Experimental and theoretical verification of focusing in a large, periodically loaded transmission line negative refractive index metamaterial

We have previously shown that a new class of Negative Refractive Index (NRI) metamaterials can be constructed by periodically loading a host transmission line medium with inductors and capacitors in a dual (high-pass) configuration. A small planar NRI lens interfaced with a Positive Refractive Index (PRI) parallel-plate waveguide recently succeeded in demonstrating focusing of cylindrical waves. In this paper, we present theoretical and experimental data describing the focusing and dispersion characteristics of a significantly improved device that exhibits minimal edge effects, a larger NRI region permitting precise extraction of dispersion data, and a PRI region consisting of a microstrip grid, over which the fields may be observed. The experimentally obtained dispersion data exhibits excellent agreement with the theory predicted by periodic analysis, and depicts an extremely broadband region from 960MHz to 2.5GHz over which the refractive index remains negative. At the frequency at which the theory predicts a relative refractive index of -1, the measured field distribution shows a focal spot with a maximum beam width under one-half of a guide wavelength. These results are compared with field distributions obtained through mathematical simulations based on the plane-wave expansion technique, and exhibit a qualitative correspondence. The success of this experiment attests to the repeatability of the original experiment and affirms the viability of the transmission line approach to the design of NRI metamaterials.

A Multilayer Negative-Refractive-Index Transmission-Line (NRI-TL) Metamaterial Free-Space Lens at X -Band

A volumetric free-space negative-refractive-index (NRI) transmission-line (TL) metamaterial lens is described that employs fully printed interdigitated capacitors and meandered inductors designed to exhibit NRI properties at X-band (8-12 GHz). The volumetric topology is realized in a layer-by-layer fashion without any vias, which facilitates easy and rapid fabrication. The fabricated lens was tested for its transmission and dispersion properties using a free-space X-band measurement system consisting of an Agilent network analyzer, standard gain horn antennas, and Rexolite dielectric lenses fabricated in-house, and showed good correspondence with simulations. The focusing ability of the multilayer NRI-TL lens was also verified using a free-space field probing system based on small shielded-loop antennas affixed to a computer-controlled xyz-translator apparatus. Arguably, these results represent the first experimental evidence of coupling between TL-based metamaterials and sources in free space.

Free-Space Imaging Beyond the Diffraction Limit Using a Veselago-Pendry Transmission-Line Metamaterial Superlens

Focusing using conventional lenses relies on the collection and interference of propagating waves, but discounts the evanescent waves that decay rapidly from the source. Since these evanescent waves contain the finest spatial details of the source, the image suffers a loss of resolution and is referred to as ldquodiffraction-limited.rdquo Superlensing is the ability to create an image with fine features beyond the diffraction limit, and can be achieved with a ldquoVeselago-Pendryrdquo lens made from a metamaterial. Such a Veselago-Pendry superlens for imaging in free space must be stringently designed to restore both propagating and evanescent waves, but meeting these design conditions (isotropic n = epsivr = mur = -1) has proven difficult and has made its realization elusive. We demonstrate free-space imaging with a resolution over three times better than the diffraction limit at microwave frequencies using a Veselago-Pendry metamaterial superlens based on the negative-refractive-index transmission-line (NRI-TL) approach, which affords precise control over its properties and is also less susceptible to losses than other approaches. A microwave superlens can be particularly useful for illumination and discrimination of closely spaced buried objects over practical distances by way of back-scattering, e.g., in tumour or landmine detection, or for targeted irradiation over electrically small regions in tomography/hyperthermia applications.

Mechanisms of subdiffraction free-space imaging using a transmission-line metamaterial superlens : An experimental verification

This work presents experimental verification of free-space subdiffraction imaging using a Veselago-Pendry superlens [Sov. Phys. Usp. 10, 509 (1968); Phys. Rev. Lett. 85, 3966 (2000)] based on the negative-refractive-index transmission-line approach. The superlens is able to resolve two sources λ0∕3 apart not only at the design frequency of 2.40GHz, where the metamaterial possesses μ and ϵ equal to −μ0 and −ϵ0, respectively, but also at 2.08GHz, where the metamaterial experiences a resonance in μ.

A three-dimensional isotropic transmission-line metamaterial topology for free-space excitation

This work proposes a three-dimensional isotropic transmission-line metamaterial topology for free-space excitation whose negative-refractive-index properties are validated by full-wave simulation. The topology is based on a physical realization of the symmetrical condensed node of transmission-line matrix modeling, but reverses the positions of the inductive and capacitive lumped elements in order to yield an isotropic negative refractive index and good matching to free space.

Volumetric layered transmission-line metamaterial exhibiting a negative refractive index

We examine a new class of volumetric metamaterials based on two-dimensional (2D) transmission-line layers that exhibit a negative refractive index (NRI). The dispersion characteristics of a single 2D layer are revealed through a periodic analysis, and the effective-medium response of the volumetric layered topology is predicted by an intuitive equivalent circuit model. Dispersion and transmission characteristics are also obtained for various designs by using full-wave finite-element method (FEM) simulations, including one design meeting the requirements of Veselagos slab lens in free space, and suggest an isotropic NRI over bandwidths anywhere from 25% to 45%. Finally, the potential to implement these metamaterials from terahertz to near-infrared and optical frequencies is discussed.

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.