Abstract
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.
Highlights
IntroductionWe present theoretical and experimental data describing the focusing and dispersion characteristics of a larger device consisting of a 105mm×200mm Negative Refractive Index (NRI) lens interfaced with a 105mm×105mm PositiveRefractive-Index (PRI) medium, the latter of which is constructed using an unloaded microstrip grid
In 2002, a different class of Negative Refractive Index (NRI) metamaterials was introduced, based on the periodic reactive loading of a 2-D transmission line (TL) host medium in a dual configuration [5]. This approach to the design of NRI metamaterials offers several advantages: these structures do not utilize Split Ring-Resonators (SRRs) and are inherently broadband; their unit cells are connected through a transmission line network and they may be equipped with lumped elements, making them simultaneously scalable and compact; and they are truly planar
This dispersion relation reveals a well-defined region of backward-wave propagation extending from the Bragg frequency (960MHz) to approximately 2.5GHz that exhibits a distinct NRI characteristic [5,6], largely in excellent agreement with that predicted by periodic analysis of the corresponding infinite structure presented in Fig. 2
Summary
We present theoretical and experimental data describing the focusing and dispersion characteristics of a larger device consisting of a 105mm×200mm NRI lens interfaced with a 105mm×105mm PositiveRefractive-Index (PRI) medium, the latter of which is constructed using an unloaded microstrip grid. This new prototype offers significant improvements over the smaller structure originally used to verify focusing; the edge effects are minimized to more clearly. The dispersion characteristics of the device are compared with the results of periodic analysis of microwave networks, and we attempt to predict the field distributions through mathematical simulations based on the plane-wave expansion technique
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