I. Introduction
Metamaterials are artificial materials with anomalous electromagnetic properties, synthesized by embedding specific inclusions in a host medium. The attention of the microwave community for these complex media has recently increased significantly, mainly due to the discovery of some of the anomalous properties of these materials that may potentially lead to overcoming some fundamental limits of the present technology, and also due to technological improvements in the manufacturing process. In particular, the current interest is mainly focused on quasi-homogeneous metamaterials in which the effective permittivity and permeability have negative real parts (in a given range of frequency), also named “double negative” (DNG) materials [1]. Some of the main anomalous properties of the wave interaction with these materials, predicted almost forty years ago in a pioneering paper by Veselago [2], have been verified experimentally at the microwave frequencies by the group from UC San Diego [3], who, inspired by the work of Pendry [4], constructed such a DNG composite medium by arranging arrays of small metallic wires and split ring resonators. Various aspects of this class of metamaterials are now being studied by several groups worldwide, and many ideas and suggestions for potential applications of these media have been presented in the technical literature. In particular, we have proposed several ideas in which the pairing of DNG materials with standard dielectrics may overcome the diffraction limit in some different potential applications. For instance, we have shown how sub-wavelength cavities and waveguides loaded with such pairs may in principle support no cut-off resonant and guided modes [5], how in an analogous way open waveguides of different geometries may similarly squeeze the guided beam in a sub-wavelength transverse cross section [6], how sub-wavelength scatterers constituted by such pairings may go into an anomalous resonance drastically enhancing the total scattered power [7], or how such sub-wavelength planar bi-layers may be employed as covers in front of small apertures to dramatically enhance their radiation features [8].