Microwave and Millimeter Wave Technologies from Photonic Bandgap Devices to Antenna and Applications

Abstract

Materials that exhibit negative index (NI) of refraction have several potential applications in microwave technology. Examples include enhanced transmission line capability, power enhancement/size reduction in antenna applications and, in the field of nondestructive testing, improved sensitivity of patch sensors and detection of sub-wavelength defects in dielectrics by utilizing a NI superlens. Since NI materials do not occur naturally, several approaches exist for creating NI behaviour artificially, by combinations of elements with certain properties that together yield negative refractive index over a certain frequency band. Present realizations of NI materials often employ metallic elements operating below the plasma frequency to provide negative permittivity ( ), in combination with a resonator (e.g. a split-ring resonator) that provides negative permeability ( ) near resonance. The high dielectric loss exhibited by metals can severely dampen the desired NI effect. Metallic metamaterials also commonly rely on periodic arrays of the elements, posing a challenge in fabrication. A different approach is to employ purely dielectric materials to obtain NI behaviour by, for example, relying on resonant modes in dielectric resonators to provide and near resonance. Then, the challenge is to design a metamaterial such that the frequency bands in which both and are negative overlap, giving NI behaviour in that band. Two potential advantages to this approach compared with NI materials based on metallic elements are i) decreased losses and ii) simplified fabrication processes since the NI effect does not necessarily rely on periodic arrangement of the elements. This chapter explains the physics underlying the design of purely dielectric NI metamaterials and will discuss some ways in which these materials may be used to enhance various microwave technologies.