Properties of waveguides filled with anisotropic metamaterials

Abstract

Metamaterials are artificially structured composite materials that show unusual properties not usually available in natural materials. In general, metamaterial structures and properties are anisotropic. A waveguide filled with an anisotropic metamaterial shows unique properties not achievable in conventional waveguides, such as propagation of backward waves and modes below the cut-off frequencies of the conventional fundamental mode, zero group velocity etc. The waveguide filler material can be anisotropic with the tensorial permittivity and permeability components having positive or negative values, and combinations thereof, giving rise to a rich variety of phenomena. Further, modes in a cylindrical waveguide filled with a hyperbolic metamaterial are described by unusual Bessel modes of complex orders. In many situations, the wave propagating region is isotropic, and it is enclosed by anisotropic metamaterials with different thicknesses and contrarily the propagating region is might be anisotropic that is enclosed by isotropic in some other situations. Various metamaterial waveguide geometries like a pair of parallel plates, waveguides with rectangular or cylindrical cross-section filled with anisotropic metamaterials as well as hollow-core waveguides with metamaterial claddings or linings have been demonstrated experimentally. The anisotropy can be uniaxial or biaxial depending on the orientation of structure. Here we review the advances in the theory and applications of waveguides filled with subwavelength structured metamaterials with anisotropic or even hyperbolic properties across the electromagnetic spectrum. By examining the field behaviour in such waveguides, connection is made to the extraordinary transmission of light through arrays of subwavelength sized apertures in a metallic screen. Potential applications range from enhanced MRI imaging and electromagnetic shielding at radio frequencies to intriguing imaging applications and efficient coupling of the emitted radiation from small sources into waveguides at optical frequencies.

Keywords

• Metamaterials
• Structured waveguides
• Anisotropic materials
• Hyperbolic dispersion
• Split ring resonator
• Thin wire media