Demonstration of Negative Refraction in a Cutoff Parallel-Plate Waveguide Loaded With 2-D Square Lattice of Dielectric Resonators

A 2D negative-refractive index metamaterial is proposed and investigated, which is composed of a parallel-plate waveguide loaded with a square lattice of disc-type dielectric resonators. Collective and macroscopic behavior of the dielectric resonator lattice under the fundamental TE resonance gives negative effective permeability, whereas the parallel-plate waveguide below the cutoff for TE modes provides negative effective permittivity. Thus, the double-negative condition for the propagated waves with the appropriate polarizations can establish the left-handedness. Equivalent-circuit models are shown to give a good insight into the physical mechanism of the guided waves. Numerical simulation of the 2D dispersion diagram verifies the existence of the left-handed (LH) guided modes along with the isotropic characteristics. A triangular prism of the proposed LH structure that is sandwiched by the right-handed parallel-plate waveguides is designed and fabricated to directly observe the negative refractive index of the LH waveguide. The numerical and experimental results validate the negative refraction for the proposed structure.

I. Introduction

Negative-Refractive-Index metamaterials or left-handed (LH) metamaterials [1], [2] have been intensively investigated for applications to planar super lenses, near-field imaging, antenna, and so forth that covers microwave, millimeter-wave, terahertz, as well as optical regions. The LH metamaterials have negative effective permittivity and permeability simultaneously, resulting in the negative refractive index and support of backward wave propagation. Most conventional LH metamaterials are classified mainly into two types of structures: one is the combination of high- polarization-selective magnetic and electric resonators, such as a combination of split-ring resonators and thin wires [2], and the other is a transmission line loaded with series capacitances and shunt inductances [3]–[6]. It is noted that they are both made of metals. However, metals are no longer a good conductor and can cause some serious insertion loss when they are used in resonant sections at higher frequencies such as the terahertz region and beyond. Therefore, it is desirable to propose and design new configurations of LH materials in these frequency regions.