Broadband gradient index microwave quasi-optical elements based on non-resonant metamaterials.

Utilizing non-resonant metamaterial elements, we demonstrate that complex gradient index optics can be constructed exhibiting low material losses and large frequency bandwidth. Although the range of structures is limited to those having only electric response, with an electric permittivity always equal to or greater than unity, there are still numerous metamaterial design possibilities enabled by leveraging the non-resonant elements. For example, a gradient, impedance matching layer can be added that drastically reduces the return loss of the optical elements due to reflection. In microwave experiments, we demonstrate the broadband design concepts with a gradient index lens and a beam-steering element, both of which are confirmed to operate over the entire X-band (roughly 8-12 GHz) frequency spectrum.

An efficient broadband metamaterial wave retarder.

Metamaterials with anisotropic electromagnetic properties have the capability to manipulate the polarization states of electromagnetic waves. We describe a method to design a broadband, low-loss wave retarder with graded constitutive parameter distributions based on non-resonant metamaterial elements. A structured metamaterial half-wave retarder that converts one linear polarization to its cross polarization is designed and its performance is characterized experimentally.

Analytical design of conformally invisible cloaks for arbitrarily shaped objects.

To design conformally invisible cloaks for arbitrarily shaped objects, we use the nonuniform rational B -spline (NURBS) to represent the geometrical modeling of the arbitrary object. Based on the method of optical transformation, analytical formulas of the permittivity and permeability tensors are proposed for arbitrarily shaped invisible cloaks. Such formulas can be easily implemented in the design of arbitrary cloaks. Full-wave simulations are given for heart-shaped invisible cloaks and perfectly electrical conducting (PEC) objects, in which we observe that the power-flow lines of incoming electromagnetic waves will be bent smoothly in the cloaks and will return to their original propagation directions after propagating around the object. We also show that the scattered field from the PEC object coated with the invisible cloak is much smaller than that from the PEC core. The application of NURBS in the design of arbitrary cloaks shows transformation optics to be a very general tool to interface with commercial softwares like 3D STUDIOMAX and MAYA.

Experimental demonstration of electromagnetic tunneling through an epsilon-near-zero metamaterial at microwave frequencies.

Silveirinha and Engheta have recently proposed that electromagnetic waves can tunnel through a material with an electric permittivity (epsilon) near zero (ENZ). An ENZ material of arbitrary geometry can thus serve as a perfect coupler between incoming and outgoing waveguides with identical cross-sectional area, so long as one dimension of the ENZ is electrically small. In this Letter we present an experimental demonstration of microwave tunneling between two planar waveguides separated by a thin ENZ channel. The ENZ channel consists of a planar waveguide in which complementary split ring resonators are patterned on the lower surface. A tunneling passband is found in transmission measurements, while a two-dimensional spatial map of the electric field distribution reveals a uniform phase variation across the channel--both measurements in agreement with theory and numerical simulations.

Description and explanation of electromagnetic behaviors in artificial metamaterials based on effective medium theory.

We present a general theory of effective media to set up the relationship between the particle responses and the macroscopic system behaviors for artificial metamaterials composed of periodic resonant structures. By treating the unit cell of the periodic structure as a particle, we define the average permittivity and permeability for different unit structures and derive a general form of discrete Maxwell's equations on the macroscale, from which we obtain different wave modes in metamaterials including the propagation mode, pure plasma mode, and resonant crystal band-gap mode. We explain unfamiliar behaviors of metamaterials from the numerical S parameter retrieval approach. The excellent agreement between theoretical predictions and retrieval results indicates that the defined model and method of analysis fit the physical structures very well. Thereafter, we propose a more advanced form of the fitting formulas for the effective electromagnetic parameters of metamaterials.

Realization of a super waveguide for high-power-density generation and transmission using right- and left-handed transmission-line circuits.

In an earlier work [Cheng and Cui, Phys. Rev. B 72, 113112 (2005)], we have shown theoretically that extremely high power densities can be generated and transmitted in a super waveguide which is filled with homogeneous bilayers of right- and left-handed materials. In this paper, we realize such a super waveguide using right-handed transmission-line (RHTL) and left-handed transmission-line (LHTL) circuits. After a rigorous design of the RHTL-LHTL structure, we observe the generation and transmission of high-power densities in the super circuit waveguide from accurate simulation results. Both lossless and lossy cases have been studied for the LHTL circuit. From the simulation results and the rigorous analysis of energy speeds, we show that high-power flows with opposite directions are excited in the RHTL and LHTL parts of the super waveguide, which form the energy vortices in the waveguide cross section.