SUPERCONDUCTING ARTIFICIAL MATERIALS WITH A NEGATIVE PERMITTIVITY, A NEGATIVE PERMEABILITY, OR A NEGATIVE INDEX OF REFRACTION

Abstract

Artificial materials are media made of inclusions such that the sizes and spacing of the inclusions is much smaller than the incident electromagnetic radiation. This allows a medium to act as an effective bulk medium to electromagnetic radiation. Artificial materials can be tailored to produce desired values of the permittivity, permeability, and index of refraction at specific frequencies. The applications of this tailoring include electromagnetic cloaking, and, theoretically, subwavelength imaging resolution. However, the success of these applications depends on their sensitivity to loss.

This research uses superconducting niobium (Nb) metals to create arrays of wires, split-ring resonators, and a combination of wires and split-ring resonators, with very low loss. These arrays are used to investigate properties of a medium with an index of refraction that contains a bandwidth of frequency where the real part is negative. The Nb wire arrays produce a frequency bandwidth with a negative real part of the permittivity, while the Nb split-ring resonators produce a frequency bandwidth with a negative real part of the permeability. The combination of Nb wires and Nb split-ring resonators creates an artificial medium with a negative real part of the index of refraction.

The electromagnetic transmission of the wires, split-ring resonators, and combination medium is measured in a waveguide as a function of frequency, and models of the permittivity and permeability are used to fit this data. For a single Nb split-ring resonator, the change in the resonant frequency and quality factor with temperature is measured and fit with a two-fluid model of superconductivity. The change in the resonant frequency and quality factor with an applied dc H field and applied power is also measured and compared to, respectively, magneto-optical imaging and laser scanning photoresponse measurements. Bianisotropy and perturbations in the resonant frequency are investigated, and simulated with commercial electromagnetic modeling software. The electromagnetic transmission of a single Nb split-ring resonator is compared to resonators made of YBCO, Copper, and a Nb closed-ring resonator. Similar measurements are made with the single resonators embedded in a metallic wire array.