(a) Split ring resonator for high-frequency metamaterials (outer ring diameter is 2.26mm). (b) Spiral resonator for radio frequency metamaterials (outer diameter is 6mm and has 40 turns).
Metamaterials (so called because of their engineered electromagnetic properties) hold great promise for new applications in the megahertz to terahertz bands, as well as optical frequencies. Examples including super-resolution imaging, cloaking, hyperlensing, and optical transformation. Conventional metamaterials are limited in their ability to demonstrate these phenomena because of their ohmic and dielectric losses and dissipation. However, a new class of metamaterials sidestep these problems and have the added benefit of being much smaller, more tunable, and more nonlinear than their ordinary counterparts. University of Maryland researchers are pioneering superconducting metamaterials.
Among the metamaterial designs that feature both negative permittivity and permeability (and thus a negative index of refraction), split-ring resonators (SRRs) placed in a wire array medium have drawn a great deal of attention. These devices generally employ normal metal films on a dielectric substrate, and operate in the gigahertz frequency spectrum and higher
The superconducting materials show zero DC resistance and minimal ohmic losses at finite frequency when they are cooled below their superconducting transition temperature (Tc).
We fabricate our superconducting SRRs from 200nm-thick niobium (Nb) thin films magnetron-sputtered onto quartz single crystals. Microwave transmission measurements are performed by placing the Nb SRRs into a wire-loaded Nb X-band waveguide to create a negative permeability medium and to observe high-Q resonant features below the Tc of the Nb film. These SRRs give sharp peaks (dips) at about 10.77GHz when their resonance frequency is below (above) the cutoff frequency of the waveguide. Since the superconducting state is extremely sensitive to both temperature and magnetic field, we have managed to carry out precise tuning of the permeability and refractive index simply by changing the ambient temperature, the radio frequency (RF) input power, or applying an external DC magnetic field