Evanescent wave amplification has been predicted under the ideal condition that the index of refraction, n=−1+i0 precisely, but is difficult to observe in practice because current metamaterials suffer from high losses. We present experimental results on a metamaterial that employs superconducting Nb metals and low-loss dielectric materials. Results include transmission data on a wire, split-ring resonator, and a combination medium at temperatures between 4.2 and 297K. Evidence of negative effective permittivity, permeability, and a negative effective index passband are seen in the superconducting state between 50MHz and 18GHz. We find a dielectric loss of εeff,2=2.6×10−3 in a superconducting wire array at 10.75GHz.

Metamaterials are artificial structures with unique electromagnetic properties, such as relative dielectric permittivity and magnetic permeability with values less than 1, or even negative. Because these properties are so sensitive to loss, we have developed metamaterials comprised of superconducting waveguides, wires, and split-ring resonators. An important requirement for applications of these metamaterials is the ability to tune the frequency at which the unique electromagnetic response occurs. In this paper we present three methods (unique to superconductors) to accomplish this tuning: temperature, dc magnetic field, and rf magnetic field. Data are shown for dc and rf magnetic field tuning of a single Nb split-ring resonator (SRR). It was found that the dc field tuning was hysteretic in the resonant frequency data, while the quality factor, Q, was less hysteretic. The rf power tuning showed no hysteresis, but did show supression of the Q at high power. Magnetooptical images reveal inhomogeneous magnetic vortex entry in the dc field tuning, and laser scanning photoresponse images for a YBa2Cu3O 7−δ SRR reveals the current distribution in the rings.

We investigate the microwave electrodynamic properties of a single superconducting thin film split-ring resonator ͑SRR͒. Transmission data showed a high-Q stop band for a single Nb SRR ͑Q ϳ 4.5ϫ 10 4 at 4.2 K͒ below T c , and no such feature for a Cu SRR or closed Nb loops. Adding SRRs increased the bandwidth, but decreased the insertion loss of the features. Placing the Nb SRR into an array of wires produced a single, elementary negative-index passband ͑Q ϳ 2.26ϫ 10 4 at 4.2 K͒. Changes in the features due to the kinetic inductance were observed. Models for SRR permeability and wire dielectric response were used to fit the data.

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