  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 hysteritic in the resonant frequency data,
while the quality factor, $Q$, was less hystertic. The rf power tuning showed
no hysteresis, but did show supression of the $Q$ at high power.
Magneto-optical images reveal inhomogeneous magnetic vortex entry in the dc
field tuning, and laser scanning photoresponse images for a
YBa$_2$Cu$_3$O$_{7-\delta}$ SRR reveals the current distribution in the rings.
