  We present a superconducting metamaterial saturable absorber at terahertz
frequencies. The absorber consists of an array of split ring resonators (SRRs)
etched from a 100nm YBaCu3O7 (YBCO) film. A polyimide spacer layer and gold
ground plane are deposited above the SRRs, creating a reflecting perfect
absorber. Increasing either the temperature or incident electric field (E)
decreases the superconducting condensate density and corresponding kinetic
inductance of the SRRs. This alters the impedance matching in the metamaterial,
reducing the peak absorption. At low electric fields, the absorption was
optimized near 80% at T=10K and decreased to 20% at T=70K. For E=40kV/cm and
T=10K, the peak absorption was 70% decreasing to 40% at 200kV/cm, corresponding
to a modulation of 43%.
