Terahertz saturable absorption in superconducting metamaterials

We present a superconducting metamaterial saturable absorber at terahertz frequencies. The metamaterial was designed to have a resonant absorption peak at 0.5 THz for T = 10 K. The absorber consists of an array of split ring resonators (SRRs) etched from a 100 nm YBa2Cu3O7 film. A polyimide spacer layer and gold ground plane are placed above the SRRs using the metamaterial tape concept, 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, broadening the resonance and reducing the peak absorption. At low electric fields, the experimental absorption was optimized near 80% at f = 0.47 THz for T = 10 K and decreased to 20% for T = 70 K. For E = 40 kV/cm and T = 10 K, the peak absorption was 70%, decreasing to 40% at 200 kV/cm, corresponding to a modulation of 43%.