  We investigate the nonlinear effect of a planar superconducting metamaterial
made from niobium nitride (NbN) at terahertz frequencies. As the variation of
the incident intense terahertz field alters the intrinsic conductivity in the
NbN, a consequent giant amplitude modulation is observed due to the strong
nonlinearities. The high sensitivity of the chosen metamaterial even allows
observing the nonlinear behaviors at various temperatures, but the resonance
modulation induced by the nonlinear effect was distinct from that induced by
the heating effect. The presented results illustrate a clever implementation of
strongly enhanced nonlinearities and thus may bring nonlinear metamaterials
into novel applications.
